تجزیه و تحلیل عملکرد از سیستم پمپ حرارتی CO2 ترکیبی خورشیدی زمین گرمایی برای گرمایش مسکونی

A simulation study of hybrid solar-geothermal heat pump system for residential applications using carbon dioxide was carried out under different operating conditions. The system consists of a solar unit (concentric evacuated tube solar collector and heat storage tank) and a CO2 heat pump unit (three double-pipe heat exchangers, electric expansion valve, and compressor). As a result, the differential of pressure ratio between the inlet and the outlet of the compressor increases by 19.9%, and the compressor work increases from 4.5 to 5.3 kW when the operating temperature of the heat pump rises from 40 °C to 48 °C. Besides, the pressure ratio of the compressor decreases from 3 to 2.5 when the ground temperature increases from 11 °C to 19 °C. The operating time of the heat pump is reduced by 5 h as the daily solar radiation increases. As the solar radiation increases from 1 to 20 MJ/m2, the collector heat rises by 48% and the maximum collector heat becomes 47.8 kWh. The heating load increases by 70% as the indoor design temperature increases from 18 °C to 26 °C. However, the solar fraction is reduced from 11.4% to 5.8% because of the increases of the heating load.

Recently, hydrocarbon shortage and global energy crisis have 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 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 or geothermal) for refrigeration and air conditioning applications 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 cycle refrigerant because of large irreversibility during compression and gascooling. Moreover, system reliability is very low due to large performance variations with operating conditions. Hence, system performance and operating characteristics of a CO2 heat pump should be investigated according to operating conditions. During the last decade, a number of studies have been conducted by some researchers on design, modeling and testing of heat pump systems with a view of using solar and geothermal energy for residential heating. The characteristic of an integral-type solar-assisted heat pump (ISAHP) has been examined by Huang and Chyng [1]. Their ISAHP experimental system includes a reverse Rankine refrigeration cycle and a thermosiphon loop that integrated in a combined package heater. Both solar and ambient air energies are absorbed at the collector/evaporator and pumped to the storage tank through the reverse Rankine refrigeration cycle and the thermosiphon heat exchanger. Yumrutas et al. [2] have studied the annual performance of a solar associated heat pump system (SAHPS) with seasonal underground energy storage and annual water temperature distribution in the storage tank using an iterative computational procedure based on the analytical solution of the problem. The results showed that earth type and system size had considerable effects on system performance. Kuang et al. [3] have carried out an experimental study on SAHPS performance and concluded that the thermal storage tank was an important component in solar heating systems, which could modulate the mismatch between solar radiation and the heating load. Besides, Chiasson et al. [4] performed a 20-year life-cycle cost analysis to evaluate the economics of ground heat pump systems coupled to thermal solar collectors for six different climates in the U.S. They concluded that GSHP system combined with solar collectors is economically viable for heating-dominated climates. Han et al. [5] investigated different operational modes of a solar assisted GSHP system in the presence of latent heat energy storage tanks, and they indicated that using storage tanks increases system performance by 12.3%. Recently, Kjellsson et al. [6] analyzed five alternatives to supply solar energy to a ground source heat pump(GSHP) system and compared them against a base case without solar collectors. Wang et al. [7] have performance prediction of a hybrid solar ground-source heat pump system(HSGSHPS) and the electrical energy demand of the system could be reduced by 32%. Xi et al. [8] have experiment of long term operation of a solar assisted ground coupled heat pump system for space heating and domestic hot water. Solar assisted heating and solar coupled ground heat pump heating modes are energy saving modes that supply 22% of the total heating load. Chaturvedi et al. [9] and Aziz et al. [10] have performed thermodynamic analysis of two-component, two-phase flow in solar collectors with application to direct expansion SAHP. Their results showed that changes in the mass-flow rate and the absorbed solar heat flux had significant effects on the collector tube length and refrigerant heat transfer coefficient. Ozgener and Hepbasli [11] have investigated the performance of a solar assisted ground-source heat pump greenhouse heating system (SAGSHPGHS) with R-22 refrigerant in the heating mode by using exergy analysis. Stene [12] has carried out theoretical and experimental study of a residential brine-to-water CO2 heat pump system for combined space and hot water heating. The CO2 heat pump was equipped with a unique counter-flow tripartite gas cooler for preheating of domestic hot water (DHW), low-temperature space heating and reheating of DHW. In earlier studies, however, the performance of the hybrid solar and geothermal heat pump system has been mainly studied assuming CFC or HCFC refrigerant for residential heating. Only few studies have considered heat pump systems with CO2 refrigerant for hot water and space heating systems. Therefore, it is very important to analyze performance characteristics of the hybrid solar-geothermal CO2 heat pump system (HSG-CHPS) for residential heating and find out optimal operating conditions for variable conditions. In this study, a CO2 heat pump model with solar and geothermal heating system for residential heating has been developed. For efficient use of the CO2 heat pump, an operating characteristics and performance analysis are required in order to save energy and increase reliability. To address this problem, the performance data of the hybrid solar-geothermal CO2 heat pump system have been investigated against the pump operating temperature, indoor design temperature, daily solar radiation and outdoor temperature.

Performance characteristics of the hybrid solar-geothermal CO2 heat pump system (HSG-CHPS) have been analyzed under varying operating conditions using a simulation model. The system consists of a solar unit (concentric evacuated tube solar collector and heat storage tank) and a CO2 heat pump unit (three double-pipe heat exchangers, electric expansion valve, and compressor). It was found that when the heat pump operating temperature increases from 40 °C to 48 °C, the pressure ratio between the inlet and the outlet of the compressor rises by 19.9% and the compressor work increases from 4.5 to 5.3 kW. Also, the solar heat fraction decreases from 9% to 7.6% and the heating time is reduced by approximately 2.7 h. As the ground temperature increases from 11 °C to 19 °C, the compressor pressure ratio decreases from 3 to 2.5. At this, the outlet temperature of the compressor decreases by 23 °C and the heat fraction of heat pump increases by 5.1%. Besides, the maximum heating COP can reach 2.81. The elevation of the ground temperature can significantly reduce the refrigerant temperature at the outlet of the compressor, which can improve system performance and reliability. Moreover, the system performance of a solar hybrid CO2 heat pump is very sensitive to heat pump operating temperature. As the daily solar radiation increases, the heat pump operating time and the heat pump heat can be reduced by 5 h and 22.5%. With solar radiation increase from 1 to 20 MJ/m2, the collector heat and the solar heat fraction rise by 48% and 22%, respectively. The maximum collector heat at daily solar radiation of 20 MJ/m2 is 47.8 kWh. The solar fraction increases by 5.7% on average per 5 MJ/m2 rise of daily solar radiation. As the indoor design temperature rises from 18 °C to 26 °C, the heating load increases by 70%. However, the solar fraction reduces by 5.9% due to the rise of the heating load. Besides, the heat pump operating time rapidly increases. Therefore, the design of proper indoor temperature for variable outdoor conditions is very important in order to maintain high system performance and reliability in the hybrid solar-geothermal CO2 heat pump system. It was found that the performance of a CO2 heat pump can improve significantly by using solar and geothermal system and it can supply sufficient heat to the space during winter season.