Thermodynamics of fuel cell systems

I believe fuel cell vehicles will finally end the hundred-year reign of the internal combustion engine as the dominant source of power for personal transportation. If s going to be a winning situation all the way around—consumers will get an efficient power source, communities will get zero emissions, and automakers will get another major business opportunity—a growth opportunity. 

Thermodynamics is the study of equilibrium at a macroscopic level. When a system is in mechanical equilibrium, there is no net force imbalance that causes motion. Complete thermodynamic equilibrium is more extensive and requires not only mechanical equilibrium but also thermal, phase, and chemical equihbrium. We can use classical thermodynamics to analyze chemically reacting and nonequiUbrium flows, such as those in fuel cells, but are restricted to only the quasi-equilibrium beginning and end states of the process, with no details of the reaction itself. Thermodynamics can tell us the potential for reaction and direction of spontaneous reaction, but not how fast the reaction will occur. Classical thermodynamics also assumes a continuous fluid, meaning that there are enough molecules of a substance to yield accurate values of thermodynamic variables like pressure and temperature. As such, classical thermodynamics is generally inappropriate for use with microscopic-level molecular charge transfer processes and electrochemical reactions. 

In this chapter, the fundamentals of classical thermodynamics as it applies to the study of fuel cells is introduced. Although the reader is assumed to have a background in basic thermodynamics, this chapter includes a review of the physical meaning of several parameters used frequently in electrochemistry and how calculations of their values can be made. This chapter concludes by applying the thermodynamic concepts presented to determine the maximum expected thermodynamic efficiency and open-circuit voltage expected for a fuel cell at a given condition. 

There are many methods to correct for nonideal gas behavior, including use of empirically or semiempirically modified EOSs. Actually, hundreds of EOSs have been developed to describe the pressure-density-temperature relation for a wide variety of gas-, liquid-, and solid-phase substances. For additional background, the reader is referred to a fundamental thermodynamics textbook [e.g., 1]. An early attempt to improve the ideal gas EOS was 

While the van der Waals EOS has improved accuracy compared to the ideal gas law and is historically quite important, it is not frequently utilized because more accurate approximations are now available, especially for behavior near the critical point. Other approaches include two or more parameters that are empirically defined by fitting experimental data, or the so-called virial EOSs, which have a series expansion form with coefficients based on molecular theory, statistical mechanics, or experimental data. 

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Barclay F J, 2002, Fundamental thermodynamics of fuel cell, engine, and combined heat and power system efficiencies. Proceedings of the Institution of Mechanical Engineers, Part A, Journal of Power and Energy, 216, 407-417. 

A number of papers propose using the thermodynamic property of exergy to evaluate processes of fuel-cell systems [20-23]. Exergy is defined by... 

Barz, D.P. Tragner, U.K. Schmidt, V.M. Koschowitz, M. Thermodynamics of hydrogen generation from methane for domestic polymer electrolyte fuel cell systems. Fuel Cells 2004, 3 (4), 199-207. 

A more detailed description of different types of batteries and other electric energy storage systems for electric vehicles can be found in Sect. 5.3, while a description of the main characteristics and properties of fuel cells for automotive application is given here, starting from some basic concepts of electrochemistry and thermodynamic, and focusing the attention on the operative parameters to be regulated to obtain the best performance in the specific application. 

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