1 INTRODUCTION. Keywords: ground source heat pump; energy pile; geothermal; geophysics; ground temperature

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1 *Corresponding author: nottingham.ac.uk Comparison of a modelled and field tested piled ground heat exchanger system for a residential building and the simulated effect of assisted ground heat recharge... C.J. Wood *, H. Liu and S.B. Riffat Institute of Sustainable Energy Technology, Department of the Built Environment, Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, UK... Abstract The ground source heat pump can reduce electrical energy requirements for the heating of buildings by up to 70% compared with electrical resistive heating; however, the cost of ground loop installation can be a disincentive to their use. Foundation structures, such as concrete piles, are particularly applicable for loop incorporation and as such provide a cost-effective solution for the installation of ground loops for new buildings. It is now considered that residential energy piles can not only be economically installed but also provide the necessary heat requirement, which is sustainable over the life of the building. A test plot of 72 m 2 (ground floor area) was produced with 16 perimeter 10 m deep concrete piles, with a single U-tube in each. Over the heating season 2007/2008, a heat pump extracted heat from the pile loop circuit and the heat load was controlled as to simulate the heat demand of a modern detached fourbedroom low-energy residential dwelling. The ground physical and thermal properties were investigated by means of laboratory tests and an onsite thermal response test. These results were used within the borehole field software modelling program Earth Energy Designer (EED) as to computationally model the evolution of the mean circulating glycol temperature across the season. These results were compared against the measured results from the field test. EED was further utilized to investigate the effect of ground heat recharging, where an increase in circulating fluid temperature is observed. Keywords: ground source heat pump; energy pile; geothermal; geophysics; ground temperature Received 20 January 2010; revised 23 March 2010; accepted 27 March INTRODUCTION The energy required to provide hot water and space heating for the UK s residential housing stock amounts to 27% of the total UK primary energy consumption. Therefore, any decreases in building energy usage will have a significant effect on the overall UK CO 2 emission reduction target [1]. In an attempt to mitigate a rise in overall residential CO 2 emissions, it is required that new build homes utilize heating appliances, which are able to supply heat at a much greater efficiency than conventional gas or electrical resistive boilers. A ground source heat pump (GSHP) is an energy efficient method of supplying heat to a building. To extract heat from A draft of this paper was first presented at SET2009 8th International Conference on Sustainable Energy Technologies. Aachen, Germany, 31 August to 3 September the ground, the GSHP cools and circulates a glycol/water solution through closed-circuit ground loops. The heat extracted by the heat pump can then be transferred to a hydronic heating system such as under-floor heating. Electrical power input is required to pump the circulating ground loop fluid and to drive the heat pump compressor. The instantaneous measure of the efficiency of the heat pump is known as the coefficient of performance (COP): COP ¼ heat output from heat pump ðkwþ electrical power input for operation ðkwþ The COP depends on the temperature difference between the heat source and the heat sink; warmer ground (and thus warmer ground loop fluid temperature) and lower heating delivery temperatures result in a higher COP. ð1þ International Journal of Low-Carbon Technologies 2010, 5, # The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oxfordjournals.org doi: /ijlct/ctq015 Advance Access Publication 29 April

2 C.J. Wood et al. The aggregated COP over a full heating season is known as the seasonal performance factor (SPF) and in heat pump systems, which provide heating only, SPF of 400% can be achieved. Conventionally, ground loops are installed in either deep vertical boreholes ca. 100 m deep or horizontal trenches ca. 1.5 m deep. The installation cost and/or land required for such installations can be prohibitively expensive, and therefore novel methods of ground loop installation are required. One method is to utilize a building foundation by incorporating ground loops within the structure. The vertical concrete pile is one such structure, which is particularly suitable for loop inclusion, and indeed, many commercial buildings have used this technology with so-called energy piles. A highly informative review of heat pump-coupled foundation technology is provided by Brandl [2]. Residential buildings present a different situation in comparison to commercial buildings, due to the fact that the pile separation distance