The improvement of collector efficiency in solar air heaters by simultaneously air flow over and under the absorbing plate

The performance of double-flow type solar air heaters, in which air is flow ing simultaneously over and under the absorbing plate, is more effective th an that of the devices with only one flow channel over or under the absorbi ng plate because the heat-transfer area in double-flow systems is double. T he effect of the fraction of mass how rate in the upper or lower flow chann el of a double-flow device on collector efficiencies, has also been investi gated theoretically and experimentally. Considerable improvement in collect or performance is obtained by employing a double-flow type solar air heater , instead of using a single-flow device, if the mass flow rates in both flo w channels are kept the same. (C) 1999 Elsevier Science Ltd. All rights res erved.