태양열과 태양열-지열 하이브리드 이산화탄소 열펌프 시스템의 성능에 관한 해석적 연구

Abstract

Recently, global warming problem and energy crisis aroused great interest in alternative energy supplies. This is especially true for South Korea that badly depends on imported energy resources. However, most alternative energy technologies are faced with difficulties when it comes to application for community facilities because of the regional restrictions and operating cost. Therefore, researches on energy saving and optimal operation of residential heat pump systems are urgently required. To this end, using renewable energy (e.g. solar and geothermal) for refrigeration becomes increasingly important and draws considerable attention. As for working fluids, carbon dioxide is a natural climate-friendly refrigerant as it does not deplete ozone layer and has a low direct global warming potential with reference value 1. Generally, the performance of a heat pump using carbon dioxide is lower than that of a system using a subcritical refrigerant because of large irreversibility during compression and gascooling. Moreover, system reliability is very low due to large performance variations with operating conditions. Hence, studies on system performance according to various heat sources have carried out continuously. For efficient use of the CO2 heat pump an optimal operation control method is required in order to save energy and increase reliability. To address these problems, the performance data of the CO2 heat pump have been analysed against the pump operating temperature. In this study, the analytical model of S-HCHP(solar hybrid CO2 heat pump) system and the SG-HCHP(solar-geothermal CO2 heat pump) system for residential heating were developed to compare the system performance for given operating condition. A simulation study on S-HCHP and SG-HCHP system for residential applications were carried out for varying operating conditions. The system consists of a solar system(concentric evacuated tube solar collector and heat storage tank) and a CO2 heat pump system(double-pipe heat exchanger, fin-tube heat exchanger, electric expansion valve, and compressor). In the simulation of S-HCHP system, the solar collector efficiency and solar collector heat were decreased by 7.6% and 2.7 kWh, respectively, when the heat pump operating temperature rose by 2oC. As the outdoor temperature rose by 2oC, the heat pump heating COP increased by 6.5% and heating load decreased by 7.6%, on average. Also, the heating load increased by 14.3% with a rise of indoor setting temperature of 2oC. The solar hybrid CO2 heat pump system in this study had maximum solar fraction of 24.5%. In the simulation of SG-HCHP system, when the heat pump operating temperature rises from 40oC to 48oC, the heating COP is decreased from 2.77 to 2.12, heating time is reduced about 40%, and heat pump operating time is increased about 1.7 hours due to the increase of the compressor pressure ratio. In addition to, the heat pump COP is increased about 6.5% and heat fraction of heat pump is increased by 5.1% as the ground temperature increased from 11oC to 19oC, The system performance can be improved by adapting geothermal system into a solar hybrid heat pump. The comparison study of performance characteristics between the S-HCHP system and the SG-HCHP system was performed with heat pump operating temperature. As a result, the compressor work and heating capacity of the SG-HCHP system decreased and increased compared to those of the S-HCHP system, resulting that the heating performance of the SG-HCHP system improved. For basic operating condition, the heating performance of the SG-HCHP and S-HCHP system was 2.405 and 1.91, respectively. Besides, increment of compressor work of the S-HCHP system was lower by 9.1% than that of the SG-HCHP system. The heating COP of the SG-HCHP system improved approximately 2.6% compared to that of the S-HCHP system. Therefore, the SG-HCHP system has a good performance and a high reliability for residential application with various operating conditions. Throughout this study, we will provide basic performance data for S-HCHP system and SG-HCHP system, and the optimal control of the operation parameter to maintain high performance.