0

“Thermal response test and

geothermal modelling of

the soil”

Supervisors:

Inga Sorensen

Henrik Bjorn

María Alberdi

Students:

David Canosa Vaamonde

Martín Amado Pousa

Miguel Salgado Pérez

Pedro Rico López

1

ABSTRACT The study of the thermal properties of the soil is one of the most important things when trying to

improve the performance of borehole heat exchangers (BHE) as part of ground source heat

pump (GSHP) systems.

This project is focused on the study of thermal response test (TRT) execution and the

geothermal modelling of the soil behaviour.

Thermal response test is the indirect method to know the most important thermal properties,

thermal conductivity of the soil and thermal resistance of the BHE.

Geothermal modelling is the theoretical study of the soil temperature behaviour along time. This

behaviour is influenced by the amount of energy that can be extracted or stored through the

BHE.

2

ACKNOWLEDGEMENTS Firstly we want to sincerely thank to our families for their support and understanding through the

hard moments of this year.

In relation with VIA University College we are really thankful to our supervisor, María Alberdi

Pagola for sharing her knowledge, help in the execution of the TRT and her supporting in all the

parts of this project. We know we are your favourite group!

We also want to thank Søren Erbs Poulsen for his effort trying to teach us interesting and useful

things, for his help with FEFOW and also for trying to help us to develop our knowledge and our

future career.

Thanks also to Inga Sørensen for her wise advice and support during the development of the

project.

Finally we also want to thank all the people we met in Denmark this year that in one way or

another became part of our lives helping us and becoming our friends.

Thanks to all for all these good moments and experiences shared during this project and during

this whole academic year in Horsens 2013-2014.

3

NOMENCLATURE μ Dynamic viscosity of the carrier fluid [Pa s] � Speed of the carrier fluid inside the pipes [m/s]

Ρ Density [kg m-3]

λ Thermal conductivity [W m-1 K-1]

λeff Effective thermal conductivity

α Thermal diffusivity [m2 s-1]

δ Carrier fluid density in Kg/m3

h Local coefficient of heat transfer [W m-2 K-1]

ɣ Euler’s constant

∆θ Average change in the temperature of the ground

rarray Effective array radius

Bi Biot number

ηH Theoretically maximum specific heat extraction

Tfl Temperatures of the fluid [ºC or K]

ti Temperatures initial of the fluid [ºC or K]

tf Temperatures final of the fluid [ºC or K]

Tf Average or mean BHE temperature [ºC]

Tb Borehole wall temperature [ºC]

T0 Undisturbed ground temperature [ºC]

t Time [s]

T∞ Temperature of the surroundings [K]

T Temperature [ºC or K]

SVC Volumetric heat capacity [MJ m-3K-1

]

SC Specific heat capacity [J K-1

]

Rg Thermal resistance of the surrounding ground [ºC]

Rf Thermal resistance of the carrier fluid inside the pipes [K m W-1

]

Re Reynolds number

Rb Borehole Thermal Resistance [K m W-1

]

rb Borehole Radius [m]

q Heat Transfer Rate from Body to Fluid of surface area [W m-1

]

qi Heat flow density [W /m2]

Q Heat Flow [kW]

k Slope of the Function

Ø Inner Diameter of the Pipe [m]

A Cross-Sectional Area [m2]

ΔƬ Temperature Increment [K]

Rbwh Thermal Resistance of the Grouting Material [K m W-1

]

Rbhf Thermal Resistance of the Pipe Wall [K m W-1

]

4

ACRONYMS BHE Borehole heat exchanger

BTES Borehole thermal energy storage

COP Coefficient of performance

DTH Down the hole hammer

GSHP Ground source heat pump

LS Line source model

SPF Seasonal performance factor

SCW Standing column well systems

TES Thermal energy storage

TRT Thermal response test

UTES Underground thermal energy storage

5

REPORT OUTLINE This project is part of the requirements to get the BSc degree of Civil Engineering with 18 ECTS

credits per student. The main purpose of the project is to reinforce the skills and knowledge

acquired during the studies.

The project was carried out at the Civil Engineering department (energy specialization), at VIA

University College in Horsens (Denmark) during the spring semester of 2014. This final project consists of:

- Bibliographic research: in order to understand the basic concepts regarding geothermal

energy, ground source heat pump, borehole heat exchanger, thermal response test and

energy storage.

- Preliminary study: overview of the current status regarding renewable energy in

Denmark and the description of the energy park at VIA University College.

- Thermal Response Test: with the experimental section, the results and the

interpretation.

- Thermal Energy Storage: briefly introduction and prior assumptions to end up with the

results and interpretation.

- Conclusion and further research proposals.

6

TABLE OF CONTENTS 1. INTRODUCTION ................................................................................................................. 8

2. BIBLIOGRAPHIC RESEARCH ............................ .............................................................. 9

2.1. GEOTHERMAL ENERGY INTRODUCTION .............................................................................. 9 2.1.1. Ground source heat ............................................................................................... 9 2.1.1. Heat properties of the soil ..................................................................................... 9 2.1.1. Heat transfer ........................................................................................................ 11 2.1.2. Geothermal gradient ............................................................................................ 12 2.1.1. Conductive heat flow ........................................................................................... 13

2.2. GROUND SOURCE HEAT PUMP SYSTEMS (GSHP) .......................................................... 14 2.2.1. Heat pumps ......................................................................................................... 14 2.2.2. GSHP systems .................................................................................................... 15

2.3. BOREHOLE HEAT EXCHANGERS (BHE) ............................................................................ 18 2.3.1. Definition .............................................................................................................. 18 2.3.2. BHE installation ................................................................................................... 18 2.3.3. BHE Systems ...................................................................................................... 20 2.3.4. Thermal resistance concept ................................................................................ 21

2.4. THERMAL RESPONSE TEST (TRT) ................................................................................... 24 2.4.1. Definition .............................................................................................................. 24 2.4.2. Operation of the test ............................................................................................ 25 2.4.3. Test evaluation .................................................................................................... 26 2.4.4. Limitations of thermal response tests .................................................................. 27

2.5. UNDERGROUND THERMAL ENERGY STORAGE (UTES) ..................................................... 27 2.5.1. Borehole thermal energy storage (BTES) ........................................................... 28

3. PRELIMINARY STUDY ................................. ................................................................... 30

3.1. CURRENT STATUS OF RENEWABLE ENERGY IN DENMARK ................................................. 30 3.1.1. Shallow Geothermal Energy Status in Denmark ................................................. 30 3.1.2. Groundwater and Environmental Protection and Legislation .............................. 31 3.1.3. Geological frame in Denmark .............................................................................. 32

3.2. FACILITY DESCRIPTION .................................................................................................. 32 3.2.1. Location of project area ....................................................................................... 32 3.2.2. Description of VIA Energy Park ........................................................................... 33 3.2.3. Borehole description and geology ....................................................................... 34

4. EXPERIMENTAL SECTION OF THERMAL RESPONSE TEST ..... ................................ 36

4.1. THERMAL PROPERTIES ESTIMATION ................................................................................ 36 4.2. UNDISTURBED GROUND TEMPERATURE ........................................................................... 37 4.3. THERMAL RESPONSE TEST .............................................................................................. 39

4.3.1. Analysis method .................................................................................................. 39 4.3.2. Process summary ................................................................................................ 42 4.3.3. Experimental setup for thermal response test ..................................................... 43 4.3.4. Soil thermal conductivity and borehole thermal resistance calculations ............. 45

5. RESULTS, INTERPRETATION AND COMPARISON OF TRT ..... .................................. 48

5.1. RESULTS ....................................................................................................................... 48 5.2. INTERPRETATION OF RESULTS AND COMPARISON ............................................................. 51

5.2.1. Comparison of the results by intervals of time .................................................... 51 5.2.2. Comparison of the results with GeRT software ................................................... 52 5.2.3. Comparison of the results with previous TRT ..................................................... 54 5.2.4. Comparison of the results with FEFLOW simulation ........................................... 56

6. STUDY AND RESULTS OF THERMAL ENERGY STORAGE ....... ................................ 59

6.1. ENERGY STORAGE IN VIA 14 AND EXTRACTION IN VIA 13 .................................................. 59

7

6.1.1. Volumetric heat capacity calculation ................................................................... 59 6.1.2. Calculation according VDI 4640 .......................................................................... 60 6.1.3. Line source method ............................................................................................. 61 6.1.1. FEFLOW model ................................................................................................... 63

6.2. SEASONAL THERMAL ENERGY STORAGE IN VIA 14 ............................................................ 66

7. INTERPRETATION OF THERMAL ENERGY STORAGE RESULTS .. ........................... 72

8. CONCLUSIONS AND FURTHER RESEARCH PROPOSAL ......... ................................. 73

9. REFERENCES .................................................................................................................. 74

10. APPENDIX ........................................................................................................................ 79

8

1. INTRODUCTION

Background

VIA University College facilities concerning renewable energy systems and sources give the

opportunity to carry out several kinds of research projects in the Energy Park and the

laboratories.

Currently, shallow geothermal energy is having a high development and acceptance due to its

big potential. This fact together with the possibility to use VIA technical facilities regarding

materials, equipment and installations set up the perfect scenario to develop this research

project about the study of the soil as an energy source and also for heat energy storage

purposes.

In this case the project will be focused on two borehole heat exchangers installed in the Energy

Park. To calculate the thermal conductivity of the soil and the thermal resistance of the borehole

a thermal response test by GeRT machine will be executed.

To carry out this research it is necessary to know some points such as, the effective heat

transfer capacity of the borehole, sizes and configuration, backfill materials and grouting of

BHEs, as well as the geology of the soil around the well.

Objectives

The present project has two main goals.

The first objective consists in a thermal response test in the BHE VIA 14 with the aim to get the

thermal conductivity of the soil around the borehole and the thermal resistance of the borehole

heat exchanger. These results will be interpreted and compared with other calculations and

taken into consideration by different approaches. This part ends with a conclusion about the

reliability of preceding tests and the new results as well as the trustworthiness of equipment.

The second part is developed with the information provided by the TRT. This part consists in a

numerical modelling with FEFLOW software. The aim is to estimate how much energy can be

extract from the BHE VIA 14, how much energy can be stored the BHE VIA 13 and how the soil

will behave along time when heat is being stored or extracted.

To carry out this research project, VIA University College provides the following:

- Thermal Response Test TRT equipment (GeRT).

- Software: FEFLOW and EDD.

- VIA 14 and VIA 13 borehole heat exchangers.

- Different literature sources.

- Digital thermometer.

9

2. BIBLIOGRAPHIC RESEARCH

2.1. GEOTHERMAL ENERGY INTRODUCTION

2.1.1. Ground source heat

Geothermal energy is, literally, the heat contained within the earth that generates geological

phenomena on a planetary scale. Nowadays the term “geothermal energy” is used to describe

the heating energy of the earth that can, or could, be taken advantage of by man. (Dickson &

Fanelli 2004)

Geothermal energy can be divided in high-temperature and low-enthalpy heating energy.

The terms “geothermal energy” or “geothermics” are used to describe the high-temperature

energy that is delivered from the earth’s deep interior or the energy finds in very deep boreholes

or in certain specific locations in the earth’s crust (Banks 2012)

The terms “thermogeology” or “ground source heat” are denominated to describe the low-

enthalpy energy that can be exploited by man at normal temperatures in the shallow subsurface

which is generated by solar energy that has been absorbed and stored in the subsurface and

also can contain a component of geothermal energy from the deep-earth heat flux. (Banks

2012)

However, the European Union has announced that shallow ground source heat is also classed

as geothermal energy. The EU Renewable Energy Directive 2009/28/EC states that

“geothermal energy” means energy stored in the form of heat beneath the surface of solid earth.

(Banks 2012)

In North America, shallow geothermal technology is also known under the term “geoexchange”.

(Geotrainet 2011)

To use the undisturbed low temperatures of the ground, the basis of the shallow geothermal

systems, there are two options:

- Increase or decrease the temperature of geothermal heat to a usable level using

ground source heat pumps.

- Increase or decrease the temperature in the ground by storing heat or extracting

heat in underground thermal energy storage systems.

As far as the terms to describe geothermal energy is concerned, will be used the terms

"geothermal energy" and "ground source heat" to denominate the shallow geothermal energy by

this project.

2.1.1. Heat properties of the soil

Specific heat capacity and heat storage

Specific heat capacity (Sc) is the ability of a medium (soil) to store heat and is defined how the

quantity of heat have to be administrated or extracted to a unit of mass of the substance to

increase or decrease its temperature in 1 degree. The specific heat capacity is expressed by

J·kg-1·K-1.

10

Heat capacity can be expressed also per unit volume. This is termed volumetric heat capacity

(SVC) and is obtained multiplying specific heat capacity by density (kg/m3):

SVC = ρ Sc (J·m-3·K-1)

The exceptionally high volumetric heat capacity of the water, suppose that very porous water

saturated rocks have higher volumetric heat capacities than dry sediments and soils. Many

rocks are composed by solid (matrix), air and water and the effective volumetric heat capacity

can be calculated like an arithmetic mixing model (Banks 2012).

Thermal conductivity

Thermal conductivity (λ) is the capacity of the material to transfer heat by conduction, as

described by Fourier’s’ law (Banks 2012). The thermal conductivity capacity is expressed by

W·m-1·K.

Fourier’s Law of heat conduction defines heat flow density (qi), the vector of specific energy flow

rate, as the product of the thermal conductivity tensor (λij), and the temperature gradient vector

dT/dx.

Equation 2-1: Fourier’s Law

�� = ��� ��� Thermal conductivity for many rocks is, to a good approximation, isotropic, particularly for

volcanic and plutonic rocks. In these cases heat flow will be predominantly vertical, and it is

sufficient to consider only the vertical component of. In contrast to this, thermal conductivity of

many sedimentary and metamorphic rocks is strongly anisotropic, and lateral heat flow will be

significant. (Clauser & Huenges 1992)

The thermal conductivity of rocks depends of the mineral composition, porosity and water

content. In mixed materials, bulk thermal conductivity is some average of the different

constituents. To estimate the porous rock’s bulk thermal conductivity have been proposed

numerous models, but all have their disadvantages. Most of them are valid only for a specific

range of volume ratios (or porosities).

