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Experimental study of a ground-coupled heat pump combined with thermal solar collectors
Energy and Buildings 38 (2006) 1477–1484 www.elsevier.com/locate/enbuild
Experimental study of a ground-coupled heat pump combined with thermal solar collectors V. Trillat-Berdal *, B. Souyri, G. Fraisse LOCIE, ESIGEC, Universite´ de Savoie, Campus Scientifique, Savoie Technolac, 73376 Le Bourget du Lac, France Received 28 February 2006; received in revised form 18 March 2006; accepted 3 April 2006
Abstract This paper presents the experimental study of a ground-coupled heat pump used in a 180 m2 private residence and combined with thermal solar collectors. This process, called GEOSOL, meets domestic hot water and heating–cooling building energy needs. Solar heat is used as a priority for domestic hot water heating and when the preset water temperature is reached, excess solar energy is injected into the ground via boreholes. This system has the advantage to contribute to the balance of the ground loads, increasing the operating time of the solar collectors and preventing overheating problems. After 11 months in operation, the power extracted and injected into the ground had average values of 40.3 and 39.5 W/m, respectively. Energy injected into the ground represents 34% of the heat extracted, and the heat pump’s coefficient of performance (COP) in heating mode had an average value of 3.75. In addition, the domestic hot water solar fraction had an average value higher than 60% for the first 11 months in operation. # 2006 Elsevier B.V. All rights reserved. Keywords: Ground-coupled heat pump; Thermal solar collectors; Experimental study
1. Introduction If we wish to prevent substantial changes in the climate, we must decrease our greenhouse gas emissions. In this context, the building sector produces one-third of the greenhouse gas effect. In addition, in 2000,the INSEE (Institut National de la Statistique et des Etudes Economiques) [1] reported that detached houses make up more than half of residential buildings (56%). Clearly, geothermal energy and solar energy systems are two means to produce energy in the private residence sector. Solar energy systems produce energy for heating domestic hot water (DHW). Geothermal heat pumps with vertical heat exchangers, also called ground-coupled heat pumps (GCHP), are good solutions for heating or cooling buildings thanks to their high-energy efficiency. Low-temperature geothermics are based on the use of the heat contained in the soil via embedded heat exchangers and heat pumps, which are usually of the water-to-water type [2]. In a zone ranging between 6 and 46 m in depth, the ground has a constant temperature throughout the year approximately equal
* Corresponding author. E-mail address: [email protected] (V. Trillat-Berdal). 0378-7788/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2006.04.005
to the annual air temperature [3]. This phenomenon results from complex interactions between the heat coming from the surface and that coming from the depths of the earth. The temperature of this zone corresponds to the average temperature of the site. Above this zone (at a depth below 6 m), the ground temperature directly relates to the climatic conditions; below that zone (at a depth above 46 m), the ground temperature starts to increase because of the geothermic gradient (2–3 8C/100 m). Heat pumps increase the outgoing fluid temperature of the embedded heat exchanger to values on the order of 35 8C, which can be used to heat buildings directly in a low-temperature heating system such as a heating floor. In these conditions, because the small difference in temperature between the heating floor and the ground, the average value of the heat pump coefficient of performance (COP) is expected to be above 3. Heat pumps can also be reversible (in heating mode or cooling mode) and thus be used for cooling in the summer. Most embedded heat exchangers for private residences are installed either horizontally or vertically in the ground. Interest in vertical heat exchangers, also called borehole heat exchangers (BHE), has increased in the housing sector over the last decade because they offer better performances. Indeed, horizontal heat exchangers are directly affected by local climatic conditions [4,5] as they are buried at depths between 0.80 and 1.50 m, while boreholes can exploit the ground
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Nomenclature a efficiency of the heat pump’s compressor BHE borehole heat exchanger COP coefficient of performance Cp solution specific heat (J/kg K) DHW domestic hot water GCHP ground coupled heat pump m solution flow rate (kg/s) ˙ Dm absolute error of m ˙ ˙ (kg/s) P calculated power of each circuit (W) DP absolute error of P (W) Pheat pump electrical power of the heat pump’s compressor (W) Pload power provided by the heat pump to the heating floor (W) Ploop power extracted from the ground (W) P1, P2, P3, P4, P5 circulation pumps DTin absolute error of Tin (8C) Tin, Tout inlet and outlet temperatures in each circuit for the calculation of P (8C) DTout absolute error of Tout (8C) TBHE1, TBHE2 outlet and inlet boreholes temperatures (8C) TGD5, TGD45, TGD85 5, 45 and 85 m depth temperature at the center of one of the two boreholes (8C) THW0 domestic hot water tank temperature (8C) TSC1 outlet solar collector temperature (8C) T1, T2 inlet and outlet evaporator temperatures (heat pump) (8C) T3, T4 inlet and outlet condenser temperatures (heat pump) (8C) temperature regularity below 6 m in depth, which ensures good performance throughout the year whatever the local climatic conditions. However, it should be pointed out that the high cost of boreholes is the major drawback of BHE systems, as installation requires drilling technologies. Nevertheless, the soil surface area occupied by a BHE is very small compared to that occupied by a horizontal ground heat exchanger, an advantage in areas of high land prices. In Europe, the utilization of GCHP is increasing, especially in Germany, Austria and Switzerland. For example, in Switzerland, more than 25,000 GCHPs have been installed [6] and an Ozgener estimate [7] indicates that there are over 140,000 GCHPs in the USA. Nevertheless, the use of a geothermal heat pump (GHP) with BHE to heat and/or to cool buildings can create annual imbalances in the ground loads [3,8]. In the case of heatingdominated buildings, a thermal heat depletion of the soil can occur, which progressively decreases the heat pump’s entering fluid temperature. On the contrary, cooling-dominated buildings heat the soil, which progressively increases the heat pump’s entering fluid temperature [9]. As a consequence, the heat pump’s performance coefficient decreases and the installation gradually becomes less efficient.
