471-2004-1-Concentrating collector-Serkan Kapucu

The stagnation temperature tmax is given by

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Unformatted text preview: stagnation temperature Tmax is given by: temperature gamma.Ainqin = U.Arec(Tmax - Ta). For example, if the optical efficiency is gamma = 0.8, gamma the incident solar irradiation is qin = 800W/m2, the the the ambient temperature is Ta = 30°C, and the heat loss ambient coefficient is U = 10W/m2K, then a concentration ratio coefficient K, Ain/Arec = 1 (no concentration) gives Tmax = 94°C, and a concentration ratio Ain/Arec = 10 gives Tmax = 670°C. concentration The collector efficiency etac at operating temperature T is eta etac=Qout/Ainqin = F'[gamma-U.Arec(T -Ta)/Ainqin] = F''gamma(Tmax - T)/(Tmax - Ta). F ). The available mechanical power from the thermal power output of the collector that would be obtained using a Carnot cycle is Qout(1 - Ta/T), where the temperatures are absolute cycle /T), temperatures. temperatures. The second law efficiency eta2 of a heat engine is second eta defined by defined eta2=(mechanical power delivered) /(available mechanical power). /(available Suppose a heat engine with second law efficiency eta2 eta uses as input the thermal power Qout from the solar uses collector. The first law efficiency of the engine is collector. eta1 = (mechanical power delivered)/Qout = eta2(1 - Ta/T), eta where Tmax depends on the design of the collector and where on the solar radiation input qin. Now, given F', gamma, on Now, gamma eta2, Ta, and Tmax, we can find the maximum efficiency eta we obtainable, and the optimum operating temperature Topt obtainable, from the condition d(eta)/dT = 0. This occurs at the from )/dT optimum temperature optimum ½ Topt = [TmaxTa],, and the maximum efficiency is obtained by putting T = Topt in the equation eta = etac.eta1. eta For example, putting F' = 0.9, gamma = 0.8, eta2 = 0.6, gamma eta Ta = 30°C = 303K, we get the efficiencies etamax for eta different degrees of concentration shown in Table 6.1. Very low overall efficiencies are obtained unless operating temperatures greater than 500°C are used. Expensive concentrating systems are needed to reach these high temperatures, so commercial viability is difficult [12]...
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This document was uploaded on 02/21/2014 for the course MEE 599 at Federal University of Technology.

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