Current status of ground-source heat pumps in China

ARTICLE IN PRESS Energy Policy 38 (2010) 323–332

Contents lists available at ScienceDirect

Energy Policy journal homepage: www.elsevier.com/locate/enpol

Current status of ground-source heat pumps in China Wei Yang a, Jin Zhou a,, Wei Xu b, Guoqiang Zhang a, a b

Key Lab of Building Safety and Energy Efficiency, MOE, College of Civil Engineering, Hunan University, Changsha, Hunan 410082, PR China China Academy of Building Research, Beijing, PR China

a r t i c l e in fo

abstract

Article history: Received 2 May 2009 Accepted 21 September 2009 Available online 17 October 2009

As a renewable energy technology, the ground-source heat pump (GSHP) technologies have increasingly attracted world-wide attention due to their advantages of energy efficiency and environmental friendliness. This paper presents Chinese research and application on GSHP followed by descriptions of patents. The policies related to GSHP are also introduced and analyzed. With the support of Chinese government, several new heat transfer models and two new GSHP systems (named pumping and recharging well (PRW) and integrated soil cold storage and ground-source heat pump (ISCS&GSHP) system) have been developed by Chinese researchers. The applications of GSHP systems have been growing rapidly since the beginning of the 21st century with financial incentives and supportive government policies. However, there are still several challenges for the application of GSHP systems in large scale. This paper raises relevant suggestions for overcoming the existing and potential obstacles. In addition, the developing and applying prospects of GSHP systems in China are also discussed. & 2009 Elsevier Ltd. All rights reserved.

Keywords: Ground-source heat pump (GSHP) Current status China

1. Introduction As the largest developing country in the world, China’s rapid economic growth is accompanied with serious environmental challenges such as pollution, energy shortage and climate change (Li et al., 2007c). Energy shortage is the most serious problem. The increases of energy consumption in building sector potentially threat the end users of energy. The building sector accounted for about 27.8% (Crompton and Wu, 2005; Document of the World Bank, 2005; Li et al., 2007c) of the total energy consumption, which was just inferior to industrial sector. On the other hand, the structure of energy consumption in China is dominated by coal consumption; and clean energies only represent a very small percentage (as shown in Fig. 1). As shown in Table 1, when comparing the energy consumptions between China and the world, coal accounted for about 69.5% (National Bureau of Statistics of China, 2009) of the national total energy consumption in 2007, while this percentage for the whole world was only 28.6% (BP, 2008) in the same year. Considering the coal emitted more CO2 and other pollutant than other energy resources, it can be concluded that energy conservation in China is much more closely related with environment than that in other countries. It was estimated that, heating, ventilation and air-conditioning (HVAC) accounted for about 65% of the energy consumption in the Chinese building sector (Liu et al., 2008b; Yao et al., 2005).

 Corresponding authors. Tel.: + 86 73188823900; fax: + 86 73188821005.

E-mail addresses: [email protected] (J. Zhou), [email protected] (G. Zhang). 0301-4215/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enpol.2009.09.021

Therefore, it is strongly desired to reduce energy consumption in HVAC systems. Professionals and policymakers have been making great efforts in this aspect. Ground-source heat pump (GSHP) technology is regarded as an effective technology for this purpose. Chinese research and practices on GSHP in China started relatively much later than that in developed countries. The end of 1980s saw the beginning of experiments and tests on the performance of the GSHP systems. Qingdao Technological University, Tianjin University of Commerce and Tianjin University are the first three universities, which conducted relevant research on GSHP technologies. At the end of 1990s, theoretical and experimental studies in all aspects of GSHP were carried out, mainly supported by the National Natural Science Foundation of China. More and more universities and academes, such as Hunan University, Harbin Institute of Technology, Tongji University and Beijing University of Technology, joined in the research work. Some significant achievements have been attained ever since, so that the application and development of GSHP systems are being boosted greatly. The beginning of the 21st century is a period of rapid growth of the application of GSHP systems. However, due to China’s vast territory and differentiated climatic zones as well as the restrictions on water resource and environmental policies, Chinese research and practices on GSHP have their own characteristics. Investigations in the characteristics of the research and application of GSHP in China become a necessity for academia and policy development. This paper discusses the research and the state-of-art practices on GSHP systems in China. Then it introduces the Chinese patents on GSHP. The GSHP development policies of China, including incentive mechanism, relevant codes and regulations, are also introduced

ARTICLE IN PRESS 324

W. Yang et al. / Energy Policy 38 (2010) 323–332

3.5% 7.3% Coal Oil

19.7%

Gas Hydro power, Nuclear, and Wind power

69.5%

Fig. 1. Energy consumption structure of China in 2007. Note: data in Fig. 1 sources from National Bureau of Statistics of China (2009).

Table 1 Primary energy consumption profile in China and the world in 2007 (MTOE)a. World Amount (MTOE) Coal Oil Gas Hydro power, nuclear, and wind power Total

3177.5 3952.8 2637.7 1331.2c 11,099.3

China Proportion (%) 28.6 35.6 23.8 12.0 100

Amount (MTOE)b 1292.1 366.2 65.1 135.7 1859.1

Proportion (%) 69.5 19.7 3.5 7.3 100

a Data in Table 1 sources from National Bureau of Statistics of China (2009) and BP (2008). b The data of China’s energy consumption is transformed from MOCE to MTOE by multiplying a conversion factor of 0.7000. c The data of 1331.2 in Table 1 only concludes Hydro power and nuclear.

and analyzed. At last, the application prospects of GSHP systems in China are analyzed, and some propositions are provided.

