The exergy of thermal radiation and its relevance in solar energy conversion
Date
2018-05-02
Authors
Wright, Sean
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Abstract
Driven by the importance of optimizing energy systems and technologies, the field of exergy analysis was developed to better illuminate process inefficiencies and evaluate performance. Exergy analysis provides important information and understanding that cannot be obtained from energy analysis. The field of exergy analysis is well formulated and understood except for thermal radiation (TR) heat transfer. The exergy flux, or maximum work obtainable, from TR has not been unambiguously determined. Moreover, many thermodynamic textbooks are misleading by incorrectly implying that the entropy and exergy transport with TR is calculated by using the same expressions that apply to heat conduction.
Research on the exergy of TR was carried out by Petela. However, many researchers have considered Petela's analysis of the exergy of TR to be irrelevant to the conversion of TR fluxes. Petela's thermodynamic approach is considered irrelevant because, others argue, that it neglects fundamental issues that are specific to the conversion of fluxes, issues that are unusual in the context of exergy analysis. The purpose of the research in this thesis is to determine, using fundamental thermodynamic principles, the exergy flux of TR with an arbitrary spectrum and its relevance to solar radiation (SR) conversion.
In this thesis it is shown that Petela's result can be used for the exergy flux of blackbody radiation (BR) and represents the upper limit to the conversion of SR approximated as BR. The thesis shows this by resolving a number of fundamental issues: (1) Inherent Irreversibility; (2) Definition of the Environment; (3) Inherent Emission; (4) Threshold Behaviour; (5) Effect of Concentrating TR. This thesis also provides a new expression, based on inherent irreversibility, for the exergy flux of TR with an arbitrary spectrum. Previous analysis by Karlsson assumes that reversible conversion of non-blackbody radiation (NBR) is theoretically possible, whereas this thesis presents evidence that NBR conversion is inherently irreversible.
In addition the following conclusions and contributions are made in the thesis: (1) Re-stated the general entropy and exergy balance equations for thermodynamic systems so that they correctly apply to TR heat transfer. (2) Provided second-law efficiencies for common solar energy conversion processes such as single-cell Photovoltaics. (3) Showed that Omnicolor (infinite cell) conversion, the widely held ideal conversion process for SR, is not ideal by explaining its non-ideal behaviour in terms of exergy destruction and exergy losses. (4) Presented an ideal (reversible) infinite stage thermal conversion process for BR fluxes and presented two-stage thermal conversion as a practical alternative. (5) Showed that Prigogine's minimum entropy production principle cannot be used as a governing principle in atmospheric modeling, and that in general, it may have little significance. (6) Presented a graybody model of the planet that may prove useful in understanding the thermodynamics of the Earth system. (7) Showed that the expression derived from the Clausius equality for reversible processes is applicable, whereas the statement for irreversible processes is not applicable, when there is significant heat transfer by TR. (8) Showed that the 4/3 coefficient in the BR entropy expression can be obtained by simply using the concept of equilibrium and the experimentally observable relationship for BR energy (energy x T⁴).
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Keywords
Heat, Radiation and absorption, Heat,Transmission