Fuel cells

A fuel cell converts chemical energy of a fuel into electricity directly, with no intermediate combustion cycle. Since there is no intermediate 'heat to work' conversion, the efficiency of fuel cells is not limited by the second law of thermodynamics, unlike conventional 'fuel ^ heat ^ work ^ electricity' systems. The efficiency of conversion from chemical energy to electricity by a fuel cell may theoretically be 100%. Although not strictly 'storage' devices, fuel cells are treated in this chapter because of their many similarities to batteries, Section 16.5, and their possible use with H2 stores, Section 16.3.2. In a 'hydrogen economy', fuel cells would be expected to be used not only for stationary electricity generation but also for powering electric vehicles. Therefore we shall discuss only fuel cells using H2, although other types exist.

Like a battery, a fuel cell consists of two electrodes separated by an electrolyte, which transmits ions but not electrons. In the fuel cell, hydrogen (or another reducing agent) is supplied to the negative electrode and oxygen (or air) to the positive electrode (Figure 16.5). A catalyst on the porous anode causes hydrogen molecules to dissociate into hydrogen ions and electrons. The H+ ions migrate through the electrolyte, usually an acid, to the cathode, where they react with electrons, supplied through the external circuit, and oxygen to form water.

The efficiencies of practical fuel cells, whether hydrogen/oxygen or some other gaseous 'couple', are much less than the theoretical 100%, for much the same reasons as for batteries. In practice the efficiency is perhaps 40% for the conversion of chemical energy to electricity, but this is not dependent

Figure 16.5 Schematic diagram of a fuel cell. Hydrogen and oxygen are combined to give water and electricity. The porous electrodes allow hydrogen ions to pass.

on whether or not the cell is working at its full rated power. This contrasts with most diesel engines, gas turbines and other engines.

Since the efficiency of an assembly of fuel cells is nearly equal to that of a single cell, there are few economies of large scale. Therefore small localised plants of 1-100 kW capacity are a promising proposition. Using the fuel cell as a CHP source, a single building could be supplied with both electricity and heat (from the waste heat of the cells), for the same amount of fuel ordinarily required for the thermal demand alone. The main reason why fuel cells are not in wide use for such applications is their capital cost (>$2000 kW-1).

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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