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Content archived on 2024-04-16

Photovoltaic energy conversion : an endoreversible process


Thermodynamic analyses will be made of solar cell efficiencies for various materials under various illuminations. Appropriate functions will be used for the illumination as a function of frequency, the density of states as a function of the electron energy, etc. Endoreversible thermodynamics and computer algebra will be utilised.
Research relevant to the development of thermodynamic analyses of photovoltaic energy conversion has been carried out.As a tool, the concept of endoreversible thermodynamics of photovoltaics has been introduced. Endoreversible processes are a special class of irreversible processing; the irreversibilities are all located in the transport of heat from the heat sources to the heat engine and from the heat engine to the heat sinks. It was demonstrated that photothermal solar energy conversion could be modelled as an endoreversible process. A photovoltaic converter proved to be more complicated than a photothermal one. A more general model of endoreversible engines was introduced, where thermodynamic reservoirs were characterized not merely by a temperature, but by a temperature and a chemical potential, and where not only energy but energy and matter were exchanged between reservoirs.

The conversion efficiency of hybrid photothermal and photovoltaic convertors has been calculated. It was demonstrated that a hybrid converter could realise higher efficiencies than pure photovoltaic or photothermal converters. For a given light concentration and a given bandgap, a pure photothermal converter has only one degree of freedom, its temperature. Analogously, a pure photovoltaic converter has only degree of freedom, its bias voltage. A hybrid converter has 2 degrees of freedom, a temperature and a voltage. The maximum power point proved to be at negative voltage.

A computer algebra has also been developed to handle symbolic equations in solar energy theory with speed and accuracy.
The photovoltaic energy conversion process will be treated thermodynamically taking into account the limited angle subtended by the sun at the earth's surface, the radiation emitted by the cell and its surroundings, the effect of the energy gap and the occurrence of non-black radiation. It is hoped to extend the theory to include the effect of an arbitrary number of energy gaps. The results will be applied to various technologies, notably crystalline and amorphous. The effect of entropy generation will also be considered in various ways. For example a model can incorporate explicit expressions for the entropy generation rate (S) in the converter. Alternatively one can use a model which merely uses the fact that S 0.

Among the technical tools to be used we include endoreversible thermodynamics. This deals with irreversible cycles such that the irreversibilities are located in the transport of heat between the heat engine and its reservoirs. The theoretical efficiencies are of course lower than the Carnot efficiency and investigations of such cycles exist and will be used or extended.

Another technical aid to be used is computer algebra. This is a computer tool to solve algebraic problems by manipulation of symbols instead of numbers. The technique was invented in the 1970's by people working in quantum electrodynamics and general relativity. It has only recently been introduced into other fields of physics and technology. As far as we know it has not yet been applied to solar energy conversion. We are envisaging the use of the language ''Reduce'' although others are not excluded.


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