J. Cent. South Univ. (2012) 19: 809-815 

DOI: 10.1007/s11771-012-1076-4

Heat balance of solar-soil source heat pump compound system

CHEN Jin-hua(陈金华)¹, BAO Xiu-bi(包修碧)², PENG Yun-lin(彭运林)², JIA Yu(贾宇)²

1. Key Laboratory of The Three Gorges Reservoir Region’s Eco-Environment of Ministry of Education,Chongqing University, Chongqing 400045, China;

2. Faculty of Urban Construction and Environmental Engineering,Chongqing University, Chongqing 400045, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: Based on the state-of-the-art studies of solar-soil source heat pump compound system, operation patterns of solar-soil compound system were analyzed, particularly the advantages of parallel operation pattern. It is found that parallel operation pattern is better for solar-soil compound system. Furthermore, the heat balance issue of solar-soil compound system was emphatically analyzed from four aspects, which were annual analysis of heating and cooling load, the heat exchange of ground heat exchanger, capacity determination of solar-assisted heat source and heat balance calculation of solar-soil compound system. Moreover, annual rate of heat balance in a solar-soil source heat pump compound system was calculated with a case study. It is shown that the annual heat unbalance ratio is 19%, which is less than 20%. As a result, the practical solar-soil compound system can basically maintain the heat balance of soil.

Key words: solar-soil source heat pump compound system; heat balance; ground-coupled heat pump

1 Introduction

The earth’s energy comes from external and internal. The external energy is the energy of earth surface launched by sun. And the internal energy comes from the decay of radioactive elements [1]. Energy balance is maintained between the two parts of energy, that is to say, heat balance is maintained. Soil is a huge regenerator [2-4]. If cumulative extraction of heat is larger than cumulative emission of heat in ground-coupled heat pump, the heat balance will be damaged and the cumulative extraction will be continuously increased. As a result, soil temperature will be reduced year by year with operation of the system, which is the same to heating energy efficiency ratio. Eventually, the ground-coupled heat pump system will paralyze [5-6]. In consequence, keeping heat balance is extremely important to successful designs and stable operations in a solar-soil source heat pump compound system.

In recent years, scholars have done a large number of researches on theoretical simulation, experiments and economic analysis of solar-soil source heat pump compound system [7]. Studies on solar and soil composite heat source, efficiency of solar collectors, performance of heat pump system, distribution of soil temperature, utilization of solar and other aspects are constantly expanded [8-10]. However, actual cases of solar-soil source heat pump compound system are rare. Besides, the investigations about this aspect are basically aimed at a special case in a special area, which are not universal without sufficient basic data. It cannot provide a solid theoretical foundation for the application and promotion of solar-soil source heat pump compound system [11].

“Energy crisis” had inspired the research needs of ground-coupled heat pump in Europe and America. As a result, demonstration projects which were funded by Europe and America government had been built up gradually. In 2003, CHIASSON and YAVUZTURK [12] carried out a simulation study on solar-soil source heat pump compound system, using TRNSYS as a platform. What’s more, the simulation objects were Chicago, Saline Lake, Denver city and other three cities whose heating loads were leading. Furthermore, the simulation cycle was 20 years, and the energy saving effect of the compound system was proved.

Researches on solar-soil source heat pump compound system were less in China. In 2001, soil temperature change of intermittent operation was simulated in Harbin Industrial University on the basis of Kelvin line theory. Therefore, the best running time distribution proportion of solar-soil source heat pump combined heating system was determined [13-14]. In 2003, solar deep soil heat storage was studied in Harbin Industrial University, which was directed towards running problems of ground source heat pump in northern cold regions. It was believed that the utilization of solar cross seasons was feasible [15]. In 2005, alternate operating performances of solar-soil source heat pump system were numerically simulated in Southeast University. As a consequence, the running time distribution proportion of combined heating system was obtained [16-17].

Heat balance, operation pattern, characteristic analysis of cooling and heating loads, heat exchange determination of buried tube heat exchanger and capacity  determination of solar auxiliary heat source are discussed in this work, which are universalistic and can be used in general solar-soil source heat pump compound system. In addition, the heat balance of compound system is carefully analyzed which combines an actual case with adequate data base in Chongqing, China. This work intends to provide a reliable theory and basis for further promotion of solar-soil source heat pump compound system.

