Reversible fuel cell

The concept of the reversed fuel cell, as shown schematically, consists of two parts. One is the already discussed direct oxidation fuel cell. The other consists of an electrochemical cell consisting of a membrane electrode assembly where the anode comprises Pt/C (or related) catalysts and the cathode, various metal catalysts on carbon. The membrane used is the new proton-conducting PEM-type membrane we developed, which minimizes crossover. 

Fig. 2.1 The reversible fuel cell, its energy balance and its system boundary.

The first and the second law of thermodynamics allow the description of a reversible fuel cell, whereas in particular the second law of thermodynamics governs the reversibility of the transport processes. The fuel and the air are separated within the fuel cell as non-mixed gases consisting of the different components. The assumption of a reversible operating fuel cell presupposes that the chemical potentials of the fluids at the anode and the cathode are converted into electrical potentials at each specific gas composition. This implies that no diffusion occurs in the gaseous phases. The reactants deliver the total enthalpy J2 ni Hi to the fuel cell and the total enthalpy J2 ni Hj leaves the cell (Figure 2.1). 

Fig. 2.12 The reversible fuel cell -U heat engine hybrid system.

The combination of a SOFC with a heat engine allows highest electric efficiencies. This is caused by the comparable low entropy production within high temperature fuel cells. Generally, the combination of a reversible fuel cell and a reversible heat engine, as represented by the Carnot cycle, results in a reversible process at any operating temperature of the fuel cell. This combination can be used as refer-... 

See color insert following page 140.) A single cell of my reversible fuel cell (RFC) design, using the basic proton electrolyte membrane (PEM) type fuel cell. 

See Figure 4.2 for a description of the reversible fuel cell (RFC) and Section... 

The one area where oxyhydrogen combustion is desirable is where high flame temperatures are required, such as in welding. In all other applications the most efficient system is to use fuel cells and the least expensive configuration is to use my new reversible fuel cell design (Section 1.3.5.4), which can operate both in the electrolyzer and the fuel cell modes and uses free solar energy to drive the electrolyzer. 

The fuel used to make solar-hydrogen is free (sunshine) and unlimited, the raw material for H2 is water, and the emission when burning the H2 in fuel cells, internal combustion engines, or in power plants is distilled water. The cost of building the solar-hydrogen plants will be known once the demonstration power plant described in this book is built. It might turn out that this cost is already competitive but whatever it is, we know that it will drop by an order of magnitude when the mass production of ultrathin-film solar collectors and reversible fuel cells is started. 

On the H2 side of the plant, the main inventions involve the fuel cells. They include the idea of using the "dual-function" electrolyzer and fuel cell combination units, which I call reversible fuel cells (RFCs) (Figure 4.2). This way, in the electrolyzer mode, the RFCs convert electricity into H2/ and in the fuel cell mode, they generate electricity from the H2 in storage. 

R/A Reverse acting RCP Recycle compressor RFC Reversible fuel cell R/P Resource to production ratio... 

Reversible fuel cells are also the focus of much research. A fuel cell can be run backward to take an external source of electricity and turn it into hydrogen, which can then be stored and later used by the fuel cell to generate electricity. If it turns out to be more cost-effective than other means of storing electricity, such as batteries, this strategy could have a variety of potential applications in military and space missions. Reversible fuel cells could also help address one of the barriers to more widespread use of renewable electricity—its intermittency. For instance, excess electricity generated by windmills during windy times could be stored and used during less windy times. 

Intrinsic tests were performed on the electrolyser and fuel cell of the test bench system for their characterisation (electrical and thermal behaviour, Faraday efficiency, gas purity). Additionally simulations were performed using the Matlab/Simulink software in order to develop a numerical model for such a kind of reversible fuel cell . The system storage efficiency was estimated at 40-42%. 

Electricity is very expensive to store, so it is generated as needed. Hydrogen is somewhat easier to store and, as discussed elsewhere in this report, hydrogen could be used in conjunction with the electric system as backup storage, so that hydrogen would be generated at times of ample power in a reversible fuel cell and reconverted as needed (see Chapter 8 and Appendix G in this report). 

Milliken, C.E. and Ruhl, R.C., Low-Cost High-Efficiency, Reversible Fuel Cell System, paper presented at Proceedings of the 2002 U.S. DOE Hydrogen Program Review, NREL/CP-610-32405. 

Water electrolysis reverse fuel cell operation... 

Figure 3.2 shows the general layout of a fuel cell, based on the free energy change AG = -7.9 x 10" J for the reaction (left-to-right for fuel cell production of electricity, right-to-left for reverse fuel cell performing electrolysis) ... 

A different approach is to reconsider the airship as a means of air travel. A first approach to this is considering an airship for high-altitude cruising (or as a stratospheric platform) powered by photovoltaic panels and using a reversible fuel cell system to store surplus solar power and use it when the sim is not visible. In this way, carrying possibly heavy batteries may be avoided. The envisaged relative shares of direct use of solar power, of elec-trolyser operation and of fuel cell power production are shown in Fig. 4.12. So far, testing of the equipment sketched in Fig. 4.12 has been performed on a 1-kW scale in the laboratory and in simulated airship conditions. 

Figure 4.12. Power modes and their time-shares for reversible fuel cell (RFC) plus solar cell system proposed as a power source for a stratospheric airship. (From K. Eguchi, T. Fujihara, N. Shinozaki, S. Oka-ya (2004). Current work on solar RFC technology for SPF airship. In Proc. 15 World Hydrogen Energy Conf., Yokohama. 30A-07, CD Rom. Copyright attributed to the Hydrogen Energy Soc. Japan.)...

Figure 5.3. Layout of a decentralised, building-integrated hydrogen and fuel cell system based on intermittent primary power sources (such as wind or solar energy), reversible fuel cells and local stores, including stationary and maybe vehicle-based stores, and possibly capable of interchanging hydrogen with users in other buildings through pipelines (Sorensen, 2002a).

The building integration concept used in the decentralised scenario of section 5.5 is shown in Fig. 5.3. The concept is based on availability of reversible fuel cell technology, which as mentioned in Chapter 3, section 3.5.5 seems a realistic assumption, at least for a future scenario. At an increased cost, the reversible fuel cell could of course be replaced by a pair of a power-... 

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