To remedy this problem, two main solutions exist. First, the total length of boreholes can be increased, but this is not the most economical solution. Second, hybrid systems can be used. These systems incorporate supplementary components that decrease the thermal load of the boreholes. In the case of cooling-dominated buildings, cooling towers are often used to evacuate a part of the heat. This type of installation has received attention in recent years. Yavuzturk and Spitler [9] presents a number of studies investigating these systems, which have already been used in buildings for several years [10], showing that the problem is on its way to being solved for coolingdominated buildings. However, for heating dominated-buildings, it is less obvious, as it is more difficult to create than evacuate heat without consuming a considerable amount of energy. An alternative solution could be combining solar collectors and the GCHP. This type of system has been increasingly recognized since the oil crisis in the 1970s [3], but the technology has not been widely adopted. In addition, experimental and theoretical results on the combination of thermal solar collectors with a GCHP used in heatingdominated detached houses are relatively scarce. Chiasson [11] presents a system simulation approach to assess the feasibility of ground heat pump coupled with solar thermal collectors in heating-dominated buildings. The loads for the exemple building were calculated for six U.S. cities using typical meteorological year weather data. He shows that combining solar collectors with a GCHP can help to reduce the borehole length at the design step, with a reduction per solar collector area ranging from 4.5 (weather data of Cheyenne and Wyoming) to 7.7 m/m2 (weather data of Omaha and Nebraska). A Swedish theoretical study [12] showed that recharging the ground with solar heat is particularly useful if the system is undersized and has active boreholes that are too shallow or if the boreholes are so close together that they influence each other. Moreover, according to a recent experimental study [13], injecting the excess solar heat into the ground is not worthwhile for a small number of boreholes; its main advantage is to prevent problems with solar collectors overheating. The project reported herein, known as GEOSOL, studied an experimental system based on the combination of a GCHP (with boreholes) and thermal solar collectors. We present the working system, its behaviour and its energy balances since the installation was started up. 2. Description of the GEOSOL process The schematic diagram of the GEOSOL process is presented on Fig. 1. Solar heat is used in priority to heat DHW and is injected into the ground via boreholes only when the DHW temperature setting is reached. The advantage of this operation is that it contributes to the balance of the ground loads, optimizes the use of solar heat provided by solar collectors and prevents overheating problems. The heat pump can be used in heating mode or in cooling mode. In cooling mode, heat is injected into the ground which also contributes to the balance of the ground loads. Nevertheless, in this paper, we only present the behavior in heating mode as the occupants have never
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Fig. 1. Schematic diagram of the GEOSOL process.
activated the heat pump in cooling mode. A water-antifreeze mixture – 35% propylene glycol solution (antifreeze to 18 8C) – circulates throughout the installation because the solar collectors are subject to the risk of freezing. A high concentration of propylene glycol increases the electricity consumption of circulation pumps but has very little impact on the heat pump performance [14]. As the propylene glycol solution circulates throughout the boreholes, there is a risk of leakage into the environment. Nevertheless, as opposed to ethylene glycol which is a toxic chemical, propylene glycol is generally considered as a safe chemical [15]. In addition, should leakage occur, propylene glycol is rapidly degraded, and it is not expected to persist or bioaccumulate in aquatic organisms. The half-life of propylene glycol in water is estimated to be between 1 and 4 days under aerobic conditions and between 3 and 5 days under anaerobic conditions. The half-life of propylene glycol in soil is expected to be equal to or slightly less than that in water. Vapors released into the atmosphere readily undergo rapid photochemical oxidation via reaction with hydroxyl radicals, with an estimated half-life of 0.8 days [15]. 3. Control system To optimize the use of electricity, circulation pumps operate sequentially. We do not use three-way valves, as they generate higher maintenance costs and make the control system more difficult to operate. Compared to conventional heating or cooling, system controlling this installation is relatively complex. The power provided by geothermal energy is nearly the same throughout the year, as opposed to solar energy for which the power provided depends on solar radiations. To ensure proper operation of the installation, it is simpler to use two existing control systems: one adapted to ground-coupled heat pump systems and another adapted for solar heating systems. However, these two control
systems must have good operational flexibility in order to ensure that the GCHP system combines well with the solar collector system and also to ensure that all circulation pumps turn off if all preset temperatures are reached. For the GEOSOL process, the operating mode of the solar collectors directly relates to the DHW tank temperature (THW0), the outlet solar collector temperature (TSC1) and the outlet borehole temperature (TBHE1). Circulation pump P1 is activated if (TSC1 THW0) > 6 8C and is turned off if (TSC1 THW0) < 2 8C or if THW0 > 72 8C. In this case, the DHW preset temperature is reached and the excess solar heat is injected into the ground by operating circulation pumps P2 and P3. These pumps are activated if THW0 > 72 8C and also if (TSC1-T5) > 18 8C and are turned off if (TSC1-T5) < 12 8C. These two temperatures (18 8C and 12 8C) have been determined experimentally by studying the behavior of the process and allow to maintain a high level of the solar collector’s temperature, otherwise TSC1 never exceed THWO which prevents the heating of the DHW with the solar energy by operating the circulation pump P1. The heat pump and P3, P4 and P5 circulation pumps are activated in accordance with the indoor temperature and the outlet heating floor temperature (T4). If the indoor temperature (19 8C) is not reached, P5 circulation pump is activated and, in heating mode, the heat pump and P3 and P4 circulation pumps are turned on if T4 < 26 8C. In heating mode, P3 and P4 circulation pumps and the heat pump are turned off if T4 > 30 8C, which ensures a low difference in temperature between the heating floor and the ground. Circulation pump P5 is turned off when the indoor temperature is reached. 4. Installation of the process The process was tested in a 180 m2 single-family house, constructed in 2004. The GEOSOL process has been operational since October 2004.
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Table 1 Characteristics of the installation Components
Characteristics
DHW tank
Volume: 500 l; insulation thickness: 4 cm polyurethane; heating power: 5000 W; surface area of internal heat exchanger: 1.06 m 2 Water-to-water heat pump; heating power: 15.8 kW in heating mode for an inlet temperature to the condenser of 40 8C and an inlet temperature to the evaporator of 5 8C Number 2; individual depth: 90 m; composed of two U-tubes; fill material: sand Limestone with marl veins; the rock is dry, there is no underground water flow Total surface: 154 m 2 Total surface: 180 m2; heat transfer coefficient (Ubat) equal to 0.628 W/(m2 K); the site is at 600 m altitude; Inside temperature set at 19 8C
Heat pump Boreholes Ground Heating/cooling floor Building
The solar collector area is oversized with respect to the domestic hot water requirements alone so that the excess solar energy is routed to the boreholes to favour thermal ground recovery. Rooftop thermal solar collectors covering 12 m2 were installed, although 6 m2 would have been sufficient in relation to the DHW needs. The boreholes length needs to be calculated for the building’s load profile. One common way of sizing ground heat exchangers is to use the methodology for borehole’s length calculation proposed by Bernier [16], which is summarized by equation (1), where L is the total borehole length required (m). L¼
qh Rb þ qy R10y þ qm R1m þ qh R6h ðTg þ Tp Þ
Tin;ground þTout;ground 2
(1)
In the numerator, the variables qh , qm and qy represent the hourly, monthly and yearly values of the ground loads and the variables R10y , R1m , R6h represent effective thermal resistances for ten-years, one-month and six-hours thermal pulse while Rb is the effective borehole thermal resistance. The denominator contains four temperatures: the undisturbed ground temperature (Tg ), a temperature penalty (Tp ) associated with thermal interference between adjacent bores, and the term (Tin;ground þ Tout;ground )/2 represents the average fluid temperature in boreholes. Table 1 presents the design values used for the calculation of the total borehole length. In heating mode, the ground loads are given by relation (2). 1 qground load ¼ qbuilding load 1 (2) COPheat pump It is assumed that the heating COP of the heat pump is constant at 3.44 which corresponds to its average value. To calculate qh and qm , building design loads are not sufficient and annual hourly energy calculations are required. For our study, the hourly building loads have been obtained in heating mode by simulating the single family house in the TRNSYS [17] modeling environment. The other process characteristics are presented in Table 1. 5. Measurement system The measurement system set-up provides precise tracking of the energy flows in each circuit using temperature PT1000 sensors and volumetric water meters. The power of each circuit
is given by relation (3): P ¼ mC ˙ p ðTin Tout Þ ½W
(3)
The variables m ˙ and Cp represent the solution flow rate (kg/s) and the solution-specific heat (3700 J/kg K at 10 8C for the 35% propylene glycol solution), while Tin and Tout are the inlet and outlet temperatures of each circuit. For each circuit, a volumetric water meter gives variable m, ˙ and two temperature sensors give Tin and Tout. For these data, the acquisition time step is 30 s. This time gives a good relation between the memory necessary for data storage and the precision needed for studying the behaviour of the system. While considering that the absolute specific heat error is negligible, the relative error of P is given by relation (4): DP Dm DTin DTout ˙ ¼ þ þ jPj jmj jTout Tin j jTout Tin j ˙
(4)
The term Dm=j ˙ mj ˙ is the relative error of the volumetric water meter. According to the manufacture’s data, this error does not exceed 2%. Variable DTin is calculated with relation (5), where the value 0.005 corresponds to the relative error of the temperature sensors used: DTin ¼ 0:005Tin
(5)
With these assumptions, the relative error of P for each circuit does not exceed 15%. By integrating the power (P) of each circuit, we obtain its energy balance (Q) in W h. Four electricity meters give the power consumption of the circulation pumps, the heat pump compressor and the DHW tank electric resistance heater. 6. Behaviour and energy balances of the process Fig. 2 shows possible operating modes available over a single day. The day begins with the heating of the building by the heat pump, that corresponds to the oscillation of the temperatures T3, T4, TBHE1 and TBHE2 and of the power of the condenser and of the evaporator. By mid-day, solar heat first enables DHW preset temperature to be reached and then recharges heat into the ground. The total DHW extraction was equal to 138 l for that particular day.