2 Solar-soil source heat pump compound system

There is a large difference between the heat extraction and rejection in ground-coupled heat pump system when the heating load demand of a building is larger than the cooling load demand. A compound ground source heat pump system is usually used to achieve the balance of heating and cooling. Solar system is used as an auxiliary heating source while cooling tower is used as an auxiliary cooling source, which undertakes part of load and basically keeps heat balance. This kind of ground-coupled heat pump system with auxiliary heating source or cooling source is known as a hybrid system, which is also called a compound system [18]. It is known as solar-soil source heat pump compound system when solar system is used as an auxiliary heating source [19-20].

3 Heat balance of solar-soil source heat pump compound system

3.1 Annual analysis of heating and cooling loads

System modes are determined by the heating and cooling loads of buildings. It is the unbalance of annual load that makes the geotechnical temperature of soil source heat pump system rise or fall continuously. Thus, it affects the heat exchanger performance of underground pipe and reduces the operating efficiency of system. Therefore, it is believed that the effect of load throughout one year should be considered when designing the buried tube system. It is necessary for a ground heat exchanger system to have a year-round dynamic load simulation and calculation. Besides, the minimum simulation period should be one year [21].

Heating and cooling loads of buildings can be calculated by software, for example, DOE and DEST. As a result, annual cumulative value of loads, peak value of loads and distribution of loads are calculated.

3.2 Heat exchange of ground heat exchanger

Heat Q_P released underground corresponds with the cooling load of a building, including three parts. The first part is heat Q₁ released into the circulating water by water-source heat pump unit in all air-conditioning districts, which equals the air conditioning cooling load added by the consumption of compressor power. The second part is heat Q₂ which is obtained through the transmission of circulating water. Besides, the third part is the heat Q₃ which is released in circulating water by pumps. The released heat of circulating water in cooling status can be calculated by adding up Q₁, Q₂ and Q₃. Equation (1) can be used to calculate the released heat of ground heat exchanger:

    (1)

where q₁ represents annual accumulative air conditioning cooling load of each partition, and C_P represents the coefficient of performance of ground-coupled heat pump units under cooling conditions.

Heat Q_X obtained underground corresponds with the heating load of a building, also including three parts. The first part is heat  retrieved from the circulating water by water-source heat pump unit in all air-conditioning districts, which equals the air conditioning heating load subtracting the consumption of compressor power. The second part is heat  which is released through the transmission of circulating water. Besides, the third part is the heat  which is released in circulating water by pumps. The retrieved heat of circulating water in heating status can be calculated by adding up  and  and then subtracting . Equation (2) can be used to calculate the retrieved heat of ground heat exchanger:

    (2)

where represents annual accumulative air conditioning heating load of each partition, and  represents the coefficient of performance of ground- coupled heat pump units under heating conditions.

It is rational to ignore the obtained and lost heat in the process of transportation through the pipe heat preservation for an air conditioning system. The heat released by pumps is negligible compared with the transport energy of the entire system. What’s more, annual accumulative energy consumption can be simulated and calculated by simulation software of cooling and heating loads. Then, Eqs. (1) and (2) can be simplified as Eqs. (3) and (4), respectively:

           (3)

           (4)

For sanitary hot water system, the heat loss coefficient of tank is controlled less than 20% due to setting hot water storage tanks. It is considered that heat loss coefficient changes with the effect of seasons. So, 10% is considered as the heat loss coefficient in the entire year. Equation (2) can be simplified as

   (5)

Heat exchange of buried tube in soil source heat pump system can be got by Eqs. (1)-(5) which are universal [22].

3.3 Capacity of solar-assisted heat source

According to annual accumulative load value simulated by the simulation software, Eqs. (3), (4) and (5) are used to calculate the heat extraction and rejection of underground pipes. It is extremely significant to keep the heat balance of emission and extraction in a ground-coupled heat pump system within the calculation cycle. When there is a large gap between cumulative extraction of heat and cumulative emission of heat, auxiliary heating is a sensible solution. It has many advantages, cutting the heating peak of underground pipes, reducing the length of pipes and significantly reducing the drilling cost.

