J. Cent. South Univ. (2012) 19: 664-668

DOI: 10.1007/s11771-012-1054-x

Case study of underground pipe ground coupled heat pump system

ZHENG Min(郑敏)¹, LI Bai-yi(李百毅)², QIAO Zheng-yong(乔振勇)³

1. Faculty of Urban Construction and Environment, Chongqing University, Chongqing 400030, China;

2. Faculty of Architecture, Southwest Jiaotong University, Chengdu 610031, China;

3. Architecture Science Institute of Sichuan Province, Chengdu 610000, China

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

Abstract: Aiming to give some advices on the ground coupled heat pump system design in Sichuan Province, China, a typical ground source heat pump (GSHP) system in Sichuan Province was tested in a whole operational year, and the parameters of temperature and flow rate in different parts of system were measured during this period. The seasonal energy efficiency ratio was calculated and the performance of heat pump system in summer was compared with that in winter. The result shows that the coefficient of performance of the system reaches 3.63 in summer and 3.49 in winter, respectively. The heat balance in underground rock mass is acquired basically throughout the year, and the heat accumulation in the earth tends to be zero.

Key words: ground coupled heat pump; operational test; parameters measurement

1 Introduction

In recent years, ground source heat pump (GSHP) system has obtained increasing attractions in cooling and heating buildings. In China, more than 110×10⁶ m² buildings applied this technology [1]. The wide application motivated researches on ground source heat pump [2-4].

Ground coupled heat pump (GCHP), a kind of heat pump, recognized by U.S. Environmental Protection Agency, considers ground as the heat source because the temperature of the ground becomes relatively constant with depth, which is closer than the ambient air to the temperature desired for human comfort during the course of the year. Heat exchangers with horizontally installed pipes can be placed 1-1.5 m below ground horizontally or pipes with various shapes can also be placed into vertical position by boreholes which can be up to 200 m in depth [5-7].

In fact, the ground temperature would change when GCHP system works. In winter, heat pumps extract heat from the ground to elevate temperature and supply heat to buildings, while the temperature of soil or rock around the underground pipe decreases. In summer, heat pumps extract the heat from the building and discharge to ground, and surrounding temperature of underground pipe increases. The endothermic or exothermic process of ground heat exchanger in ground will change the initial temperature of ground. The surrounding ground temperature not only changes with the extension of space, but also with time. Therefore, heat transfer process in ground is typical of non-steady-state heat transfer process. For a given designing situation, the transient effect of heat transfer between the surrounding and underground pipe greatly affects the performance of entire air conditioning system. Thus, the performance of heat exchanger in the ground is the core of ground coupled heat pump technology.

A number of investigations have been conducted in designing and testing of ground source heat pump. HEALY and UGURSAL [8] investigated the effect of various system parameters on GSHP system using a computer model. INALLI and ESEN [9] evaluated the effects of parameters, such as the buried depth of earth coupled heat exchanger. PULAT et al [10] investigated the performance of the horizontal closed-loop water to air ground source heat pump system including the effect of various parameters, such as leaving temperature of ground heat exchanger unit and outdoor temperature for winter climate. In addition, extensive research efforts including field tests and theoretical analysis have been applied to estimate ground properties [11-16].

In China, more attentions have been paid to the development of heat pump. However, the studies, in operation and effect of ground coupled heat pump, are still unsatisfied [17]. It is important that the properties of geologic structure in different districts have significant influence on the performance of heat exchanger of heat pump.

In this work, the performance evaluation of a GCHP system with a vertical ground heat exchanger is conducted in Sichuan Province, China. The objective is to give some advice on the ground source heat pump system design in this district, and provide a theoretical support for local government to push and develop the application of ground coupled heat pump.

2 Field test of project

This project was located in City of Chengdu, Sichuan Province, China, with a total construction area of 3.8×10⁴ m². It was comprised of a total of 200 sets of high-class apartments with two floors. Ground coupled heat pump system was used for resident to provide central air conditioning and hot water. U-tube of GCHP system was buried under the underground garage. Operation room was located in the underground garage equipment room. Each room was equipped with individually controlled fan coil system. Each household was installed with a billing system, which can be used to calculate the cold and heat energy for household use and household charges. Ground coupled heat pump was equipped with heat recovery units using waste condensing heat of chillers in summer to get free living hot water. This project was completed in 2005. The main related technical parameters are listed in Table 1.

Table 1 Main technical parameters of system

The test of system was carried out in the period of May 2007 to August 2007, and December 2007 to March 2008, including a whole heating and cooling season throughout the year. Operational data of GCHP system was acquired in real-time, such as the water temperature of inlet and outlet of heat exchanger, flow rate, the inlet and outlet water temperature of end-use of air-conditioning system, flow rate, ground temperature and indoor temperature, outdoor air temperature. Electricity consumptions of water pumps and end users were measured to get total electricity consumption of the system for economic analysis.