2. The research on GSHP in China Ground-source heat pump (GSHP) is applied to a variety of systems that use the ground, groundwater, or surface water as a heat source and sink. Included under this general term are ground-coupled (GCHP), groundwater (GWHP), and surface water (SWHP) heat pumps (ASHRAE, 2003; Muhammad, 2004). The GSHP technology can achieve higher energy efficiency for airconditioning than conventional air-conditioning systems because the underground environment provides lower temperature for cooling and higher temperature for heating and experiences less temperature fluctuation than ambient air temperature change. As a renewable energy technology, the GSHP’s high energy efficiency and low environmental impact characteristics have already drawn a fair amount of attention in China. Due to China’s diversification in climatic zones, restrictions of water resource and environmental policies, the research of GSHP in China has its own characteristics. The main subjects researched by Chinese researchers on GSHP technologies are presented and analyzed as follows. 2.1. Popular research subjects Different types of GSHP systems have their own characters, leading to different research subjects, which are analyzed as follows. 2.1.1. Ground-coupled heat pump Due to large population and high population density, the GCHP systems in China are mainly installed with vertical pipes. More-

over, the geothermal heat exchanger is not only the fundamental part of the GCHP systems but also the most critical and difficult subject of the GCHP technologies. Therefore, the heat transfer theory and numerical simulation of geothermal heat exchanger is a popular research subject in recent years, which has resulted in fruitful achievements on this aspect. Bi et al. (2002) developed a heat transfer model of a vertical double spiral coil ground heat exchanger, and the underground temperature distribution of the coil was solved numerically. Their numerical results were valid and effectively supported by the experimental data. Zeng et al. (2002, 2003) and Qu (2004) presented a new quasi-three-dimensional model for vertical ground heat exchangers (GHE) that accounted for the fluid axial convective heat transfer and thermal ‘‘short-circuiting’’ among U-tube legs. Analytical solutions of the fluid temperature profiles along the borehole depth were obtained. Calculation results showed that the double U-tube boreholes were superior to those of the single U-tube with reduction in borehole resistance of 30–90%. In their studies, calculations on typical GHE boreholes indicated that the U-tube shank spacing and the thermal conductivity of the grout were the prevailing factors in all the configurations considered in determining the borehole thermal resistance. The authors validated that the quasi-three-dimensional model was more accurate than the other current existing models. An analytical solution to the finite line source was also developed by them which considered the influences of the finite length of the borehole and the ground surface as a boundary. This analytical model approximates the borehole with a U-tube as a finite line source with radial heat flow. When time approaches infinity, the temperature rise of the Kelvin’s theory approaches infinity, whereas the temperature from the finite line source model approaches steady state, which corresponds to the actual heat transfer mechanism. It is obvious that the analytical solution can be computed much faster than the numerical solution of the heat conduction problem in the semi-infinite domain with long duration. According to the presented model, a GHE design procedure was then presented. The quasi-three-dimensional model and the analytical solution has been complied in the later design and simulation software (named Geostar) developed by them. Qu (2004), Diao and Fang (2006), and Cui et al. (2006) developed a transient three-dimensional heat conduction model to describe the temperature response in the ground caused by a single inclined line source. Comparisons between the GHEs of typical rectangular patterns with inclined or vertical boreholes showed that the temperature rise on the borehole wall of the inclined GHE can be 10–35% lower than that of the vertical GHE for long-term performance in commonly encountered conditions in engineering practice. Diao et al. (2004) presented a heat transfer model of GHE which took water advection into account, and an analytical transient solution was obtained for a line heat source in an infinite medium by means of the Green function analysis. This solution indicated that the impact of moderate groundwater flow on the heat transfer process may be prominent. Li et al. (2005) presented an inner heat source model of underground heat exchanger based on the heat and mass transfer theory in soil. Moisture movement in soil, soil type, and soil property were modeled in their study. Diao and Fang (2006) systemically summarized and investigated the heat transfer theory of GHEs, and the construction and design methods were also introduced in their study, which can be used as guidance for the design of GHE for GCHP systems. A number of researchers investigated the thermal performance of GHEs according to different types and conditions in recent years. Li et al. (2006c) presented both the numerical simulations and experiments on the thermal performance of U-vertical ground-coupled heat exchanger. The variation of the ground

ARTICLE IN PRESS W. Yang et al. / Energy Policy 38 (2010) 323–332

325

Reture water pipe Supply water pipe

Reture water pipe Supply water pipe

Steel tube

Steel tube

Grout

Grout

Clapboard

Clapboard

Diving pump

Diving pump

Bore hole

Bore hole

Filter tube

Filter tube

Fig. 2. Schematic diagram of the PRW.

temperature and heat balance of the system were analyzed and compared in different operating modes in the numerical simulation. Zhao et al. (2007) investigated the thermal performance of saturated soil around coaxial GHE. A theoretical model with Darcy’s natural convection was developed and some numerical solutions were achieved using the Keller-Shooting method. This theoretical model greatly reduced the limitations of experimental setup and revealed the heat transfer performance within a wider range. The inlet temperature, initial temperature of porous medium and the flow rate were major factors affecting heat transfer. There was a linear relationship between dimensionless temperature gradient along the outer wall of GHE and the dimensionless height. Gao et al. (2008) investigated the heat transfer performance of pile-foundation ground heat exchangers by experimental and numerical methods. Simulation results showed that under the thermal imbalance ratio of 10% and 3%, total elevated mean ground temperature of five-year running was 2.77 and 0.81 1C, respectively. Li et al. (2006b) investigated the thermal performance of ground heat exchanger in different conditions of inlet temperature, flow rate, soil type, backfill materials, number of U-pipes and operating modes. The previous studies on the heat transfer theory of GHEs provide fundamentals for the design/simulation of the GCHP systems in engineering applications. In addition, these studies are helpful in improving the calculation precision of ground heat exchanger.

2.1.2. Groundwater heat pump Since the GWHP system has some obvious advantages including low initial cost and minimal requirement for ground surface area over other GSHP systems, the groundwater heat pump system has become the most wildly used type of GSHP in China, which is a popular research subject in recent years in China. Li et al. (2006a, 2007a, 2007b) and Liu et al. (2008a) developed a mathematical model of piezometric head of the groundwater in the confined aquifer for the standing column well

(SCW) system and an analytical solution for the piezometric head was obtained by using the integral transformation method, which provides a foundation for the heat transfer analysis for the standing column well systems. Meanwhile, a number of factors seriously restrict the wide application of the GWHP systems, such as the limited availability of groundwater. In addition, many legal issues arise over groundwater withdrawal and re-injection in some regions, which also restrict their applications to a large extent. In recent years, a new ground-source heat pump system called pumping and recharging well (PRW) was firstly presented by Chinese researchers. Some patents were applied based on this technology (Xu, 2002; Lv, 2008). Fig. 2a shows the principle of the PRW system. As shown in Fig. 2, groundwater is drawn from underneath of a well and then re-injected to the upside of the same well after transferring heat with heat pumps. A clapboard is built in the middle part of the well to prevent thermal interferences between the supplied water and recharged water. To improve the thermal performance, two clapboards are built in the system as is shown in Fig. 2b. Fig. 3 shows another type of PRW with the tube in tube heat exchanger. The groundwater is drawn and re-injected at the same plane spot but at different depth of the well in the system. However, the probability of heat transfixion is increased in the system. Thus, it is highly important to analyze the thermal characteristics of the systems. The first model for groundwater heat transfer of PRW with constant circulating water flow was built by Ni et al. (2006a) and Ni and Ma (2005, 2006b, 2006c). The analytic seepage equation of PRW was obtained based on principle of superposition. Based on the model, a numerical simulation was done for a typical fine sand confined aquifer. Ni and Ma (2005) analyzed the effect of heating load on the groundwater heat pump system with pumping and recharging in the same well. Furthermore, the effect of aquifer thickness, ratio of permeability coefficient and different aquifers on groundwater seepage and heat transfer caused by groundwater heat pump system with pumping and recharging in the same well was also analyzed through numerical simulation in their studies. By the end of 2005,

ARTICLE IN PRESS 326

W. Yang et al. / Energy Policy 38 (2010) 323–332

Reture water pipe Supply water pipe

Steel tube

Grout

2.2. The hybrid systems

Diving pump

Bore hole

Filter tube

Fig. 3. Schematic diagram of the PRW with the tube in tube heat exchanger.

