Fuel utilisation

J. N. Harris, A. R. Russell, and J. B. Mi. fotd,Mir ualif Implications of Methanol Fuel Utilisation, SyPE Paper 881198, Society of Automotive Engineers, Warrendale, Pa., 1998. 

The physiological usefulness of this method for identifying the fuels that are used and their rates of utilisation in different cells is discussed in other chapters (Chapter 3). For example, measurement of the activity of the enzymes hexokinase and glutaminase in immune cells showed, for the first time, that glucose and glutamine are the major fuels utilised by these cells. This finding has had clinical significance (Chapter 17). 

Table 13.2 Properties of human muscle fibre types and their capacities for fuel utilisation ...

Quantitative studies of energy metabolism and fuel utilisation in very prolonged physical activity were performed by Mike Stroud during his Antarctic expedition with Ranulph Fiennes in 1992. One part of the study was measurement of energy expenditure by the dual isotope techniques (Chapter 2). Calculated over the whole expedition, the average daily energy expenditure of Stroud and Fiennes was 29 MJ but on particularly arduous days it increased to... 

Figure 16.7 Pattern of fuel utilisation during a short fast. This is the metabolic condition after, for example, the overnight fast.

Figure 16.11 Pattern of fuel utilisation during prolonged starvation. The major metabolic change during this period is that the rates of ketone body formation and their utilisation by the brain increases, indicated by the increased thickness of lines and arrows. Since less glucose is required by the brain, gluconeogenesis from amino acids is reduced so that protein degradation in muscle is decreased. Note thin line compared to that in Figure 16.9.

The reaction product H20 is mixed with the anode gas and its concentration increases with higher fuel utilisations Uf as shown in Figure 2.2. The fuel utilisation Uf is defined as the ratio of the utilised fuel and the maximum available fuel,... 

The influence of the fuel utilisation on the reversible voltage Vpcrev, which is similar to the Nemst voltage Ey, can be calculated by the change of the partial pressures of the components within the system [2,3], We can write Equation (2.4) more precisely as... 

With Equation (2.27) the ideal Nemst voltage C v can be expressed as a function of the fuel utilisation U/ with... 

Equation (2.49) shows that the current I is proportional to the fuel utilisation U/... 

This implies that the electrolyte is free of electric leaks and the electrodes are free of any parallel reactions. The fuel (2 for H2) determines the number of transferred electrons nel. The Faraday constant F is a constant value and the fuel inlet flow fi is the only variable influencing the relation between fuel utilisation Uf and current I. Fuel utilisation Uf and current I deliver the same expression if the fuel flow is kept a constant. 

Uf < 1 in the real operation regime of a SOFC. This model based on the thermodynamic equilibrium already shows the principal influences of the system pressure p, SOFC temperature sofc, excess air X and fuel utilisation Uf on the Nemst voltage En. 

The Nemst voltage En is shown as a function of the fuel utilisation Uf in a SOFC in Figure 2.4 with H2 as a fuel and with the system pressure p as a parameter. The excess air and the SOFC temperature are the fixed parameters. The range of practical interest between Uf = 0.1 and Uf = 0.9 can be well approximated with the model of the ideal gas. The dotted lines show the adaptation of the model for a high fuel utilisation. The amount of the water fraction and the decrement in the hydrogen and oxygen fraction within the SOFC reduces En between Uf = 0.1 and Uf = 0.9 by about more than 200 mV An increment of the system pressure from... 

The fuel utilisation U/ is defined as quotient of the converted hydrogen and the maximum convertible hydrogen according to Equation (2.35). The maximum convertible hydrogen can be expressed as maximum available electrical current /max, as well... 

Generally, high fuel utilisations are aspired to achieve high efficiencies, whereas different Nemst voltages occur along the cell area. Thus, the dependency of the fuel utilisation from the electrical current (Equation (2.56)) has to be implemented in Equation (2.57). 

Different conditions under test procedures and practical operation require different calculation procedures for the evaluation of the test results. In this subsection, three cases at different distributions of the Nemst voltage are considered with regard to the solution of the integral cell area and integral fuel utilisation, respectively. 

Case B Nemst voltage changes inversely proportional with the fuel utilisation. Case C Nemst voltage changes according to Equation (2.27). 

If the fuel is utilised within a range of hydrogen to water pressure ratios (/m2/pn2o) between 0.7 and 0.3, the Nemst voltage changes approximately inversely proportional with the fuel utilisation. Thereby, a linear approach is used to determine the... 

Concerning practical applications, high fuel utilisations result in low of hydrogen to water pressure ratios at the outlet of the anode, whereas high hydrogen pressures occur at the entry of the anode. Thereby, the non-linear dependency of the Nemst voltage from the fuel utilisation has to be taken into account with respect to the integral solution of Equation (2.59). 

In Case B, the hydrogen flow rate is chosen with 2.72-10-6 mobs to approximate a linear change in the molar hydrogen fraction from 0.7 at the entry to 0.299 at the outlet of the anode along the cell area. In this range of the molar fraction, the Nernst voltage changes approximately inversely proportional with the fuel utilisation. 

Even if the Nernst voltage, the overpotential, the current density and the power density change inversely proportional with the fuel utilisation, the local distribution... 

If the electrical performance of the cell Pei, the average cell voltage Vcell and the fuel utilisation uf are requested parameters,... 

A solution to avoid these situations is the implementation of limitations on the power produced from renewable generators, depending on the operating condition of the system. Such a measure can improve the power quality and stability of the APS. On the other hand, bearing in mind that the introduction of renewable sources in an APS fed by diesel generators aims at the improvement of the operational efficiency and the minimisation of the fuel utilisation, this solution is far from being optimal. 

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