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Author: Sunita Satyapal
Hydrogen and fuel cell technologies are part of the US Department of Energy’s (DOE) balanced portfolio of research and development activities. Significant progress has enabled market entry, with an estimated 35,000 fuel cells shipped worldwide just last year. Fuel cells are now being used for backup power, primary power, and early market applications such as forklifts and even cell phone chargers. Fuel cell cars are starting to be leased and automakers have announced plans for commercial sales as early as 2015. Given all the recent advances, this article provides an overview of the advantages and disadvantages of hydrogen and fuel cells, the current status, and a summary of progress and remaining challenges.
Fuel cells can use diverse fuels and generate electricity directly through an electrochemical reaction rather than through combustion, which can waste more than two-thirds of the fuel energy content as heat. Figure 1 lists different types of fuel cells, primarily distinguished by their electrolytes (e.g., polymer electrolyte membranes or solid oxide ion conducting electrolytes), and diverse applications ranging from portable power at a scale of just a few watts to large, stationary, multimegawatt central power generation.
For automotive applications, the fuel cell of choice is the proton-exchange (also called polymer-electrolyte) membrane (PEM) fuel cell, which operates at around 80°C and has rapid startup and response times and high power densities. Key advantages of hydrogen-fuelled transportation applications are that hydrogen can be produced from diverse domestic resources, and the only emissions from the point of use are water and a small amount of heat. Because there is no combustion involved, fuel cells are highly efficient. In contrast, just over 20 percent of fuel energy content is actually used to move gasoline-powered automobiles, taking into account losses such as heat, air drag, rolling friction, and brake losses (Chu and Majumdar 2012).
Fuel cell electric vehicles (FCEVs) are one approach among a number of others (e.g., hybrid vehicles, higher-efficiency combustion engines, electric vehicles, and plug-in hybrids) being pursued to improve the efficiency of light-duty vehicles. Global automakers have spent billions of dollars over more than a decade to bring FCEVs to the market and are just starting to lease commercial vehicles and announce plans for sales in 2015–2017.
In addition to the efficiency improvement—by about a factor of two—of fuel cells over conventional gasoline internal combustion engines, vehicle performance can be on par or better. The driving range of FCEVs can exceed 250 miles on a single tank, with refueling times of just a few minutes, as demonstrated with more than 180 FCEVs and 3.5 million miles of driving (Wipke et al. 2012). Some models were capable of up to 430 miles without needing to refuel, as shown by independent on-road validation (Wipke et al. 2009), demonstrating that range is no longer an issue.
Moreover, since full torque is available from a standing start, FCEVs do not require multiple shifting of gears to get up to speed, and acceleration is smooth and quiet without the noise associated with a continuously variable transmission. Automakers such as General Motors have repeatedly emphasized that FCEVs can eliminate conventional drive train components such as transmissions, axles, and mechanical linkages, potentially simplifying vehicle manufacturing (Burns et al. 2002).
While fuel cells provide benefits in terms of performance, efficiency, response time, and emissions, there are also challenges, primarily associated with the hydrogen infrastructure required to fuel automotive fuel cells.
Enthusiasts point out that hydrogen has the highest energy content of all known fuels (33.3 kWh/kg or 120 MJ/kg), but this is accurate only on a mass basis (nearly 3 times more than gasoline). On a volumetric basis, the energy content in liquid hydrogen (2.36 kWh/L or 8.5 MJ/L) is nearly four times lower than that of gasoline, and gaseous hydrogen at 700 bar pressures is six times lower (Berry et al. 2004; McWhorter et al. 2011). Since hydrogen is typically stored on board FCEVs as a high-pressure gas, this adds cost and complexity both on the vehicle and at the refueling station.
Advocates also point out that hydrogen is the most abundant element in the universe. But molecular hydrogen cannot be found on earth: it is bound in the form of water and numerous other compounds, and although it can be produced from water, using electrolysis, the typical conversion efficiency of producing hydrogen from water is at most about 70 percent (NPC 2012), which means that for every unit of energy input, roughly a third is wasted.
