In any future hydrogen-based economy, key economic determinants will be the cost and safety of the fuel distribution system from the site of manufacture of the hydrogen to the end user. This is true of any fuel, but hydrogen presents unique challenges because of its high diffusivity, its extremely low density as a gas and liquid, and its broad flammability range relative to hydrocarbons and low-molecular-weight alcohols. These unique properties present special cost and safety obstacles at every step of distribution, from manufacture to, ultimately, on-board vehicle storage. Also critical is the form of hydrogen being shipped and stored. Hydrogen can be transported as a pressurized gas or a cryogenic liquid; it can be combined in an absorbing metallic alloy matrix or adsorbed on or in a substrate or transported in a chemical precursor form such as lithium, sodium metal, or chemical hydrides. Carbon-bound forms of hydrogen such as today’s gasoline, natural gas, methanol, ethanol, and others are not considered in this report, since their properties and use are well understood. However, comparisons with such conventional fuels will be made when necessary to help clarify the issues related to hydrogen.
Any analysis of hydrogen distribution, transportation, and storage must encompass both centralized manufacture at sites remote from the user points (these could include large central station plants or midsize plants for regional markets, cases that are considered in the cost analysis presented in Chapter 5) and distributed manufacture at the vehicle filling facilities. The centralized manufacture of molecular hydrogen requires a means of transportation and distribution as well as intermediate storage capabilities, while distributed manufacture will likely require only storage at the vehicle filling facility. The use of a chemical hydrogen carrier requires centralized manufacture of that material, shipment to the user site, and then disposal or recycling of the waste materials after the hydrogen is released on board the vehicle.
References to storage in the preceding comments relate only to storage in transit from the production site and at the vehicle filling facility. On-board vehicle storage is discussed separately because its requirements are potentially quite different, even though some of the same technologies, modified for vehicle use, may be employed—for example, high-pressure cylinders or liquid hydrogen containers. On-board reforming of fuels such as gasoline, methanol, or ethanol to produce molecular hydrogen is attractive in principle because it allows use of the existing fuel distribution infrastructure and consequently, if practical, could speed the widespread use of fuel cell vehicles without waiting for safe, cost-effective hydrogen storage technologies to be developed. A few companies are pursuing this technology, but significant technical barriers exist, such as size, weight, cost, and long start-up times.1 (On-board reforming is discussed in Chapter 3.)
The kind of manufacture, transportation, and distribution infrastructure required to support a hydrogen-based fuel cell vehicle will be tied directly to the form of hydrogen used on board the vehicle. For example, on-board storage of molecular hydrogen allows a broad spectrum of raw material precursors to manufacture hydrogen. With a chemical carrier, however, molecular hydrogen may not be needed, and the manufacture, transportation, distribution, and storage systems would be quite different.
In the following sections, various scenarios describe the process of going from the manufacture of hydrogen or its carrier to the on-board storage systems of the vehicle. The major cost and technology barriers to making this process as safe and efficient as possible are presented. Comments are also made on the infrastructure scenarios—that of getting the hydrogen economy started (during the next 10 to 15 years), followed by the intermediate stage as significant numbers of fuel-cell-powered light-duty vehicles are produced (2020 to 2030), and finally, the steady-state scenario
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 37
The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs 4 Transportation, Distribution, and Storage of Hydrogen INTRODUCTION In any future hydrogen-based economy, key economic determinants will be the cost and safety of the fuel distribution system from the site of manufacture of the hydrogen to the end user. This is true of any fuel, but hydrogen presents unique challenges because of its high diffusivity, its extremely low density as a gas and liquid, and its broad flammability range relative to hydrocarbons and low-molecular-weight alcohols. These unique properties present special cost and safety obstacles at every step of distribution, from manufacture to, ultimately, on-board vehicle storage. Also critical is the form of hydrogen being shipped and stored. Hydrogen can be transported as a pressurized gas or a cryogenic liquid; it can be combined in an absorbing metallic alloy matrix or adsorbed on or in a substrate or transported in a chemical precursor form such as lithium, sodium metal, or chemical hydrides. Carbon-bound forms of hydrogen such as today’s gasoline, natural gas, methanol, ethanol, and others are not considered in this report, since their properties and use are well understood. However, comparisons with such conventional fuels will be made when necessary to help clarify the issues related to hydrogen. Any analysis of hydrogen distribution, transportation, and storage must encompass both centralized manufacture at sites remote from the user points (these could include large central station plants or midsize plants for regional markets, cases that are considered in the cost analysis presented in Chapter 5) and distributed manufacture at the vehicle filling facilities. The centralized manufacture of molecular hydrogen requires a means of transportation and distribution as well as intermediate storage capabilities, while distributed manufacture