Greenhouse covering materials measurement and modelling of thermal properties using the hot box method, and condensation effects

Two isothermal hot boxes were designed, built, and set up in the open to investigate the overall heat transfer coefficient or K factor (Wm(2)K), of different greenhouse covers. Tests were made on plastic films and glass, both dry and in the presence of condensation. Experiments were conducted with she same cover on two boxes, one with condensation and the other without. Condensation was found to be favourable for plastic films as it reduced the overall heat transfer coefficient and hence heat losses. Conversely, condensation increased heat losses with glass. Antidrop films were found to degrade with age, in terms of preventing drop formation, but this did not significantly modify the K factor. A model for the hot boxes was developed which allows simulation of the behaviour of plastic films or glass under predetermined conditions. Curves and simple regressions were computed which gave relationships between the K factor and the emissivity of the cover, the temperature difference, the wind speed and sky temperature. The accuracy of the external hot box method is within 2%, of the measured value of K. (C) 1996 Silsoe Research Institute.