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Fuel-Cell Power Systems Incorporating Mg-Based H2 Generators

Hydrogen would be generated from magnesium and steam.

Two hydrogen generators based on reactions involving magnesium and steam have been proposed as means for generating the fuel (hydrogen gas) for such fuel-cell power systems as those to be used in the drive systems of advanced motor vehicles. The hydrogen generators would make it unnecessary to rely on any of the hydrogen-storage systems developed thus far that are, variously, too expensive, too heavy, too bulky, and/or too unsafe to be practical.

The two proposed hydrogen generators are denoted basic and advanced, respectively. In the basic hydrogen generator (see figure), steam at a temperature ≥330 °C would be fed into a reactor charged with magnesium, wherein hydrogen would be released in the exothermic reaction Mg + H2O → MgO + H2. The steam would be made in a flash boiler. To initiate the reaction, the boiler could be heated electrically by energy borrowed from a storage battery that would be recharged during normal operation of the associated fuel-cell subsystem. Once the reaction was underway, heat from the reaction would be fed to the boiler. If the boiler were made an integral part of the hydrogen-generator reactor vessel, then the problem of transfer of heat from the reactor to the boiler would be greatly simplified. A pump would be used to feed water from a storage tank to the boiler.

Only a small fraction of the heat generated in the reaction would be needed for boiling: For every kilogram of hydrogen produced, about 44.5 kW⋅h of heat would be generated, while only about 6 kW⋅h would be needed to boil the requisite amount of water. The remaining 38.5 kW⋅h of high-grade heat would have to be dissipated via a radiator. Optionally, the flow of heat from the reactor to the radiator could be intercepted by a thermoelectric converter, thereby increasing the overall electric power generated; the net increase in the overall efficiency of the fuel-cell power system has been estimated to be equivalent to that afforded by an increase of 19 percent in the amount of hydrogen generated.

The generated hydrogen would be sent through a cooler to reduce its temperature to 80 °C as required for the fuel cell(s). During operation of a hydrogen-burning fuel cell, water is generated in liquid and vapor forms. The liquid water could easily be recycled to the storage tank. Depending on detailed calculations yet to be performed, it may be advantageous to also recycle the water vapor by condensing it and pumping the resulting liquid water to the storage tank, provided that the weight of the condenser did not exceed the weight of the water saved. At 80 °C, the waste heat from the fuel cell would be of too low a grade to be useful for most purposes; however, if the fuelcell power system were part of a motor vehicle, the waste heat could be used to heat the passenger compartment in winter.

In the advanced hydrogen generator, the reactor vessel (or one of two or more vessels, depending on the design) would be initially charged with magnesium hydride. The advanced hydrogen generator would exploit two reactions: the aforementioned exothermic reaction at a temperature ≥330 °C plus the endothermic reaction MgH2 → Mg + H2 at a temperature ≥300 °C. Once the initial heating was complete and both reactions under way, the Mg produced in the endothermic reaction would be consumed in the exothermic reaction, which, in turn, would generate sufficient heat to maintain the endothermic reaction. The main advantages of the advanced hydrogen generator over the basic one would be that (1) it would produce twice the amount of hydrogen for a given amount of magnesium, but (2) the cost of operation is likely to be less than that of the basic hydrogen generator because it is likely that MgH2 could be produced at less than twice the cost of the corresponding amount of Mg.

The main waste product of both the basic and advanced systems would be MgO, which has extremely low toxicity. MgO could be safely and easily recycled in a magnesium-refining plant for less than the cost of Mg because MgO is an intermediate product of the refining process.

This work was done by Andrew Kindler and Sri R.. Narayan of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
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Refer to NPO-43554, volume and number of this NASA Tech Briefs issue, and the page number.