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Performance study of underground thermal storage in a solar-ground coupled heat pump system for residential buildings
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Energy and Buildings 40 (2008) 1278–1286 www.elsevier.com/locate/enbuild
Performance study of underground thermal storage in a solar-ground coupled heat pump system for residential buildings Huajun Wang *, Chengying Qi School of Energy and Environment Engineering, Hebei University of Technology, Tianjin 300401, PR China Received 23 September 2007; accepted 30 November 2007
Abstract This paper is performed to analyze the performance of underground thermal storage in a solar-ground coupled heat pump system (SGCHPS) for residential building. Based on the experimental results, the system performance during a longer period is simulated by the unit modeling, and its parametric effects are discussed. The results show that the performance of underground thermal storage of SGCHPS depends strongly on the intensity of solar radiation and the matching between the water tank volume and the area of solar collectors. Compared with the solar radiation, the variations of the water tank temperature and the ground temperature rise lag behind and keep several peaks during the day time. For the case of Tianjin, the efficiency of underground thermal storage based on the total solar radiation and absorbed solar energy by the collectors can reach over 40% and 70%, respectively. It is suggested that the reasonable ratio between the tank volume and the area of solar collectors should be in the range of 20–40 L/m2. # 2007 Elsevier B.V. All rights reserved. Keywords: Geothermal heat pump; Solar energy; Underground thermal storage; Efficiency
1. Introduction The energy requirements needed for the space heating of buildings in winter can be supplied in part or in whole by solar radiation, using different patterns of seasonal thermal storage. Usually, either a water mass or a volume of ground is used as the storage medium. Bose [1] early studied the performance of a lowcost solar-assisted heat pump system (SAHPS) based on the geothermal energystorage.The resultsverified that such a coupled system could be active to improve the whole energy efficiency. Similar application examples were also achieved extensively in Sweden, Germany, Italy, the Netherlands and other countries [2–5]. These systems contribute significantly to improved energy efficiency. Thus the use of fossil fuels and CO2, SOx and NOx emissions to the atmosphere can be reduced considerably. For decades, the optimization and simulation on the solarground coupled heat pump systems (SGCHPS) have been paid much academic attention. Oliveti [6] proposed a calculation method of the accumulated probability curves from the solar fraction provided by plants with seasonal solar energy storage.
* Corresponding author. Tel.: +86 22 26564525; fax: +86 22 26564525. E-mail address: [email protected] (H. Wang). 0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2007.11.009
Based on Markov’s matrix approaches, the daily available energy was simulated and compared with the experimental results. Ozgener [7,8] conducted an experimental performance study of a SGCHPS for greenhouse heating with a 50-m vertical U-shaped geothermal heat exchanger. The COP values and energy efficiency for the whole system were compared in an attempt to assess individual performances and its potential for improvements. Yang [9] simulated the performance of a SGCHPS operated at alternate or combined mode. Based on the comparison of the energy-saving rates, some preferable operation modes were suggested. Recently, Trillat-Berdal [10] also described a GEOSOL project installed in an 180 m2 private residence. This system combined a reversible geothermal heat pump with thermal solar collectors for building heating and cooling and the production of domestic hot water. The energy-related behavior was analyzed using TRNSYS software. It can be seen that a general goal and major task of these efforts is to improve the energy efficiency and decrease the system cost as much as possible. In China, the idea of SGCHPS has been widely accepted under the driving force of the rapidly increasing applications of ground source heat pump (GSHP). It has been recognized as being among the cleanest, most energy efficient and cost effective systems for space heating and cooling [11]. Due to a
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Nomenclature a0, a1, a2 equation coefficients A area, m2 As surface area of the tank or amplitude, m2 b equation coefficient cp specific heat, J/(kg 8C) d bore diameter, m F0 Fourier number G G function H borehole depth, m I solar radiation, W/m2 Kv vegetation coefficient m mass, kg M water mass, kg p radius ratio Q heat energy, J Rcon heat-transfer resistance, (m 8C)/W Rfill resistance of the ground or refilled materials, (m 8C)/W Rpipe conduction resistance of pipe walls, (m 8C)/W Rtotal equivalent borehole resistance, (m 8C)/W t temperature, 8C Us heat-transfer coefficient, W/(m2 8C) x depth, m V volume, L Greek symbols a thermal diffusivity, m2/s h efficiency, % ls thermal conductivity, W/(m 8C) t time, s Subscripts a ambient or equation coefficient c solar collector g ground in inlet loss heat loss m average out outlet s water tank w pipe wall 1 far field
higher initial cost, however, SGCHPS is usually used for some public urban buildings, such as offices, hospitals, hotels and shopping centers. In these cases, due to the unbalance between heat injection and extraction from and to the ground, the ground temperature surrounding the geothermal heat exchangers may potentially rise over a number of years, resulting in a lowering of performance of the heat pump as the fluid temperature rises [12–15]. In the present work, an experimental SGCHPS is built in a residential building in the countryside of Tianjin, China. Different from the urban, the cooling load of residential buildings is much lower than the heating load, due to an
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especial climate feature with cool summer, cold winter and short transition seasons. Under this circumstance, a usual high initial ground temperature is favorable for the heat extraction by geothermal heat exchangers during the space heating. Without the extra heat injection into the ground in summer and transition seasons, the ground temperature would tend to be decreasing gradually. Thus a basic task of solar collectors in SGCHPS is to prevent such an undesired situation. The interest of this paper lies in the analysis of the preliminary experimental results. Further, the performance characteristics of underground thermal storage of SGCHPS is simulated based on the system modeling, and the effects of design parameters are discussed, which are expected to be helpful for the next improvement work. 2. Experimental investigations 2.1. Overview of experimental system A general view of the experimental SGCHPS is shown in Fig. 1. It is installed in a 120-m2 family house in Jinghai county, Tianjin (Fig. 2, latitude 39.138N, longitude 117.28E). It mainly consists of the solar collection system, the underground thermal storage system, the indoor air-conditioning system and a data acquisition system. The solar collection system mainly includes the rooftop solar thermal collectors, a circulating pump, an 800-L water storage tank, a 50-L expansion water tank and the connecting pipes. With a 25-m2 effective collection area, all the solar collectors are south-oriented and the tilted angle is 408. During the operation, an ON/OFF circulating controller is used, which means that the mass flow rate has two allowable values: maximum and zero. In the present work, the maximum mass flow rate is 800 kg/h. When the outlet fluid temperature of solar collectors exceeds a set value (e.g. 50 8C), the circulating pump starts, and the absorbed solar heat is transferred to the water storage tank through the heat exchanger with coil. Once the temperature difference between the outlet and inlet of solar collectors is lower than a set value (e.g. 3 8C), the circulating pump stops. For the cold conditions in winter, the water– antifreeze mixture (35% glycol solution with the freezing point of 21 8C) is considered for use. The underground thermal storage system mainly includes four geothermal heat exchangers, a DMR-020 type heat pump unit and two circulating pumps. The maximum mass flow rate of circulating pumps is 1800 kg/h. The rated cooling and heating capacity of heat pump unit are 11 kW and 10.45 kW, and the corresponding input powers are 2.2 kW and 2.7 kW, respectively. Geothermal heat exchangers are composed of double U-shaped high-density polyethylene pipes (HDPE) with the nominal outer/inner diameter of 32/25 mm. The diameter and depth of each borehole are 220 mm and 50 m. The configuration of four boreholes is square, with an equal spacing of 2 m. In order to measure the variation of the ground temperature distributions, six Pt1000-type temperature sensors with 0.1 8C accuracy are embedded at the depth of 3 m, 5 m, 10 m, 20 m, 35 m and 50 m of each borehole. Before the
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Fig. 1. Principle diagram of SGCHPS. 1: solar collectors; 2: geothermal heat exchangers; 3: water filter; 4: temperature sensor; 5: circulating pump (P1); 6: valve (V1); 7: heat pump unit; 8: circulating pump (P2); 9: valve (V2); 10: valve (V3); 11: auxiliary tank; 12: indoor coil pipes; 13: water storage tank; 14: circulating pump (P3); 15: expansion tank; 16: calorimeter; 17: valve (V4); 18: pressure meter.
underground installation, all sensors are calibrated by XLR-1 type constant-temperature bath with the range of 30 to 80 8C and 0.01 8C accuracy. The indoor air-conditioning system mainly includes four FP68LMY type fan-coil heat exchangers, which are mounted at a front room and three bedrooms, respectively. The rated air output, cooling and heating capacity of fan-coil units are 680 m3/h, 3.87 kW and 6.35 kW, respectively. Besides the ground temperature sensors, data acquisition system also includes several GRP-type calorimeters, which can record in a timely manner the accumulated heating or cooling energy. Each calorimeter consists of one flow meter with the minimum flow rate of 0.05 m3/h and two Pt1000 temperature sensors with 0.1 8C accuracy. All power consumptions are recorded by the wattmeter. 2.2. Operation modes Based on the requirements of residential owners, four operation modes are designed in the present SGCHP system.