tends to be much smaller and the volume of ground contained around the piles is much less. Owing to the close spacing of residential piles, it is considered that the thermal interference between neighbouring piles will have an effect of significantly reducing the local ground temperature and thus a lowering of the circulating glycol mean temperature. The consequence of this will be a corresponding fall in the COP of the heat pump. However, the heat load of the new build residential dwelling is significantly lower than buildings of only a few years ago and its decreasing trend is expected to continue. It is therefore possible that the required ground heat can be satisfied by a typical pile length, which is required for structural purposes, i.e. no excess length is needed for heat extraction purposes. The purpose of the experimental study was to monitor the ground and pile temperatures, and the associated heat pump running parameters, while extracting heat to fulfil the heat demand of a residential dwelling, which would be categorized as a low-energy building of today. The pile system was specified for structural reasons only, with the only modification being the inclusion of a single U-tube within the pile centre for ground heat extraction. Preliminary results concerning ground temperature changes and heat pump performance have been given in previous papers [3,4]. Investigations were also conducted to determine the thermal and physical properties of the ground in the vicinity of the energy pile plot. Proprietary software, such as Earth Energy Designer (EED) [5], is available for modelling the effect of heat extraction on the heat storage of the ground for vertical borehole systems and subsequently determines the evolution of the mean circulating glycol temperature. The mathematical work of Eskilson [6] and Hellstrom [7] concerning borehole field heat movement and storage is the basis for the calculations performed by the software, EED. This paper reports the changes of the mean glycol temperature obtained with the experimental tests over the heating season and compared with those predicted by EED. 2 EXPERIMENTAL SYSTEM A72m 2 test plot house foundation was constructed, which had a layout of m deep continuous flight augered piles (300 mm diameter) as shown by the numbered locations in Figure 1. The spacing between piles was applicable to a loadbearing foundation as typically installed by foundation engineers, Roger Bullivant Ltd. Each pile had a 32 mm OD U-tube absorber pipe inserted to a depth of 10 m. The temperatures of each pile and the surrounding ground (at the lettered locations) were monitored at various depths as can be seen in Figure 1. The test foundation plot did not have a dwelling built upon it, so a heat rejection system was utilized to expel the heat delivered by the GSHP. In order to extract a ground heat load, which would be representative of that required by a modern low-energy residential dwelling, a seasonal heat loading profile had to be used to control the heat pump running times. The heat pump used had a heat output of 5.7 kw nominal [EN255 (358C flow temperature)] and therefore cycled at calculated intervals to provide a daily heat extraction load (kwh), which would be in line with the daily heat requirement for the simulated building as shown in Table 1. Owing to initial system tests combined with the changes which had to be made to the pile header circuit, the heat load did not follow a smooth profile across the entire season. The actual heat loading is seen in Table 1. Figure 1. Pile and thermocouple array layout. Table 1. Monthly heat loading. Month Ground heat extraction (MWh) (based upon SPF ¼ 3.62) October 0 0 November December January February March April May Total Heat output ( Building load ) per month (MWh) 138 International Journal of Low-Carbon Technologies 2010, 5,

3 Modelled and field tested piled ground heat exchanger system Figure 2. Schematic of a GSHP with energy piles. Figure 2 illustrates the energy pile heat pump system and the dry cooler which expels the generated heat from the heat pump so to represent a residential dwellings heat requirement. Heat pump efficiency and performance were observed by monitoring the usual heat pump parameters of electrical energy input, water and glycol flow rates and temperatures. The parameter known as the glycol equilibrium temperature (GET) was analysed as a key indicator. GET is a daily average of the flow and return glycol temperatures to the energy piles from the heat pump. This average is calculated from recorded temperatures while the heat pump is operational within a single day. The GET provides a long-term indication of the evolution of glycol temperature as opposed to instantaneous mean measurements, which are influenced by how long the heat pump has been running in single heat pump on/off cycle. The long-term analysis of GET is indicative of the changing thermal influence of the ground. Following preliminary testing, it was decided to only utilize the