With the pros and cons of various technologies, there is no single one that meets all current needs. A portfolio of options is needed to significantly reduce petroleum use and carbon emissions. While advanced combustion, hybrids, and electric vehicles are important to pursue, fuel substitution is essential to meet national goals.
Fuel cells have high electrical efficiencies—with up to 59 percent demonstrated in real-world driving (Wipke et al. 2012)—but because hydrogen production requires energy it is not sufficient to emphasize fuel cell efficiency alone. The energy input and associated carbon emissions involved in fuel production, delivery, compression, dispensing, on-board storage, and on-board use must be taken into consideration.
Figure 2 shows the total “well-to-wheels” emissions for a variety of vehicles using a range of assumptions for advanced future technologies such as FCEVs, incorporating both conservative and optimistic advances in each technology and the availability of renewable electricity projected in 2035 (Nguyen et al. 2013). The numbers in the horizontal bars represent the base case and each end of the bar represents the minimum and maximum estimate for carbon emissions based on assumptions for efficiency, fuel pathway, electricity mix, and technology advances.
According to the data depicted in Figure 2, an average conventional midsize light-duty passenger vehicle operating on gasoline emits roughly 430 grams of carbon dioxide per mile, whereas all the advanced technologies show potential for reduced emissions. In the case of FCEVs, even when using distributed natural gas at fueling stations to generate the hydrogen, the total well-to-wheels emissions is less than half that of conventional gasoline-powered vehicles (Nguyen et al. 2013). And if natural gas is used to produce hydrogen at a central plant that includes carbon capture, the total emissions can be significantly less.
Clearly the use of renewables for hydrogen production or for charging battery electric vehicles is needed to achieve the greatest reduction in emissions. The key challenge is to produce hydrogen in a clean, low-cost, and environmentally responsible way.
Costs and Technical Challenges
More than 50 million metric tons of hydrogen are produced worldwide, primarily by steam methane reforming of natural gas (DOE 2013a). Most of it is used in petroleum refining (to reduce sulfur content) and ammonia production.
With the large central production of hydrogen from natural gas, the cost of hydrogen is less than $2/kg (NPC 2012). This equates to about $2 per gallon gasoline equivalent (gge) because in terms of energy content (i.e., lower heating value), 1 kg of hydrogen is about the same as 1 gallon of gasoline (a convenience of nature so no conversion is needed). Hydrogen must also be produced from renewable sources at a sufficiently low cost to be competitive with gasoline and other fuels.
Although hydrogen production costs from natural gas at large central plants may be low (particularly since shale gas development has enabled a drop in feedstock costs), the hydrogen still needs to be transported, compressed, and dispensed at a refueling station for use in vehicular storage tanks. The additional cost of these steps can be as high as $3/gge, even at volume (Parks et al. 2014), resulting in a hydrogen cost of about $5/gge dispensed at the pump (untaxed) even if produced at scale and using optimistic assumptions. In practice, at today’s low volumes, the cost is substantially higher and varies depending on supplier, region, and application. Moreover, because compressors have not achieved consistent reliability, stations have to have more than one to ensure that customers can get the fuel they need when they need it.
The estimated cost of a vehicular carbon fiber compressed hydrogen storage system at 700 bar is about $3,000 if manufactured at 500,000 units per year—or more than $6,000 at volumes of 10,000 units per year, even with optimistic assumptions (James et al. 2012). Compression is a key contributor to cost, and therein lies a key challenge: to carry a sufficient mass of hydrogen on board a typical passenger car (roughly 5 kg) and achieve a driving range of at least 300 miles, automakers are pressurizing it to 700 bar (about 700 atmospheres or 10,000 psi, the pressure agreed on by major global automakers). Safety requirements add cost as the tanks must be built to withstand more than twice their fill pressure (or more than 1,400 atmospheres of hydrogen) and undergo drop tests, bonfire tests, and even gunfire tests to ensure safety.
