Water vapour transfer in a plastic house equipped with a dehumidification heat pump

Abstract

Improvement of greenhouse insulation, for example through using a double inflated wall or thermal screens, leads to changes in the microclimate of the protected cultivation. The low air exchange rate induces an increase in the inside air humidity and the development of condensation on the walls and roof which causes water to drip on to the crops. Consequently, the occurrence of fungal diseases and physiological disorders increases and greenhouse air dehumidification is necessary to combat these. In this paper, the greenhouse air dehumidification possibilities are analysed with both experimental and theoretical evaluations of the water vapour transfer between the different sub-systems (soil, crop, inside air, roof, outside air, heat pump), considered as sources or sinks of water vapour. The water vapour fluxes were estimated from measurements of temperature, heat flow, humidity and radiation. A dynamic model of water vapour exchange was developed and compared with the experimental results. It showed that the heat pump did not reduce significantly the inside air humidity, but eliminated almost completely the water condensation on the roof (and dripping of water on to the crops). This model allows the determination of the size of the dehumidification system required, according to the characteristic parameters of the greenhouse and of the crop, namely, air temperature and humidity set points, air changes, crop leaf area index, stomatal and aerodynamic resistance.