SIMULATION STUDY ON THE PERFORMANCE OF SOLAR AND GEOTHERMAL HYBRID R22 HEAT PUMP

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1 SIMULATION STUDY ON THE PERFORMANCE OF SOLAR AND GEOTHERMAL HYBRID R HEAT PUMP Byun Kang 1, Jae-Kyeong OH, Cha-Sik Park 3, and HongHyun Cho 1 Graduate school of Mechanical Engineering / Chosun University, Kwangju (korea) Graduate school of Mechanical Engineering / Chosun University, Kwangju (korea) 3 Deartment automotive Engineering / Hoseo University, Asan (korea) Deartment Mechanical Engineering / Chosun University, Kwangju (korea) ABSTRACT A simulation study on the erformance of the solar and geothermal hybrid heat um system by using R was carried out with a variation of oerating conditions. The system was consisted of solar system (concentric evacuated tube solar collector, heat storage tank) and geothermal heat um system (double ie heat exchanger, electric exansion valve and comressor). As a result, the heating caacity is linearly decreased from 13. kw to 11 kw as the heat um oerating temerature increases from 0 o C to o C. Besides, the heating COP decreases by 13.6% from. to 3. when the ground temerature raises 13 o C to 17 o C. The heating caacity is increased by.7% from 11.5 kw to 1. kw and the heating COP rises by 19% from.7 to 5.6. KEYWORKDS Heat um, R, COP, Hybrid, Solar, Geothermal A area (m ) c 1 -c 5 coefficients in eq. (6) D F R F R τα diameter (m) collector heat removal factor intercet of the efficiency curve h enthaly (KJ kg -1 ) I t solar radiation (W/m ) L N S P length in EEV (m) mass flow rate (kg/s) RPM Relative iston stroke length ressure (kpa) T temerature ( o C) T i inlet temerature at the collector ( o C) T a outlet temerature at the collector ( o C) U L collector overall heat loss coefficient NOMENCLATURE

2 V com comressor volume (kg/m 3 ) V s comressor inlet volume (kg/m 3 ) W comressor work (kw) density (kg/m 3 ) comression ratio 1. INTRODUCTION Recently, awareness of the effect of human activities on the environment and the develoing debate on climate change has heightened and interested in non-fossil energy source. Since the limitation of energy usage and energy crisis became the roblem in the world, the necessity of develoing new energy resources has been a hot issue for the whole world in related to an alternative energy. Among the renewable energy, solar energy has advantages such as the cheaer maintain cost, clean and limitless. However, solar energy has serious roblems like the low density solar energy, the imact of climate change and unstable energy sulies. To solve these roblems, solar and geothermal hybrid heat um system has been develoed in this study. As solar radiation is enough, solar and geothermal hybrid heat um system absorbed solar energy by solar collection. On the other hand, when the solar radiation is not enough to oerate, the required energy can be filled by using heat um. Besides, geothermal heat um system can be used a constant temerature for year. Furthermore, the energy usage could be decreased by using this system for all season. In this study, a heat um model with solar and geothermal heating system for residentail heating was develoed. For efficient use of the solar and geothermal hybrid heat um an otimal oeration control emthod is required in order to save enery and increase reliablility. To address these roblem, the erformance date of the solar and geothermal hybird heat um system have been analysed against the um oerating temerature and goethermal temerature.. SYSTEM MODELING The solar and geothermal hybrid heat um system (SG-HHPS) consists of a solar heat system and a geothermal heat um system used refrigerant R. The solar heat system has a concentric evacuated tube collector and a heat storage tank. The geothermal heat um system consists of two double-ie heat exchangers (high- & low-tem), a double ie tye evaorator, an EEV (electric exansion valve) and a comressor. For modeling of system comonents and calculating of thermodynamic refrigerant roerties, EES (engineering equation solver) is used. A schematic diagram of the SG-HHPS used in this study is shown in Fig.1..1 Solar collector and heat storage tank modeling The solar collector has eight concentric evacuated tube collectors which can be reliably oerated for getting heat for residential alication. As a working fluid water-roylene glycol mixture (water to roylene glycol ratio of 0:0) is used. The solar collector model was develoed based on test results of the concentric evacuated tube collector by Korean Institute of Energy Research. In this study, the efficiency of solar collector is calculated by eq.(1). * ( ) + * ( ) + (eq. 1) Solar hybrid systems show some deendability roblems because of the time lag when solar energy is acquired and when it is used. To solve this roblem, the heat storage tank (1.0 ton) was designed, and total collection heat was calculated by eq.().

