However, water electrolysis is still a small segment of the total hydrogen-generation capacity, because capital costs are high and have not been seriously reduced to date, and operating costs are high; together these costs lead to high-cost hydrogen. Except for selected military and industrial-based uses, electrolysis systems have been built without volume and cost-reduction benefits due to the lack of high-volume manufacturing.
Even with these limitations, conventional water electrolysis exhibits high efficiencies percentages in the 70s versus lower heating value [LHV] and long lifetimes using multiple chemistries and processes, and electrolysis systems are available commercially in low- and high-volume production of gas. Such advancements have the potential to impact the energy requirements that are over and above what is needed to split the water from the perspective of a fundamental electrochemistry requirement. Specifically, membrane development could possibly impact the resistance of the stack, thereby reducing the ohmic losses if the membrane has enhanced conductivities over those of currently used materials.
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In all cases, such energy-consumption-reduction and capital-cost-reduction efforts will have to succeed without seriously impacting the efficiency and lifetimes currently exhibited in commercial and military electrolyzers. Lastly, such developments must make progress against the goals for hydrogen cost. Presented in Table are the DOE cost targets for distributed hydrogen generation from water electrolysis. Higher-risk, longer-term new technologies have been recently reported Farmer, and include both high-temperature electrolysis and the photo-electrolysis of water.
High-temperature electrolysis has advantages in the reduction of energy requirements to split water owing to the lower voltage requirements to dissociate water, but it requires high-temperature materials that are challenging. The high-temperature option, using solid oxide technology, also has the potential to make use of waste thermal energy, thereby making nuclear power plants attractive locations for centralized generation.
Another aspect of the long-term potential of the electrolysis process is that there are variations of the engineering configurations, making it even more attractive by possibly reducing system complexity. For example, in selected cases, hydrogen may be generated at substantial pressures, thereby reducing the need for follow-on mechanical compressors DOE, Furthermore, from a final cleanup perspective, as electrolysis generates relatively pure hydrogen, the final cleanup stage from a pressurized system may use alternative, existing know-how to remove residual oxygen and moisture efficiently e.
In such cases, the electrolyzer may be able simply to generate pressure and purity without significantly impacting electrical consumption and capital. If such is accomplished, refueling hydrogen may be available in remote and non-methane-accessible regions that meet hydrogen purity specifications. The DOE recognizes that water electrolysis may play an important role in the hydrogen infrastructure, and the DOE is supporting numerous electrolysis efforts related to capital, electrocatalytic processes, and configuration and engineering.
In addition, a number of systems analyses now include water electrolysis.
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Furthermore, photoelectrolysis has the potential to improve the efficiency of water splitting, but fundamental research is needed to establish the feasibility of this approach relative to conventional electrolysis DOE, Water electrolysis should remain an integral part of the future hydrogen infrastructure development. The DOE should continue to fund novel water electrolysis materials and methods, including alternative membranes, alternative catalysts, high-temperature and -pressure operation, advanced engineering concepts, and systems analysis.
Additional efforts should be placed on advanced integration concepts in which the electrolyzer is co-engineered with subsequent upstream and downstream unit operations to improve the overall efficiency of a stand-alone system. Commercial demonstrations should be encouraged for new designs based on established electrolytic processes.
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For newer concepts such as high-temperature solid oxide systems, efforts should remain focused on laboratory evaluations of the potential for lifetime and durability, as well as on laboratory performance assessments. The DOE continues to study at NREL opportunities to couple wind and solar energy with electrolysis, and it has several projects to improve the efficiency of electrolyzers. The program has recently demonstrated about 70 to 71 percent efficiency at the stack level. Higher-pressure electrolyzers could be a thrust for the future and could reduce the compression energy for storage and vehicle refueling.
The hydrogen storage can be used to offset at least in part the intermittent and variable nature of the wind and solar resource. This approach can be employed with three different energy pathways: wind to grid; wind to electrolysis unit to hydrogen; and hydrogen to fuel cell to grid. These outputs can be varied if there is not enough demand for hydrogen for vehicle fueling. Some of the challenges with a wind- and solar-driven electrolysis approach include efficient power electronics for dc-to-dc and ac-to-dc conversion, and controllers and communications protocols to match the source to the electrolyzer.
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Work on close coupling of wind and solar energy with electrolysis should be continued with stable funding. Further improvements in electrolyzers, including higher stack pressure, and in power electronics will benefit this application. This subsection covers the discussion of programs oriented toward using solar energy to split water. Included are biological and photoelectrochemical hydrogen production. The production of hydrogen using microorganisms, utilizing energy by absorbing incident light and nutrients, can be a carbon-neutral process.