Fig. 2. View of residential buildings for the experiments.
Operation modes are switched by the control on different valves. They can be summarized as follows: (1) Mode of solar underground thermal storage: In this mode, V1, V2 and V3 are closed, while V4 is open. Solar radiation absorbed by the solar collectors is transferred into the water storage tank, and then injected into the ground through the interaction between geothermal heat exchangers and the ground. The whole ground temperature keeps an increasing tendency. Like the circulating pump (P3) on the side of the solar collectors, the operation of geothermal heat exchangers also depends on an ON/OFF controller based on a set temperature value (e.g. 30 8C). In the present work, the mode of underground thermal storage is expected to take effect during the non-heating periods. (2) Cooling mode by heat pump: In this mode, V1 and V3 are open, while V2 and V4 are closed. Under the operation of heat pump unit, the cooling requirements inside the residential buildings are satisfied completely by the indoor fan-coil heat exchangers. At the same time, the heat is transferred and then injected into the ground by geothermal heat exchangers. Like the mode of underground thermal storage, the whole ground temperature at the cooling mode by heat pump also keeps an increasing tendency. Besides, in order to avoid the frequent action of heat pump unit, an auxiliary tank is designed to replace a part of cooling load. (3) Heating mode by heat pump: In this mode, V1 and V2 are open, while V3 and V4 are closed. When the inlet temperature of the heat pump unit is lower than a set value (e.g. 40 8C), the unit comes into action. At this time, the heat injected in summer is abstracted again from the ground by geothermal heat exchangers in order to satisfy the requirements for the space heating of residential buildings. The whole ground temperature at the heating mode by heat pump keeps a decreasing tendency.
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(4) Solar direct-heating mode: In this mode, V2 is open, while V1, V3 and V4 are closed. Compared with the heating mode of heat pump, when the inlet temperature of heat pump unit is higher than a set value (e.g. 45 8C), the unit stops its operation. Instead, the solar water storage tank is used directly for the fan-coil heat exchangers. In the present work, the mode of solar direct-heating is expected to reduce the operation time of the compressor and prolong its service lifetime. The whole ground temperature keeps a natural decreasing tendency. An important purpose of the SGCHP system is to obtain an improved integration between the renewable sources available and residential thermal requirements, while guaranteeing a satisfactory level of comfort and quality of use under all situations. In this paper, the performance analysis is mainly focused on the mode of underground thermal storage. 2.3. Experimental results and discussions 2.3.1. The natural ground temperature and its distributions For determination of the thermal interaction of geothermal heat exchangers with the ground, precise knowledge of the natural ground temperature is required. The ground temperature distributions are usually affected by many factors, including (1) the ground structure and physical properties, (2) ground surface cover (e.g. bare ground, lawn, etc.) and (3) weather conditions (e.g. ambient temperature, wind speed, solar radiation, relatively humidity, etc). Popiel [16] presented a theoretical model based on the solution for a transient heat conduction in a semi-infinite solid where the temperature of the ground surface (x = 0) is varying periodically with time (t = As cos[2p(t t0)/365]). The model can be expressed as tðx; tÞ ¼ðtm Dtm Þ 1:07K v As exp ð0:00031552xa0:5 Þcos 2p ðt t0 þ 0:018335xa0:5 Þ 365
(1)
where the vegetation coefficient Kv depends mainly on the proportion of vegetation projective shade cover. In this paper, Kv is equal to 1.0 for the bare ground in full sun. Based on Eq. (1), Fig. 3 shows the seasonal variations of the ground temperature distributions in Jinghai, Tianjin. Initially, the ground temperature close to the region of ground surface is higher than in the deeper regions. This means that the heat is transferred down from the surface. At the end of September the situation reverses and the heat starts to be transferred up to the ground surface. Therefore, deeper ground not only experiences less extreme seasonal variations in temperature, but the changes that occur lag farther behind those of the shallower ground. This shifts the ground temperature profile later in the year, such that it more closely matches the demand for heating and cooling. In the present work, besides the theoretical calculations, the ground temperatures under different depths are also measured by Pt1000 sensor with 0.1 8C accuracy. The test period ranges from 20 July to 22 July 2007. It can be seen from Fig. 4
Fig. 3. Seasonal variations of the ground temperature distributions.
that the results of measurements and prediction with Eq. (1) of the ground temperature distributions show a good agreement. From the point of view of the temperature distribution, we can distinguish the following ground zones: (1) surface zone reaching a depth of about 3–5 m, where the ground temperature is very sensitive to the short time changes of weather conditions; (2) shallow zone extending from the depth of about 5–20 m where the ground temperature is close to the average annual ambient temperature. In this zone, the ground temperature distributions depend mainly on the seasonal cycle weather conditions; (3) deep zone below 20 m. As shown in Fig. 5, the ground temperature at the depth below x = 20 m tends to be constant and equal to 14.5 8C. 2.3.2. Underground thermal storage during the operation Fig. 5 shows the experimental results of the ground temperature during 2–22 August 2007. It can be seen that, at the depth ranging from 10 m to 50 m, the variation of the ground temperature is almost the same, which is determined mainly by the difference of the initial temperatures. Different from the deeper ground, besides the heating flux of geothermal heat exchangers, the temperature variation at the surface zones (e.g. at 3 m depth) is also affected by its thermal interaction
Fig. 4. Comparison on the measurement and prediction results of the ground temperature.
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Fig. 5. Experimental results of the ground temperature during 7–22 August. (a) 7–22 August (b) 24 h on 15 August.
with the ambient. During the heat injection, the average temperature rise is 1.2 8C (the maximum is 2.4 8C). As shown in Fig. 5(b), the daily effective period of underground thermal storage keeps 5 h (10:00 a.m.–15:00 p.m.). Usually, this period depends strongly on the daily variation of solar radiation. As for the weather conditions in Tianjin, the above period can range from 5 h to 6 h. Besides, it can be seen that there exist multiple temperature peaks during the underground thermal storage. These peaks are mainly caused by the fact that the temperature rise of the water tank and the underground heat-transfer process always lag behind, compared with the variation of solar radiation. Naturally, the peak number is also associated with the intensity of solar radiation. In the case of 15 August, the average peak number is 4. Usually, the peak number tends to be increasing with the growth of solar radiation. When there is no solar radiation, the ground temperature keeps a natural decreasing tendency. Fig. 6 shows the comparison on the accumulated energy storage and consumption by the end of 30 August. It can be
seen that, if based on the absorbed solar energy (4033.55 MJ), the average efficiency of underground thermal storage is 76.55%. The proportion of energy consumption of the solar circulating pump and the ground circulating pump is 0.44% and 1.66%, respectively. Therefore, the SGCHPS in the present work can reach its objective of design efficiency (70%). In spite of this, we can see that the total energy loss reaches 19.45%, except about 4% energy storage in the water tank and the solar collectors. So, in the next work the whole system is expected to be optimized from the point of view of the improvement on the tank structure and control strategies. Besides the mode of solar underground thermal storage, the preliminary experiments of the cooling mode by heat pump are also conducted. Experimental results show that the average COP of the heat pump unit and the system is 5.27 and 3.75, respectively. Through the above experimental investigations, the preliminary performance of the SGCHPS is analyzed. In order to obtain the results with wider operation range, however, it is necessary to further build up a theoretical model and perform the simulations. Another purpose of theoretical approaches is to seek for the effects of some design parameters on the performance of solar-ground thermal storage. 3. Theoretical approaches 3.1. Unit modeling 3.1.1. Solar collectors Under a certain solar radiation, the thermal performance of solar collectors can be expressed as Qc ¼ hAIðtÞ
(2)
where h is the transient efficiency, which can be obtained according to the related national standards, such as Test Method for Thermal Performance of Flat-plate Solar Collectors (GB/ T4271-2000) [17]. Usually, the efficiency is regressed as h¼ab
tc;in ta I
(3)
Besides, the heat gain of solar collectors can be obtained directly through tests and further expressed as the function of the solar radiation and the temperature difference (tc,in ta) between the inlet fluid and the ambient. Qc ¼ a0 þ a1 I þ a2 ðtc;in ta Þ
(4)
According to the principle of energy conservation, the outlet fluid temperature of solar collectors can be expressed as follows: tc;out ¼ tc;in þ
Fig. 6. Comparison on energy storage and consumption during the operation.
