# Simulation of Hybrid Photovoltaic Solar Assisted Loop Heat Pipe/Heat Pump System
## 1 Introduction  
A photovoltaic/thermal (PV/T) hybrid solar system is a combination of a photovoltaic (PV) and solar thermal system which produces both electricity and heat from one integrated system. By cooling the PV module with a working fluid, the electricity yield can be improved and the heat pick up by the fluid can be used for space or water heating. Hence, the system becomes increasingly attractive in solar energy utilizations. The concept of the PV/T was originally initiated by [PERSON] and [PERSON] in 1978 [1]. Soon after this, various PV/T systems or collectors for water heating have been theoretically and experimentally studied [2]. A PV/T system adopting a flat-box absorber design was constructed and tested by [PERSON] et al. [3]. The results indicated that daily thermal efficiency could reach around 40%. [PERSON] et al. [4] simulated a hybrid PV/T system in residential buildings in Hong Kong. It was found that the fabric-integrated PV/T system could achieve a good electricity conversion and heat collecting efficiency. [PERSON] et al. [5] developed a model to estimate the performance of a hybrid PV/T system. It was concluded that the configuration of the PV/T system significantly affects its thermal and electrical output. Using a heat pump, lower-grade heat energy extracted from a PV/T system could be upgraded to an appropriate temperature for heating purposes. One recent development in the integration of heat pump and photovoltaic technology lies in the use of photovoltaic solar assisted heat pump (PV-SAHP) systems [6; 7; 8]. [PERSON] et al. [9] first proposed the conception of a solar-assisted heat pump system usinga photovoltaic/thermal (PV/T) collector as its evaporator. The results indicated that a coefficient of performance (COP) of the heat pump as high as six could be obtained when the water temperature at the inlet of the condenser was at 40 \({}^{\circ}\)C in the daytime in winter. Recently, [PERSON] et al. [10] experimentally investigated the dynamic performances of a PV-SAHP system. It was found that its thermal efficiency was 46% higher than a traditional air source heat pump. In addition, [PERSON] et al. [11] numerically studied a PV/T heat pump system that had a modified collector/evaporator, and showed that this new system could efficiently generate electricity and thermal energy simultaneously both in Nanjing and Hong Kong all-year-round. Although a PV-SAHP system is able to achieve a higher efficiency, the continuous operation of the compressor consumes considerable amounts of electricity when heating water. In order to cut down its power consumption, researchers combined a PV-SAHP system with a loop heat pipe (LHP) system in recent years. The LHP is a two-phase heat-transfer measure with the working fluid circulating in a loop, thus enabling remote and passive heat transfer at enhanced capacity. Due to its simple operation and environmental effect, LHP has been widely used in the thermal control of satellites and electronics [12; 13; 14] and it has gradually been gaining attention in the field of solar energy collection and transportation [15; 16; 17]. The LHP mode is passive, which means it does not consume work. Therefore, the hybrid photovoltaic solar assisted loop heat pipe/heat pump (PV-SALHP/HP) system could heavily reduce power consumption, raise the utilization ratio of solar energy, and promote energy saving. For the hybrid PV-SALHP/HP system, the loop heat pipe (LHP) mode will be utilized when solar radiation is strong and the temperature of the working medium in the PVT evaporator is higher than that in the condenser. Correspondingly, the heat pump (HP) mode will be started when solar radiation is weak or the temperature difference of the working medium in the PV/T evaporator and the condenser cannot satisfy the condition of the LHP mode. [PERSON] et al. [18] designed a hybrid PV-SALHP/HP system and separate operation modes of the system were studied. The results showed that the thermal efficiency of the LHP system and HP system were respectively 35.55% and 74.49%.