Furthermore, it can not only make the system more economic but also avoid the change of soil temperature caused by the heat imbalance.

In this work, solar system is intended to use as auxiliary heating source of a ground-coupled heat pump. Both the solar and soil provide living hot water. And the soil can also provide hot and cold water for air conditioning. The capacity and operation of auxiliary heating source should be determined according to the specific circumstances, through more detailed analysis and calculation.

3.3.1 Operating pattern of solar-soil compound system

In order to facilitate operation and adjustment, the design of solar-soil compound system should be able to realize serial operation, parallel operation and independent operation between solar system and ground-coupled heat pump system.

1) Serial operation pattern

When solar system and ground-coupled heat pump system are in serial operation, it can be divided into two patterns by the through path of contained fluid. The first pattern A is from solar energy collector to ground heat exchanger, while the second pattern B is from ground heat exchanger to solar energy collector. The efficiency of solar energy collector in the former pattern is higher, while the efficiency of ground heat exchanger in the latter pattern is better. The serial operation pattern of the compound system is shown in Fig. 1.

Fig. 1 Serial operation pattern: (a) Pattern A; (b) Pattern B

It may realize the serial operation pattern when closing valves 1, 3 and opening valves 2, 4. Besides, it may realize independent operation of solar energy collector when closing valves 1, 4 and opening valves 2, 3. Furthermore, it can realize independent operation of ground-coupled heat pump when closing valves 2, 3 and opening valves 1, 4.

2) Parallel operation pattern

It is efficient to use parallel operation pattern when single heating source can independently meet the needs of hot water or air conditioning water in a period of time. The parallel operation pattern is shown in Fig. 2.

Fig. 2 Parallel operation pattern

It may realize parallel operation pattern when opening valves 1, 2, 3 and 4. It may realize the independent operation pattern of ground-coupled heat pump when closing the valves 1, 3 and opening valves 2, 4. Besides, it can realize the independent operation pattern of solar collector when closing the valves 2, 4 and opening valves 1, 3.

Solar-soil source heat pump compound system has a lot of advantages in parallel operation pattern. 1) It can also serve as auxiliary cooling equipments [23] when the system runs at night in the summer. As a result, the rejection of heat to soil could be reduced obviously. 2) Due to the increase of evaporation temperature, not only energy efficiency of units are improved but also outlet temperature of water or air is increased in the users’ side. So the users’ comfort is greatly improved. 3) It can obtain the soil heat balance easily when reasonable intermittent running time of ground heat exchanger is considered for buildings with different functions. Solar collector system has improved the safety of ground source heat pump system, making ground source heat pump system achieve intermittent operation [24] and also recovering the soil temperature. 4) It is usable to improve the reliability of ground-source heat pump system. 5) Ground heat exchanger is almost unaffected by weather conditions, urban electricity, gas and other factors. In consequence, it can choose to run the solar collector systems more efficient in high heat period, which is a “time optimal” operation pattern to improve the efficiency of solar energy systems.

3.3.2 Determination of capacity of solar-assisted heat source

Heat consumption Q_* of design-hour hot water can be calculated by

Q_*=K_bmq_rc(t_r-t_l)ρ_r/86 400                      (6)

where m is the water unit number, or number of beds, q_r is the fixed daily water consumption, c is the  water specific heat, c=4 187 J/(kg·°C); K_b is the hour coefficient variation of hot water, t_r is the temperature of hot water, t_r=60 °C and t_l is the temperature of cold water.

Design-hour hot water volume q_r* of buildings can be calculated by

                         (7)

where q_r* is the design-hour hot water volume, L/h; ρ_r is the density of water, kg/L.

The volume of design-hour hot water can be calculated by Eqs. (6)-(7). Water storage capacities of the heating system are obtained by the empirical formula.

So, it can be set to a heating tank and a solar collector tank. Water tank is used to supply heating hot water in daily requirements, while solar thermal storage tank is responsible for both the needs of solar collector and filling water of heating water tank.

In addition, ground-coupled heat pump system could also supply heat for hot water system considering the difference between heating load and hot water load. As a result, Eq. (8) [15] is used to calculate the load Q_g of ground heat exchanger, taking the effect of constant temperature heating water tank into consideration:

                   (8)

where Q_g is the hour heat of volume heater, W; η is the capacity coefficient of heat storage, 0.75; V_t is the total heat storage capacity, L; T is the design-hour duration, and according to the specification it is typically 2-4 h.

