the solar energy flux, enormous collection areas are required, whose manufacture and maintenance are outside the existing possibilities. Note that figures 4 a and b could also be used to evaluate the flux of concentrated solar radiation at normal incidence. By taking into account eqns (1) and (10) this flux is obtained by multiplying the unconcentrated flux for 0 = 0 and the concentration ratio. Of course, this simple procedure couldn't be used in case of 0*0. Maximum Efficiency of Thermodynamic Solar Power Generation As it is well known solar energy could be used to produce work. Two techniques are typical. First, there is the direct conversion of solar energy into electrical work (photovoltaic conversion). Second, there is the so called "thermodynamic" conversion. Solar concentrators reflect the flux of solar radiation towards an absorber where a working fluid is heated up to drive conventional thermal engines. Usually, thermodynamic cycles envisaged are Brayton, Rankine or Stirling cycles. Electrical energy could be generated by alternators coupled to these thermal engines [10], Only this second technique will be considered in this paper. Let us apply the Gouy-Stodola theorem to the assembly solar concentrator + receiver - thermal engine. Then the flux W of available work provided by the engine per unit collection area is given by [14]: where is the energy flux of solar radiation at the level of concentrator aperture, Tq is the aosolute ambient temperature while S-dot is the entropy generation rate per unit collection area during the irreversible process i (i.e. reflection by the mirror, absorption by the receiver, conversion of heat into work etc.) The efficiency T| of solar energy conversion into work is defined as The entropy generation rate is always non-negative. Consequently, from equation (14) we learn that an upper bound of T| given by the energy efficiency flex. Note that T]ex could be reached only when all the processes involved are reversible.
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