Influence of Condensation and Evaporation on the Greenhouse Climate and its Regulation

A dynamic greenhouse climate model was used to simulate the effect of condensation and evaporation on the auxiliary heating requirements, on the inside air humidity and temperature and on the vegetation temperature in greenhouses covered with 12 different cladding materials. Condensation was shown to increase the auxiliary heating requirements for materials having a far infrared radiation transmittance lower than 0.18, while it reduces them for all other materials. Savings ranged between +25 % for PP and -17 % for standard glass. The thermal insulation properties of dry claddings were shown to depend on their far infared radiation transmittance, while in the presence of condensation, the outside surface emittance was predominant. Materials reflecting all far infrared radiation could be shown to be best insulating under both wet and dry conditions. It was also demonstrated that neglecting condensation and evaporation gives rise to an overestimation of the yearly mean inside air relative humidity of about 10 % for most materials. Since condensation fluxes were found to be lowest in greenhouses cladded with low emissivity materials, yearly average relative humidities were somewhat lower in this kind of greenhouses, while the relative contribution of condensation to the nighttime water vapour removal from the inside air was found to be lower for low emissivity cladded greenhouses (79-83 %) than for the other greenhouses (88-89 %). Evaporation fluxes from the cover as well as condensation fluxes to the floor were shown to be negligible when compared to condensation fluxes to the cover. Neglection of condensation was shown to have nearly no effect on the simulated temperature of an active greenhouse, whereas it can result in an overestimation or an underestimation of the vegetation temperature, according to the claddings far infrared radiation transmittance, high transmittances giving rise to underestimates. Since these errors were found to vary throughout the year, it was argued that the vegetation temperature should also be controlled by greenhouse climate systems and that models used for inclusion in such systems will have to describe condensation phenomena in the greenhouse in a detailed way.