5

7

6

1

2

3

However, the thermal characteristics of surface water bodies are significantly different from those of the ground or groundwater. Some unique applications are possible, though special precautions may be warranted (ASHRAE, 2003). According to different heat sources and sinks, sea water heat pump and sewage water heat pump can be included in the SWHP. A great number of SWHP systems were installed, but few of them were well studied. The economic, energy and environmental impacts of a seawater heat pump install in Dalian were analyzed by Li et al. (2007d). Their study showed that under favorable conditions, investment in a SWHP system was generally profitable. Chen et al. (2006b, 2006c) investigated the performance of a SWHP system in Xiangtan, a middle south city of China. A simplified twodimensional model was also developed to simulate the steady lake water temperature (LWT) distribution during continuous operation by them.

8

4

12 9

10

11 Fig. 4. Schematic diagram of the ISCS&GSHP system.

there had been 180 competed projects which used the GWHP system with PRW, covering floor area of more than 2,500,000 m2 (Ni and Ma, 2005).

2.1.3. Surface water heat pump SWHP system can be either closed-loop systems similar to GCHP system or open-loop systems similar to GWHP system.

As is known, China has a territory of about 9.6 million km2. Approximately 98% of the land area stretches between a latitude of 201N and 501N. In terms of the thermal design of buildings, there are five major climates, namely severe cold, cold, hot summer & cold winter, mild, and hot summer & warm winter (National Standard of the People’s Republic of China, 1993). In the northeast of China, the heat pumps operate mainly in the heating mode, but in the southeast, the cooling load is dominating. Thus, the soil temperature increases/decreases gradually after yearly operation of the GSHP system because of the inefficient recovery of soil temperature as the result of imbalance loads. The heat buildup within the ground will definitely increase the ground temperature, which can consequently deteriorate the system performance over time. To maintain a high operating performance, a much larger GHE size is always required. However, the high initial cost and large land area required for the GHE installation restrict a wider application of the GCHP technology in buildings with imbalanced loads. Therefore, in order to deal with this problem and keep the high performance of GSHP, hybrid systems are suggested. Incorporating a supplemental heat rejecter/absorber can reduce a fair amount of heat rejected/extracted into/from the ground and then effectively balance the ground thermal loads, which can consequently reduce the first cost. When the GCHP systems are applied to heating-dominated buildings, the cooling tower is also used to meet the peak cooling needs of the building. In recent years, the hybrid systems have been paid a lot of attention in China by researchers. A new system, called the integrated soil cold storage and ground-source heat pump (ISCS&GSHP) system, was presented by Fan et al. (2007, 2008) and Yu et al. (2004, 2006, 2007, 2008). The ISCS&GSHP system was operated under the following three conditions: (a) storing cold energy into soil (charging) during off-peak period at night in summer; (b) providing air-conditioning by releasing the cold energy stored in soil (discharging) at daytime, where thawing and freezing took place periodically in the soil surrounding the GHE during summer air-conditioning period; (c) supplying heat to buildings in winter. Fig. 4 shows the principle of the ISCS&GSHP system. The ISCS&GSHP system is basically applicable to the cooling-dominated buildings. A mathematical model of the GHE considering the impact of coupled heat conduction and groundwater advection on the heat transfer between the GHE and its surrounding soil was developed in their studies. The operating performances of the GHE in an ISCS&GSHP system with a vertical dual-function GHE had been studied by simulation and were reported by them. Their simulation results were helpful for

ARTICLE IN PRESS W. Yang et al. / Energy Policy 38 (2010) 323–332

327

the design of a GHE buried in soil and widely used in GSHP systems or ISCS&GSHP systems. On the other hand, in the heating-dominated buildings, the GSHP system should be integrated with other heat source to keep heat balance. Among all possible candidates, the solar energy seems to be the most promising one. The theoretical and experimental studies, performed by some Chinese researchers (Bi et al., 2004; Yang et al., 2006a; Wang and Qi, 2008; Wang et al., 2009; Han et al., 2008), promote the design and application of the solar assisted ground-source heat pump systems.

Zhao et al. (2003c) developed a novel approach named temperature-matching method, which provides a direction in selecting high-performance working fluids for GSHPs. It was shown from the results that the mean COPs of binary and ternary mixtures adopted by them were 4.85 and 4.74, respectively, but that of pure refrigerants was 4.12 under the same ambient condition. And the novel approach to high-performance working fluids can be conveniently introduced into other working conditions.

2.3. Improvement in the GSHP systems’ performance

There are also some other scientific achievements that have been applied in China, which are listed as follows: the optimization of GSHP systems (Yang et al., 2002, 2006b), the recharging technology of GWHP system (Wu, 2004), the grouting materials (Chen et al., 2006a; Zhuang et al., 2004), and the control methods of GSHP systems (Xue and Shi, 2006). All these researches are beneficial to the application of GSHP systems.

