Special Report: Fuel Cell Vehicles

• The fuel cell is the future

• DaimlerChrysler is examining various fuel options for fuel cell vehicles

• Sodium borohydride is a safe, clean solution offering independence from oil

• The distribution and recycling infrastructure is the major challenge

• Presentation by Bernard I. Robertson, Senior Vice President, Engineering Technologies & Regulatory Affairs, DaimlerChrysler Corporation, at the DaimlerChrysler Innovation Symposium on June 11/12, 2002 in Sindelfingen near Stuttgart

Fuel cells are considered the most promising alternative drive systems. They operate very efficiently while producing very few emissions – or none at all – depending on the fuel used. One of the key challenges is how to get hydrogen to the fuel cell. Experts are searching for the optimum solution – and have found 6 major contenders, each with their pros and cons. Sodium borohydride is an exciting option for delivery. In Auburn Hills, researchers have built a prototype minivan based on the Chrysler Town & Country with a range of 300 miles (500 km). Called Natrium and completed in a period of only 18 months, this project is part of an enterprise-wide program for evaluating new fuel and drive technologies. Dubbed "soap" by the press, sodium borohydride is an earth-friendly, non-toxic, non-flammable substance that offers the benefits of zero emissions, and freedom from reliance on petroleum and oil imports.

Drive system of the future

Experts agree: Fuel cells have the best chance of becoming the drive systems of the future. They combine the operating range of conventional internal combustion engines with high efficiency, low fuel consumption and minimal emissions. They are also very quiet and offer a smooth powertrain. And since they can be powered by fuels that can be obtained from renewable energy sources – such as hydrogen and alcohol – they could end the current dependency on petroleum.

Finding the right fuel

Hydrogen-powered fuel cells themselves are completely emission-free in operation. No pollutants are generated by the reaction of hydrogen with oxygen, which generates electrical energy, heat, and pure water vapor. The problem is that the label "emission-free" only applies if the fuel cell is directly supplied with hydrogen – which may not be the most viable solution. There are various options for the delivery of hydrogen – and, essentially, the more environmentally friendly they are, the more costly they are to implement in terms of infrastructure. The main pros and cons of the 6 major options are covered in the following, along with brief notes on related DaimlerChrysler projects:

• Liquefied hydrogen

• Compressed hydrogen

• Methanol

• Gasoline reformation

• Metal hydride

• Sodium borohydride

A brief review of the options:

Liquefied hydrogen

The energy required for cooling in cryogenic tanks (to minus 253 degrees Celsius) has a negative impact on the economy and efficiency of the liquefied hydrogen fuel cycle, as does "boiling off" required to prevent excessive temperatures. Cost of cryogenic tanks is also a major disadvantage. Liquefied hydrogen also requires a new distribution infrastructure and sophisticated on-board cooling equipment. The NECAR 4 (New Electric Car) fuel cell vehicle uses liquefied hydrogen.

Compressed hydrogen

Compressed hydrogen requires expensive heavy tanks at pressures of up to 10,000 psi – and also requires a new infrastructure for distribution. A major disadvantage is the large amount of space required to package sufficient hydrogen to achieve adequate driving range. Tanks are non-conformable. Equal range requires approximately 8 times the volume of gasoline. Also, there is a strong – but unwarranted – public perception that this form of storage of compressed hydrogen is dangerous. There are no emissions – except heat and water vapor – from the vehicle. DaimlerChrysler’s experimental fuel cell vehicle NECAR 4a is powered by compressed hydrogen.

Methanol

Although methanol requires on-board reformation at a temperature of around 260 degrees Celsius to produce hydrogen, a methanol-fuel fuel cell is 30 percent more efficient than a normal combustion engine. There are very few emissions from the vehicle, but while methanol is a popular alternative in Europe, it is a non-starter in North America because of its toxicity, ground water contamination risk, invisible flame characteristics, and negative rating by environmentalists. Used as a denaturant in alcohol, methanol is extremely poisonous when taken internally by drinking or inhaling vapors. The key benefit: It can be transported, stored and handled much like gasoline or diesel fuel, allowing the existing infrastructure to be used. The NECAR 5 uses on-board methanol reformation, as did the Jeep® Commander 2 luxury sport utility vehicle unveiled in 2000. DaimlerChrysler is also working on technology called "direct methanol", in which methanol is fed directly into a modified fuel cell. This technology – as demonstrated by a go-kart at the last Innovation Symposium in November 2000 – is still under investigation.

Gasoline reformation

Onboard gasoline reformation requires very high temperatures (600 degrees Celsius), is less efficient and produces more emissions than methanol. The benefit of being able to use the existing fuel and distribution infrastructure is offset by the fact this approach does not offer independence from imported oil. The Jeep® Commander SUV concept vehicle – unveiled in 1999 – demonstrated gasoline-fueled fuel cell engine technology.