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Fig. 2. Behaviour of the GEOSOL process (Thursday, 14th April 2005).
On Fig. 2, the heat pump COP is calculated every 30 s with relation (6): COPheat pump ¼
Pload Pload ¼ Pheat pump ðPload Ploop Þ=a
(6)
where Pload is the power provided by the heat pump to the heating floor (condenser); Ploop the power extracted from the ground and used by the heat pump (evaporator); Pheat pump the electrical power of the heat pump’s compressor and a is the efficiency of the heat pump’s compressor. The value used for our calculations (a = 0.97) was obtained from the manufacturer’s catalogue data. On Fig. 2, the heat pump COP has an average value of 3.8. This value corresponds to the performance announced by the manufacturer for the same operation temperatures.
Fig. 3 shows the heat exchanges with the ground since the installation was started. During the first heating season (from November 2004 to May 2005), 5987 kW h were extracted from the ground, i.e. 33 kW h/m, for an operating time of 838 h in heating mode. These values are low and show that it would be possible to extract more heat from the ground [18]; the geothermic potential of the site is thus underexploited. At the end of September, once the summer was over and after restarting the heat pump in heating mode, 6253 kW h were extracted from the ground and 2121 kW h of solar heat was injected into the ground, i.e. 34% of the heat extracted. The operation lasted 298 h. The values of the heat pump COP in heating mode from the beginning of November to the end of September are presented on Fig. 4, with an average value of 3.75. The results show that this COP decreased as the heating season advanced. Indeed, at the start-up, the heat pump COP had a value of 4.05 compared to
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Fig. 3. Heat exchanges with the ground since the installation start-up. Fig. 5. Experimental DHW tank energy balance and solar fraction.
3.5 in April. As a consequence, the thermal heat depletion of the ground during the heating season decreased the heat pump COP by about 14%. On Fig. 4, we see that the average heat pump COP in May was better by approximately 10%, with a value of 3.87. This improvement is due to the fact that in May, 277 kW h of solar heat were injected into the ground, while 140 kW h were extracted. Consequently, the ground temperature near the boreholes was higher in May, which increased the heat pump COP. In June, the solar energy injected into the ground was very low due to the failure of an electrical component. The system’s COP in heating mode, including the electricity consumption of P3, P4 and P5, is not lower than 3.2 when these circulation pumps are working at the same time as the heat pump, and the electrical consumption of the circulation pumps make up 14% of the heat pump’s electrical consumption. However, it is important to note that if only one of these circulation pumps, e.g. P5, works continuously, the system’s COP decreases to an average value of 2.6. As a consequence, even if circulation pump power seems to be low in relation to heat pump compressor power, the choice of the control system is essential to optimize the system’s COP. Finally, Fig. 5 shows the energy balance and the solar fraction of the DHW tank. According to international standard ISO/FDIS 9488 [19], solar fraction is defined by the energy supplied by the solar part of a system divided by the total system load. From the beginning of November to the end of September, the average solar fraction value was 68%. For our system, this value should be improved by increasing the thermal insulation of the DHW tank.
Fig. 4. Heat pump and system performance.
Indeed, the DHW tank storage has an experimental average loss coefficient of about 10 W/K, whereas its manufacturer value is 2.89 W/K. Even after the application of a correction factor for imperfections [20], the DHW reaches a maximum theoretical value of only 4.2 W/K which is less than the experimental value. As a consequence, we could say that the thermal insulation manufacturer value is not always met, DHW tanks may have fabrication flaws. The low DHW tank insulation factor explains the high values of DHW tank electricity consumptions in relation to DHW energy needs, especially in November–January. 7. Behaviour of the boreholes Here we present the behaviour of the boreholes when the heat pump is working in heating mode or when solar heat is injected into the ground. Fig. 6 shows that during heating mode, the power extracted from the ground was not regular. Indeed, the longer the operating time, the less power was extracted from the ground, and as a consequence the lower the heat pump COP. This can be clearly seen on the last graph of Fig. 2 where the heat pump COP significantly decreases after working for several hours. When the heat pump is turned off during several minutes, the heat flux of the ground warms up the boreholes and its coolant, which explains why the heat pump performs better when the installation is started again. As a consequence, the continuous working of heat pumps during several hours is not optimal in terms of performance. It would be interesting to find a good compromise between the operating time and the shutdown phases to optimize performances while meeting building energy needs.
Fig. 6. Power extracted from the ground over time (Thursday, 14th April, 2005).
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Fig. 7. Power injected into the ground over time (Thursday, 14th April, 2005).
From the beginning of November 2004 to the end of September 2005, the power extracted from the ground had an average value of 40.3 W/m, which is low in relation to the geological composition of the ground, principally limestone. Indeed, in this type of soil, a power of at least 55 W/m can be expected [2]. Nevertheless, the theoretical values can vary significantly due to rock fabric such as crevices or foliation. The composition and the quality of the refill material of the boreholes is also a significant parameter [21]. As a result, it is difficult to predict experimental values of power heat extraction, unless the geological composition of the ground and the characteristics of the refill material are known precisely. Therefore in situ measurements of ground thermal properties [22,23] are advised when designing GCHP used elsewhere than in detached houses. The behaviour of boreholes during the injection of solar heat is presented on Fig. 7. After 10 min which are necessary to renew completely the borehole coolant and to exhaust the solar heat stored in solar collectors during their shut-down, the power injected into the ground fluctuated in relation to solar radiation; its average value was approximately 37 W/m, as shown on Fig. 7. From the beginning of November to the end of September, the power injected into the ground had an average value of 39.5 W/m, approximately the same as the average power of the heat extracted from the ground.
