A Theoretical Study of the Thermal Performance of the TermoDeck Hollow Core Slab System

A theoretical study of the thermal performance of theTermoDeck hollow core slab system

P. Barton, C.B. Beggs *, P.A. Sleigh

School of Civil Engineering, University of Leeds, Leeds LS2 9JT, UK

Received 26 October 2001; accepted 17 April 2002

Abstract

The TermoDeck hollow core slab system is a versatile energy storage technique for controlling the en-vironment within large and medium sized buildings. It utilises the hollow cores within pre-cast concretefloor slabs as ventilation ducts to produce an environment which is thermally stable. Although manyTermoDeck systems have successfully been installed in Scandinavia, the United Kingdom and in othernorthern European countries, the thermal performance of the system is not fully understood. This paperpresents the results of a theoretical study, using a numerical model, into the thermal performance of theTermoDeck system. In particular, the role of the bends in the system is investigated and the conclusionreached that their impact on overall heat transfer is minimal. It is also concluded that greater thermalattenuation is achieved by using a five-core pass system in comparison with a three-core system.� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: TermoDeck; Hollow core slab; Thermal storage; Fabric heat storage; Fabric energy storage; Pre-cast

concrete; Heat transfer; Thermal performance

1. Introduction

The TermoDeck system was developed in Sweden [1] and has been used successfully in manylocations throughout northern Europe. Recently in the United Kingdom (UK) a number of highprofile TermoDeck buildings have been constructed, including the Elizabeth Fry Building at the

Applied Thermal Engineering 22 (2002) 1485–1499www.elsevier.com/locate/apthermeng

*Corresponding author. Tel.: +44-113-233-2303; fax: +44-113-343-2265/233-2265.

E-mail address: [email protected] (C.B. Beggs).

1359-4311/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S1359-4311(02)00059-5

mail to: [email protected]

University of East Anglia [2] and the Kimberlin Library Building at De Montfort University [3].The TermoDeck system involves pushing ventilation air through hollow core tubes in pre-castconcrete floor slabs. It is an environmentally benign system which produces buildings which arethermally stable and comfortable without the need for any refrigeration. The TermoDeck systememploys low air velocities (i.e. approximately 1 m/s) with the result that buildings using thissystem tend to consume little energy. For example, the Elizabeth Fry Building consumes very littleenergy; its average electrical energy consumption for 1997 was only 61 kWh/m2 and its gasconsumption was 37 kWh/m2 [2], figures which are less than half of the targets values for goodpractice air conditioned office buildings in the UK [4]. In addition, it is perceived by its occupantsto be a particularly comfortable building [5].Although the TermoDeck system has been successfully employed in northern Europe, Australia

and even Saudi Arabia, relatively little is known about its thermal performance. In particular,there appears to be confusion as to the influence of the bend section on the overall performance ofthe TermoDeck system [6–8]. The authors therefore developed a numerical model and undertooka study to investigate the thermal performance of various components of the TermoDeck system.This paper presents the results of this study.

Nomenclature

Tair air temperature at node point (�C)Tsurface surface temperature at node point (�C)Ti;j node point temperature (�C)h1 core convective heat transfer coefficient (W/m2 K)A core surface area (m2)_mm air mass flow rate through core (kg/s)Cpair specific heat capacity of air (kJ/kgK)qair density of air (kg/m3)vair velocity of air (m/s)dcore diameter of core (m)l dynamic viscosity of air (Pa s)kair thermal conductivity of air (W/mK)kslab thermal conductivity of concrete slab (W/mK)a thermal diffusivity of concrete slab (m2/s)Re Reynolds numberPr Prantl numberNu Nusselt numberFo Fourier numberBi Biot numbert time (s)Dt time step (s)Dx, Dy spaces steps (m)

1486 P. Barton et al. / Applied Thermal Engineering 22 (2002) 1485–1499

2. The TermoDeck system

It is possible to cool buildings, without the use of refrigeration plant, by utilising night ven-tilation. Night venting involves flooding the building interior with cool outside air during thenighttime, so that heat accumulated by the structure during the daytime is purged. The coolstructure can then be used to absorb heat from the room space by radiation and natural con-vection during the daytime and also to cool outside air as it enters the building. For night ventingto be successful good thermal coupling must exist between the air and the mass of the building.The TermoDeck system achieves this objective well by ensuring a high degree of thermal contactbetween the air and the building mass by pushing ventilation air through the hollow cores inproprietary concrete floor slabs, as shown in Fig. 1. By forming perpendicular coupling airwaysbetween the hollow cores, it is possible to form a 3 or 5 pass circuit through which supply air maypass. During periods in which cooling is required, outside air at ambient temperature is blownthrough the hollow core slabs for as much as 24 h of the day. Overnight the slab is cooled toapproximately 18–20 �C, so that during the daytime warm incoming fresh air is pre-cooled by theslab before entering the room space. By exposing the soffit of the slabs it is also possible to absorbheat radiated from occupants and equipment within the space.The TermoDeck system achieves good heat transfer between the incoming air and the concrete