2.3.2. Utilization of non-azeotropic refrigerants Since the characteristics of refrigerants have a great influence on the performance of the refrigeration system, an appropriate refrigerant is of vital importance to the GSHP. But in some instances when pure working fluid is not suitable, non-azeotropic refrigerants are suggested to be used in order to improve the performance of GSHP. The fact that some areas need district heating through heat pumps and at the same time recover residual heat from geothermal water presents a new working condition: feed water temperature of heat network 80 1C, return water temperature 65 1C; discarded geothermal water temperature 40 1C and its emission standard temperature below 30 1C. R123/R290 (50/50 by mass %) and R123/R290/R600a (40/50/10 by mass %) were the working fluids adopted by Zhao et al. (2002) and Zhao (2004) in order to meet the working conditions. The results in their researches showed that the non-azeotropic mixture working fluid was superior to R22 or R123. A ternary mixture of R124/R142b/R600a, named HTR01, was used by Liu et al. (2005) for moderately high temperature heat pumps. Their tests of material compatibility and oil miscibility showed that the mixture could be used with a R22 compressor in an HTR01 heat pump system. Their test also showed that the condenser outlet water temperature could reach and hold on about 90oC with a high coefficient of performance. %

3. Patents related to GSHP There has been increasing number of patent applications related to GSHP over the past two decades. From 1985 to 2007, there are 630 in total, in which 278 of them are issued patents, 157 of the total is under examination, the other 258 has been rejected (Xie, 2008). Fig. 5 shows the number of the patents applied from 1985 to 2007. As is shown in Fig. 5, the patent applications are in the rapid development phase from 1996 to 2003. Since 2003, the number of application patent is about 80 each year, which indicates that the GSHP in the maturity stage in China. Among all the 630 patents, 59% were about making use of water as heat source and sink, and 41% of the patents were related to GCHP system. As the application of SWHP and GWHP are restricted by environmental policy, water source, and recharge technology, the related number of patents is decreased in recent years. Meanwhile, the number of the patent related to the GCHP is increased. These patents are mainly used for air-conditioning and hot water. The percentages of the patents usage are shown is Fig. 6.

4. The application of GSHP systems in China The GSHP systems have already been widely used in developed countries. By 2005, the installed capacity in the world was

100 Number of patents related to GSHP

2.3.1. Influence of systematic parameters on the GSHP system performance and system performance evaluation The systematic parameters have great influence on the performance of GSHP. The relationships among the compressor frequency, the water flow rate and other important parameters such as coefficient of performance (COP), heat capacity and compressor power input have been investigated by some Chinese researchers. Zhao et al. (2003a, 2003b) investigated the influence of some systematic parameters, including the water flow rate inside the condenser and the compressor input frequency, on the GSHP performance. Their experiments were performed on a small-scale GSHP system, and their experimental results showed that the COPs of the GSHP system fluctuated greatly when the condenser water flow rate changed. Lin et al. (2007) investigated the influence of hot water temperature, power of circulating pump and environment temperature on the COPs of several types of hybrid GSHP systems. Reasonable running modes of the hybrid systems were suggested by them. On the other side, energy consumption, economy, and environmental benefit were the major indexes that were adopted by Chinese researchers to evaluate the performance of GSHP systems (Jiang et al., 2003; Zhu et al., 2005; Tyagi et al., 2008). The performance of GSHP system was usually compared with other HAVC systems, such as air source heat pumps and boiler systems. These efforts can help the public and Chinese government to realize the benefits of GSHP systems.

2.4. Other fields

Patent applications Issued patents 80

60

40

20

0 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Year Fig. 5. Number of patents related to GSHP in China. Note: data in Fig. 5 sources from Xie (2008).

ARTICLE IN PRESS 328

W. Yang et al. / Energy Policy 38 (2010) 323–332

1.6%

80 7.2%

5.8%

0.2%

Air conditioning Refrigeratory Heating Others Hot water

84.8%

Snow melt

Fig. 6. Application of the patents related to GSHP. Note: data in Fig. 6 sources from Xie, 2008.

15,384 MWt and the energy use was 87,503 TJ/year. Most of the above systems were installed in North America and Europe (John et al., 2005). In European countries such as Sweden and Switzerland, the GSHP systems are only used for space heating, and the GCHP are mainly installed with horizontal pipes. In North America and Europe, the GSHP systems are mainly used in residential buildings and low load commercial buildings, so the GSHP systems are mainly small sized. But in China, the GSHP systems are being installed not only for space heating in winter but also for space cooling in summer. Furthermore, the GCHP systems are mainly being installed with vertical pipes, and the large-sized projects are very popular.

Totle floor area (million m2)

0.4%

70 60 50 40 30 20 10 0 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Year

Fig. 7. Total floor area of GSHP projects from 1998 to 2007. Note: data in Fig. 7 sources from Xu (2008).

4.1. Development of the GSHP application in China The last decade of the 20th century was the first phase of the application of GSHP in China. During that period, policymakers and the public began to realize the advantages of GSHP through several engineering projects of GSHP. The office building of Shanghai Minhang Economic and Technological Development Zone, completed in October 1989, was the first project using the GCHP system in China. The heating and air-conditioning system covered about 4305 m2 floor area, and the heating and cooling load was 231 and 4532 KW, respectively. A closed vertical typed ground heat exchanger with 135 boreholes of 35 m in depth was constructed for the GCHP system in this project (Liu, 2007; Chen, 2007). In 1997, China and USA started technology cooperation in energy efficiency and GSHP was one of the most important fields. Then three demonstration projects were built in Beijing, Guangzhou and Hangzhou from the beginning of the 21st century, which promote the application of GSHP in China greatly (Gao et al., 2009; Ding et al., 2002). Since the beginning of the 21st century, with the establishment of financial incentive mechanism and corresponding supportive policies, the application of GSHP systems came to a rapid development phase. In this period, the quantity of the GSHP projects and the installed capacity had a rapid increase simultaneously. By 2005, the installed capacity in China reached 631 MWt and the annual energy use reached 6569 TJ/year (John et al., 2005; Zheng et al., 2005). Fig. 7 shows the total floor area of GSHP projects in China from 1998 to 2007 (Xu, 2008). 4.2. Features of the GSHP application in China Different types of GSHP systems are applied in different climate zones. The GWHP systems are widely used in Beijing, Liaoning, Hebei and Shandong Province which are mainly in cold climate zone. Meanwhile, the application of GCHP systems is mainly expanding in Beijing, Hubei and Jiangsu provinces which

Fig. 8. Projects distribution classified by provinces and climatic zones

mainly in ‘‘cold’’ and ‘‘hot summer & cold winter’’ climate zones. In addition, the SWHP systems are mainly used in the region where there are abundant groundwater and surface water resources. According to the statistic of 3859 GSHP projects investigated by ‘‘Construction and Design for Project’’ (Ma and Lv, 2006), GSHP systems are geographically distributed in almost all Chinese provinces. Fig. 8 shows the project distribution on different climatic zones of China. It can be seen that the GSHP projects are mainly built in cold zone which takes account for about 70%, while few GSHP projects are used in the temperate zone. On the other hand, the GSHP projects are mainly distributed in the middle and east part of China, where the population is larger and is relative richer than the west. The floor area and investment cost of these completed GSHP projects are shown in Figs. 9 and 10 (Lv et al., 2005). It can be concluded from these figures that most of the completed projects are small and medium-sized. The main possible reason is that before the phase of extensive application, projects are usually executed as the demonstration plants which are studied by scholars from universities and research institutions. The relevant