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one of the two boreholes. After the installation start-up, these temperatures did not directly reflect the thermal condition of the ground, but reflected the extraction and the injection of heat into the ground. Before the installation start-up, we can see on Fig. 8 that the ground temperature at a depth of 5 m fluctuated over the year: the maximum ground temperature occurred about 6 months later than the average maximum temperature of the surface in summer. At the 45 m depth, the TGD45 temperature was constant over the year, with a value of 11.6 8C, which corresponds approximately to the average annual air temperature of the borehole site. The heat extraction influence can clearly be seen between 3/11/2004 and 10/03/2005. After 10/03/2005, the injection of solar heat was substantial; its effect was also clearly significant on TGD45 (Fig. 8). In order to determine the average temperature of the ground near the ground heat exchangers, this borehole was closed down in June. Five days after heat-extraction shut-down (or heat injection), the average temperature of the ground near the borehole was about 11.3 8C compared to 11.6 8C, the initial ground temperature. At the end of August, the average temperature near the boreholes was about 12 8C, which is higher than the initial ground temperature. As a consequence, solar heat energy injected into the ground seemed to improve the natural ground heat recovery near the borehole. So, if the heat pump started in heating mode again, its COP would be approximately the same as at the initial start-up. This idea is validated by the comparison of the heat pump COP values from November 2004 and September 2005, which are equal to 4.05 and 4, respectively. Nevertheless, this level of performance would not be constant over a long period of time, because it can be assumed that the ground reached 12 8C only near the boreholes and not throughout the volume concerned by heat exchange with boreholes during the heating season, given that at the end of September, the solar heat injected represented only 34% of the heat extracted. 9. Conclusion
8. Evolution of the ground temperature near the boreholes TGD5 and TGD45 sensors measure temperatures at a depth of 5 and 45 m and at the centre (between the two U-tubes) of
After 11 months in operation, the experimental study has shown that the combination of renewable energies such as thermal solar energy and geothermal energy in a single system should make it possible to meet a residence’s heating and hot
Fig. 8. Ground temperature after and before the installation start-up.
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water requirements, while guaranteeing a satisfactory level of comfort. For this period, the heat pump COP had an average value of 3.75 in heating mode. In parallel, 6253 kW h were extracted from the ground by the GCHP in heating mode and 2121 kW h of solar heat were injected into the ground, i.e. 34% of the heat extracted. The power extracted and injected into the ground had average values of 40.3 W/m and 39.5 W/m, respectively. In addition, the system reached a high value of 68% for the domestic hot water solar fraction even though the hot water tank had an excessive loss coefficient. During the heating season, the ground temperature near the boreholes decreased, which penalized the heat pump COP with a difference of about 14% between the installation start-up (November) and April. From the beginning of March, the amount of solar heat injected into the ground increased but was not sufficient to improve the heat pump COP. Its effect was particularly visible in May, with an increase of about 10%. Near the boreholes, the natural thermal ground heat recovery was completed by the injection of solar heat, especially during the summer period, which made it possible to obtain the equivalent of the heat pump COP of the installation’s first start-up on the second start-up. Thus, in case the ground is without underground water flow, recharging the ground with solar heat is indeed a way to balance the ground loads of heating-dominated buildings. To optimize the system’s COP, choosing the best circulation pump control system is essential. In our system, if only one circulation pump worked continuously, the average system’s COP would have a value of 2.6 versus 3.35 if all circulation pumps worked only when the heat pump worked. Consequently, operating circulation pumps non-stop must be avoided to preserve the main advantage of GCHP: to low energy consumption. Our solution initially devised for private buildings may be extended to public buildings and the tertiary sector; the thermal solar collectors could help to reduce the number of boreholes and the investment cost of the installation. To determine whether the electricity consumption of the additional circulation pumps provides a lower total system seasonal performance as compared to the system without ground injection, an additional study must be conducted. Finally, theoretical results generated by simulations with TRNSYS [17] are awaited so that the system can be technically and economically optimized. Acknowledgements This study was financially supported by the ADEME (Agence Franc¸aise de l’Environnement et de la Maıˆtrise de l’Energie). The project partners are CIAT, CLIPSOL and ECO’ALTERNATIVE. References [1] Paul Champsaur, INSEE (Institut National de la Statistique et des Etudes Economiques), Paris, France, Recensement de la Population 1999-Des logements Plus Grands et Plus Confortables, 2000, p. 4 ISSN 0997-3192.