Metal hydrides

Metal hydrides are another possibility for hydrogen storage because they are safe and offer very high storage capacity and performance. Currently available rare-earth metal hydrides have a hydrogen content of 1.5 percent to 2.5 percent by weight, making them more volume efficient than compressed gas; however, the weight of a metal hydride system capable of a 300-mile range in a minivan weighs twice as much as compressed hydrogen. Metal hydrides require delivery of gaseous hydrogen for replenishment, as the hydride is a storage medium, not a fuel. This results in significant infrastructure requirements for this fuel, too. The system currently appears to be the most expensive alternative. Nanotubes have significant theoretical advantages but are not yet really into a credible hardware demonstration phase.

Sodium borohydride – easy to handle, earth friendly

Sodium borohydride (NaBH4) is the most benign fuel under consideration – this benefit being offset by the fact that it would require a new infrastructure for distribution and recycling. Sodium borohydride is a compound of sodium, boron, and hydrogen. It is used in a variety of chemical industries, including the paper and pulp industries (as a bleach), in wastewater processing and in pharmaceutical synthesis. NaBH4 is a dry powder that is safe and easy to transport and offers much better volumetric storage density than compressed hydrogen, and so requires less space on board the vehicle for a given range. Sodium borohydride is hydrogenated sodium borate (NaBO2), chemically equivalent to borax. This benign substance is used as a laundry detergent ingredient. The sodium borate (NaBO2) is not consumed in the process, but merely acts as a carrier for the hydrogen. So the in-vehicle process is "clean", emitting only heat and water. The idea of using sodium borohydride as a fuel is not new – it has long been known that boron hydrides store more energy than similar hydrocarbons. Back in the 1960s, work on the fuel was abandoned because at that time the fuel was intended for combustion, which represented an insurmountable engineering challenge. The development of catalyst technology for controlled release of hydrogen and of the hydrogen-powered fuel cell have allowed engineers to take a fresh look at this possibility. Sodium borohydride offers the key benefit of eliminating dependence on oil imports. Hydrogen for this fuel can be generated by sources such as including natural gas, wind power, solar power, nuclear, and hydroelectric power stations. It is also non-toxic, non-explosive and non-flammable.

The process

When dissolved in water the NaBH4 solution can be stored in a lightweight plastic tank at ambient temperature and pressure. This liquid is non-toxic, non-flammable and non-explosive – and has low environmental impact. Hydrogen is released on demand by a catalyst. Following removal of hydrogen, the recyclable slurry is returned to a collection bladder in the fuel tank. The spent fuel is pumped out for recycling when the vehicle is refueled and shipped back to the chemical factory in the tank delivery trucks for rehydrogenation.

Natrium – the testbed vehicle

18 months ago, DaimlerChrysler initiated a project to demonstrate the feasibility of this technology. The Natrium vehicle is based on the Chrysler Town & Country minivan. The name comes from the Latin and German word for sodium, as reflected in its chemical symbol Na. The Natrium also shares much of the fuel cell technology developed and tested by DaimlerChrysler and its partner Ballard Power Systems. The Natrium was unveiled at EVAA annual convention on December 12, 2001 in Sacramento, CA and at North American International Auto Show, Detroit, in January 2002. It was also presented to President Bush on February 25, 2002. A demonstration vehicle is available for drive and ride at the Innovation Symposium. One of the impressive things about the vehicle is its appearance: Unless you pop the hood or crawl underneath, the Natrium looks like a normal minivan. The tanks, catalyst, fuel cell and motor do not encroach in any way on the passenger or luggage space. The Natrium has a 53-gallon tank and achieves a range of 300 miles (500 km). It accelerates from 0 – 60 mph (0 – 100 km/h) in 16 seconds. The vehicle has a top speed of 80 mph (129 km/h) and achieves an equivalent fuel economy of 30 mpg (7.8 l/100 km). The Natrium also features regenerative braking and a 40 kW SAFT Lithium-Ion battery pack. Drive is provided by a 35 kW Siemens AC motor.

Hydrogen on Demand™

Hydrogen is extracted by a Hydrogen on Demand™ catalyst developed by Millennium Cell of Eatontown, N.J. This is a dynamic, on-demand process which requires no storage of the hydrogen gas – hydrogen liberation ceases as soon as the fuel pump stops. The basic chemical reaction is given by:

The fuel cell

The fuel cell module used in the Natrium is supplied by DaimlerChysler’s Canadian fuel cell partner Ballard Power Systems. It is the same fuel cell as used in the NECAR 5 methanol-powered fuel cell vehicle.

