Measurement of COPs for Ground Source Heat Pump (GSHP) System in Heating and Cooling Mode

Transcription

1 Measurement of COPs for Ground Source Heat Pump (GSHP) System in Heating and Cooling Mode Jae-Han Lim 1 and Seung-Gi Won 2 1 Cheong-Ju University, Cheong-Ju, Korea 2 ES Co. Ltd, Cheong-Ju, Korea Corresponding limit0@cju.ac.kr ABSTRACT Today, ground source heat pump (GSHP) systems have been used for one of the most growing applications of renewable energy in Korea. A GSHP system has three major components such as a heat pump, a ground heat exchanger and an interior heat distribution system. These components are all affecting the efficiency of a GSHP system, the Coefficient of Performance (COP). This study aims at measuring the COPs for GSHP system in heating and cooling mode. We have measured electric power consumption of heat pump and all pumps in the GSHP system, and also measured the water flow rates through heat distribution circuits and water temperature differences in the heat distribution circuits. In conclusion, the overall system COPs in cooling mode range from 2.9 to 3.3 and those in heating mode range from 2.85 to 3.3. Comparing the COPs of an existing air-source heat pump system, which has a COP of around 2, we could expect GSHP systems are more energy-efficient than current heating and cooling system, i.e. ASHPs or high-efficiency gas-fired boilers. However, in comparison with other alternatives, the total costs of the energy consumed during the building operation and the energy rate of gas and electricity should be considered together. INTRODUCTION For many years, ground source heat pump (GSHP) systems have been widely used in many countries of the world, including Turkey, India, North America and Europe. In Korea, due to the Promotion Law of the New and Renewable Energy Development, Use and Dissemination, which was enacted in 2004 and imposed an obligatory installation of space heating and cooling systems using new and renewable energy sources including geothermal energy for newly constructed public buildings, GSHP systems have spread quickly up to about 60 % of the total public installation of new and renewable energy equipment between 2004 and 2007 [1]. Starting with 35.2 kw of two buildings in 2000, the total system capacity has been MW installed in 551 buildings as of August A GSHP system uses the thermal energy of the ground or groundwater as the heat source and heat sink for space heating and cooling. Typically they cost more to install than conventional systems, however they are expected very low energy costs and can also provide reliable and environmentally friendly heating and cooling with buildings [2]. GSHPs typically were known to have higher efficiencies than airsource heat pumps (ASHP). This is because they extract heat from the ground or groundwater which is about 10 to 15 ºC at a relatively constant temperature all times of the year. The GSHP system mainly comprises a heat pump, ground heat exchanger and an interior heat distribution system. These components are all affecting the efficiency of a GSHP system, the Coefficient of Performance (COP). According to the ASHRAE s classification of GSHP system, they could be subdivided into ground-coupled heat pumps (GCHPs), groundwater

2 heat pumps (GWHPs) and surface water heat pumps (SWHPs) with relation to ground heat exchanger. GCHP system is the closed loop system, and GWHP system is the open loop system. Also as shown in Figure 1, GWHP systems in an semi-open loop arrangement are commonly known as standing column well (SCW) systems. SCW systems use groundwater circulated from wells as a heat sink or source. The ground heat exchanger in these systems consists of a vertical borehole that is filled with groundwater up to the level of the water table. And water is circulated from the well using the submersible pump in an open-loop pipe circuit. A large proportion of water is returned to the well. Compared with other GSHP systems, shorter borehole depths and more stable water temperatures make the SCW system an attractive commercial and industrial design approach [5]. Among the installed GSHPs between 2000 and August 2008 in Korea, closed loop system (GCHP) and open loop system (GWHP), including the semi-open loop system (SCW) occupy about 68.6% and 30.5% respectively [1]. Especially, SCW systems have recently received more attention in Korea because the re-injection of pumped groundwater minimized the amount of extracted groundwater and there generally is competent rock below a few meters of subsurface soils. So this study focuses on the performance evaluation on SCW system. This study aims to assess energy performance of GSHP system through the measurement of COPs in heating and cooling mode. In this work, current researches about GSHP system were reviewed firstly, and after the installation of GSHPs using SCW system in Kwang-Yang city which is located in the southern area of Korea, the heating and cooling capacity and the electrical power input including heat pump, circulation pumps and submersible pump were evaluated to determine the heating and cooling performance of the GSHP system. Figure 1. Schematic diagram of GWHP system using SCW (Standing column well) method. LITERATURE REVIEWS REGARDING PERFORMANCE OF GSHPs GSHPs are attractive alternatives to conventional heating and cooling systems owing to their higher energy efficiency. These systems can be an efficient means of saving money and saving carbon emissions if carefully designed for space heating of an appropriately designed building. The efficiency of a GSHP is evaluated by the Coefficient of Performance (COP). In spite of the first law of thermodynamics, which tells us that every kinds of energy can neither be created nor destroyed, GSHPs in a good installation can yield up to about four units of heat for each unit of electricity consumed. These systems are not creating heat energy, but merely transferring the heat energy from the ground into warmth for heating and vice versa for cooling. The COP can vary with each installation, but the lower the output temperature to the