perimeter 16 piles and these were connected by means of four parallel circuits, with each circuit consisting of four piles in a series string. The four circuits can be seen in Figure 1. The specific heat extraction per linear metre of pile was 26 W/m. The geology of the site was typical of a brown field site. The first 3 m depth was characterized by an inhomogeneous over-burden. A homogenous layer of very soft clay with was seen between 3 and 10 m depth. The moisture content in the clay layer varied between 16% and 23%. EED requires the input of the following parameters of ground type, monthly heat loads and borehole field layout (or in this case, the energy pile layout). The software also requires knowledge of the borehole thermal resistance (R b ) (m K/W) which can be entered as a constant, if known from experimentation or can be calculated by defining the circulating fluid type and flow velocity, pipe type and sizing and also borehole details such as grout type and leg spacing (shank spacing) of the U-tube. A limitation of the software is that the borehole field geometries available for simulation are pre-defined and the borehole separation is constant in both directions. In the case of simulating the energy pile plot, a 55 borehole open rectangle was used, as in agreement with the real situation. However, as a constant pile separation had to be defined, an average value of 2.2 m was used, which resulted in an open rectangular layout of 77 m 2. 3 RESULTS AND DISCUSSION The U-tubes did not have a fixed shank spacing and calculations of R b performed by EED showed that varying the shank spacing altered the value of R b over a range of +15% from the mean value. Two extreme cases were therefore calculated and the average of these two values was used at 0.27 W/mK. The ground thermal conductivity testing had provided an experimentally determined value of R b (0.22 W/mK) which was shown to be slightly lower than that calculated by EED; therefore, a lower value of 0.22 W/mK was also used in EED for the simulation. Figure 3 shows that there is a general agreement in the trend of GET between the experimental results and the EED prediction. The line plot of GET in the period of December to mid-january does not agree with that of EED, although in this time period, initial header circuit problems were encountered, leading to a number of piles receiving no fluid flow. The header circuit was redesigned in mid-january 2008 and the subsequent increase in GET can be seen. It appears that EED predicts a slightly worse condition in this first year period, which may be an indicator that ground water in the area contributes heat to the system and therefore slightly elevates the mean glycol temperature. The actual load within the testing period was altered as and when required and not necessarily inline with the beginning or end of a month, whereas discrete monthly load data are inputted into EED and the program delivers an end of month mean glycol temperature. In this respect, it is therefore seen that the actual and simulated line plots are not consistently in phase with respect to time. A polynomial trend line of actual GET data (discounting the period International Journal of Low-Carbon Technologies 2010, 5,

4 C.J. Wood et al. Figure 3. Actual GET and EED predicted glycol temperatures across the heating season. 9th December to 15th January) is shown as a comparison. The mean seasonal GET value obtained by test in this same period was 2.28C, while EED calculates values of 2.1 and 2.88C for R b values of 0.27 and 0.22 mk/w, respectively. It must be remembered that EED neglects the thermal capacitance of the borehole; however, in the case of concrete energy piles, this stored heat is significant. Tests have shown that the temperature on the outside of the pile was significantly higher than the glycol temperature; hence, the concrete thermal mass would be providing significant heat which should not be neglected. This is a possible reason why EED delivers colder glycol temperatures than that which were shown by the experiments. The mathematical approach of EED assumes a linear profile of the radial heat transport to and from the borehole and neglects the end effects of the borehole, i.e. heat transfer from the ground surface and from beneath the end of the borehole. This simplifies the calculation and provides suitable results for long boreholes; however, the end effects proportionately increase with shorter depths. B. Sanner ( personal communication) suggests that beyond 10 m depth, the end effects can discounted, so it is understood that the simulation in the case of energy piles is on the boundaries of an acceptable depth. A limitation of EED is that an SPF figure is required to calculate the heat extraction load on the ground per month. In reality, the ground heat load is not directly proportional to the SPF at each point in time but is a function of the glycol temperature and heating side leaving water temperature (LWT), both of which have a thermodynamic effect