Energy conserving dehumidification of greenhouses

As greenhouses become better insulated and increasingly airtight, the humidity of the inside air rises easily and may become unfavourably high. Therefore, most greenhouses frequently open their vents to remove the moisture excess. When heated, opening the vents will increase the energy consumption. In case the greenhouse is dehumidified by internal moisture withdrawal, the loss of sensible heat via ventilation can be prevented. Moreover, condensation of moisture at an internal surface regains latent heat. Thus, internal condensation reduces the sensible heat loss of the greenhouse and regenerates part of the heat demand associated with the crop evaporation. The prevention of sensible heat loss and the capture of latent heat enables the reduction of energy consumption. The principle is not new, but the economic feasibility depends on the proper selection of capacities and engineering details and, of course, on energy costs. In order to explore and to demonstrate the potentials of this technique, Wage-ningen UR Greenhouse Horticulture employed such an internal dehumidification system in a 500 m2 experimental greenhouse growing a tomato crop year round. Energy consumption of the greenhouse and water condensed in the dehumidification unit was measured. Parallel to these measurements, an existing simulation model was extended to include the description of the dynamic behaviour of internal dehumidification on a cold surface. This work gives an overview of the practical results in the experiments and shows the sensitivity of the technique on important parameters, like greenhouse air temperature and humidity setpoint. It appears that the system has a high potential for energy saving in the moderate and cold climates