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The collector efficiency factor F', besides the collector heat loss coefficient [U.sub.L], characterizes the thermal quality of a solar collector. As F' is strongly influenced by the tube distance w and the absorber plate thickness [delta], 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, [delta], F' and material content are presented in a new type of nomograph. This nomograph indicates the values of w and [delta] 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.sub.L], [delta] and the Reynolds number. The technique chosen for contacting tube and absorber has only a minor influence on F'. Keywords: Collector efficiency factor; Material content; Flat-plate solar collector

Three different optimization procedures, which are expected to improve the performance of large solar thermal systems, are described in this paper. The first procedure concerns the planning phase of the system. The second one considers its operation and should be carried out after about one year of data collecting. The third one examines the daily performance considering predictions of weather and hot water consumption and the actual temperature level in the buffer store. For all three optimization steps the feasibility and the energetic potential have been investigated. For these studies validated system models of two solar domestic hot water systems in Germany have been implemented in TRNSYS. In combination with the simulations, both classical algorithms and Evolutionary Algorithms have been applied for the optimization procedures.

A method to calculate the short-term dynamic behavior of solar collectors, working with varying fluid-flow rate, has been developed. It is based on a system of first-order partial differential equations. The method was applied to model a large unglazed collector used for heating a public outdoor swimming-pool in Marburg (Germany). A validation with data measured at this collector showed good short- and long-term accuracy of the model at constant and varying flow rates. Furthermore, the dynamic behavior of the detailed model was compared to that of several model approximations especially at varying flow rate. In the case of unglazed collectors a simple dynamic model, based on an ordinary differential equation, gives accurate results in most practical applications even at varying flow rate. The extension of the method to flat-plate collectors is demonstrated as well.