Fig. 5. Absorbing sphere concept. Since both concepts utilize thermal absorption, concentrated laser radiation must be employed to obtain the high temperatures needed for efficient operation. If concentrating optics are to be avoided, then the ground based laser spot should be reduced to the limitations imposed by diffraction, turbulence, thermal blooming, pointing accuracy and jitter. Concentrating optics are undesirable from two standpoints. First, environmental degradation of the reflecting surface will cause power losses and a concomitant decrease in system efficiency. Second, large-area precision optics will be expensive, especially if a high mirror figure is required to obtain very large power densities in the conversion device. For any heat engine receptor concept, the absorbing surface should possess a high absorptance but a small hemispherical emittance at the characteristic operating temperature. In this manner, re-radiation losses due to greybody radiation to the ambient environment can be minimized. One concept (“light bucket”) for maximizing the absorption of incoming radiation while minimizing thermal losses is shown in Figure 5. Re-radiated energy can only escape through the entrance aperture, which purposely subtends a small solid angle. Most importantly, this concept does not employ high quality optical surfaces and, as such, is not subject to environmental degradation. Hence, the absorbing sphere concept is preferred provided that the focal spot size at the receptor is small enough to obtain the large radiation power density necessary for high-temperature (i.e., high thermal-efficiency) operation. Using a receptor aperture of approximately 30 m in diameter, the average incident power density on the internal (absorbing) surface is estimated to be roughly 35 kW/m2. System Chain Efficiencies If the laser-SPS system efficiency is defined as the ratio of power available at the user grid to power produced by the solar photovoltaic array (taken from the power commutator rings), then values of 14% and 10% are estimated for the CO and CO2
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