Qc mc cp;c
(5)
If the heat losses on the pipes connecting the solar collectors and the storage tank are neglected, Qc can be treated as the net thermal energy transferred to the tank form the primary circuit.
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3.1.2. The water storage tank In the present work, an indirect heat exchange water storage tank with coils is employed. Based on the fist law of thermodynamics, the governing equation applied to the water in the storage tank is described as M s cp;s
dts ¼ Qin Qloss Qout dt
(6)
where Qin is the net thermal energy transferred to the tank form the primary circuit, Qout is the net heat transferred to the secondary circuit and Qloss is the heat loss through the walls of the water storage tank. Combined with the operation mode, the heat Qin transferred from the solar collectors to the water storage tank is given by Qin ¼
expressed as tw t1 ¼
Qg GðF 0 ; pÞ ls H
(10)
where p is the radius ratio, and F 0 is Fourier number, defined as F0 ¼
4at d2
(11)
The group functions of G(F 0, p) provide us with a relatively simplified analytical solution of cylindrical source problem. When p = 1, p = 2 and p = 5 G ¼ 10½0:89129þ0:36081lgF0 0:05508lg G ¼ 10½1:4541þ0:8933lgF 0 0:31193lg 2
0 mc cp;c ðtc;out tc;in Þ
Pump P3 stops Pump P3 operates
G ¼ 10½3:007þ2:25606lgF 0 0:7928lg (7)
In the secondary circuit, it is assumed that the water tank is fully mixed. This assumption is reasonable, taking into account the small mass of water and large mass flow rate used in the geothermal exchangers. Then, the heat Qout in the secondary circuit is given by Qout ¼
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0 Pump P1 stops mg cp;g ðtg;out tg;in Þ Pump P1 operates
(8)
The heat loss between the water in the storage tank and the ambient is given by Qloss ¼ U s As ðts ta Þ
(9)
where As and Us are the total surface area of the tank and the heat-transfer coefficient, respectively. In this paper, a value Us = 0.33 W/(m2 K) is used, which is in agreement with some experimental results [18,19]. 3.1.3. Geothermal heat exchangers In the present work, double vertical U-shaped HDPE pipes are used as geothermal heat exchangers, and the typical cylindrical source analytical model is adopted. For decades, modeling on the geothermal exchangers has been an important GSHP research area, which allows system simulations to be performed. The various approaches used for modeling the geothermal heat exchangers can be subdivided into two major categories. Some models mainly deal in short time-step (an hour or so) simulations [20–22]. Bernier [23] did a complete GSHP system simulation using the model and obtained a good agreement with that by DST model in TRNSYS. Other models are used for designing heat exchangers, and simplifications are carried out to make them computationally efficient for performing long-term performance (usually up to 20 years) evaluations of GSHP systems [24–26]. In spite of this, the cylindrical heat source theory, first presented by Ingersoll [27], and later refined by Kavanaugh [28], is relatively easy to comprehend and the resulting analytical solution can easily be programmed. Usually, the transient heat-transfer process can be
2
2
F 0 þ3:596103 lg3 F 0
F 0 þ0:06119103 lg3 F 0
F 0 þ0:134293103 lg3 F 0
(12) (13) (14)
The accumulation effect of continuous heat inject loads can be calculated through the dimensionless temperature response factors presented by Yavuzturk [22]. The govern equations of energy conservation inside the geothermal heat exchanges can be expressed as Qg ¼ mg cp;g ðtg;out tg;in Þ
(15)
The heat transfer between the fluid and the pipe wall can be express as Qg ¼
Hðtw tf Þ Rtotal
(16)
where Rtotal is the equivalent borehole resistance, which can be calculated as Rtotal ¼ Rcon þ Rpipe þ Rfill
(17)
where Rcon, Rpipe and Rfill are the heat-transfer resistance of the fluid inside the pipe, the conduction resistance of pipe walls, and the resistance of the ground or refilled materials, respectively. 3.2. Simulation results and discussions 3.2.1. Analysis of typical simulation results In the present work, a simulation program is developed based on the Visual Basic software. Main parameters for simulation calculations are listed in Table 1. The average CPU time for every calculation is about 6 min. The simulation period ranges from 1 May to 27 October (totally 180 days). As the input of ambient conditions, the hourly data from the typical meteorological year (TMY) in Tianjin is used [29]. The coefficients of the efficiency equation of solar collectors are taken as 0.75 and 4.5, respectively. The start fluid temperature of P1 and P3 is set as 30 8C (T1) and 50 8C (T3), respectively. According to the experimental results, the initial ground temperature starts from 14.5 8C. Fig. 7 gives the daily simulated results during the operation period. It can be seen that the curve characteristics of solar radiation, the water tank temperature and the ground temperature are similar to a great extent, which indicates that the ground temperature is determined strongly on the intensity of solar
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Table 1 List of main parameters for calculations Items
Unit
Value
Solar collectors Area Titled angle Flow rate
m2 8 m3/h
Water tank Volume Heat loss coefficient
L W/(m2 K)
Ground Initial temperature Density Specific heat Thermal conductivity
8C kg/m3 J/(kg K) W/(m K)
14.5 2200 1500 1.2
Geothermal heat exchangers Inner diameter Outer diameter Density Specific heat Thermal conductivity Flow rate
mm mm kg/m3 J/(kg K) W/(m K) m3/h
32 25 1100 1465 0.43 1.8
Boreholes Number Depth Diameter
– m mm
25 40 0.8
800 0.33
Fig. 8. Hourly simulated results in 3 and 4 May.
radiation. The water tank temperature ranges from 24.9 8C to 40.4 8C, and the average is 31.9 8C. This range is relatively close to the variation of the ambient temperature. Compared the latter, however, the ground temperature has an obviously increasing tendency. For this simulation case, the average temperature amplitude is 3.7 8C (from 14.5 8C to 18.2 8C). Taking 3 and 4 May as references, Fig. 8 shows the simulated results for 48 h. It can be seen that, during the first day, the solar radiation is relatively weak due to a cloudy weather condition. Thus, the solar collectors cannot reach its set operation temperature, and the temperatures of the water storage tank and the ground temperature keep a natural decreasing tendency. During the second day, the solar radiation and ambient temperature begin to rise again. The first peak of the ground
temperature appears at 9:30 a.m., which lags behind than the maximum peak time (8:30 a.m.) of solar radiation. Soon with the coming of the next peak of solar radiation, the second and third ground temperature peak reaches at 11:30 a.m. and at 14:00 p.m., respectively. If there is a sufficient solar radiation, such a process will continue repeatedly until the solar radiation is low enough so that the water storage tank cannot get the energy supplement from solar collectors. In this case, the underground thermal storage ceases after 16:00 p.m. The total effective period of underground thermal storage is 5 h or so, which is in agreement with the experimental results. It is also found that the peak number of the ground temperature and the temperature of the water storage tank is always lower than 4, depending strongly on the variation of solar radiation. Besides, the ratio (V/A) between the water tank volume and the area of solar collectors, as well as operation modes, are also influential factors. Fig. 9 shows the comparison on the accumulated energy storage and consumption during the whole operation. It can be seen that, if based on the total solar radiation (73.50 GJ) and the absorbed solar energy (41.94 GJ), the average efficiency of underground thermal storage is 40.64% and 71.24%, respectively. Compared with the experimental results shown in Fig. 6, the simulation efficiency is lower with 6.9%. The simulated total energy loss reaches 16.42%, lower than the experimental results. It is also found that the absorbed efficiency of solar collectors is 57.06%, which means that about 43.94% of solar radiation is unavailable by solar collectors during the whole operation. Therefore, for the design of SGCHPS, the improvement of solar collectors should be an important topic.
