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Reducing Heat Losses in Polyethylene Greenhouses

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Abstract

The use of curtains to reduce heat loss from double-filmed air-inflated polyethylene greenhouses has been investigated. Tests in an environmental chamber and a small prototype greenhouse indicate that substantial heat savings can be obtained. Evaluations of a number of curtain materials and installation techniques predict savings of up to one-half of the total heat requirement.
Article
Theoretical and experimental evaluations are made of thermal curtains applied to double-glazed greenhouse structures. The reflectance and emittance of the curtain is permitted to be different on the two surfaces: It may be partially transparent or opaque to thermal radiation, and it may have thick blanket insulative properties. The effect of cloud cover upon the night-sky radiation temperature is included in the model. Convection coefficients are prescribed for the curtain and glazed surfaces, and the effect of air leakage into the space between the curtain and inner layer of glazing is included as an empirically defined parameter. The thermal balance for the system is obtained in terms of the radiative, convective and air-leakage parameters relevant to each component. The resulting set of thermal balance equations are solved iteratively using existing matrix-solution programs at the University of Waterloo. An experimental rig was constructed to determine nighttime heat loss from a conventional double-walled polycarbonate glazing material which is partially transparent to thermal radiation. The rig was operated with and without an insulating curtain. An opaque multilayer curtain and a semitransparent bubble film curtain were tested. The agreement between the thermal model and the test rig was within ±5% for all cases. Cloud cover was found to be a significant parameter that alters heat loss upwards by 11% from fully cloudy to clear-sky conditions. Intermediate cloud cover conditions were investigated as well and were found to be well predicted by an existing correlation for sky temperature vs cloud cover. The analysis is extended to provide a simple resistance coefficient for heat loss from the glazed structure based upon inside and outside temperatures for a specified cloud cover. It is found that the semiopaque curtain and double-glazed combination has 3.74 times the thermal resistance of a single glass structure as compared to a value of 3.81 for the opaque curtain and double-glazed arrangement. In addition the semiopaque curtain can be used for solar shading during the summer time, thereby serving a dual purpose that significantly increases its economic advantage.
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