Thermodynamic energy conversion efficiencies

Landsberg P. T and Tonge G. (1980), Thermodynamic energy conversion efficiencies , J. Appl. Phys. 51, R1-R20. 

Table 4.1 The transferred electrons (Ne), electromotive force ( °), volume energy density (IVe), and thermodynamic energy conversion efficiency (erev) of electrooxidation of selected alcohols at standard conditions.

Figure 3. Plot showing the variation of solar energy conversion efficiency with threshold wavelengths at sea level. The solid vertical lines represent the energies required for decomposition of water using one or two photons per molecule of water respectively (4 photons would be at 2000 nm). The dashed vertical lines represent computed values for these threshold wavelengths taking into account unavoidable thermodynamic losses (adapted from ref. 8)...

In 1894, the German physical chemist Wilhelm Ostwald came forward in the Zeitschriftfur Elektrochemie with the proposal to build devices for a direct oxidation of natural kinds of fuel with air oxygen by an electrochemical mechanism without heat production (the so-called cold combustion of fuels) (Fig. 17.1). He wrote In the future, the production of electrical energy will be electrochemical, and not subject to the limitations of the second law of thermodynamics. The conversion efficiency thus will be higher than in heat engines. This paper of Ostwald was basic and marked the beginning of huge research into fuel cells. 

The SSTAR (24) and STAR-LM (25) lead cooled reactor concepts are based on nitride fuel and use a higher core outlet temperature to drive a supercritical CO2 Brayton cycle at 550 to 600°C, with a potential to gain energy conversion efficiencies of 43% at these temperatures. Moreover, the outlet temperature on the cool side of the recuperator can lie in the range of 70 to 125°C with only weak influence on the efficiency. As the inlet to the compressor is just above 31°C, these conditions facilitate installation of bottoming cycles for district heating, seawater desalination, or process heat production, using the heat otherwise rejected in thermodynamic cycle (see Annexes XXII and XXIII). The supercritical CO2 Brayton cycle lacks an industrial experience base this non-conventional Bra)don cycle will require R D. 

For methane oxidation, the highest energy conversion efficiency that is thermodynamically possible is close to 100%, according to equation (1.6). 

The energy conversion efficiency is usually discussed in terms of the enthalpy-based conversion rate. For example, theoretical efficiency is defined as the ratio of the Gibbs energy change to the enthalpy change for fuel cell reaction. Therefore, the values just discussed should be transferred to the enthalpy-based ones. For this purpose, the thermodynamically theoretical conversion efficiency should be defined in a manner that enables comparison with other energy convertors such as heat engines. Flere, we start with methane as the common fuel. In Fig. 2.4(a), we compare several cases as a function of temperature ... 

The theoretical solar conversion efficiency of a regenerative photovoltaic cell with a semiconductor photoelectrode therefore depends on the model used to describe the thermodynamic and kinetic energy losses. The CE values, which consider all the mentioned losses can generally only be estimated the full line in Fig. 5.65 represents such an approximation. Unfortunately, the materials possessing nearly the optimum absorption properties (Si, InP, and GaAs) are handicapped by their photocorrosion sensitivity and high price. 

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