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PhD Thesis - Energy Systems Research Unit - University of Strathclyde

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5.6.4 If Fuel Availability is Less Than Required<br />

If there is less fuel than is required to run the fuel cells at the desired loading,<br />

the maximum output that may be achieved must be ascertained. If only one fuel<br />

cell has been specified, or if the efficiency is at a maximum at full load, the<br />

maximum output achievable with the available fuel is calculated in the same<br />

way as described for engines in Section 5.2.6. Otherwise, where the fuel cell<br />

efficiency increases with decreasing load, the method outlined below is used.<br />

This is shown graphically in Figure 5.20.<br />

Firstly, the fuel consumption at the optimum percentage load and the other given<br />

percentage loads are calculated using Equation 5.33, 5.35 or 5.36 as appropriate.<br />

The optimum number <strong>of</strong> fuel cells amongst which the load may be divided can<br />

be determined using the factor calculated in Equation 5.40.<br />

Factor = Fuel Available (5.40)<br />

Optimum Fuel Consumption<br />

Where Fuel Available = Amount <strong>of</strong> fuel available (litres, kg or kWh)<br />

Optimum Fuel Consumption = Fuel consumption at the optimum<br />

percentage loading (litres, kg or kWh).<br />

If five fuel cell sets have been specified, and this factor is equal to an exact<br />

integer <strong>of</strong> five or below, that number <strong>of</strong> fuel cells may be run at their optimum<br />

percentage load. If this factor is greater than five, the best load spread with the<br />

fuel available is achieved by dividing the fuel available equally between all five<br />

fuel cells. If the factor is between four and five, the available fuel is divided<br />

equally between four fuel cells, and the percentage load possible is calculated by<br />

interpolation between the fuel consumption figures calculated for the given<br />

percentage loads. If the factor is between three and four, two and three, or one<br />

and two, the same process is carried out to compare the efficiency values gained<br />

by running three or four, two or three, or one or two, fuel cells respectively. If<br />

the factor is less than one, the percentage load is calculated. If this is less than<br />

minimum load, no fuel cells are run. Otherwise, one fuel cell is run at the load<br />

possible with the available fuel.<br />

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