On the correlations between collector efficiency factor and material content of parallel flow flat-plate solar collectors

The collector efficiency factor F ′, besides the collector heat loss coefficient U L, characterizes the thermal quality of a solar collector. As F ′ is strongly influenced by the tube distance w and the absorber plate thickness δ, F ′ is also correlated with the material content of absorber plus tubing. Due to the future mass production of collectors and to the restricted copper resources (in the literature, a range until 2026 is given), the role of material savings can be expected to become more and more important. This paper focuses on the correlations between F ′ and the material content of absorber and tubing for flat-plate collectors with the fin-and-tube geometry. The correlations between w, δ, F ′ and material content are presented in a new type of nomograph. This nomograph indicates the values of w and δ that minimize the material content (for a given F ′). For a typical absorber with F ′=0.90, material savings of 25% can theoretically be achieved without any deterioration of F ′, by reducing the absorber plate thickness and the tube distance. The resulting plate thickness is below 0.1 mm; the respective tube distance will be about 7 cm. Practical restrictions are discussed. In a sensitivity analysis, the influence of different parameters on F ′ is investigated. The most important parameters are w, U L, δ and the Reynolds number. The technique chosen for contacting tube and absorber has only a minor influence on F ′.