will likely require only storage at the vehicle filling facility. The use of a chemical hydrogen carrier requires centralized manufacture of that material, shipment to the user site, and then disposal or recycling of the waste materials after the hydrogen is released on board the vehicle. References to storage in the preceding comments relate only to storage in transit from the production site and at the vehicle filling facility. On-board vehicle storage is discussed separately because its requirements are potentially quite different, even though some of the same technologies, modified for vehicle use, may be employed—for example, high-pressure cylinders or liquid hydrogen containers. On-board reforming of fuels such as gasoline, methanol, or ethanol to produce molecular hydrogen is attractive in principle because it allows use of the existing fuel distribution infrastructure and consequently, if practical, could speed the widespread use of fuel cell vehicles without waiting for safe, cost-effective hydrogen storage technologies to be developed. A few companies are pursuing this technology, but significant technical barriers exist, such as size, weight, cost, and long start-up times.1 (On-board reforming is discussed in Chapter 3.) The kind of manufacture, transportation, and distribution infrastructure required to support a hydrogen-based fuel cell vehicle will be tied directly to the form of hydrogen used on board the vehicle. For example, on-board storage of molecular hydrogen allows a broad spectrum of raw material precursors to manufacture hydrogen. With a chemical carrier, however, molecular hydrogen may not be needed, and the manufacture, transportation, distribution, and storage systems would be quite different. In the following sections, various scenarios describe the process of going from the manufacture of hydrogen or its carrier to the on-board storage systems of the vehicle. The major cost and technology barriers to making this process as safe and efficient as possible are presented. Comments are also made on the infrastructure scenarios—that of getting the hydrogen economy started (during the next 10 to 15 years), followed by the intermediate stage as significant numbers of fuel-cell-powered light-duty vehicles are produced (2020 to 2030), and finally, the steady-state scenario 1 Bill Innes, ExxonMobil Research and Engineering Corporation, “Issues Confronting Future Hydrogen Production and Use for Transportation,” presentation to the committee, June 12, 2003.
OCR for page 37
The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs when such vehicles achieve major market penetration (2050). (See Chapter 6.) MOLECULAR HYDROGEN AS FUEL Molecular hydrogen is currently receiving the most attention and financial support as the starting point for fuel cell energy supply. The literature and the many presentations that the committee heard indicate that the manufacture of molecular hydrogen is the consensus approach favored by the majority of leadership within the government, at universities, and in industry. It is favored because it allows the use of a variety of hydrogen sources, ranging from coal and natural gas to biomass, solar, wind, and nuclear energy, as well as a multitude of relatively well understood manufacturing approaches ranging from small to large reformers, water-gas-shift reactors, electrolytic devices, thermal processes, and so on. (See Chapter 8 and Appendix G for a discussion of the various hydrogen production technologies.) In the early stages of a transformation to a hydrogen economy, molecular hydrogen will probably be obtained from existing sources such as chemical plants and petroleum refineries. Today, about 9 million tons of hydrogen are manufactured annually in the United States2 and transported for chemical and fuel manufacturing as a low- or high-pressure gas via pipelines and trucks or even as a cryogenic liquid (DOE, 2002a). Much experience worldwide has been achieved over many years to make these transportation modes safe and efficient. However, if the volume of hydrogen use grows, new safety and cost issues will surface, requiring major infrastructure changes. The committee found the analysis presented by Joan Ogden, among others, to be reasonable.3 These analysts contend that in the very early stage of transition to the hydrogen economy, supplying of hydrogen for use in fuel-cell-powered vehicles would rely predominantly on over-the-road shipment of cryogenic liquid hydrogen or possibly hydrogen in high-pressure cylinders from existing chemical and petroleum refining plants.4 Because of the high cost of such shipment modes, government subsidies would probably be needed to help fuel-cell-powered vehicles approach cost parity with gasoline-powered cars. It is also possible that pipelines could be used from existing manufacturing facilities, but this would only be possible where location dictated favorable economics as compared with costs for road shipment. The committee believes that as the volume of demand grows, however, this approach will evolve to the use of local distributed hydrogen production based on natural gas reformers and electrolytic units. These alternatives are less capital-intensive than that of building special pipelines coupled to large, dedicated hydrogen manufacturing plants, and are undoubtedly more economic than continued over-the-road shipping. Whether molecular hydrogen is manufactured centrally or locally, a number of transportation, distribution, and storage requirements pose significant technical, cost, and safety problems. These various requirements could necessitate the use of interim storage facilities at plant sites for inventory or to compensate for demand swings and plant interruptions; the possible use of storage along pipelines and at distribution hubs; storage at the fuel cell vehicle loading stations; and, most critically, storage on board the vehicles themselves. For clarity, on-board vehicle storage is addressed separately from off-board storage, which is