Equation 2-2: Arithmetic mixing model

� = ∑ Ø� ������∑ Ø�����

Equation 2-3: Geometric mixing model

� = � �������

Equation 2-4: Harmonic mixing model

� = ∑ Ø�����∑ Ø�������

11

Diffusivity

Thermal diffusivity (α) is the ratio between the thermal conductivity and the volumetric heat

capacity, it means the ability of a material to conduct thermal energy relative to its ability to store

thermal energy and is expressed by m2·s-1.

Equation 2-5: Diffusivity

α = �Svc

Once known the capacity of the material to transfer heat by conduction (thermal conductivity),

the capacity of the material to store heat (volumetric heat capacity) and the relationship

between these two values (diffusivity) it’s important know how the heat moves through the

subsoil and how it is possible to take advantage of it.

2.1.1. Heat transfer

Heat conduction

The amount of heat transported by conduction from a body with higher temperature to an area

of lower temperature is controlled mainly by two factors: the temperature difference between the

areas considered, and the properties of the material in between the two areas (Geotrainet

2011). It is governed by Fourier’s Law:

Equation 2-6: Fourier’s Law

� = −� � ���

Where Q is the heat flow (W), A is the cross-sectional area (m2), T is the temperature (K), z is

the depth (m) and λ the thermal conductivity (W m-1 K-1).

Heat convection

It happens when heat transfers between a surface and a moving fluid which are at different

temperatures (Pagola 2013).

The rate of convection heat transfer between the body and its environment at that time can be

determined from Newton’s law of cooling as:

Equation 2-7: Heat convection ���� = ℎ �� [��� − ∞] Where Q is the heat transfer (W), h is coefficient of heat transfer (W·m-2·K-1), As is the effective

area of the body (m-2), T(t) is the temperature of the body as a function of time (K) and T∞ is the

temperature of the surroundings (K) (Conduction & Systems n.d.).

When a solid body is being heated by the hotter fluid surrounding it, like could be happened in a

BHE by groundwater, heat is first transferred by convection to the body and subsequently

12

conducted within the body. The Biot number (Bi) is the ratio of the internal resistance of a body

to heat conduction to its external resistance to heat convection. (Conduction & Systems n.d.)

The heat transfer by convection is also important in the design of BHE, where the fluid into the

pipes should work in turbulent flow to decrease the borehole resistance and favour the heat

transfer between the soil and the BHE.

Heat advection

Advection is the transport of heat produced by movement of the media. As far as the heat

extraction and heat storage is concerned, the presence of groundwater movement is very

important due to the heat transfer by advection appears within the soil. Heat advection by

groundwater benefits heat extraction through the BHE but works against heat storage into the

soil.

Heat dispersion

Heat transfer between the groundwater and the soil that arises from the nature of pore water

flow and the heterogeneity of the hydraulic properties of the soil. Dispersion increases with the

average pore water velocity and decreases with lower thermal diffusivity of the soil. Dispersion

increases also in tortuous pore structures of the soil. Compared to the bulk thermal conductivity,

the contribution from dispersion is insignificant at low velocities but could become significant

even dominant at higher velocities.

Heat radiation

Inside soil and rock, heat radiation can be neglected. Therefore, only two transport mechanisms

need to be considered. In many cases, the actual heat transfer in the underground is a mixture

of both conduction and convection, but, for example, in solid rocks without pore space, heat

transfer occurs by conduction only. (Geotrainet 2011)

As far as the heat extraction and heat storage within the soil is concerned, sun radiation might

be important only in the zone of seasonal fluctuation into the soil (Figure 2-1).

2.1.2. Geothermal gradient

The geothermal gradient expresses the increase in temperature with depth in the Earth's crust.

Down to the depths accessible by drilling with modern technology, the average geothermal

gradient is about 2.5-3 °C/100 m. There are, however, vast areas in which the geothermal

gradient differs from the average value. In areas in which the deep rock basement has

undergone rapid sinking, and the basin is filled with geologically “young” sediments, the

geothermal gradient may be lower than 1º C/100 m. On the other hand, in some “geothermal

areas” the gradient is more than ten times the average value. (Dickson & Fanelli 2004)

13

2.1.1. Conductive heat flow

The difference in temperature between deep hotter zones and shallow colder zones generates

a conductive flow of heat from the former to the latter, with a tendency to create uniform

conditions, although, as often happens with natural phenomena, this situation is never actually

attained. The mean terrestrial heat flow of continents and oceans is 65 and 101 m·W·m-2,

respectively. These values are based on 24,774 measurements at 20,201 sites covering about

62% of the Earth's surface. Due to empirical estimators, referenced to geological map units,

heat flow in areas without measurements can be estimated. For example, the University of

North Dakota is currently providing access via internet to an updated heat flow database

comprising data on oceanic and continental areas (Dickson & Fanelli 2004).

Figure 2-1: Geothermal gradient into the soil (Geotrainet 2011)

A constant geothermal gradient supposes undisturbed temperatures within the soil, the basis of

the shallow geothermal systems. The constant geothermal gradient appears at the depth where

the influence of geothermal heat flux is higher than the influence of sun radiation.

14

2.2. GROUND SOURCE HEAT PUMP SYSTEMS (GSHP)

2.2.1. Heat pumps

A heat pump is a device used for heating or cooling purposes whose objective is extracting heat

from a location and delivers it in any other. Heat is conducted through a heat carrier fluid that is

subjected to pressure variations to get changes of state from liquid to gas (and vice versa)

increasing this way the amount of energy transported for the fluid because of the latent heat

necessary to carry out the status changes.

The heat pump system is divided in the next parts:

- Compressor: Device that compress the heat carrier fluid increasing its pressure and

inducing its movement through the heat pump circuit (Geotrainet 2011).

- Condenser: Heat exchanger that extracts heat from its surroundings (Geotrainet

2011).

- Expansion valve: Device that decreases the heat carrier fluid pressure (Geotrainet

2011).

- Evaporator: Heat exchanger that delivers heat from heat carrier fluid to its

surroundings (Geotrainet 2011).

The refrigerant cycle in a heat pump system can be divided in four different parts:

1. Refrigerant circulates through the evaporator in a liquid state with low pressures

provided by the expansion valve. This conditions favours the status change to

steam for which it’s necessary a great heat absorption that is obtained from the

surroundings (Banks 2012) (process 4-1 in Figure 2-2).

2. Heat carrier fluid passes through a compressor that increases its pressure and its

temperature but keeping its vapour state (Banks 2012) (process 1-2 in Figure 2-2).

3. After that, refrigerant crosses the condenser, where a status change from vapour to

liquid occurs. The surroundings absorbs the heat necessary to let the status change

happens and at the end of the compressor the heat carrier fluid is in liquid state and

with a high pressure (Banks 2012) (process 2-3 in Figure 2-2).

4. Finally, heat carrier fluid is induced to a pressure decrease in the expansion valve

and, due to that, the temperature drop too (Banks 2012)

(process 3-4 in Figure 2-2).

Figure 2-2 represents the heat pump cycle in a graph which vertical axis measures pressure (P)

and its horizontal axis enthalpy (h) (Geotrainet 2011).

15

Figure 2-2: Thermodynamic process in a heat pump

For measuring the heat pump efficiency a relation between the useful energy (H) and the power

electricity consumed to harness this energy (E) is done (Banks 2012). This term is called

coefficient of performance (COP) and it can be estimated on the next way:

Equation 2-8: Heating COP

#$%&'()*+, = -. = ∆ℎ01+2'+3'4∆ℎ01564'3314 7 1

Equation 2-9: Cooling COP

#$%0119*+, = -. = ∆ℎ':(614()14∆ℎ01564'3314 7 1

In case of heating useful energy is provided by the condenser and in cooling it’s extracted by

the condenser.

The biggest advantages in heat pump systems are that the efficiency (COP) is over one

hundred per cent, that is to say, the energy consumption (electricity used by the compressor) is

much lower than the energy provided by the heat pump, and the heat pump can reverse the

“natural” heat flow, it means, heat transfer is produced from a cold source to a hot destination

(Geotrainet 2011).

2.2.2. GSHP systems

A ground source heat pump (GSHP) is a system which harnesses the relatively constant

temperature of a ground source (soil, ground water or surface water) to perform a heat

exchange with it through a heat pump device. Energy is transferred by, at least, one heat carrier

fluid, but in most of cases complementary heat exchangers are established and more than a

fluid is used (Huttrer 1997; Banks 2012). Although ground source temperatures are quite

steady, some variations are produced altering the amount of energy that can be extracted from

the ground source or delivered to it. For this reason, the efficiency of the heat GSHP (COP) is

not a completely reliable data to know the efficiency of the systems taking into account that it

represents the efficiency in an instantaneous portion of time. In order to get a more realistic

estimation of the efficiency a long-term average COP is established, it’s known as seasonal

16

performance factor (SPF). A seasonal performance factor can be defined for a heating or

cooling season taking into account the total useful energy (H), the electrical energy consumed

by the compressor (E), the supplementary pump consumption (Epump) and any backup

immersion (Ebackup) or supplementary energy transmission that could be necessary take into

account (Banks 2012).

Equation 2-10: Seasonal performance factor

;%< = -�. + .6>56 + .?(0@>6 + ⋯ �

There are different kinds of GSHP systems depending on the way the heat exchange is carried

out. The main types are:

Ground water heat pump systems:

These systems are characterized for extracting heat from ground water which is conducted

through pipes and then delivered again in the subsoil. Its establishment depends directly of an

aquifer able to provide enough water to support the heating or cooling loads (Sanner et al.

2003; Banks 2012).

- Open loops: In open loop systems, water is extracted through a well and, after

extracting energy from it, it’s delivered to the source again, it can be done through

another well or other methods, but never through the same well. These systems are

not expensive and its efficiency is quite high, however, its maintenance is more

laborious, because of the requirement of keeping the heat exchanger clean of

organic matter or dirt that could prevent the operation of the heat pump, and the

discharge site must be carefully chosen (Banks 2012; Huttrer 1997; Deng 2004).

- Standing column wells (SCW): An only well is used for extracting and injection of

groundwater. Water is extracted from near the bottom of the well and it is returned

to the top after. It could be logical thinking that water in SCW would tend to warm or

chill (depending if heating or cooling) but the thermal inertia of so big volume of

water moderates the temperature (Huttrer 1997; Deng 2004).

Closed loop systems

Those in which the heat transfer is produced through a ground heat exchanger (Sanner et al.

2003; Banks 2012).

- Direct circulation systems: The heat exchange is produced directly between the

evaporator (condenser for cooling) of the heat pump and the soil. The main

advantage of this system is its relatively high efficiency in relation with other kinds;

however, its use is fallen because of environmental reasons. The heat exchanger

buried in the soil is usually made of cupper, thus susceptible to corrosion or

damage, and the refrigerant which circulates through it could be dangerous in case

of spill (Banks 2012).

17

- Indirect circulation systems: The heat exchange between the heat pump and the

soil is produced through a closed loop in which a water-based carrier fluid circulates

to interchange energy. Efficiency slightly drops because of the energy losses

generated in the heat exchange. Indirect circulation systems can be subdivided in

the next typologies:

⋅ Horizontal closed loops: The heat exchanger is disposed in a 1-2

meters depth trench, because of that, it collects solar energy absorbed

by the shallowest part of the earth. Its installation is cheap but a big

area is needed to cover the necessary heat loads (Huttrer 1997; Banks

2012).

⋅ Pond and lake loops: The heat exchanger is installed in a pond or a

lake with which energy is interchanged. In order to prevent

environmental issues the lake must be large enough to avoid

temperature variations that damage autochthonous fauna (Banks

2012).

⋅ Vertical closed-loops arrays or borehole heat exchangers (BHE): The

heat exchanger is disposed in a vertical well that usually reaches depth

between 40 and 180 meters. Its installation is more expensive due to

deeper excavations but the necessary area of soil is much lower

(Huttrer 1997; Banks 2012). A deeper explanation of BHE performance,

characteristics and typologies will be done in point 3.3.

⋅ Energy piles: These systems take advantage of the large drillings made

in many geotechnical situations for structural reasons disposing heat

exchangers inside the piles which shape the foundation of the building

(Banks 2012).

⋅ Heat pipes: This system is pretty similar to BHE system explained

before but it has a notorious difference, it doesn’t need a pump in the

heat exchanger for producing the refrigerant circulation. The borehole

bases its performance in convection streams that are formed because

of the temperature differences between the shallowest and the deepest

parts of the borehole. The main disadvantage of this system is that it

can only be used for heating (Banks 2012).

- Hybrid closed loop: When energy exchanged from the soil is not enough to cover

the expected loads, a supplementary system can be installed covering this lack.

This kind of systems is known as hybrids. Most common uses of hybrid system are

supplementary cooling ponds and cooling towers for cooling demands and solar

collector addition for heating demands (Earth River Geothermal n.d.).

18

2.3. BOREHOLE HEAT EXCHANGERS (BHE)

2.3.1. Definition

A borehole heat exchanger (BHE) is a shallow geothermal method to extract heat energy of the

soil through heat exchanger installed into the borehole. The heat exchanger is formed by close

loop pipes to operate by conduction. To extract energy of the soil, a brine (pure water or with an

antifreeze additive) flows along the pipes connected to a heat pump where the heat exchange

happens. The BHE needs a pump to return the fluid from the deepest point of the borehole to

the top in order to keep a constant movement of the system brine.

The main parts of the borehole are:

- Heat exchanger: slim plastic (polyethylene) pipes filled with brine that absorbs heat from

the ground and is delivered to the heat pump.

- Grouting: a sealant, thermal enhancer and water. Grouting is used to increase the

thermal conductivity between the surrounding underground and the heat exchanger.

2.3.2. BHE installation

Before the installation of BHE it is necessary to do a study of the soil in order to obtain the

information about the geology.

Other factor to take into account is the cost of the drill. Its represent the biggest part of the

installation budget.

To install the BHE is necessary to drill a vertical or sloped well with a diameter between 75-

200mm with between 30-300m of depth.

The meanly methods to do the drill are by percussion, rotary and roto-percussion. There are

also other relatively recent techniques like coil tubing, sonic drilling, horizontal directional drilling

(HDD).

Percussion

The most important method of percussion is cable tool percussion. This method uses a heavy

metal piece joined to a cable. The metal piece hit the soil repeatedly, lifted and dropped

breaking the soil by a cable tool rig. With each hit the cable turns the metal piece to facilitate the

drill in each stroke. Also water is added if the well is dry to do faster the job. The length of the

cable is adjusted as progresses the deep of the well. The rate of perforation is usually less than

10m per day.