3 heat distribution system the higher the COP will be. If an output temperature of 60 C is needed to heat radiators, the COP could be likely to fall to level of only about 2.5. If the heat distribution is to a well designed radiant floor heating system that works well at an output temperature of 40 C then the COP could rise to a level of 4 or higher. The input temperature from the ground heat exchanger is also critical to the COP of the heat pump. The higher the input temperature from the ground, the lower the amount of work needed from the heat pump, the higher the COP will be obtained. Although a heat pump can be more efficient than conventional heating and cooling systems, the COP is critical because electricity is more expensive than gas in Korea. If the system do not get a high COP from heat pump, it could be cheaper to use a conventional system for heating and cooling. So many researchers have reported the performance analysis of GSHP system. In Korea, Hwang et al. [4] reported that the COP hp and COP overall of GSHP with ground heat exchanger of the vertical closed type installed in a school building were about 8.3 and 5.9 respectively. In USA, O Neill et al. [5] presented the thermal and economic performance of standing column wells based on the computer simulation technique. Also in Turkey, Hepbasli et al. [6,8] conducted the experimental study on the COPs of GSHP, but these values were extremely low compared to other heat pumps operating under conditions at or near design values due to the parameters such as GSHP size and capacity, depth, spacing and pipe size of ground heat exchanger below grade, and heat transfer fluid. Another researches in Turkey showed the COPs of the two different cases were obtained to be about 2.7 and 2.0 respectively [7,9]. In cold climate, Healy et al. [10] reported that the performance of the GSHP system can also be influenced by many parameters. Thus, they concluded it would be necessary to conduct a pre-design analysis to determine optimal system parameters that would ensure minimum energy consumption and favorable economics. Basically, all of these previous researches evaluated the performance of GSHP system based on laboratory scale measurements or simulation techniques because the evaluation on the COP was related to the appropriate system design and economics. As a fundamental research about performance of GSHP using SCW system, this study aims at assessing the COPs in heating and cooling operations. EXPERIMENTAL SETUP AND COP CALCULATION In this study, to evaluate the performance of GSHPs using SCW system, it was installed in the business center in Kwang-Yang city in Korea. The building had totally twenty floors and total floor area of 18,000m 2. In this building, there were several commercial and institutional offices, educational facilities, banking facilities, a restaurant, multi-purpose hall and meeting room, etc. Figure 2 describes the schematic diagram of the GSHP system for this experimental setup installed at Kwang-Yang city. Briefly, the GSHP system consisted of a water-to-water heat pump unit, the ground heat exchangers using two submersible pumps, two circulation pumps for hot and cold water (one for urgent backup), two secondary circulation pumps (one for urgent backup), and a secondary titanium heat exchanger. In this study, heat pump was controlled automatically according to the water temperature of buffer tank (on/off control), and the circulation pump is continuously operated during the experimental period. This COP measurement is based on the calculation of the heating and/or cooing capacity and measurement of electric power consumption of the compressor in heat pump. To study thermal performance of GSHP system, empirical tests were conducted at heating and cooling operation. In the beginning of the experiments, mass flow rates in the pipe circuits and electric power supplied for each pump were measured at one time. During the experimental periods, the temperatures at primary inlet and outlet of heat pump, the temperatures at secondary inlet and outlet of heat pump, the temperatures at inlet and outlet of building load,

