Performance of photovoltaic and ground source heat pump system for daytime cooling of mushroom greenhouse during summer: Preliminary analysis

Mushrooms represent typical high quality and high-price niche products. Especially in summer, the mushroom price is usually high due to the limited production. The only way to cultivated mushrooms, during the hot season, is to decrease the air temperatures and increase the humidity content through use of cooling technologies in controlled environment. Unfortunately, the traditional systems used for greenhouse conditioning and the cooling cost are not economically sustainable in summer and the thermal levels reached inside the greenhouse are too low. Alternative solutions are represented by renewable energy plants as photovoltaic and geothermal integrated systems for greenhouse daytime cooling. In this paper, a small capacity heat pump (6.5 kW) for cooling a greenhouse in south of Italy is analysed. The heat pump is coupled to a vertical closed loop geothermic borehole and the electric energy necessary to feed the heat pump is supplied by photovoltaic array. The average energy efficiency ratio of the heat pump, the operating parameters of the heating systems and the weather conditions outside and inside the greenhouse are investigated. The data collection and mushroom cultivation period was from 08 to 25 July 2016, but the results were reported from 21 to 25 July 2016. The analysis shows that the greenhouse environmental conditions reached in the greenhouse are sustainable for mushroom cultivation only when the heat pump is used. Thanks to the geothermic borehole the average energy efficiency ratio of the hat pump in cooling mode ranges from 6 to 8, this value is double than the efficiency achieved by the traditional air to air source heat pumps. The results underline that the geothermal heat pump with high energy efficiency ratio improves the environmental conditions inside the greenhouse and represents a sustainable solution during the summer for high-price crops.