The FreedomCAR and Fuel Partnership identifies four main biological production pathways: photolytic direct water splitting , photosynthetic bacterial solar-aided organic decomposition , dark fermentative organic decomposition , and microbial-aided electrolysis electric power-aided organic decomposition DOE, b. There are many barriers to technical success that, if overcome, would result in a process competitive with other pathways for hydrogen production.
Thus, a possible application identified for this approach is to generate hydrogen from dilute feedstock in waste streams from other processes that would not be captured otherwise. The technical barriers include, among others, lack of information on microorganisms with suitable characteristics for biological hydrogen production; efficiency in light utilization; efficiency in feedstock utilization; cost; and product purity.
In spite of the difficulties, the Partnership reported some noticeable progress—for example, the successful cloning of the Tla2 gene to enable 15 percent absorbed solar-to-chemical-energy conversion efficiency in microalgae. Internationally, this approach is pursued actively. Photoelectrochemical water splitting, utilizing electrolysis, converts solar energy directly into chemical energy in the form of hydrogen.
Barring spectacular breakthroughs, the potential impact of biological and photoelectrochemical hydrogen production will be limited and far in the future. Support of this approach has been by BES, which is appropriate because of its exploratory nature and because discoveries could just as likely have applications other than for the Partnership.
The committee finds no clearly defined targets or vision of the photolytic approach that will contribute to the overall hydrogen production goals, and as a result it is unclear whether the Partnership should retain this approach in its portfolio of activities. Alternatively, hydrogen could be stored for use when there is no sunlight. This would also be a barrier, given the issues with hydrogen storage discussed in this report.
The Partnership should examine the goals for the photolytic approach to producing hydrogen using microorganisms and formulate a vision with defined targets. Otherwise, this approach should be deemphasized as an active research area for hydrogen production. A significant factor in fuel cost and source-to-wheels efficiency for fuel-cell-powered vehicles is the means for delivering, storing, and dispensing hydrogen, especially compared to the petroleum delivery system, which is low-cost and efficient.
The distribution costs are of even greater concern in the transition period when there is a lack of demand, particularly when hydrogen from centralized production is available. In such cases distribution could easily cost more than production. Dispensing systems for gaseous hydrogen must be designed to prevent excessive temperature increases in the vehicle tank during pressuring and filling, particularly for bar approximately 70 MPa or 10, psi operation. As a result, communication between the vehicle and the refueling dispenser is required so that pressure and temperature can be monitored and controlled.
Given the importance of the area, all of these have been studied in the program. The DOE program on the delivery, storage, and dispensing of hydrogen is comprehensive and includes aggressive cost targets see Table Given that all of the physical steps involved in delivery and dispensing have been practiced for decades by the gas industry, the committee continues to question whether it will be possible to reduce costs to the target levels, but clearly significant cost reductions are very important to the outlook for hydrogen-powered vehicles.
Funding of this important program has been variable. In spite of this inconsistent funding, progress has continued to be made. Gardiner and J. Dispensing at refueling site a.
Progress has been made in all areas of the program. Delivery models have been developed that predict delivery and dispensing costs for different methods as a function of market penetration. Hydrogen compression has been direction-ally advanced by investigating a centrifugal compressor design and also electrochemical compression. Promising aging studies on a fiber-reinforced polymer pipe material were completed. In addition, studies of carbon-fiber composites and glass fibers indicate that the capacity of tube trailers can be increased by a factor of two to three, leading to a potential cost reduction for these trailers, according to DOE estimates, of up to 50 percent.
Past funding in the area of hydrogen delivery and dispensing has not been based on program needs but apparently on budget constraints. Otherwise, any chance of meeting the target will be forgone. Hydrogen delivery, storage, and dispensing should be based on the program needed to achieve the cost goal for If it is not feasible to achieve that cost goal, emphasis should be placed on those areas that would most directly impact the decision regarding commercialization.
The program framework is based on a target date for getting all of the technical information needed by the automotive manufacturers to make decisions on commercialization of fuel-cell-powered vehicles. Presumably, these decisions will involve in part the assurance of the availability of hydrogen at a sufficient number of locations to provide the fuel needed. Compression will take on even greater importance as the delivery pressure is raised from to bar about 5, to 10, psi, or 35 to 70 MPa.
Liquid hydrocarbon fuels made from biomass and used in an internal combustion engine ICE are another pathway that can have an effect on oil imports and CO 2 emissions. This pathway differs from the other two primary pathways being developed by the Partnership in that the light-duty vehicle LDV and drive system do not require new technology for this pathway to grow. The hydrogen fuel cell pathway and the battery electrification pathway hybrid electric vehicle [HEV], PHEV, and BEV both require new drivetrain technology for them to begin and to grow.