Technology Status of Hydrogen Road Vehicles

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difficulties must be overcome: ! High capital cost, probably subject to steep economies of production, but saddled with precious catalysts both scarce and expensive. ! Low current densities at electrodes, increasing bulk and weight, decreasing useful load and decreasing vehicle efficiency at the wheels. ! Hydrogen storage. Although these drawbacks make FCs impractical for industrial use today, the space applications (the Shuttle has three 12-kW FCs) and increasing transport demonstrations (see Tables 1 and 2) are putting them in better perspective: the potential economies of future mass production are undeniable, and Pt loadings at the electrodes have decreased by one to two orders of magnitude in the past 10 years; current densities are increasing rapidly, and have improved threefold in recent years; while hydrogen storage can be roughly half that of ICE vehicles with the same range, and significant progress is being made as discussed in Chapter 3. If we compare this perspective with that of the ICE at a similar stage in its development--say 100 years ago-there is every reason to be optimistic about FC development, provided it receives the concentrated industrial attention that the ICE enjoyed. This being said, one item could halt the large-scale deployment of FCs: their use of strategic and scarce catalysts. If the relative loadings cannot be brought to acceptable levels, not only the price but ultimately the unavailability of the present favorites (Pt, Pd, etc.) could be decisive. A3.2 Types of Fuel Cells The main distinguishing feature of FCs is the type of electrolyte. Ideally, the electrolyte should allow easy passage of the hydrogen ions without allowing electrons through. At the moderate temperatures preferred for transport applications, a strong acid or strong alkali solution is indicated and led to earlier choices of phosphoric acid or potassium hydroxide alkali as electrolytes. Known respectively as PA FC and A FC, they have found some limited application. However, the actual experience has not been favorable and they are being abandoned for what is becoming the almost unique choice: an acidtype in the form of a solid polymer known then as a SPE FC, but now increasingly called the PEM FC. The solid polymer electrolyte of the PEM FC comes in very thin sheets (only a few tenths of a micron) and, although it must be kept humid, it is not subject to liquid loss. The sheets are under proprietary trademarks or patents, are very expensive considering the bulk plastics industry behind them, and are presently marketed by Asahi, Ballard, De Nora, Dow, DuPont and probably others. Since only these thin sheets and relative humidity paths separate the electrodes, the stacking density (cells/cm) is relatively high but a limit is posed by the electrode thickness which demands inversely-proportional catalyst loading. The current density (A/cm 2 ) of these sheets is relatively high, and appears to be increasing rapidly. The result is -and the evolution is strongly in favor of the PEM FC- that it provides power densities (W/kg) over 260, as compared to about 100 for PA FC or 65 for A FC, all at efficiencies of 50%-55%. Nonaqueous electrolytes are also being developed for FCs, the most important being molten carbonates, which operate at 650°C, and solid oxides, which operate at 1000°C. The high temperatures encourage the electrochemical reactions, and catalysts can be dispensed with, but other problems can arise on road vehicles and, so far, no project has emerged. Considerable interest is shown in these high-temperature FCs for stationary power generation. Attention will be restricted in the following to the PEM FC, but much will be applicable to others, especially the aqueous low-temperature ones. A3.3 FC System Optimization 38
The optimization of an FC has to proceed hand-in-hand with the system that transfers the FC power to the road wheels safely, economically, and reliably, and of course provide the usual driver comforts and satisfaction with no adverse environmental impact. The FC stack will consist of a number of cells to give a compact unit, easy to accommodate in the available spaces, yet not too big to discourage stack substitution if one or more cells develop problems. The number of stacks will depend on the power requirements, which depend not only on the type of vehicle and loads, but on the drive system, which can be FC-only (to meet all power demands) or FC in hybrid coupling with storage devices such as batteries (allowing FC power to be more than halved). Not surprisingly, many projects are choosing the hybrid route. The literature reveals little about stack components and performance. Besides the membrane, which is the main proprietary item, the quantities and disposition of catalysts, the electrode characteristics, and so on are confidential--a good sign of competition. Some academic sources (Anand et al. 1994; Lamy and Leger 1994) have compared membrane performance in terms of voltage/current density, but developments underway can easily change the results. New suppliers are obviously entering the field, a trend which could well continue. The hydrogen supply is another major system choice to be optimized with the fuel stack. If pure hydrogen is the choice, either as LH 2 or GH 2 or MH, it can be regarded as an independent variable with little influence on FC performance. Certainly it is less than for the ICE whose performance, as illustrated in Appendix 2, can be significantly influenced by cryogenic temperatures. The hydrogen storage, refilling, and safety questions are otherwise similar to ICE situations. For whatever reason, more attention is being given to methanol as a hydrogen-rich storage medium with FCs than with ICEs. After reforming, which needs significant heat input (at least 15% of FC power), the hydrogen is fed to scrubbers where the CO and other impurities are removed; CO is particularly poisonous at the anode (
Page 1 and 2: IEA Agreement on the Production and
Page 3 and 4: 1.0 Introduction This report was co
Page 5 and 6: Memories of the Hindenburg disaster
Page 7 and 8: During the past 5 years, a carbon s
Page 9 and 10: Wetzel (1996) gives a comprehensive
Page 11 and 12: The 11th WHEC shows increasing inte
Page 13 and 14: ! There is higher efficiency at low
Page 15: Some overall observations may be ap
Page 20 and 21: Europe--Fuel Cells Table 2. Hydroge
Page 22 and 23: 11, Florence, 1992. Dufour, A.; B.G
Page 24 and 25: Lamy, C; J.-M. Leger, “Les piles
Page 26 and 27: Hydrogen Energy Progress XI, 1569-1
Page 28 and 29: Fischer and Eichert (1995) provide
Page 30 and 31: ! Take cryogenic precautions. ! Det
Page 32 and 33: A1.4 The Challenger Accident Certai
Page 34 and 35: The air on the inlet stroke is ofte
Page 36 and 37: of approach: ! A mechanical/hydraul
Page 38 and 39: y Furahama et al. (1991) for high-p
Page 42 and 43: Turning finally to the electrical t
Page 44 and 45: The most abundant and accessible hy
Page 46 and 47: more air in the combustion chamber
Page 48 and 49: noticed.... 7. Environmental Impact
Page 50 and 51: 8. Liquid Hydrogen and Hydride Vehi
Page 52 and 53: 8.4 The cost of liquefying hydrogen
Page 54 and 55: tensions and conflict related to im
Page 56 and 57: Proceedings of the 11th World Hydro
Page 58 and 59: Moore, R.B.; V. Raman, “Hydrogen
Page 60 and 61: ! Cryogenic GH 2 is still in the R&
Page 62 and 63: Pizak, V. et al., “Investigations
Page 64 and 65: Initial tests on a representative e
Page 66 and 67: Cleghorn, S., et al., “PEM Fuel C
Page 68 and 69: vehicles (as well as stationary) ap
Page 70 and 71: To ease the introduction of a new t

Technology Status of Hydrogen Road Vehicles

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