In medium and high latitude countries the amount of solar energy is not usually enough and artificial light and heat supply is needed in most months of the year. The visible spectrum of solar radiation affects illumination, while the infrared part causes mainly the heating effect when absorbed by the building elements. In addition, the distribution of daylight on external and internal building surfaces, results in most cases to non-uniform energy flow and therefore solar control is often necessary. These constructions are used to replace artificial illumination and thus to save electricity, but daylight plays a more important role considering visual comfort, communication and other aspects.
In addition to typical windows, the sunspace, the atrium, the gallery or other light-guide designs are used in architecture to provide more solar radiation into the building. The daylight that penetrates the transparent apertures of a building affects illumination and temperature of the interior spaces. Regarding the effect of the suggested system to building space cooling, the results showed a satisfactory temperature reduction, exceeding 10 ☌ for cold water circulation through the absorber. By using thermal absorbers and for low operating temperature, efficiency of about 50% can be achieved, while considering photovoltaics, satisfactory electrical output can be obtained. The Fresnel lenses can be combined with thermal, photovoltaic, or hybrid type photovoltaic/thermal absorbers to collect and extract the concentrated solar radiation in the form of heat, electricity or both. In low intensity solar radiation, the absorber can be out of focus, leaving all light to come in the interior space and to keep the illumination at an acceptable level. The collection of 60–80% of the transmitted solar radiation through the Fresnel lenses on linear absorbers leaves the rest amount to be distributed in the interior space for the illumination and thermal building needs. Laboratory scale experimental results are presented, giving an idea about the application of this new optical system. In this paper, the Fresnel lens concept is suggested for solar control of the buildings to keep the illumination and the interior temperature at the comfort level. The advantage to separate the direct from the diffuse solar radiation makes Fresnel lenses suitable for illumination control of building interior space, providing light of suitable intensity level and without sharp contrasts. Fresnel lenses are optical devices for solar radiation concentration and are of lower volume and weight, smaller focal length and lower cost, compared to the thick ordinary lenses.
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