---- -a- ----- - . -/a ■ >— — - Fig. 4. The dependence of the total CO laser system efficiency, t),, on the discharge efficiency, 17,, using the thermodynamic models of a closed-cycle EDL. including photovoltaic cells (2), tuned optical diodes (29,30), various classes of laser engines [boiler (2), resonance absorption (31-35), and photon (36-38)], and ther- moelectronic cells (39-44). Ir photovoltaic cells and tuned optical diodes are only in the preliminary research phase, and uncertainties exist concerning their cost effectiveness, conversion efficiency, power handling capability, weatherability, and reliability. The resonance-absorption and photon engines and thermoelectronic cells require the use of a transmissive window to isolate the working fluid and may also require the use of concentrating optics at the receptor device, all of which are subject to environmental degradation and possible problems with reliability. In keeping with the exclusive utilization of current or near-term technology, only the boiler laser engine is sufficiently advanced and workable to satisfy the present laser-SPS requirements for conversion of 5-jzm or 9-^tm laser radiation into electric power. The boiler laser engine relies upon absorption of the incident radiation at a surface and conduction of the resulting heat to a working fluid. An advanced Brayton cycle may achieve an efficiency of 40% (2). The use of an energy exchanger in the thermodynamic cycle promises to significantly improve this value (35). The energy exchanger/binary cycle concept developed in Ref. (3) uses a high-temperature Brayton cycle coupled to a bottoming Rankine cycle. The efficiency has been calculated to be 73%; however, the necessary high temperatures in the primary loop require the use of a liquid alkali as the working fluid, which may present difficulties in materials selection. The explicit receptor design will depend upon the available power density at the focal spot and constraints imposed by high-temperature materials.
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