The efficiency of an air-heating flat-plate solar collector can be increased by reducing the absorber plate temperature by providing it with extended surfaces. An experimental study was conducted to determine the temperature depression and the increase in friction factor, by providing a uniformly heated plate with pin fins. Data were obtained with three spacings of the fins for five mass flow rates of air and two values of heat flux. Significant temperature depressions can be obtained leading to increase in efficiency but these are acompanied by increases in friction factor. Employing different length scales and the concept of pumping power factor, it is shown that the data can be correlated by j = 1.018 Re−0.49 and Nu = 1.03 F0.19.

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