4 and the temperatures at inlet and outlet of ground heat exchanger were continuously recorded at intervals of 10 minutes. Also the electric power supplied to the compressor was continuously measured. Figure 2. Schematic diagram of GSHP system installed at Kwang-yang city, Korea. Table 1. Specifications of the measurement equipments Measurement Equipments Specification Data logger DA 400 (Yokogawa) Temperature sensor T-type thermocouple Electric power meter CW 140 (Yokogawa) Ultrasonic flow meter PT 868 (Panametrics, GE) Pressure gague Oil pressure-type pressure gague Table 2. Temperature sensor positions Sensor Number Symbols Sensor positions T1 T2 T3 T4 T5 T6 T7 T8 Primary inlet of heat pump (condenser inlet) Primary outlet of heat pump (condenser outlet) Secondary inlet of heat pump (evaporator inlet) Secondary outlet of heat pump (evaporator outlet) Inlet of building load Outlet of building load Inlet of ground heat exchanger Outlet of ground heat exchanger a) b) c) Figure 3. Measurement equipments. a) Electric power meter, b) Installation of electric power meter, c) Untrasonic flow meter.

5 Generally, heating and cooling performance of heat pump is represented by the coefficient of performance (COP) and energy efficiency ratio (EER) respectively. COP is a measure of efficiency in the heating mode that represents the ratio of total heating capacity to electrical energy input resulting in a dimensionless value (no units). EER is a measure of efficiency in the cooling mode that represents the ratio of total cooling capacity to electrical energy input, which is defined by the cooling effect in Btu per hour divided by the power use in watts for the peak day. COP could be converted to the EER using the following Equation (1). (1) where the units of EER are BtuW -1 h -1. In this study, the cooling coefficient of performance of the heat pump coefficient of performance of the overall GSHP system Equation (2) and (3) respectively., the cooling, are estimated by (2) (3) where is the cooling capacity of heat pump, and is the electric power input to the compressor, and is the electric power input to the circulation pump for hot and cold water, and is the electric power input to the submersible pump in the borehole. Likewise, the heating coefficient of performance of the heat pump COP hp,h, the heating coefficient of performance of the overall GSHP system COP overall,h, are estimated by Equation (4) and (5) respectively. (4) (5) and were calculated by Equation (6) and (7) respectively with the measurement results of temperature at primary inlet and outlet of heat pump. where is the flow rate of hot and cold water, and is the specific heat of water. In this study, power input to the compressor was continuously measured during the experimental period, and power input to the water circulation pump, and power input to the submersible pump were measured at one time in the beginning of the experiment. (6) (7) EXPERIMENTAL RESULTS To evaluate the efficiency of GSHP, empirical tests were conducted at each of heating and cooling operation. As results of these empirical tests, variations of temperature, thermal energy rate, and electric power input were analyzed at different heating and cooling operation.

6 Table 3 shows the results of water flow rate and electric power consumption in circulation pump for hot and cold water, circulation pump in secondary loop, and submersible pump. To calculate the thermal energy rate and COP of overall GSHP system, these values were assumed to be constantly maintained during the experimental period. Table 3. Measurement results of water flow rate and electric power consumption Components Water flow rate (lpm) Electric power consumption (kwh) Circulation pump for hot and cold water Circulation pump in secondary loop Submersible pump In the calculation of the and the, electric power consumption of compressor in heat pump was just considered. And in the calculation of the and, electric power consumption of one circulation pump for hot and cold water, one circulation pump in secondary loop, and two submersible pumps were also included. Basically, the and the of GSHP system were found to be lower than the and because and included the energy consumed by additional water circulation pumps and submersible pumps. Results for heating operation Figure 5 a) shows the temperature of secondary inlet and outlet of heat pump, and the temperature differences between the inlet and outlet have maintained about 2 C for heating operation. Also, as shown in Figure 5 b), the temperature differences between the inlet and outlet of ground heat exchanger have remained about 2 C. In case of the building load, the temperature of inlet varies from 43 to 46 C, and the temperature of outlet ranges from 40 to 43 C. As shown in Figure 5 d), the electric power consumption of overall system was stepped because the two compressors in heat pump were automatically operated in accordance with the building load. Figure 5 e) shows the variation of circulating water temperature at the inlet and outlet of the heat pump and the heating capacity of heat pump. As shown in Figure 5 e), the temperature difference between primary inlet and outlet of heat pump is about 3 C, and the heating capacity of heat pump ranges from 95 to 190 Mcal/h. Figure 5 f) shows COP variation of heat pump and overall GSHP system. From this figure, the value of heat pump COP at heating operation has the range from 4.3 to 4.85, and the value of overall GSHP COP has the range from 2.85 to 3.3. Results for cooling operation Figure 6 a) shows the variation of circulating water temperature at the inlet and outlet of the heat pump and the cooling capacity of heat pump. As shown in Figure 6 a), the temperature difference between primary inlet and outlet of heat pump is about 3 C, and the cooling capacity of heat pump ranges from 75 to 160 Mcal/h. Figure 6 b) shows COP variation of heat pump and overall GSHP system. From this figure, the value of heat pump COP at cooling operation has the range from 4.5 to 5.0, and the value of overall GSHP COP has the range from 2.9 to 3.3. The in cooling operation range from 2.9 to 3.3 and those in heating operation range from 2.85 to 3.3. Comparing the COPs of an existing air-source heat pump system, which has a COP of around 2, we could expect GSHP systems are more energy-efficient than current heating and cooling system, i.e. ASHPs or high-efficiency gasfired boilers. However, in comparison with other alternatives, the total costs of the energy consumed during the building operation and the energy rate of gas and electricity should be considered together.

