Installation of a Solar Collector

1 Installation of a Solar Collector

Lydia Pforte - Emilie Girard Folkecenter

2008

Lydia Pforte

University of Karlsruhe

Germany

Emilie Girard

Ecole des Mines de Nantes

France

Installation of a Solar Collector



Table of Contents

1. Introduction .............................................................................................................................. 3

2. Solar Panel Adjustment ............................................................................................................. 5

2.1. Location of the Solar Panels ......................................................................................................... 5

2.2. Orientation and Tilt ...................................................................................................................... 7

3. The Solar Collector System ......................................................................................................... 8

3.1. System Sizing ................................................................................................................................ 8

3.2. The Solar collector ........................................................................................................................ 8

3.3. Frame Construction .................................................................................................................... 10

3.4. Installation of the solar collectors .............................................................................................. 14

4. The Hydraulic System of the solar installation ........................................................................... 18

4.2. Water tank and heat exchanger ................................................................................................. 19

4.3. The Check Valve ......................................................................................................................... 20

4.4. The Circulating pump ................................................................................................................. 20

4.5. The Differential Thermostat ....................................................................................................... 22

4.6. The Expansion tank .................................................................................................................... 23

4.7. The Circulation Pipes .................................................................................................................. 23

4.8. The Anti-freezing fluid ................................................................................................................ 23

5. Measurements ........................................................................................................................ 25

6. Financial Analysis .................................................................................................................... 37

7. Conclusion .............................................................................................................................. 39

8. Reference List ......................................................................................................................... 40

9. Annex I ....................................................................................................................................41



1. Introduction

The Straw Bale House

Figure 1: Straw Bales House with its solar installation

The house of interest was a Straw Bale construction, a building method that uses straw bales

as structural elements, insulation, or both. It is commonly used in natural building and has

advantages over some conventional building systems as it is cheaper and easy available. Another

advantage is its high insulation value.

In terms of electricity, the Straw Bale house possesses its own power centre. It produces its own

supply of electricity from a small windmill (2,2 kW) installed in the garden. Charger regulator, battery

storage and inverter supply 230V electricity. Consequently the house is not connected to the public

grid, it is auto sufficient.

Figure2: Batteries installation

Concerning the hot water and the heating, the Straw Bale house possesses a wood boiler.

Technically, 2/3 of this wood fire capacity is drained off as hot water.



Figure3: Wood boiler Figure4: Tank installation

This boiler was connected to a 1000 Litre storage tank for hot water supply; hence the house

was already auto sufficient in terms of hot water supply before our solar panel installation. However,

in order to take advantage of the summer period, the decision to install solar collectors which would

run from April to October was made. Moreover, since heating of the house is not needed during

summer period, the main aim of those collectors is the supply of hot water.

The Folkecenter offered four solar panels for our installation, which we had to build up in the

backyard of the house. This report will describe the different tasks of our project, starting with the

choice in the solar collectors’ localization, tilt and orientation. Then it will deal with the construction

of the support and its installation followed by the main details about the hydraulic system. Finally an

analysis of the measurements of heat output and efficiency done during 2 extreme days in August is

included.



2. Solar Panel Adjustment

2.1. Location of the Solar Panels

The Location of the Solar Panels is an important aspect of planning. Hereby not just the

factor of shadowing should be considered, but also aesthetical problems of a solar collector of 8 m²

and the connectivity to existent pipes should be included. In our considerations we made 5

assumptions of possible positions (Figure 5). All 5 options are placed facing the South to receive the

maximum solar radiation.

• Position Number 1 was moved inwards so that there were no shadowing effects, neither by the

windmill nor by the bushes. It was still close to the pipes. However aesthetically it would have

looked disturbing.

• Position Number 2 is placed next to the windmill. It would have stand in front of a flower bed

so that the view of the occupant would not have be hindered. The small distance to the

connection pipes was also an advantage of this position. However, this location was not perfect

due to shadowing effects of the windmill.

• Position Number 3 was standing free in the North-West of the garden. There were no shadows

affecting the performance of the Solar Panels from the existent Windmill. However, it was

planned to construct smaller windmills along the Western pathway in the near future, so there

would have been shadowing effects. Other disadvantages are the far distance from existent

pipes and the visual hindrance for occupants.

• Option Number 4 stood in the South-West corner of the garden. Here the solar panels were

also harboured against shadowing. The aesthetical criteria could also have been achieved. The

only disadvantage was the large distance to the existent pipes which would have to be

extended if this location would have been chosen.

• Option Number 5 was situated in the lower part of the garden, separated through a small

pathway. An advantage was the free area with no shadowing obstacles. However it was not

considered to be appropriate due to the large distance to the pipes and the aesthetically

separated appearance.

Table 1: Summary of Advantages and Disadvantages for the different locations

Proposed

Panel Advantages Disadvantages

1 - Close to the pipes (~ 4m)

- no shadow - Aesthetically not very appealing

2 - Close to the pipes

- Aesthetically very good

- small Shadow effects from the

big windmill

3 - no shadow from the bigger windmill and

bush

- shadow from future small

windmills

- optic

4 - no shadow

- optically good

- far away from the pipe

5 - no shadow - far away from the pipe (10m)

- looks separated



Option 2:

Panel Location (Simulation)

3

12

4

5

.

. .

.

.

Figure 5: The different considerations of the location of the Solar Panel



After discussing all advantages and disadvantages of different locations of our Solar Collector, we

decided to build the Solar Panel on Position Number 2, according to the fact that the shadow effect

will have no major impact on the collector efficiency.

2.2. Orientation and Tilt

Choosing a proper angle and direction of a solar collector is a very important step, without which the

system will loose efficiency. When determine the direction of the solar collector the following

considerations should be included.

• In the Northern Hemisphere: The collector should face South

• In the Southern Hemisphere: The collector should face North

Figure 6: Direction and angle solar installation

Secondly, and with similar importance a proper angle to mount the solar collectors has to be

determined. Generally speaking the best angle should roughly be equal to the latitude of the

location. Denmark has a latitude of 56° North, therefore the collector should face South at 56°.

However, if the tilt is lower than the latitude, upper than standard performance can be achieved

during summer.

