Effect of reflection of sunlight on illuminance and energy gain of greenhouses

Abstract

A reflecting wall was built into a greenhouse which provided for the reflection back to the ground of sun rays arriving at the northern wall of a greenhouse. Thus, the ground illuminance/irradiance could be substantially increased. The effect is particularly significant in winter when the sun is low throughout the day, so that most of the radiation goes through the greenhouse and the ground radiant power is small. Four types of greenhouses with reflecting walls were considered with increasing efficiency of insolation. A theory was developed, showing that a several-fold enhancement of the ground irradiance and radiant energy should be achievable. The effect of sky (diffuse) radiation is also considered, both in the case of a clear and of a cloudy day. An experimental model as well as a full-scale greenhouse with a reflecting wall with the provision for measuring the ground illumination was built and exposed to the sun. Measurements were made over an extended period of time and the results confirmed qualitatively those obtained from the theory.