drogen produced at low current density are small, resulting in very high capital costs per unit of hydrogen produced. As shown in Figure G-9, cell stack efficiencies drop to 75 percent when current densities rise into the range of 1000 amps per square foot (ASF). As previously stated, the electrochemical efficiency of KOH systems is higher over a broader range of current densities, but this higher reaction efficiency is offset at least in part by higher compression and purification costs, as well as by higher costs associated with managing the liquid electrolyte itself.

The committee believes that current technology is capable of producing an electrolyzer-based fueling facility having the capacity to produce 480 kg/day, or 20 kg/hour. This plant would be capable of fueling 120 cars per day, assuming an average purchase of 4 kg per car. A plant of this scale would of necessity today be a KOH system, but with additional development, PEM technology should be capable of providing systems of comparable scale.

Electrolyzer systems of this scale should be capable of operating with an overall efficiency of 63.5 percent lower heating value [LHV], including all parasitic loads other than compression. The electrolyzer is assumed to be able to generate hydrogen at an internal pressure in the 150 psi range; supplementary compression will be required to raise the pressure to automotive fueling pressures in the 7000 psi (400 atm) range. The electrical requirement associated with compression is assumed at 2.3 kW/kg/hour, adding about 5 percent to the plant’s electrical consumption and bringing overall efficiency down to about 59 percent.

Equipment Costs

Regarding capital cost recovery, the cost of the 480 kg/ day system, excluding compression and dispensing, is assumed at $1000/kW input. The cost of the complete fueling system is summarized in Table G-5.

The total cost of a system at this scale would be about $2.5 million. It is anticipated that electrolysis technology scales with an 85 percent factor, so smaller-scale systems, with somewhat higher unit costs, are entirely feasible. For example, a facility with half the fueling capability (60 cars per day) would cost about $1.25 million, plus a 15 percent scaling factor. The scalability of electrolysis is one of the important factors relating to its likely use in early-stage fuel cell vehicle adoption. The electrochemical efficiency of electrolysis is essentially independent of scale.

FIGURE G-9 Electrolysis cell stack energy consumption as a function of cell stack current density.

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