Table 2: Theoretical daily energy gain in Estonia (60°N) (in kWh.m².day-1)

Months 30° 45° 60°

April 3.95 3.95 3.74

May 4.96 4.84 4.46

June 5.40 5.20 4.74

July 5.08 4.92 4.50

August 4.28 4.23 3.96

September 2.94 3.02 2.93

This table clearly shows that for a tilt lower than the latitude, the energy gain is higher. The

values are taken from Estonia which has latitude close to the one in Denmark. Consequently, since

the objective of the solar collectors for the straw bale house was to receive the most possible

available energy for the period from April to September, a lower tilt than 56° seemed to be the best

option. From Annex I one can notice that in average the highest global irradiance for the period April-

September is obtained for a 46° tilt at 56°N. According to all those data, we decided to tilt our solar

collectors at 45°.



3. The Solar Collector System

3.1. System Sizing

For our study we assumed that the Straw Bale House was constructed for a 4-person family

composed of 2 adults and 2 children. According to Esbensen Consulting Engineers the domestic hot

water consumption in Denmark is around 40L/person per day. Hence the domestic hot water supply

for the Straw Bale house would be around 160L/day. This value matches with the data for a large

family in the following Table 3. According to this table, the house would have needed about 6m² of

solar collector to supply all the domestic hot water (DHW) and 250L storage. On the other hand, in

northern United States, a rule of thumb for sizing collector allows 2m² of collector area for each of

the first two family members and 1.1 to 1.3 m² for each additional family member [1] . This leads to

6-7m² for the family. However, taking into account the fact that the hot water consumption is higher

in US (around 50L/person/day), it can be concluded that 6m² of solar collector should be enough to

supply DHW of the Straw Bale House [2].

Table 3: Solar System Sizing

Hot water consumption Solar collector Area Storage

Small Family 80 – 140 L 3 – 4 m² 140 – 230L

Large Family 140 – 200L 5 – 7 m² 230 – 300L

3.2. The Solar collector

The 4 solar panels were glazed Flat Plate Collectors. In other words, the collectors were

insulated and weatherproof boxes containing a dark absorber plate under a glass or plastic cover.

There are many types of flat plate collectors, which differ from one to the other by their tubing

arrangements (Figure 7). The type we used is represented on the figure, but consists of a metal black

plate which is filled with a fluid (either water or an antifreeze solution), hence it does not use pipes.

As the sunlight hits the dark absorber plate the black plate heats up and conducts this heat to the

fluid passing through the plate. The flat plate collector is by far the most common and the flooded

metal plate kind is known to be more efficient than the tube flat plate [3, 16].

Figure 7: Flat plate collector design Figure 8: Our flooded Flat Plate Collector

The efficiency of a solar collector is defined as the quotient of usable thermal energy versus received

solar energy. Besides thermal loss there is always optical loss as well.



Figure 9: Efficiency graph of solar collector performance

The heat loss is indicated by the thermal loss factor, given in Watt/m² collector surface and

the particular temperature difference (in °C) between the collector and the ambient air. The smaller

the temperature difference, the less heat is lost. Above a specific temperature difference, the

amount of heat loss equals the energy yield of the collector, so that no energy at all is delivered to

the solar circulation system. As a conclusion, a good collector has a high conversion factor and low

thermal loss. According to the typical Danish Weather, the heat loss should not be very high for our

installation. Some typical values for these factors are shown in Table 3.

Table 3: The different factors concerning different types of Collectors [4]

Type of Collector Conversion Factor Thermal Loss Factor

(W/m²°C)

Temperature Range

(°C)

Absorber (uncovered) 0,82 to 0,97 10 to 30 up to 40

Flat-plate collector 0,66 to 0,83 2,9 to 5,3 20 to 80

Evacuated-plate

collector 0,81 to 0,83 2,6 to 4,3 20 to 120

Evacuated-tube

collector 0,62 to 0,84 0,7 to 2,0 50 to 120

Figure 10: Percentage of monthly solar coverage (Annual Value: 65%)

Generally it is said that a properly dimensioned system can cover 50 to 65% of the yearly hot water

demand. In most cases in summer even the entire demand for hot water could be provided by the

solar heating system. Then the conventional heating system can be shut off completely. This is

particularly advantageous due to the fact that in this time period the heating system would work

with a low rate of capacity utilization due to the lack of heating demand. Thus, there is a larger

conformity between DHW demand and the solar energy supply than with the utilization for heating.

The objective of our system is therefore to supply all the hot water of the house during the summer



months as well as to supply a part of the energy for the heating of the house in parallel with the

boiler. The measurements were meant to give an idea of the efficiency of the installation and gave

the percentage of hot water which can be produced.

3.3. Frame Construction

A frame should stabilize and adjust the solar collector. Therefore planning the frame is one

very important part of the installation of the solar panel. The material we used was made of

galvanized iron-steel. The galvanization prevents the frame from rusting. We had to cut and drill all

the pieces in order to connect them together and to the ground. Therefore we also used a galvanized

paint to protect those parts, where the galvanization had been damaged. We had two possibilities

for the design of the frame. The first one was an isosceles one, similar to the installation for the solar

collector presented at the Folkecenter area (Figure 11). However, as we used only recycled materials

and our concrete blocks were not large enough we decided to build a perpendicular support with

feet (Figure 12).

Figure 11: Picture of a solar frame at Folkecenter.

To find out the size of all pieces of the frame, we had to use the Pythagoras theorem. Considering the

length of the panel (2.1m) and the fact that we wanted a 45° tilt for the panel, the height was given

by the classical trigonometric formula:

AB = AC x cos 45° (1)

2.1m 45°

C B

A

1.48m

0.15m

1.63m

Figure 12: The values for our

perpendicular support.



The result of the calculation amounted AB = 1.48m. However, taking into consideration that our

panels had water inlet fittings at the bottom, we needed to raise the height of the bottom of the

solar collector up to 15 cm from the ground, which gave a final length for the 4 top feet of 1.63m and

0.15m for the 4 bottom feet.

Figure 13: The values for each of the 4 solar collectors and the total area needed.

We decided to do one support for the total of all four panels, which gave us a total length for the

frame of 4.15 meters. This value takes in account a 1cm space between all the collectors. This small

space has been chosen to prevent any frictions due to material expansion. Finally we ended up to a

simple design, shown on the figure below.