Fig. 7. Simulated results during the operation period of 180 days.
Fig. 9. Diagram of the distribution of energy flow.
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Fig. 10. The effect of the operation period on the performance of thermal storage.
3.2.2. Effects of design parameters Fig. 10 shows the effect of operation periods on the performance of underground thermal storage. It can be seen that with the accumulation of the thermal storage time, the absorbed energy and the ground temperature rise increase at first and then tend to be steady. The efficiency of underground thermal storage based on the total solar radiation varies weakly. It reaches the maximum of 41.77% at the end of June, and decreases to 42.27% and 40.64% at the end of September and October, respectively. This is in agreement with the seasonal variation of solar radiation in Tianjin. Therefore, for the operation of SGCHPS in Tianjin, the period of underground thermal storage is rather sufficient, ranging from May to October, which is helpful to improve the whole use performance of renewable energy utilization. Fig. 11 shows the performance of thermal storage under different water tank volumes. With the increase of water tank volumes, the absorbed efficiency of solar collectors, the efficiency of underground thermal storage and the ground temperature increase at first and then decay gradually. It is also found that within the water tank volume of 400–1000 L, the increase of the water tank volume do not affect the performance of thermal storage obviously. If the water tank volume continues to increase, it will not be favorable for the heat injection into the ground by the geothermal heat exchangers, due to a lower and
Fig. 11. The effect of the tank volume on the performance of thermal storage.
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lower fluid temperature. In fact, under this circumstance the thermal storage mode based on the water tank becomes dominant (this is another topic). At the same time, the system cost-up also has to be taken into full consideration. Badescu [30] suggested that the ratio V/A might be changed in the range 16.5–67 L/m2 with no significant influence on the system performance, and the optimum ratio is about 33 L/m2. Considering that the area of solar collector is 25 m2 in the present work, the corresponding water tank volume should be 412.5–1675 L, and the optimum volume should be 825 L. Obviously, the above simulation results are in agreement with Badescu’s suggestions. For the SGCHPS in the present work, it is suggested that the reasonable ratio V/A should range form 20 L/m2 to 40 L/m2. Fig. 12 shows the effect of the set ground inlet temperature T1 on the performance of thermal storage under different area of solar collectors. It is found that within a certain temperature range, the system performance has a tiny fluctuation. In this case, when the area of solar collectors is 25 m2 and 50 m2, the above temperature range is 30–40 8C and 20–30 8C, respectively. If exceeding such a range, the system performance (especially the ground temperature rise) decays rapidly. This means that, when the water tank volume is fixed, the temperature range should decrease with the increase of the area of solar collectors. Besides, the ground temperature rise tends to be increasing with the area of solar collectors, which indicates that the amount of underground
Fig. 12. The effect of the set ground inlet temperature on the performance of thermal storage. (a) 25 m2 (b) 50 m2.
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storage energy is increasing. Compared with the increase of the solar radiation, however, this amount is lower, which can further explain the reason why the efficiency of underground thermal storage experiences a decreasing tendency. According to the simulated results mentioned above, when the area of solar collectors increases to 50 m2, the optimum water tank volume should be 1650 L. Therefore, for a given climate condition, the thermal performance of SGCHPS depends mainly on the matching between the water tank volume and the area of solar collectors. 4. Conclusions In this present work, the performance of underground thermal storage in a SGCHPS for residential building is experimentally analyzed. Then the system performance during a longer period is simulated by the unit modeling, and its parametric effects are discussed. From above experimental and simulation results, the following conclusions can be obtained: (1) The performance of underground thermal storage of SGCHPS depends strongly on the intensity of solar radiation and the matching between the water tank volume and the area of solar collectors. Compared with the solar radiation, the variations of the water tank temperature and the ground temperature rise lag behind and keep several peaks during the day time. (2) For the case of Tianjin, the efficiency of underground thermal storage based on the total radiation at the surfaces of solar collectors and absorbed solar energy can reach over 40% and 70%, respectively. It is suggested that the reasonable ratio between the tank volume and the area of solar collectors should be in the range of 20–40 L/m2. Acknowledgement The authors are grateful for the support provided by Key Scientific Support Foundation of Tianjin, China (No. 07ZCKFSF00400), and National Scientific and Technical Support Foundation (No. 2006BAJ03A06). References [1] J.E. Bose, C.W. Ledbetter, J.R. Partin, Experimental results of a low cost solar-assisted heat pump system using earth coil and geo-thermal well storage, in: Proceedings of the Fourth Annual Heat Pump Technology Conference, Stillwater, Oklahoma State University, 1979. [2] E. Grabrielsson, Seasonal storage of thermal energy-Swedish experience, Journal of Solar Engineering-Transactions of the ASME 110 (3) (1988) 203–207. [3] T.P. Bokhoven, De Geus, Large Solar Heating System with Seasonal Storage for Bulb Drying in Lisse, vol. 1, Eurosun’96, The Netherlands, (1996), pp. 77–82. [4] G. Oliveti, N. Arcuri, Prototype experimental plan for the interseasonal storage of solar energy for the winter heating of buildings: description of plant and it function, Solar Energy 54 (8) (1995) 85–97. [5] R. Yumrutas, M. Unsal, Analysis of solar aided heat pump systems with seasonal thermal energy storage in surface tanks, Energy 25 (12) (2000) 1231–1243.
