where Ee is the energy radiated (per unit area) by the device at temperature Te, i.e., Ee = aTe4, and Ae is the area over which the emission takes place. For our present purpose, we assume that Ae = AH. Since the entropy of the device is constant, we must require that entropy-in equals entropy-out, i.e., At the location of the Earth 17, numerically equals 97%. This result is impressive but is also somewhat misleading. An examination of 17, indicates that the further from the Sun the device is located, the more efficiently it operates. This results from the condition that entropy-out must equal entropy-in. Under such a restraint the ratio of energy-loss (E,,) to energy-gain (E^ must go at Te/Ts. Since T, is fixed, high efficiency demands low Te which, as Eq. 6 shows, means large value of r. In fact, in the limit of very great distance the device will operate with (essentially) 100% efficiency upon (essentially) zero radiation. Since the ultimate goal of any project to utilize such devices presumably would be to maximize power output rather than efficiency as defined (in the normal way) by 17^, one should consider 17,, rather than 17^. By its definition, a maximum value of rjj, will correspond to maximum power extractable from the Sun. Standard mathematical analysis shows that the maximum of 17,, occurs when Equations 4 and 6 taken together lead to the conclusion that For comparison, at the location of the Earth (r —200R), 17 = .002% and at the location of Mercury (r = 80R), i79 = .02%. REFERENCES 1. P.T. Landsberg, Thermodynamics and Statistical Mechanics, Oxford University Press, Oxford, 1978, p. 243. 2. W.H. Press, Theoretical maximum for energy from direct and diffuse sunlight, Nature, 264, 734-735, 1976. 3. M. Planck, The Theory of Heat Radiation, Dover, New York, 1959.
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