associated with distribution from the hydrogen-manufacturing site to the vehicle filling facilities. The committee notes that resilience to terrorist attack has become a major performance criterion for any infrastructure system. In the case of hydrogen, neither the physical and operating characteristics of future infrastructure systems nor the timing of their construction can be understood in sufficient detail to permit an analysis of their vulnerability. However, the committee does observe that public concerns with terrorism seem likely to influence the choice of any future energy system and that resilience to deliberate attack is best designed in at the beginning. Centralized Production of Molecular Hydrogen Table 4-1 underscores key aspects of the costs of moving molecular hydrogen from its place of manufacture to the place where it is used as compared with the same types of costs for today’s conventional fuels such as gasoline and natural gas. The table presents a series of cases that the committee developed for purposes of understanding costs and indicating where research or technology development might play a useful role in reducing them. The increased costs for transportation of molecular hydrogen versus those for conventional fuels are the direct result of the fundamental physical and thermodynamic properties of molecular hydrogen compared with today’s liquid fuels. Molecular hydrogen is a uniquely difficult commodity to ship on a wide scale, whether by pipeline, as a cryogenic liquid, or as pressurized gas in cylinders. On a weight basis, hydrogen has nearly three times the energy content of gasoline (120 megajoules per kilogram [MJ/kg] versus 44 MJ/kg), but on a volume basis the situation is reversed (3 megajoules per liter [MJ/L] at 5000 pounds per square inch [psi] or 8 MJ/L as a liquid versus 32 MJ/L for gasoline). Furthermore, the electric energy needed to compress hydrogen to 5000 psi is 4 to 8 percent of its energy content, depending on the starting pressure; to liquefy and store it is of the order of 30 to 40 percent of its energy content.5 Pipe- 2 Jim Hansel, Air Products and Chemicals, Inc., personal communication to Martin Offutt, National Research Council, October 3, 2003. 3 Joan Ogden, Princeton University, “Design and Economics of Hydrogen Energy Systems,” presentation to the committee, January 23, 2003. 4 Joan Ogden, Princeton University, “Design and Economics of Hydrogen Energy Systems,” presentation to the committee, January 23, 2003. 5 Joan Ogden, Princeton University, “Design and Economics of Hydrogen Energy Systems,” presentation to the committee, January 23, 2003.
OCR for page 37
The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs TABLE 4-1 Estimated Cost of Elements for Transportation, Distribution, and Off-Board Storage of Hydrogen for Fuel Cell Vehicles—Present and Future Case Production Costs ($/kg) Distribution Costs ($/kg) Dispensing Costs ($/kg) Total Dispensing and Distribution Costs ($/kg) Total Costs ($/kg) Total Energy Efficiency (%) Centralized Production, Pipeline Distribution Natural gas reformer Today 1.03 0.42 0.54 0.96 1.99 72 Future 0.92 0.31 0.39 0.70 1.62 78 Natural gas + CO2 capture Today 1.22 0.42 0.54 0.96 2.17 61 Future 1.02 0.31 0.39 0.70 1.72 68 Coal Today 0.96 0.42 0.54 0.96 1.91 57 Future 0.71 0.31 0.39 0.70 1.41 66 Coal + CO2 capture Today 1.03 0.42 0.54 0.96 1.99 54 Future 0.77 0.31 0.39 0.70 1.45 61 Distributed Onsite Production Natural gas reformer Today 3.51 56 Future 2.33 65 Electrolysis Today 6.58 30 Future 3.93 35 Liquid H2 Shipment Today 1.80 0.62 2.42 Future 1.10 0.30 1.40 Gasoline (for reference) $0.93/gal refined $0.19/gal $1.12/gal Well to tank: 79.5% NOTES: The energy content of 1 kilogram of hydrogen (H2) approximately equals the energy content of 1 gallon of gasoline. Details of the analysis of the committee’s estimates in this table are presented in Chapter 5 and Appendix E of this report; see the discussion in this chapter. line transmission of hydrogen is expected to be more capital-intensive than pipeline transmission of natural gas because of the need for pipes at least 50 percent greater in diameter to achieve the equivalent energy transmission rate, and because of the likelihood that more costly steel and valve metal seal connections will be required for pipelines for hydrogen in order to avoid long-term embrittlement and possibilities of leakage. As the shipments of hydrogen grow from today’s low levels to the amounts required to support full-fledged fuel cell vehicle use, major transportation safety code revisions will undoubtedly be required (see Chapter 9). Table 4-1 presents selected data from the committee’s estimates for the costs to deliver hydrogen to fuel cell vehicles (see Chapter 5 and Appendix E). The table summarizes the committee’s assessment of today’s technology costs and possible future costs based on improvements through development and research for the following cases: Centralized production, followed by pipeline distribution and dispensing of gaseous molecular hydrogen. Natural gas and coal are the raw materials, and costs are given with and without CO2 by-product capture and storage.6 Distributed onsite production by natural gas reforming or electrolysis of water. Over-the-road shipment costs of cryogenic liquid hydrogen. This mode is expected to be used in the early stages of hydrogen supply to filling depots and stations. Gasoline distribution and dispensing via today’s infrastructure is shown for reference. 6 The cost of capturing CO2 in a natural-gas-to-hydrogen plant is roughly three times that of a coal-gasification-to-hydrogen plant owing to greater added capital costs related to CO2 capture in the natural gas plant (monoethanolamine [MEA] scrubber plus CO2 compressor) versus that of the coal plant (compressor only). In addition, the natural gas reformer plant pays a greater efficiency penalty than does the coal plant (relative to the case in which CO2 is vented), so its increase in variable costs (feed and fuel) is greater.