[6] G. Oliveti, N. Arcuri, S. Ruffolo, Effect of climatic variability on the performance of solar plants with inter-seasonal storage, Renewable Energy 19 (1) (2000) 235–241. [7] O. Ozgener, A. Hepbasli, Experimental performance analysis of a solar assisted ground-source heat pump greenhouse heating system, Energy and Buildings 37 (1) (2005) 101–110. [8] O. Ozgener, A. Hepbasli, Performance analysis of a solar assisted groundsource heat pump system for greenhouse heating: an experimental study, Building and Environment 40 (1) (2005) 1040–1050. [9] W. Yang, M. Shi, H. Dong, Numerical simulation of the performance a solar-earth source heat pump system, Applied Thermal Engineering 26 (17) (2006) 2367–2376. [10] V. Trillat-Berdal, B. Souyri, G. Achard, Coupling of geothermal heat pumps with thermal solar collector, Applied Thermal Engineering 27 (10) (2007) 1750–1755. [11] H. Wang, J. Zhao, Center solar heating technology with seasonal thermal storage, Solar Energy 108 (4) (2005) 27–31. [12] E. Hahne, M. Hornberger, Experience with a solar heating ATES system for a university building, ASME Journal of Solar Energy Engineering 116 (8) (1994) 88–93. [13] S.P. Kavanaugh, Impact of operating hours on long-term heat storage and the design of ground heat exchangers, ASHRAE Transactions 109 (1) (2003) 187–192. [14] D.S. Breger, Thermal energy storage in the ground: comparative analysis of heat transfer modeling using U-tubes and boreholes, Solar Energy 56 (6) (1996) 493–503. [15] H. Wang, J. Zhao, Numerical study on the ground temperature field around large-area compact U-shaped pile-buried heat exchangers, in: Proceedings of Third Asian Conference on Refrigeration and Air-conditioning, Gyeongju, Korea, (2006), pp. 109–112. [16] C.O. Popiel, J. Wojtkowiak, Measurements of temperature distribution in ground, Experimental Thermal and Fluid Science 25 (5) (2001) 301–309. [17] National Standard Bureau, Test Method for Thermal Performance of Flatplate Solar Collectors (GB/T4271-2000), China Building Industry Press, Beijing, 2000. [18] V. Badescu, N. Sicre, Renewable energy for passive house heating. II. Model, Energy and Buildings 35 (15) (2003) 1085–1096. [19] D.W. Mather, K.G.T. Holland, Single and multi-tank energy storage for solar heating systems: fundamentals, Solar Energy 73 (1) (2002) 3–13. [20] V.C. Mei, C.J. Emerson, New approach for analysis of ground-coil design for applied heat pump system, ASHRAE Transactions 91 (2) (1985) 1216–1224. [21] P. Eskilson, Thermal analysis of heat extraction boreholes, Doctoral Thesis, University of Lund, Department of Mathematical Physics, Lund, Sweden, 1987. [22] C. Yavuzturk, J.D. Spitler, S.J. Ree, A short time step response factor model for vertical ground loop heat exchangers, ASHRAE Transactions 105 (2) (1999) 475–485. [23] M. Bernier, Ground-coupled heat pump system simulation, ASHRAE Transactions 107 (1) (2001) 605–616. [24] S.P. Rottmayer, W.A. Beckman, J.W. Mitchell, Simulation of a single vertical U-tube ground heat exchanger in an infinite medium, ASHRAE Transactions 103 (2) (1997) 651–659. [25] C. Yavuzturk, J.D. Spitler, S.J. Ree, A transient two-dimensional finite volume model for the simulation of vertical U-tube ground heat exchangers, ASHRAE Transactions 105 (2) (1999) 465–474. [26] L. Lamarche, B. Beauchamp, A new contribution to the finite line-source model for geothermal boreholes, Energy and Buildings 39 (2) (2007) 188– 198. [27] L.D. Ingersoll, H.J. Plass, Theory of the ground pipe heat source for the heat pump, ASHVE Transactions 47 (7) (1948) 339–348. [28] S. Kavanaugh, A design method for commercial ground-coupled heat pumps, ASHRAE Transactions 101 (2) (1995) 1088–1094. [29] Tsinghua University, Chinese Typical Meteorological Data for Analysis of Thermal Environment of Buildings, China Construction Industry Publishing House, Beijing, 2005, pp. 20–40. [30] V. Badescu, Optimal control of flow in solar collector systems with fully mixed water storage tanks, Energy Conversion and Management 49 (2) (2008) 169–184.
Energy and Buildings 40 (2008) 1278–1286 www.elsevier.com/locate/enbuild
Performance study of underground thermal storage in a solar-ground coupled heat pump system for residential buildings Huajun Wang *, Chengying Qi School of Energy and Environment Engineering, Hebei University of Technology, Tianjin 300401, PR China Received 23 September 2007; accepted 30 November 2007
Abstract This paper is performed to analyze the performance of underground thermal storage in a solar-ground coupled heat pump system (SGCHPS) for residential building. Based on the experimental results, the system performance during a longer period is simulated by the unit modeling, and its parametric effects are discussed. The results show that the performance of underground thermal storage of SGCHPS depends strongly on the intensity of solar radiation and the matching between the water tank volume and the area of solar collectors. Compared with the solar radiation, the variations of the water tank temperature and the ground temperature rise lag behind and keep several peaks during the day time. For the case of Tianjin, the efficiency of underground thermal storage based on the total solar radiation and absorbed solar energy by the collectors can reach over 40% and 70%, respectively. It is suggested that the reasonable ratio between the tank volume and the area of solar collectors should be in the range of 20–40 L/m2. # 2007 Elsevier B.V. All rights reserved. Keywords: Geothermal heat pump; Solar energy; Underground thermal storage; Efficiency
1. Introduction The energy requirements needed for the space heating of buildings in winter can be supplied in part or in whole by solar radiation, using different patterns of seasonal thermal storage. Usually, either a water mass or a volume of ground is used as the storage medium. Bose [1] early studied the performance of a lowcost solar-assisted heat pump system (SAHPS) based on the geothermal energystorage.The resultsverified that such a coupled system could be active to improve the whole energy efficiency. Similar application examples were also achieved extensively in Sweden, Germany, Italy, the Netherlands and other countries [2–5]. These systems contribute significantly to improved energy efficiency. Thus the use of fossil fuels and CO2, SOx and NOx emissions to the atmosphere can be reduced considerably. For decades, the optimization and simulation on the solarground coupled heat pump systems (SGCHPS) have been paid much academic attention. Oliveti [6] proposed a calculation method of the accumulated probability curves from the solar fraction provided by plants with seasonal solar energy storage.
* Corresponding author. Tel.: +86 22 26564525; fax: +86 22 26564525. E-mail address: [email protected] (H. Wang). 0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2007.11.009
Based on Markov’s matrix approaches, the daily available energy was simulated and compared with the experimental results. Ozgener [7,8] conducted an experimental performance study of a SGCHPS for greenhouse heating with a 50-m vertical U-shaped geothermal heat exchanger. The COP values and energy efficiency for the whole system were compared in an attempt to assess individual performances and its potential for improvements. Yang [9] simulated the performance of a SGCHPS operated at alternate or combined mode. Based on the comparison of the energy-saving rates, some preferable operation modes were suggested. Recently, Trillat-Berdal [10] also described a GEOSOL project installed in an 180 m2 private residence. This system combined a reversible geothermal heat pump with thermal solar collectors for building heating and cooling and the production of domestic hot water. The energy-related behavior was analyzed using TRNSYS software. It can be seen that a general goal and major task of these efforts is to improve the energy efficiency and decrease the system cost as much as possible. In China, the idea of SGCHPS has been widely accepted under the driving force of the rapidly increasing applications of ground source heat pump (GSHP). It has been recognized as being among the cleanest, most energy efficient and cost effective systems for space heating and cooling [11]. Due to a
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Nomenclature a0, a1, a2 equation coefficients A area, m2 As surface area of the tank or amplitude, m2 b equation coefficient cp specific heat, J/(kg 8C) d bore diameter, m F0 Fourier number G G function H borehole depth, m I solar radiation, W/m2 Kv vegetation coefficient m mass, kg M water mass, kg p radius ratio Q heat energy, J Rcon heat-transfer resistance, (m 8C)/W Rfill resistance of the ground or refilled materials, (m 8C)/W Rpipe conduction resistance of pipe walls, (m 8C)/W Rtotal equivalent borehole resistance, (m 8C)/W t temperature, 8C Us heat-transfer coefficient, W/(m2 8C) x depth, m V volume, L Greek symbols a thermal diffusivity, m2/s h efficiency, % ls thermal conductivity, W/(m 8C) t time, s Subscripts a ambient or equation coefficient c solar collector g ground in inlet loss heat loss m average out outlet s water tank w pipe wall 1 far field
higher initial cost, however, SGCHPS is usually used for some public urban buildings, such as offices, hospitals, hotels and shopping centers. In these cases, due to the unbalance between heat injection and extraction from and to the ground, the ground temperature surrounding the geothermal heat exchangers may potentially rise over a number of years, resulting in a lowering of performance of the heat pump as the fluid temperature rises [12–15]. In the present work, an experimental SGCHPS is built in a residential building in the countryside of Tianjin, China. Different from the urban, the cooling load of residential buildings is much lower than the heating load, due to an
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especial climate feature with cool summer, cold winter and short transition seasons. Under this circumstance, a usual high initial ground temperature is favorable for the heat extraction by geothermal heat exchangers during the space heating. Without the extra heat injection into the ground in summer and transition seasons, the ground temperature would tend to be decreasing gradually. Thus a basic task of solar collectors in SGCHPS is to prevent such an undesired situation. The interest of this paper lies in the analysis of the preliminary experimental results. Further, the performance characteristics of underground thermal storage of SGCHPS is simulated based on the system modeling, and the effects of design parameters are discussed, which are expected to be helpful for the next improvement work. 2. Experimental investigations 2.1. Overview of experimental system A general view of the experimental SGCHPS is shown in Fig. 1. It is installed in a 120-m2 family house in Jinghai county, Tianjin (Fig. 2, latitude 39.138N, longitude 117.28E). It mainly consists of the solar collection system, the underground thermal storage system, the indoor air-conditioning system and a data acquisition system. The solar collection system mainly includes the rooftop solar thermal collectors, a circulating pump, an 800-L water storage tank, a 50-L expansion water tank and the connecting pipes. With a 25-m2 effective collection area, all the solar collectors are south-oriented and the tilted angle is 408. During the operation, an ON/OFF circulating controller is used, which means that the mass flow rate has two allowable values: maximum and zero. In the present work, the maximum mass flow rate is 800 kg/h. When the outlet fluid temperature of solar collectors exceeds a set value (e.g. 50 8C), the circulating pump starts, and the absorbed solar heat is transferred to the water storage tank through the heat exchanger with coil. Once the temperature difference between the outlet and inlet of solar collectors is lower than a set value (e.g. 3 8C), the circulating pump stops. For the cold conditions in winter, the water– antifreeze mixture (35% glycol solution with the freezing point of 21 8C) is considered for use. The underground thermal storage system mainly includes four geothermal heat exchangers, a DMR-020 type heat pump unit and two circulating pumps. The maximum mass flow rate of circulating pumps is 1800 kg/h. The rated cooling and heating capacity of heat pump unit are 11 kW and 10.45 kW, and the corresponding input powers are 2.2 kW and 2.7 kW, respectively. Geothermal heat exchangers are composed of double U-shaped high-density polyethylene pipes (HDPE) with the nominal outer/inner diameter of 32/25 mm. The diameter and depth of each borehole are 220 mm and 50 m. The configuration of four boreholes is square, with an equal spacing of 2 m. In order to measure the variation of the ground temperature distributions, six Pt1000-type temperature sensors with 0.1 8C accuracy are embedded at the depth of 3 m, 5 m, 10 m, 20 m, 35 m and 50 m of each borehole. Before the
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Fig. 1. Principle diagram of SGCHPS. 1: solar collectors; 2: geothermal heat exchangers; 3: water filter; 4: temperature sensor; 5: circulating pump (P1); 6: valve (V1); 7: heat pump unit; 8: circulating pump (P2); 9: valve (V2); 10: valve (V3); 11: auxiliary tank; 12: indoor coil pipes; 13: water storage tank; 14: circulating pump (P3); 15: expansion tank; 16: calorimeter; 17: valve (V4); 18: pressure meter.
