cycle gas turbines which use inert gases such as He or He-Xenon mixture up to 100 psi. Alternate materials exist such as TD nickel (DuPont) for a tubular heat exchanger in the cavity receiver. Alternate cavity designs exist such as a porous graphite receiver slab in a high temperature glass pipe such a concept would be operated at lower temperatures due to glass strength limits (Ref. 7). Open cycle (air) Brayton cycles are also being considered with various porous ceramic receivers. The dish collector can be used to heat a variety of fluids for the central generation system, or a gas for the distributed generation system. Although the materials, temperature level and specific design will be different, a cavity receiver similar to that for inert gases can be used. Thus, the cavity receiver is attractive for several reasons. It is probably not necessary to use a vacuum to minimize convection heat leaks which both simplifies the design as well as eliminates the transmission decrease of an additional glass surface. Radiation heat leak is limited to only the small amount reradiated through the small aperture. The heat transfer problems which result from such high concentration ratios are minimized since the amount of tubing surface inside the cavity is relatively independent of the concentration ratio and aperture size. This is especially important when a gas heat transfer medium is used. A collection efficiency prediction of 70% is estimated for 815°C at a concentration ratio (CR) of 1000 as shown in Figure 7. The collection efficiency approaches 80% at temperatures of 550°C. This study makes the simplifying assumption that the absorber fluid exit temperature is relatively close (<50°F) to the effective cavity temperature. The collector temperature of Figure 7 is therefore considered to also be the fluid temperature. This assumption applies very well to good heat transfer fluids such as NaK and boiling water. Poorer fluids such as helium and subcooled water and superheated steam do require significant temperature differences between the cavity and
RkJQdWJsaXNoZXIy MTU5NjU0Mg==