Calculation and Measurement Methods for the Performance of Solar Collectors. Models of Flat Plate Collectors, Transparent Insulation and Efficiency Factors

University dissertation from Lund University, Lund Institute of Technology, Div. of Energy and Building Design, P.O. Box 118, SE-221 00 LUND, Sweden

Abstract: This thesis deals with models of flat plate collectors, transparent insulation and efficiency factors. An equation for the energy output from a glazed flat plate collector is derived by modelling the collector efficiency factor, F´ as the sum of a constant and a temperature dependent part. An alternative way of testing a flat plate collector, based on this model, is suggested. A complete set of algorithms for calculating the energy output for a flat plate collector with flat films or honeycombs between the cover and the absorber is presented. Algorithms for calculating the radiation heat exchange between the different sheets or layers in a honeycomb glazing are given. Formulas for calculating the absorbed solar energy in each layer are also given. The algorithms can be used in a computer program for determining the energy output, the efficiency and collector characteristic parameters. Measurements of heat conductivity and total heat loss coefficients of glazings, performed for different transparent insulation materials, are presented and compared with calculations. Measurements of directional-hemispherical solar transmittance are also presented. For a concentrating collector with an uneven irradiation on the absorber, the efficiency factor for the gain term of the energy output equation, here called the optical efficiency factor, F´c, is different from F´ and is a function of the irradiation distribution on the absorber. Formulas for calculating F´c(x) for the location x on a fin absorber with constant fin thickness are derived. The average optical efficiency factor, F´c,a, can then be calculated from F´c(x) and the absorbed intensity distribution. Formulas for calculating the temperature distribution across the absorber for the case of uneven irradiation are also derived. A method for accurate measurements of F´ is presented and tested. The method uses accurate temperature measurements across the absorber plate, in the heat carrier fluid and in the ambient air for heat losses without irradiation. Two absorbers of different types were tested. For laminar flow, the measured values of F´ for both absorbers were slightly higher than the calculated values, while in the transition region, the measured values were slightly lower.

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