OCR for page 37
The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs Obviously the future costs given in Table 4-1 are speculative and were based on the committee’s consensus views of what might be possible. They are to some extent optimistic. Table 4-1 also includes a column on overall efficiency from raw material to final product at the pump, which is interesting for showing how difficult it is to approach today’s gasoline refining and delivery efficiencies. The complete cost data sets with assumptions for the cases in Table 4-1 are given in Appendix E. These cost estimates also include estimates of future improvements through technology refinements and basic research; these results are not listed in Table 4-1 because they do not change the overall conclusions with respect to where the critical areas for cost improvement lie for the distribution and dispensing of hydrogen in a future fuel cell economy. According to the committee’s analysis, the most efficient means of producing hydrogen in the long run is via large-scale, centralized plants that use pipeline distribution networks. Strikingly, while hydrogen can be produced today at costs ranging from $1.22 to $1.03/kg H2 from natural gas, and from coal at $1.03 to $0.96/kg H2 with and without carbon sequestration, respectively, pipeline shipment and dispensing adds an estimated cost of $0.96/kg H2, which is essentially equal to the cost of production. Even with possible future improvements in shipping and distribution, this cost is much more than today’s gasoline dispensing and distribution costs, at $0.19/gal. This analysis demonstrates the realities of shipping H2 gas versus the much more efficient shipment of a liquid. If and when extensive new hydrogen transmission pipelines are needed in the decades ahead, research in such areas as lower-cost pipeline materials, technology for dual-use natural gas-and-hydrogen pipeline connection techniques, layout optimization, and even pipeline emplacement technologies may be of significant value. However, the committee sees this as a priority research area only to the extent that such efforts directly benefit distributed production techniques, which are expected to dominate over the next 20 to 30 years. The energy needed to pressurize hydrogen for pipeline transmission and for local storage at filling facilities where it is stepped up to vehicle on-board storage needs will be significant in terms of capital and electricity; this area may benefit from the development of new technologies. Those used today are mature and have not been improved significantly for many years. Here, too, the committee believes that this is not a near-term priority research area unless it is related to distributed hydrogen production systems, as mentioned above. In the initial phases of hydrogen infrastructure development, the transportation of cryogenic liquid hydrogen via trucking or rail could play a significant role. Table 4-1 shows that over-the-road shipment of liquid hydrogen and dispensing at a vehicle filling site is estimated to add anywhere from $2.42 to $1.40/kg H2 to the production costs. The process of liquefying molecular hydrogen consumes up to 40 percent of the energy content of the weight shipped and may represent an opportunity for technology development. If that could be reduced to a 20 percent loss through some sort of breakthrough, there could be an incremental decrease in cost relative to today’s liquefaction costs, somewhere in the range of $0.20/kg. Research to reduce the liquefaction costs for hydrogen could potentially benefit its cost of shipment by truck, ship, or rail, but could also be advantageous for storage at plant sites to guard against unplanned shutdowns. The committee views this research as more appropriate for nearer-term investment, since this mode of shipment could dominate in the early stages of fuel cell vehicle introduction. In addition to the shipping considerations already discussed, the centralized manufacture of molecular hydrogen will require a series of storage facilities as it makes its way to the consumer. A large-volume, centrally placed manufacturing plant site will require storage for 1 to 5 days’ supply of production to accommodate demand fluctuations and short-term outages. If hydrogen were stored as a pressurized gas, the most economical method at the manufacturing site would probably be underground caverns. A few such caverns have been used in Europe, although they depend for their utility on appropriate underground formations, such as depleted petroleum reserves or wet salt caverns (Ogden, 1999). Clearly, widespread use of such storage would engender much government regulation and careful permitting procedures that in the long run might render them uneconomic as compared with the more-capital-intensive insulated tanks that use liquefied hydrogen as the plant buffer. Whether the hydrogen was stored as pressurized gas or liquid hydrogen, there would also be a need for local storage at the filling facilities and possibly secondary regional distribution sites. For local storage of liquid hydrogen, there would be the need for insulated tanks with tall evaporation dispersement stacks or other means to capture and reliquefy the vaporized hydrogen. For gaseous hydrogen, arrays of high-pressure cylinders probably would be needed. Shipment of compressed hydrogen gas also requires local step-up compressors to bring the pipeline-delivered pressures (100 to 200 psi) or the mobile truck cylinder pressure (2,500 psi) to the needed on-board vehicle pressures of 5,000 to 10,000 psi. The capital and energy-loss costs of all these steps present formidable obstacles to justifying hydrogen as an energy carrier when compared with today’s liquid fuels. Safety issues related to the placement of filling facilities near population centers are also of major concern. Measures to address safety should be a major part of near-term R&D expenditures (see Chapter 9). At a briefing to the committee from representatives of DOE’s Office of Energy Efficiency and Renewable Energy (EERE) on June 10, 2003, cost ranges were given for pipeline and liquid shipment of hydrogen that were somewhat higher than the results shown in Table 4-1. Comparison of the assumptions used for EERE’s and the committee’s cal-
