Lighter than air
Efficient fuel cells could turn hydrogen fuel into a realistic option, discovers E&T, but only if we can figure out how to make it cleanly and distribute it to users.
With over 600 million cars on the road today, and that number predicted to double to 1.2 billion by the end of the next decade, it is clear why attention is increasingly being focused on reducing exhaust emissions.
Just consider that in the United States alone, 420 million gallons of petrol are burnt each day - that's almost two million tonnes of CO2 released into the atmosphere every 24 hours. There are many options available to reduce the consequent damage to the atmosphere, but as yet there is no consensus on which solution or solutions to go for. Electric vehicles, biofuels and improved performance from the venerable internal combustion engine are all in the race, but one solution that is gaining favour is using hydrogen as a fuel - either by burning it directly in a traditional engine, or by using a hydrogen fuel cell to turn it into electricity.
Demonstration cars in places like California and Norway have indicated that hydrogen is a viable option. And in the public transport arena, hydrogen-powered buses are plying their trade from London to Melbourne, and from California to China.
"My enthusiasm for hydrogen goes up and down," says John Wright, sustainable energy partnerships advisor to the Australian Commonwealth Scientific and Research Organisation. "We really need to develop a fuel cell that is robust, cost-effective and reliable, as well as having hydrogen available when and where it is needed - which we don't have at the moment. I would have to say that it's a long way off, but another sustained oil spike could push that forward."
Barriers to hydrogen
Wright, a long-time supporter of hydrogen as a fuel, admits to a growing sense of frustration. "People have been saying for 50 years that we will see hydrogen cars on the road within the next decade, and we are still not there yet," he says. "We must accept that we still have a lot of barriers to overcome."
One company at the leading edge of automotive technology is UK-based Ricardo. "Our view is that the fuel cell is a promising but challenging technology that certainly has a future and probably in road transport," Nick Owen, its senior manager technology, explains. "Although there has been a lot of progress in areas such as low-temperature operation, durability and cost, the industry is nowhere near ready to hit the go button when it comes to profitable commercialisation of fuel cells for cars."
In 2008 it became fashionable to talk about electric vehicles again, but such vehicles face severe challenges. Many people still believe there must be one golden bullet out there that solves all the environmental problems. "The public has believed in the past that hydrogen and fuel cells was that golden bullet; then it was biofuels and now it seems to be the electric vehicle," Owens says. "I think the answer is that it's all of them."
In the US, the Sustainable Transportation Energy Pathways (STEPS) programme was established at UC Davis in 2007 to inform the public debate and to assist the private sector and government agencies by providing tools and knowledge concerning sustainable transportation alternatives. "There has been a lot of progress in hydrogen vehicles just over the last several years. Most of the major motor companies have developed prototype hydrogen vehicles, and several of them are actually placing large numbers of hydrogen cars with consumers to test them out," explains STEPS' Dr Joan Ogden. "The major players here are Daimler, GM, Honda and Toyota, and those automakers have announced plans to place several hundred fuel cell cars with consumers in southern California over the next couple of years.
"So hydrogen cars are very good performers; I have driven several of them. They get over 250 miles range at present, they refuel very quickly, they are fun to drive around, and of course they are zero emission."
After some early efforts to reform the hydrogen on board from petrol or ethanol the industry has broadly settled on using fuel cells as the way forward. There are various options for fuel cells including alkaline, phosphoric acid, solid oxide, molton carbonate and solid polymer.
Most manufacturers are concentrating on solid polymer, mainly because of its higher power density, rapid start-up ability and low temperature operation. For larger vehicles there is also the potential for the use of solid oxides.
"Fuel cells are electrochemical converters and you feed in fuel - in this case hydrogen and oxygen from air - and it makes electricity directly," Ogden says. "There are different chemistries that you can use, but the one that has really won out is the proton exchange membrane fuel cells. The reason for that is that they are very lightweight relatively speaking, so you can make a compact fuel cell to power cars small enough to fit under the hood."
The fuel cell is only half the issue, however. Once you have a cost-effective, reliable car, you also need the infrastructure to supply fuel for it. There is a wide range of options for producing hydrogen, mostly off vehicle, although as mentioned earlier, on-board reforming of petrol and methanol is possible.
"There are various ways to make hydrogen. For a while people were looking at making the hydrogen on board the vehicle, and that would have solved one problem because it is harder to store hydrogen than it is to store a liquid fuel," Ogden says. "That approach was tried quite a lot in the 1990s and early 2000s, but it is really has been abandoned now. All the prototype cars that I am aware of are direct hydrogen, so they store compressed hydrogen gas - or in one case liquid hydrogen - on the car and just feed that direct to the fuel cell.
"Hydrogen is like electricity, you can make it from lots of different things; you can generate electricity from natural gas, coal, solar, wind or nuclear power," Ogden continues. "With pretty much all of these sources you could also make hydrogen and that is a benefit, because one of the issues with our current transportation fuels is they are 98 per cent reliant on crude oil. With hydrogen or electricity, you can use quite an array of resources so you would have in that sense more energy security."
