2.52 Other Thermal System Concepts In addition to the working fluid cycles, two other conversion concepts were considered in the study. The thermionic concept is based on the phenomenon of electron emission from a high-temperature (approximately 3000F) electrode across a gap filled with cesium vapor to enhance the flow of electrons, with collection at a low-temperature electrode. The efficiency of the device is a function of emitter temperature, and efficiencies of 7 to 10% can be achieved with an emitter operating at 2500°F, as compared to 14 to 17% efficiency at 3000°F. Some research has been initiated to try to reduce emitter temperatures without lowering efficiency, and progress has been made by orienting the crystal axes of the tungsten electrodes in certain ways. For a given efficiency, this has allowed a reduction in emitter temperature of 200 to 300°F, but not under 2200°F, Several converters have been operating for periods over one year, but consistent results have not been achieved. The main problems with this concept are: (1) very high temperatures are required to produce efficiencies of only half that of the Brayton cycle, and these temperatures are too high for the 30-year life requirement of the SPS; (2) the gap between electrodes must be very small (0.01 inch) which presents a significant thermo- structural problem; (3) it is difficult to control the vapor pressure of the cesium at the required level; and (4) cesium leakage further complicates the system. For these reasons this system will not be further considered in the study. Another concept considered briefly was the magnetohydrodynamic (MHD) generator, a high-temperature conversion device using an electrically conducting fluid (liquid metal, liquid metal vapor, or a gas) which is ionized and flows through a channel at right angles to an applied magnetic field, producing a voltage across the channel, Highest efficiencies reached so far with liquid metal systems have been 56% (generator only) on a small liquid metal unit, with projected generator efficiencies of 70-75% for large generators, but present systems have an overall efficiency of less than 10% at a peak temperature of 2000°F. A problem with achieving acceptable electrical conductivity exists with the gas systems. The life limiting component is the current-carrying channel. Also, more work is needed to improve the electrical conductivity of the fluid by electron acceleration techniques. Another problem for space units is the high strength of magnetic field required. This involves the use of large magnets and heavy supports. The development state of the MHD concept is not considered sufficiently advanced to merit further consideration in this study. 2.6 Reflector-Absorber Subsystems While analysis techniques are available for the design of solar reflectorabsorber subsystems9, much development work remains to be done in these two related areas. The intent of this section is not to concentrate on
RkJQdWJsaXNoZXIy MTU5NjU0Mg==