IRRADIATION ASPECTS FOR ENERGY BALANCE IN GREENHOUSES

In this paper we present irradiation aspects for the use of glass type fresnel lenses instead of typical glass or plastic covering materials of greenhouses. They can be mounted stationary on the greenhouse roof, combined with linear absorbers to receive and convert the concentrated solar radiation into heat, electricity or both. The absorbers are of small width, depending on the selected concentration ratio and are properly moved to truck the concentrated beam solar radiation, with the diffuse solar radiation not received by the absorber. The advantage of the linear fresnel lenses to separate the direct from the diffuse solar radiation makes them suitable for lighting and temperature control of the greenhouse interior space, providing light of suitable intensity level and without sharp contrasts. The incident beam solar radiation is concentrated on the absorber and can be taken away from the glazed space, achieving lower illumination level and avoiding the overheating of it. In low intensity solar radiation, the absorber can be out of focus leaving the light to come in the interior space and keep the irradiation at an acceptable level for the cultivated plants. The distribution of solar radiation on the focal plane of linear fresnel lenses and the collected solar radiation regarding absorber width and incidence angle of solar radiation are given. The study includes also design concepts for thermal and photovoltaic absorbers, emphasised to hybrid photovoltaic/thermal type linear absorbers, which convert solar radiation simultaneously into electricity and heat.