To be competitive with gasoline internal combustion engines, an automotive fuel cell system must cost $30/kW or less; the DOE has set a target of $40/kW by 2020 if produced at scale for early markets. However, based on state-of-the-art technology demonstrated in the laboratory (not yet in FCEVs), the projected cost if manufactured at high volume—500,000 units per year—is about $55/kW (Spendelow and Marcinkoski 2013). The current rate of production is substantially lower, resulting in a much higher unit cost of $280/kW, based on automaker references (Greene and Duleep 2013), which should be reported in conjunction with the high-volume projection. One of the key contributors to cost is platinum, the primary catalyst required for the electrochemical reaction. As discussed below, alternative materials are being studied.
Last, although performance and power density have been steadily improving, the durability of fuel cells does not yet meet the target of 5,000 hours or 150,000 miles, the expectation of today’s automobile driver.
Recent Progress in Fuel Cells
Despite the challenges, significant progress has occurred, especially over the last decade, spurred by both government funding and private sector developments.
Federal Funding and Commercial Adoption
On the government side, the DOE fuel cell program began in the mid-1970s with a small group of researchers and managers at a Los Alamos National Laboratory workshop, when the oil embargo had stimulated increased attention to alternative energy and fuel technologies. These innovative thinkers paved the way for what was to become DOE’s Fuel Cell Technologies Office, which now funds a roughly $100 million annual portfolio of research, development, and demonstration (RD&D) activities through universities, industry, and national laboratories, enabling innovations now being implemented in commercial systems for various applications.
In DOE’s Office of Energy Efficiency and Renewable Energy (EERE) fuel cell activities have enabled more than 450 patents, the introduction of 40 commercial technologies in the market, and 65 emerging technologies that are expected to be market ready in 3–5 years (DOE 2013b). In addition to funding RD&D activities, EERE has cost shared the deployment of roughly 1,600 fuel cells for backup power at cell phone towers and for forklifts, resulting in industry purchases of more than 11,000 fuel cells (Devlin and Kiuru 2013a,b).
Furthermore, major companies such as FedEx, Walmart, Sysco, Wegman’s, and Coca-Cola are beginning to purchase fuel cell forklifts for their warehouses, and Sprint, AT&T, and others are deploying fuel cell backup power units for their cell phone towers, all of which will create demand for a hydrogen infrastructure. In addition, large commercial and industrial buildings as well as data centers are using fuel cells for reliable power or combined heat and power. Figure 3 shows that fuel cells are no longer a laboratory research project: an estimated 35,000 units were shipped worldwide in 2013—up from about 15,000 just four years earlier—primarily in the stationary fuel cell market for combined heat and power (Satyapal 2014).
Materials and System Innovations
The platinum group metal (PGM) loading in PEM fuel cells has decreased by 2 orders of magnitude since the 1960s and 1 order of magnitude since the mid-1980s (Spendelow and Papageorgopoulos 2011). Advances such as nanostructured thin films by 3M and core-shell catalysts by Brookhaven National Laboratory (which contain a less expensive core metal such as nickel and a layer of platinum skin or alloy) have contributed to recent progress. Based on these and other advances, automotive fuel cell cost has decreased approximately 30 percent since 2008 and 50 percent since 2006 (Spendelow and Marcinkoski 2013).
Other innovations have paved the way for greater interest in hydrogen and fuel cell technologies. For example, DOE and state agency and private sector partners funded the demonstration of the world’s first “trigeneration” system, a 300 kW, high-temperature, molten carbonate fuel cell that can convert biogas or natural gas to power, heat, and hydrogen. This system provides three simultaneous coproducts for use across sectors: stationary power generation, industrial or building heating and cooling, and hydrogen for transportation or other applications such as backup power or disaster mitigation. The system was demonstrated at a wastewater treatment plant and could be useful at other sites such as sewage treatment plants and landfills as well as numerous industrial facilities.