Design-hour hot water of buildings and load of ground heat exchanger can be got by Eqs. (6)-(8) which is universal [25].

According to fixed daily water demand of hot water, the total amount of hot water daily water is calculated. Besides, daily supplied water of ??solar system is determined considering both the area of ??solar collector and solar energy resource in actual situation. The ground-coupled heat pump system and solar collector system share the hot water load. And the ground-coupled heat pump system is in charge of the part of hot water load that the solar system cannot satisfy.

3.4 Heat balance of solar-soil compound system

It is very important to maintain consistent of released heat and retrieved heat in long-running for solar-soil source heat pump compound system.

According to Thermal Environment-Specific Meteorological Data, China Meteorological Library, Chinese Academy of Meteorological Information Center, statistical method is used to analyze the running of solar hot water system. The number of days, the total production capacity of hot water and the amount of hot water are calculated. Thus, it is apparent that how long in each year the solar collector system can meet the hot water supply of the entire building and satisfy the half need of the building.

It is a premise that determining the heating ratio of solar collector system and underground heat exchanger reasonably. Then, cumulative extraction of heat for underground heat exchanger is scientifically determined. It is feasible to calculate the cumulative heat within one-year period.

By this analysis, it is easy to know that the annual hot water G_s provided by solar collector system. Learning total amount of hot water G_L, the proportion β(β=G_S/G_L) that the solar collector occupies in the total water is clear. That is to say, the proportion (1-β) is known. Equations (5) and (9) can be available to the cumulative extraction of heat:

        (9)

The rate of balance in solar-soil source heat pump compound system can be got by

                            (10)

According to Eq. (10), it is easy to know the rate of imbalance by

Η_UB=|1-η_B|                                 (11)

It is adverse if the rate of imbalance is more than 20% [26]. If the rate of imbalance can be less than 20%, the solar-soil source heat pump compound system will have a long-term and stable operation.

4 Analysis of actual case

4.1 Project overview

The designed building is a green building demonstration project located in Shapingba, Chongqing, China. The height of the building is about 20 m. The construction land is relatively flat and neat with no adverse geological conditions and surrounding environmental pollution. The main technical and economic indicators are as follows: Land area is ??      13 837.8 m², and total construction area is 11 609 m². Besides, the area of ??energy-saving demonstration is    11 609 m². In addition, the building has five floors and one basement. Also, it has a green area of 5 960 m².

Air conditioning cooling and heating loads of the building are entirely borne by ground-coupled heat pump, while hot water load is shared by solar collector system and ground-coupled heat pump.

4.2 Cooling and heating loads of building

Firstly, VisualDOE Version 4.1.2 is used to simulate and calculate the annual load. The peak load of air conditioning is given in Table 1. Annual hourly air-conditioning cooling and heating loads are given in Fig. 3, and the cumulative monthly value is seen in Table 2 and Fig. 4.

Table 1 Peak load of air conditioning

Fig. 3 Annual hourly air-conditioning cooling and heating loads

Table 2 Cumulative monthly values

4.3 Exchanged heat of ground heat exchanger

It could be calculated that the accumulation of annual value in summer and winter are 748.6 MW·h and 239.2 MW·h by Eqs. (3) and (4), respectively.

4.4 Determining capacity of solar heat source

In summer, ground-coupled heat pump and solar heat source are running in the serial pattern. Part of cooling water first flows through hot water heat pump unit and then mixes with other water returning from underground heat exchanger. The mixed water     flows through the air conditioning unit on the purpose of reducing temperature of inlet cooling water. Besides,   efficiencies of heating and cooling systems are ensured. In winter, ground-coupled heat pump and solar heat source are running in parallel pattern avoiding the decrease of operating efficiency. Furthermore, indoor air- conditioning and hot water system are the same way as a conventional system.

Fig. 4 Cumulative monthly values

According to the design parameters provided by drainage and HVAC design group, design-hour hot water heat loss calculations of building are given in Table 3.