Rotary

The process consists in drill by rotation the soil with a sharp spinning drill bit in the bottom.

There are different kinds of drill bit types, each one according to the hardness of the soil

condition. As the perforation progresses pieces of metal are assembled to the drill bit. The

excavated material is extracted with a bolt, keeping the hole opened.

To prevent the collapse of the well during the drilling process a steel casing is used.

19

Other system use mud and other additives. This mix is circulated down through the well to clean

the bottom of the hole. The mix is prepared and store in a tack in the surface.

Roto-percussion

Roto-percussion uses the percussion and rotation to do the borehole drilling using a

jackhammer or a hydraulic hammer to transmit intensive impact energy into the rock breaking

the soil formation with repeated hits, between 500-2000 per minute. On every stroke the drill

torques the point of impact doing more easily the disaggregation of the rock. The cuttings are

expulsed toward the surface by water of compressed air.

This system is classified according to the point of impact:

Top hammer: The strokes are transmitted from the top of the drill pipe to the base. This system

is used to do well lesser than 50m.

Down the Hole Hammer (DTH): unlike of top hammer the DTH has a piston into the hammer in

the bottom just above of the drill bit.

One the well has been done the next step is install the heat exchanger into the well.

During the installation is necessary to do a visual inspection to detect damages on the BHE

pipes. The installation of the BHE has to be as vertically as possible. To facilitate, weigh is

added on the bottom of the BHE.

After the insertion of the heat exchanger, the pipes are cut to the required length and filled with

the brine (water and antifreezes).

Grouting

The grouting is a mixture of a sealant (usually bentonite) a thermal enhancer (usually quartz

flour because has a high thermal conductivity 3,00 W/mk and water. Grouting is used to have a

good thermal conductivity between the surrounding underground and the heat exchanger. The

grouting also has other important function such as:

- Avoiding pollution from the surface

- Avoid cross contamination between aquifers

- Avoid artesian overflow

Grouting is mixed in tanks and then pumped through the injection pipe into the borehole. During

the injection the steel casing prevents the borehole from collapsing. It is essential that the

grouting fill completely from the bottom to the top of the well avoiding the presence of voids.

Finally the casing can be extracted or not.

A wide range of grouting material with different physical properties exists on the market

according to the criteria of design of the BHE. Some of them have a higher thermal resistance

against freeze, higher thermal conductivity, etc.

After the installation two tests should be performed:

- Flushing the BHE: water is pumped through each BHE circuit to flush out any dirt

particles, preferably from both sides, until each circuit is completely flushed once.

20

- Flow test: the aim of the flow test is to prove that the tested BHE circuit does not

have an increased pressure drop, an increased hydraulic resistance.

2.3.3. BHE Systems

The borehole heat exchanger can be classified according to the number of pipes and

distribution used to transport the heat from the ground to the heat pump. The main heat

exchangers are:

Single U-Pipe

In Figure 2-3 the cold brine flows through the pipe from the top to the deepest point in the

borehole absorbing the heat of soil and returning to the surface.

Advantages:

- Easy to install

- Reliable, proven technology

- Low cost

Disadvantages:

- High borehole thermal resistance

- Not suitable for deep boreholes

Figure 2-3: Single U-Pipe

Double U-Pipe

The system is the same that single U-pipe but employing two pipes as it can be seen in Figure

2-4.

Advantages:

- Lower borehole thermal resistance

- Lower flow resistance

Disadvantages:

- More difficult to install

- More expensive

21

Figure 2-4: Double U-Pipe

Coaxial U-Pipe

The Figure 2-5 shows how this system is formed by one concentric supply pipe and other

external return pipe around the first one. The external supply pipe can be formed for one pipe o

several radial pipes.

Advantages:

- Very low borehole thermal resistance

- Low thermal short-circuiting

- Suitable for deep borehole in rock

Disadvantages:

- High cost

Figure 2-5: Concentric pipe

2.3.4. Thermal resistance concept

The thermal resistance [K m W-1] is the capacity of any material to oppose to heat transfer

through itself. Because the aim of the borehole heat exchanger is to transport the heat from the

soil to the surface this parameter has a great influence on the well performance of the system.

Each material involved in the design of the borehole has itself thermal resistance. This is

necessary to optimize the heat transfers between the undisturbed ground around the borehole

and the fluid carried into the pipes.

As far as the BHE is concerned the heat transfer is represented by two stages:

Thermal resistance of the surrounding ground Rg: where the thermal conductivity of the soil

around the borehole and the groundwater flow are parameters to take into account.

Equation 2-11: Surrounding ground thermal resistance ? − B = C, D �

Where:

22

- Tb: Temperature of the borehole wall

- T0: Temperature of the ground

- q: specific heat extraction rate q [W/m]

Borehole thermal resistance Rb: this parameter represents the relationship between the specific

heat transfer and the difference of temperature between the wall of borehole and the fluid inside

the pipes.

Equation 2-12: Specific heat extraction rate E − ? = C? D �

� = E − ?C?

Where:

- Tb: Temperature of the borehole wall

- Tf: Temperature of the fluid

- q: specific heat extraction rate q [W/m]

As it is shown in the Figure 2-6, the borehole thermal resistance (Rb) is the sum of other thermal

resistances: thermal resistance of the grouting (Rbhw), thermal resistance of the pipes (Rbhf) and

the thermal resistance of the fluid inside the pipes (Rf).

Equation 2-13: Borehole thermal resistance

Rb= Rbhw+ Rbhf+ Rf

Figure 2-6: Borehole thermal resistance section (Monzo 2011)

23

According with these formulas the ideal value to Rb would be Rb =0. But as it is impossible the

only way to get a good performance of the BHE system is keeping Rb lowest as possible.

The parameters that can be modified by the designer to kept the thermal resistance as low as

possible according to (Geotrainet 2011):

- Grouting: the filling material is one of the most important elements to considerer in

the design of BHE system.

On the Figure 2-7 there are some common filling materials and their thermal conductivities.

Figure 2-7: Thermal conductivities for filling materials (Geotrainet 2011)

- Pipe material and diameter.

- Number of pipes: The single U-pipe has a highest thermal resistance of all possible

configurations and coaxial configuration has the lowest.

- Shank spacing: The thermal resistance is affected according the position of the

pipes inside the grouting. Figure 2-8 shows a single U-pipe with three different

positions between pipes.

Figure 2-8: Relation between borehole thermal resistance and thermal conductivity of filling material

(Geotrainet 2011)

24

The thermal resistance increases with distance between the pipes and the borehole

wall.

- Borehole depth: The short-circuiting increases with depth due to the internal heat

transfer between the inlet and outlet channels.

- Borehole diameter: A larger borehole diameter results in a lower ground thermal

resistance.

- Fluid flow rate: The brine flow rate (litre/sec) affects directly the Rb. A low flow rate

increases the thermal resistance as well as the short circuiting between the

channels influencing adversely (Figure 2-9). Is very important to keep the fluid

inside the pipes with a turbulent flow with Reynolds number between 2500 and

4000 in order to ensure an optimal and reasonable balance between heat pump

efficiency and energy requirement.

Figure 2-9: Borehole thermal resistance relation with carrier fluid flow rate (Geotrainet 2011)

2.4. THERMAL RESPONSE TEST (TRT)

2.4.1. Definition

The thermal response test is a suitable method to determine the effective thermal conductivity

of the underground and the borehole thermal resistance (Gehlin 2002).

As an answer to the problem that there is not a direct method to measure the thermal properties

of boreholes in situ, Mogensen (1983) presented a method to calculate these values, the

thermal response test.

It is possible to reduce the borehole thermal resistance improving or modifying the elements

and the design of the borehole; however the thermal conductivity of the ground is specific for

every site and cannot be influenced anyway. With this test it is possible to know both properties.

Mogensen designed a system where a fluid is circulated through the BHE. TRT method is

based in the principle that with a known input power and tracking the mean temperature

development over time, it is possible to measure the heat transported to the ground. This test

gives us the temperature development through time as a curve that can be evaluated. Applying

25

a mathematical model it will be possible to extract the thermal conductivity and the borehole

thermal resistance and use this data to design GSHP systems or UTES systems. The properties

of the rock and borehole collector are technical key parameters (Gehlin 2002).

This method has been developed in Europe and North America since then. There are two main

streams of investigation, the line source theory and the cylinder source model. The first one is

mainly used in Europe and is simpler and faster to use however the second one is more

common in North America. Here the authors will focus on the line source theory.

As the ground is an inhomogeneous medium, it has complicated structures and also

groundwater can be found easily creating disturbances or transporting heat by advection. All

this parameters will affect the measurements of the TRT, so It may be more correct to speak of

an “effective” thermal conductivity λeff (Sanner et al. 2005).

2.4.2. Operation of the test

Before starting the TRT is necessary to know the undisturbed ground temperature (To). This

temperature increases with depth because of the geothermal gradient.

There are several ways to measure the ground temperature. The most common method is to

circulate the brine through the BHE circuit for half an hour before the heater is switched on for

the thermal response test (Gehlin 2002). Doing this, it will give us the temperature of the

ground, which will be in equilibrium with the borehole and the brine. This temperature should be

interpreted as an average of the temperature surrounding the borehole.

It is crucial to set up the system correctly and to minimize external influences (Sanner et al.

2005) before the test starts to avoid problems and disturbed results.

Once the thermal response test has started it will give us the mean fluid temperature (Tf)

development over time. Analysing this temperature is possible to get information about the

thermal properties of the borehole and the ground.

Tf is defined as the average of the inlet and outlet temperatures of the BHE. The estimated

injected heat is used to calculate the average borehole temperature (Tb). When injecting a

constant heat pulse, the temperatures Tf and Tb will vary over time, but after a short initial

period, the temperature difference ΔT = Tf – Tb reaches a constant value. This condition is the

so called steady-flux state, for which ΔT is proportional to the injected heat rate q (Wm-1) per

meter of BHE (Gehlin 2002).

E − ? = C? D �

The time to reach the point where the heat flow through the ground is stable and is not

influenced by the borehole is a key factor and it has to be enough. It is important to ensure this

point to get the real thermal conductivity of the soil, otherwise it will be influenced by the

borehole. Times range from 12 hours to 48 but due to economic reasons it should be reduced to

the minimum possible. As it was said before, it is recommended to prolong these tests to ensure

correct results and a stable heat flow influenced only by the ground.

26

2.4.3. Test evaluation

There are different models, analytical and numerical, to evaluate thermal response tests. With

these models it is possible to predict how the BHE will behave but is necessary to compare the

model with real data. In this case the authors will focus on the line source analytical model but

there are many other options available.

When applying this model three assumptions are made;

- Heat transport in the ground is purely conductive.

- Radial symmetry around the borehole axis.

- Heat conduction in the direction along the borehole axis is negligible (Gehlin 2002).

Line source model requires less information about the BHE properties than cylinder model and

numerical models.

The input data for the line source model is:

- Effective borehole depth.

- Borehole diameter.

- Undisturbed ground temperature (T0).

- Injected power (q).

- Volumetric heat capacity of the ground (cp).

With this data is possible to derive an expression for the temperature as a function of time (t)

and radius (r):

Equation 2-14: Line source model

��, G� = �4I� D .J D �GK/4M��

For the equation, q is the constant heat injection performed during the TRT.

E1 is the exponential integral that can be approximated with the following simple relation:

.J�GK/4M�� = ln �4M�/GK� − P M�/GK 7 5

Euler’s constant, P = 0.5772

Diffusivity, a=λ/cp where λ is the ground thermal conductivity and cp is the ground specific heat

capacity.

The fluid temperature is evaluated by taking the line source temperature at the borehole radius

(r = rb) and adding the effect of the borehole thermal resistance (Rb) between the fluid and the

borehole wall. Thus the fluid temperature as a function of time can be written:

E��� = �4I� D Vln V4M�GK W − PW + � D C? + B

27

2.4.4. Limitations of thermal response tests

TRT can be affected by several conditions which is important to take into consideration. The

accuracy of these tests depends on:

- The variations of the voltage in the grid affect the resistance heater, so fluctuations

are provoked in the power injected into the subsoil.

- Ground water flow due to aquifers or even ground water flow upwards and

downwards in the borehole in poorly grouted BHE.

- Weather external conditions can affect the test.

- Bad insulation of the system.

- Fluxing of the system is really important because the measurement devices can fail

if there is air in the system and the TRT machine can stop due to this.

2.5. UNDERGROUND THERMAL ENERGY STORAGE (UTES)

Thermal energy storage (TES) is the ability to preserve heat in order to use it in a future time.

Several reasons can be hold to carry out a TES and deserve a special mention minimizing

thermal energy losses, attaining high energy recovery (Pavlov & Olesen 2011) getting low-cost

(or even free) energy, obtaining energy independence (Kekelia 2012), prevent greenhouse

effect (Hellström & Larson 2001) or meeting a demand that otherwise would not be met (Hadorn

2004).

Storing heat involves energy losses that depend directly of storage time, temperature, volume

and geometry besides thermal properties of the storage medium. In order to get inexpensive

media for storing large volumes, underground thermal energy storage (UTES) systems were

designed to harness the thermal capacity and the relatively low conductivity of the soil (Pavlov &

Olesen 2011; Kekelia 2012). Four types of UTES systems can be highlighted:

- Aquifer thermal energy storage: Aquifers constrained by impervious layers and with a

low or non-existent flow water can be used for heat storage. The system is shaped by

two wells, one of them extracts water from the aquifer and the other one inject the same

water again after the energy exchange was produced (Pavlov & Olesen 2011).

- Water tank thermal energy storage: It’s based in a steel or reinforced pre-stressed

concrete tank, fully or partially buried in the ground, which contains water in which

stores energy (Pavlov & Olesen 2011).

- Water-gravel pit storage: Systems consisting in a pit, which is waterproofed and

insulated at least in the side walls and on the top, fill of water in which energy is stored.

Its efficiency is lower than water tanks (Pavlov & Olesen 2011).

- Borehole thermal energy storage (BTES): These systems store energy directly in the

soil through borehole heat exchangers. Its main advantage is in reversible heat pumps

in which energy is stored during cooling uses extracting it during heating utilization

(Pavlov & Olesen 2011).

