Absorber for concentrating solar heat collectors

Abstract

Open volumetric absorbers have been tested and evaluated with the objective to determine their ability to heat air for small scale concentrating solar systems, more exactly a solar oven where a stove is heated up by a rock bed storage provided with air above 220°C. The absorbers were tested in terms of different size, shape and material. Heating up air to the target temperature has been a challenge for years, but was achieved with good margin with an experimental setup based on the flaws of previous test setups. At a concentration factor of 300 and a parabolic dish aperture area of 1.07 m2, steady state air temperatures at 300°C were achieved with a stainless steel fiber mesh absorber and a silicon carbide honeycomb absorber which has been exposed for extensively testing in solar towers. The air temperatures were achieved at a flow rate of 1.96*10^-3 kg/s, and as the flow rate was increased the air temperature decreased. At increased flow rate the absorber temperature was reduced and caused less radiation and convection loss which resulted in increased heat transfer between absorber and air. Efficiency defined as air energy increase through absorber divided by the normal direct solar irradiance ranges between 50 % and 80 %, where the efficiency peaked at the highest flow rate employed during the tests. The greatest average air temperature measured was 350°C which was achieved by employment of the honeycomb absorber at a concentration factor of 600 and a mass flow of 1.5*10^-3 kg/s.