17 - Fuel Cells Engineering thermodynamics concentrates on...

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Fuel Cells Engineering thermodynamics concentrates on the production of work through cyclic devices, e.g. the power to drive a vehicle as produced by a reciprocating engine; the production of electricity by means of a steam turbine. As shown previously, these devices are all based on converting part of the Gibbs function of the fuel to useful work, and if an engine is used some energy must be ‘thrown away’: all engines are limited by the Carnot, or Second Law, efficiency. However, there are some devices which are capable of converting the Gibbs energy of the fuel directly to electricity (a form of work); these are called fuel cells. The advantage of a fuel cell is that it is not a heat engine, and it is not limited by the Carnot efficiency. The thermodynamics of fuel cells will be developed below. The concept of the fuel cell arises directly from the operating principle of the electric cell, e.g. the Daniel1 cell, and as early as 1880 Wilhelm Ostwald wrote: ‘I do not know whether all of us realise fully what an imperfect thing is the most essential source of power which we are using in our highly developed engineering - the steam engine.’ He realised that chemical processes could approach efficiencies of 100% in galvanic cells, and these were not limited by the Carnot efficiency. Conversion efficiencies as high as 60-80% have been achieved for fuel cells, whereas the practical limit even for sophisticated rotating machinery is not much above 50%. A further benefit of the fuel cell is that its efficiency is not reduced by part load operation, as is the case for all heat engines. Hence, if a fuel cell operating on hydrocarbon fuels can be achieved it will improve ‘thermal efficiency’ significantly and reduce pollution by CO, and NOx. The current situation is that successful commercial fuel cells are still some way from general use but small ones have been used in specialist applications (e.g. space craft) and large ones are being developed (e.g. 1 MW by Tokyo Gas, Japan). Figure 17.1 shows a proposal for a fuel cell to power a motor vehicle using methanol (CH,OH) as its primary fuel. In this case the methanol is used to generate hydrogen for use in a hydrogen-oxygen fuel cell. Figure 17.l(a) shows the hydrogen generator which is based on the water gas reaction, and Fig 17.l(b) depicts the fuel cell itself. This arrangement has the advantage that the fuel can be carried in its liquid phase at atmospheric conditions, which is much more convenient than carrying hydrogen either in gas, liquid or hydride form. The theory of fuel cells can be developed from the previously derived thermodynamic principles, and it shows how equilibrium reversible thermodynamics can be interwoven with irreversible thermodynamics. Before developing the theory of the fuel cell itself it is
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346 Fuel cells Fig. 17.1 Proposed hydrogen fuel cell for vehicle applications necessary to consider simpler electrical cells.
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