ARTICLE IN PRESS W. Yang et al. / Energy Policy 38 (2010) 323–332

operation and design data of these demonstration plants would then be collected to guide the design of following projects. With the gradually wide use of GSHP systems, more and more large projects are constructed. The Beijing Olympic Village is the most remarkable one, which adopted a sewage water heat pump system for its HVAC system. The sewage water heat pump system covered about 393,000 m2 floor area with investment of 117 million RMB. The project can save coal by 3600 tons and reduce CO2 emission by 8600 tons a year. Table 2 shows some typical GSHP projects in China. Since the gradually wide application of GSHP systems, some issues, such as the heat balance must be resolved properly. Accordingly, we should make use of GSHP systems based on different locations and climatic zones, and the hybrid systems are suggested to be used. Furthermore, it should be made sure that the groundwater and surface water, where the GWHP and SWHP systems are being used, should be kept away from pollution. 4.3. Geothermal energy companies There are many companies in China that specialize in GSHP systems production and installation. The scale of these companies

16% 42%

>50,000 [10,000, 50,000] <10,000 42%

Fig. 9. Percentage of the completed projects according to different floor area. Note: data in Fig. 9 sources from Lv et al. (2005).

17%

21%

>10 [5,10] <5 62% Fig. 10. Percentage of the completed projects according to different investment cost. Note: data in Fig. 10 sources from Lv et al. (2005).

329

ranges from 1 million RMB to more than 100 million RMB. However, most of the companies are middle- and small-sized, which indicates that the geothermal energy companies in China are in the early development phase.

5. The government policies for GSHP in China In China, the state encourages and supports the development and utilization of renewable energies including using GSHP. Renewable energy is promoted by the financial incentive policies and relevant codes or criterions. The major existing financial incentives include subsidies, tax-related incentives, custom duties, and pricing incentives, and the government is moving toward more comprehensive quantity- and price-based support mechanisms (NREA, 2004). Table 3 lists existing significant policy documents, which are related to GSHP technologies, promulgated by Chinese government. These documents indicated the government efforts on promoting the application of GSHP systems in China. ‘‘Main points of industry development plan in new and renewable energy resources from year 2000 to 2015’’ indicated that ‘‘China will actively promote the development and utilization of geothermal and ocean energies. Geothermal energy resources will be used rationally-promoted so as to meet environmental protection and water resource protection requirements. Geothermal energy will be used for heating, hot water supply, and geothermal heat pumps. The heat pump technology will be widely promoted for space heating and cooling in regions in need of space cooling in summer and space heating in winter. The target of annual geothermal energy utilization will be 4 metric tons carbon equivalent (Mtce) by 2010 and 12 Mtce by 2020.’’ The ‘‘Water-source heat pumps (GB/T19409-2003)’’ was promulgation and enforcement in 2003. The basic parameters, technical requirements, test methods, inspection rules of watersource heat pumps were introduced in the standard, and the mark, package, transportation, and stockpile requirements were also present. ‘‘Technical Code for Ground Source Heat Pump System (GB50366-2005)’’, has been promulgated and took effect on January 1, 2006. The code plays a significant role for the development of geothermal heat pump industry. The code is composed of eight parts, including general principles, technical terms, engineering exploitation, buried pipeline heat exchange systems, groundwater heat exchange systems, surface water heat

Table 2 Typical GSHP projects in China. Location

Province

Type

Beijing Friendship Hospital Qingdao Yingshengtai International Commercial Port Building and Chengyang Trade Center Beijing Haidian foreign language school Beijing Police Institute National Center for the Performing Arts Dalian Xinghai Business District (First-stage construction) Dragon Moon Bay Hotel Xiangtan Center Zone Yinfeng Tourist Zone Xikou Fenghua Nanjing Landsea International Block Beijing Yongyou software center Xi’an ‘‘Gate of the city’’ Office Building of Ningbo Yinzhou Municipal State Taxation Bureau

Beijing Shandong

GWHP GWHP

Beijing Beijing Beijing Liaoning Zhejiang Hunan Zhejiang Jiangsu Beijing Shanxi Zhejiang

GWHP GWHP GWHP SWHP SWHP SWHP SWHP GCHP Hybrid system Hybrid system Hybrid system

Floor area (m2)

Cooling load (Kw)

53,000 84,400

4200 6751

50,900 178,000 220,000 300,000 33,760 135,000 29,500 68,115 185,000 101,200 19,000

4581 16,000 10,325 25,680 2664 12,190 4534 2334 15,784 8582 2400

Heating load (Kw)

Investment cost (million RMB)

Annual operating cost (RMB/m2)

3800 4051

– 29.9

27.0 19.4

4717 15,000 11,340.6 19,560 2131 6953 3505 1828 13,391 5700 1600

– – – – – – 7.5 8.0 42.0 26.0 9.6

35.9 88.2 34.0 19.6 (44.1, 50.9) – 32.0 24.6 36.0

ARTICLE IN PRESS 330

W. Yang et al. / Energy Policy 38 (2010) 323–332

7.6%

Table 3 Existing significant policy documents related to GSHP technologies. No. 1 2 3 4 5 6 7 8 9 10 11 12 13

Title Energy Conservation law of People’s Republic of China Renewable Energy Law of People’s Republic of China Agenda of China 21st Century New and renewable energy resource development outline (1996–2010) Main points of industry development plan in new and renewable energy resources from year 2000 to 2015 Regulation of civil building energy saving (2005) Decision about strengthening energy efficiency tasks Opinions on promoting application of renewable energy in buildings A tentative management method of special funds for renewable energy development China’s national climate change program Medium and Long-Term Development Plan for Renewable Energy in China Technical code for ground source heat pump system Water-source heat pumps

exchange systems, indoor systems, equipment operation and trial running, etc (Gao et al., 2009). The Chinese government also implements the financial incentive policies, such as Special Fund System and Preferential Tax System to support the development and utilization of GSHP. In September 2006, ‘‘Opinions on promoting application of renewable energy in buildings’’ and ‘‘A tentative management method of special funds for renewable energy development’’ were promulgated by the Ministry of Construction cooperating with the Ministry of Finance. In the two documents, half of the 8 major technologies which are emphatically supported by the state have connections with GSHP. The four technologies are listed as follows:

21.4% GCHP

31.8%

GWHP SWHP 39.3%

Hybrid system

Fig. 11. Projects distribution according to different types. Note: data in Fig. 11 sources from Xu (2008).