slab by ensuring turbulent airflow through the hollow cores. This is achieved by using a core airvelocity of approximately 1 m/s, which enables heat to be stored at a rate of between 10 and 40 W/m2 of floor area, depending on the air temperatures involved [1,9].

3. Finite difference model

In order to investigate the heat transfer mechanisms associated with the TermoDeck system, atwo-dimensional numerical model was developed. The TermoDeck system relies on heat being

Fig. 1. The TermoDeck system.

P. Barton et al. / Applied Thermal Engineering 22 (2002) 1485–1499 1487

exchanged between the air in the core to the concrete of the slab, through the interface which is thesurface of the core. Heat is conducted radially through the concrete from this interface until itreaches the surface (top and bottom of the slab) or to where the temperature gradient becomessmall––in the horizontal direction between cores. As the temperature difference between the coresis relatively low, the high heat capacity of the concrete means that it is possible to neglect theinfluence of adjacent cores. This means that a two dimensional model may be constructed that is,one that models flow along the core and vertically through the concrete. This is the form of themodel used in the studies described in this paper. The model equations (which are presentedbelow) were programmed using Microsoft Excel incorporating Visual Basic for Applications(VBA) programming code which enabled a great deal of flexibility to be introduced into themodel. An explicit finite difference methodology was used to simulate variations in temperature atincremental slab lengths and depths over time. Transient analysis was performed for each caseusing a cyclic 24 h sinusoidal air temperature distribution at the inlet boundary and for the room.The model incorporated features to allow investigation of the effect of the convective heat

transfer at the bend sections in the TermoDeck system and also the effect of changing the numberof cores utilised. Because hollow core concrete slabs are manufactured in a range of geometries itwas decided that the dimensions of the study slabs should correlate with those used in previousresearch [1,6–8,10,11], thus enabling published results to be utilised during the model validationprocess. Fig. 2 shows the geometry used in the study; the slab thickness was 270 mm and the corediameter was 180 mm. The length of the hollow cores was 4000 mm and the bends were assumedto have an equivalent length of 415 mm.The model utilised a regular spaced finite difference mesh with interlinking nodes in both the x

(along the slab core) and y (vertically down through the slab) directions, as shown in Fig. 3.Although the heat transfer from core is radial preliminary numerical simulations showed thiscould be approximated well by plane conduction due to the small curvature of the core surfaceand the short distance to the boundary. With the assumption of constant temperatures in theupper and lower room spaces this meant that the system could be simulated by modelling heattransfer through one half of the slab depth, although the complete surface area of the core wasused to take in to account all of the heat transfer.

Fig. 2. Standard TermoDeck cross-section core geometry.


The model equations used are those derived from standard heat transfer considerations ofconduction and convection. They incorporate the considerations of the heat capacities and re-spective abilities to transfer heat within and between the air and concrete. Here we will presentonly the final model equations in finite difference form, derivation of the equations and their finitedifference form can be found in many standard books on heat transfer, for example, [12].The finite difference model incorporated Eqs. (1)–(4) to determine the temperature distribution

through the concrete slab and the air temperature variation along the length of the hollow core. Inthese equations the subscripts i and j represent x and y nodal positions respectively. The super-scripts n and nþ 1 signify the current and subsequent time step with current time defined ast ¼ tstart þ nDt. The core node air temperature was determined by using Eq. (1).

Tairiþ1 ¼ Tairi �h1AðTairi � TsurfaceiÞ

_mmCpair

!ð1Þ

The core surface temperature at the surface nodal points was determined by using Eq. (2).

T nþ1i;j ¼ Foð2T n

i;j�1 þ T ni�1;j þ T n

iþ1;j þ 2BiT ni;jþ1Þ þ ð1� 4Fo� 2BiFoÞT n

i;j ð2Þ

These two equations incorporate a heat transfer coefficient, h1, to relate heat transfer at theboundary between the core air and the concrete slab.The interior node temperature at the nodal points was determined by using Eq. (3).