underground installation, all sensors are calibrated by XLR-1 type constant-temperature bath with the range of 30 to 80 8C and 0.01 8C accuracy. The indoor air-conditioning system mainly includes four FP68LMY type fan-coil heat exchangers, which are mounted at a front room and three bedrooms, respectively. The rated air output, cooling and heating capacity of fan-coil units are 680 m3/h, 3.87 kW and 6.35 kW, respectively. Besides the ground temperature sensors, data acquisition system also includes several GRP-type calorimeters, which can record in a timely manner the accumulated heating or cooling energy. Each calorimeter consists of one flow meter with the minimum flow rate of 0.05 m3/h and two Pt1000 temperature sensors with 0.1 8C accuracy. All power consumptions are recorded by the wattmeter. 2.2. Operation modes Based on the requirements of residential owners, four operation modes are designed in the present SGCHP system.
Fig. 2. View of residential buildings for the experiments.
Operation modes are switched by the control on different valves. They can be summarized as follows: (1) Mode of solar underground thermal storage: In this mode, V1, V2 and V3 are closed, while V4 is open. Solar radiation absorbed by the solar collectors is transferred into the water storage tank, and then injected into the ground through the interaction between geothermal heat exchangers and the ground. The whole ground temperature keeps an increasing tendency. Like the circulating pump (P3) on the side of the solar collectors, the operation of geothermal heat exchangers also depends on an ON/OFF controller based on a set temperature value (e.g. 30 8C). In the present work, the mode of underground thermal storage is expected to take effect during the non-heating periods. (2) Cooling mode by heat pump: In this mode, V1 and V3 are open, while V2 and V4 are closed. Under the operation of heat pump unit, the cooling requirements inside the residential buildings are satisfied completely by the indoor fan-coil heat exchangers. At the same time, the heat is transferred and then injected into the ground by geothermal heat exchangers. Like the mode of underground thermal storage, the whole ground temperature at the cooling mode by heat pump also keeps an increasing tendency. Besides, in order to avoid the frequent action of heat pump unit, an auxiliary tank is designed to replace a part of cooling load. (3) Heating mode by heat pump: In this mode, V1 and V2 are open, while V3 and V4 are closed. When the inlet temperature of the heat pump unit is lower than a set value (e.g. 40 8C), the unit comes into action. At this time, the heat injected in summer is abstracted again from the ground by geothermal heat exchangers in order to satisfy the requirements for the space heating of residential buildings. The whole ground temperature at the heating mode by heat pump keeps a decreasing tendency.
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(4) Solar direct-heating mode: In this mode, V2 is open, while V1, V3 and V4 are closed. Compared with the heating mode of heat pump, when the inlet temperature of heat pump unit is higher than a set value (e.g. 45 8C), the unit stops its operation. Instead, the solar water storage tank is used directly for the fan-coil heat exchangers. In the present work, the mode of solar direct-heating is expected to reduce the operation time of the compressor and prolong its service lifetime. The whole ground temperature keeps a natural decreasing tendency. An important purpose of the SGCHP system is to obtain an improved integration between the renewable sources available and residential thermal requirements, while guaranteeing a satisfactory level of comfort and quality of use under all situations. In this paper, the performance analysis is mainly focused on the mode of underground thermal storage. 2.3. Experimental results and discussions 2.3.1. The natural ground temperature and its distributions For determination of the thermal interaction of geothermal heat exchangers with the ground, precise knowledge of the natural ground temperature is required. The ground temperature distributions are usually affected by many factors, including (1) the ground structure and physical properties, (2) ground surface cover (e.g. bare ground, lawn, etc.) and (3) weather conditions (e.g. ambient temperature, wind speed, solar radiation, relatively humidity, etc). Popiel [16] presented a theoretical model based on the solution for a transient heat conduction in a semi-infinite solid where the temperature of the ground surface (x = 0) is varying periodically with time (t = As cos[2p(t t0)/365]). The model can be expressed as tðx; tÞ ¼ðtm Dtm Þ 1:07K v As exp ð0:00031552xa0:5 Þcos 2p ðt t0 þ 0:018335xa0:5 Þ 365
(1)
where the vegetation coefficient Kv depends mainly on the proportion of vegetation projective shade cover. In this paper, Kv is equal to 1.0 for the bare ground in full sun. Based on Eq. (1), Fig. 3 shows the seasonal variations of the ground temperature distributions in Jinghai, Tianjin. Initially, the ground temperature close to the region of ground surface is higher than in the deeper regions. This means that the heat is transferred down from the surface. At the end of September the situation reverses and the heat starts to be transferred up to the ground surface. Therefore, deeper ground not only experiences less extreme seasonal variations in temperature, but the changes that occur lag farther behind those of the shallower ground. This shifts the ground temperature profile later in the year, such that it more closely matches the demand for heating and cooling. In the present work, besides the theoretical calculations, the ground temperatures under different depths are also measured by Pt1000 sensor with 0.1 8C accuracy. The test period ranges from 20 July to 22 July 2007. It can be seen from Fig. 4
Fig. 3. Seasonal variations of the ground temperature distributions.
that the results of measurements and prediction with Eq. (1) of the ground temperature distributions show a good agreement. From the point of view of the temperature distribution, we can distinguish the following ground zones: (1) surface zone reaching a depth of about 3–5 m, where the ground temperature is very sensitive to the short time changes of weather conditions; (2) shallow zone extending from the depth of about 5–20 m where the ground temperature is close to the average annual ambient temperature. In this zone, the ground temperature distributions depend mainly on the seasonal cycle weather conditions; (3) deep zone below 20 m. As shown in Fig. 5, the ground temperature at the depth below x = 20 m tends to be constant and equal to 14.5 8C. 2.3.2. Underground thermal storage during the operation Fig. 5 shows the experimental results of the ground temperature during 2–22 August 2007. It can be seen that, at the depth ranging from 10 m to 50 m, the variation of the ground temperature is almost the same, which is determined mainly by the difference of the initial temperatures. Different from the deeper ground, besides the heating flux of geothermal heat exchangers, the temperature variation at the surface zones (e.g. at 3 m depth) is also affected by its thermal interaction
Fig. 4. Comparison on the measurement and prediction results of the ground temperature.