3 Result and analysis

Calculated by the testing data, the main technical parameters would be acquired, such as coefficient of performance (COP) of GCHP system, heating capacity, heating capacity of the pipe unit and pipe, and the changes of ground temperature.

3. 1 Operation condition in summer

Ground coupled heat pump systems take the underground rock and soil as cold/heat source, discharge heat to it in summer and extract the heat from the source in winter for supply heat for end user. If the heat balance of discharging and extracting could not be achieved throughout the year, the temperature of ground will change, which will influence the GCHP performance greatly. Therefore, the observation of ground temperature change is also extremely important. The average temperature of ground could be acquired by thermal resistance buried in the underground and the outside air temperature could be gotten from meteorological data (daily average), as shown in Fig. 1.

Fig. 1 Underground rock and soil temperature and outside air temperature

As shown in Fig. 1, underground rock and soil temperature in summer during the running time presented an increasing trend, and the maximum and minimum values were 23.4 °C and 17.9 °C, within the normal range. It can also be found that the underground rock and soil temperature was not associated with the outdoor air temperature (daily average), which indicates that the temperature of rock and soil was mainly affected by the heat effect on the rock and soil. The longer the running time, the more the accumulated heat and the rock and soil temperature increases. But the range of rock and soil temperature was insignificantly affected by the outdoor temperature.

Figure 2 shows that during the test phase in summer the supply and return water temperature of user side and pipe side fluctuated within the normal range. The return water temperature of pipe was at 30.4 °C, and the water temperature of the user side was 10.28 °C. The average temperature difference between supply and return water temperature of underground pipe and the user side were 2.31 °C and 2.57 °C, respectively, and the    fluctuation was very small. It can be seen in Fig. 3 that, the higher the air temperature in summer, the larger the unit depth heat rejection, and the more the cooling capacity supply to the users. The maximum and minimum values were 92 W and 35 W, respectively, with the average of 58 W.

Fig. 2 Supply and return water temperature variation of end-user (a) and buried pipe (b) in summer (1-Return water temperature; 2-Supply water temperature; 3-Outside air temperature)

Fig. 3 Heat discharge capacity of unit hole depth in summer

It can be seen in Fig. 4 that the COP of heat pump and system fluctuated largely. This is because the COP is of a certain relationship with outdoor temperatures, the occupancy rate and utilization of air conditioning at the end. After calculation, the average COP of heat pump during the summer reaches 4.68, and the system average COP is 3.63.

Fig. 4 COP variation of heat pump and system

3. 2 Operation condition in winter

Figure 5 indicates that there was no abnormal data in the temperature of the earth throughout the test period. The maximum and minimum values are 14.3 °C and 10.1 °C, respectively. It can also be found that the underground rock and soil temperature had the same trends with the outdoor temperature basically, because the lower the air temperature was, the more the heat required for, and the more heat extracted from rock and soil, the lower rock soil temperature would be.

Fig. 5 Variation of underground temperature and outside air temperature

As shown in Fig. 6, during testing phase in winter, supply and return water temperature of end user fluctuated within the normal range. The temperature of outlet water of underground pipe reaches 30.4 °C, supply water temperature on the user side in winter is 38.3 °C. The average temperature difference between supply and return water of underground pipe and the user side are 1.67 °C and 2.08 °C, respectively. At the same time, it can also be seen from Fig. 7, in some section, the lower the air temperature in winter, the greater the temperature difference between supply and return water of underground pipe side and the end user side, and the greater the exchange heat under a given flow rate, which could meet the needs of users. The lower the outside air temperature, the greater the unit hole depth absorbing from earth. The maximum and minimum values are 71 W and 24 W, respectively, with the average of 51 W.

Fig. 6 Supply and return water temperature variation of end-user (a) and buried pipe (b) in winter

Fig. 7 Heat discharge capacity of unit hole depth in winter

It can be seen in Fig. 8, during the winter, the average COP of heat pump is 4.42, and the average COP of system is 3.49. In some sections, the views which put forward by some researchers [7] could be verified that the higher the outside air temperature, the lower the COP of the system and heat pump. In partial, the phenomenon was inconsistent with the law mentioned above, the reason for which was that COP was of a certain relationship with the occupancy rate and end user usage.

Fig. 8 COP variation of heatpump and system in winter

4 Analysis of operational fee

By the sample survey of indoor environment, it could be found that the design requirements could be met under normal circumstances. In a few extreme weather (hot or cool) when the indoor temperature will be on the high side or on the low side, the main reason was due to insufficient underground pipe for use, which was rooted from the deficiency of exhibition area for pipe.

