Fuel cell electric work

Note that the electrical work potential consistently declines with temperature, while the Carnot work potential must increase. Fuel cell electrical work potential and Carnot work potential are added in Gardiner (1996) and termed the combined work . That example is followed here. The combined work has to be calculated separately, and added to the circulator work potential. The total is the fuel chemical exergy,... 

Using Chase etal. (1998) for (AG, S) values, the increase of Carnot cycle work versus the reduction in fuel cell electrical work is now calculated for both fuel electrochemical reactions, at conditions FgTo and above. Included in brackets are (—AGWsmol 5kJmol ) ... 

In an electrochemical cell, the electrical work is done owing to the flow of electrons through an electrical load circuit under the effect of a potential difference between the two electrodes or the voltage of the cell. When electrical charges flow through an external load circuit owing to the potential difference E, the fuel cell electrical work is given as... 

Solid alkaline membrane fuel cell (SAMFC) working at moderate temperatures (20-80 °C) for which an anion-exchange membrane (AEM) is the electrolyte, electrically conducting by, for example, hydroxyl ions (OH ). 

The OCV seems to be of less practical use. It is much more useful for a fuel cell to work under a load, with power being delivered to the users. With the electric energy output, the fuel cell voltage will decrease as the electric current load increases. A popular way to evaluate a fuel cell is to measure its polarization curve (abbreviated as I-V curve ), which is a plot showing the cell voltage change with current or current density. Figure 1.20 shows a typical polarization curve obtained with a PEMFC. 

The model evaluates the overall thermal efficiency of the power system, as shown in Figure 3. The system efficiency is the hydrogen stored and electric power supplied to the load, divided by the methane input to the reformer and the power consumed by the compressor. The system efficiency is highest when the reformer is producing hydrogen to be stored during the low-load periods. In contrast, the system efficiency drops when the combined reformer and fuel cell are working at capacity to supply the peak load. 

The first of these possibilities has the advantage of keeping the fuel cell at a higher temperature and thus realizing a better electrical performance. Shukla et al. (1995) have described such a direct methanol fuel cell. A temperature of 200 C was maintained in an evaporator for a 2.5 M aqueous methanol solution. The fuel cell s working temperature was lower. At a temperature of 100°C current density of 200 mA/cm could be realized at a voltage of about 0.5 V (the total platinum content of both electrodes was 5 mg/cm ). 

A Direct Methanol Fuel Cell (DMFC) works creating an electric potential by the reaction between methanol and oxygen, specifically it produces electricity through an electrochemical process without combustion and without the need for a reformer system for the fuel [82]. 

Dry cells (batteries) and fuel cells are the main chemical electricity sources. Diy cells consist of two electrodes, made of different metals, placed into a solid electrolyte. The latter facilitates an oxidation process and a flow of electrons between electrodes, directly converting chemical energy into electricity. Various metal combinations in electrodes determine different characteristics of the dry cells. For example, nickel-cadmium cells have low output but can work for several years. On the other hand, silver-zinc cells are more powerful but with a much shorter life span. Therefore, the use of a particular type of dry cell is determined by the spacecraft mission profile. Usually these are the short missions with low electricity consumption. Diy cells are simple and reliable, since they lack moving parts. Their major drawbacks are... 

Finally, the energy available from the above reaction might be used to operate a fuel cell such as those involved in the space program. In that case, as much as 818 kj/mol of useful electrical work could be obtained relatively litde heat is evolved. Summarizing this discussion in terms of an energy balance (per mole of methane reacting) ... 

The theoretical efficiency of a fuel cell is given by the ratio between the Gibbs free energy (AG) which is the maximum electrical work that can be obtained, and the enthalpy (A//) of the fuel (Equation 6.3). 

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