NUMERICAL MODELING OF REFLUX SOLAR RECEIVERS

Using reflux solar receivers to collect solar energy for dish-Stirling electric power generation systems is presently being investigated by several organizations, including Sandia National Laboratories, Albuque rque, N. Mex. In support of this program, Sandia has developed two num erical models describing the thermal performance of pool-boiler and he at-pipe reflux receivers. Both models are applicable to axisymmetric g eometries and they both consider the radiative and convective energy t ransfer within the receiver cavity, the conductive and convective ener gy transfer from the receiver housing, and the energy transfer to the receiver working fluid. The primary difference between the models is t he level of detail in modeling the heat conduction through the receive r walls. The more detailed model uses a two-dimensional finite control volume method, whereas the simpler model uses a one-dimensional therm al resistance approach. The numerical modeling concepts presented are applicable to conventional tube-type solar receivers, as well as to re flux receivers. Good agreement between the two models is demonstrated by comparing the predicted and measured performance of a pool-boiler r eflux receiver being tested at Sandia. For design operating conditions , the receiver thermal efficiencies agree within 1 percent and the ave rage receiver cavity temperature within 1.3 percent. The thermal effic iency and receiver temperatures predicted by the simpler thermal resis tance model agree well with experimental data from on-sun tests of the Sandia reflux pool-boiler receiver. An analysis of these comparisons identifies several plausible explanations for the differences between the predicted results and the experimental data.