New heat transfer factors for flat plate solar collectors

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New expressions for the heat transfer factor F R based on inlet, outlet and mean fluid temperature are derived for use with the Hottel-Whillier collector model by the use of restrictive heat transfer assumptions. The new factors are simple algebraic functions which are shown to be equivalent to the generally accepted exponential relationships in realistic collector applications. Their utility is demonstrated through a set of equations by which efficiency curves based on inlet, outlet or mean fluid temperatures may be converted to an alternate basis if the flow rate and the slope and intercept of one efficiency curve are known.