Performance of Unglazed Flat Plate Heat Exchangers at Low Temperatures

Abstract

The heat source of a heat pump for space heating or domestic hot water production is in general ambient air. It was proposed to use unglazed flat plate heat exchangers, so called “solar absorbers”, in the heat pump cycle to gain heat from the outdoor air and radiation from the sun. It is shown to what extend “solar absorbers” get energy from the sun and the ambient air. Therefore six different flat plate heat exchangers were experimentally investigated under outdoor-conditions. Results show, that the flat plate heat exchangers absorb between 53% and 88% of the solar irradiance, when they are working with fluid temperatures near at ambient temperature. The overall heat transfer coefficients are between 11 W/m2K and 32 w/m2K. With heat transfer by forced convection from the ambient air (wind speed about 4 m/s) the overall heat transfer coefficient reaches up to 4o W/m2K. Using these characteristic values, the yearly obtained useful heat was calculated with weather conditions, measured in Stuttgart 1981 and 1982. All absorbers operated 5 K below the ambient temperature. Results show, that an unshaded absorber gains about loo to 13o% more than a shaded absorber, in case the absorber ist back isolated. If the absorber is ventilated from the back the unshaded ones gain about 60 to 80% more than the shaded absorbers.