Figure 14: Initial design

of the frame

1.03m 0.01m

2.1m

4.15 m



This structure has been evaluated to be strong enough to support the total weight of our

four solar collectors, admitting that one solar panel weighs around 50kg. However, a protection for

the panels from being lifted by the strong winds occurring in this area was missing. Hence, we

decided to add 8 metal angles on the bottom of the support and 2 on each side to maintain the

structure of the panels on the frame. The way the solar collectors were attached to the structure is

illustrated in Figure 16 (right site) as well as the connection of the frame to the concrete block (left

site). This figure also shows that two even pieces of iron-steel have been added between the inside

and outside feet of the top. Their role is to reduce the degree of freedom of the structure, to conduct

the pressure to other points and to make the frame more stable.

All the cuts and drills of the pieces were made in the workshop following our calculation. However,

when it was time to join all the pieces together on the site, the two lines of concrete blocks were not

exactly on the same level. Consequently the feet at the bottom were too short to be linked to the

structure. We finally had to cut four new feet of 35cm length instead of 15cm.

Figure 15: Final dimension of the frame



Figure 16: Details of the frame



3.4. Installation of the solar collectors

After finding the right place for the Solar Panel in the garden and building up the frame, the

construction work was started. At first old plans about the location of the existent hot and cold water

tubes were studied. These tubes with a diameter of 26 mm were previously used for water transport.

The tubes had to be removed in a 3 m long line from the windmill to the place on the solar panel

(Figure 17). Also a trench for the Temperature Sensor had to be dug from the Differential Thermostat

in the house to the Solar Panel. The trench was about 30 cm deep and 20 cm thick. The tube with the

sensor wire was laid on a 5 cm thick layer of sand. It was then covered with another 10 cm of sand

and a safety belt warning of the high voltage (Figure 17). Eventually the trench was closed.

Figure 17: The Installation of the Controller Wire and the Supply Tubes

For the fundament of the frame 6 concrete blocks were required. The concrete blocks were

about 110 cm long, 30 cm wide and 20 cm deep. At first the fundament was excavated with a

distance between the individual excavations in one line of 30 cm and between the front and the back

of 1, 40 m. The concrete blocks were placed and corrected to be immersed in water and exact

direction. An important detail was to base the concrete blocks about 5-10 cm above the ground level

so that the iron-frame is not exposed to standing water, accelerating the corrosion (Figure 18).

After the fundaments were placed accurately and closed, boreholes were drilled into the

concrete block of a diameter of 16 mm. These boreholes were used to fit in expansion bolts. The

expansions bolts had a diameter of 10 mm and a length of 100 mm, whereby the enclosed part was

30 mm long. These bolts have the characteristic that they expand when enforced into the concrete

and therefore are stable to hold the frame. They can also be used to adjust the frame; however, this

was not necessary for our frame (Figure 19).

Figure 18: The construction of the concrete fundament



Figure 19: Drilling the boreholes and constructing the frame

As seen in Figure 19 the primed pieces of the frame were then built on the expansion bolts.

First the basement was installed, followed by the feet and the connecting parts. The bolts used were

10 mm hexagon bolts with a length of 5 cm and 10 mm countersink screws with 2 cm length. About

50 bolts were used, the same amount as nuts; washers were just used for the hexagon bolts.

After finishing the frame, the 4 solar panels were mounted. Important hereby was to leave

1cm space for the expansion of the solar material at high temperatures. The Solar Panels have a rail

on the upper backside. This rail was fitted to the frame. To prevent the panels to be lifted at strong

winds, angles were attached at the bottom and fixed on the frame with screws (Figure 20).

Figure 20: The mounted Solar Collectors

In case of any exigency to remove the panels the following instruction should be abided. Due to the

fact that we had to screw the protection angle on the iron frame, we used countersink screws which

were than overlapped by the solar panels (Figure 21). When the angels have to be removed than the

Solar panel has to be lifted and a piece of wood has to be slid into the hollow space. Then the Solar

panel can be released and the screwdriver can counter the removal of the nut.

Figure 21: The approach to remove the

protection angels. A piece of wood has to be

placed between angle and frame

and then the screw can be removed.



The next step of the Installation was to connect the individual solar collectors with the tube

leading to the house. Therefore copper tubes were chosen as this material tolerates high

temperatures above 100°C. The connection coming out of the Flat plate collector was an 18 mm tap.

The 4 Solar Collectors were connected in parallel, each having its own “cold-water”-connection, so

that the risk of overheating was diminished. As seen in Figure 22 does the water inlet on the bottom

of the Solar Panel go into the Panel, gets there heated and reunites on the top, where it is led into

the water outlet.

Figure 22: The connection of the copper tubes.

The Water Inlet was not connected to the Solar panel taps on the right side directly, but was

led through a 4 m long copper tube to the other side where it was fed in. The reason for this type of

flow conditions is that if it would have been fed into the solar panels directly, the colder water would

have gone the easiest way through the first panel on the right hand side. The water would not be

going through the 3 other solar panels and therefore not heated adequate. With our kind of system,

the waters easiest way to go on the bottom is the left panel. However, on the top this way is the

catchiest as there are water jets coming from the other panels hindering the unhampered passage.

For the water jet going through the right panel the bottom way is the most hampered one, while the

way on the top is the easiest.

The copper tubes were connected with five 1/2 Inch T-Fittings, one Elbow and 2 Reducer, reducing

the diameter of the plastic tube (26mm) to the diameter of the copper tubes (18mm)

In case one has to buy new Reducers for the Plastic Tube, a 26 mm wide tube is very rare today; in the

area it is only available in Bedsted.

Due to the negative effect of air in the system (artificial high pressure, limited heat conduction)

an additional Air Release Valve was installed to release the air produced in the beginning phase when

water is added to the system and when temperatures get so high that the water is evaporated. On



the bottom a water outlet valve was also installed so that in case of a necessity to empty the solar

collector, the water can be released easily. On the top the body with the attached sensor was also

installed and connected to the wire leading to the controller in the house (Figure 22).

After all fittings were connected several test runs were done. Leaking fittings were repaired or

screwed up.



4. The Hydraulic System of the solar installation

There are actually two ways of using the sun power to heat water. One is called Passive solar

heating. Passive solar refers to the usage of sunlight for energy without active mechanical systems.