As far as this research is concerned, BTES is going to be analysed deeper meanwhile

remaining solutions are going to be set aside.

28

2.5.1. Borehole thermal energy storage (BTES)

BTES are characterized for storing heat directly in the undisturbed soil through borehole heat

exchangers. BTES are constituted by two basic elements: the geological medium, which

provides the thermal storage capacity, and the heat exchanger, which is responsible of the heat

transfer. Thus, the storage capacity of the system depends directly of the thermal properties of

the ground (volumetric heat capacity and thermal conductivity) and the borehole thermal

resistance (Hellström 1991; Gehlin 1998). The size of storage will be of decisive importance to

the heat loss and storage efficiency, being excessive the heat losses and too low the efficiency

in smaller systems (Meurs 1985).

In order to increase the efficiency and the storage capacity of the system boreholes arrays are

set. According VDI (2001) these systems have the next characteristics:

- Dynamic steady state is reached quickly by the temperatures.

- Interferences between boreholes are not so important than in BHE, that’s why closer

space between boreholes is possible.

- In order to avoid thermal losses, a compact arrangement (circular, hexagonal…) is

preferable to an open or linear array.

- Groundwater flow must be avoided to prevent heat losses.

- More extreme peak temperatures may be possible or even desirable.

According Banks (2012), the total thermal storage (H) of a cylindrical array of boreholes (like the

one shown in Figure 2-2) with a depth D, an effective array radius rarray, a volumetric heat

capacity of the aestifer material SVC and an average change in the temperature of the ground

enclosed by the borehole array ∆θ, may be estimated by:

Equation 2-15: Total thermal storage of a cylindrical array of boreholes - = ;XY D I D G(44(ZK D [ D ∆\

29

Figure 2-10: Borehole heat exchanger array

BTES can also be used to balance heating systems, it means, the source can be chilled during

winter seasons, for extracting energy from it, and heated in summer seasons when the system

is used for cooling. This way, extreme temperatures are avoided, the seasonal performance

factor is increased and the possibilities of any long-term drift in the carrier fluid or the ground are

minimized. Some examples of surplus heat that can be stored are natural solar radiation

absorbed by the ground surface, solar radiation injected into the ground via solar panels, heat

extracted from buildings by cooling systems in summer season or surplus energy from industrial

processes (Banks 2012).

30

3. PRELIMINARY STUDY

3.1. CURRENT STATUS OF RENEWABLE ENERGY IN DENMARK Currently the fossil fuels (Coil, natural gas and petrol) represent the 70,6 % of the energy

demand in Denmark (data from The World Bank Group 2013). Despite energy reservoirs of this

fossil fuel continue to be abundant, the hazard of climate change have done necessary

concentrate of renewable energies in order to reduce of CO2 emissions.

Nowadays Denmark is one of the first countries when it comes to fighting climate change. The

government of Denmark is carrying out plans to supply of 100 % of its energy needs with

renewable sources reducing its energy dependence. The government’s energy policy

milestones up to 2050 are:

2020 Half of the traditional consumptions of electricity will be covered by windy power.

2030 Coal and oil burners will be phased out from Danish power plant.

2035 The electricity and heat supply will be covered by renewable energy.

2050 All energy supply (electricity, heat, industry and transport) will be covered by

renewable energy.

The most used renewable energies in Denmark are: wind, geothermal, waves, solar, biomass

and waste biodegradable. Being biomass, waste and wind en the most important sources of

energy representing 70% of the consumption of renewable energy (data from Danish Ministry of

Climate, Energy and Building)

3.1.1. Shallow Geothermal Energy Status in Denmark

As it have been said, the aim to reduce CO2 emissions and change fossil fuels by renewable

energy sources in 2050, makes that shallow geothermal energy has a large potential to

contribute towards that goal. However, the potential of this type of energy, nowadays in

Denmark, is relatively limited compared to Germany or Sweden (Mahler et al. 2013).

Denmark is situated in a sedimentary basin dominated by soft sediment and a variable depth in

the water table. This gives a 40% lower energy extraction rate than other favourable geological

conditions.

One research carried out to estimate de possible effect of the geologic variation in energy

extraction in Denmark can be consulted in Appendix 1.

Currently the number of ground source heat pumps installed in Denmark is around 27.000. This

figure is increasing 5.000 units per year. Most of these installations are horizontal closed loop

systems being 400-500 of these borehole heat exchangers. Nowadays this figure is changing

significantly; more than a hundred boreholes are installed every year (Figure 3-1).

31

Figure 3-1: Number of shallow geothermal boreholes reported to the national borehole database Jupiter.

Other important advance is the combined use of STES with district heating plants. In Brædstrup

a small village in east Jutland, the district heating company has extended their solar plant with a

pilot project developing a borehole storage system. This has been carried out installing 48

boreholes with 45 meters deep each one. The borehole storage system is heating

approximately 19.000 m3 of soil which is equivalent to 5000 m3 of hot water with a storage

capacity of 275 MWh approximately.

Source: Per Kristensen, Director Braedstrup District Heating Company

3.1.2. Groundwater and Environmental Protection and Legis lation

In Denmark the groundwater aquifers supply all drinking water. Protection of the environment

and groundwater is very important in shallow geothermal projects. Therefore, is very important

the protection and mapping of catchment areas.

The main issues are leakage of water with antifreeze, filtration of surface water along the

borehole wall, cross-connecting different aquifers and unwanted thermal effect on the aquifers.

Aside the temperature of the groundwater in existing aquifers cannot be increased more than 5

ºC being necessary numeral modelling in order to document the temperature.

The legislation also specifies that the anti-freeze agents must be non-toxic and biodegradables.

Safety distances to other heat exchangers and to extraction wells for drinking water are

described too. Furthermore the municipalities has the possibility to increase the requirement of

safety distance to water wells and stipulate special conditions to protect areas against

contamination (Ditlefsen & Vangkilde-Pedersen; 2012).

The lack of hydro-geological background in the administration permits often appears to vary

from one municipality to another. To avoid this administrative heterogeneity, GeoEnergy

prepared guidelines and recommendations for typical hydrogeological situations.(Ditlefsen et al.

2013)

Currently to preserve the Danish environment some specific legislation has been established.

32

- Order on Heat Abstraction and Groundwater Cooling Plants (BEK-1206, 24/11/2006)

- Order on Groundwater Heating (BEK-1203, 20/11/2006)

3.1.3. Geological frame in Denmark

The study of the geology and hydrogeology is fundamental. To estimate the possible energy

extraction, the potential flow of energy is determined by the thermal conductivity and heat

capacity of the rocks or sediments surrounding the borehole. For more information about the

heat capacity for different rocks and sediment Appendix 1 can be consulted.

In Denmark the geology is dominated by soft sediment with a variable depth to the groundwater

table.

The sediment consist in a sedimentary basin dominated by shallow marine to deep marine

clastic and biogenic sediment cover quaternary deposit. In some areas the quaternary cover is

slender and limestone, mud and sand are present near the surface. In the northern and south-

western of the country succession of quaternary sediment are common. Aside thick succession

of quaternary deposit is found in valleys (Ditlefsen et al. 2013). The variation in the geological

thick of the quaternary deposits is high so is necessary to do studies and estimate the possible

energy extraction from the borehole.

Other important information regarding shallow geothermal energy is:

- Net solar insolation: 400 kWh m-2 /year.

- Heat flux from earth core: from 0,20 to 0,35 kWh m-2/year.

- Geothermal gradient: from 25 to 30 ºC km.

- Seasonal variation of the ground temperature due to change in ambient temperature

reach 15 m depth. (Gehlin 2002)

3.2. FACILITY DESCRIPTION

3.2.1. Location of project area

The location of the project is in Horsens. Is a Danish city in east Jutland (Figure 3-2). This

project has been carried out in Energy Park an experimental area with several installations

about renewable energies that belong to Via University College.

33

Figure 3-2: VIA University College

3.2.2. Description of VIA Energy Park

VIA Energy Park is located at VIA University College Campus. These facilities have several

installations focusing in renewable energy sources (Figure 3-3).

VIA provide the tools and workspace necessary to carry out student research projects focusing

in sustainable energy. Here, VIA's experts can collaborate with students, external research

institutions and private partners.

34

Figure 3-3: VIA Energy Park facilities

Renewable energy installations in the park focus in photovoltaic solar panels, thermal solar

panels and borehole heat exchangers.

To develop this project part of the borehole heat exchanger are used as well as some Geo

Laboratory equipment.

In VIA Energy Park there were 6 boreholes, solar energy installations, 3 water tanks and 2 heat

pumps into the laboratory building.

These are the existing boreholes:

- VIA 10: Single U configuration system borehole (30 m depth with sensors placed

around)

- VIA 11: Single U configuration system borehole (30 m depth with sensors placed

around)

- VIA 12: Single U configuration system borehole (100 m depth)

- VIA 13: Double U configuration system borehole (96 m depth)

- VIA 14: Single U configuration system borehole (100 m depth)

- VIA 15: Coaxial system borehole (100 m depth)

Our project is about geothermal energy and it’s focused only in two BHE: VIA 13 and VIA 14.

3.2.3. Borehole description and geology

As has been said, in Denmark, the geology is dominated by soft sediment with a variable depth

to the groundwater table.

35

The geology were the boreholes are located correspond to the Oligocene and the thickness of

the quaternary sediment is between 50-80 meters according with the data provided by GEUS

(Geological Survey of Denmark and Greenland). The water level in the area is approximately

15m, being the nearest point of water the lake Nørrestrand. For more information about the

sediment and its thickness in Denmark the Appendix 1 shows this information.

The geological description has been obtained from Geus Jupiter (2014).

Due to the high alteration grade of the topsoil in the first meter, and due to the lack of

information, that part has been neglected for the calculations.

The two studied boreholes have been performed with the same drilling system and they have a

separation distance around 10 meters between them. However, each of them has a different

pipe configuration. VIA 13 has a Double U configuration system, while the VIA 14 has a Single

U.

In Appendix 1 a scheme of stratigraphic column of each borehole is given.

The next table provides the most important characteristics of each borehole studied:

Borehole name VIA 13 VIA 14

Borehole number 107,1605 107,1606

Cote level 16,93 17,27

Depth (m) 96 100

Diameter (m) 0,16 0,16

Pipe type Double U SINGLE U

Drilling method Rinse Drilling Rinse Drilling

Water level 15,35 15,35

Lithology Almost

Everything Sand, Glacial Layer.

Almost Everything Sand,

Glacial Layer.

Grouting material Dantonit Dantonit

Pipe material PE Polyethilene PE Polyethilene

Pipe diameter (m) 0,032 0,040

Shank distance (cm) 6 6

Table 3-1: Borehole heat exchangers characteristics.

36

4. EXPERIMENTAL SECTION OF THERMAL RESPONSE TEST

In the experimental section of this project, the authors will develop all the processes to end up

calculating the thermal properties of the ground, thermal conductivity (λ) and the borehole

thermal resistance (Rb). This entire process involves some stages prior to the TRT.

In order to be ready to start the TRT first it is necessary to estimate the thermal properties of the

ground; thermal conductivity and volumetric heat capacity. Secondly the undisturbed ground

temperature is also needed.

4.1. THERMAL PROPERTIES ESTIMATION

For the thermal properties estimation, relevant literature values, the stratification of the borehole

and the results of needle prove tests will be used.

The geological description has been obtained from Geus website with Google Earth.

Because of the works to build the borehole modify the structure and the composition of the

surface of soil, the first meter will not be taken into consideration for the calculations.

Knowing all the λ and Svc values for each of the layers, we will use the arithmetic mean method

to calculate λ and Svc.

Depth (m) Layer thickness

(m) λ (W/mK)

Svc (MJ/m³K)

From To

1 3 2,0 1,54 2,40

3 6 3,0 1,00 1,60

6 9 3,0 2,36 2,20

9 12 3,0 1,40 1,90

12 15 3,0 1,00 1,50

15 18 3,0 2,35 2,50

18 24 6,0 1,40 1,90

24 27 3,0 1,74 2,40

27 45 18,0 1,00 2,00

45 48 3,0 1,31 2,00

48 51 3,0 1,10 2,00

51 54 3,0 1,80 2,40

54 57 3,0 2,40 2,50

57 100 43,0 1,00 2,00

Total depth (m)

99,0

λ (ari) Svc (ari)

1,23 2,03

Table 4-1: Thermal conductivity and volumetric heat capacity estimation

37

Equation 4-1: Thermal conductivity

� = Ʃ*�J^ ∅* ∙ �*Ʃ*�J^ ∅*

Equation 4-2: Volumetric heat capacity

;XY = Ʃ*�J^ ∅* ∙ ;XY*Ʃ*�J^ ∅*

The result for the arithmetic mean value of the thermal conductivity (λ) of the ground

surrounding the borehole is predicted as 1,23 W·m-1·K-1 while the arithmetic mean value of

volumetric heat capacity (Svc) is 2,03 MJ·m-3·K-1.

The value of thermal conductivity is considered not a good estimation. Comparing the result to

the values extracted from the thermal response test the authors assume that the values taken

into consideration on the borehole profile are significantly lower than the reality. This means that

the real soil has a better thermal conductivity than the one estimated previously, so thermal

conductivity estimation cannot be taken as a real value to the design of a GSHP system.

Thermal conductivity estimation value is only used to calculate the break time for steady state

heat flux in the borehole.

4.2. UNDISTURBED GROUND TEMPERATURE

The second step before the TRT is to measure the undisturbed ground temperature.

A good estimate of the undisturbed ground temperature is necessary for a correct design of the

ground heat exchanger (Gehlin 2002).

At the same time the authors also measure the water table level in the ground where the

borehole is placed which gave a result of 15,05 meters depth.

For the undisturbed ground temperature the method performed was:

- Measure the temperature in each meter of the borehole depth.

- 4 minutes interval between steps.

- The arithmetic mean will give an average temperature for the borehole.

The undisturbed ground temperature mean result was 9,56 ºC (Figure 4-1).

38

Figure 4-1: Undisturbed ground temperature along borehole profile

There is another method to measure the undisturbed ground temperature. Circulating the brine

through the borehole for 30 minutes and tracking the temperatures of the brine. However, even

though no heat is injected by the heater during this period, there will always be some heating of

the water from the pump work (Gehlin 2002).