[2] D. Pahud, Geothermal Energy and Heat Storage, SUPSI-DCT-LEEE, Laboratorio di Energia, Ecologia ad Economia, 2002,, p. 133. [3] A.D. Chiasson, Advances in Modelling of Ground-source Heat Pump Systems, Oklahoma State University, 1999 168 pp. [4] M. Reuss, B. Sanner, Design of closed loop heat exchangers, in: International Summer School on Direct Application of Geothermal Energy, 2000, 147–156. [5] E. Bose, D. Smith, D. Spitler, Advances in ground source heat pump systems an international overview, in: Proceedings of the Seventh International Energy Agency Heat Pump Conference, vol. 1:313:314, Beijing, (2002), pp. 313–324. [6] Presentation of Low-temperature Geothermics. Available from http:// www.geothermal-energy.ch, 2005. [7] O. Ozgener, A. Hepbasli, A review on the energy and energy analysis of solar assisted heat pump systems, Renewable and Sustainable Energy Reviews (2004) 15. [8] W.J. Eugster, L. Rybach, Sustainable production from borehole heat exchanger systems, in: Proceedings of the World Geothermal Congress, 2000, pp. 825–830. [9] E. Yavuzturk, J.D. Spitler, Comparative study of operating and control strategies for hybrid ground-source heat pump systems using a short time step simulation model, ASHRAE Transactions 106 (2) (2000) 192– 209. [10] Federal Energy Management Program: Assessment of hybrid geothermal heat pump systems, DOE/EE-0258, 2001, 12 pp. in: http://www.eren. doe.gov. [11] A. Chiasson, C. Yavuzturk, Assessment of the viability of hybrid geothermal heat pump systems with solar thermal collectors, ASHRAE Transactions 109 (2) (2003) 487–500. [12] E. Kjellsson, Solar Heating in Dwellings With Analysis of Combined Solar Collectors and Ground Source Heat Pump, Report TVBH 3047, Dept. of Buildings Physics, Lund University, Sweden, 2004, 173 pp. [13] D. Pahud, B. Lachal, Mesure des Performances Thermiques d’une Pompe a` Chaleur Couple´e sur des Sondes Ge´othermiques a` Lugano (TI), SUPSIDCT-LEEE, Laboratorio di Energia, Ecologia ad Economia, Office Fe´de´ral de l’Energie Suisse (OFEN), 2004,, p. 51. [14] M. Bernier, O. Ladang, J. Hulot, Etude du choix du fluide caloporteur sur l0 e´nergie de pompage des syste`mes de pompes a` chaleurs ge´othermiques, 7e`me Colloque Inter-universitaire Franco-Que´be´cois sur la thermique des syste`mes, IUT de St Malo, 2005, pp. 103–108. [15] Agency for Toxics Substances and Disease Registry (ATSDR), Toxicological Profile for Ethylene Glycol and Propylene Glycol, US Department of Health and Human Services, Public Health Service, Atlanta, GA, 1997, 4 pp. [16] M. Bernier, Uncertainty in the design length calculation for vertical ground heat exchanger, ASHRAE Transactions (108) (2002) 939– 944. [17] S.A. Klein, et al., TRNSYS—A Transient Systems Simulation Program, Version 15.1, Solar Energy Laboratory, University of Wisconsin-Madison, 2000. [18] W.J. Eugster, L. Rybach, Sustainable production from borehole heat exchanger systems, in: Proceedings of the World Geothermal Congress, Japan, (2000), pp. 825–830. [19] Comite´ technique ISO/TC 180 Energie solaire, Norme ISO/FDIS 9488:199 (E/F), International Standard Solar Energy Vocabulary, 1999. [20] L.J. Shah, A Solar Combisystem-based on a Heat Storage with Three Internal Heat Exchanger, Sags Rapport BYG DTU Sr-02-19, 2002. [21] M.L. Allan, S.P. Kavanaugh, Thermal Conductivity of Cementitious Grouts and Impact On Heat Exchanger Length Design for Ground Source Heat Pumps, 1999, p. 11. [22] J.D. Spitler, C. Yavuzturk, S.J. Rees, In situ measurement of ground thermal properties, in: Proceedings of the Terrastock, vol. 1, Stuttgart, (2000), pp. 165–170. [23] D. Pahud, B. Matthey, Comparison of the thermal performance of double U-pipe borehole heat exchangers measured in situ, Energy and Buildings (2001) 5.
Experimental study of a ground-coupled heat pump combined with thermal solar collectors V. Trillat-Berdal *, B. Souyri, G. Fraisse LOCIE, ESIGEC, Universite´ de Savoie, Campus Scientifique, Savoie Technolac, 73376 Le Bourget du Lac, France Received 28 February 2006; received in revised form 18 March 2006; accepted 3 April 2006
Abstract This paper presents the experimental study of a ground-coupled heat pump used in a 180 m2 private residence and combined with thermal solar collectors. This process, called GEOSOL, meets domestic hot water and heating–cooling building energy needs. Solar heat is used as a priority for domestic hot water heating and when the preset water temperature is reached, excess solar energy is injected into the ground via boreholes. This system has the advantage to contribute to the balance of the ground loads, increasing the operating time of the solar collectors and preventing overheating problems. After 11 months in operation, the power extracted and injected into the ground had average values of 40.3 and 39.5 W/m, respectively. Energy injected into the ground represents 34% of the heat extracted, and the heat pump’s coefficient of performance (COP) in heating mode had an average value of 3.75. In addition, the domestic hot water solar fraction had an average value higher than 60% for the first 11 months in operation. # 2006 Elsevier B.V. All rights reserved. Keywords: Ground-coupled heat pump; Thermal solar collectors; Experimental study
1. Introduction If we wish to prevent substantial changes in the climate, we must decrease our greenhouse gas emissions. In this context, the building sector produces one-third of the greenhouse gas effect. In addition, in 2000,the INSEE (Institut National de la Statistique et des Etudes Economiques) [1] reported that detached houses make up more than half of residential buildings (56%). Clearly, geothermal energy and solar energy systems are two means to produce energy in the private residence sector. Solar energy systems produce energy for heating domestic hot water (DHW). Geothermal heat pumps with vertical heat exchangers, also called ground-coupled heat pumps (GCHP), are good solutions for heating or cooling buildings thanks to their high-energy efficiency. Low-temperature geothermics are based on the use of the heat contained in the soil via embedded heat exchangers and heat pumps, which are usually of the water-to-water type [2]. In a zone ranging between 6 and 46 m in depth, the ground has a constant temperature throughout the year approximately equal
* Corresponding author. E-mail address: [email protected] (V. Trillat-Berdal). 0378-7788/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2006.04.005
to the annual air temperature [3]. This phenomenon results from complex interactions between the heat coming from the surface and that coming from the depths of the earth. The temperature of this zone corresponds to the average temperature of the site. Above this zone (at a depth below 6 m), the ground temperature directly relates to the climatic conditions; below that zone (at a depth above 46 m), the ground temperature starts to increase because of the geothermic gradient (2–3 8C/100 m). Heat pumps increase the outgoing fluid temperature of the embedded heat exchanger to values on the order of 35 8C, which can be used to heat buildings directly in a low-temperature heating system such as a heating floor. In these conditions, because the small difference in temperature between the heating floor and the ground, the average value of the heat pump coefficient of performance (COP) is expected to be above 3. Heat pumps can also be reversible (in heating mode or cooling mode) and thus be used for cooling in the summer. Most embedded heat exchangers for private residences are installed either horizontally or vertically in the ground. Interest in vertical heat exchangers, also called borehole heat exchangers (BHE), has increased in the housing sector over the last decade because they offer better performances. Indeed, horizontal heat exchangers are directly affected by local climatic conditions [4,5] as they are buried at depths between 0.80 and 1.50 m, while boreholes can exploit the ground
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Nomenclature a efficiency of the heat pump’s compressor BHE borehole heat exchanger COP coefficient of performance Cp solution specific heat (J/kg K) DHW domestic hot water GCHP ground coupled heat pump m solution flow rate (kg/s) ˙ Dm absolute error of m ˙ ˙ (kg/s) P calculated power of each circuit (W) DP absolute error of P (W) Pheat pump electrical power of the heat pump’s compressor (W) Pload power provided by the heat pump to the heating floor (W) Ploop power extracted from the ground (W) P1, P2, P3, P4, P5 circulation pumps DTin absolute error of Tin (8C) Tin, Tout inlet and outlet temperatures in each circuit for the calculation of P (8C) DTout absolute error of Tout (8C) TBHE1, TBHE2 outlet and inlet boreholes temperatures (8C) TGD5, TGD45, TGD85 5, 45 and 85 m depth temperature at the center of one of the two boreholes (8C) THW0 domestic hot water tank temperature (8C) TSC1 outlet solar collector temperature (8C) T1, T2 inlet and outlet evaporator temperatures (heat pump) (8C) T3, T4 inlet and outlet condenser temperatures (heat pump) (8C) temperature regularity below 6 m in depth, which ensures good performance throughout the year whatever the local climatic conditions. However, it should be pointed out that the high cost of boreholes is the major drawback of BHE systems, as installation requires drilling technologies. Nevertheless, the soil surface area occupied by a BHE is very small compared to that occupied by a horizontal ground heat exchanger, an advantage in areas of high land prices. In Europe, the utilization of GCHP is increasing, especially in Germany, Austria and Switzerland. For example, in Switzerland, more than 25,000 GCHPs have been installed [6] and an Ozgener estimate [7] indicates that there are over 140,000 GCHPs in the USA. Nevertheless, the use of a geothermal heat pump (GHP) with BHE to heat and/or to cool buildings can create annual imbalances in the ground loads [3,8]. In the case of heatingdominated buildings, a thermal heat depletion of the soil can occur, which progressively decreases the heat pump’s entering fluid temperature. On the contrary, cooling-dominated buildings heat the soil, which progressively increases the heat pump’s entering fluid temperature [9]. As a consequence, the heat pump’s performance coefficient decreases and the installation gradually becomes less efficient.