Table 3 Design-hour hot water heat loss calculations

When daily water consumption q_r=34 080 L/d is fixed into Eqs. (6) and (7), the design-hour hot water heat consumption and the design-hour hot water capacity are calculated to be 484.7 kW and 7 967 L/h, respectively. According to empirical formula, the heating of heat storage tanks should be more than 90 min, that is to say, storage water is required to achieve 11.95 t.

To take full advantage of solar energy, total water heating tank is designed to be 16 t, which is divided into 6 t of heating water tank and 10 t of solar thermal storage tank. Heating tank is responsible for directly supplying hot water for users, and solar thermal storage tank is responsible for water heating tank, sharing the peak load together with heating water tank.

Considering the existence of 16 t capacity under the conditions of constant temperature heating water tank, design load of underground heat exchanger can be calculated by Eqs. (6) and (8). And the load equals 484.7 kW subtracted by 243.3 kW, which is 241.4 kW.

4.5 Calculation of heat balance

The hot water for the building is supplied for 120 d in summer, 120 d in winter and 125 d in transition season. As a result, accumulative consumption of 677.31 MW·h is needed to produce annual hot water.

It is known that hot water provided by the solar collector occupies 36.9% in total water, that is to say, 63.1% of hot water is borne by ground-coupled heat pump. In part-time, solar energy can provide hot water for the load of whole building, and there is no need to heat pump units. By Eqs. (9) and (10), heat to soil obtained by hot water system and the heat balance ratio are 363.4 MW·h and 81%, respectively. The heat unbalance ratio can also be calculated to be 19%.

5 Conclusions

1) When designing solar-soil source heat pump system, simulation of energy consumption within one year must be done for the designed buildings. It is the target to accurately simulate and calculate accumulative cooling and heating loads. Further, accumulative released heat and obtained heat are calculated reliably. According to accumulative released heat and obtained heat, ground heat exchanger system and solar system can be designed and placed more reasonably.

2) The parallel operation pattern of solar-soil source heat pump compound system could ensure the efficiency of units, improve the comfort of users, benefit the recovery of soil temperature and effectively improve the efficiency of a solar system. At the same time, the parallel operation pattern can improve the reliability of the compound system. Consequently, it is proposed that using parallel operation pattern is better.

3) According to the daily radiation data of solar, the provided living hot water ratio of solar system and also the provided living hot water ratio of soil system can be obtained. As a result, heat balance ratio of solar-soil source heat pump compound system can be designed and calculated. What’s more, the heat unbalance ratio of the compound system can also be checked.

4) Combined with the analysis of an actual case, the heat unbalance rate of the solar-soil source heat pump compound system is 19%, which is less than 20%. In consequence, accumulative released heat and obtained heat could keep a basic balance, and the compound system can almost meet the requirements of soil heat balance after a long term operation.

References

[1] WANG Xun-chang. Thoughts on development of ground-source heat pump systems [J]. Heating Ventilating & Air Conditioning, 2007, 37(3): 38-43. (in Chinese)

[2] ZHANG Xiao-ming, WU Jian-kun. The application of ground source heat pump system and its economic analysis [J].Journal of Shenyang Jianzhu University: Social Science, 2011, 13(1): 35-39. (in Chinese)

[3] MA Zui-liang, L? Yue. Ground source heat pump system design and application [M]. Beijing: Mechanical Industry Press, 2007: 1-25. (in Chinese)

[4] Energy Saving Office of Tianjin Construction Committee, Solar Energy Society of Tianjin, Heat Energy Engineering of Mechanical College in Tianjin University. Tianjin construction of renewable energy projects report [J]. China Construction Heating & Refrigeration, 2007(7): 16-22. (in Chinese)

[5] ESEN H, INALLI M, ESENA M. A techno-economic comparison of ground-coupled and air-coupled heat pump system for space cooling [J]. Building and Environment, 2007, 42: 1955-1965.

[6] DIKICI A, AKBULUT A. Exergetic performance evaluation of heat pump systems having various heat sources [J]. International Journal of Energy Research, 2008, 32: 1276-1296.

[7] ZHANG Fang-fang. Study on the compound system of solar energy and ground source heat pump with seasonal heat storage [D]. Shandong: Shandong Architecture University, 2010. (in Chinese)

[8] INALLI M, UNSAL M, TANYILDIZI V. A computational pattern of a domestic solar heating system with underground spherical thermal storage [J]. Energy, 1997, 22(12): 1163-1172.