39

4.3. THERMAL RESPONSE TEST

4.3.1. Analysis method

The subsurface and BHE thermal properties (λ and Rb) are calculated by reproducing water

temperature variations between inlet and outlet observed at the heat exchanger pipes during

the TRT. A borehole and adjacent sediments is a composite of cylinders with different thermal

properties. There are a large number of analytical models for simulating heat transport in

different geometries and composites. As regards thermal response test analysis, the heat flow

to or from the BHE is represented as an infinitely long heat source with negligible influence of

heat flows in a direction along the BHE axis. The analysis assumes that the thermal process

depends only on the radial distance from the BHE axis. Line source model and cylinder source

model simulate the heat transfer between the BHE (described as an infinite source) and the

surrounding area (described as a medium of infinite radial extent). These methods need several

simplifications about the geometry of the BHE and heat exchanger pipes. (Gehlin 2002;

Raymond et al. 2008) In this case, line source model (LS) will be used for the analysis of

Thermal Response Test.

Cylinder source model

The cylinder source model approximates the BHE as an infinite cylinder with constant heat flux

and the heat exchanger pipes are represented by an equal diameter cylinder. Line source

model is a simplified variation of cylinder source model.

Line source model (LS)

The line source model is the most common model used for the analysis of TRT and was the

model used in this project. This model is the analytical solution to the equation of transient heat

conduction in a homogeneous and isotropic medium, expressed as function of time and radius

around a line source with constant heat injection rate (Figure 4-2). The soil is considered as a

medium with constant thermal properties and an initial ground temperature known (previously

calculated). The TRT is executed in the range of 50 h.

40

Figure 4-2: Line source model

Equation 4-3: Line source model

�4 = �a = �b = � ∙ �G ∙ G ∙ �\ ∙ �� − ��ac2a + �4c24 + �bc2b� = d D #6 D ��� ∙ �G ∙ G ∙ �\ ∙ ��

Where:

- q = Heat flux (W/m)

- ρ = Density (kg/m³)

- Cp = Specific Heat Capacity (J·kg-1·K-1)

- ρ·Cp = Volumetric Heat Capacity (J·m-3·K-1)

- A = Heat Production (W/m)

�ac2a = �a + ��a�\ ∙ �\

�4c24 = �4 + ��4�G ∙ �G

�bc2b = �b + ��b�� ∙ ��

− ��a�\ �\ − ��4�G �G − ��b�� �� + � ∙ �G ∙ G ∙ �\ ∙ �� = d D #6 D ��� ∙ �G ∙ G ∙ �\ ∙ ��

Fourier’s Law describes thermal conduction on the macroscopic scale and is described:

� = �� D ��

Where:

41

- T = Temperature (K)

- x = Distance (m)

Applying Fourier’s Law in each direction:

�a = ��G ∙ �G ∙ �� ∙��\

�4 = �� ∙ G ∙ �\ ∙ �� ∙ ��G

�b = �� ∙ G ∙ �G ∙ �\ ∙ ���

Replacing:

��G D V�G ∙

��GW =

1G ∙

��\ D V� ∙

��\W =

��� D V� ∙

���W ∙ G = � ∙ G = d D #6 ∙ ���

The analysis assumes that the thermal process depends only on the radial distance and

influence of heat flow in direction along the BHE axis is considered negligible (Figure 4-1):

��G D V�G ∙

��GW = � ∙ G = e D #6 ∙ ���

Integrating the previous formula:

�²�G² =

1G D��G =

d D #6� D ���

To solve this equation, it is assumed that the temperature in the system at the beginning (t=0)

and in the surroundings located at infinite distance from the heat source (r=∞) is constant:

- T (r, t=0) = T0 (undisturbed ground temperature)

- T (r=∞, t) = T0

The soil is considered like a homogenous and isotropic medium and the infinite line source is

located at r=0 with constant heat injection rate. The solution for this equation is expressed as

temperature increment:

g�G, �� = �4 D h D � D i

jk>l

m4²nD(D)

�l

Where:

- l = 4onD(D) - g = �G, �� − B

g�G, �� = �4 D h D � D .J p GK4 D M D �q

Where:

- .J �� = −P − ln�� + ∑ �kJ��rst�DV uovDwDxWr*!D*m*�J

- .J z 4onD(D){ = ln znD(D)4o { − P; if (D)4² 7 5

- λ= thermal conductivity (W·m-1·K-1)

- a =λ/ ρ·Cp = Thermal diffusivity (m2·s-1)

- γ = 0,5772… = Euler’s constant

42

- t = time (seconds)

g�G, �� = �4 D h D � D Vln V

4 D M D �GK W � PW

The average fluid temperature induced by a specific flow rate is calculated at borehole radius

distance from line infinite source and taking into account the borehole resistance:

Equation 4-4: Line source model approximation

E��� = �4 D h D � D Vln V4 D M D �G|K W − PW + � D C| + B

Where:

- r = rb (borehole radio)

- q = Heat flux (W/m)

- RB = borehole resistance

- T0 = undisturbed ground temperature

- E = }~�c}���K (average between the inlet and outlet carrier fluid temperatures)

To estimate the soil thermal conductivity along the time when Tf is learned is necessary to know

the slope of the function through the horizontal axis as logarithm of time (t).

Equation 4-5: Thermal conductivity

� = �4 D h D �

Where:

- � = �� �)o�k�� �)t�� �)o�k� �)t� (slope of the function)

To calculate the borehole resistance along the time once the soil thermal conductivity was

estimated is used the next equation:

Equation 4-6: Thermal borehole resistance

C| = E��� − B� − 14 D h D � D Vln V

4 D � D �G|K D ;XYW � PW

Where:

- SVC = ρ·Cp

- a = λ/ SVC

One of the restrictions of line source model is that the analysis requires a previous estimation of

volumetric heat capacity (SVC).

4.3.2. Process summary

Thermal response test was executed following the next process:

- Measurement of the undisturbed ground temperature.

- Bibliographic estimation of the thermal properties of the ground.

- Thermal response test execution with a constant heat power injection during 50 hours.

43

The data of the inlet and outlet heat carrier fluid obtained have been taken into consideration for

the calculation of the soil thermal properties.

4.3.3. Experimental setup for thermal response test

Equipment

The equipment used to carry out the TRT consists in a device known as GeRT that is produced

by the German company UBeG. The device was elaborated with the specific aim of performing

thermal response tests and its operation consists in heating a known fluid and circulating it

through the borehole circuit while the needed data for the future calculations are measured and

saved.

The main hardware of GeRT is:

- Circulation pump: Device which makes the fluid flow along the closed loop. GeRT’s

circulation pump has an adjustable power between 9 and 130 w.

- Heater: Its function is heating the carrier fluid. GeRT’s heater can provide until 9 Kw

power.

- Vessel: Device responsible of pressure changes regulation.

- Data logger: System which measures carrier fluid’s temperature and flow rate through

sensors and stores the data obtained. Besides, it calculates the consumed amount of

energy based in the data obtained.

- Safety devices: Devices that prevent the equipment from damages. GeRT is equipped

with three different safety devices:

· Paddle flow switch: It disconnects the heater when the flow rate is too low.

· Thermostat: It disconnects the heater when the carrier fluid reaches

temperatures higher than 60º C.

· Pressure control: It prevents the system of overpressures.

Initial assumptions for TRT

Before starting the TRT some assumptions must be taken into account:

- The borehole and the ground must be in thermal equilibrium, so the TRT must not be

done after, at least, 3-5 days after the borehole installation. This interval of time could

be longer in low conductivity formations (Banks 2012).

- Any pipe of the closed loop that’s placed in the surface must be well-insulated and

prevented of any other heat sources than the heater.

- The system must be purged for removing all the air that could alters the results or even

damage the equipment.

- The pressure inside the closed loop must be regulated, being the common values

between 1.00 and 2.00 bars.

- In order to increase the heat convection, and favour this way the heat transfer, a

turbulent flow must be set. Reynolds number (Re) defines the kind of flow the fluid has,

44

and this one is turbulent when Reynolds number is bigger than 4000. This way is

obtained:

Equation 4-7: Reynolds number

Cj = e D ˅ D [�� = � D [��� > 4000 That means:

� > 4000 D ��[� Where δ is the carrier fluid density in Kg/m3, v is its velocity in m/s, DH is the pipe’s hydraulic

diameter in meters, μ the viscosity of the carrier fluid in Pa·s, � the kinematic viscosity of the

carrier fluid in m2/s, A is the area of the pipe’s inner section and Q the flow rate in m3/s.

Heat power must be set high enough to avoid interferences with the atmosphere and for

getting a temperature difference into the inlet and the outlet pipe between 3 and 7ºC.

This is obtained with a heat power of 30-80 w per meter of BHE (Witte et al. 2002;

Sanner et al. 2005).

- As regards TRT length, there are some different theories that differ from each other.

The most tolerant is Perry (1999) theory that consider enough a duration between 12

and 20 hours. According the ASHRAE (2007) the duration must be between 36 and 48

hours meanwhile Sanner (2005) and Gehlin (2002) consider 50 and 60 hours

respectively as the most appropriated duration.

- Data logger make measurements every 10 seconds but, in order to avoid getting a huge

amount of data, other intervals of time can be chosen for receiving an average value of

all the data obtained in that interval. The interval needed must be chosen properly

before starting the test.

After the test

Once the test has been completed, the data must be extracted from the data logger to a

computer. The data obtained for each interval of time, which was set before starting the TRT,

with the equipment used in this project are:

- Date: Date in which measurement has be done.

- Heating work: Energy used by the heater since the TRT has started expressed in

Megawatts hour.

- Water volume: Overall volume of water that goes through a fixed point expressed in

cubic meters.

- Input temperature: Heat carrier fluid temperature during its entrance to the BHE

expressed in Celsius degrees.

- Put temperature: Heat carrier fluid temperature during its departure of the BHE

expressed in Celsius degrees.

- Flow rate: Volume of water per time unit flowing through the closed loop expressed in

litres per hour.

45

- Heating power: Thermal power used by the heater expressed in kilowatts.

Once obtained the data, calculations must be done. GeRT has its own software that allows

doing a fast estimation of the ground thermal conductivity and the borehole thermal resistance.

Software calculation is done after the introduction of some initial parameters and it takes the

data obtained as a basis and Kelvin’s line source theory as method of evaluation.

As far as this project is concerned, calculations will be done by hand for a subsequently

comparison with the results of the software.

4.3.4. Soil thermal conductivity and borehole thermal resi stance calculations

The BHE analysed in this project is known as VIA14 and it’s placed in the energy park of VIA

University College. Its main characteristics are:

- Heat exchanger type ..................................................................................... Single U-pipe - Active length ............................................................................................................. 100 m - BHE diameter ........................................................................................................ 160 mm - Grouting

· Type .......................................................................................................... Dantonit · Thermal conductivity ............................................................................ 2,35 w/mK

- Pipe · Material ................................................................................. PE 100-RC S5 PN16 · Thermal conductivity ............................................................................ 0,42 w/mK · External diameter ....................................................................................... 40 mm · Hydraulic diameter .................................................................................. 32,6 mm

A BHE profile with the different layers of the ground is known, so its thermal properties can be

estimated. They are:

- Thermal conductivity ......................................................................................... 2,10 w/mK - Volumetric heat capacity................................................................................ 2,15 MJ/m3K

Before starting the TRT the groundwater level and the undisturbed ground temperature were

measured obtaining the next results:

- Groundwater level ................................................................................................. 15,05 m - Undisturbed ground temperature ............................................................................ 9,56ºC

Some values are notated in the moment of starting the TRT and in the moment of finishing. The

values obtained are:

46

Starting values Final values

Input temperature 9,62 ºC 26,56 ºC

Output temperature 9,63 ºC 23,43 ºC

Selected heating power 75% -

Date 07/04/2014 09/04/2014

Time 11:03 13:00

Actual heating power 5,8 Kw 5,7 Kw

Flow rate 1,572 m3/h 1,572 m3/h

Total flow volume 283,90 m3 361,94 m3

Total electric work 673 Kwh 958 Kwh

Pressure 2 bar 2 bar

Table 4-2: Thermal response test conditions

From this data we obtain:

- Total duration: 50,8 h

- Average Reynolds number: 17020,60

- Average heat input rate: 58 w/m

- Initial ti: 9,625 ºC

- Final tf: 24,995 ºC

Although the data are measured for all the TRT length, the one obtained in the first hours must

be rejected because the heat transfer is not steady for that interval of time interfere in the linear

logarithmic evolution of the temperature during the test. There are some different criteria for the

time that must be neglected, being the most notorious the next one:

- Most of authors, among who are Sanner (2005) or Banks (2012), hold that the time

rejected must be:

� > 5G?K∝

- On the other hand some authors like Ingersoll (1954) or Gehlin (2002) supports a more

restrictive theory:

� > 20G?K∝

Where:

- t: time in seconds

- rb: radius of the borehole in meters

- α: thermal diffusivity in m2/s.

The second method has been rejected for this project because too many hours of TRT would be

needed and a so high accuracy is not necessary for this project.

As far as the first method is concerned, some different intervals were analysed before electing

the time rejected. The different intervals are:

47

- From 6 hours after starting to the end.

- From 9 hours after starting to the end.

- From 12 hours after starting to the end.

- From 9 hours after starting to 45 hours after starting.

- From 9 hours after starting to 40 hours after starting.

48

5. RESULTS, INTERPRETATION AND COMPARISON OF TRT

5.1. RESULTS

In this section the authors will give the results obtained from the TRT. More information is

provided in the Appendix 2.

Thermal Response Test VIA 14:

In the next table there is a resume of the results obtained from the TRT developed by the

authors.

Heat Exchanger Type Single U

Installation Number VIA 14

Active Length (m) 100

Borehole Diameter (mm) 160

Undisturbed Ground Temperature (°C) 9,56

Mean power rate input (W/m) 56,85

Thermal Conductivity (W/mK) 2,03 ± 0,03

Borehole Thermal Resistance (mK/W) 0,1079 ± 0,0020

Table 5-1: Thermal response test results

Plotting the mean temperature measured through time is possible to see how the temperature

increases with the power input. Also is possible to see the temperature calculated using the

Line Heat Source theory and compare both results (Figure 5-1).

49

Figure 5-1: Thermal response test (9-50h)

Plotting the mean temperature against the time logarithm is possible to know the slope of the

trend line that will be used to calculate the thermal conductivity (Figure 5-2).

