The impacts of a concept greenhouse with highly insulating double glass and a new method for greenhouse dehumidification management on energy use

In order to reduce the dependency on fossil fuels, the Dutch horticultural sector puts a lot of effort in the reduction of energy demand. By using multiple thermal screens, a modest temperature regime and allowing high humidities, the energy consumption of a greenhouse can be reduced substantially, but for additional savings, a considerable increase of the insulation is needed. In an earlier study (Hemming et al., 2009), we showed that newly developed coverings using anti-reflective coatings to increase light transmission and low-emission coatings to reduce energy losses resulted in minor light losses inside the greenhouse, which paves the way for double glazing with these materials. Such highly-insulating glass was integrated in a total greenhouse concept resulting in a 500 m2 research facility, realised in summer 2010 at the research station of Wageningen UR Greenhouse Horticulture in Bleiswijk. Besides increasing insulation value, the design of this facility also paid special attention to dehumidification. In general, three possible dehumidification systems can be used: a balanced ventilation system with heat recovery; condensation on a cold surface; and a hygroscopic fluid. The disadvantage of a balanced ventilation system is the loss of sensible heat during its operation. In the case where the greenhouse is dehumidified by internal moisture withdrawal, the loss of sensible heat via ventilation can be prevented. Condensation of moisture on an internal surface gathers latent heat associated with the excess moisture. Thus, internal condensation of excess humidity by means of a heat pump refrigeration cycle reduces the sensible heat loss of the greenhouse and regenerates (part of) the heat demand associated with the crop evaporation. For this facility, we decided on using a balanced system. Since the completion of the greenhouse, two years of year round tomato production resulted in energy savings of up to 60% even with a higher rate of production, as compared with the practice in commercial greenhouses. The greenhouse showed a natural gas consumption of 460 MJ¿m-2 and 6 kWh¿m-2 for the balanced ventilation system compared with natural gas consumption of over 1100 MJ¿m-2 in common practice.