When a lifecycle analysis is undertaken, a hydrogen-powered fuel cell vehicle is still less 'polluting' than a conventional internal combustion engine. However, the extent to which emissions are reduced is dependent on the method used to produce the hydrogen.
How much greenhouse gas reduction is achieved by switching to hydrogen fuel depends how the hydrogen is produced. Centralised natural gas reforming produced the greatest reductions. However decentralised production has only slightly greater emissions, poses fewer technical challenges and is expected to be the most cost effective hydrogen system, since it can be expanded as fuel cell vehicles increase in numbers.
To secure the maximum environmental benefits, hydrogen production should be carbon-free. There are several ways of producing carbon-free hydrogen: fossil fuels and carbon sequestration; electrolysis of water using carbon-neutral electricity, the electricity being produced either from fossil fuels with carbon sequestration, from renewable energy or from nuclear power; biomass, either by the chemical or thermal reformation of biomass feedstocks or the biological reformation of biomass using micro-organisms; solar photoelectrolysis, the direct splitting of water using light with special catalysts or extreme heat.
"The most common way today is making it from natural gas," Ogden explains. "If you make hydrogen from natural gas that is still a fossil fuel, but if you use that hydrogen in a fuel cell vehicle you will cut greenhouse gas emissions compared to a gasoline vehicle by about 50 per cent.
"So it is an interim step, but in the long term you can make hydrogen from renewable energy like biomass or wind power or solar, and then you would have a full fuel cycle that was true zero emission. With hydrogen you don't get anything out of the tail pipe, but you have to make the hydrogen somehow so when you count up the emissions you have to count up the whole fuel cycle and making the hydrogen as well as using it."
Hydrogen is already used in industrial processes such as fertiliser production, and the refining of metals, plastics and petrochemicals. "If you could somehow divert and purify all that hydrogen, it would be sufficient to fuel a third of all European vehicles - though then you would have no fertiliser, plastic or metals," Owens says. "The point here is that hydrogen is not a rare gas that can only be bought in a sample bottle. Industrial quantities are already quite large and most is steam-reformed from natural gas.
"Clearly producing hydrogen from gas is not a sustainable option, but the plans are to move on to use renewable sources of energy such as solar or wind to produce the hydrogen. In that case, there is a strong argument for using the sustainable fuel for reducing the carbon footprint of the electricity grid by feeding the energy into it, rather than using it to power vehicles. Almost without exception, if more renewables become available, or more nuclear or carbon-capture coal plant capacity comes on stream, these would be the best course of action."
Economies of scale
"If you look at going up the chain in terms of clean vehicles, going to a gasoline hybrid you would probably have a $1,000 or $2,000 increment, and then going to a fuel cell it might be a $3,600-$6,000 increment, according to a study by the National Academy of Engineering in the US," Ogden says.
"In terms of the cost of hydrogen, initially it will be expensive. The study thought that hydrogen will probably end up costing between the equivalent of $3-4 per kg hydrogen, which is equivalent to $3-4 a gallon of gasoline on an energy basis. Fuel cells are more efficient than gasoline cars by a factor of two, so if hydrogen costs the equivalent of $4 per gallon on an energy basis then on a cent per mile basis it is $2 per gallon. In other words you can go twice as far on each unit of energy in a hydrogen car."
According to Owen, at least 10 per cent of cars across the EU would need to be powered by fuel cells before economies of scale bring the cost down enough. That's 1.5 million fuel cell vehicles before they become profitable for the manufacturer and affordable for the user. "It's an enormous leap and a much greater volume step than the type of demonstration projects seen today - typically large city transport authorities buying or leasing limited fleets of fuel cell buses," he says.
"The frustrating thing is that if we could jump to the point where there are a larger number of these vehicles, while at the same time solving many of the technical hurdles, you could produce something that's commercially successful. But for now whoever does it first is going to have to sell at a loss for quite some time and it is likely to require a lot of government support in the form of subsidies."
"Hydrogen will be part of the mix in the future, probably with electric vehicles for passenger cars - you need to remember that hydrogen cars are in essence an electric vehicle that carries the generator around with them," Wright adds. "To cut greenhouse gases, Electric vehicles depend on producing low emission electricity at the right price - electric has one advantage in that it has an infrastructure already in place, which hydrogen lacks.
"If a manufacturer were to produce a fuel cell that was half the price with twice the life it might change things, but with the current crop of fuel cells it will be difficult. Even if you managed that you would need to get the distribution right - either generate it at source or develop a transportation system."
So it seems unlikely we will ever see a world that is only populated by hydrogen fuel cell vehicles. It's a fundamental law of nature that when resources get scarce you have to diversify. That law applies to road transport every bit as much as it does to plant and animal species on a desert island.