15.78 million m2. Fig. 11 shows the distribution of 144 projects according to different types. Meanwhile, local governments of provinces and cities have also developed some financial incentive policies to stimulate the development and application of GSHP systems. ‘‘Guiding opinions on the development of heat pump systems’’, promulgated by Beijing city, prescribes that ‘‘Since July 1, 2006, 35 Yuan per square meter can be compensated as a policy subsidy if the ground (surface) water heat pump systems are adopted in the new projects, and 50 Yuan per square meter can be compensated if the ground-coupled heat pump or sewage water heat pump is adopted.’’ Now, it is proved that with the support of this policy, the application situation of GSHP systems has been greatly and effectively promoted in Beijing. Both the Chinese central and local governments have also taken a series of actions to promote the application and extension of GSHP systems. The applications of GSHP systems have been growing rapidly since the beginning of the 21 century with financial incentives and encouraging policies.

6. Prospects of GSHP technologies in China

 In the area where there is abundant groundwater and surface   

water resources, water-source heat pump is recommended to be used. In the coastal areas, sea water heat pump is recommended to be used. Ground-coupled heat pump is recommended to be used. Sewage heat pump is recommended to be used.

In the ‘‘A tentative management method of special funds for renewable energy development’’, Article Three indicated that ‘‘Special funds for Development are used to finance following activities: (i) scientific and technological research, standards formulation and demonstration projects for the development and utilization of renewable energy; (ii) localized manufacture of equipment and devices that facilitate the development and utilization of renewable energy resource.’’ And the special funds for Development shall be mainly used to support the development and utilization of renewable energy with huge potential and good prospects, such as petroleum substitutes, space heating & cooling and power generation (Article Five). Meanwhile, for the development and utilization of renewable energy resources in terms of building heating and cooling, emphasis will be placed on the popularization and application of solar energy and geothermal energy in buildings (Article Seven). In addition, Ways of using special funds for development include grants and loan with deducted interests (Article Seventeen). Since 2006, 212 demonstration engineering projects of renewable energy on building have been released by the Ministry of Construction and the Ministry of Finance, in which 144 are related to GSHP. The total floor area of the 144 projects is about

The GSHP technologies can be useful to adjust Chinese energy structure by remarkably reducing reliance on fossil fuels. With the support of Chinese government, the research work of the GSHP technologies is recognized as one focus issue in the HVAC field in China. Consequently, these technologies enjoyed a boom in China, and some achievements have been made in recent years, which promoted the application of GSHP. Though China has not yet set up a fully developed financial incentive system for GSHP, some laws, codes and regulations have been set up to promote the development of the GSHP systems. In addition, the advantages of GSHP technologies also evoked a strong attraction to the users. Therefore, as presented early, the application of GSHP systems will have an upward tendency at its primary stage in China. It is also believed that the GSHP systems will have broad prospects in China. On the other hand, the application of GSHP systems should be based on different locations and climatic zones. The GCHP systems are suggested to be mainly used in cold, cold winter & hot summer and temperate zones. If the heat extraction and heat rejection cannot be balanced, the hybrid systems are suggested to be used. In the southeast and coastal areas, where there is abundant groundwater and surface water resource, the SWHP are suggested to be used while in the cold and cold winter and hot summer zone, the GWHP are suggested to be used.

7. Conclusions and suggestions Implementing the GSHP systems is one of the promising efforts to reduce the fossil fuels consumption and CO2 emissions. The GSHP systems have been increasingly paid attention in China and

ARTICLE IN PRESS W. Yang et al. / Energy Policy 38 (2010) 323–332

the number of applications is growing rapidly in recent years. Achievements have been obtained in several aspects. The typical achievements in research are shown as following:

 A new quasi-three-dimensional model for vertical ground heat

 

exchangers (GHE) that accounted for the fluid axial convective heat transfer and thermal ‘‘short-circuiting’’ among U-tube legs was developed. Analytical solutions of the fluid temperature profiles along the borehole depth were also obtained. A new ground-source heat pump system which called pumping and recharging well (PRW) was first presented. A new hybrid system, called the integrated soil cold storage and ground-source heat pump (ISCS&GSHP) system, was developed.

Since the beginning of the 21st century, the application of GSHP systems has been growing rapidly. However, there are also some challenges. The following suggestions are provided in this paper in order to overcome these encumbrances and promote the market growth of GSHP in China.

 Further research should be conducted on the heat transfer theory 

    

of outdoor heat exchanger (OHE) to improve the calculation precision, and to improve the design precision of OHE; The optimization methods of GSHP systems should be consummated in order to improve the performances of GSHP systems. In addition, simple optimization methods are suggested to be developed for the engineers and technicians to use; Strengthen the technical cooperation on GSHP research, including domestic and international cooperations, to reduce repetitive work; Increase support for R&D on GSHP, including domestic and international partnerships; The application of GSHP systems should base on different location and climatic zones in China and the hybrid systems are suggested to be used; A fully developed financial incentive system should be established as soon as possible to promote the R&D and application of GSHP systems; Technical guidelines, contractor certifications, quality awards, etc. should be set into force to protect the industry and the consumers against poor quality and insufficient longevity of ground-source heat pump systems.

Acknowledgments The work of this paper is financially supported by the National Key Technologies R&D Program of China during the 11th Five-year Plan Period (no: 2006BAJ04B04, 2006BAJ04A05, and 2006BAJ04A13). References ASHRAE, 2003. Handbook—HVAC Applications. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA. Bi, Y.H., Chen, L.G., Wu, C.H., 2002. Ground heat exchanger temperature distribution analysis and experimental verification. Applied Thermal Engineering 22, 183–189. Bi, Y.H., Guo, T.W., Zhang, L., Chen, L.G., 2004. Solar and ground source heat-pump system. Applied Energy 78, 231–245. BP, 2008. BP Statistical Review of World Energy. /http://www.bp.com/productland ing.do?categoryId=6929&contentId=7044622S. Chen, W.C., Liu, Q.L., Jia, L.Q., Sun, L.L., Fang, Z.H., 2006a. Experimental research on high-performance backfill materials for ground heat exchangers. Heating Ventilating and Air Conditioning 36 (9), 1–6 (in Chinese). Chen, X., Zhang, G.Q., Peng, J.G., Lin, X.J., Liu, T.T., 2006b. The performance of an open-loop lake water heat pump system in south China. Applied Thermal Engineering 26, 2255–2261.

