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The goal of this project was to design three different energy storage systems utilizing reversible solid oxide fuel cells. Two of the systems use gaseous feed stocks and require storage for the gas produced by electrolysis. In the third design, molten antimony oxide is reduced to pure antimony during electrolysis rather than storing the energy in a gas. The three systems discussed in this report use electric power during off-peak demand hours to electrolyze a chemical feedstock. The resulting products are then stored for use during peak demand time in fuel cell operation. The systems were designed to store 1 MW of electrical power over a 12 hour period of off-peak demand and release power during a 12 hour period of peak demand. In the two gaseous systems, the fuel cell is made of an yttria-stabilized-zirconia (YSZ) electrolyte with porous nickel at the anode and YSZ-LaMnO3 composite at the cathode.
In the electrolysis of steam to form hydrogen, the hydrogen is stored in pressurized vessels at 100 psi. During fuel cell mode, the hydrogen gas is oxidized in the fuel cell to form water, which is again stored for use in electrolysis the following day. The operating temperature for both electrolysis and fuel cell operation is 1472°F. The overall efficiency for the hydrogen system is 52.4%, with the main losses occurring as heat supplied to the electrolyzer.
The second design electrolyzes a mixture of steam to hydrogen and carbon dioxide to carbon monoxide at 1292°F and 147 psia. The resulting syngas is fed through a methanation reactor to produce a methane rich stream. The overall efficiency of the methane-based system is 55.7%, with the main losses coming from compression and heating for electrolysis mode.
The molten antimony fuel cell uses an equimolar mixture of antimony and antimony trioxide as the feedstock for electrolysis. The electric current in the electrolyzer reduces the antimony trioxide to form a stream of pure molten antimony. Both the electrolyzer and fuel cell operate isothermally at 1292°F and 14.7 psia. The overall efficiency for the antimony design is 53.7%, with the main losses coming from heat supplied to the electrolyzer.
For profitability analysis, off peak electricity was priced at $0.06/kWh and peak power was priced at $0.20/kWh. Under these optimistic assumptions, the return on investment (ROI) for the hydrogen design was calculated to be -26.1%. For the methane system, the ROI was calculated to be -19.2%. For the antimony case, the ROI was found to be -34.2%. These designs serve as a framework for future work with electrical energy storage. However, we believe that with improvements in system efficiency and reductions in the initial capital investment, future reversible fuel cell systems will be profitable and competitive with other forms of electrical energy storage.
Date Posted: 24 July 2012