OCR for page 37
The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs culations reveals that the difference lies principally in the length of the transmission pipes, their diameters, and their cost compared with natural gas pipelines. Additionally, EERE’s calculations lumped costs for dispensing with those for transmission and did not include costs of buffer storage at the centralized production facility. Both groups’ assumptions are reasonable, and both lead to the same conclusion for future research targets. Distributed Manufacture of Molecular Hydrogen In the intermediate stages of expansion of fuel cell vehicle use (in the 2010–2020 time frame), local distributed generation with small-scale natural gas reformers or by electrolysis of water will probably make the most economic sense before large, central, dedicated plants with pipeline distribution can be justified economically. The delay of large capital investments for centralized H2 production through distributed manufacture is a significant advantage when fuel cell vehicle density is low, but there are drawbacks in terms of the higher costs associated with current distributed H2 generation technology as well as in the inability to capture CO2 emissions in the case of local reformers. There will undoubtedly also be many new safety and code issues related to the manufacture of hydrogen adjacent to or in urban areas. In the case of local manufacture, however, there appears to be opportunity for important technological improvements in costs and efficiencies for distributed reformers and electrolytic hydrogen generators. Over the next 5 years, improved small reformers with lower operating costs, higher energy efficiency, and lower investment deserve priority (see Chapter 8). If economic means of capturing CO2 on a small scale could also be found, this capability might be a strong incentive for local manufacture in the long run. The committee believes that reformer research aimed at the distributed market should be emphasized now in order to provide hydrogen manufacturing options in the 2010–2030 period. Exploratory research to improve electrolyzer efficiency should also be supported. If it were possible to develop electrolyzers that could lower the cost of local ancillary equipment, such as compressors, or reduce the need for components of storage facilities and improve safety, such advances could significantly benefit the intermediate stages of a hydrogen economy. The committee believes that distributed manufacturing technologies deserve significantly increased research investment over the next 10 to 15 years (see Chapter 9). Solid-State Transport of Hydrogen and Off-Board Hydrogen Storage Means other than pressurized gas or cryogenic liquid theoretically exist for useful transportation and storage of molecular hydrogen. They principally include pressurized absorption in metallic alloys and on or in carbon or other substrates. There are many possibilities, perhaps hundreds (see Thomas  and DOE [2003e] for excellent assessments of the many possibilities under study or suggested as areas for future work). None of these technologies are serious contenders for shipment from centralized manufacturing sites because they are inefficient on a weight and/or volume basis in comparison with cryogenic liquid hydrogen and pipeline-transmitted hydrogen. However, they are still in contention for possible local storage or on-board vehicle storage. Some of the technologies in this category have been used in demonstration projects, but none have come close to being practical for light-duty vehicles. Problem areas include the overall weight of the storage alloys, the limited capacity of the alloys and carbon materials, the difficulties in liberating hydrogen from the carriers, and the high overall system costs. Nevertheless, the committee believes that absorption, adsorption, and related dense-phase hydrogen carrier technologies are a fruitful area for sustained exploratory research primarily because of their promise of safety for off-board and on-board vehicle applications. Almost as important as the need to study this area is the need to narrow the field of technology options as quickly as possible rather than spreading a limited development budget too thinly. The committee makes this point based on the observation of the great number of proposed concepts vying for support. The committee is pleased that the requested DOE budgets in these areas have been increased substantially over the next several years (DOE, 2003a), but it is concerned that continuing existing programs on pressurized tanks and liquid hydrogen approaches may limit more exploratory areas (described above and in the next subsection). On-Board Storage of Molecular Hydrogen Viable options to provide acceptable and adequate on-board vehicle storage of molecular hydrogen for at least a 300-mile driving range follow directly from the preceding discussion. These options include, for example, containment in high-pressure cylinders, in cryogenic dewars with controlled bleed-off and the ability to accommodate significant pressure buildup to slow losses, and in metal alloy matrices or some type of solid absorbent or adsorbent. In the case of 5,000 to 10,000 psi cylinders, the principal issues are concern for public acceptance of their safety, the cost to manufacture such containers (which today are made as multishelled structures that use fiber-wound composite technologies), the time and complexity of the filling operations, and the space that such tanks with the needed capacity would occupy on board the vehicle (see Table 4-2). For example, for more than a 200-mile driving range, today’s natural gas vehicles usually require two tanks, which use up much of the trunk. A hydrogen-fueled vehicle with 5,000 psi tanks would probably require two tanks, or, if the tank was 10,000 psi, a small vehicle might need one tank. Several companies are trying to develop these tanks, but none has