Figure 5-2: Trend line (9-50h)

Then, thermal conductivity can be calculated with the slope of the trend line. In the next graph

the thermal conductivity is represented along the TRT time and taken into consideration the

power input (Figure 5-3).

0,00

5,00

10,00

15,00

20,00

25,00

30,00

0 10 20 30 40 50 60

Time (h)

Tf(°C) LHS (°C) Effect (kW) Flow (m³/h)

y = 2,2229ln(x) + 16,219R² = 0,9963

20,00

21,00

22,00

23,00

24,00

25,00

26,00

5,00 50,00

Tem

pera

ture

(°C

)

Time logarithm (h)

Tf (ºC) Trend Line (9-50h)

50

Figure 5-3: Thermal Conductivity (λ)

As it is shown in the graph there is a direct relation between the effect of the heater and the

value of the thermal conductivity. This is because in the formula the slope of the trend line is a

constant so the only thing that can change the value of the thermal conductivity is the heating

effect introduced by the heater.

Using the formulas described before it is also possible to know the borehole thermal resistance.

In the next graph, Rb is plotted along the time of the TRT (Figure 5-4).

5

5,2

5,4

5,6

5,8

6

6,2

6,4

6,6

6,8

7

1,80

1,85

1,90

1,95

2,00

2,05

2,10

2,15

9,00 19,00 29,00 39,00 49,00

Effe

ct (

kW)

The

rmal

con

duct

ivity

(W

/mK

)

Time (h)

λ Effect (kW)

51

Figure 5-4: Borehole Thermal Resistance (Rb)

5.2. INTERPRETATION OF RESULTS AND COMPARISON

To interpret the TRT the authors will use different intervals of time calculations, the results of

GeRT software and previous TRT done in the same borehole. This way it is possible to

compare results and find possible differences and mistakes.

5.2.1. Comparison of the results by intervals of time

First we will begin with the results of the 5 intervals of time for the current TRT.

VIA 14 RESULTS

Time interval

6h-50h 9h-50h 12h-50h 9h-45h 9h-40h

Thermal Conductivity (W/mK)

2,00 ± 0,03 2,03 ± 0,03 2,08 ± 0,03 2,05 ± 0,03 2,03 ± 0,03

Borehole Thermal Resistance (mK/W)

0,1060 ± 0,0020

0,1079 ± 0,0020

0,1101 ± 0,0020

0,1088 ± 0,0019

0,1079 ± 0,0019

Table 5-2: Thermal response test results comparison between time intervals

0,1000

0,1050

0,1100

0,1150

0,1200

9,00 14,00 19,00 24,00 29,00 34,00 39,00 44,00 49,00

Bor

ehol

e th

erm

al r

esis

tanc

e (m

K/W

)

Time (h)

Rb

52

This table shows the different values for thermal conductivity and borehole thermal resistance

depending on the time interval taken into consideration.

The formula for the break time until the steady state flux gives a result of 9 hours to dismiss so

the authors take the second interval from 9 hours to the end of the TRT as the most accuracy

value, 2,03 W/mK for thermal conductivity and 0,1079 mK/W for borehole thermal resistance.

Comparing these results with the values for the first time interval that starts at 6h, 2,00 W/mK

and 0,1079 mK/W, these are higher. This means that maybe in the first interval the heat flux is

not steady yet and because of this it gives a lower thermal conductivity.

Thermal conductivity for the third interval (12h to 50h) gives even a higher value, 2,08 W/mK

which means a 2,2% increase about second interval and 4,1% about first interval. Borehole

thermal resistance is also higher

It can be deducted from these 3 time intervals results that, the more hours dismissed at the

beginning of the TRT, the higher thermal conductivity and borehole thermal resistance results

will be. From a scientific perspective the goal should be try to avoid more hours to get to a more

accuracy result. However from the economic perspective the goal should be try to make the

experiment as short as it can be.

The reason to the fourth and fifth time intervals calculations is to analyse how thermal

conductivity and borehole thermal resistance vary taken out of consideration the last hours of

the TRT. As it can be seen in the table the values are pretty similar to the second interval so it

can be assumed that the results don’t change too much even taken 10 hours out of

consideration. The most relevant part of the TRT and which will have the biggest impact in the

results are the first hours during which the heat flow is not steady.

5.2.2. Comparison of the results with GeRT software

It is possible also to compare the results analysis of the TRT done by the authors with the

results given by GeRT machine, with which the TRT was done. The machine includes software

to analyse the data and give results without the necessity of further work.

53

Figure 5-5: GeRT results

As it can be seen in GeRT results, the program also dismiss the first 9 hours to calculate the

thermal conductivity and borehole thermal resistance, so it can be assumed that our previous

criterion is in concordance with the software.

54

The results given by GeRT software don’t differ too much from the results given by the authors.

Thermal conductivity result is 2,01 W/mK for the software which is 1% less.

The borehole thermal resistance result is 0,109 mK/W, 1% more than the result given by the

authors.

5.2.3. Comparison of the results with previous TRT

A comparison with older tests can help to know the accuracy of the results, so that a relation

with an older test in VIA14 BHE is established.

The results obtained in both tests are summarized in the table X, which is exposed below:

Results Previous TRT Current TRT

Thermal Conductivity (W/mK) 1,75 ± 0,05 2,03 ± 0,03

Borehole Thermal Resistance (mK/W) 0,1128 ± 0,0049 0,1079 ± 0,0020

Table 5-3: Comparison with previous thermal response test

As it’s shown, the values differ hugely so that differences between both tests must be analysed

to know which one has more reliable values. The main difference between both tests is the

device used, in the first one it is an experimental machine constructed by VIA University staff

meanwhile in the current project a professional device is used. Based on this, a higher accuracy

is supposed for the current project because of the higher quality of its components and the

security systems incorporated by GeRT that avoid the system working in inappropriate

conditions. Besides, GeRT’s software is a warranty for the results calculation accuracy.

Other important issues to take into account are the differences in test conditions (Table 5-4;

Figure 5-6).

Past test Current test

Starting Date 03/07/2013 07/04/2014

Starting time 18:00 10:10

Finishing date 07/07/2013 09/04/2014

Finishing time 16:33 13:00

Total duration (h) 51,25 50,8

Undisturbed ground temperature (ºC) 9,90 9,56

Groundwater level (m) 15,15 15,05

Average heating power (w) 2180 5626

Average flow rate (l/h) 1121,70 1554,75

Table 5-4: Comparison with previous TRT conditions

55

Figure 5-6: Comparison with previous TRT

As it’s shown, the power of the heater differs notoriously between the tests, and it involves a

contrast between heat carrier fluid temperatures, anyway it shouldn’t interfere in the results

because borehole, heat carrier fluid and ground thermal properties are equal and it implies that

temperature gradient ought to be proportional.

Figure 5-7: Trend lines comparison

However, after analysing temperature evolution in a time logarithm scale (Figure 5-7) a

discrepancy between trend lines’ slopes is noticed and, as it interferes directly in thermal

conductivity and borehole thermal conductivity values, this is a prove that something is wrong in

0

3

6

9

12

15

18

21

24

27

-5 0 5 10 15 20 25 30 35 40 45 50

Time (h)

Past TRT temperature (ºC) Current TRT temperature (ºC)

Past TRT power (Kw) Current TRT power (Kw)

y = 0,9748x + 4,356

R² = 0,9766

y = 2,2229x - 1,9832

R² = 0,9963

10

12

14

16

18

20

22

24

26

10,25 10,75 11,25 11,75 12,25

Tem

pera

ture

(ºC

)

ln (s)

Past TRT Present TRT Linear (Past TRT) Linear (Present TRT)

56

one of them. Temperature differences are lower in past TRT and it involves a higher possibility

of error, anyway thermal properties variation over the time must be analysed (Figure 5-8).

Figure 5-8: Thermal properties’ comparison along TRT

The thermal properties’ evolution is much more stable in the current test and, taking into

account the device used in the past hasn’t a security system for avoiding it works without a

proper purge, the presence of air inside the loop in the past TRT could explain these thermal

properties variations and, therefore, the high difference in the results.

5.2.4. Comparison of the results with FEFLOW simulation

Taking into account that a computational model of the BHE is done as a second part of the

project, the authors consider that a simulation of the TRT is an interesting method to check the

reliability of the model and, at the same time, it could help to corroborate that the estimated

parameters in the analysis of the test are right.

The model is set as a quadrangular prism of soil with 20 meters sides and 120 meters depth.

The soil is modelled as a homogenous material whose properties are an average of the real

layered soil for considering it sufficiently accurate for the purpose of the project. Remaining

parameters are established as truthfully as possible, including a simulation of the power in real

time, that is to say, the variations in the heat power of the TRT are the same that GeRT

measured in the real one.

In overall, four different calculations of the model have been done (Table 5-5).

0,10

0,11

0,12

0,13

0,14

0,15

1,00

1,25

1,50

1,75

2,00

2,25

9 14 19 24 29 34 39 44 49 54

Bo

reh

ole

th

erm

al r

esi

sta

nce

(m

K/w

)

So

il t

he

rma

l co

nd

uct

ivit

y (w

/mK

)

Time (h)

λ past TRT λ present TRT Rb past TRT Rb Present TRT

57

λ grouting

(w/m·K)

Shank spacing (mm)

Svc soil (MJ/m3·K)

Model 1 2,35 80 2,03

Model 2 1,50 80 2,03

Model 3 1,50 60 2,03

Model 4 1,50 60 3,00

Table 5-5: Comparison between different TRT models

All the changes carried out in the models are justified in the obtainment of more reliable values

for the borehole characteristics, unless the change in the volumetric heat capacity that was an

intention of approximating to reality an estimated value obtained as an arithmetic mean of

literature’s values.

The curve of the model is approaching to the real one with every change done until obtaining a

result close enough for the project requirements (Figure 5-9).

From this comparison can be deduced that every new data obtained was more veracious than

the old one, as the curves demonstrate in any step, and that estimated volumetric heat capacity

is not as precise as it should have been.

58

Figure 5-9: Thermal response tests comparison

9

11

13

15

17

19

21

23

25

27

0 5 10 15 20 25 30 35 40 45 50 55

Tem

pera

ture

(ºC

)

Time (h)

TRT Model 1 Model 2 Model 3 Model 4

59

6. STUDY AND RESULTS OF THERMAL ENERGY STORAGE

Seasonal Thermal Energy Storage systems (STES) work in a way that, half of the year the

system extracts energy from the ground to supply an energy demand and the other 6 months

the system stores in the ground exceeding energy from another resource, in most cases solar

heating panels.

This system allows us to use the exceeding energy from solar panels even when there is not a

real demand.

The influence of the selected type of borehole heat exchanger is slight compared to the

influence of the surrounding underground. (VDI 4640)

For the theoretical study of thermal energy storage and the ground behaviour model around the

borehole, some assumptions have to be taken into consideration:

• The poor data available of the ground stratification in the boreholes gives low values for

the λ (1,23 W/mK) and the SVC (2,03 MJ/m2K). Checking these results with the real

data of the TRT for λ (2,03 W/mK) can be assumed that the values taken before are

lower than the real ones.

• Also ground water table was measured 15,05 m below ground surface. Below this point

the values for λ and SVC will increase due to the content of water in the soil pores.

Anyway the authors will consider the soil as one layer with homogeneous

characteristics.

• The temperature of the heat carrier fluid in the borehole should not exceed the limiting

range of ±11 K during the different cycles (VDI 4640). This condition will be used in

further calculations to set boundary conditions in 0 ºC as a minimum temperature and

20 ºC as a maximum temperature.

• As a last assumption to simplify the procedure, ground water movement is neglected.

For systems without storage this can be a plus because in further research this should

be taken into consideration because heat transport by advection can change the results

of the simulations.

6.1. ENERGY STORAGE IN VIA 14 AND EXTRACTION IN VIA 13

6.1.1. Volumetric heat capacity calculation

Taken into consideration a certain volume of soil around the borehole heat exchanger it is

possible with the volumetric heat capacity of the soil and the temperature difference to know

how much energy can be extracted or stored in this volume.

In this example the volume of soil is assumed to be a close system without losses and heated

or cooled uniformly.

Heat energy extraction through BHE VIA 13

The BHE VIA 13 depth is 96 m, the volumetric heat capacity of the soil is 2,03 MJ/m3K, the

undisturbed temperature is 9,56 ºC and the minimum temperature established in heat

60

extraction is 0 ºC. Then different scenarios depending on the radius of soil around the borehole

are calculated.

Temp. Diff. (K) Radius

(m) Volume

(m³) Energy

(MJ) Energy (MWh)

-9,56 3,40 3486 -67660 -18,794

Svc (MJ/m³K) 3,45 3590 -69665 -19,351 3,50 3695 -71699 -19,916

2,03 3,55 3801 -73762 -20,489

Borehole depth (m) 3,60 3909 -75854 -21,071 3,65 4018 -77976 -21,660

96 3,70 4129 -80127 -22,257

Table 6-1: Extraction based on volumetric heat capacity

Heat energy storage through BHE VIA 14

The BHE VIA 14 depth is 100 m, the volumetric heat capacity of the soil is 2,03 MJ/m3K, the

undisturbed temperature is 9,56 ºC and the maximum temperature established in heat storage

is 20 ºC. Then different scenarios depending on the radius of soil around the borehole are

calculated.

Temp. Diff. (K) Radius

(m) Volume

(m³) Energy

(MJ) Energy (MWh)

10,44 3,10 3019 63984 17,773

Svc (MJ/m³K) 3,20 3217 68178 18,938 3,30 3421 72506 20,141

2,03 3,40 3632 76967 21,380

Borehole depth (m) 3,45 3739 79247 22,013 3,50 3848 81561 22,656

100 3,60 4072 86288 23,969

Table 6-2: Storage based on volumetric heat capacity

6.1.2. Calculation according VDI 4640

In the case of systems with a heat pump heating capacity of up to 30kW, which are only used

for heating operation the design can be carried out using specific heat extraction values (in

W/m) (VDI 4640).