The second solution, Active solar heating, requires a pump to run the anti-freezing product through

the circuit. It needs additional components which make it more complicated but also give more

control over the system. In the Straw Bale House the Active Solar Solution was chosen. Additionally

there are two basic designs used in an active solar heating system: Open Loop and Closed Loop

systems. Open loop systems heat and circulate household (potable) water directly in the collectors

before it is distributed in the household. Closed loop systems use a heat-transfer fluid to collect heat

and a heat exchanger to transfer the heat to household water. This fluid is usually a glycol-water

mixture raising the freezing temperature and therefore making closed-loop systems effective in areas

with freezing weather. For this reason, closed loop systems are preferred in Denmark.

4.1. The Straw Bale House’s hydraulic system

The active closed loop circulates the anti-freezing product through the solar collectors to a

heat exchanger which transfers the heat to the water storage tank. This system uses a small

Circulating pump activated by a differential thermostat controller that senses when heat is available

in the solar collectors. Technically, this sensor sets off the pump when the temperature in the solar

collector is hotter than in the water tank.

Figure 23 below shows the usual way to link all parts together. The Circulating Pump is hereby

located at the “cold” pipe. However, for the Straw Bale House, where the tank and the expansion kit

were already installed, we had some place issues to install the pump at the “cold” pipe. Therefore, it

has been added on the “hot” pipe, which should not create any problems since it can works until

110°C. The only problem caused is that the pump is heated additionally by the hot pipe rather than

being cooled by the cold pipe. This will probably shorten its lifetime.

Figure 23: General Scheme of a solar heating system



4.2. Water tank and heat exchanger

As can be seen in Figure 25 a heat exchanger should be existent in a tank. Its function is to

transfer the heat from the Antifreeze liquid passing through the Heat exchanger to the tank water.

The volume of the tank should be about 1.5 – 2 times greater than the daily water consumption. As

the water consumption in the Straw Bale house amounts 160 L/day, the tank should have a volume

of about 230 L for the warm water storage [5]. Because the heating system should also be delivered

with hot water from the tank and as the house is completely self-sufficient, we decided to install a

bigger tank with a storage capacity of 1000 L. It is a Solus II from Consolar, a German producer. The

table below gives important technical data.

Table 4: Technical data for the storage tank [6]

Technical Data - SOLUS II 1000

Storage capacity [V] 1000 L

Empty weight [m] 225 kg

Diameter without isolation [D] 85 cm

Diameter with isolation [D] 111 cm

Height with isolation [H] 206 cm

Isolation Cover: 15 cm

Sides: 10 cm +2.5 cm

Max storage temperature [T] 90°C

Collector area [A] 8 – 16 m²

The tank has a tall cylindrical form to develop temperature

stratification. This allows an optimal usage of the heated

water in the upper area without heating the complete

content.

Solus tanks are also characterized by the special layer

system whereby the specific flow conditions of the warm

water allow 2- 3 times more water to be heated. Another

advantage of Solus storage systems is the low volume of

the heat exchanger, which amounts 3 – 15 L [7]. Hence the

warm water is heated very fast in the flow path and

therefore more hygienic even when the water stays longer

in the tank (Figure 25).

Figure 24: The existent hot water

tank, a Consolar Solus II . [6]



Figure 25: The assembly of the Solus tank [7]

4.3. The Check Valve

A check valve permits the fluid to flow in one direction only. It prevents heat loss at night by

convective flow from the warm storage tank to the cool collectors through the Return. Check valves

may be of the "swing" type or the "spring" type. At our installation a check valve was already existent

in the pipes provided in the house.

4.4. The Circulating pump

In order to pump the solar collector liquid from the solar collector through the heat

exchanger into the storage tank and back again, a small circulation pump has to be used when the

storage tank cannot be placed higher than the solar collector. In the Straw Bale House, this pump

was placed on the “hot” pipe because of place issue. But it is usual to install it on the “cold” pipe to

prevent it from high temperatures during operation. Eventually, stop valves were mounted in front

of and behind the pump so that the entire system did not have to be emptied when replacing a

defective pump.

The circulating pump is, with the controller, the only component which needed to be powered by

electricity. Therefore a high energy efficiency pump was the best option to consume as less

electricity as possible. The choice of the pump also took into account the flow of water which had to

go through it and the head of the highest point of the system. The prevalent flow rate in small solar

heating systems amounts 30 to 50 L/h*m² of the collector surface [5]. Considering the 8 m² of solar



collector surface, a flow between 0.25 and 0.4 m³/h was required for a maximum height of 2m.

According Figure 26, the model ALPHA2 from the brand GRUNDFOS is the model needed.

Figure 26 : Pump Types for a given flow and head [8]

a)

b)

c)

d)

Figure 27: Data about the Pump Grundfos Alpha 2 25 – 40. a) A picture of the pump b) An explanation what the

different data mean c) the energy category is A, the best category one can achieve in Europe d) Several details

about the pump with the max Flow, the Head, the temperature it can be used for and the operation pressure.

Thus the circulating pump we decided to install was a GRUNFOS ALPHA2 25- 40.It is an energy label

A, which indicates that the energy-saving level of the pump is the highest possible. The advantage of



this pump is that it will reduce the power consumption considerably, reduce noise from thermostatic

valves and similar fittings and improve the control of the system.

From the GRUNDFOS company, the price of this device is 323€ - which is around 2,410 DKK (phone

call). Therefore we decided to buy it on a discount website for only 177.5€ or 1,324.27 DKK. [9]

Finally, this pump has been declared by the Folkecenter to be too efficient

and too expensive for working only on day time and only during the summer.

Therefore another pump was bought for the Straw Bales House. This pump

was an UPS 25-40. It is not an “Energy Label A” pump; consequently, it was

decided to run the pump at the first speed in order to consume the least

electricity possible. Indeed there are three speeds, which consume 30W,

45W and 60W respectively. This pump did cost 750DKK (100€) VAT, which is

600DKK (80€) pre-tax [10].

Figure 28: The final

pump

4.5. The Differential Thermostat

The Differential Thermostat, or controller, is one of the most important devices of a solar

heating system. Its main function is to regulate the functioning of the pump. When the fluid in the

solar panels is not heated sufficient, the warmer water in the tank will be replaced by cooler water

from the solar collectors as long as the transfer fluid pump works. In order to guard against this loss,

the pump has to be switched off. Likewise it has to be switched on when the temperature in the solar

collectors rises higher than that of the tank. Usually the temperature of the collector should be 5 –

8°C higher than the tank temperature until the controller sets up to start the pump. When this

temperature difference sinks to 2 - 3°C, then the controller should shut off the pump [11].

