Experimental and numerical evaluation of the performance and flow stability of different types of open volumetric absorbers under non-homogeneous irradiation

Numerical results found in the literature predict the capability of volumetric receivers to produce gas outlet temperatures of more than 1000 degrees C. Other approaches found the tendency for an inherent flow instability for volumetric absorbers at high outlet temperatures. Both aspects, which are based on one-dimensional approaches, do not fit the experimental experience available even for semi-commercial volumetric receivers in the case of non-homogeneous flux distribution. Since this is the general case in solar power plants, a new analytical approach which takes into consideration a three-dimensional irradiance distribution and its influence on fluid flow and radial heat transfer is presented in this article. A comparison with experimental data for four totally different volumetric absorber types coincides well with the measured overall thermal efficiency as well as with the local temperature distribution. The model helps to explain how thermal efficiencies and temperature distributions are strongly influenced by nonhomogeneous irradiation. Moreover, it demonstrates how flow instabilities are partly prevented by radial heat transfer. Finally, it is shown that a volumetric receiver consisting of sufficiently small absorber modules which are equipped with an additional orifice plate at the rear side will always run under stable flow conditions. (C) 1997 Elsevier Science Ltd.