BINARY CONVERSION CYCLES FOR CONCENTRATING SOLAR POWER TECHNOLOGY

It is recognized that the temperature potential of concentrated solar energy is much higher than needed by standard conversion cycles. High temperature solar receivers are in the development stage hopefully leading to the use of solarized gas turbines or of solar combined cycles. These systems are analyzed and taken as a reference standard. Binary alkali-metal steam cycles are shown to be intrinsically more efficient than combined cycles owing to their fully condensing nature. Even at top temperatures of about 600 °C typical for steam cycles the binary cycle allows, in principle, a significant efficiency gain (49.5% against 43% of a steam cycle). However, the binary high tem- perature systems are investigated featuring either a direct vaporization of the metal within the receiver or a liquid receiver cooling loop with the working fluid vaporized in a proper heat exchanger. With reference to the second option, the computed efficiency is 56% at a top cooling loop temperature of 1000 °C (the same efficiency is attained in a direct vaporization loop at 720 °C). A 60% thermal efficiency is within the potential of the technology. The above figures can be compared with a combined cycle efficiency of 50% at 1200 °C turbine inlet temperature. Available alkali metals are reviewed for the use of working fluid: potassium being the best known fluid but rubidium (or cesium) offer- ing, in perspective, a better overall performance. Material problems connected with the containment of alkali metals at high temperature are reviewed. Experimental evidence suggests that up to 800–850 °C stainless steel is an adequate material, while for higher temperatures, up to 1200 °C, refractory metals should be used. With reference to heat storage the availability of appropriate high temperature substances either as liquids or as melting solids, storing energy as sensible or as latent heat respectively, is discussed. Finally the critical issue of metal vapour turbine design is considered. The results of a number of computations are presented giving the basic geometrical data of some potassium, rubidium and cesium expanders. Rotor diameters tend to be comparatively large. With reference to a 50 MW overall plant output the maximum tip diameter is 3.9 m for a potassium and 2.8 m for a rubidium turbine.