9.1 CONCENTRATION LIMITS FOR NONIMAGING TERMINAL CONCENTRATORS IN Central RECEIVER APPLICATIONS
In this chapter, a preliminary analysis that carries out various performance trade-offs involved in the design of a two-stage central receiver plant that could achieve ultrahigh concentrations with a nonimaging CPC type secondary is presented. It should be noted that the study was carried out in the context of using a solar central receiver plant to generate hydrogen from the direct thermochemical splitting of water. However, the analysis is independent of that context and applies to any and all central receiver (or the so-called power-tower) configurations. This analysis shows that such a two-stage central receiver plant can achieve geometric concentrations approaching the maximum theoretical limits and provides a methodology for evaluating various geometrically dependent performance trade-offs. The concept of using a secondary (terminal) concentrator in central receiver systems has been around for some time (Rabl, 1976a; Athavaley, Lipps, and Vanthull, 1979) but has not been seriously investigated until relatively recently (Spirkl, Timinger, Ries, Kribus, and Mus – chaweck, 1998; Kribus, Huleihil, Timinger, and Ben-Mair, 2000; O’Gallagher, and Lewandowski, 2005).
The objective is to design a large-capacity (multimegawatt) plant, and it is clear that such a large scale with >1 MW power requirements can only be delivered by some kind of central receiver. We wish to consider the incorporation of a nonimaging secondary into such a configuration. In CPC designs, the geometry known to be optimal is a cylindrical, axially symmetric configuration, which, for a central receiver has a central tower surrounded by a circular area containing heliostats (Athavaley, et al., 1979) such as illustrated schematically in Figure 9.1a.