F

*

*

-2.8 kW 5000 psi, 2-stase

+2.2 %

* same value as in base case.

METHANOL SYSTEMS: The base case for the methanol-fueled FCVs consists of a steam reformer operating at 260'C and 3 atm using Cu-Zn catalysts, with a steam to carbon ratio of 1.5:1, generating a total yield of 2.87 moles of hydrogen for each mole of methanol(3). The fuel cell is conservatively assumed to operate economically at no greater than 78% utilization, with a voltage penalty due to reformate dilution of 3%(4). This also assumes that CO is reduced to tolerable ppm levels by addition of 2% oxygen in a preferential oxidation stage following reforming reactions and the water-gas shift reactions.

In such a system, fuel processing constitutes a significant heat load, roughly 12 kWa for the reforming reactions and 27 kWft for the evaporation and preheat of the methanol/water mixture, so the degree of thermal integration will dramatically affect overall system efficiency. In the cases considered, the efficiency of heat exchangers was assumed to be 87%. As displayed in Table 2, the base case assumes that hydrogen remaining in the anode exhaust is combusted to provide heat for the evaporator and reformer, but that no compressive work is recovered in an expander. In order to provide sufficient heat, the hydrogen utilization is set at 64%. In case A, a combustor and turbo expander (operating at an inlet temperature of 1100 K) recover compressive work from the anode and cathode exhaust streams, satisfying the compressive requirements of the air stream and providing additional power. Because energy in the exhaust is converted to mechanical work in the expander, less remains for providing heat to other processes and the utilization must be reduced to

59% in order to meet heat requirements. The net affect is a 7.0% improvement in system efficiency, despite the decreasing hydrogen utilization. In case B, the fuel utilization is set at the maximum value of 78%, and methanol is burned directly to supply heat. The anode exhaust is then combusted without supplying heat to other processes and expanded through a higher temperature turbo expander («1400 K). This yields an additional 2.8% efficiency gain over case A. Case C attempts to exploit the turbo expander further, recompressing the exhaust streams to 5.5 atm before combusting in order to produce more power, but yields only an additional 0.9% efficiency gain.

Modelling of the methanol-fueled systems suggests that recovery of compressive power from the fuel cell exhaust is likely to be desireable, and that the expander technology (inlet temperatures, size, cost, efficiency etc) may determine the optimal operating point with respect to apportioning the energy in anode exhaust for heat vs power.

Table 2. Net system efficiencies for base case and variations of 50 kW methanol FCV.

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