Exergy ratio

In addition to these standard definitions, it is useful to define a quantitative measure of the quality of energy that can be delivered as a result of a change from State 1 to 2 this is represented by the exergy ratio (o ) defined as... 

According to this definition, the quality of mechanical or electrical energy is equal to unity and that of thermal energy at a temperature, T, is equal to the Carnot factor, 1 - TQ/T. For chemical reactions, the exergy ratio (a-) represents that fraction of the delivered energy that could be converted to thermodynamic work by a reversible process and has a value most often (but not always) between zero and unity. 

Exergy Ratios of Primary and Secondary Energy Sources... 

The calculated values of exergy ratios for various primary and secondary sources are shown in Table 1. It 1s to be noted that all the fossil fuel sources are inherently of very high quality. The value of aE greater than unity for coal stems from the fact that a reversible oxidation of carbon at ambient temperature has a negative TQ S and results in production of more work than the enthalpy of combustion, with an associated absorption of heat from the environment. In spite of these high values of < r, when fossil fuels are combusted, say for electrical generation, the maximum temperature is limited not by thermodynamic considerations but by materials constraints. The effective < under this limitation is restricted to values below 0.65. t... 

In order to understand the underlying reasons for the higher overall efficiencies afforded by the CHP systems, it is necessary to re-examine the four options in the framework of the thermodynamic quality. Figure 3 shows semi-quantitative plots of Individual steps for each of these options, using exergy ratios as the ordinate. 

Figure 3. Exergy ratios of energy conversion steps.

Second-law thermodynamic analyses have been shown to be of considerable value when applied to systems where an efficient energy interconversion is important. Using the chemical energy transport systems as examples, the use of exergy ratios as a measure of thermodynamic quality has been shown to give Important Insights Into the efficiency or inefficiency inherent in any conversion of one energy form to another. 

The key energy transfer steps for which we need the exergy ratios are ... 

One last point to be noted pertains to a comparison between the steam-reforming reaction (Case 4) and the methanol cracking reaction (Case 5). From the exergy ratio calculations, the reforming reaction appears to be superior in its ability to produce lower quality fuel. However, the overall efficiency calculations show a lower value for Case 4 than that for Case 5. The main reason for this reversal is due to the fact that nearly 25% of the recuperated energy for Case 4 is in the form of the heat of evaporation of H 0 and is not recovered from the exhaust gases. The result 1s an increase in the stack losses. 

The option of lowering the exergy ratio of fuels through... 

Table 3. Exergy Ratios for Key Energy Transfer Steps...

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