T nþ1i;j ¼ FoðT n

iþ1;j þ T ni�1;j þ T n

i;jþ1 þ T ni;j�1Þ þ ð1� 4FoÞT n

i;j ð3Þ

The slab surface temperature at the slab/room boundary was determined by using Eq. (4).

T nþ1i;j ¼ Foð2T n

i;jþ1 þ T ni�1;j þ T n

iþ1;j þ 2BiT nroomÞ þ ð1� 4Fo� 2BiFoÞT n

i;j ð4Þ

Fig. 3. A section of the two-dimensional finite difference mesh.


The Fourier number (Fo), Biot number (Bi) and convective heat transfer coefficient (h1) are de-fined as follows:

Fo ¼ aDtDx2

ð5Þ

a is the thermal diffusivity of the concrete, which is the ratio of thermal conductivity to the heatcapacity.

Bi ¼ h1Dxkslab

ð6Þ

h1 ¼kairNudcore

ð7Þ

where

Nu ¼ 0:023Re0:8Prn ð8Þ

Pr ¼ Cpairlkair

ð9Þ

Re ¼ qairvairdcorel

ð10Þ

As the solution method for the model equations was explicit in time the solution is not uncon-ditionally stable. It was necessary to choose a time step, Dt, sufficiently small to ensure stability.The criterion used to ensure this was

Foð2þ BiÞ6 12

This expression implies a relationship between Dx and Dt as well as thermal properties of the airand concrete.

3.1. Validation of the model

The model was validated by comparing the results it produced with those published by Williset al. [11]. Willis’s data, comprised boundary condition data for the core inlet air temperature,together with core surface and outlet air temperatures for a 3 day experimental period. Willis’sinlet air temperature data was modified slightly to create a sinusoidal diurnal temperature dis-tribution, and was applied to the model. Fig. 4 shows a direct comparison between core airtemperatures published by Willis et al. and core air temperatures generated by the two-dimen-sional model for a three-core pass through a TermoDeck slab. The slab and air temperature datashown is for the third day of continuous operation and represents a situation where thermalstabilisation has been achieved.The results produced by the model corroborate those published by Willis et al. with the model

producing peak core outlet air temperatures that were within 0.2 �C of Willis’s data and a core airtemperature range that was within 0.65 �C of the experimental values. These results confirm that


the developed methodology provides a good correlation with the actual behaviour of theTermoDeck slab operating under the conditions applied.As well as to demonstrate that the model can simulate known data, it is also important to

ensure that solutions obtained are as accurate as possible using the equations chosen. The usualprocedure is to increase the number of nodes in the model until solutions with an increasednumber of nodes change little. From this the minimum number of nodes required can be deter-mined. This procedure was carried out for this study with the conclusion that using a node spacing(equal in x and y directions) of 25 mm gave only a 0.05% difference between solutions with 12.5mm spaced nodes. It meant that five nodes would be placed in the concrete enough to give atemperature profile through the slab. All test were subsequently run with the 25 mm node spacing.

4. Bend study

Having demonstrated the viability of the model under sample operating conditions a para-metric study was undertaken to assess the relative impact of various components of the Termo-Deck system. The study utilised both steady state and transient temperature profiles to analyse thebehaviour of the slab and to provide data which could be compared with that produced byprevious research.

Fig. 4. Comparison between results achieved by the two-dimensional model and Willis et al. for the third day of

operation.


There is some uncertainty as to the effect that the bend sections in the hollow core have on theoverall performance of the TermoDeck system. Previous researchers [6–8] have suggested thatbecause of increased turbulence, the heat transfer coefficient experienced at the bends is muchgreater than that for the straight sections. Ren and Wright [8] suggests that the heat transfercoefficient in this region is approximately 50 times that for the straight sections, whilst Winwoodet al. [6,7] states that the bend heat transfer coefficient is approximately 15 times that of thestraights. Despite the wide discrepancy between these figures, no additional research has beenpublished to corroborate either value. Consequently, it was decided to undertake a study using themodel described above to assess the influence that the bends have on the performance of theTermoDeck system.The model representation of the TermoDeck system is illustrated in Fig. 5. In this study it was

assumed that air flowed through a continuous straight tube. However, where bend sections

Fig. 5. A simplified representation of the TermoDeck effective core length.


