Performance characteristics of rectangular honeycomb solar-thermal converters

Abstract

An array of rectangular cells, properly shaped with highly reflecting specular walls absorbing to infrared radiation, is shown to be an effective device to limit heat losses when placed over a solar absorber. Theoretical relationships are presented which permit the prediction of the performance of closed-cell and air transpiration systems. The theory rests on analytical and experimental studies of radiant transmission through cells with specularly reflecting, polarizing walls coated with an absorbing film and studies of the suppression of natural convection in cells heated from below. Good agreement was obtained between theory and experiments for the air transpiration system. Three experimental honeycombs of different geometries were tested over a 1-sq ft blackened fiberglass porous absorber, assembled in a well-insulated and glazed housing mounted on a tilting platform to permit any desired orientation. Experiments over a 1-yr period, and theoretical predictions, indicated that the thermal efficiency could be given as a linear function of the ratio of temperature difference between the average temperature of the working fluid and ambient air to the incident solar power, virtually independent of flow, angle of tilt and time of collection. For the best honeycomb tested, the thermal efficiency ranged from 82 to 63 per cent for values (T av − T a) G s equal to 0·10 and 0·30°F/ (B.t.u./hr ft 20, respectively. The honeycomb-porous bed solar-air heater is shown to perform as well as the best previously reported solar-air heater at moderate temperatures. At higher temperatures, of the order of 200°F, it is greatly superior.