A temperature difference of 6°C has been chosen to start the pump of our system and 4°C to turn it

off.

This kind of controller we installed switches off/on the pump automatically when a certain

temperature in the tank and solar collectors is passed. There are also simpler forms of controlling the

pump, whereby the pump is started and stopped by a time switch or in accordance with light

intensity. However, those methods of control are less efficient, that’s why we decided to use the

Differential thermostat.

Figure 29: The differential

thermostat Resol Deltasol BS [6]

For our project we decided to include the

differential thermostat Resol DeltaSol BS/3 (Figure

29). This controller has two standard-relays and

one additional thermostat function. The limitation

of the tank temperature amounts 20°C to 95°C.

The power supply amounts 115V, the power

consumptions 2 VA.

We ordered the Controller at Varmt vand fra

solen in Denmark, the prize amounted 1,390DKK-

which is about €186- [12].



4.6. The Expansion tank

The liquid in the solar collector as any liquid expands when heated up. Therefore, to prevent

overpressure in the system an expansion tank is necessary. In the Straw Bale House, the pressure

expansion tank was placed on the “hot” pipe where overpressure can occur. The operating pressure

of the solar heating systems, which is controlled by a nanometer, was set down to 1bar. The safety

valve, on the top of expansion tank should open at approximately 0.3 bar triggering pressure. The

expansion tank keeps the pressure in the system stable and takes up the amount of exceeded heat-

transfer fluid that is caused by a temperature difference. For safety reasons, the volume of the

expansion tank has to be sufficiently large. It should be able to take up the entire volume of heat-

transfer fluid. An expansion tank of 25L capacity was already installed in the Straw Bale House.

4.7. The Circulation Pipes

Within the solar heat circulation, heat is transported from the collector to the hot water

storage tank. In order to minimize heat loss, the distance from the collector to the tank should be as

short as possible.

For systems in family homes, copper pipes with a circumference of 15 mm to 18 mm are enough to

guarantee an optimal transportation of heat. In our system copper pipes of 18 mm diameter were

used [5]. The fitting for the copper pipes were ½ Inches. Finally pipes were sufficiently insulated with

a 30 mm – polyurethane foam pipe. The insulation had to be able to withstand high temperatures

and the outdoor section had to be UV and weather-resistant.

Figure 30: The polyurethane foam

pipes used for insulation

4.8. The Anti-freezing fluid

The collector loop circulates an antifreeze solution. The used Propylene glycol is hereby the

most common heat transfer fluid. It is a non-toxic substance and more commonly used as food

additive, although it is not considered to be a potable fluid. Propylene glycol was mixed with 60 %

water. Inhibitors may be added to increase the lifetime of the fluid, which breaks down over time

due to overheating, creating a sludgy deposit that can clog the collector loop, as well as reduce the

solution's effectiveness as an antifreeze. This inhibitor has not been used in our case since the anti-

freezing solution is changed frequently.



Figure 31: Scheme of the installation in the Straw Bale House



5. Measurements

The straw bale house is occupied by 4 persons, 2 adults and 2 children. Together with the

electricity produced by the two windmills it demonstrates a decentralised solution for producing

heat and electricity by ones own. But how much heated water will be produced? And when will be

the Payback time? In our 2nd part of the report we tried to answer these questions. The values that

were measured are shown in table 5.

Table 5: Measurements done for the straw bale house

Value Unit Explanation

Irradiance W/m²

To receive not only an average value, but to have a specific value

for a specific day in this area, the Irradiance was measured every

5 min with a Hand Pyranometer 98 HP

Temperature

Collector °C

The temperature of the water at the hottest point of the Solar

Collector was measured with a temperature sensor

Temperature hot

part of the Tank

(Sensor 1)

°C By measuring the temperature on the tube leading to the tap,

we can say what the hottest point of the tank

Temperature cold

point of the Tank

(Sensor 2)

°C The temperature of the return water (outlet), which is almost

similar to the cold bottom water of the tank.

Air temperature °C The temperature of the air is important to calculate the

efficiency, it was measured with a thermometer

Energy flow kWh To know how much Energy was produced in one day, the power

was measured with a Picocal Heat Meter

Water flow l/h The flow of water through the system was also measured with

the heat meter

Volume m³

Temperature Inlet

Tank °C

The temperature was here measured before entering the tank.

Hence the heat loss of the tubes between collector and house

could be measured.

The different positions of the sensors are shown in figure 32.



H ot w ater from collector

R eturn

Position of Sensor 1 from 13/08/08

Initial Position of Sensor 1

Position of Outlet Sensor of H eat Meter

Position of In le t Sensor of H eat Meter (Sensor 2)

Tube going to show er

Position of In let Sensor of Heat Meter

Position of Outlet Sensor of Heat Meter (Sensor 2)

Initia l Position of Sensor 1

Position of Sensor 1 from 13/08/08

Tube going to shower

Return

Hot water from collector

Figure 32: The tank, tubes and different sensors which were used.

To understand the dependence of the production of hot water and the irradiance we did some

measurements on different days with different weather condition. However in this report we only

included the results of our last two measurements on the 13/08/08, a cloudy day and on the

15/08/08, a sunny day. The measuring times were from 9:10 am till 5:10 pm with a 5 minute

frequency for the Irradiance and a 10 minute frequency for all other values. Unfortunately the

measurement cannot be 100% correct, as the temperature of the tank cannot be measured directly.

The tank temperature is calculated through taking the average of the hot part of the tank (Sensor 1)

and the cold part of the tank (Sensor 2). As the temperature is stratified in the tank, the average of

these two extreme values will give the most reliable actual tank temperature.

Sensor 1 was hereby placed at the top tube coming out of the tank and going to the tap. Therefore

we used the sensor as a contact sensor touching the copper tube and isolated it with mineral wool

against external impacts. As copper is a very good heat conductor, good results were expected.

Sensor 2 was placed at the cold return from the tank to the solar collector. This also led to an error as

the temperature of the Return is the temperature of the glycol, which is decreasing when the pump

stops as it gives all its heat to the tank. The actual tank temperature at the bottom however, is

increasing as it receives heat from the heat exchanger and therefore also from the return.

The methodology of linking one sensor to the tank water temperature (Sensor 1) and one sensor to

the circulating system temperature (Sensor 2) presents a disadvantageous situation as of course tank

water and the glycol of the collector system behave unequal to some conditions. However, we

decided that this was the most precise form of taking temperature measurements.

