Abstract
Exergy is a thermodynamic property that represents the quantification of the maximum useful work that can be extracted from a system interacting with the environment. Regarding solar radiation, radiative exergy has been a matter of study over the last 60 years where the main models applied describe the radiation as undiluted and diluted. The exergy of solar radiation is useful in the preliminary assessment of the performance of solar technologies, since the efficiency of the system depends directly on this value. The present paper describes a review of the main models reported in the literature considering these two approaches, analysing the main differences between the models and the main assumptions applied. A comparative analysis is carried out for the models of diluted and undiluted radiation, where the behaviour of every expression is discussed in detail. For the undiluted expressions, the behaviour of every model within a temperature range is analysed. For black-body radiation at a source temperature of 6000 K, the model proposed by Jeter determines an exergy factor of 0.96, while Spanner, Petela, Press and Badescu calculate a value of 0.93. Parrott’s model obtains a value of 0.99, which is above the value for Carnot efficiency. The diluted exergy expressions were evaluated according to wavelength and temperature range, where the trend in each comparison was that the exergy calculated from Karlsson, Candau and Petela was always the lowest. This result is attributed to the fact that these expressions consider the spectral entropy of the medium the radiation passes through. Finally, some new approaches are analysed which consider empirical correlations based on meteorological variables to model the exergy of solar radiation.
Highlights
In thermodynamics, exergy is a property that describes the maximum useful work possible during a process that brings a system into equilibrium with the environment
Of the different models previously described, the one that has achieved a consensus over the years, that gives the exergy factor of the undiluted solar radiation, is the model proposed by Petela [24,40]
Different kinds of studies have been developed to estimate the exergy of solar radiation, as can be seen in Table 2, including attenuation effects, which can be divided into two main groups: those studies that consider a thermodynamic analysis between a radiation source and a sink, and those that consider an electromagnetic approach based on Planck equations
Summary
Exergy is a property that describes the maximum useful work possible during a process that brings a system into equilibrium with the environment. The present review is organised according to how models commonly consider the exergy of solar radiation including those which focus on exergy of undiluted radiation, exergy of diluted radiation, and empirical models for specific locations.In addition, a comparative analysis is carried out considering models that have similarities in their conception and assumptions, allowing evaluation of the differences in the results delivered by each model. It is proposed that a better approach should be the evaluation of the exergy of solar radiation considering the undiluted exergy expression
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