OCR for page 37
The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs achieved the required performance. Table 4-2 summarizes the minimum performance needs for hydrogen on-board storage as expressed by representatives of the automotive industry (DOE, 2002b). Table 4-2 also includes the targets established by the DOE with the FreedomCAR Hydrogen Storage Technical Team (DOE, 2003b). Cryogenic pressurized storage technologies are less developed than high-pressure gas storage cylinders are, but have been used in some demonstration vehicles. The use of liquid hydrogen as fuel on board a light-duty passenger vehicle seems unlikely to meet the capacity and size requirements acceptable to the automotive industry. In addition, further obstacles to this approach include the high energy requirements for liquefying molecular hydrogen, safety concerns related to continuous hydrogen boil-off, and the escalating number of delivery trucks that would be on the road to meet demand in the middle years of scale-up. If molecular hydrogen is to be used on board small personal vehicles, it seems most likely that some sort of reversible solid system must be developed. Currently, many concepts are under study for this type of system. These include a wide variety of metal alloys that form reversible hydrides, hydrogen adsorbers based on various forms of carbon and other high-surface-area materials, high-energy chemical compounds such as sodium borohydride that react with water or even alcohols, and a whole series of early concept ideas that aim to store and then liberate hydrogen when it is heated or reacted (Thomas, 2003). None of the concepts under study has achieved the minimum objectives set by industry (see Table 4-2). Even if the capacity and percent-by-weight goals can be demonstrated, there are major issues around costs of the carrier materials, filling times, and heat management during filling and hydrogen liberation to meet the fluctuations in electrical demand associated with normal driving. Heat management during hydrogen uptake (fueling) and hydrogen desorption during vehicle operation need further study. If the heat of desorption per mole of molecular hydrogen is large, two important implications follow. First, a large surface area for the heat exchangers is required, and it will add weight and volume; if waste heat is not available at the needed temperature and rate, a significant fraction of the fuel energy will be wasted. This also means that the fuel cell must operate at a higher temperature than the desorption temperature for hydrogen. Current proton exchange membrane fuel cells (PEMFCs) operate at approximately 80°C; consequently, the desorption temperature must be substantially lower. This relationship suggests that important research is needed either to raise the fuel cell operation temperature or to lower the H2 desorption temperature. New materials concepts have an important role to play in finding a solution for the hydrogen release problem. Heat management during uptake and release is a critical area requiring attention. Device designs that can load vehicles in an acceptable time with fail-safe safety controls and then release hydrogen at the rates demanded are vital to the success of this approach. The committee views these areas, although still in their infancy, as very important. In summary, the committee questions the use of high-pressure tanks aboard mass-marketed private passenger vehicles from cost, safety, and convenience perspectives. The committee is also concerned about the complexity and capital intensity of the filling station equipment. The committee has a similar view of the use of liquid hydrogen. Exploratory budgets for the development of dense-phase materials as hydrogen carriers are being expanded, as mentioned above, but goals for this research need to be sharpened toward the objective of focusing on a few options that have real promise, and then on accelerated early-stage development. Without such a commitment to show encouraging progress in this critical area, private sector enthusiasm toward the development of fuel-cell-powered light-duty vehicles could wane substantially. Alternatives to Molecular Hydrogen Transportation, Distribution, and Storage The preceding discussion is based on the assumption that the cost and safety problems associated with transportation, distribution, and on- and off-vehicle storage can be satisfactorily solved with molecular hydrogen at every stage of its scale of use, and that there is no better approach available. However, the committee was presented with several intriguing “game-changing” possibilities (JoAnn Milliken, Department of Energy, “Hydrogen Storage,” presentation to the TABLE 4-2 Goals for Hydrogen On-Board Storage to Achieve Minimum Practical Vehicle Driving Ranges Energy Density General Motors Minimum Goals Compressed/Liquid Hydrogen (Currently) DOE Goal Megajoules per kilogram 6 4/10 10.8 Megajoules per liter 6 3/4 9.72 NOTES: Energy densities are based on total storage system volume or mass. Energy densities for compressed hydrogen are at pressures of 10,000 psi. SOURCES: DOE (2002b, 2003b).