7 a) b) c) d) e) f) Figure 5. Measurement results in heating operation. a) temperature of secondary inlet and outlet of heat pump, b) temperature of inlet and outlet of ground heat exchanger, c) temperature of inlet and outlet of building load, and heating capacity, d) electric power consumption of overall system, e) Variation of water temperature of inlet and outlet and heating capacity of heat pump, f) Variation of COP in heating operation a) b) Figure 6. Measurement results in cooling operation. a) Variation of water temperature of inlet and outlet and cooling capacity, b) Variation of COP in cooling operation

8 CONCLUSIONS Today, ground source heat pump (GSHP) systems have been used for one of the most growing applications of renewable energy in Korea. A GSHP system has three major components such as a heat pump, a ground heat exchanger and an interior heat distribution system. These components are all affecting the efficiency of a GSHP system, the Coefficient of Performance (COP). As a fundamental research about performance of GSHP using SCW system, this study aims at assessing the COPs in heating and cooling operations. In conclusion, the overall system COPs in cooling operation range from 2.9 to 3.3 and those in heating operation range from 2.85 to 3.3. Comparing the COPs of an existing air-source heat pump system, which has a COP of around 2, we could expect GSHP systems are more energyefficient than current heating and cooling system, i.e. ASHPs or high-efficiency gas-fired boilers. However, in comparison with other alternatives, the total costs of the energy consumed during the building operation and the energy rate of gas and electricity should be considered together. ACKNOWLEDGEMENT "This work was supported by the Grant of the Korean Ministry of Education, Science and Technology"(The Regional Core Research Program/Biohousing Research Institute) REFERENCES 1. Jin-Yong, Lee Current status of ground source heat pumps in Korea. Renewable and Sustainable Energy Reviews. Article in press. 2. Stephen P. Kavanaugh Ground-Source Heat Pumps. ASHRAE. 3. Ladislaus Rybach and Burkhard Sanner Ground-source heat pump systems, the European experience. GHC Bulletin. Geo-Heat Center, Oregon Institute of Technology. 4. Yujin, Hwang, Jae-Keun, Lee, Young-Man, Jeong, et al Cooling performance of a vertical ground-coupled heat pump system installed in a school building. Renewable Energy. Vol 34, pp Zheng Deng O Neill, Jeffrey D. Spitler and Simon J. Rees Performance analysis of standing column well ground heat exchanger systems. ASHRAE Transactions. Vol 112, Part 2. pp Arif Hepbasli, Ozay Akdemir and Ebru Hancioglu Experimental study of a closed loop vertical ground source heat pump system. Energy Conversion and Management. Vol 44, pp Mustafa Inalli and Hikmet Esen Experimental thermal performance evaluation of a horizontal ground-source heat pump system. Applied Thermal Engineering. Vol 24, pp A. Hepbasli Performance evaluation of a vertical ground-source heat pump system in Izmir, Turkey. International Journal of Energy Research. Vol 26, pp Mustafa Inalli, Hikmet Esen Seasonal cooling performance of a ground-coupled heat pump system in a hot and arid climate. Renewable Energy. Vol 30, pp P.F. Healy and V.I. Ugursal Performance and economic feasibility of ground source heat pump in cold climate. International Journal of Energy Research. Vol 21, pp Jun-Un Park and Nam-Choon Baek Geothermal heat pump performance characteristics test in transient condition. Proceedings ISEA Asia-Pacific. Korean Solar Energy Society. pp