The technical challenges

The key technical challenges for the use of sodium borohydride are the stabilizing agent and recycling. Because the reaction is exothermic (releasing heat) and spontaneous when water is added, it has to be controlled. Currently, sodium hydroxide is used for stabilization, but this substance has the disadvantage of being highly alkaline. Researchers are testing possibilities for a more benign stabilizing agent – and for alternative approaches, such as purging surplus hydrogen – with the aim of keeping handling simple. The second major challenge is recycling. Various multi-stage rehydrogenation processes are being tested, and the economic viability and energy required are under investigation. The efficiency of the recycling process is probably the biggest single hurdle for sodium borohydride, as it will be the major factor in determining the fuel price. DaimlerChrysler’s partner Millennium Cell is working on heat management, reduction of catalyst costs, improving catalyst durability and reducing the size of the catalyst package. Because there is no doubt that it is the power source of the future, DaimlerChrysler and Ballard are developing fuel cell technology in leaps and bounds. Apart from continuing to make the packaging of the fuel cell module smaller and lighter, one of the major challenges is the warm-up time. Currently, warm-up takes around two minutes. Anything longer than 20 seconds is unacceptable to the general public – and the goal is to make the warm-up time as short as for diesel engines, so around 2 seconds. However, when the system is a hybrid electric powertrain as in Natrium, the driver can drive away immediately on battery power while the fuel cell warms up.

The economic challenge

The biggest drawback of sodium borohydride is the fact that it would require the establishment of a new infrastructure for distribution – somewhat like adding unleaded gasoline with separate tanks and shipping. The fuel cycle requires two tanks both in the vehicle and in refueling stations: one for fresh fuel and one for spent fuel. Also, spent fuel would have to be returned to a facility for recycling – and the recycling plant would either have to produce hydrogen, or be supplied with hydrogen or with electricity for electrolysis. Although some argue that the infrastructure requirement is an insurmountable obstacle, others point to the power of economic incentive: The number of gasoline stations in the US rose from 12,000 in 1921 to 143,000 just eight years later, an increase of over 1,100 percent. The hydrogen from the catalyst can also be used to power a modified regular gasoline internal combustion engine. Adopting this approach – as an interim supporting measure – could increase acceptance of the fuel and accelerate the establishment of the distribution and recycling infrastructure, while at the same time speeding independence from imported petroleum. Successful incorporation of this idea requires resolution of the high NOX levels that currently result from using H2 in an IC engine. The intrinsically high humidity of hydrogen derived from sodium borohydride is helpful in this regard.

Fueling the cycle

A recently released study of global borate reserves by U.S. Borax Inc. indicates that even if all new cars were to adopt sodium borohydride as the energy carrier, there would be plenty of borax reserves. The study reports 600 million metric tons of borates in viable deposits worldwide. If 50 million new cars built each year were supplied with borates, this would require some 20 million metric tons – about 3 percent of known reserves. And very little additional borate would be required, as the spent "fuel" is recycled.

Concerted research and development effort

Fuel cell development is a closely coordinated R&D effort within DaimlerChrysler. The Research and Technology group in Germany is responsible for the design of the stack and the reformer, and engineer teams on both sides of the Atlantic draw on this expertise for their research projects. There is healthy internal competition between the sodium borohydride technology being developed by the Auburn Hills team and the methanol and compressed H2 alternatives being developed in Stuttgart, but close cooperation and division of labor eliminate duplication of effort, while achieving the common goal of implementing fuel cell vehicles.

Outlook

DaimlerChrysler is fully committed to staying on the leading edge of this exciting new technology and to continuing to research the optimum fuel or fuels to power it, investing more than $1 billion in fuel cell technology research and development until 2004. To encourage development and commercialization of fuel cell vehicles DaimlerChrysler is a founding member of the California Fuel Cell Partnership, an industry-government program based outside Sacramento. DaimlerChrysler is committed to putting a number of fuel cell vehicles on the road for this program by the end of 2002. Of the first five vehicles, one will be a Natrium. DaimlerChrysler has already sold the first limited-production series of fuel cell urban buses (30 Mercedes-Benz Citaros) to 10 operators in Western Europe cities. Forming part of the European Fuel Cell Bus program, the buses will be delivered over coming years. DaimlerChrysler plans to launch fuel cell passenger cars in very limited volume in 2004. Although the fuel cell is the most serious contender for escaping imported oil dependency, there is no clear front-runner in the race for a fuel to power fuel cells. Because of the various pros and cons and differing requirements in regional markets, it may well be that more than one fuel type will be used. This may sound like a compromise, but it reflects the pragmatism that has led to the use of two key fuels – gasoline and diesel – today. Sodium borohydride offers many advantages – such as low emissions, zero carbon, harnessing of renewable energy sources, high level of safety, and independence from oil imports. The technical challenges have largely been overcome. The key challenge will be economic viability – and that depends almost entirely on the price of gasoline. Without a tax concession, sodium borohydride will not be a viable alternative while gasoline prices remains around US $1.25 per gallon. Experience has shown that very few US buyers are prepared to pay for fuel economy if there is no payback. Since September 11, 2001, energy security has once again become an issue of paramount concern. This situation can only serve to hasten development of alternative energy sources.