OCR for page 37
The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs committee, December 2, 2002; Thomas, 2003) that it believes should be vigorously examined for their potential. Here again, narrowing the field as quickly as possible to focus on those few prospects with the most potential is a vital component of any research investment strategy. All alternatives to molecular hydrogen relate to the manufacture of energetic metals or their hydrides, which, when reacted with water, emit hydrogen (Thomas, 2003). These materials would be shipped from centralized manufacturing sites by conventional truck, rail, or ship and distributed to consumer fuel cell vehicle filling facilities. Vehicles would be equipped with devices for reacting the compounds with water in order to generate fuel-cell-quality hydrogen and for storing the waste reactants. Waste would then need to be recycled or disposed of in an environmentally acceptable manner. The principal game-changing features of these materials are the elimination of most safety and cost issues that high-pressure or cryogenically liquefied molecular hydrogen has, and the possibility of a major safety and range enhancement for on-board storage of hydrogen. Several small-vehicle demonstrations of the efficacy of this approach and its ability to provide acceptable driving range, hydrogen purity, and delivery rate and vehicle space efficiency have been successfully made (Bak, 2003). The use of 20 to 30 percent by weight of alkali-stabilized aqueous solution of sodium borohydride as fuel, which is pumped over a catalyst to generate hydrogen instantaneously, was demonstrated recently by DaimlerChrysler in its Chrysler Town and Country Natrium fuel cell minivan vehicle.7 This approach demonstrated the potential for meeting vehicle mileage, weight, and volume goals.8 The principal current shortcomings of these chemical methods for generating hydrogen are the high cost of manufacture of the chemicals and the not-yet-demonstrated technology for recycling or disposing of waste products effectively. Secondary issues include catalyst longevity over the vehicle life, fuel stability on board the vehicle, and the ability to meet automotive range and reliability requirements. However, all of these shortcomings, with the exception of the cost of recycling and initial manufacture, have had encouraging real-world demonstrations in full-sized passenger vehicles, as for example with the Natrium fuel cell vehicle. The committee believes that this is an important area for further research and that it should be pursued vigorously to find the best chemicals for this use and to improve the economics of their manufacture and regeneration. The DOE should also continue to encourage other game-changing concepts because of the pivotal importance of this need to the future of fuel-cell-powered vehicles. THE DEPARTMENT OF ENERGY’S HYDROGEN RESEARCH, DEVELOPMENT, AND DEMONSTRATION PLAN The committee was pleased to be given an early draft of the DOE Office of Energy Efficiency and Renewable Energy’s “Hydrogen, Fuel Cells and Infrastructure Technologies Program: Multi-Year Research, Development and Demonstration Plan” (dated June 3, 2003) (DOE, 2003b). The following are the committee’s comments on this document regarding the areas of off-board storage, transportation, and distribution of hydrogen (see DOE [2003b], pp. 3-30 through 3-55). Fundamentally, the committee agrees with the DOE’s assessment of the research needs in these important areas, especially those relative to pipeline costs and the need to improve the energetics of hydrogen compression and liquefaction. The committee differs with the DOE on near-term priorities. The committee believes that the requested increased funding in these areas should be prioritized to strongly favor solid or dense-phase storage of hydrogen, especially for on-board vehicle use, since on-board storage appears to be one of the primary obstacles to fuel cell vehicle practicality, along with the needed fuel cell cost reduction and reliability improvements. FINDINGS AND RECOMMENDATIONS The following findings and recommendations are based on the idea that some research and technology investments are at present more important than others in criticality and in time. This prioritization reflects the need to invest in overcoming the technology gaps that might be major stumbling blocks to immediate progress and to delay or reduce investment in those activities that, while very important, can wait for several years because they are not critical to near-term progress. Finding 4-1. It seems likely that in the relatively near term (the next 10 to 30 years), distributed rather than centralized production of hydrogen will be a driver for the continued expansion of fuel-cell-powered private vehicles. Needs in the very early period are expected to be covered by shipment of pressurized or liquefied molecular hydrogen, but as volume requirements grow, such an approach may be deemed too expensive and/or too hazardous for continued widespread use. Distributed manufacture of molecular hydrogen seems most likely to be best done with small-scale natural gas reformers or by electrolysis of water. At present both technologies are capital-intensive and relatively energy-inefficient. Without such distributed manufacture, it seems likely that the very large centralized production and pipeline distribution investments will be difficult to justify and could slow conversion to hydrogen markedly. It seems possible that, in comparison with today’s state-of-the-art technology, the new 7 The spent fuel cartridges would be regenerated at a central location. 8 Additional information is available online at www.h2cars.biz/artman/publish/article_144.shtml. Accessed December 4, 2003.