Certain fuel cell systems can also be used to separate/purify carbon dioxide to enable carbon capture. If tied to relevant future electricity generation, transmission, and storage infrastructure, a more holistic approach for hydrogen generation (e.g., trigeneration or natural gas reforming) could be coupled with local capture and utilization of carbon byproducts.
Hydrogen can also be used to enable the more widespread use of intermittent renewables such as solar or wind, electrolyzing water and storing the hydrogen for use either as a fuel or feedstock or to feed back to the grid via turbines or fuel cells to generate electricity. A number of such projects are under way at a large scale in Germany and other countries that have substantial deployment of renewables.
More Research Is Needed
The widespread commercialization and acceptance of hydrogen and fuel cell technologies will depend on advances enabled by further research and development (R&D). The cost of hydrogen from renewables and low-carbon sources must be reduced to meet the DOE target of $4/gge by 2020. Innovative approaches such as direct photoelectrochemical conversion of water to produce hydrogen, biological (including photobiological) approaches, and high-temperature thermochemical methods that can use heat from either nuclear or solar power are just some of the technologies that require more R&D. Although preliminary research (Elgowainy et al. 2014) shows that water and environmental impacts can be minimal, strategic and well-defined studies are necessary to ensure production of the required amount of hydrogen regionally with minimal ecological impacts.
Once hydrogen is produced at a large scale, high-pressure tube trailers can reduce the cost of compression at the station and provide a viable option in the near to midterm. However, in the long term hydrogen pipelines will need to be built—currently only 1,200 miles of hydrogen pipeline exist in the United States, compared to more than 1 million miles of pipeline for natural gas (USDRIVE 2013).
For hydrogen storage, 700-bar tanks allow market entry with a 300-mile driving range for several types of vehicles, but low-pressure materials–based options would enable all vehicle platforms to achieve that range and without the infrastructure challenges associated with delivery of high pressure to the vehicle. Regardless of the type of technology used, codes and standards must be developed to allow the smooth market entry and social acceptance of hydrogen.
Fuel cell technologies also require more R&D to reduce or even eliminate PGM content without compromising performance or durability. From a vehicle systems perspective, one approach may be to use a small fuel cell (e.g., 8 kW rather than the nominal 80 kW) for an FCEV in conjunction with a larger battery as a range extender to allow the fuel cell to operate at constant load. This approach would enable greater durability and provide the extra driving range that BEVs cannot provide with smaller amounts of hydrogen at pressures lower than 700 bar. These and other innovative options should be considered even as early models are provided to customers.
Summary and Outlook for the Future
With carmakers already announcing plans for commercial FCEVs—and Hyundai already leasing the first production-volume FCEV in California as of June 2014—the industry is poised to make headway in the next few years, both nationally and internationally.
In 2013 DOE and industry stakeholders launched H2USA, a public-private partnership of more than 30 federal and state government agencies, global carmakers, hydrogen providers, trade associations, and other stakeholders committed to the deployment of hydrogen infrastructure. In May 2014 the California Energy Commission announced nearly $47 million in new funding for an additional 28 stations in the state, with a total of close to 50 to be completed before the end of 2015. Hawaii and Massachusetts are also developing scenarios for hydrogen infrastructure, and eight states (California, Connecticut, Maryland, Massachusetts, New York, Oregon, Rhode Island, and Vermont) recently signed a memorandum of understanding for 3.3 million zero-emission vehicles on the road by 2025.
On the international front even more aggressive plans are being made. Japan and Germany have announced plans for 100 stations each by 2015 and public-private partnerships to assess options for a much greater number in the coming years. The International Partnership for Hydrogen and Fuel Cells in the Economy, which includes the United States and 16 other countries as well as the European Commission, was established in 2003 to coordinate activities and accelerate progress toward widespread commercialization of hydrogen and fuel cell technologies.
There has been significant progress over the past decade, but sustained efforts in both RD&D and deployments are needed to continue the progress and enable the environmental, economic, and energy security benefits that could be realized with hydrogen and fuel cell technologies.
The author thanks the many researchers, stakeholders, and managers that are part of the DOE Hydrogen and Fuel Cells Program.
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