The electric power consumption reached 4.27×10⁵ kW·h in summer, according to the unit electrical fee in average 0.75 RMB Yuan/(kW·h), and electricity fee reached 3.2×10⁵ RMB Yuan, equivalent to 0.09 RMB Yuan/(m²·d^-1). The total power consumption in winter was 5.14×10⁵ kW·h, according to the same unit fee and the electricity fee reached at 3.86×10⁵ RMB Yuan, equivalent to 0.103 RMB Yuan/(m²·d^-1). Obviously, the cost was lower than the centralized heating and cooling system.

5 Conclusions

1) The performance of vertical ground coupled heat pump system was investigated experimentally. The experimental results indicate that the heat pump COP and COP　of all system are approximately 4.42 and 3.49, respectively in winter, while 4.68 and 3.63, respectively, in summer, nearly 40%, larger than air-source heat pump.

2) Underground rock mass temperature in the summer tends to increase, and the maximum temperature is about 5 °C greater than the undisturbed area. Temperature tends to decrease in winter, and the minimum value is bout 4 °C lower than the undisturbed areas. This indicates that the accumulation of heat is not obvious because the summer heat discharge and winter heat absorption were balanced for underground rock mass. In order to balance the heat accumulation in ground, the intermittent operation mode is suggested in ground coupled heat pump system.

References

[1] XU Wei. Report on China ground source heat pump [M]. Beijing: China Architecture and Building Press, 2008: 33-34. (in Chinese)

[2] GAO Qing. Underground thermal energy storage and its effect on ground source heat pump system [J]. Journal of Jilin University: Engineering and Technology Edition, 2006, 36(4): 497-501. (in Chinese)

[3] XIE Xiao-na, LIU Xiao-hua, JIANG Yi. Design and analysis of the annual running of ground source heat pump system [J]. Acta Energy Solaris Sinica, 2008, 29(10): 1218-1224. (in Chinese)

[4] CHENG Heng-Sheng, GONG Wei-Sheng, LIU Xiang-Long. The utilization of ground-source heat pump system in east China [J]. Journal of Hunan University: Natural Sciences, 2009, 36(2): 27-30. (in Chinese)

[5] ESEN H, INALLI M, ESEN M. Numerical and experimental analysis of a horizontal ground-coupled heat pump system [J]. Building and Environment, 2007, 42(10): 1126-1134.

[6] ESEN H, INALLIB M, ESEN M, SENGUR A. Energy and exergy analysis of a ground-coupled heat pump system with two horizontal ground heat exchangers [J]. Building and Environment, 2007, 42(10): 3606-3615.

[7] HEPBASLI A, BALTA M T. A study on modelling and performance assessment of a heat pump system for utilizing low temperature geothermal resource in buildings [J]. Building and Environment, 2007, 42(10): 3747-3756.

[8] HEALY P P, UGURSAL V L. Performance and economic feasibility of ground coupled heat pumps in cold climate [J]. International Journal of Energy Research, 1997, 21(10): 857-70.

[9] INALLI M, ESEN H. Experimental thermal performance evaluation on of a horizontal ground source heat pump system [J]. Applied Thermal Engineering, 2004, 24(14/15): 2219-2232.

[10] PULAT P, COSKUN S, UNLU K, YAMANKARADENIZ A. Experimental study of horizontal ground source heat pump performance for mild climate in Turkey [J]. Energy, 2009, 34(9): 1284-1295.

[11] TATSUAKI K. Heat conduction model of saturated soil and estimation of thermal conductivity of soil solid phase [J]. Soil Science, 1984, 138(3): 240-247.

[12] GOUAL M S, BALI A, QUENEUDEC M. Effective thermal conductivity of clayey aerated concrete in the dry state: Experimental results and modeling [J]. Applied Physics, 1999, 32(33): 3041-3046.

[13] ABUHAMDEH N H, RANDALL C R. Soil thermal conductivity: effects of density. Moisture, salt concentration, and organic matter [J]. Soil Science Society of America Journal, 2000, 64(4): 1285-1290.

[14] CHAROENVISAL K. Energy performance and economic evaluations of the geothermal heat pump system used in the knowledge works I and II buildings [D]. Blacksburg, Virginia: Virginia Polytechnic Institute and State University, 2008.

[15] AUSTIN W A, YAVUZTURK C, SPITLER J D. Development of an in-situ system for measuring ground thermal properties [J]. Ashare Transactions, 2000, 106(1): 365-379.

[16] THEIS C V. The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well, using groundwater storage [J]. Transaction of American Geophysical Union, 1935, 2(1): 518-524.

[17] WANG Hong-zhong. Current situation of the development and prospect of ground source heat pump system in Chengdu area [J]. Sichuan Architecture, 2007, 27(6): 64-65. ( in Chinese)

(Edited by DENG Lü-Xiang)

Foundation item: Project(50838009) supported by the National Science Key Foundation 1 tem

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

Corresponding author: ZHENG Min, PhD; Tel: +86-13281061685; E-mail: zmiamhere@yahoo.com.cn