PERFORMANCE PENALTIES CAUSED BY THERMAL COUPLING IN SOLAR PIPING LOOPS

Solar heating systems often employ conduits to carry cool fluid to the collector array and to carry the heated fluid back to the storage tan k. In systems in which these two conduits are bundled together in one cover (in what has come to be known as the ''life-line'') there is a t hermal performance penalty caused by heat transfer from the hot condui t to the cold conduit. This cross heat transfer results in a penalty i n system performance, and this paper is about evaluating this penalty. We show that the standard Hottel-Whillier-Bliss (HWB) equation can be modified to simultaneously take into account both the pipe heat losse s to the ambient environment and the cross heat transfer between the h ot and cold streams. Parameters in these equations are the thermal res istances between the fluids and between each fluid and the ambient. Me thods are presented for both calculating and measuring these thermal r esistances. We carry out sample calculations of the parameters in the modified HWB equation for a representative solar DHW system equipped w ith either of two different life-lines of commercial design. System si mulations using these parameters reveal that the thermal effects of th ese life-lines are to reduce the net delivered solar energy by 6-14%, and that heat loss to the ambient is more detrimental to the system pe rformance than heat transfer from the hot to the cold conduit. Copyrig ht (C) 1996 Elsevier Science Ltd.