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Fig. 5. Experimental results of the ground temperature during 7–22 August. (a) 7–22 August (b) 24 h on 15 August.
with the ambient. During the heat injection, the average temperature rise is 1.2 8C (the maximum is 2.4 8C). As shown in Fig. 5(b), the daily effective period of underground thermal storage keeps 5 h (10:00 a.m.–15:00 p.m.). Usually, this period depends strongly on the daily variation of solar radiation. As for the weather conditions in Tianjin, the above period can range from 5 h to 6 h. Besides, it can be seen that there exist multiple temperature peaks during the underground thermal storage. These peaks are mainly caused by the fact that the temperature rise of the water tank and the underground heat-transfer process always lag behind, compared with the variation of solar radiation. Naturally, the peak number is also associated with the intensity of solar radiation. In the case of 15 August, the average peak number is 4. Usually, the peak number tends to be increasing with the growth of solar radiation. When there is no solar radiation, the ground temperature keeps a natural decreasing tendency. Fig. 6 shows the comparison on the accumulated energy storage and consumption by the end of 30 August. It can be
seen that, if based on the absorbed solar energy (4033.55 MJ), the average efficiency of underground thermal storage is 76.55%. The proportion of energy consumption of the solar circulating pump and the ground circulating pump is 0.44% and 1.66%, respectively. Therefore, the SGCHPS in the present work can reach its objective of design efficiency (70%). In spite of this, we can see that the total energy loss reaches 19.45%, except about 4% energy storage in the water tank and the solar collectors. So, in the next work the whole system is expected to be optimized from the point of view of the improvement on the tank structure and control strategies. Besides the mode of solar underground thermal storage, the preliminary experiments of the cooling mode by heat pump are also conducted. Experimental results show that the average COP of the heat pump unit and the system is 5.27 and 3.75, respectively. Through the above experimental investigations, the preliminary performance of the SGCHPS is analyzed. In order to obtain the results with wider operation range, however, it is necessary to further build up a theoretical model and perform the simulations. Another purpose of theoretical approaches is to seek for the effects of some design parameters on the performance of solar-ground thermal storage. 3. Theoretical approaches 3.1. Unit modeling 3.1.1. Solar collectors Under a certain solar radiation, the thermal performance of solar collectors can be expressed as Qc ¼ hAIðtÞ
(2)
where h is the transient efficiency, which can be obtained according to the related national standards, such as Test Method for Thermal Performance of Flat-plate Solar Collectors (GB/ T4271-2000) [17]. Usually, the efficiency is regressed as h¼ab
tc;in ta I
(3)
Besides, the heat gain of solar collectors can be obtained directly through tests and further expressed as the function of the solar radiation and the temperature difference (tc,in ta) between the inlet fluid and the ambient. Qc ¼ a0 þ a1 I þ a2 ðtc;in ta Þ
(4)
According to the principle of energy conservation, the outlet fluid temperature of solar collectors can be expressed as follows: tc;out ¼ tc;in þ
Fig. 6. Comparison on energy storage and consumption during the operation.
Qc mc cp;c
(5)
If the heat losses on the pipes connecting the solar collectors and the storage tank are neglected, Qc can be treated as the net thermal energy transferred to the tank form the primary circuit.
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3.1.2. The water storage tank In the present work, an indirect heat exchange water storage tank with coils is employed. Based on the fist law of thermodynamics, the governing equation applied to the water in the storage tank is described as M s cp;s
dts ¼ Qin Qloss Qout dt
(6)
where Qin is the net thermal energy transferred to the tank form the primary circuit, Qout is the net heat transferred to the secondary circuit and Qloss is the heat loss through the walls of the water storage tank. Combined with the operation mode, the heat Qin transferred from the solar collectors to the water storage tank is given by Qin ¼
expressed as tw t1 ¼
Qg GðF 0 ; pÞ ls H
(10)
where p is the radius ratio, and F 0 is Fourier number, defined as F0 ¼
4at d2
(11)
The group functions of G(F 0, p) provide us with a relatively simplified analytical solution of cylindrical source problem. When p = 1, p = 2 and p = 5 G ¼ 10½0:89129þ0:36081lgF0 0:05508lg G ¼ 10½1:4541þ0:8933lgF 0 0:31193lg 2
0 mc cp;c ðtc;out tc;in Þ
Pump P3 stops Pump P3 operates
G ¼ 10½3:007þ2:25606lgF 0 0:7928lg (7)
In the secondary circuit, it is assumed that the water tank is fully mixed. This assumption is reasonable, taking into account the small mass of water and large mass flow rate used in the geothermal exchangers. Then, the heat Qout in the secondary circuit is given by Qout ¼
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0 Pump P1 stops mg cp;g ðtg;out tg;in Þ Pump P1 operates
(8)
The heat loss between the water in the storage tank and the ambient is given by Qloss ¼ U s As ðts ta Þ
(9)
where As and Us are the total surface area of the tank and the heat-transfer coefficient, respectively. In this paper, a value Us = 0.33 W/(m2 K) is used, which is in agreement with some experimental results [18,19]. 3.1.3. Geothermal heat exchangers In the present work, double vertical U-shaped HDPE pipes are used as geothermal heat exchangers, and the typical cylindrical source analytical model is adopted. For decades, modeling on the geothermal exchangers has been an important GSHP research area, which allows system simulations to be performed. The various approaches used for modeling the geothermal heat exchangers can be subdivided into two major categories. Some models mainly deal in short time-step (an hour or so) simulations [20–22]. Bernier [23] did a complete GSHP system simulation using the model and obtained a good agreement with that by DST model in TRNSYS. Other models are used for designing heat exchangers, and simplifications are carried out to make them computationally efficient for performing long-term performance (usually up to 20 years) evaluations of GSHP systems [24–26]. In spite of this, the cylindrical heat source theory, first presented by Ingersoll [27], and later refined by Kavanaugh [28], is relatively easy to comprehend and the resulting analytical solution can easily be programmed. Usually, the transient heat-transfer process can be
2
2
F 0 þ3:596103 lg3 F 0
F 0 þ0:06119103 lg3 F 0
F 0 þ0:134293103 lg3 F 0
(12) (13) (14)
The accumulation effect of continuous heat inject loads can be calculated through the dimensionless temperature response factors presented by Yavuzturk [22]. The govern equations of energy conservation inside the geothermal heat exchanges can be expressed as Qg ¼ mg cp;g ðtg;out tg;in Þ
(15)
The heat transfer between the fluid and the pipe wall can be express as Qg ¼
Hðtw tf Þ Rtotal
(16)
where Rtotal is the equivalent borehole resistance, which can be calculated as Rtotal ¼ Rcon þ Rpipe þ Rfill
(17)
where Rcon, Rpipe and Rfill are the heat-transfer resistance of the fluid inside the pipe, the conduction resistance of pipe walls, and the resistance of the ground or refilled materials, respectively. 3.2. Simulation results and discussions 3.2.1. Analysis of typical simulation results In the present work, a simulation program is developed based on the Visual Basic software. Main parameters for simulation calculations are listed in Table 1. The average CPU time for every calculation is about 6 min. The simulation period ranges from 1 May to 27 October (totally 180 days). As the input of ambient conditions, the hourly data from the typical meteorological year (TMY) in Tianjin is used [29]. The coefficients of the efficiency equation of solar collectors are taken as 0.75 and 4.5, respectively. The start fluid temperature of P1 and P3 is set as 30 8C (T1) and 50 8C (T3), respectively. According to the experimental results, the initial ground temperature starts from 14.5 8C. Fig. 7 gives the daily simulated results during the operation period. It can be seen that the curve characteristics of solar radiation, the water tank temperature and the ground temperature are similar to a great extent, which indicates that the ground temperature is determined strongly on the intensity of solar
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Table 1 List of main parameters for calculations Items
Unit
Value
Solar collectors Area Titled angle Flow rate
m2 8 m3/h
Water tank Volume Heat loss coefficient
L W/(m2 K)
Ground Initial temperature Density Specific heat Thermal conductivity
8C kg/m3 J/(kg K) W/(m K)
14.5 2200 1500 1.2
Geothermal heat exchangers Inner diameter Outer diameter Density Specific heat Thermal conductivity Flow rate
mm mm kg/m3 J/(kg K) W/(m K) m3/h
32 25 1100 1465 0.43 1.8
Boreholes Number Depth Diameter
– m mm
25 40 0.8
800 0.33
Fig. 8. Hourly simulated results in 3 and 4 May.
