Heat loss study of trapezoidal cavity absorbers for linear solar concentrating collector

There should be minimum heat loss from the absorber to achieve better efficiency of the solar collector. Overall heat loss coefficients of the trapezoidal cavity absorber with rectangular and round pipe were studied in the laboratory. Two identical rectangular pipe absorbers (section size: 100 × 23 mm, thickness: 2.5 mm and length 2170 mm) and two round pipe absorbers (a set of six mild steel round tubes of 16 mm diameter and 2.5 mm thickness brazed together in single layer making 100 mm width) were fabricated. A rectangular and a round pipe were painted with ordinary mat black paint (emissivity at 100 °C = 0.91) and one pipe of each type was coated with black nickel selective surface (emissivity at 100 °C = 0.17). Overall heat loss coefficient of the absorber was studied by circulating hot oil through it at different temperatures. The heat loss coefficient was increased with the absorber temperature. The heat loss coefficients for ordinary black coated and selective surface coated round pipe absorbers were varied from 3.5 to 7.5 W/m2/°C and 2.7–5.8 W/m2/°C respectively. The rectangular pipe section has marginally higher heat loss coefficients as compared to round pipe absorber. Selective surface coating on the absorbers reduced heat loss coefficient significantly by 20–30% as compared to ordinary black coating. The double glass cover also reduced heat loss coefficient by 10–15% as compared to single glass cover. The overall heat loss coefficients were also estimated analytically by parallel plate correlation and cavity correlations. The trend of variation of estimated heat loss coefficients by both methods was similar to experimental values. However, estimated values by cavity correlation were closure and uniformly distributed at all temperature range.