Taking into consideration; 100m of borehole depth, a specific heat extraction value of 60 W/m to

our type of soil and 1800h/a of heat pump work. The energy extracted from the borehole heat

exchanger can be calculated;

Borehole depth (m)

Specific heat extraction

(W/m)

Running time (h)

Energy (MWh)

100 60 1800 10,8

Table 6-3: Extraction based on VDI 4640

61

6.1.3. Line source method

Theoretical calculation

According to the infinite line source model the heat flux of BHE can be calculated as function of

the time. The amount of heat energy can be extracted within the soil through the BHE VIA 13

and how much energy can be storage within the soil through the BHE VIA 14 are estimated

during 1 year.

The heat flux in W/m can be isolated from the line source model method formula as a function

of time. Taking into account the borehole depth and the time have been working, the amount of

energy extracted in BHE VIA 13 or stored in BHE VIA 14 can be calculated. To simulate each

situation with FEFLOW, heat flux was calculated per day during 1 year.

Equation 6-1: Heat flux based on line source model method

E��� = �4 D h D � D Vln V

4 D M D �G|K W � PW + � D C| + B

E��� − B = � D �ln V4 D M D �G|K W − P4 D h D � + C|�

� = �E��� − B� D �ln V4 D M D �G|K W − P4 D h D � + C|�kJ

� = � D �� D �3,6 D 10�

Where:

- q = heat flux (W/m)

- r = rb (borehole radio in m)

- RB = borehole resistance (m·K·W-1)

- T0 = undisturbed ground temperature (ºC)

- E = }~�c}���K (average between the inlet and outlet carrier fluid temperatures in ºC)

- γ = 0,5772… = Euler’s constant

- t = time (seconds)

- λ= thermal conductivity (W·m-1·K-1)

- SVC = Volumetric Heat Capacity (J·m-3·K-1)

- a =λ/ ρ·Cp = Thermal diffusivity (m2·s-1)

- zB= borehole depth

- Q= heat energy amount (MWh)

Heat energy extraction through BHE VIA 13

To calculate the amount of energy can be extracted through the BHE VIA 13 the minimum

temperature was established in 0 ºC. Then, all values of the equation about the heat energy

extraction are known:

62

Energy extraction (BHE VIA 13)

r (m) λ

(W·m-1·K-1) Rb

(m·K·W-1) SVC (J·m-3·K-1) a (m2·s-1) γ

T0

(ºC)

Tf

(ºC)

0,16 2,03 0,0899 2030000,00 0,000001 0,577215665 9,56 0,00

Table 6-4: Energy extraction previous conditions

These results are obtained from the calculations that can be consulted in appendix 3:

Figure 6-1: Energy extraction in BHE VIA 13

Figure 6-1 shows the variation of the heat flux in W/m during the year according to the previous

premise that the temperature should be higher than 0 ºC for heat energy extraction. The figure

also exposes the amount of energy is possible store along the year.

According to this theoretical results obtained from the line source method calculations, 20,07

MWh of heat energy could be extracted through the BHE VIA 13 during 1 year.

Heat energy storage through BHE VIA 14

To calculate the amount of energy can be stored through the BHE VIA 14 the maximum

temperature was established in 20 ºC. Then, all values of the equation about the heat energy

extraction are known:

-22,00

-20,00

-18,00

-16,00

-14,00

-12,00

-10,00

-8,00

-6,00

-4,00

-2,00

0,00

-60,00

-55,00

-50,00

-45,00

-40,00

-35,00

-30,00

-25,00

-20,00

-15,00

-10,00

0 30 60 90 120 150 180 210 240 270 300 330 360

Q (

MW

h)

q (W

/m)

Time (days)

Heat flux (W/m) Energy extracted (kWh)

63

Energy storage (BHE VIA 14)

r (m) λ

(W·m-1·K-1) Rb

(m·K·W-1) SVC (J·m-3·K-1) a (m2·s-1) γ

T0

(ºC)

Tf

(ºC)

0,16 2,03 0,1079 2030000,00 0,000001 0,577215665 9,56 20,00

Table 6-5: Energy storage previous conditions

These results are obtained from the calculations that can be consulted in appendix 3:

Figure 6-2: Energy storage in BHE VIA 14

Figure 6-2 shows the variation of the heat flux in W/m during the year according to the previous

premise that the temperature should be lower than 20 ºC for heat energy storage. The figure

also exposes the amount of energy is possible store along the year.

According to this theoretical results obtained from the line source method calculations, 21,85

MWh of heat energy could be stored through the BHE VIA 14 during 1 year.

6.1.1. FEFLOW model

The FEFLOW model was calculated from the values of heat flux (W/m) obtained from line

source method estimation. The values of heat flux were estimated in W/m per day during 1

year. The heat energy extraction from BHE VIA 13 and the heat energy storage to BHE VIA 14

was modelled using these values of heat flux. All the values of this table were introduced in

FEFLOW for de model.

0,00

2,00

4,00

6,00

8,00

10,00

12,00

14,00

16,00

18,00

20,00

22,00

10,00

15,00

20,00

25,00

30,00

35,00

40,00

45,00

50,00

55,00

60,00

0 30 60 90 120 150 180 210 240 270 300 330 360

Q (

MW

h)

q (W

/m)

Time (days)

Heat flux (W/m) Energy storage (kWh)

64

Heat exchanger type Double U-pipe Single U-pipe Installation number VIA 13 VIA 14 Grout type Dantonit Dantonit

Grout thermal conductivity (W m-1 K-1)

1,50 1,50

Borehole diameter (mm) 160 160 Active length (m) 96 100

External pipe material PE 100 -RC S5 PN16 PE 100 -RC S5

PN16 External pipe outer Ø (mm) 32,0 40,0 External Pipe Inner Ø (mm) 26,2 32,6 External pipe wall thermal conductivity (W·m-1·K-1) 0,42 0,42

Undisturbed soil temperature (ºC)

9,56 9,56

Soil thermal conductivity (W·m-1·K-1)

2,03 2,03

Soil volumetric heat capacity (MJ·m3·K-1)

2,03 2,03

Refrigerant thermal conductivity (W·m-1·K-1)

0,44 0,44

Refrigerant specific heat capacity (J·kg-1·K-1)

4250 4250

Refrigerant density (kg·m-3) 960 960

Refrigerant volumetric heat capacity (J·kg-1·K-1)

4080000 4080000

Refrigerant viscosity (kg·m-1·s-1)

0,0076 0,0076

Freezing point (ºC) -15,00 -15,00

Flow rate (m3·d-1) 112,58 70,07

Reynolds number 4030 4001

Table 6-6: Boreholes heat exchangers previous conditions

After the simulation with FEFLOW software the results of the BHE outlet / inlet temperatures are

shown in the figure 6-3.

65

Figure 6-3: BHE outlet / inlet temperatures

Figure 6-3 shows the evolution of the BHEs temperatures during 1 year according to the heat

flux previously calculated. The evolution of the BHEs temperatures is according to the main

premises established before de calculation of the heat flux along the year.

Figure 6-4: FEEFLOW soil temperature simulation after 1 year

The figure 6-4 shows the influence of the heat energy extraction through the BHE VIA 13 and

the heat energy storage to BHE VIA 14 along 1 year. There is no heat transfer between the

boreholes during 1 year (Figure 6-4).

-5,00

0,00

5,00

10,00

15,00

20,00

25,00

0 30 60 90 120 150 180 210 240 270 300 330 360

Tem

pera

ture

(ºC

)

time (days)

Inlet VIA 13 Outlet VIA 13 Inlet VIA 14 Outlet VIA 14

66

6.2. SEASONAL THERMAL ENERGY STORAGE IN VIA 14

The authors estimate the energy consumption of the World Flex House in the energy park.

Heat energy consumption of the house is supposed taking into account the data about heat

loads and DHW consumption:

Heat energy consumption Heating system

(kWh) DHW (kWh)

TOTAL (kWh)

Jan 1468 200 1668 Feb 1017 200 1217 Mar 688 200 888 Apr 197 200 397 May 0 200 200 Jun 0 200 200 Jul 0 200 200 Aug 0 200 200 Sep 198 200 398 Oct 208 200 408 Nov 892 200 1092 Dec 1432 200 1632 YEAR 6100 2400 8500

Table 6-7: World Flex House heat energy consumption estimated

Heat energy production is supplied with a heat pump connected to the BHE VIA 14 and with

thermal solar panels. The heat pump production is the difference between the total heat energy

consumption and thermal solar panels energy production.

Balancing the system with thermal solar panels then it is easy to know the exceeding energy

produced by these panels in summer. Four 2,5 m2 of area and 0,79 of optical efficiency thermal

solar panels be selected for the estimation.

The heat energy extraction through the BHE VIA 14 is calculated according to the COP of the

heat pump.

The thermal solar panels excess production is stored within the soil through the BHE VIA 14.

The next tables show the parameters of the seasonal heat energy extraction and heat energy

storage.

67

Thermal solar panels Heat Pump Sun radiation (kWh/m²)

Heat energy production (kWh)

Excess production (kWh)

Consumption (kWh)

Extraction (COP = 4,65)

Jan 29,1 166,3 0,0 1501,7 1235,9 Feb 44,8 256,1 0,0 960,9 790,8 Mar 112,0 640,2 0,0 247,8 203,9 Apr 158,0 903,2 506,2 0,0 0,0 May 174,0 994,7 794,7 0,0 0,0 Jun 170,0 971,8 771,8 0,0 0,0 Jul 167,0 954,6 754,6 0,0 0,0 Aug 152,0 868,9 668,9 0,0 0,0 Sep 119,0 680,3 282,3 0,0 0,0 Oct 78,7 449,9 41,9 0,0 0,0 Nov 37,8 216,1 0,0 875,9 720,9 Dec 23,5 134,3 0,0 1497,7 1232,6 YEAR 1265,9 4824,3 3820,3 5083,9 4184,1

Table 6-8: World Flex House heating system previous conditions

Knowing the power rate injection and extraction in the borehole along the year, a simulation

about the ground behaviour was done with FEFLOW.

Month time (days) Power (J/d) Q (kWh) Jan 31 -148304706,3 -1235,9 Feb 59 -94899848,9 -790,8 Mar 90 -24468914,29 -203,9 Apr 120 60743702,4 506,2 May 151 95359267,2 794,7 Jun 181 92615376 771,8 Jul 212 90557457,6 754,6 Aug 243 80267865,6 668,9 Sep 273 33870763,2 282,3 Oct 304 5026059,36 41,9 Nov 334 -86506647,95 -720,9 Dec 365 -147910853,5 -1232,6

Table 6-9: FEFLOW simulation previous conditions

68

BHE temperatures evolution along 1 year

Figure 6-5 analyses the evolution of the BHE inlet and outlet temperatures during 1 year.

Figure 6-5: BHE outlet / inlet temperatures along 1 year

The evolution of the BHE temperatures corresponds to the heat energy extraction in winter

months and the heat energy storage in summer months (Figure 6-5).

4,00

5,00

6,00

7,00

8,00

9,00

10,00

11,00

12,00

13,00

0 30 60 90 120 150 180 210 240 270 300 330 360

Tem

pera

ture

(ºC

)

time (days)

Inlet VIA 14 Outlet VIA 14

69

Ground temperatures evolution

The next figures show the temperature of the soil around the BHE in different seasons during

the year:

- Figure 6-6: Temperature of the soil in 31th of January. This figure shows the cooling of

the ground after the first month of heat energy extraction.

Figure 6-6: Soil temperatures on 31/01

- Figure 6-7: Temperature of the soil in 31th of May. This figure shows how the

temperature of the ground is balance after the second month of heat energy storage.

Figure 6-7: Soil temperatures on 31/05

70

- Figure 6-8: Temperature of the soil in 30th of September. This figure shows the heating

of the temperature of the ground after the heat storage season.

Figure 6-8: Soil temperatures on 30/08

- Figure 6-9: Temperature of the soil in 31th of December. This figure shows the cooling

of the ground after the second month of heat energy extraction.

Figure 6-9: Soil temperatures on 31/12

71

BHE temperatures evolution along 3 year

Figure 6-10: BHE outlet / inlet temperatures along 3 years

The evolution of the BHE temperatures corresponds to the heat energy extraction in winter

months and the heat energy storage in summer months during 3 years (Figure 6-10).

Stored heat energy influence along 3 year

Figure 6-11: Stored heat energy influence into the soil

Stored heat energy into the soil corresponds to the values obtained from FEFLOW after the

simulation of the heating system model along 3 years (Figure 6-11).

4,00

5,00

6,00

7,00

8,00

9,00

10,00

11,00

12,00

13,00

0 90 180 270 360 450 540 630 720 810 900 990 1080

Tem

pera

ture

(ºC

)

time (days)

Inlet VIA 14 Outlet VIA 14

-4500,00

-4000,00

-3500,00

-3000,00

-2500,00

-2000,00

-1500,00

-1000,00

-500,00

0,00

0 90 180 270 360 450 540 630 720 810 900 990 1080

Sto

red

heat

ene

rgy

(MW

h)

time (days)

Stored heat energy variation (MWh)

72

7. INTERPRETATION OF THERMAL ENERGY STORAGE RESULTS

Heat energy extraction in BHE VIA 13

According to the theoretical results obtained from the line source method calculations, 20,07

MWh of heat energy could be extracted through the BHE VIA 13 during 1 year.

Taking into account calculations with the volumetric heat capacity, 20,07 MWh of energy

extracted is equivalent to the cooling from 9,56 ºC to 0 ºC of a cylinder of soil with radio

between 3,50 and 3,55 m.

The model simulation with FEFLOW shows the influence of the temperature on the ground,

along the year during the heat energy extraction of 20,07 MWh. Due to the soil is an infinite

medium, the influence of the heat energy extraction through the ground is different depending

on the distance to the BHE. After 1 year, considering the influence around 1 m of the BHE, the

temperature of the soil drops until 4ºC.

Heat energy storage in BHE VIA 14

According to the theoretical results obtained from the line source method calculations, 21,85

MWh of heat energy could be stored through the BHE VIA 14 during 1 year.

Taking into account calculations with volumetric heat capacity, 21,85 MWh of energy stored is

equivalent to the heating from 9,56 ºC to 20 ºC of a cylinder of soil with radio between 3,40 and

3,45 m.

The model simulation with FEFLOW shows the influence of the temperature on the ground,

along the year during the heat energy storage of 21,85 MWh. Due to the soil is an infinite

medium, the influence of the heat energy storage through the ground is different depending on

the distance to the BHE. After 1 year, considering the influence around 1 m of the BHE, the

temperature soil increases until 13.5ºC.