radiation. The water tank temperature ranges from 24.9 8C to 40.4 8C, and the average is 31.9 8C. This range is relatively close to the variation of the ambient temperature. Compared the latter, however, the ground temperature has an obviously increasing tendency. For this simulation case, the average temperature amplitude is 3.7 8C (from 14.5 8C to 18.2 8C). Taking 3 and 4 May as references, Fig. 8 shows the simulated results for 48 h. It can be seen that, during the first day, the solar radiation is relatively weak due to a cloudy weather condition. Thus, the solar collectors cannot reach its set operation temperature, and the temperatures of the water storage tank and the ground temperature keep a natural decreasing tendency. During the second day, the solar radiation and ambient temperature begin to rise again. The first peak of the ground
temperature appears at 9:30 a.m., which lags behind than the maximum peak time (8:30 a.m.) of solar radiation. Soon with the coming of the next peak of solar radiation, the second and third ground temperature peak reaches at 11:30 a.m. and at 14:00 p.m., respectively. If there is a sufficient solar radiation, such a process will continue repeatedly until the solar radiation is low enough so that the water storage tank cannot get the energy supplement from solar collectors. In this case, the underground thermal storage ceases after 16:00 p.m. The total effective period of underground thermal storage is 5 h or so, which is in agreement with the experimental results. It is also found that the peak number of the ground temperature and the temperature of the water storage tank is always lower than 4, depending strongly on the variation of solar radiation. Besides, the ratio (V/A) between the water tank volume and the area of solar collectors, as well as operation modes, are also influential factors. Fig. 9 shows the comparison on the accumulated energy storage and consumption during the whole operation. It can be seen that, if based on the total solar radiation (73.50 GJ) and the absorbed solar energy (41.94 GJ), the average efficiency of underground thermal storage is 40.64% and 71.24%, respectively. Compared with the experimental results shown in Fig. 6, the simulation efficiency is lower with 6.9%. The simulated total energy loss reaches 16.42%, lower than the experimental results. It is also found that the absorbed efficiency of solar collectors is 57.06%, which means that about 43.94% of solar radiation is unavailable by solar collectors during the whole operation. Therefore, for the design of SGCHPS, the improvement of solar collectors should be an important topic.
Fig. 7. Simulated results during the operation period of 180 days.
Fig. 9. Diagram of the distribution of energy flow.
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Fig. 10. The effect of the operation period on the performance of thermal storage.
3.2.2. Effects of design parameters Fig. 10 shows the effect of operation periods on the performance of underground thermal storage. It can be seen that with the accumulation of the thermal storage time, the absorbed energy and the ground temperature rise increase at first and then tend to be steady. The efficiency of underground thermal storage based on the total solar radiation varies weakly. It reaches the maximum of 41.77% at the end of June, and decreases to 42.27% and 40.64% at the end of September and October, respectively. This is in agreement with the seasonal variation of solar radiation in Tianjin. Therefore, for the operation of SGCHPS in Tianjin, the period of underground thermal storage is rather sufficient, ranging from May to October, which is helpful to improve the whole use performance of renewable energy utilization. Fig. 11 shows the performance of thermal storage under different water tank volumes. With the increase of water tank volumes, the absorbed efficiency of solar collectors, the efficiency of underground thermal storage and the ground temperature increase at first and then decay gradually. It is also found that within the water tank volume of 400–1000 L, the increase of the water tank volume do not affect the performance of thermal storage obviously. If the water tank volume continues to increase, it will not be favorable for the heat injection into the ground by the geothermal heat exchangers, due to a lower and
Fig. 11. The effect of the tank volume on the performance of thermal storage.
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lower fluid temperature. In fact, under this circumstance the thermal storage mode based on the water tank becomes dominant (this is another topic). At the same time, the system cost-up also has to be taken into full consideration. Badescu [30] suggested that the ratio V/A might be changed in the range 16.5–67 L/m2 with no significant influence on the system performance, and the optimum ratio is about 33 L/m2. Considering that the area of solar collector is 25 m2 in the present work, the corresponding water tank volume should be 412.5–1675 L, and the optimum volume should be 825 L. Obviously, the above simulation results are in agreement with Badescu’s suggestions. For the SGCHPS in the present work, it is suggested that the reasonable ratio V/A should range form 20 L/m2 to 40 L/m2. Fig. 12 shows the effect of the set ground inlet temperature T1 on the performance of thermal storage under different area of solar collectors. It is found that within a certain temperature range, the system performance has a tiny fluctuation. In this case, when the area of solar collectors is 25 m2 and 50 m2, the above temperature range is 30–40 8C and 20–30 8C, respectively. If exceeding such a range, the system performance (especially the ground temperature rise) decays rapidly. This means that, when the water tank volume is fixed, the temperature range should decrease with the increase of the area of solar collectors. Besides, the ground temperature rise tends to be increasing with the area of solar collectors, which indicates that the amount of underground
Fig. 12. The effect of the set ground inlet temperature on the performance of thermal storage. (a) 25 m2 (b) 50 m2.
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storage energy is increasing. Compared with the increase of the solar radiation, however, this amount is lower, which can further explain the reason why the efficiency of underground thermal storage experiences a decreasing tendency. According to the simulated results mentioned above, when the area of solar collectors increases to 50 m2, the optimum water tank volume should be 1650 L. Therefore, for a given climate condition, the thermal performance of SGCHPS depends mainly on the matching between the water tank volume and the area of solar collectors. 4. Conclusions In this present work, the performance of underground thermal storage in a SGCHPS for residential building is experimentally analyzed. Then the system performance during a longer period is simulated by the unit modeling, and its parametric effects are discussed. From above experimental and simulation results, the following conclusions can be obtained: (1) The performance of underground thermal storage of SGCHPS depends strongly on the intensity of solar radiation and the matching between the water tank volume and the area of solar collectors. Compared with the solar radiation, the variations of the water tank temperature and the ground temperature rise lag behind and keep several peaks during the day time. (2) For the case of Tianjin, the efficiency of underground thermal storage based on the total radiation at the surfaces of solar collectors and absorbed solar energy can reach over 40% and 70%, respectively. It is suggested that the reasonable ratio between the tank volume and the area of solar collectors should be in the range of 20–40 L/m2. Acknowledgement The authors are grateful for the support provided by Key Scientific Support Foundation of Tianjin, China (No. 07ZCKFSF00400), and National Scientific and Technical Support Foundation (No. 2006BAJ03A06). References [1] J.E. Bose, C.W. Ledbetter, J.R. Partin, Experimental results of a low cost solar-assisted heat pump system using earth coil and geo-thermal well storage, in: Proceedings of the Fourth Annual Heat Pump Technology Conference, Stillwater, Oklahoma State University, 1979. [2] E. Grabrielsson, Seasonal storage of thermal energy-Swedish experience, Journal of Solar Engineering-Transactions of the ASME 110 (3) (1988) 203–207. [3] T.P. Bokhoven, De Geus, Large Solar Heating System with Seasonal Storage for Bulb Drying in Lisse, vol. 1, Eurosun’96, The Netherlands, (1996), pp. 77–82. [4] G. Oliveti, N. Arcuri, Prototype experimental plan for the interseasonal storage of solar energy for the winter heating of buildings: description of plant and it function, Solar Energy 54 (8) (1995) 85–97. [5] R. Yumrutas, M. Unsal, Analysis of solar aided heat pump systems with seasonal thermal energy storage in surface tanks, Energy 25 (12) (2000) 1231–1243.
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