An investigation into the energy balance of solar greenhouses

Abstract

Utilising the sensible and latent energies in air vented from a greenhouse in order to reduce its auxiliary heating energy requirement would appear to be attractive at Christchurch given the temperate and sunny climate that exists there. All greenhouses, to some extent, utilise solar energy for reducing the auxiliary heating energy requirement. Solar greenhouses, in comparison to conventional greenhouses, have greater solar transmittance, less heat loss and more effective thermal energy storage, or at least two of these characteristics. Designing a solar greenhouse without careful consideration of the other factors, in particular yield, that affect greenhouse investment economics would be of little benefit to the greenhouse industry.

Yield, income, light transmittance and energy balance models were developed so that the internal rate of return could be predicted for any given greenhouse design. The energy balance model was a modified version of Kimball's (l986c) MEB programme. Modifications included treating solar transmittance as a variable, instead of a constant parameter. A light transmittance programme using the ray-tracing technique was developed. This programme was used to predict light transmittance in uncropped and cropped greenhouses. A 3-step process was used for finding transmittance of a cropped house. This enabled the estimation of the additional light intercepted by tall crops to be made.

A tomato yield model was developed. Yield was found from PAR and the unvented radiation fraction.

The energy and economic performance characteristics of a number of designs are presented. Of the designs tested, single-spans with an opaque rear reflecting wall appeared to be the most suitable design for local conditions.

An 'M' shaped greenhouse with an LPG CO₂ generator with heat recovery from the CO₂ gases was selected as the best investment. A prototype M greenhouse was built and the energy use and yield were monitored over one cropping season. Daily light transmittance exceeded 0.85 during sunny periods and was predicted to exceed 1.0 for a longer greenhouse where end effects are less important.