Unglazed Transpired Solar Collectors: Heat Loss Theory

Unglazed transpired solar collectors offer a potentially low cost, high-efficiency option for once-through applications such as preheating air for ventilation, crop drying, and desiccant regeneration. This paper examines the major heat loss mechanisms associated with this concept. Radiation heat loss is determined by considering losses to both the sky and the ground. Convective heat losses are obtained by integrating the product of the temperature and velocity profiles in the boundary layer at the downwind edge of the collector. This convective heat loss is then expressed in terms of the thermal equivalent length of irradiated absorber, and analysis shows that this loss can be very low for large collectors even under windy conditions. These results are incorporated into a simple computer model which predicts collector efficiency as a function of suction velocity, wind speed, ambient temperature, and radiation. Remaining research issues are discussed.