To remedy this problem, two main solutions exist. First, the total length of boreholes can be increased, but this is not the most economical solution. Second, hybrid systems can be used. These systems incorporate supplementary components that decrease the thermal load of the boreholes. In the case of cooling-dominated buildings, cooling towers are often used to evacuate a part of the heat. This type of installation has received attention in recent years. Yavuzturk and Spitler [9] presents a number of studies investigating these systems, which have already been used in buildings for several years [10], showing that the problem is on its way to being solved for coolingdominated buildings. However, for heating dominated-buildings, it is less obvious, as it is more difficult to create than evacuate heat without consuming a considerable amount of energy. An alternative solution could be combining solar collectors and the GCHP. This type of system has been increasingly recognized since the oil crisis in the 1970s [3], but the technology has not been widely adopted. In addition, experimental and theoretical results on the combination of thermal solar collectors with a GCHP used in heatingdominated detached houses are relatively scarce. Chiasson [11] presents a system simulation approach to assess the feasibility of ground heat pump coupled with solar thermal collectors in heating-dominated buildings. The loads for the exemple building were calculated for six U.S. cities using typical meteorological year weather data. He shows that combining solar collectors with a GCHP can help to reduce the borehole length at the design step, with a reduction per solar collector area ranging from 4.5 (weather data of Cheyenne and Wyoming) to 7.7 m/m2 (weather data of Omaha and Nebraska). A Swedish theoretical study [12] showed that recharging the ground with solar heat is particularly useful if the system is undersized and has active boreholes that are too shallow or if the boreholes are so close together that they influence each other. Moreover, according to a recent experimental study [13], injecting the excess solar heat into the ground is not worthwhile for a small number of boreholes; its main advantage is to prevent problems with solar collectors overheating. The project reported herein, known as GEOSOL, studied an experimental system based on the combination of a GCHP (with boreholes) and thermal solar collectors. We present the working system, its behaviour and its energy balances since the installation was started up. 2. Description of the GEOSOL process The schematic diagram of the GEOSOL process is presented on Fig. 1. Solar heat is used in priority to heat DHW and is injected into the ground via boreholes only when the DHW temperature setting is reached. The advantage of this operation is that it contributes to the balance of the ground loads, optimizes the use of solar heat provided by solar collectors and prevents overheating problems. The heat pump can be used in heating mode or in cooling mode. In cooling mode, heat is injected into the ground which also contributes to the balance of the ground loads. Nevertheless, in this paper, we only present the behavior in heating mode as the occupants have never
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Fig. 1. Schematic diagram of the GEOSOL process.
activated the heat pump in cooling mode. A water-antifreeze mixture – 35% propylene glycol solution (antifreeze to 18 8C) – circulates throughout the installation because the solar collectors are subject to the risk of freezing. A high concentration of propylene glycol increases the electricity consumption of circulation pumps but has very little impact on the heat pump performance [14]. As the propylene glycol solution circulates throughout the boreholes, there is a risk of leakage into the environment. Nevertheless, as opposed to ethylene glycol which is a toxic chemical, propylene glycol is generally considered as a safe chemical [15]. In addition, should leakage occur, propylene glycol is rapidly degraded, and it is not expected to persist or bioaccumulate in aquatic organisms. The half-life of propylene glycol in water is estimated to be between 1 and 4 days under aerobic conditions and between 3 and 5 days under anaerobic conditions. The half-life of propylene glycol in soil is expected to be equal to or slightly less than that in water. Vapors released into the atmosphere readily undergo rapid photochemical oxidation via reaction with hydroxyl radicals, with an estimated half-life of 0.8 days [15]. 3. Control system To optimize the use of electricity, circulation pumps operate sequentially. We do not use three-way valves, as they generate higher maintenance costs and make the control system more difficult to operate. Compared to conventional heating or cooling, system controlling this installation is relatively complex. The power provided by geothermal energy is nearly the same throughout the year, as opposed to solar energy for which the power provided depends on solar radiations. To ensure proper operation of the installation, it is simpler to use two existing control systems: one adapted to ground-coupled heat pump systems and another adapted for solar heating systems. However, these two control
systems must have good operational flexibility in order to ensure that the GCHP system combines well with the solar collector system and also to ensure that all circulation pumps turn off if all preset temperatures are reached. For the GEOSOL process, the operating mode of the solar collectors directly relates to the DHW tank temperature (THW0), the outlet solar collector temperature (TSC1) and the outlet borehole temperature (TBHE1). Circulation pump P1 is activated if (TSC1 THW0) > 6 8C and is turned off if (TSC1 THW0) < 2 8C or if THW0 > 72 8C. In this case, the DHW preset temperature is reached and the excess solar heat is injected into the ground by operating circulation pumps P2 and P3. These pumps are activated if THW0 > 72 8C and also if (TSC1-T5) > 18 8C and are turned off if (TSC1-T5) < 12 8C. These two temperatures (18 8C and 12 8C) have been determined experimentally by studying the behavior of the process and allow to maintain a high level of the solar collector’s temperature, otherwise TSC1 never exceed THWO which prevents the heating of the DHW with the solar energy by operating the circulation pump P1. The heat pump and P3, P4 and P5 circulation pumps are activated in accordance with the indoor temperature and the outlet heating floor temperature (T4). If the indoor temperature (19 8C) is not reached, P5 circulation pump is activated and, in heating mode, the heat pump and P3 and P4 circulation pumps are turned on if T4 < 26 8C. In heating mode, P3 and P4 circulation pumps and the heat pump are turned off if T4 > 30 8C, which ensures a low difference in temperature between the heating floor and the ground. Circulation pump P5 is turned off when the indoor temperature is reached. 4. Installation of the process The process was tested in a 180 m2 single-family house, constructed in 2004. The GEOSOL process has been operational since October 2004.