331

Chen, Xiao, Zhang, Guoqiang, Peng, Jianguo, et al., 2006c. Utilization of artificial lake as heat source-sink for open-loop surface water heat pump system in hunan. Journal of Refrigeration 27 (3), 10–13 (in Chinese). Chen, Y.H., 2007. The application of GSHP system in Wuhan. Construction Conserves Energy 3, 40–43 (in Chinese). Crompton, P., Wu, Y., 2005. Energy consumption in China: past trends and future directions. Energy Economics 27, 195–208. Cui, P., Yang, H.X., Fang, Z.H., 2006. Heat transfer analysis of ground heat exchangers with inclined boreholes. Applied Thermal Engineering 26, 1169–1175. Diao, N.R., Li, Q.Y., Fang, Z.H., 2004. Heat transfer in ground heat exchangers with groundwater advection. International Journal of Thermal Sciences 43, 1203–1211. Diao, N.R., Fang, Z.H., 2006. Ground-coupled heat pump technology, first ed China Higher Education Press, Beijing. Ding, L.X., Chen, J.F., Guo, H., 2002. Advancement prospects of GSHP air conditioning system in HSCW zone of China. In: Proceedings of the Seventh International Energy Agency Conference on Heat Pump Technologies, Beijing, China, pp. 1054–1064. Document of the World Bank, 2005. Project document on a proposed global environment facility (GEF) grant of US$18 million to the PRC for the heat reform and building energy efficiency project. Report no. 20747-cn, pp. 1–9. Fan, R., Jiang, Y.Q., Yao, Y., Deng, S.M., Ma, Z.L., 2007. A study on the performance of a geothermal heat exchanger under coupled heat conduction and groundwater advection. Energy 32, 2199–2209. Fan, R., Jiang, Y.Q., Yao, Y., Ma, Z.L., 2008. Theoretical study on the performance of an integrated ground-source heat pump system in a whole year. Energy 33, 1671–1679. Gao, J., Zhang, X., Liu, J., Li, K.S., Yang, Jie, 2008. Thermal performance and ground temperature of vertical pile-foundation heat exchangers: a case study. Applied Thermal Engineering 28, 2295–2304. Gao, Q., Li, M., Yu, M., et al., 2009. Review of development from GSHP to UTES in China and other countries. Renewable and Sustainable Energy Reviews 13, 1383–1394. Han, Z.W., Zheng, M.Y., Kong, F.H., et al., 2008. Numerical simulation of solar assisted ground-source heat pump heating system with latent heat energy storage in severely cold area. Applied Thermal Engineering 28, 1427–1436. Jiang, B.C., Wang, Y.B., Li, B.X., 2003. Economic evaluation of ground source heat pump. Journal of Harbin Institute of Technology 35 (2), 195–202 (in Chinese). John, W. Lunda, Derek, H. Freeston, Tonya, L. Boyd, 2005. Direct application of geothermal energy: 2005 worldwide review. Geothermics 34, 691–727. Li, M., Diao, N.R., Fang, Z.H., 2006a. Analytical solution of seepage flow in a confined aquifer with a standing column well. Journal of Shandong Jianzhu University 21 (1), 1–5 (in Chinese). Li, M., Diao, N.R., Fang, Z.H., 2007a. Analysis of seepage flow in a confined aquifer with a standing column well. Journal of Hydrodynamics 19 (1), 84–91. Li, M., Diao, N.R., Fang, Z.H., 2007b. Study on numerical heat transfer models of standing column well. Acta Energiae Solaris Sinica 28 (12), 1394–1400 (in Chinese). Lin, J., Hu, Y.N., Li, Z.J., et al., 2007. Study on compound ground source heat pump system. Acta Energiae Solaris Sinica 23 (6), 687–691 (in Chinese). Liu, G.L., 2007. Discussions of the design and application of ground source heat pump system. Guangxi Journal of Light Industry 1 57–58,65 (in Chinese). Liu, Hui, Diao, Nairen, Fang, Zhaohong, 2008a. Theoretical flow model and analysis of the underground heat exchanger in standing column well. Journal of Shandong Jianzhu University 28 (11), 1206–1212 (in Chinese). Liu, N.X., Lin, S., Han, L.Z., Zhu, M.S., 2005. Moderately high temperature water source heat-pumps using a near-azeotropic refrigerant mixture. Applied Energy 80, 435–447. Liu, Yang, Joseph, C. Lam, Tsang, C.L., 2008b. Energy performance of building envelopes in different climate zones in China. Applied Energy 85, 800–817. Li, X.G., Chen, Y., Chen, Z.H., et al., 2006b. Thermal performances of different types of underground. Heat exchangers. Energy and Buildings 38, 543–547. Li, X.G., Chen, Z.H., Zhao, J., 2006c. Simulation and experiment on the thermal performance of U-vertical ground coupled heat exchanger. Applied Thermal Engineering 26, 1564–1571. Li, X.G., Zhao, J., Zhou, Q., 2005. Inner heat source model with heat and moisture transfer in soil around the underground heat exchanger. Applied Thermal Engineering 25, 1565–1577. Li, Z.S., Zhang, G.Q., Li, D.M., Zhou, Jin, Li, L.J., Li, L.X., 2007c. Application and development of solar energy in building industry and its prospects in China. Energy Policy 35, 4121–4127. Li, Zhen, Lin, D.M., Shu, H.W., et al., 2007d. District cooling and heating with seawater as heat source and sink in Dalian, China. Renewable Energy 32, 2603–2616. Lv Haijun, 2008. A device for pumping and recharging well [P] (in Chinese). Lv, Y., Yang, L.P., Zhou, M., Mo, R., 2005. The ground source hot pump uses the survey report of the situation domestically. Construction and Design for Project 6, 5–10 (in Chinese). Ma, Z.L., Lv, Y., 2006. The Design and Application of GSHP Systems, first ed China Machine Press, Beijing. Muhammad, H.K., 2004. Modeling, simulation and optimization of ground source heat pump systems. Doctoral Thesis, Bachelor of Science in Mechanical Engineering University of Engineering and Technology, Pakistan. National Bureau of Statistics of China, 2009. China Statistic Yearbook-2008, first ed China Statistic Press, Beijing.