[9] CHWIEDUK D. Analysis of utilization of renewable energies as heat sources for heat pumps in building sector in Poland [J]. Renewable Energy, 1996, 9(1-4): 720-723.

[10] NORDELL B, HELLSTROM G. High temperature solar heated seasonal storage system for low temperature heating of buildings [J]. Solar Energy, 2000, 69(6): 511-523.

[11] WU Xiao-han. Research and simulation analysis of combing solar collector and ground source heat pump heating system [D]. Jilin: Jilin University, 2008. (in Chinese)

[12] CHIASSON A D, YAVUZTURK C. Assessment of the viability of the hybrid geothermal heat pump systems with solar thermal collectors [J]. ASHRAE Trans, 2003, 109(2): 487-500.

[13] YU Yan-shun, LIAN Le-ming. Determination of the solar energy assurance factor of a solar energy-ground soil-source heat pump system in frigid regions [J]. Journal of Engineering for Thermal Energy and Power, 2002, 17(4): 393-395. (in Chinese)

[14] YU Yan-shun, LIAN Le-ming. Approach of operation condition in solar-ground source heat pump system in cold climate [J]. Acta Energiae Solaris Sinica, 2003, 24(1): 111-115. (in Chinese)

[15] LIN Yuan. Numerical simulation and experiment research of solar deep soil [D]. Harbin: Harbin Industrial University, 2003. (in Chinese)

[16] YANG Wei-bo, SHI Ming-heng, DONG Hua. Numerical simulation on performance of alternate operation of solar-earth source heat pump system [J]. Journal of Thermal Science and Technology, 2005, 4(3): 228-232. (in Chinese)

[17] YANG Wei-bo, DONG Hua, ZHOU En-ze, HU Jun. Research on approach of combined operation in solar-earth source heat pump system [J]. Fluid Machinery, 2004, 32(2): 41-45. (in Chinese)

[18] CAI Jing-jing, CHEN Ru-dong. Discussion about thermal equation of ground-coupled heat pump [J]. China Construction Heating & Refrigeration, 2009(4): 32-35. (in Chinese)

[19] WANG Xiao, ZHENG Mao-yu, ZHANG Wen-yong, ZHANG Shu, YANG Tao. Experimental study of a solar-assisted ground-coupled heat pump system with solar seasonal thermal storage in severe cold areas [J]. Energy and Buildings, 2010, 42: 2104-2110.

[20] LI Hong, YANG Hong-xing. Study on performance of solar assisted air source heat pump systems for hot water production in Hong Kong [J]. Applied Energy, 2010, 87: 2818-2825.

[21] The Ministry of Construction of People’s Republic of China. Technical code for ground-source heat pump system [M]. Beijing: China Architecture & Industry Press, 2009: 10. (in Chinese)

[22] DIAO Nai-ren, FANG Zhao-qing. Ground-coupled heat pump technology [M]. Beijing: Higher Education Press, 2006: 104-105. (in Chinese)

[23] ZHOU Xue-wen. Problems and solutions for thermal balance of vertical ground heat exchanger in ground source heat pump [J]. New Energy & Environment, 2009, 37(1): 64-66. (in Chinese)

[24] ZHANG Qiao-jun, SHANG Bei-shi, LU Man. Energy saving potential analysis of ground source heat pump operation management [J]. China Housing Facilities, 2011(2): 51-53. (in Chinese)

[25] Shanghai Urban and Rural Construction and Transportation Committee. Code for design of building water supply and drainage [M]. Beijing: China Planning Press, 2009: 81-91. (in Chinese)

[26] WEI Tang-li. Study on characteristics of buried pipe-in-pipe heat exchanger in ground source heat pump system [D]. Chongqing: Chongqing University, 2001. (in Chinese)

(Edited by YANG Bing)

Foundation item: Project(50838009) supported by the National Natural Science Foundation of China; Project(2010DFA72740-05) supported by the International Science & Technology Cooperation Program of China

Received date: 2011-07-26; Accepted date: 2011-11-14

Corresponding author: CHEN Jin-hua, Associate Professor; Tel: +86-13708315562; E-mail: c66578899@126.com