3 Fig. 1: Schematic diagram of the SG-HHPS (solar and geothermal hybrid heat um system) (eq. ). Comressor modeling The recirocating tye comressor used for the simulation analysis used as the heat um system of using R for develoing the analytical model. In addition to, the mass flow rate of the comressor was calculated by using volumetric efficiency and isentroic efficiency, resectively. m V N v com (eq. 3) V s N 0.05S.9 10 N v S (eq. ) i S N 0.55S.5710 N (eq. 5).3 Heat exchanger modeling The heat um system consists of three double-ie heat exchanger (high & low temerature condenser and evaorator), an EEV (electric exansion valve), and a recirocating tye comressor. In the heating mode, the double-ie heat exchanger acts as an evaorator to exchange heat between the refrigerant and water that is sulied from the GHX (ground heat exchanger). And two double-ie heat exchangers lay the role of the condenser. The high temerature heat exchanger is laced at the comressor outlet. It exchanges the heat from high-temerature water to the middle of the heat storage tank so as to suly the hot water to the user. The low temerature double-ie heat exchanger exchanges the heat from low-temerature water at the bottom of heat storage tank to suly the mean temerature water for heating. Table 1 shows roerty reference data for heat exchanger simulation used in this study.. EEV modeling The EEV with a diameter of 1.6 mm is used as an exansion device so as to calculate the mass flow rate

4 Tye Tab. 1: Proerty references for simulation Refrigerant heat transfer coefficient Refrigerant ressure dro Water transfer coefficient Condenser Double Gnielinski (1976) Churchill (1977) Dittus and Boelter Evaorator ie Shah (1979) Churchill (1977) (195) through the exansion device. In this study, the exansion rocess is assumed to be isenthalic. The mass flow rate of the EEV is calculated by using 6-hysical and -geometrical variables based on Buckingham-π theory as shown below in eq. (6). The constants of eq. (6) are shown in Table. ( ) ( ) ( ) ( ) ( ) (eq. 6) Tab. : Constants in EEV correlation of eq. (6) Constant Value C C C C C Simulation conditions To simulate heating load, the indoor sace of 66.5 m was assumed. The heat loss on inner walls was ignored, but losses on roof and outside walls with windows have been considered. Besides, the daily usage of the hot water was assumed to be 0L for four-member family according to ASHRAE standard 116 (193). The design arameters of heating load and hot water load are shown in Tables 3 and. In this study, the erformance analysis of the SG-HCHP system with heat um oerating temerature was carried out in order to comare the erformance characteristics. The heat um oerating temerature means the setting temerature that can suly the heat to heat storage tank by the heat um when the water temerature is reduced below the designed temerature. Simulation conditions used in this study are shown in Table 5. Tab. 3: Heating load design Tab. : Hot water load design Tab. 5: Simulation conditions Parameters Conditions Time Usage water (L) Parameter Conditions Indoor sace Thermal conductivity (W/m o C) Windows (cm) Wall (cm) Roof (cm) 65.5 m 09 : Brick: 0.53 Styrofoam: 0.0 Glass: : cm Glass EA Brick : 15 cm Styrofoam: 5 cm 1 : Brick: 0 cm Heat um oerating tem. ( o C) Ground tem. ( o C) 0,,, 6, 11, 13, 15, 17, 19

5 3. RESULTS AND DISCUSSION 3.1The effect of heat um oerating temerature on system erformance Fig. shows the variations of the otimal EEV oening, mass flow rate, heating caacity, and COP with heat um oerating temerature. The heating caacity is roortionally decreased from 13. kw to 11 kw as the heat um oerating temerature increases from 0 o C to o C. Besides, the heating COP decreases by 13.6% from. to 3. EEV oening of the heat um is one of the most imortant control factors to determine system erformance because it affects the mass flow rate of the refrigerant and the comressor discharge ressure. In this study, when the heat um oerating temerature increase from 0 o C to o C, the otimal EEV oening decreases from 6% to 0%, and mass flow rate decreases from 76 g/s to 65 g/s. Fig. 3 shows the variations of the comressor work, ressure ratio, condenser outlet temerature and quality with heat um oerating temerature. Mass flow rate has to tendency to decrease with the rise of heat um oerating temerature. Besides, the ressure ratio increases from.61 to 5.5 and the comressor work decreases by 0.9% from.9 to.. Generally, heat exchanging rate is reduced at the condenser with the rise of heat um oerating temerature. Thus, the temerature and ressure of refrigerant through the EEV are increased roortionally with the outlet temerature at the condenser. Therefore, the quality of refrigerant at the inlet of evaorator is increased. Furthermore, the entering ressure at the comressor inlet decreases and comressor work increases owing to the increase of ressure ratio between inlet and outlet of comressor. In this study, when the heat um oerating temerature increases from 0 o C to o C, the condenser outlet temerature increases from 93. o C to 100 o C and quality increases from 0.31 to Since heat um oerating temerature is very effective to system erformance, the oerator should control to maintain the otimal heat um oerating temerature according to oerating condition in order to save energy and increase system reliability. 3.The effect of ground temerature on system erformance Fig. shows the variations of the otimal EEV oening, mass flow rate, heating caacity, and COP with ground temerature. The heating erformance can imrove as the ground temerature rises. As the ground temerature increases from 13 o C to 17 o C, the heating caacity is imroved by.7% from 11.5 kw to 1. kw and the heating COP increases by 19% from.7 to 5.6. The temerature and ressure at the comressor inlet increases as the ground temerature elevates. At this case, the mass flow rate of refrigerant increases because of the high temerature and ressure. Hence, the otimal EEV oening increases with a rise of ground temerature. In this simulation result, the otimal EEV oening increase from % to 6% when the ground temerature increases from 13 o C to 17 o C. The mass flow rate of refrigerant is also increased from 6 g/s to 7 g/s. Fig. 5 shows the variations of the comressor work, ressure ratio, evaorator outlet temerature and evaorator caacity with ground temerature. The comressor outlet temerature decreases with reduction of comressor ressure ratio when the ground temerature elevates. Since the evaorator is directly affected by the ground temerature, the ressure of comressor inlet increases, and the ressure ratio of a comressor reduces as the ground temerature rises. Besides, the comressor work decreases during comression rocess due to rise of comressor efficiency. Therefore, the system erformance imroves under the same indoor heating load condition with the increase of ground temerature. As the simulation result, the comressor ressure ratio is reduced from 5 to.6 and comressor work decreases from.9 to. kw when the ground temerature increases from 13 o C to 17 o C. The evaorator caacity increases by 7.7% from 9.0 to 9.7 kw and evaorator outlet temerature increases from 6.5 o C to 7.7 o C. From the simulation result, it can be deduced that the heating caacity can be maintained over the wanted heating caacity whole of winter season and system erformance is also imroved significantly by adating the solar geothermal hybrid heat um system.