ARTICLE IN PRESS 332

W. Yang et al. / Energy Policy 38 (2010) 323–332

National Standard of the People’s Republic of China, 1993. Thermal Design Code for Civil Building, GB50173-93. China Plan Press. Ni, L., Ma, Z.L., 2005. Effect of heating load on groundwater source heat pump system with pumping and recharging in same well. Heating Ventilating and Air Conditioning 35 (3) 12–14, 23 (in Chinese). Ni, L., Ma, Z.L., Sun, L.Y., 2006a. Thermal characteristics of groundwater heat pump with pumping and recharging in the same well. Journal of Harbin Engineering University 27 (2), 195–222 (in Chinese). Ni, L., Ma, Z.L., 2006b. Effect of aquifer parameters on groundwater heat pump with pumping and recharging in the same well. Journal of Tianjin University 39 (2), 229–234 (in Chinese). Ni, L., Ma, Z.L., 2006c. Study on the seepage theory for groundwater heat pump system with pumping and recharging in the same well. Acta Energiae Solaris Sinica 27 (12), 1219–1224 (in Chinese). NREA, 2004. Renewable Energy Policy in China: Financial Incentives. /http:// www.nrel.gov/docs/fy04osti/36045.pdfsS. Qu, Y.X., 2004. Modeling and simulation for ground source heat pumps system. Doctoral Thesis, School of Environmental and Municipal Engineering, Xi An University of Architecture and Technology, China (in Chinese). Tyagi, S.K., Wang, S.W., Park, S.R., Atul, Sharma, 2008. Economic considerations and cost comparisons between the heat pumps and solar collectors for the application of plume control from wet cooling towers of commercial buildings. Renewable and Sustainable Energy Reviews 12, 2194–2210. Wang, H.J., Qi, C.Y., 2008. Performance study of underground thermal storage in a solar–ground coupled heat pump system for residential buildings. Energy and Buildings 40, 1278–1286. Wang, H.J., Qi, C.Y., Wang, E.J., et al., 2009. A case study of underground thermal storage in a solar–ground coupled heat pump system for residential buildings. Renewable Energy 34, 307–314. Wu, X.B., 2004. Development of ground water loop for ATES and ground water source heat pump systems. Heating Ventilating and Air Conditioning 34 (1), 19–22 (in Chinese). Xie, X.Y., 2008. Analysis of GSHP industries & technologies development. China Invention and Patent 2, 40–43 (in Chinese). Xue, Z.F., Shi, L., 2006. Investigation on optimal control of economical operation of water source heat pump. Fluid Machinery 34 (6), 46–50 (in Chinese). Xu shengheng, 2002. A pumping and recharging well for heat source and sink [P] (in Chinese). Xu, Wei, 2008. Report on China Ground-Source Heat Pump, first ed China Architecture & Building Press, Beijing. Yao, R., Li, B., Steemers, K., 2005. Energy policy and standard for built environment in China. Renew Energy 30, 1973–1988. Yang, W.B., Shi, M.H., Dong, H., 2006a. Numerical simulation of the performance of a solar–earth source heat pump system. Applied Thermal Engineering 26, 2367–2376. Yang, Z., Zhang, S.G., Sun, Z., et al., 2002. Optimization of a ground water-source heat pump. Acta Energiae Solaris Sinica 23 (6), 687–691 (in Chinese).

Yang, Z., Zhao, H.B., Hu, Y.F., Wu, K., 2006b. Research on annual performance factor of water–water heat pump system. Journal of Refrigeration 27 (3), 24–29 (in Chinese). Yu, Y.S., Ma, Z.L., Yao, Y., 2004. Simulative research on soil thermal storage in integrated soil thermal storage and ground-coupled heat pump system. Acta Energiae Solaris Sinica 25 (6), 820–825 (in Chinese). Yu, Y.S., Ma, Z.L., Yao, Y., et al., 2006. Simulation and experimental validation of soil cool thermal charging and discharging process. Acta Energiae Solaris Sinica 27 (10), 1063–1068 (in Chinese). Yu, Y.S., Ma, Z.L., Yao, Y., et al., 2007. Energy analysis of cool storage and discharging process in soil. Acta Energiae Solaris Sinica 27 (10), 1407–1412 (in Chinese). Yu, Y.S., Ma, Z.L., Li, X.T., 2008. A new integrated system with cooling storage in soil and ground-coupled heat pump. Applied Thermal Engineering 28, 1450–1462. Zeng, H.Y., Diao, N.R., Fang, Z.H., 2002. A finite line-source model for boreholes in geothermal heat exchangers. Heat Transfer Asian Research 31 (7), 558–567. Zeng, H.Y., Diao, N.R., Fang, Z.H., 2003. Heat transfer analysis of boreholes in vertical ground heat exchangers. International Journal of Heat and Mass Transfer 46, 4467–4481. Zhao, J., Wang, H.J., Li, X.G., Dai, C.S., 2007. Experimental investigation and theoretical model of heat transfer of saturated soil around coaxial ground coupled heat exchanger. Applied Thermal Engineering 28, 116–125. Zhao, L., 2004. Experimental evaluation of a non-azeotropic working fluid for geothermal heat pump system. Energy Conversion and Management 45, 1369–1378. Zhao, L., Zhang, T.J., Zhang, Q., Ding, G.L., 2003a. Influence of two systematic parameters on the geothermal heat pump system operation. Renewable Energy 28, 35–43. Zhao, L., Zhao, L.L., Zhang, Q., Ding, G.L., 2003b. Theoretical and basic experimental analysis on load adjustment of geothermal heat pump systems. Energy Conversion and Management 44, 1–9. Zhao, P.C., Zhao, L., Ding, G.L., Zhang, C.L., 2002. Experimental research on geothermal heat pump system with non-azeotropic working fluids. Applied Thermal Engineering 22, 1749–1761. Zhao, P.C., Zhao, L., Ding, G.L., Zhang, C.L., 2003c. Temperature matching method of selecting working fluids for geothermal heat pumps. Applied Thermal Engineering 23, 179–195. Zheng, K.Y., Zhang, Z.G., Zhu, H.Z., Liu, S.B., 2005. Process and prospects of industrialized development of geothermal resources in China – country update report for 2000 – 2004. In: Proceedings of WGC2005. Zhuang, Y.C., Sun, Y.H., Xie, K.H., 2004. Study on performance of backfilled soil for vertical-buried closed-loop GSHP. Acta Energiae Solaris Sinica 25 (2), 216–220 (in Chinese). Zhu, Y., Yang, L., Li, Z.L., 2005. Energy saving and techno-economic analysis of ground-source heat pump. Gas and Heat 25 (3), 73–76 (in Chinese).