Seasonal thermal energy storage in BHE VIA 14

As far as seasonal thermal energy storage is concerned, the theoretical results obtained with

FEFLOW software show that the temperature of the soil is balanced along the year. To draw

conclusions about the model simulated, thermal energy storage that happens from April to

October must be taken into account. The first season extracting energy through the borehole

causes that the BHE temperature drops until 5 ºC, the second season extracting energy after

the seasonal thermal energy storage causes the BHE temperature drops until 6 ºC and during

the third season extracting energy the BHE temperature drops until 5,5 ºC. This supposes that

the thermal energy storage balances the installation, the BHE temperatures and the heat

energy stored into the soil along the time.

73

8. CONCLUSIONS AND FURTHER RESEARCH PROPOSAL

VIA University College is trying to be one of the leading universities in Denmark researching

about shallow geothermal energy. This project focuses on the shallow soil underground use as

a heat source and as a storage system.

Thermal response tests are a demand to design ground source heat pump systems and in this

project the authors developed one TRT in VIA 14 borehole heat exchanger.

For the TRT one of the main problems is that, the volumetric heat capacity of the soil has to be

estimated comparing literature data and the poor field data available. Line heat source

calculation can be fitted to the real Tf values of the TRT by adjusting the volumetric heat

capacity of the soil but this is just a guess and it can lead to wrong assumptions. However, this

problem leaves the door open to develop many projects in this particular area and achieve more

reliable values for the volumetric heat capacity.

To simulate the heat behaviour of the ground, used as a storage system and a heat source, the

authors developed several simulations using FEFLOW software. For these simulations the

amounts of energy stored or extracted are just theoretical estimations in ideal conditions but for

further projects better calculations should be done. For example, World Flex House heating

system simulated with FEFLOW could support new field investigations in real conditions taking

advantage of the Energy Park installations. The comparison of these simulations with real

experiments will end up in great information that should be analysed to achieve better results in

further projects for the design of ground source heat pumps.

The coordination between groups and projects is also very important to combine researches

and share information taking advantage of all the project results and to contrast them. Perhaps

these lacks of information can be solved by more efficient managing procedures by VIA and

better collaboration between project groups, because all will end up in better results.

74

9. REFERENCES

ASHRAE, 2007. ASHRAE Handbook. Heating, ventilating and air-conditioning applications, American Society of Heatinh, Refrigerant and Air-Conditioning Engineers, Inc.

Banks, D., 2012. An Introduction to Thermogeology: Ground Source Heating and Cooling, John Wiley & Sons. Available at: http://books.google.com/books?id=kKQ1t-9ZfzgC&pgis=1 [Accessed March 17, 2014].

Clauser, C. & Huenges, E., 1992. Thermal Conductivity of Rocks and Minerals. , (1).

Conduction, T.H. & Systems, M., Transient heat conduction. In pp. 217–284.

Deng, Z., 2004. Modeling of standing column wells in ground source heat pump systems. , pp.1–9.

Dickson, M.H. & Fanelli, M., 2004. What is Geothermal Energy ? , (February).

Ditlefsen, C. et al., 2013. GeoEnergy – a national shallow geothermal research project. , (June).

Ditlefsen, C. & Vangkilde-Pedersen;, T., 2012. Shallow geothermal energy in Denmark. In 74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012. Copenhaguen, Denmark.

Earth River Geothermal, Geothermal HVAC Systems — An In Depth Overview.

Gehlin, S., 1998. Thermal Response Test - In Situ Measurements of. Lulea University of Technology.

Gehlin, S., 2002. Thermal Response Test. Method development and evaluation. Lulea University of technology.

Geotrainet, 2011. Geotrainet training manual for designers of shallow geothermal systems M. M. Corry, ed., Brussels: GEOTRAINET.

Hadorn, J., 2004. Storage solutions for solar thermal energy, Geneve.

Hellström, G., 1991. Thermal analysis of duct ground heat storage. In Thermastock ’91- 5th International Conference on Thermal Energy Storage. Schevingen, The Netherlands, p. 6.

Hellström, G. & Larson, Å., 2001. Seasonal thermal energy storage – the HYDROCK concept. Bulletin of Engineering Geology and the Environment, 60(2), pp.145–156.

Huttrer, G.W., 1997. Geothermal heat pumps: An increasingly successful technology. Renewable Energy, 10(2-3), pp.481–488.

Ingersoll, L.R., 1954. Heat conduction: with engineering, geological and other applications 2nd editio., New York: McGraw-Hill.

Kekelia, B., 2012. Heat transfer to and from a reversible thermosiphon placed in porous media. University of Utah.

75

Mahler, A. et al., 2013. Geothermal Energy Use , Country Update for Denmark. , (June), pp.3–7.

Meurs, G.A.M. van, 1985. Seasonal Heat Storage in the Soil,

Pagola, A., 2013. Master Final Dissertation Investigation of Soil for Shallow Geothermal María Alberdi Pagola. VIA University College.

Pavlov, G. & Olesen, B., 2011. Seasonal solar thermal energy storage through ground heat exchangers–Review of systems and applications. … of Energy, Water and Environment Systems.

Perry, M.S. and R., 1999. In-situ testing and thermal conductivity testing. In Proc. of the 1999 GeoExchange Technical Conference & Expo. Stillwater, Oklahoma: Oklahoma State University.

Raymond, J. et al., 2008. A review of thermal response test analysis using pumping test concepts. Ground water, 49(6), pp.932–45. Available at: http://www.ncbi.nlm.nih.gov/pubmed/21306358 [Accessed May 8, 2014].

Sanner, B. et al., 2003. Current status of ground source heat pumps and underground thermal energy storage in Europe. Geothermics, 32(4-6), pp.579–588.

Sanner, B. et al., 2005. Thermal Response Test – Current Status and World-Wide Application. In Proceedings World Geothermal Congress. pp. 24–29.

VDI, 2001. Utilization of the subsurface for thermal purposes: Underground thermal energy storage, Düsseldorf.

Witte, H., Gelder, G. Van & Spitler, J., 2002. In situ measurement of ground thermal conductivity: a Dutch perspective. Ashrae Transactions, 108(1), pp.1–21.

76

LIST OF FIGURES FIGURE 2-1: GEOTHERMAL GRADIENT INTO THE SOIL (GEOTRAINET 2011)................................................................. 13 FIGURE 2-2: THERMODYNAMIC PROCESS IN A HEAT PUMP ....................................................................................... 15 FIGURE 2-3: SINGLE U-PIPE .............................................................................................................................. 20 FIGURE 2-4: DOUBLE U-PIPE ............................................................................................................................ 21 FIGURE 2-5: CONCENTRIC PIPE .......................................................................................................................... 21 FIGURE 2-6: BOREHOLE THERMAL RESISTANCE SECTION (MONZO 2011) ................................................................... 22 FIGURE 2-7: THERMAL CONDUCTIVITIES FOR FILLING MATERIALS (GEOTRAINET 2011) ................................................. 23 FIGURE 2-8: RELATION BETWEEN BOREHOLE THERMAL RESISTANCE AND THERMAL CONDUCTIVITY OF FILLING MATERIAL

(GEOTRAINET 2011) ............................................................................................................................... 23 FIGURE 2-9: BOREHOLE THERMAL RESISTANCE RELATION WITH CARRIER FLUID FLOW RATE (GEOTRAINET 2011) ............... 24 FIGURE 2-10: BOREHOLE HEAT EXCHANGER ARRAY ................................................................................................ 29 FIGURE 3-1: NUMBER OF SHALLOW GEOTHERMAL BOREHOLES REPORTED TO THE NATIONAL BOREHOLE DATABASE JUPITER. 31 FIGURE 3-2: VIA UNIVERSITY COLLEGE ............................................................................................................... 33 FIGURE 3-3: VIA ENERGY PARK FACILITIES ........................................................................................................... 34 FIGURE 4-1: UNDISTURBED GROUND TEMPERATURE ALONG BOREHOLE PROFILE .......................................................... 38 FIGURE 4-2: LINE SOURCE MODEL ...................................................................................................................... 40 FIGURE 5-1: THERMAL RESPONSE TEST (9-50H) .................................................................................................... 49 FIGURE 5-2: TREND LINE (9-50H) ...................................................................................................................... 49 FIGURE 5-3: THERMAL CONDUCTIVITY (Λ) ........................................................................................................... 50 FIGURE 5-4: BOREHOLE THERMAL RESISTANCE (RB) .............................................................................................. 51 FIGURE 5-5: GERT RESULTS .............................................................................................................................. 53 FIGURE 5-6: COMPARISON WITH PREVIOUS TRT ................................................................................................... 55 FIGURE 5-7: TREND LINES COMPARISON .............................................................................................................. 55 FIGURE 5-8: THERMAL PROPERTIES’ COMPARISON ALONG TRT ................................................................................ 56 FIGURE 5-9: THERMAL RESPONSE TESTS COMPARISON ............................................................................................ 58 FIGURE 6-1: ENERGY EXTRACTION IN BHE VIA 13 ................................................................................................ 62 FIGURE 6-2: ENERGY STORAGE IN BHE VIA 14 .................................................................................................... 63 FIGURE 6-3: BHE OUTLET / INLET TEMPERATURES ................................................................................................. 65 FIGURE 6-4: FEEFLOW SOIL TEMPERATURE SIMULATION AFTER 1 YEAR .................................................................... 65 FIGURE 6-5: BHE OUTLET / INLET TEMPERATURES ALONG 1 YEAR ............................................................................. 68 FIGURE 6-6: SOIL TEMPERATURES ON 31/01 ....................................................................................................... 69 FIGURE 6-7: SOIL TEMPERATURES ON 31/05 ....................................................................................................... 69 FIGURE 6-8: SOIL TEMPERATURES ON 30/08 ....................................................................................................... 70 FIGURE 6-9: SOIL TEMPERATURES ON 31/12 ....................................................................................................... 70 FIGURE 6-10: BHE OUTLET / INLET TEMPERATURES ALONG 3 YEARS ......................................................................... 71 FIGURE 6-11: STORED HEAT ENERGY INFLUENCE INTO THE SOIL ................................................................................ 71

77

LIST OF TABLES TABLE 3-1: BOREHOLE HEAT EXCHANGERS CHARACTERISTICS. .................................................................................. 35 TABLE 4-1: THERMAL CONDUCTIVITY AND VOLUMETRIC HEAT CAPACITY ESTIMATION .................................................... 36 TABLE 4-2: THERMAL RESPONSE TEST CONDITIONS ................................................................................................ 46 TABLE 5-1: THERMAL RESPONSE TEST RESULTS ...................................................................................................... 48 TABLE 5-2: THERMAL RESPONSE TEST RESULTS COMPARISON BETWEEN TIME INTERVALS ............................................... 51 TABLE 5-3: COMPARISON WITH PREVIOUS THERMAL RESPONSE TEST ......................................................................... 54 TABLE 5-4: COMPARISON WITH PREVIOUS TRT CONDITIONS ................................................................................... 54 TABLE 5-5: COMPARISON BETWEEN DIFFERENT TRT MODELS .................................................................................. 57 TABLE 6-1: EXTRACTION BASED ON VOLUMETRIC HEAT CAPACITY .............................................................................. 60 TABLE 6-2: STORAGE BASED ON VOLUMETRIC HEAT CAPACITY .................................................................................. 60 TABLE 6-3: EXTRACTION BASED ON VDI 4640 ..................................................................................................... 60 TABLE 6-4: ENERGY EXTRACTION PREVIOUS CONDITIONS......................................................................................... 62 TABLE 6-5: ENERGY STORAGE PREVIOUS CONDITIONS ............................................................................................. 63 TABLE 6-6: BOREHOLES HEAT EXCHANGERS PREVIOUS CONDITIONS ........................................................................... 64 TABLE 6-7: WORLD FLEX HOUSE HEAT ENERGY CONSUMPTION ESTIMATED ................................................................ 66 TABLE 6-8: WORLD FLEX HOUSE HEATING SYSTEM PREVIOUS CONDITIONS ................................................................. 67 TABLE 6-9: FEFLOW SIMULATION PREVIOUS CONDITIONS ...................................................................................... 67

78

LIST OF EQUATIONS EQUATION 2-1: FOURIER’S LAW ........................................................................................................................ 10 EQUATION 2-2: ARITHMETIC MIXING MODEL ........................................................................................................ 10 EQUATION 2-3: GEOMETRIC MIXING MODEL ........................................................................................................ 10 EQUATION 2-4: HARMONIC MIXING MODEL ......................................................................................................... 10 EQUATION 2-5: DIFFUSIVITY ............................................................................................................................. 11 EQUATION 2-6: FOURIER’S LAW ....................................................................................................................... 11 EQUATION 2-7: HEAT CONVECTION .................................................................................................................... 11 EQUATION 2-10: HEATING COP ........................................................................................................................ 15 EQUATION 2-11: COOLING COP ........................................................................................................................ 15 EQUATION 2-12: SEASONAL PERFORMANCE FACTOR .............................................................................................. 16 EQUATION 2-16: SURROUNDING GROUND THERMAL RESISTANCE ............................................................................. 21 EQUATION 2-17: SPECIFIC HEAT EXTRACTION RATE ................................................................................................ 22 EQUATION 2-18: BOREHOLE THERMAL RESISTANCE ............................................................................................... 22 EQUATION 2-23: LINE SOURCE MODEL ................................................................................................................ 26 EQUATION 2-26: TOTAL THERMAL STORAGE OF A CYLINDRICAL ARRAY OF BOREHOLES .................................................. 28 EQUATION 4-1: THERMAL CONDUCTIVITY ............................................................................................................ 37 EQUATION 4-2: VOLUMETRIC HEAT CAPACITY ....................................................................................................... 37 EQUATION 4-5: LINE SOURCE MODEL .................................................................................................................. 40 EQUATION 4-6: LINE SOURCE MODEL APPROXIMATION ........................................................................................... 42 EQUATION 4-7: THERMAL CONDUCTIVITY ............................................................................................................ 42 EQUATION 4-8: THERMAL BOREHOLE RESISTANCE ................................................................................................. 42 EQUATION 4-9: REYNOLDS NUMBER ................................................................................................................... 44 EQUATION 6-1: HEAT FLUX BASED ON LINE SOURCE MODEL METHOD ........................................................................ 61

79

10. APPENDIX