OCR for page 37
The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs technology for distributed manufacture may reduce production costs through efficiency improvements and possibly by enabling reduced capital requirements for ancillary storage and filling equipment. Recommendation 4-1. Increased research and development investment in support of breakthrough approaches should be made in small-scale reformer and electrolyzer development with the aim of increasing efficiency and reducing capital costs. A related goal should be to increase the safety and reduce the capital intensity of local hydrogen storage and delivery systems, perhaps by incorporating part or all of these capabilities in the hydrogen-generating technologies. Finding 4-2. It is clear that the vast majority of current private and governmental investments in the manufacture of hydrogen for fuel cell vehicles are aimed at the direct use of molecular hydrogen. Because of the inherent difficulties in the transportation, distribution, and storage of molecular hydrogen, it is apparent that other approaches for hydrogen generation may have advantages for transportation and for on- and off-board storage. The latter include compounds that, on reaction with water or some other reactant, generate hydrogen, and solid-state carriers that contain high concentrations of adsorbed or absorbed hydrogen that liberate the stored hydrogen through the application of heat. Many possibilities exist in these categories, but few have received significant research support. Solid-state hydrogen carriers will probably not be useful for the transportation and distribution of hydrogen, but may be valuable for local and/or on-board vehicle storage. The committee strongly supports the requested Department of Energy budget increases in the vital area of hydrogen storage. The committee believes, however, that major shifts in emphasis should be made immediately in order to make sure that the many new ideas currently available are properly examined—because without relatively near-term confidence by industry and government leaders, interest in continuing the pursuit of fuel cell vehicle transportation uses is likely to wane over time. Recommendation 4-2. The Department of Energy should halt efforts on high-pressure tanks and cryogenic liquid storage for use on board the vehicle. These technologies are in a pre-commercial development phase, and in the committee’s view they have little promise of long-term practicality for light-duty vehicles. The DOE should apply most if not all of its budgets to the new areas described in Finding 4-2 with the objective of identifying as quickly as possible a relatively few, promising technologies. Where relevant, efficient waste-recycling studies for the chemically bound approaches should be part of these studies. Even during this winnowing process the DOE should continue to elicit new concepts and ideas, because success in overcoming the major stumbling block of on-board storage is critical for the future of transportation use of fuel cells. Finding 4-3. The evolution of the transportation and delivery and storage systems for hydrogen will transition several times as hydrogen demand increases over many decades. This would of necessity mean continuous and overlapping shifts from small-scale delivery and storage, to distributed manufacture and storage, to centralized production with extensive pipeline, distribution, and storage networks. Such a complex evolution would likely benefit from systems analysis to help guide the optimum research and technology investment strategies for any given stage of the evolution and thus enable the most effective progress toward the long-term end states. Recommendation 4-3. Systems modeling for the hydrogen supply evolution should be started immediately, with the objective of helping guide research investments and priorities for the transportation, distribution, and storage of molecular hydrogen. In addition, parallel analysis of the many alternatives for other means of supplying hydrogen to fuel-cell-powered facilities and vehicles should be performed; such analysis is needed to prevent wasteful expenditures and to help focus attention on viable technology that would potentially compete with the direct supply and delivery of molecular hydrogen and that might be useful for all or portions of the future hydrogen economy. Finding 4-4. Hydrogen is particularly difficult to ship from a manufacturing site to filling facilities for vehicle servicing. In fact, the cost to ship and store can easily equal the costs of production. These costs are directly related to molecular hydrogen’s thermodynamic properties, low molecular weight, and consequently high diffusion capabilities, and to its great flammability and ability to form explosive mixtures over a wide range of concentrations. Particular concerns relate to the energy losses during compression and liquefaction and to the tendency of hydrogen to embrittle some current pipeline materials. Recommendation 4-4. Research and technology development should be carried out in support of novel concepts that promise major improvements in the cost and efficiency of compressors for molecular hydrogen and reductions in the cost of pipeline materials, valves, and other leak-prone components of its distribution system. Initial research should focus on those components that are directly related to distributed hydrogen production. In later years, research should shift to components for large, centralized production plants with extensive pipeline and storage facilities. The committee believes that current Department of Energy plans call for research that relates primarily to centralized molecular hydrogen manufacture—a need that is many decades in the future—and consequently may shortchange other, more immediate needs.