Renewable energy systems have received a lot of attention as sustainable technology in building sector. However, the efficiency of the renewable energy systems depends on the surrounding conditions, and it could gradually decrease by excessive and long-term operation. As a solution, a hybrid system can increase the reliability of energy production and decrease investment costs through by reducing the system capacity. The hybrid system operates at the ideal performance, but the design and

more »

... on method for hybrid system have not been established. In this paper, the performance of the hybrid system combined with photovoltaic/thermal (PVT) system and ground source heat pump (GSHP) system was analyzed using TRNSYS 17 and feasibility was assessed. The energy consumption and performance efficiency of hybrid system were calculated according to operating modes. Furthermore, seasonal performance factor (SPF) of hybrid system was compared with that of conventional GSHP system. System performance was analyzed in various conditions such as the usage of storage tank heating and set temperature for solar heating. As a result, the average SPF of the developed system increased about 55.3% compared with the GSHP system. utilizing annually stable underground temperatures as a heat source. The system performance depends on the underground conditions such as ground thermal conductivity, underground temperature, and groundwater level. However, excessive or long-term operations could lead to the decrease of system performance for GSHP system. Therefore, several methods with an auxiliary heat source or ground heat exchanger have been studied to prevent reducing the ground temperature. In this research, a hybrid system which consists of a photovoltaic/thermal (PVT) and a geothermal solar-heat pump (GSHP) is suggested to solve this problem. This hybrid system can enhance the reliability of the energy production and decrease the investment costs by reducing the system capacity. In addition, it is able to recover the underground temperature, and from an energy viewpoint it operates independently by utilizing the electrical and thermal production of the PVT module. Moreover, the PVT system integrated with the GSHP system has other advantages. The surface temperature of the PVT-module would be lower, so that it can increase power-production to further improve the performance efficiency. Many studies have been carried out on hybrid systems which combine a GSHP system and a solar system. Wang and Qi [7] analyzed the performance of underground thermal storage for a solar ground-coupled heat-pump system (SGCHPS) for a residential building. Based on the experiment results, the system performance during a longer period was simulated according to unit modeling, and its parametric effects were discussed. The results suggested that the performance of the underground thermal storage of the SGCHPS depends strongly on the intensity of the solar radiation and the match between the water-tank volume and the area of the solar collectors. A reasonable ratio between the tank volume and the area of the solar collectors was suggested to be in the range of 20 L/m 2 to 40 L/m 2 . An experimental study on a solar-assisted ground-coupled heat-pump system with a solar-seasonal thermal storage installed in a cold-area house was presented by Wang et al. [8] . In this case, the results provided that the system could meet the heating and cooling energy needs of the building with average system performance coefficients of 6.55 and 21.35, respectively. Further, in the heating and cooling modes of the Wang et al. 's [8] study, the operation of the heat pump was not required. Ozgener [9] used a system that comprises a solar-assisted geothermal heat pump and a small wind turbine for the heating in agricultural and residential buildings. Here, the study theoretically showed that 3.13% of the annual electrical-energy consumption of the modeled system and 12.53% of the annual electrical-energy consumption of the secondary water pumping, brine pumping, and fan coil can be met by utilizing the small wind-turbine system (SWTS). This result indicated that the combination of a modeled passive solar-pre-heating technique with the geothermal heat pump system (GHPS) and the SWTS is economically preferable to the conventional space-heating/cooling systems in agricultural and residential building-heating applications; but only if these buildings are installed in regions with sufficient wind resource. Chen and Yang [10] conducted a numerical simulation of a solar-assisted ground-coupled heat-pump system in northern China to provide both space heating and domestic hot water. The simulation results indicated that the solar-collector area of the optimized system under the specified load conditions is 40 m 2 , while the borehole depth is 264 m. Here, the annual total-heat extraction, plus 75% of the hot-water requirement, can be provided by solar energy when the optimized design is used. Wang et al. [11] presented a novel hybrid solar-geothermal solar-heat-pump system (HSGSHPS) that is composed of a ground heat-pump system (GHPS) and a solar-assisted GSHPS (SAGSHPS). The simulated results showed that the design of the proposed HSGSHPS is reasonable to solve the ground-temperature imbalance problem. A suitable control strategy for the solar collector and storage was found according to the performance of the SAGSHPS. In addition, 32% of the electrical-energy consumption in the HSGSHPS could be saved if the load-circulation pump is turned off when none of the fan coils are running. Using the previous experimental investigations, Yang et al. [12] carried out a numerical simulation regarding the performance of an SGSHPS operated in different heating modes. Accordingly, the heat-pump performance, solar-collection performance, and borehole-wall temperature were analyzed. According to Yang et al.'s [12] experiment results, the system-operation