on the COP. In the period of March to April, the COP of the actual system reduced to 3.2, for these same reasons. The resultant impact of this would be to reduce heat load on the ground and thus glycol temperature may not fall as far as EED would predict. An improvement of the software would be to provide the ability to input LWT values and associated COP levels for the heat pump being used. Following the above comparison, it is considered that EED can produce a result, which would be representative of a trend, over the following years albeit there could be disagreement in the absolute values delivered due to the reasons discussed previously. The monthly heat loads used in the following situation are those, which were calculated in relation to mean air temperature, i.e. uniform heat load profile year on year. Figure 4 shows how the glycol temperature falls, with each successive year and slowly begins to plateau at approximately year 5. Such a trend is in line with that as expressed by Eskilson [6], who showed that two-thirds of the drop to steady-state conditions occurs in the first 2 months and steady state is essentially achieved after 5 years. Nevertheless, it is seen that the glycol temperature is below 08C and prolonged time periods of this condition should be avoided when utilizing energy piles. It is seen that for each winter period, the GET falls to below zero and reaches approximately C at year 10, and as expected by year 25, the level has only reduced very slightly to C. The level below 08C typically lasts for 4 months within each winter period from December through to March. It may be considered that such a prolonged freezing temperature could be detrimental to the foundation. A system at the temperature levels predicted would be suitable for a working heat pump; however, a better system, which can attain a higher GET, would avoid possible pile freezing and provide a higher SPF. One of the fundamental limitations of the software is that the heat transfer within the ground is taken purely as a conductive mechanism. Water movement and convection are not taken into account and such mechanisms highly influence the heat being transferred to a site and the thermal transfer in the local vicinity. Hence, in such a situation of high mass water flow, the software simulation results could be somewhat inaccurate. EED assumes that the mean ground temperature is 140 International Journal of Low-Carbon Technologies 2010, 5,

5 Modelled and field tested piled ground heat exchanger system Figure 4. The evolution of GET over the running period of the initial 10 years. taken as the only input for ground temperature and this is a reasonable approach for deep boreholes. However, the first few metres depth of ground are significantly affected by the seasonal air temperature and solar irradiance, with seasonal cycles being detectable even at 10 m depth, and therefore should not be neglected. B. Sanner ( personal communication) suggests that the EED model reduces in its accuracy at depths shallower than 15 m due to the climatic influence upon ground temperature. EED is also limited to the input of one soil type; however, for shallow ground, the soil type within the first few meters is typically inhomogeneous. For deep boreholes, this layer is small compared with the overall depth and numerical analysis has shown that for a layer of,10 m, the top soil can be neglected and the thermal performance is affected by,2% [6]. Clearly with the case of short depths, inputs of different soil types are required for the great variability within this depth. As seen, the level of GET over successive years reduces until the quasi-equilibrium is reached at around year 5. As the SPF is directly dependent upon GET, which is subsequently dependent upon the local ground temperature, it can be considered that providing heat to the ground in the summer months would oppose the overall temperature reduction. Indeed, this energy pile testing has been performed in isolation, whereas real installations could be part of a high-density development, where the environmental heat recharging of the ground could be compromised due to the number of dwellings shading the ground. It is considered that assisted ground heat recharge could be performed in the summer months by absorbing solar heat from the roof by means of either a loop array behind the roofing material or alternatively by utilizing a solar thermal water heating system and then transferring this heat to the ground via a connection with the energy pile circuit. A simple preliminary assessment was performed to estimate the potential annual heat yield from absorbing heat from behind the roof fabric. A house with this footprint is calculated to have a south facing surface area of 44.3 m 2 (assuming the building is facing due south), and it is assumed that 70% of this area is available as a solar collector. Utilizing daily solar irradiance data for a location at Sutton Bonington (UK) and assuming a collector efficiency of 25%, the monthly