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Table 1 Characteristics of the installation Components
Characteristics
DHW tank
Volume: 500 l; insulation thickness: 4 cm polyurethane; heating power: 5000 W; surface area of internal heat exchanger: 1.06 m 2 Water-to-water heat pump; heating power: 15.8 kW in heating mode for an inlet temperature to the condenser of 40 8C and an inlet temperature to the evaporator of 5 8C Number 2; individual depth: 90 m; composed of two U-tubes; fill material: sand Limestone with marl veins; the rock is dry, there is no underground water flow Total surface: 154 m 2 Total surface: 180 m2; heat transfer coefficient (Ubat) equal to 0.628 W/(m2 K); the site is at 600 m altitude; Inside temperature set at 19 8C
Heat pump Boreholes Ground Heating/cooling floor Building
The solar collector area is oversized with respect to the domestic hot water requirements alone so that the excess solar energy is routed to the boreholes to favour thermal ground recovery. Rooftop thermal solar collectors covering 12 m2 were installed, although 6 m2 would have been sufficient in relation to the DHW needs. The boreholes length needs to be calculated for the building’s load profile. One common way of sizing ground heat exchangers is to use the methodology for borehole’s length calculation proposed by Bernier [16], which is summarized by equation (1), where L is the total borehole length required (m). L¼
qh Rb þ qy R10y þ qm R1m þ qh R6h ðTg þ Tp Þ
Tin;ground þTout;ground 2
(1)
In the numerator, the variables qh , qm and qy represent the hourly, monthly and yearly values of the ground loads and the variables R10y , R1m , R6h represent effective thermal resistances for ten-years, one-month and six-hours thermal pulse while Rb is the effective borehole thermal resistance. The denominator contains four temperatures: the undisturbed ground temperature (Tg ), a temperature penalty (Tp ) associated with thermal interference between adjacent bores, and the term (Tin;ground þ Tout;ground )/2 represents the average fluid temperature in boreholes. Table 1 presents the design values used for the calculation of the total borehole length. In heating mode, the ground loads are given by relation (2). 1 qground load ¼ qbuilding load 1 (2) COPheat pump It is assumed that the heating COP of the heat pump is constant at 3.44 which corresponds to its average value. To calculate qh and qm , building design loads are not sufficient and annual hourly energy calculations are required. For our study, the hourly building loads have been obtained in heating mode by simulating the single family house in the TRNSYS [17] modeling environment. The other process characteristics are presented in Table 1. 5. Measurement system The measurement system set-up provides precise tracking of the energy flows in each circuit using temperature PT1000 sensors and volumetric water meters. The power of each circuit
is given by relation (3): P ¼ mC ˙ p ðTin Tout Þ ½W
(3)
The variables m ˙ and Cp represent the solution flow rate (kg/s) and the solution-specific heat (3700 J/kg K at 10 8C for the 35% propylene glycol solution), while Tin and Tout are the inlet and outlet temperatures of each circuit. For each circuit, a volumetric water meter gives variable m, ˙ and two temperature sensors give Tin and Tout. For these data, the acquisition time step is 30 s. This time gives a good relation between the memory necessary for data storage and the precision needed for studying the behaviour of the system. While considering that the absolute specific heat error is negligible, the relative error of P is given by relation (4): DP Dm DTin DTout ˙ ¼ þ þ jPj jmj jTout Tin j jTout Tin j ˙
(4)
The term Dm=j ˙ mj ˙ is the relative error of the volumetric water meter. According to the manufacture’s data, this error does not exceed 2%. Variable DTin is calculated with relation (5), where the value 0.005 corresponds to the relative error of the temperature sensors used: DTin ¼ 0:005Tin
(5)
With these assumptions, the relative error of P for each circuit does not exceed 15%. By integrating the power (P) of each circuit, we obtain its energy balance (Q) in W h. Four electricity meters give the power consumption of the circulation pumps, the heat pump compressor and the DHW tank electric resistance heater. 6. Behaviour and energy balances of the process Fig. 2 shows possible operating modes available over a single day. The day begins with the heating of the building by the heat pump, that corresponds to the oscillation of the temperatures T3, T4, TBHE1 and TBHE2 and of the power of the condenser and of the evaporator. By mid-day, solar heat first enables DHW preset temperature to be reached and then recharges heat into the ground. The total DHW extraction was equal to 138 l for that particular day.
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Fig. 2. Behaviour of the GEOSOL process (Thursday, 14th April 2005).
On Fig. 2, the heat pump COP is calculated every 30 s with relation (6): COPheat pump ¼
Pload Pload ¼ Pheat pump ðPload Ploop Þ=a
(6)
where Pload is the power provided by the heat pump to the heating floor (condenser); Ploop the power extracted from the ground and used by the heat pump (evaporator); Pheat pump the electrical power of the heat pump’s compressor and a is the efficiency of the heat pump’s compressor. The value used for our calculations (a = 0.97) was obtained from the manufacturer’s catalogue data. On Fig. 2, the heat pump COP has an average value of 3.8. This value corresponds to the performance announced by the manufacturer for the same operation temperatures.
Fig. 3 shows the heat exchanges with the ground since the installation was started. During the first heating season (from November 2004 to May 2005), 5987 kW h were extracted from the ground, i.e. 33 kW h/m, for an operating time of 838 h in heating mode. These values are low and show that it would be possible to extract more heat from the ground [18]; the geothermic potential of the site is thus underexploited. At the end of September, once the summer was over and after restarting the heat pump in heating mode, 6253 kW h were extracted from the ground and 2121 kW h of solar heat was injected into the ground, i.e. 34% of the heat extracted. The operation lasted 298 h. The values of the heat pump COP in heating mode from the beginning of November to the end of September are presented on Fig. 4, with an average value of 3.75. The results show that this COP decreased as the heating season advanced. Indeed, at the start-up, the heat pump COP had a value of 4.05 compared to
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Fig. 3. Heat exchanges with the ground since the installation start-up. Fig. 5. Experimental DHW tank energy balance and solar fraction.
3.5 in April. As a consequence, the thermal heat depletion of the ground during the heating season decreased the heat pump COP by about 14%. On Fig. 4, we see that the average heat pump COP in May was better by approximately 10%, with a value of 3.87. This improvement is due to the fact that in May, 277 kW h of solar heat were injected into the ground, while 140 kW h were extracted. Consequently, the ground temperature near the boreholes was higher in May, which increased the heat pump COP. In June, the solar energy injected into the ground was very low due to the failure of an electrical component. The system’s COP in heating mode, including the electricity consumption of P3, P4 and P5, is not lower than 3.2 when these circulation pumps are working at the same time as the heat pump, and the electrical consumption of the circulation pumps make up 14% of the heat pump’s electrical consumption. However, it is important to note that if only one of these circulation pumps, e.g. P5, works continuously, the system’s COP decreases to an average value of 2.6. As a consequence, even if circulation pump power seems to be low in relation to heat pump compressor power, the choice of the control system is essential to optimize the system’s COP. Finally, Fig. 5 shows the energy balance and the solar fraction of the DHW tank. According to international standard ISO/FDIS 9488 [19], solar fraction is defined by the energy supplied by the solar part of a system divided by the total system load. From the beginning of November to the end of September, the average solar fraction value was 68%. For our system, this value should be improved by increasing the thermal insulation of the DHW tank.