As the 13th

of August was a very cloudy and rain-laden day, we did not expect to obtain increased

results, above all not from the tank temperature. However the temperature ought to stay constant.



Figure 33: The Irradiance for the 13/08/08, a day with many extremes.

As shown in Figure 33 did the Irradiance fluctuate very strong reaching maximum values of

1440W/m² at 2:30 pm and minimum values of 50 W/m² at 9:40 am. The average value for the 13th

August was 311.9 W/m².

The air temperature amounted between 14 and 19 °C with the Maximum at 11:05 am and the

Minimum at 12:55 am (Figure 34). The strong fluctuations were due to fast weather condition

changes between storm and clouds. The average air temperature amounted 16.3°C.

Figure 33: The highly fluctuating air temperature.

For the collector system we measured 4 different values with the temperature of the hot part of the

tank (Sensor 1), the temperature of the collector, the temperature of the Inlet and the Outlet of the

tank (Sensor 2).



Figure 34: The different temperatures of the system

In Figure 34 one can see the difference between the sensors showing the temperature of the

Collector system (Collector, Inlet and Outlet), which are more controlled by Irradiance and Air

temperature and the sensor showing the hot part of the tank temperature (red line) and the overall

tank temperature (brown line). The collector temperature of the highest Irradiance (1440 W/m²)

amounted 58.8 °C for this day and the temperature for the lowest irradiance (50 W/m²) amounted

28.3 °C. Through the whole day the temperature rise of the collector amounted 12.4 °C. As the water

of the Outlet (and the Inlet) stagnates during the night (no pump running), the temperature in the

morning equals the actual bottom tank temperature. As one can see the overall tank temperature in

the morning amounted 29°C, whereby the hot part of the tank amounted 39°C and the cold part

amounted 19°C. The temperature of the tank increased at this day about 3°C from 28.9°C in the

morning to 32 °C. However, the temperature of the hot part in the tank decreased at 1°C through the

day. The heat of the 3 kWh produced was stored in the lower part of the tank. This was due to the

fact that there was almost no pump activity at this day. Therefore only Diffusion occurred rather than

Convection.

Uncertainties do exist for the sudden increase of tank temperature at 10:55 am. This error does not

occur due to human failure by forgetting to look if the pump is running. The flow in the system which

was also measured showed only 0 L/h until 11:20 am what means that the pump was not running.

Another explanation could be, that due to fact that the pump was not running, but the collector was

heated up to 50°C at this time convection occurred, conducting heat through the system.

The difference between the Collector temperature and the temperature of the Inlet tank makes clear

that there are still high heat losses in the hot water tubes. This could be due to some tubes, which

were still not well isolated.



Figure 35: Aside from the morning, the values agree well with the features of a turned off pump

Figure 35 shows how the values behaved with the heat transfer through pumping. Even if it was not a

perfect day to show how the hot part of the tank (red line) became hotter during pumping heated

water through the heat exchanger. Particularly after 11:10 am the energy could be saved during

pumping stops with a small decrease due to heat loss. However, the first period of the day is an

outlier from the concordant values of the rest of the day. Here the temperature is decreasing rapidly.

One reason could be that there was just a residual of hot water in the top layer of the tank which was

than used by turning the tap on. The tap was turned on during every measuring period to obtain the

actual tank water and not the stagnating water in the tube. After flushing this hot layer the average

water in the tank emerged. This could also be the reason why the cold and the hot part of the tank

are concordant between 11:05 and 11:25 am. One can also see that the cold water temperature of

the tank is also decreasing during the stops of the pump. The stagnating water of the heat exchanger

conducts more heat to the tank, cooling itself even more during pumping breaks. After the pump

switched on an increase in temperature can be seen because new, less cold water is injected. The

reason for the fluctuating values even when the pump is running is due to fluctuating collector

temperatures which transfer this fluctuation to the Outlet. Hence it is important to know that the

actual cold temperature part of the tank water would not underlie such fluctuations. It is more likely

that it would behave like the hot part of the tank and conduct the obtained heat to the upper layers

of the tank. The actual tank temperature of the cold part can in this figure seen in the first part until

10:40 am, where the pump has not worked jet and hence the temperature of the Outlet is equal to

the tank temperature. Similarly does the curve behave in the last part after 4:15 pm, where the



pump has not worked for a while and therefore the temperature of the Outlet becomes similar to

the temperature of the storage tank.

To obtain the energy produced on the 13th we used besides the measuring of the heat meter a

formula to calculate the energy output Q.

Q = m * Cwater * ∆T (2)

Whereby m is the mass, Cwater is the specific heat capacity of water and ∆T the tank temperature

difference between the start and the end of our measurement.

Table 6: The values concerning the energy output for the 13/08/08

13.08.08

Energy delivered to

the tank

3 KWH

10 800 KJ

Top Temp

(°C)

Bottom

Temp (°C) Mean Temp (°C)

AM 39 19 29

PM 38 26 32

As the specific heat capacity for water is 4200 kJ/kg°C and the mass for our water to be heated

amounted 1000 kg, the energy output Q for the 13/08/08 was 3.5 kWh.

(Q= 1000 * 4200 * (32-29)

Q= 12 600 000 Joules = 3.5 KWH)

This value of 3.5 KWH coincides with the value given by the flow meter. It means that 3KWh are

needed to increase the 1000 L of water at 3°C.

However, as the hot part of the tank is not heated for this day (see Figure 36), there cannot be an

overall increase in tank temperature. To calculate the actual amount heated, we took the same

formula converting it to m.

m= �

�� ∆ (3)

Taking 3 kW for Q and only the bottom part of the tank, which was heated (∆T =26 – 19), the actual

mass heated was 429 kg. This calculation shows that all the energy delivered to the tank (3 KWH) has

been used to warm only half of it (430 Liters) from 19°C to 26°C, and not used to warm all the

capacity of the tank.

The efficiency of the tank can be calculated with the formula

η = ��

�� (4)

Using the energy of 3 kWh and the average Irradiance of 312 W/m² our efficiency for this day is

about 15%.



The 15th

of August was the completely opposite to 13/08/08. This day was very sunny with an

average Irradiance of 916.91 W/m². The Maximum Irradiance was 1278 W/m² at 10:35 am and the

Minimum Irradiance was 221 W/m² at 9:30 am (Figure 36).