occurred in the system the heat transfer coefficient for the ‘straight’ representing the bend wasincreased by a multiplication factor. In this way it was possible to analyse the effect of the bendson the core air and surface temperatures along the length of the core and thus determine theirrelative influence upon the overall behaviour of the system.Fig. 6 shows the effect that the bend sections have on core surface air temperatures for a three-

core pass TermoDeck slab under steady state conditions. It is assumed in the system shown that itis operating in cooling mode and that the heat transfer coefficient at the bends is 50 times greaterthan that for the straight cores. The air velocity through the core is assumed to be 1 m/s, with theheat transfer for the straight core sections calculated as 5.29 Wm�2 K. The core inlet air tem-perature is maintained at 28 �C and the room air temperature at 20 �C. The bend length is 415 mmand the straight core lengths are 4000 mm each.From Fig. 6 it can be seen that although the bends have a large impact on core surface tem-

perature, they have relatively little influence on the overall air temperature drop achieved by theTermoDeck system. Fig. 7 shows the results of a more detailed analysis in which bend heattransfer coefficients are multiplied by a range of factors (i.e. �10, �25, �50). The results indicatethat if the bend multiplication factor is small (e.g. �10), the core surface temperature at the bendswill be relatively low, due to the lack of heat transfer to slab. If the multiplication factor is large(e.g. �50), the heat transfer rate will be greater and thus the core surface temperature will in-creased. In contrast to the core surface temperature, variation of the bend heat transfer coefficienthas negligible impact on core air temperature, with the air temperature range at the bends beingapproximately 0.2 �C.Fig. 8 shows the transient effect of the heat transfer coefficient at the bends. The external air

temperature used in the analysis is based on meteorological data obtained for Heathrow, London.

Fig. 6. Steady state analysis of a three-core pas system incorporating a bend heat transfer factor of �50.


Fig. 7. Steady state analysis illustrating the effect of varying bend heat transfer factors upon core air and slab core

surface temperatures.

Fig. 8. Transient analysis for three-core pass TermoDeck operation with bend heat transfer factors of �1 and �50.


The room air temperature was assumed to exhibit a sinusoidal form which lagged behind theexternal temperature by two hours and which had a range from 22 to 24 �C. The nodes in the two-dimensional model were initially all set at 19 �C. From Fig. 8 it can be seen that a bend heattransfer factor as high as �50 has negligible impact on core air temperature over the 24-h period.

4.1. Core utilisation study

Although most applications utilise a three-core pass, the standard TermoDeck geometry en-ables five cores to be utilised. There has been little published research on the effect of varying thenumber of cores utilised. A study was therefore undertaken to investigate the effect of increasingthe number of cores utilised. This study assumed the standard TermoDeck geometry, with 4000mm core straights, with bends having an equivalent length of 415 mm. For five-core operation, thetotal distance travelled by the air prior to discharge was 21.66 m, and for a three-core regime thedistance was 12.83 m.Winwood et al. [6] investigated the impact of both three- and five-core operating regimes. Fig. 9

replicates results published by Winwood et al. for core air temperature prior to discharge fromboth three- and five-core pass systems.

Fig. 9. Three- and five-core TermoDeck operation (Winwood et al. data).


Fig. 10 shows results obtained from the two-dimensional model using meteorological data forHeathrow. It was assumed in the simulation that the bend heat transfer coefficient multiplicationfactor was �50. The core air temperatures stated are those prior to discharge from the slab whenutilising either a three- and five-core operating regime.Comparison between Figs. 9 and 10 reveals similarities between the modelled data and the

results published by Winwood et al. Both graphs show that the five-core regime reduces the peakcore air temperature by approximately 0.7 �C compared with the three-core regime. In addition,there is a phase shift of 60–100 min. The use of five-cores reduces the exiting air diurnal range byabout 1.5 �C compared with the three-core regime.

5. Discussion

While the bend study assumes an enhanced heat transfer coefficient in the core bend sections asdemonstrated by other researchers [6–8,11], it contradicts the findings of Willis et al. [11], whoconcluded that the majority of thermal transfer occurred in the bend sections with relatively littleoccurring along the straights.Figs. 6 and 7 indicate that the influence of the bends on air temperature is minimal. It is in the

straight core sections that the majority of the heat transfer takes place. The air temperature dropacross each the bend section varies between 0.2 and 0.5 �C depending on the location of the bendin the system and the heat transfer coefficient used in the model, while the overall temperaturedrop through the slab is nearly 4 �C. This observation can be explained as follows:

Fig. 10. Results of the two-dimensional model for three- and five-core operating regimes.


• The surface area of the bends is relatively small in comparison with the area of the straight coresections. Consequently, most of the heat transfer will take place in the straights, despite the factthat the heat transfer coefficient at the bends is much greater than that experienced in thestraights.

