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

Abstract

The rate of nocturnal heat loss from a heated plastic greenhouse is made up of a number of heat transfer processes, each of which can be described by a mathematical expression. Under a given set of climatic conditions it is possible to develop a mathematical simulation of the energy balance at the covering surface; the solution of the relevant equations yields values for the magnitude of each of the transfer processes involved and enables the total rate of heat loss to be calculated. The accuracy of the method of analysis was checked by 18 experiments in a plastic “tunnel” greenhouse. From the data obtained in these experiments, accurate values of the surface transfer coefficients were derived, and good agreement between the calculated and actual rates of heat loss was obtained. Calculations of the rates of heat loss based on commonly used values of the surfaces transfer coefficients for small surfaces produced results that underestimated the actual rate of heat loss by an average of 28%. An overestimation of 13% resulted from the use of values recommended for calculating the heating and cooling requirements of conventional buildings. The differences in the magnitude of the individual heat transfer processes caused by differences in the degree of transparency of the cover were calculated. Although the proportions of the contributions of the individual processes to the total heat loss varied widely, the effect on the total rate of heat loss was found to be small.