An Investigation of the Heating Performance of a Groundwater-Based Air Conditioning System for an Agricultural Greenhouse

Food shortages due to the decreasing arable land area, which is a consequence of the increasing global population, have brought greater attention to greenhouses. However, the cost of air conditioning in greenhouses is high. Therefore, in this study, the heating performance of a low-running-cost air conditioning system using groundwater was evaluated in winter in an agricultural greenhouse. The system consisted of a temperature control room in an agricultural greenhouse and a groundwater recirculation system. The pumped groundwater was passed through a polytube heat exchanger panel and stored in a recirculation tank. The stored water circulated back to the heat exchanger to create a water recirculation system. When operated with only a single 250 L recirculation tank, the temperature in the temperature control room was maintained at 4.9–19.4 °C, even when the maximum and minimum outdoor air temperatures were 12.6 and −2.3 °C, respectively. To achieve a higher minimum temperature in the temperature control room, a method was developed to enable the system to switch from the recirculating water to flowing groundwater when the recirculating water temperature fell below the groundwater temperature. Consequently, the minimum temperature in the temperature control room could be maintained at 8.0 °C. In an experiment in which the capacity of the recirculation tank was tripled (750 L), the minimum temperature was maintained at 7.9 °C, which is a stable temperature for cucumber cultivation. These results indicate that the heating capacity of the proposed system is equivalent to that of ACCFHES (An aquifer coupled cavity flow heat exchanger system) and other heating systems for winter heating. Therefore, this proposed method makes it possible to cultivate plants that grow in a climate similar to that of cucumbers at a low running cost. The amount of heating capacity that could be extracted simply by circulating groundwater was also revealed.