Design and exergy analysis of solar roofs: A viable solution with esthetic appeal to collect solar heat

Application of a pipeline at the lower side of copper roofs to use them as solar collectors was investigated. An optimized module capable of converting a wide fraction of solar radiation into heat, and at the same time preserving the esthetic quality of the roof, was designed. Exergy analysis, which allowed quantification of the value of collected energy, including temperature effects and friction losses of the heated fluid, was applied. The results allowed suggestion of a rectangular module with four ducts having rectangular cross section. The basic proposed version was the Unglazed Solar Roof (USR), which showed interesting levels of efficiency (30e60%) over the entire working field. Even the glazed version (GSR) was introduced and analyzed, but its performance showed significant improvements only when extensively used during cold seasons. Compared with a commercial reference flat plate collector (FPC) model, the efficiency of USR is reduced by 20% over the whole operating field. The curves of exergy efficiency showed an optimization point at temperatures well above those in flat plate collectors. For this reason, exergy analysis applied to solar collectors is worth only (I) to evaluate the lower limits in cross sectional area of the ducts, to avoid too narrow height or width which may lead to very high friction losses and (II) to manage the flowrate to extract the highest amount of exergy during the variable environmental and fluid conditions. Integration with a domestic hot water system showed a potential achievement of a yearly solar fraction of 53% with USR and 60% with GSR. Considering that both systems are fully integrated within the roof cover, differently from standard collectors, the esthetic appeal of the building roof would not be affected.