Last month's launch of a hydrogenpowered sports car could be a key step towards delivery of a nonfossil fuel economy. Dave Parker looks behind the glitz.
Forty-two years ago American sports car nut Carrol Shelby carried out one of the most outrageous transplants in motoring history. His patient was the inoffensive AC Ace, a classic British hand built sports car, powered by a modest 2 litre straight six from a basic design dating back to 1919. Shelby tossed this in the trash can and shoehorned in a massive Ford V8 in its place.
Thus was born the AC Cobra, a thunderous, gas-guzzling, barely controllable monster which has stayed in production ever since. Still largely hand built to the purchaser's specification, the current Cobra will set you back something in excess of £100,000.
Owners sensitive to the inevitable jibes from the environmentally conscious can now fork out another £30,000 or so, and buy a Cobra powered by a hydrogen bifuel engine.
And provided you live in California, Iceland or near Berlin, you will actually be able to find a filling station offering cryogenic hydrogen at -253 OC to fill up your heavily insulated fuel tank.
A number of major car manufacturers, with BMW in the lead, have already successfully converted standard liquid hydrocarbon-fuelled internal combustion engines to run on hydrogen. Assuming the in-car fuel tanks have passed all the necessary safety tests, this could be hailed as a more immediate and effective response to global warming than the less developed and much more expensive fuel cell option.
As BMW puts it: 'the internal combustion engine has been under development for 100 years. Motorists have got used to vehicles that start at the turn of a key, do at least 400km between refuelling stops, need servicing only once a year and last more than 200,000km without major component replacement.
'Fuel cell technology has hardly reached 1930s standards when looked at in these terms.'
Fuel cell powered aircraft are even further off. Yet whatever route is finally adopted, neither will have significant impact until a hydrogen production and distribution system is developed as well.
One day gaseous hydrogen will be produced in vast quantities by ecologically ideal methods - the electrolysis of water using wind, tide or solar power, for example. But hydrogen gas at ambient pressure is far from an ideal fuel.
Its energy density is such that for the same energy storage a hydrogen tank would have to be 3,000 times bigger than a petrol or diesel fuel tank. To be a practical alternative to liquid hydrocarbons, hydrogen must be converted in some way into a much higher density fuel.
Currently this is achieved by either compressing the gas to 250bar or cooling it until it liquefies. Both these options are energy hungry. Compressing or cooling the hydrogen, storing it, transporting it and so on consumes up to 50% of the potential energy in the fuel.
Even then, litre for litre, petrol and diesel contain up to 16 times as much energy as compressed hydrogen and almost four times as much as the (more expensive) liquid hydrogen, putting pressure on internal storage space within the vehicle.
A radical alternative is currently being pioneered by DaimlerChrysler and New Jersey based Millennium Cell Inc. Instead of storing pure hydrogen mechanically, Millennium Cell has opted for ambient temperature and pressure chemical storage, in the form of a water-based solution of sodium borohydride.
Sodium borohydride is produced from the mineral borax, which is found in large quantities all over the world.
It is produced mainly for the paper and pharmaceutical sectors, but in solution it actually stores energy at much the same density as liquid hydrogen - and is no more difficult to handle than liquid hydrocarbons.
In practice the solution is passed through a catalytic chamber where it is split into hydrogen and sodium metaborate solution (see diagram). Sodium metaborate is an industrial chemical but, crucially, it is not very hazardous or polluting.
Nevertheless, it has to be collected and stored on board.
The 'moist' hydrogen produced can be used in either fuel cells or internal combustion engines. Millennium Cell's vision is that vehicles would then discharge the sodium metaborate solution at the same time as they refill with sodium borohydride.
Then the same (hydrogenfuelled) tankers that brought in the sodium borohydride would return the residues back to the production plant. Once there, hydrogen derived from solar power would be reacted with the sodium metaborate to convert it back to sodium borohydride.
A similar closed loop can be achieved with somewhat less logistical complexity if hydrogen from sustainable sources is reacted with carbon from atmospheric carbon dioxide to form a liquid hydrocarbon such as methanol. This could be handled, stored and distributed using the existing liquid fuel infrastructure with minimum modification.
Internal combustion engines, including gas turbines, can run happily on methanol and its siblings, which have an energy density close to current liquid fuels. Fuel cells prefer a purer form of hydrogen, so the methanol would have to be rectified before use, preferably on demand on board the vehicle.
NOx emissions would be low.
Best of all, the inevitable carbon dioxide produced would not add to the greenhouse gas burden, as it would be replacing the CO 2taken out during the hydrocarbon production phase.
Such a system might be more compatible with wind energy than the simple generation of electricity for the national grid.
The unavoidably unpredictable nature of wind energy would be much less relevant - although there would be even more local objections to siting an onshore methanol production plant in areas of outstanding natural beauty.
Hydrogen-fuelled cars like the Cobra and BMW's trial fleet of 7 series limousines grab the headlines and help convince a sceptical public that hydrogen is an acceptable alternative fuel.
An alternative that would not involve any noticeable change in range, performance or storage capacity. What goes in the tank is still the unresolved question.
Wherever it comes from, 'green' hydrogen will only be a realistic alternative to fossil fuels when it can be transported and stored at ambient temperatures and pressures and is available at every filling station.