Thermodynamic efficiency reversible

Hydrogen is increasingly recognized as a potential fuel for industry and transport. It can be produced by electrolysis or photolysis of water, and its oxidation produces no greenhouse gases. Moreover, it is the best fuel for fuel cells, which generate electricity directly by the reversal of electrolysis. Fuel cells have been known for almost two centuries they are a type of battery in which electricity is produced by the redox reaction between H2 and O2 in solution. They offer high thermodynamic efficiencies... 

It should be noted that r y is the maximum thermodynamic efficiency obtained under reversible conditions, i.e., such that the rate of any photochemical reaction from D is infinitesimally slow. Although riy has some theoretical interest, it has no practical interest since we are interested in maximizing the rate of a photochemical reaction from D which will lead to the production of useful work. The rate of energy conversion by such a process can be defined as... 

Magnetic refrigeration offers high thermodynamic efficiencies due to the reversibility of the magneto-caloric effect. For active magnetic regeneration... 

The ideal, unrealistic, but basic limit of the thermodynamic efficiency of a process is that of the reversible process where all work available and entering the process is still available after the process. Work has simply been transferred from one carrier to another. Driving forces are infinitesimally small and the process is "frictionless" no barriers have to be taken. As a result, there is neither entropy generation nor loss of available work. The work requirements of the process can be accurately calculated from the thermodynamic properties of the equilibrium states that the process passes through. 

PROBLEM 4.27.1. Prove that for the Carnot cycle using a perfect gas as the working fluid in reversible steps, the thermodynamic efficiency is given by Eq. (4.27.1). 

A detailed study of trichloroacetonitrile addition to 2,3,4,6-tetra-O-benzyl-D-glucose showed that from the 1-oxide the -trichloroacetimidate is formed preferentially or even exclusively in a very rapid and reversible addition reaction (Scheme 16). However, this product anomerizes in a slow, base-catalyzed reaction via retroreaction, 1-oxide anomerization and renewed addition) practically completely to the a-trichloroacetimidate, with the electron-withdrawing 1-substituent in an axial position as favored by the thermodynamically efficient anomeric effect. Thus with different bases, for instance K2CO3 and... 

This result, called the Carnot efficiency or the thermodynamic efficiency, places a fundamental limit on the efficiency with which heat can be converted to mechanical work. Only if the high temperature, T, were infinite or the low temperature, T , were zero would it be possible to have a heat engine operate with 100% efficiency. To maximize efficiency, the greatest possible temperature difference should be used. Although we derived this result specifically for the ideal gas, we will show later in this section that it applies to any reversible engine operating between two temperatures. For a real engine, which must operate irreversibly, the actual efficiency must be lower than the thermodynamic efficiency. 

It is worth noting that if we reverse the operation of the heat engine (that is, use work to take heat from a cold reservoir and to release it at higher temperatures) we get thermodynamic efficiencies greater than 100 per cent, which at first sight may appear paradoxical. As... 

For a reversible separation process, LW = 0 and (- lV et) = AB. For an actual separation process, LW >0, (— IVne,) > AB, and a thermodynamic efficiency can be defined as the ratio of the change in availability function to the net work consumption for the actual process, provided that AB is positive. 

Processes that involve operations other than separation steps may result in a AB that is negative, thus indicating that it would be possible to obtain useful work from a reversible process. In that case, for the actual process it will still be true that LW>0 and -W a)> B. However, the thermodynamic efficiency should now be defined as a ratio of the net work consumption for the actual process to the change in availability function... 

Veerman, J., de Jong, R.M., Saakes, M. et al. (2009) Reverse electrodialysis comparison of six commercial membrane pairs on the thermodynamic efficiency and power density. Journal of Membrane Science, 343 (1-2), 7-15. 

Thus, the thermodynamic efficiency of the Carnot heat engine is at a maximnm when the engine is operating reversibly and can never be 1, or 100 percent, because T,. can never be zero and cannot be infinite. In other words, we can never convert heat totally into work some of it escapes into the surroundings as waste heat. 

The theoretical efficiency is sometimes also known as the thermodynamic efficiency or the maximum efficiency limit. The theoretical efficiency at different temperatures and standard pressure is shown in Figure 1.25. The data are also given in Table 1.7. It is clear that there is a conneetion between the reversible OCV of a cell and its theoretical efficiency (or maximum efficiency) based on the above equation. 

Definitions. Define briefly (a) second law of thermodynamics, (b) reversible process (two senses), (c) free energy, (d) standard free-energy change, (e) free energy of formation, (f) entropy, (g) efficiency, (h) fuel cell. 

The reversible thermodynamic efficiency of the fuel cell can thus be expressed as... 

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