Figure 36: The overall relatively high Irradiance of the 15

th of August.

As well as the Irradiance behaved the air temperature. The temperature increased from 17°C in the

morning up to 21°C in the afternoon. The average air temperature of this day amounted 21.9°C

which is several degrees higher than the 16.3°C of the 13th (Figure 37).



Figure 37: The stepwise increase of the air temperature.

The temperatures we measured at the collector, the inlet and the tank show a good consistency with

a sunny day. The collector temperature increased through the day from 35.6 °C to 59 °C in the

evening. However the highest collector temperature was reached at 1:50 pm with a value of 66.3 °C.

Looking at the collector temperature (blue line) in figure 38 a good consistency with the location of

Sun can be observed. Following this line, the highest point of Sun at about 1:30 pm led to the highest

temperature in the collector; after this point the sun moved forwards, pointing the collector not in a

perfect angle anymore and therefore leading to a decline in temperature. The Inlet temperature was

still 1°C smaller, thus showing a loss of heat on the way from the collector to the house.

The temperature of the tank was also showing good consistency to a sunny day condition. As the

heat is here added, the temperature in the tank should rise constantly. What was unclear was the

fact that in the first part of measurements the temperature of the outlet was higher than the

temperature of the hot part of the tank. This could have been due to incorrect reading of the hot

temperature at Sensor 1. The lowest tank temperature was reached at 9:30 am, where it amounted

32°C; the temperature increased till 50°C at 5:10 pm and probably increased further after the end of

our measurement. So the attained temperature over 7 hours 40 minutes for this day was 17.39°C.

That is about 6 times more than on the 13th of August.



Figure 38: The temperature rise during the day.

At 9:40 am there was a sudden temperature rise in the tank coherent with a temperature decrease in

the solar collector. Because the pump was not running until 9:40 am the tank temperatures

remained low whereas the collector was already heated during the sun. As one can see in figure 39

the pump started at 9:40 am and was followed by an increase in tank temperature, especially the

cold part. When the pump turned off for about 5 min at 9:50 one can see that the temperatures of

cold and hot part are decreasing as well and the collector temperature is increasing rapidly. After this

5 min when the pump turned on again, the collector temperature shows again a sudden decrease in

temperature (blue line, Figure 38). At this day the pump was running over the whole day. The

average flow, transported through the system was 155.13 L/h.



Figure 39: The Pump was working during almost the whole day on the 15

th of August.

On the 15th our energy output amounted according to the heat meter 20 kWh. To check this value

we included again the formula (2), adding the values delivered in table 2.

Table 7: The values important to calculate the energy output

15.08.08

Energy delivered to

the tank

20 KWH

72 000 KJ

Top

Temp (°C)

Bottom

Temp (°C)

Mean Temp (°C)

AM 34.5 28 31.25

PM 50 49 49.5

The energy output of this day amounted 21.3 kWh. As it can be seen from the table both top and

bottom temperatures in the tank reached 50°C. In other words, 20 KWh is the amount of energy

capable to warm 1000L from 30°C to 50°C.

As 1000 L of water can be warmed at 3°C using 3KWh of energy (13/08/08) or warmed at 20°C using

20KWh of energy, we concluded that 1KWh is required to rise temperature at 1°C.

It is clear that the family could only take hot water from the obtained values of the 15th. To know

how long the family could live from this heated water we included the following calculations of the

consumption.

Hereby we had to assume that no new heat is added on the following day and that 4 people

consume in average 160 L with the hot tap at 45°C and the cold tap at 15°C. To know now how much

L we use from the storage (T=50°C) we include the following formula.



T tap= ��

�� (5)

By using the following steps the amount of used tank water was calculated

45°C = ��°� ��!°��

�� and x+y = 160 L

45°C *160 L =15°C *(160L-y) +50°C y

7200 °C L= 2400°C L + 35°C y

y= 137 Liters

Hence when 160 Liters were used from the tap, only 137 Liters came from the 50°C hot tank.

Consequently, by the end of the day, 137 Liters of cold water (15°C) were added to the tank to

replace the hot water used. The tank therefore included 863 Liters of 50°C and 137 Liters of 15°C

warm water. After that the average temperature was calculated.

T average = "#$�!��$%��

�!!! = 45.2°C

The average temperature of the tank was 45.2°C. The calculation does not take in account the loss in

the tank over the night. However, our measurements have shown that there were no significant

losses.

Since the water in the tank is now 45.2°C, on the second day the hot water consumed at the tap

comes 100% directly from the tank. This means that by the end of the day 160 Liters of cold

freshwater will be added to the tank to replace the daily consumption.

Taverage = "&!&��#!��

�!!! =40.2°C

Hence on the end of the second day the tank temperature amounted 40.2°C.

Continuing this calculation on the third day we held the following water temperature in the tank.

T average = "&!&!��#!��

�!!! =31.2°C

These calculations show that after a sunny day, like Friday 15th of August, the energy delivered to

the tank is enough to supply the hot water consumption of the 4-people family during two days, if

the day after the sunny day is not profitable and does not deliver energy to the tank. This result

assumes that there is no heat loss during the night.

This outcome can be correlated with the European data about hot water consumption, which says

that the individual average energy consumption for hot water is about 950 kWh per year. At a daily

scale the consumption 2.6 kWh per day per person. Hence for our family approximately 10 kWh of

energy are needed per day and 20 kWh for two days [13].

The efficiency of the 15/08/08 was again calculated by including Irradiance, Energy produced, area of

the solar collectors and time.



η = ��

�� (4)

By taking the values 20 kWh and 917 W/m² the efficiency for the 15th of August amounted 36%. At

the maximum values of Irradiance between 11:30 am and 12:30 am the efficiency even achieved

46%.

The 2 days of measurements clearly proved that the efficiency was directly linked to the Irradiance.

The conclusion was that the higher the irradiance is, the higher is the efficiency. However, it also

shows that even with a nice sunny day with a high average irradiance (900W/m²) the efficiency does

not exceed 40% in average. Nearly 50% efficiency can be achieved at noon when the position of the

solar collectors is the best according to the sun position.



6. Financial Analysis

As our measurements were not continued over a longer distance to offer more statistical

background, we decided to include average data for Ydby in our financial analysis. Considering the

average irradiance over the year in Skive (Figure 40), located not so far from Hurup Thy, a financial

analysis has been done to determine the payback time of the installation. These calculation has been

done, given a current oil price at 0.15€/kWh and an electricty price at 0.26€/kWh.

