ACCURATE UPPER BOUND FOR THE EFFICIENCY OF CONVERTING SOLAR-ENERGY INTO WORK

A class of accurate upper bounds for the efficiency of converting sola r energy into work was derived by taking into account (i) the irrevers ibilities associated with the heat transfer inside the heat engine and (ii) details about the system considered, as the geometric (view) fac tor of the Sun, the dilution factors of solar and ambient radiation (e psilon and epsilon(0) respectively), the polarization degree of solar and absorber emitted radiation (P-s and P-a respectively) and the ther mal and optical properties of the converter (convective and conductive heat losses, concentration ratio C, effective transmittance-absorptan ce (tau a) and transmittance-emittance (tau theta)). When the usual ca se of an absorber with view factor B-a = pi is considered and the Sun is at its zenith, an accurate upper bound efficiency eta(sup) is given by: eta sup/[(tau a) (1 + (tau e)/(tau a) epsilon(o)/epsilon 1/C)] = 1 - 4/3 [T-o/T-s (4 pi(2)/Omega(s)(4 pi - Omega(s)) 2 - P-s/2 - P-a (t au e)/epsilon C(tau a) + epsilon(o)(tau e))1/4]1/2 +1/3[T-o/T-s(4 pi(2 )/Omega(s)(4 pi - Omega(s)) 2 - P-s/2 - P-a (tau(e)/epsilon C(tau a epsilon(o)(tau e))1/4](2) where T-s and T-o are Sun and ambient temper atures respectively, while Omega(s) is the solid angle subtended by th e Sun when viewed from the converter. The following constraint has to be fulfilled: T-o/T-s (4 pi(2)/Omega(s)(4 pi - Omega(s)) 2 - P-s/2 - P -a (tau e)/epsilon C(tau a) + epsilon(o)(tau e))1/4 less than or equal to 1. A simpler (but less accurate) upper bound is <(eta)over tilde>( sup) = 1 - 4/3 (T-o/T-s)(1/2) + 1/3 (T-o/T-s)(2). This upper bound is still more accurate than the upper limit efficiencies usually cited in literature.