Greenhouse dehumidification by an air-air heat pump under Mediterranean conditions

The greenhouse dehumidification by an air-to-air heat pump and heat pump efficiency under middle weather conditions were evaluated in this study. Dehumidification in a closed tunnel greenhouse was achieved by the aforementioned pump when the air relative humidity was above 80% as cucumber plants were cultivated in the soil (a second tunnel greenhouse was used as a control). Greenhouse air was dehumidifying as it was forced to pass firstly through the heat pump evaporator by a fan, where it was being cooled and left some of its moisture on the evaporator, and then through the condenser, where it was being heated and eventually returned to the greenhouse. Water vapour losses were estimated for both greenhouses. Heat pump operating parameters such as dehumidified coefficient of performance (COPdeh), thermal coefficient of performance (COPheat), specific energy consumption (SEC), depending on operating conditions and operating frequency were determined. COPdeh was increased accordingly to greenhouse air temperature. Based on Athens' climatic conditions, to maintain the relative humidity of the greenhouse air at 80%, the maximum required heat pump power was estimated at 12.3 W m^‑2, while the specific energy consumption ranged from 0.4 to 0.8 kWh kg^‑1. Greenhouse dehumidification by the heat pump increased the air temperature up to 2°C and the crop evapotranspiration (up to 73%) compared to the control greenhouse as well as caused a severe reduction in water vapour condensation in the internal surface of the greenhouse cover. Consequently, greenhouse dehumidification by an air-air heat pump is an effective method especially under middle weather conditions where water vapour losses through condensation on the cover and through infiltration are relatively low.