yields were calculated. Research performed by Medved et al. [8] has shown that simple steel roof panel construction modified with fluid circulating pipes can attain efficiencies of 25% in the summer months. It has also been assumed that the heat generated from the array, primarily offsets the domestic hot water load and excess heat above this demand is then transferred to the energy pile circuit. Figure 5 illustrates the solar heat production and it can be seen that there is only an excess heat production in the months of June to August. The result of the effective reduction in required heat load in addition to the recharge of the ground heat within the summer months can be seen in Figure 6 as a comparison against the standard system of Figure 4. It is seen that the glycol does not fall to the same low level as the standard system and this would result in an increase in system efficiency. The software uses the input data for monthly loads as base load data, and as such, this models the long-term behaviour of the ground in terms of heat extraction/injection. In reality, the solar array will provide peak inputs, which would be significant and in particular across the summer months. It is realized that by adding heat to the ground, the mean temperature of the surrounding soil will be increased from where it would have been without the recharge. This increase in temperature has a direct effect on the seasonal GET value, International Journal of Low-Carbon Technologies 2010, 5,

6 C.J. Wood et al. Figure 5. The heat load of the notional 72 m 2 house and the effect of heat input by 19 m 2 of solar thermal array. Figure 6. The evolution of GET over the running period of the initial 10 years, with and without solar thermal assistance/ground recharge. and as such, the performance and efficiency of the heat pump would increase. Increasing the COP (or SPF over the season) actually increases the heat extraction load on the ground, and as such, this would have the effect of attempting to reduce the ground temperature. Obviously, there is a long-term dynamic effect, which would converge towards a thermal equilibrium. The above model does not take this dynamic nature into account as the SPF is taken as a constant value input. It is considered that the effect of ground recharge would be to improve the SPF of the system due to the higher quality heat availability. Physically, it would be seen that the ground temperature experiences a greater rate of change from the beginning of the season, but ultimately would not fall to a level as low as it would have done otherwise. 4 CONCLUSIONS The difficulty in accurately determining the borehole thermal resistance, R b, limits the precision of the modelling results; however, good agreement in trends has been seen between the experimental and simulated data. It has also been seen that EED tends to over predict the maximum reduction in glycol temperature against observed experimental data, although the 142 International Journal of Low-Carbon Technologies 2010, 5,

7 Modelled and field tested piled ground heat exchanger system mean level of GET throughout the season determined by experimental data lay in the range of temperature predicted by EED of C. In the example showing solar recharging, it has been seen that the level of GET was only slightly elevated by the use of solar recharge; however, it must be noted that the heating load of modern buildings continues to reduce, and as such, the required ground heat extraction would also reduce. Hence, the solar recharge would increasingly become more significant as a proportion of the required annual heat load. In this respect, a greater proportional effect of the solar recharging would be evident. It is believed that the use of solar recharging in combination with a GSHP would enhance the seasonal performance of the system, by increasing the local ground temperature. ACKNOWLEDGEMENTS We would like to acknowledge Roger Bullivant Ltd for their total support in this project, without whom this field testing would not have been possible. Financial support of the EPRSC is also acknowledged. REFERENCES [1] DTI [2] Brandl H. Energy foundations and other thermo-active ground structures. Geotechnique 2006;56: [3] EED 3. buildingphysics.com, [4] Wood CJ. Investigations into novel ground source heat pumps. PhD Thesis. Dept. for the Built Environment. University of Nottingham, [5] Wood CJ, Liu H, Riffat SB. Use of energy piles in a residential building, and the effects on ground temperature and heat pump efficiency. Geotechnique 2009;59: [6] Eskilson P. Thermal analysis of heat extraction boreholes. PhD Thesis. Dept. of Mathematical Physics. University of Lund, [7] Hellstrom G. Ground Heat Storage: Thermal Analyses of Duct Storage Systems. Dept. of Mathematical Physics. University of Lund, [8] Medved S, Arkar C, Cerne B. A large-panel unglazed roof-integrated liquid solar collector energy and economic evaluation. Solar Energy 2003;75: International Journal of Low-Carbon Technologies 2010, 5,