Heat-exchange relations for unglazed transpired solar collectors with circular holes on a square or triangular pitch

Abstract

The unglazed transpired solar collector is now an established solar air heater for heating outside air directly. Sample applications include pre-heating ventilation air and heating air for crop drying. The outside air in question is drawn straight from ambient, uniformly through the whole surface of a perforated blackened plate (the absorber plate) exposed to the sun. An important parameter in fixing the collector’s efficiency is the heat exchange effectiveness, ϵ. Once ϵ is known, finding the collector’s efficiency is straightforward. This paper presents measurements of ϵ for the case where the plate is perforated with circular holes on either a square or triangular layout, covering a range of wind speeds extending from zero to 5 m/s. These data extend the earlier measurements of Kutscher to a wider range of plate thicknesses, hole spacings (pitch), suction velocities, and to include a square layout of the holes. In the region where the two experiments overlap, agreement between the two is excellent. A new predictive model is developed that is based on breaking down the total heat transfer into contributions from each of the plate sections: the front, the hole and the back. Excellent agreement was found between modelled and measured ϵ; the new measured data were predicted with a 4.2% root mean squared error (RMSE) and Kutscher’s measured data is predicted within 6.3% RMSE.