6 Oening (%) / Mass flow rate (g/s) Mass flow rate EEV oening Heating caacity COP Ground tem. = o C Indoor setting tem. = 0 o C Daily solar radiation = 10 MJ/m Outdoor tem. = 7 o C 0 6 Heat um oerating temerature ( o C) COP / Caacity (kw) Work (kw) / Pressure ratio Pressure ratio (P com,out / P com,in ) Comressor work Quality Condenser outlet temerature Ground tem. = 15 o C Daily solar radiation = 10 MJ/m Indoor setting tem. = 0 o C Outdoor tem. = 7 o C 0 6 Heat um oerating temerature ( o C) Temerature ( o C) / Quality Fig. : Variations of otimal EEV oening. mass flow rate, COP and heating caacity with heat um oerating temerature. Fig. 3: Variations of comressor work, ressure ratio, quality and condenser outlet temerature with heat um oerating temerature. Oening (%) / Mass flow rate (g/s) Mass flow rate EEV oening Comressor heating caacity COP Heat um oerating tem. = o C Indoor setting tem. = 0 o C Daily solar radiation = 10 MJ/m Outdoor tem. = 7 o C Ground temerature ( o C) COP / Caacity (kw) Work (kw) / Pressure ratio Work Pressure ratio (P com,out / P com,in ) Evaorator outlet temerature Evaorator caacity Heat um oerating tem. = o C Indoor setting tem. = 0 o C Daily solar radiation = 10 MJ/m Outdoor tem. = 7 o C Ground temerature ( o C) Temerature ( o C) / Caacity (kw) Fig. : Variations of otimal EEV oening, mass flow rate, COP and heating caacity with ground oerating temerature. Fig. 5: Variations of comressor work, ressure ratio, evaorator outlet temerature, and evaorator caacity with ground oerating temerature.. CONCLUSIONS The simulation model of solar geothermal hybrid heat um system (SG-HHPS) was develoed to investigate system erformance under variations condition. The erformance characteristics of refrigerating system were analyzed with heat um oerating temerature and ground temerature. The results of numerical analysis was obtained through this study are followings. When the heat um oerating temerature increase from 0 to o C, the otimal EEV oening decreases from 6% to 0%, and mass flow rate is also decreased from 76 g/s to 65 g/s. The heating caacity is linearly decreased from 13. kw to 11 kw. Besides, the heating COP decreases by 13.6% from. to 3.. When the ground temerature increases from 13 o C to 17 o C, the heating caacity is imroved by.7% from 11.5 kw to 1. kw and the heating COP increases by 19% from.7 to 5.6. Comressor ressure ratio is reduced from 5 to.6 and comressor work decreases from.9 to. kw. Besides, the evaorator caacity increases by 7.7% from 9.0 to 9.7 and evaorator outlet temerature increase from 6.5 o C to 7.7 o C. From the simulation result, it can be deduced that the heating caacity can be maintained over the wanted heating caacity and system erformance is also imroved significantly by adating the solar geothermal hybrid heat um system. ACKNOWLEDGE This research was suorted by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology( )

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