An analysis of the nocturnal heat loss from a double skin plastic greenhouse

Abstract

The rate of nocturnal heat loss from conventional single skin greenhouses is high because glazing materials provide low thermal resistance. A common method of increasing the thermal resistance of greenhouses is to add a second layer of glazing material—normally in the form of an inner liner of transparent plastic film. The extra convective resistance resulting from the air gap between the two sheets of glazing material results in a considerable reduction in heating requirements. It is possible to develop mathematical expressions for each of the energy transfer processes that contribute to the loss of heat, and thus develop a series of energy balance equations for the greenhouse cover. The solution of these equations yields the rate of loss of heat for any climatic condition, providing that the coefficient of convective transfer across the air gap is known. The method of analysis was applied in a series of 33 experiments in a double polyethylene greenhouse, and yielded a value of 3·85 W m −2 K −1 for the air gap convective coefficient. Energy savings averaging 32% were obtained compared with that required to heat a single skin greenhouse exposed to the same climatic conditions.