Figure 40: Monthly Irradiance in Skive [14]

Given an irradiance in Wh/m²/day, the energy produced by month has been calculated as following;

E = I * A * n * η (6)

Whereby E presents the produced Energy, I the Irradiance, A the collector area, n the number of days

per month and η the collector efficiency.

On the other hand the energy required to supply the family has been determined, according to a 2.6

kWh/day/pers energy consumption for hot water [15]. To calculate the overall family energy demand

following calculation established.

Efam = Eind * N* n (7)

Efam is hereby the Family energy demand, Eind the Individual energy demand, N the number of people

in a family and n the days per month.

In figure 41 the results for the produced energy and the energy demand are compared against each

other. One can see that the solar installation can supply the family from April to September

completely. The figure also shows that even supply more hot water can be supplied than needed by

the family during the summer period. In order to obtain the energy saving with such an installation,

only the energy demand which has been supplied by the solar collector was taken in account for the

calculation. This means that from April to September, when the solar panels supply all the demand



the energy saving would reach 1900 kWh. However, over the whole year the energy saving was

calculated from the energy produced from January to March and October to December added to the

1900 kWh energy saved during the 6 other months. This annual savings amounts 2600 kWh.

Figure 41: Average energy demand and produced energy with our solar installation

At a new acquisition the prize of our 4 solar panels would have been 40 000DKK, which is about 5 400

Euros. The current oil and electricity prices are rising and a stop in this development is not in sight.

The paybacktime was calculated by taking the actual oil price of 0.15 Euro/kWh and the actual

electrictiy price in Denmark of 0.23 Euro/kWh. Then, according to the current oil and electricity

prices and the cost of the solar collectors two types of pay back time were calculated. The first one

only considers the period the solar collcector will be in use, between April and September. The

second one is considering the whole year.

Table 8: Pay back time for different conditions.

Total energy supplied by the installation from April to Sept (kWh) 1903.2

Total energy supplied by the installation over the year (kWh) 2616.5

Price of oil : 0.15 Euro/kWh Money Saved (€/yr) PAY BACK TIME (years)

April to September 285.5 18.9

whole year 392.5 13.8

Price of electricity : 0.23 Euro/kWh Money Saved (€/yr) PAY BACK TIME (years)

April to September 437.7 12.3

whole year 601.8 9.0

Taking into consideration that the actual oil price is 0.15€/kWh, the payback time for our installation

would be 19 years if we only consider the 6months period from April to September, and nearly 14

years considering the whole year. Compared to electricty, which is actually more expensive than oil,

this payback time would be reduced to 12 and 9 years respectively. However, due to the actual

energy crisis, both oil and electricity prices are forcasted to increase in the next years, which means



that the payback time would be even more reduced. Eventually, assuming a 25 years life time for

such solar collectors, a solar installation in Hurup Thy is a not only an ecological commitment, but

also a good financial investment.

7. Conclusion

A lot of components of renewable energies argue that solar installments do not have a future in

colder countries with winter times and colder summers as Denmark is. However, this argument was

proved in our record to be wrong. Of course is a dependency on one particular energy system the

wrong way and of course can solar not deliver enough warm water during the whole year in a

country like Denmark. But with a mix of several energies like solar and wind, a balanced energy

production can be achieved. Also biomass is an important part of this backup-system as it is the only

renewable energy source with very long storage qualities. What people have to learn today is that

energy is a limited resource. For almost all Western Europeans it is seen as normal to receive energy

whenever they ask for. In the future this kind of wasting energy will not work any more, be it because

of remarkable expensive energy prices or the change of society to a more sustainable manner of

energy production. Living in a self-sustaining house means also that the inhabitants will have to live

with limited resources and limited capacity. Hence the sustainable acquaintance with our resources

is as important as to change to renewable energies.



8. Reference List

1. www.nrel.gov/docs/legosti/fy96/17459.pdf [Last visit: 12/08/08]

2. www.thermomax.com/consump.htm [Last visit: 12/08/08]

3. www.apricus.com/html/solar_typesofsolar.htm [Last visit: 11/08/08]

4. The Solarserver – The internet platform for solar energy:

www.solarserver.de/wissen/sonnenkollektoren-e.html#fla [Last visit: 13/08/08]

5. The Solarserver – The internet platform for solar energy:

www.solarserver.de/wissen/solaranlagen.html. [Last visit: 15/07/08]

6. Consolar – Solus II.: http://www.consolar.de/produkte/speicher/solus.html#c173

[Last visit: 15/07/08]

7. Solus II: www.consolar.co.uk/documents/Solus%20ll/SOLUS_TD_WEB.pdf [Last visit:

19/08/08]

8. http://www.grundfos.com/web/grfosweb.nsf/Webopslag/grundfos+alpha

9. Pumpendiscounter Germany: www.pumpendiscounter.de [Last visit: 24/07/08]

10. www.cgi.ebay.fr/Circulateur-chaffage-central-Grundfors-UPS-25-40-

130_W0QQitemZ22025836562QQihZ012QQcategoryZ92187QQcmdZViewItem [Last visit:

12/08/08]

11. http://www.solarserver.de/wissen/sonnenkollektoren-e.html#fla [Last view: 29/07/08]

12. Varmt Vand Fra Solen: http://www.varmtvandfrasolen.dk/ [Last View: 15/07/08]

13. http://ec.europa.eu/energy/atlas/html/hotdintro.html [Last View: 10/08/08]

14. http://re.jrc.ec.europa.eu/pvgis/apps/radmonth.php?lang=en&map=europe [Last view:

15/08/08]

15. http://ec.europa.eu/energy/atlas/html/hotdintro.html [Last visit: 10/08/08]

16. Twidell, J., Weir, T., 1986: Renewable Eenrgy Resources. E & FN SPON

17. www.gogreenheat.com /images/Resol%20BS%203.jpg [Last visit: 16/08/08]

18. http://www.builditsolar.com/References/SolRad/Lat56.htm [Last visit: 23/07/08]

19. http://www.engineering.com/SustainableEngineering/RenewableEnergyEngineering/

SolarEnergyEngineering/PassiveSolarSystemsSolarHotWater/tabid/3892/Default.aspx

[Last visit: 11/08/08]



9. Annex I



Documents

Installation of a Solar Collector