Fig. 4. Heat pump and system performance.
Indeed, the DHW tank storage has an experimental average loss coefficient of about 10 W/K, whereas its manufacturer value is 2.89 W/K. Even after the application of a correction factor for imperfections [20], the DHW reaches a maximum theoretical value of only 4.2 W/K which is less than the experimental value. As a consequence, we could say that the thermal insulation manufacturer value is not always met, DHW tanks may have fabrication flaws. The low DHW tank insulation factor explains the high values of DHW tank electricity consumptions in relation to DHW energy needs, especially in November–January. 7. Behaviour of the boreholes Here we present the behaviour of the boreholes when the heat pump is working in heating mode or when solar heat is injected into the ground. Fig. 6 shows that during heating mode, the power extracted from the ground was not regular. Indeed, the longer the operating time, the less power was extracted from the ground, and as a consequence the lower the heat pump COP. This can be clearly seen on the last graph of Fig. 2 where the heat pump COP significantly decreases after working for several hours. When the heat pump is turned off during several minutes, the heat flux of the ground warms up the boreholes and its coolant, which explains why the heat pump performs better when the installation is started again. As a consequence, the continuous working of heat pumps during several hours is not optimal in terms of performance. It would be interesting to find a good compromise between the operating time and the shutdown phases to optimize performances while meeting building energy needs.
Fig. 6. Power extracted from the ground over time (Thursday, 14th April, 2005).
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Fig. 7. Power injected into the ground over time (Thursday, 14th April, 2005).
From the beginning of November 2004 to the end of September 2005, the power extracted from the ground had an average value of 40.3 W/m, which is low in relation to the geological composition of the ground, principally limestone. Indeed, in this type of soil, a power of at least 55 W/m can be expected [2]. Nevertheless, the theoretical values can vary significantly due to rock fabric such as crevices or foliation. The composition and the quality of the refill material of the boreholes is also a significant parameter [21]. As a result, it is difficult to predict experimental values of power heat extraction, unless the geological composition of the ground and the characteristics of the refill material are known precisely. Therefore in situ measurements of ground thermal properties [22,23] are advised when designing GCHP used elsewhere than in detached houses. The behaviour of boreholes during the injection of solar heat is presented on Fig. 7. After 10 min which are necessary to renew completely the borehole coolant and to exhaust the solar heat stored in solar collectors during their shut-down, the power injected into the ground fluctuated in relation to solar radiation; its average value was approximately 37 W/m, as shown on Fig. 7. From the beginning of November to the end of September, the power injected into the ground had an average value of 39.5 W/m, approximately the same as the average power of the heat extracted from the ground.
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one of the two boreholes. After the installation start-up, these temperatures did not directly reflect the thermal condition of the ground, but reflected the extraction and the injection of heat into the ground. Before the installation start-up, we can see on Fig. 8 that the ground temperature at a depth of 5 m fluctuated over the year: the maximum ground temperature occurred about 6 months later than the average maximum temperature of the surface in summer. At the 45 m depth, the TGD45 temperature was constant over the year, with a value of 11.6 8C, which corresponds approximately to the average annual air temperature of the borehole site. The heat extraction influence can clearly be seen between 3/11/2004 and 10/03/2005. After 10/03/2005, the injection of solar heat was substantial; its effect was also clearly significant on TGD45 (Fig. 8). In order to determine the average temperature of the ground near the ground heat exchangers, this borehole was closed down in June. Five days after heat-extraction shut-down (or heat injection), the average temperature of the ground near the borehole was about 11.3 8C compared to 11.6 8C, the initial ground temperature. At the end of August, the average temperature near the boreholes was about 12 8C, which is higher than the initial ground temperature. As a consequence, solar heat energy injected into the ground seemed to improve the natural ground heat recovery near the borehole. So, if the heat pump started in heating mode again, its COP would be approximately the same as at the initial start-up. This idea is validated by the comparison of the heat pump COP values from November 2004 and September 2005, which are equal to 4.05 and 4, respectively. Nevertheless, this level of performance would not be constant over a long period of time, because it can be assumed that the ground reached 12 8C only near the boreholes and not throughout the volume concerned by heat exchange with boreholes during the heating season, given that at the end of September, the solar heat injected represented only 34% of the heat extracted. 9. Conclusion
8. Evolution of the ground temperature near the boreholes TGD5 and TGD45 sensors measure temperatures at a depth of 5 and 45 m and at the centre (between the two U-tubes) of
After 11 months in operation, the experimental study has shown that the combination of renewable energies such as thermal solar energy and geothermal energy in a single system should make it possible to meet a residence’s heating and hot
Fig. 8. Ground temperature after and before the installation start-up.
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water requirements, while guaranteeing a satisfactory level of comfort. For this period, the heat pump COP had an average value of 3.75 in heating mode. In parallel, 6253 kW h were extracted from the ground by the GCHP in heating mode and 2121 kW h of solar heat were injected into the ground, i.e. 34% of the heat extracted. The power extracted and injected into the ground had average values of 40.3 W/m and 39.5 W/m, respectively. In addition, the system reached a high value of 68% for the domestic hot water solar fraction even though the hot water tank had an excessive loss coefficient. During the heating season, the ground temperature near the boreholes decreased, which penalized the heat pump COP with a difference of about 14% between the installation start-up (November) and April. From the beginning of March, the amount of solar heat injected into the ground increased but was not sufficient to improve the heat pump COP. Its effect was particularly visible in May, with an increase of about 10%. Near the boreholes, the natural thermal ground heat recovery was completed by the injection of solar heat, especially during the summer period, which made it possible to obtain the equivalent of the heat pump COP of the installation’s first start-up on the second start-up. Thus, in case the ground is without underground water flow, recharging the ground with solar heat is indeed a way to balance the ground loads of heating-dominated buildings. To optimize the system’s COP, choosing the best circulation pump control system is essential. In our system, if only one circulation pump worked continuously, the average system’s COP would have a value of 2.6 versus 3.35 if all circulation pumps worked only when the heat pump worked. Consequently, operating circulation pumps non-stop must be avoided to preserve the main advantage of GCHP: to low energy consumption. Our solution initially devised for private buildings may be extended to public buildings and the tertiary sector; the thermal solar collectors could help to reduce the number of boreholes and the investment cost of the installation. To determine whether the electricity consumption of the additional circulation pumps provides a lower total system seasonal performance as compared to the system without ground injection, an additional study must be conducted. Finally, theoretical results generated by simulations with TRNSYS [17] are awaited so that the system can be technically and economically optimized. Acknowledgements This study was financially supported by the ADEME (Agence Franc¸aise de l’Environnement et de la Maıˆtrise de l’Energie). The project partners are CIAT, CLIPSOL and ECO’ALTERNATIVE. References [1] Paul Champsaur, INSEE (Institut National de la Statistique et des Etudes Economiques), Paris, France, Recensement de la Population 1999-Des logements Plus Grands et Plus Confortables, 2000, p. 4 ISSN 0997-3192.
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