Dehumidifying Heat Pump for Greenhouses

In heated greenhouses, attempts have been made to produce potted plants while reducing the amount of energy used. In fact, improvements in greenhouse structure and equipment (double walls, etc.) have led to better insulation and tightness. However, this tightness induces microclimatic changes, especially on night time humidity levels. The increase of indoor humidity leads to the development of condensation and dripping and, consequently, to the increase in the occurrence of fungal diseases and physiological disorders. It is therefore necessary to decrease the humidity in the air. To do this, growers have to dehumidify the air by ventilation-heating, involving a major increase in energy consumption. Humidity control is therefore a limiting factor for saving energy, making it necessary to find alternatives to the ventilation-heating method used to reduce indoor humidity. We analyzed the effectiveness of a dehumidifying heat pump installed in a 2350-m(2) double wall inflatable greenhouse located in northwestern France, during spring 2010. Hydrangeas in containers were placed on the ground (heated). The temporal evolution of climatic, thermodynamic and ecophysiological data is presented and analyzed. The thermodynamic efficiency of the machine is also taken into account. Depending on the weather outside, the energy consumption of the device (power: 4W m(-2)) used to dehumidify the air is three to four times lower than that which would have been used by the ventilation-heating method. By condensing approximately 30 L of water per hour during the operating phase of the heat pump, the machine balances crop evapotranspiration and avoids condensation and the occurrence of fungal diseases.