Radiation, solar, thermodynamic

Ideally, all photons with a wavelength of about 900 nm or shorter should be harvested and converted to electric current. This limit is derived from thermodynamic considerations showing that the conversion efficiency of any single-junction photovoltaic solar converter peaks at approximately 33% near the threshold energy of lAeV.1 2 There are numerous ways to convert the solar radiation directly into electrical power or chemical fuel. The silicon solar cell is the most efficient in this respect. Nevertheless, the capital cost of such devices is not attractive for large-scale applications. 

CC12FCC1F2. These compounds are non-toxic and non-flammable, and their thermodynamic properties are ideally suited for the compression/ expansion cycle in cooling and heat pump appliances. However, CFCs are chemically very inert, so when they are vented into the atmosphere, they do not react with atmospheric constituents. They diffuse unscathed first into the troposphere, then penetrate slowly into the stratosphere. There, the solar UV radiation photodissociates these compounds, liberating free chlorine atoms (the C-Cl bond is weaker than the C-F bond). The chlorine atoms react with atmospheric O3 to form chlorine oxide, which in turn reacts with atmospheric atomic oxygen regenerating chlorine atoms ... 

After the introduction, we will briefly discuss the main characteristics of solar radiation. Consistent with the scientific principles on which this book is founded, a rigorous thermodynamic analysis will then follow of the creation of wind energy, of photothermal and photovoltaic energy conversion, and of photosynthesis. Most of this chapter has been based on the monograph Thermodynamics of Solar Energy Conversion by De Vos [1]. 

Thermodynamic Availability of Solar Radiation with Special Attention to Atmospheric Rayleigh Scattering... 

To examine the available energy of solar radiation, let us briefly examine the thermodynamic results for blackbody radiation within an insulated cavity. There is no net flux of... 

As seen in Eq. 1, the water-splitting reaction has an overall energy requirement of 4.92 eV per O2 molecule formed (or +474.7 kj/mol O2 formed). The most abundant solar radiation to strike the earth s surface falls in the visible range (750-400 nm) and fortunately, these photons are energetic enough (1.65-3.1 eV)27 so that as little as two photons are required to drive this process thermodynamically. When broken down into redox half-reactions (5 and 6), the multi-electron nature of reaction 1 is readily apparent. 

The interpretation of Equation (2) is that if a solar collector is to be able to capture all of the radiation incident within the acceptance cone defined by 0max (Figure 4), at most it can have a concentration ratio given by Cmax- This rule comes from the fundamental principles of thermodynamics (Rabl, 1985) and is obeyed by all optical systems. 

Landsberg P. T. and Badescu V. (2002), Solar cell thermodynamics inclnding mnltiple impact ionization and concentration of radiation , J. Phys. D 35, 1236-1240. 

Fig. 1.1. Variation in the conversion efficiency with wavelength for photochemical devices with a threshold. Curve I is a plot of the fraction of incident solar power (in percentages) that is available at various threshold wavelength and Curve II is a plot of the thermodynamic conversion efficiencies under optimal rates of energy conversion. (Data for AM 1.2 radiation)...

The way we will prove that a solar cell must emit heat is to assume that it does not. and show that this would be a violation of the entropy generation principle (that is, the second law of thermodynamics). The steady-state energy balance on a solar cell absorbing radiation but not releasing any heat is... 

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