• It is evident from Fig. 7 that (when operating in the cooling mode) the temperature of the con-crete core surface at the bends increases dramatically as the heat transfer coefficient increases.This is because more heat is being transferred to the concrete than can be conducted away; sothe surface temperature increases. However, as the surface temperature increases, so the rate ofheat transfer decreases, because the temperature differential between the air and the concretesurface decreases.

Therefore, the assumptions made by Willis et al. [11] regarding the dominance of the bend heattransfer mechanism appear unjustified. Indeed, the modelled results suggest that even a bend heattransfer multiplication factor of �50 has little impact upon overall slab performance.The results of the core utilisation study corroborate the work of Winwood et al. [6]. The study

found that the passage of air through 5 cores, compared with a three-core pass, ensured a greaterresidence time and promoted thermal interaction with a greater proportion of the slab mass. Theuse of five-cores appears to reduce the exiting air diurnal range by about 1.5 �C compared withthe three-core regime. This is consistent with heat exchanger theory; the use of five cores increasesthe residence time of the air within the TermoDeck slab and so aids progression towards thermalequilibrium between the core airflow and the core surface temperature. In other words, the coreair temperature further approaches that of the slab surface temperature the longer the residencetime.One other interesting observation, which consistently occurs across all the studies quoted

above, including those which are specifically the subject of this paper, is the time-lag or phase shiftwhich occurs between the peak outside air temperature, the peak core surface temperature, andthe peak temperature of the air leaving the TermoDeck slab (see Figs. 4, 8, 9 and 10). Thisphenomenon occurs because of the thermal storage effect of the concrete slab. Even though thetemperature of the air entering the slab may be falling the slab surface temperature will stillcontinue to rise, albeit at a reduced rate. Consequently, the temperature of the air leaving the slabwill continue to rise for some time after the outside air temperature has peaked. The phase shifteffect becomes more pronounced the longer the air pathway. From Fig. 10 it can be seen thatphase shift increases markedly if a five-core pass is used instead of a three-core pass.During the various studies described above it was observed that the inclusion of a periodic

room temperature heavily influenced the behaviour of the TermoDeck slab. This was because theroom air temperature influenced the surface temperature of the underside of the slab and thusaffected the rate of conduction through the concrete. In turn, the air supply temperature to thespace directly influenced the room air temperature. Consequently, there is a thermal time-lagbetween the supply and room temperatures. The thermal lag associated with the room air tem-perature is dependent upon the extent to which thermal mass is present. For example, a thermallylightweight room will respond rapidly to changes in the supply air temperature, with the resultthat the thermal time-lag is minimised. By comparison, thermally massive rooms, such as thoseincorporating a TermoDeck system, tend to attenuate and delay peak internal temperaturesbecause heat is absorbed by the building structure. In the transient studies it was decided to


incorporate a 2h room temperature phase shift (see Figs. 8 and 10) into the model. A 2h thermallag was used in the analysis because rooms incorporating TermoDeck slabs are thermally stableand have a large thermal capacity.One of the limitations of the two-dimensional model is that it ignores the thermal behaviour of

the room space, with the result that the room air temperature must be assumed. In reality, thesupply air temperature and the thermal capacity of the space would determine the room airtemperature. Because the room air temperature dominates the resultant core air temperatureprofile it is important to ensure that the room air temperatures used in the analysis are realistic.However, there is strong evidence [2,3,5] that buildings using the TermoDeck system have athermally stable environment and therefore it can be safely assumed that the room air temper-ature range will be relatively small.

6. Conclusions

The two-dimensional finite difference model presented here effectively simulated the thermalperformance of a TermoDeck system and produced results consistent with previous research, andtherefore proved to be a useful analysis tool. From the studies undertaken using this model it ispossible to draw following conclusions:

• The TermoDeck slab attenuates supply air temperatures, thereby ensuring a thermally stableinternal environment. The longer the air passage, the greater the dampening effect on the airdiurnal temperature range. Consequently a five-core pass system will achieve greater thermalattenuation than a three-core system, and thus should promote a more thermally stable internalenvironment.

• The hollow core bend sections have a minimal effect on overall heat transfer within the Termo-Deck slab. This finding contradicts past research which suggested that the bend sections dom-inated heat transfer in the TermoDeck system.

• Room air temperature has a strong influence on core air temperature. Variations in room airtemperature are however not likely to be great, since the use of TermoDeck tends to producea thermally stable environment.

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Documents

A Theoretical Study of the Thermal Performance of the TermoDeck Hollow Core Slab System