Fluid circulation control in conventional and heat pipe planar solar collectors

Abstract

Planar solar collectors based on the heat pipe principle have been described by several authors (Bienert and Wolf I l l , Francken [2] ). Despi tea relatively complicated method of manufacturing as a drawback, this type of collector has several favourable properties with respect to a conventional planar collector. Advantages such as a minimum amount of material giving low material costs and a small heat capacity, the simple means for freeze protection and a reasonable resistance against corrosion, have been mentioned. As to the performance, an important quantity is the instantaneous collector efficiency. In the case of solar radiation being so high that the absorber temperature remains higher than the storage temperature, a properly designed heat pipe collector has about the same efficiency as a good conventional one. During periods of transient insolation the absorber temperature can become lower than that of the fluid in the storage tank. In order to prevent a loss of heat during these periods, the fluid circulation is usually stopped by means of a pump control unit. In this way, a reasonable increase of the overall efficiency of the conventional collector can be obtained. In contrast, the heat pipe collector has a natural, basically more efficient property to suppress heat losses, i.e. the diode effect. This effect automatically leads to a higher collector efficiency. Moreover, the efficiency appears to be nearly independent of the fact, whether the fluid circulation is stopped or not. Hence, during the day, an on/off control in the pumping system of the heat pipe collector can be omitted without a decrease of efficiency. In that case, the disturbing on and off switching of the fluid circulation is avoided and the life time of the pump increases. In this technical note, the advantage of the heat pipe collector, just mentioned, will be quantitatively illustrated. The interesting properties of both this type of collector and a conventional flat plate collector are numerically investigated for several conditions of solar radiation.