In recent years, institutional horticulture, where vegetables are grown in plastic greenhouses or traditional greenhouses, has become mainstream worldwide. However, the running costs of temperature control for conventional horticulture are extremely high, because these use electricity and heavy oil. In addition, oil is dependent on imports, its prices are subject to fluctuation. Therefore, agricultural product prices also vary. Moreover, the significant environmental impact of these practices has led to a growing demand for air conditioning systems that utilize renewable energy. Against this background, our laboratory has developed an agricultural air-conditioning system that uses groundwater. This system involves installing an air-conditioned space for growing agricultural products inside a greenhouse, where groundwater flows through to a vinyl tube, as heat exchanger, installed on the ceiling for heating in winter. In this system, the water passed through the heat exchanger is returned to the water tank and recirculated into the heat exchanger. This system has the advantage that the thermal energy stored during the day by the circulating water can be used at night. In this study, we tripled the tank capacity in this system to enhance performance. Comparing the amount of decreasing temperature in the circulating water tank from 2 pm., when the outside air temperature was at its highest, to 6 am., when the temperature began to rise again, in case of the previous study, it was 0.71℃ per hour, on the other hand, in the case of this study, it was 0.64C per hour. Focusing on the time during which the necessary 8℃ for some agricultural product cultivation could be maintained, the previous study showed it was until 11 pm. In contrast, in this study, it was extended until 6 am. By tripling the tank capacity, the heating time was extended, and this system can grow more crops even in winter.