Analysis Of Onboard Fuel Processing Designs For Pem Fuel Cell Vehicles

S.Kartha, S.Fischer, and T.Kreutz Center for Energy and Environmental Studies School of Engineering and Applied Science Princeton University, Princeton, NJ 084544

As a liquid fuel with weight and volume energy densities comparable to those of gasoline, methanol is an attractive energy carrier for mobile power systems. It is available without contaminants such as sulfur, and can be easily reformed at relatively low temperatures with inexpensive catalysts. This study is concerned with comparing the net efficiencies of PEM fuel cell vehicles fueled with methanol and hydrogen, using fuel cell system models developed using ASPEN (1) chemical process simulation software. For both the methanol and hydrogen systems, base case designs are developed and several variations are considered that differ with respect to the degree of system integration for recovery of heat and compressive work. The methanol systems are based on steam reforming with the water-gas shift reaction and preferential oxidation, and the hydrogen systems are based on compressed hydrogen. This analysis is an exercise in optimizing the system design for each fuel, which ultimately entails balancing system efficiency against a host of other considerations, including system complexity, performance, cost, reliability, weight and volume.

HYDROGEN SYSTEMS: For each fuel, we considered a "no frills" base case upon which we made incremental changes to help illuminate the point at which marginal efficiency improvements might not justify the added design complexity. This preliminary analysis considers only steady-state operation for a nominal 50 kW net fuel cell output, accounting for the main parasitic loads (i.e. compressive requirements) but not for other loads (e.g., pumping, power conditioning etc) that can be expected to be constant across all the cases considered here. In the case of hydrogen-fueled FCVs, the base case consisted of a simple system in which tanked hydrogen was stored at 5000 psi, and the fuel cell was operated(2) at 0.6 V at a temperature of 90'C, and the anode and cathode feeds were pressurized to 3 atmospheres with a pressure drop across the stack of 5 psi. The cathode stoichiometry was taken to be 2:1. The adiabatic efficiency of compressors and expanders was taken to be 75%.

As shown in Table 1, the base case included a single stage compressor for the air supply to the cathode, and included no recovery of compressive work from either the fuel cell exhaust or the high-pressure hydrogen, yielding a net system efficiency (defined as electrical power out relative to higher heating value of fiiel supplied) of 32.6%. Case A upgrades the air compressor to a 2-stage compressor with intercooling to ambient temperature. The improvement in efficiency is a nominal 0.5%, notwithstanding the relatively significant increase in cost and complexity associated with an intercooled two stage compressor. Case B includes expansion of the fuel cell cathode exhaust (after condensation of water vapor) from 3 atm to 1 aun, allowing the recovery of roughly 40% of the mechanical work embodied in the compressed stream and a system efficiency gain of almost 3%. Case C upgrades the exhaust expander to a 2-stage expander with a reheat using fuel cell waste heat, but yields only a marginal improvement over the single stage expander. Cases D, E, and F consider increasing degrees of recovery from the compressed hydrogen, considering single stage expansion from 5000 psi, single stage expansion from 8000 psi, and 2-stage expansion from 5000 psi respectively, with efficiency improvement increasing from 1.5% to 1.7% to 2.2%. While storage at 8000 psi might be attractive from the standpoint of improved vehicle range, there is no incentive from the standpoint of vehicle efficiency, since the minor improvment in recovery of compressive work is more than offset by additional compressive requirements at the refilling station. The minor benefit derived from using a 2-stage expander is unlikely to be justified by the difficulty of providing safe on-board high pressure heat exchangers for reheating the hydrogen stream between expansion stages.

This analysis suggests that there is incentive for considering a single stage expanders for the cathode exhaust and perhaps for the compressed hydrogen in light of the efficiency gains, but that double stage expanders or higher hydrogen storage pressures are probably not indicated in view of added system complexity.

Table 1. Effect of design variations on net system efficiency of 50 k\V hydrogen FCV.

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