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1989
Conference Paper
Title
Thermosolare Energienutzung mit transparenter Wärmedämmung in Einfachstanlagen
Abstract
Experience has shown that the thermal utilization of solar energy is often - in spite of high possible energy savings - economically unattractive. The aim of this project therefore was the development of systems for use in equatorial regions, where aspects like a simple construction and inexpensive materials are of primary importance. Thermal losses from the aperture area should be minimized with new transparent insulation materials. Within the project different collector systems for domestic hot water were investigated as well as two solar cookers. Experiments were carried out with five prototypes of a so-called plastic bag collector. This essentially concerns a transparent plastic bag acting as a water storage (150 l) which lies on top of a horizontal, insulated absorber. The influence of different transparent cover foils on the systems' efficiency could be shown by determining the values of Eta (0.47 - 0.60) and the overall U-values (6.0 - 7.2 W / (qm K) for usual foils, 2.2 W / (qm K) for transparent insulation). Comparative measurements with thermosiphon systems were included. With the emphasis on a high storage (heat) capacity together with a low overall U-value, a new concept of a solar cooker was developed and tested under the Freiburg weather conditions. The cooker consists of a massive storage with a transparently insulated, horizontal absorber. The choice of the materials to be used for the storage (sand, concrete) and for the opaque insulation (straw, coco fibres) depends on their actual local availability. The measured storage temperatures of two proto-types were between 100 and 110 degree C with a mean insolation of 6.6 kWh/square meter and a mean ambient temperature of 21 degree C. Optimizations of the overall U-value and the ratio of absorber area to gross area could further improve the cooker's performance.