Figure 4(a)

Model * Experimental

Figure 4(b)

The primary simplification used in the model is isothermal operation. It has been shown that non-isothermal modeling generally gives superior results (4),(5). For this reason the present attempt at modeling higher pressure operation of fuel cells was carefully done within a range of conditions such that results could be compared to published data and verified. For the air electrode supported tubular solid oxide fuel cell, of 50 cm length, Westinghouse published data on 85% fuel utilization of a hydrogen-steam mixture entering the fuel cell with six times the stoichiometric amount of air. Operation pressures ranged from 1-10 atmospheres (2). The fuel cells were operated at a nominal temperature of 1000° C. Emphasis is placed on the word "nominal", because there were temperature gradients present; however, near peak power, as can be seen in Figures 4(a)and 4(b), there was excellent agreement between isothermal model results and the empirical results. Bessette (4) indicates such good correspondence at peak power conditions. Fortunately such close compliance with field data is at current densities corresponding to peak fuel cell power, because these are the conditions under which fuel cells are expected to operate, even as components in larger energy systems.

Air electrode supported-tubular solid oxide fuel cells have shown increases in power production capability over the previous designs due to the lack of a support tube. As well, high pressure operation of fuel cells has an increase in performance beyond that of Nernst potential consideration (2), because of decreasing polarization with higher pressure. Based on these factors, an investigation was made as to the thermodynamic merit of integrating an air electrode supported-tubular solid oxide fuel cell stack within an open loop Brayton cycle. The following Figure 5 is a flow diagram of the envisioned cycle.

Figure 5

Fuel (89% H2,11% H2O) and air are compressed to the fuel cell operating pressure and heated as necessary before entering the cell stack. Both streams then pass through the fuel cell stack. Oxidation of the fuel is completed in the combustion chamber, whereupon the product gases expand through the prune mover. Usage of a fuel cell - gas turbine cycle would provide power and high quality heat to the plant The system shown in Figure 5 was modeled and analyzed with the following simplifications. Compressor and turbine efficiencies are 75% and 80%, respectively, due to their expected size ranges. The fuel cells operate isothennally at 1000°C. Based on the earlier comparisons to experimental data (2), fuel cells were operated at peak power conditions, corresponding to 85% fuel utilization and six times stoichiometric air. These conditions showed the best match between the isothermal model and actual results. In this preliminary study, thermal and performance effects of fuel cell stack geometry were not considered. The 15% of unoxidized fuel leaving the fuel cell is combusted in an adiabatic combustion chamber so that the mixture reaches an adiabatic flame temperature of 1344 K (well below the state-of-the art material limits for turbines). The hot exhaust products from the turbine may be fed back to the hydrogen production section where, after deaeration, the remaining steam can be reused (e.g. steam reformation of methane). The independent variable of interest was operation pressure, ranging up to ten atmospheres. The dependent variables of interest were net power (per fuel cell) of the cycle, thermal efficiency and a modified thermal efficiency. Thermal efficiencies were defined as:

T] = Net power / (LHVfuh+ Sensible Heat to Gas Flows) [6]

Tltnodined=Net power / (LHVFUH+ Sensible Heat to Gas Flows - Fuel Cell Heat) [7]

The modified thermal efficiency accounts for the high quality heat rejected by the fuel cells. Notice that both efficiencies account for the extra heat needed by the gas flow streams. This is particularly important when considering the needed preheating of air before entering the fuel cell stack.

Power per Fuel Cell 85% Utilization, stoichiometrics =6 •


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