In the quest to reduce automotive energy consumption and CO2 emissions, some automakers are focusing on vehicles that burn hydrogen instead of gasoline in internal combustion engines.
A closer look at hydrogen
|BMW's 750hL hydrogen ICE vehicle runs on hydrogen or gasoline.|
Frost and Sullivan
San Jose, Calif.
Last July, California Governor Gray Davis signed a bill requiring the California Air Resources Board (Carb) to develop carbon-pollution standards for vehicles, beginning with model year 2009. The bill makes California the first state to regulate carbon dioxide (CO2) emissions from passenger vehicles. Even though Carb will be setting the standards, automakers will decide what technologies will best accomplish the goal.
But auto manufacturers have been debating how to reduce energy consumption and CO2 emissions long before the California bill was passed. Some manufacturers have chosen to concentrate on fuel-cell-powered electric vehicles, but others are taking a different tack, developing vehicles that burn hydrogen instead of gasoline in internal combustion engines (ICEs).
|Cooling hydrogen to -423°F on BMW's 750hL shrinks hydrogen to a thousandths of its uncompressed volume. The hydrogen remains at this temperature thanks to high-tech insulation in the fuel tank.|
Hydrogen has a number of advantages. It is a renewable fuel that can be derived from water through electrolysis and can be extracted from petroleum or natural gas. Because there is no carbon in hydrogen fuel, engines burning it emit almost no carbon monoxide or CO2. Therefore, hydrogen may help automakers meet California's CO2 regulation. And the energy efficiency of a hydrogen ICE is said to be 20 to 25% better than that of a gasoline ICE because it can run at a lean air-to-fuel ratio and at a higher compression ratio. A hydrogen ICE can also be controlled without a throttle.
Unfortunately, hydrogen has a number of disadvantages as well. Whether as a compressed gas or supercooled liquid, hydrogen is more difficult to handle than gasoline. If it is combusted near its stoichiometric air-to-fuel ratio, the high combustion temperatures result in the formation of nitrogen oxides (NOx), which must be cleaned up by a catalytic converter. However, the catalytic converter need not deal with hydrocarbons or carbon monoxide. Running the engine on a lean air-to-fuel ratio reduces combustion temperature and mitigates the formation of NOx, but it simultaneously reduces power and performance. And CO2 is released if hydrogen is extracted from petroleum or natural gas, partially offsetting the clean combustion of hydrogen in the engine.
Fuel storage presents another problem. By volume, a vehicle uses 3.5X more liquid hydrogen than gasoline, so fuel storage takes up considerable space on a vehicle. Of course, designing a vehicle specifically for hydrogen storage would solve this problem. And even though safety is a high priority on hydrogen ICE vehicles, the high-pressure storage tanks may turn consumers off. But one company, Millenium Cell Inc., Eatontown, N.J. (www.milleniumcell.com), is already working on solving that problem. The company is researching a technology that safely generates pure hydrogen from environmentally friendly raw materials. In the process, unpressurized hydrogen bonds with sodium borohydride, a derivative of borax. The result is pure hydrogen that doesn't need to be compressed or liquefied. The only by-products are water and natural minerals called borates, which are similar to those found in laundry detergents.
|The 750hL fills up at a hydrogen refueling station.|
But another fuel problem still exists: If hydrogen ICE vehicles are to have a future, they'd need a hydrogen-refueling infrastructure.
Cost is another factor. Hydrogen may cost more on a per-mile basis than gasoline today, even when produced in large volumes for use as fuel. Liquefaction, turning gaseous hydrogen into a supercooled liquid, is particularly expensive. In addition, liquid-hydrogen tanks must be well insulated, and gaseous-hydrogen tanks have to withstand high pressure, which add to vehicle cost. Even with the right components, up to 2% of the fuel in a liquid hydrogen tank can be lost to evaporation per day.
Who's doing what?
BMW and Ford Motor Co. are two leading developers of hydrogen-fueled ICE vehicles. BMW regards ICE-powered vehicles as more suited to its performance orientation than FCEVs and doubts that fuel cells will be robust enough for automotive applications any time soon. Ford regards FCEVs as the ultimate goal, with hydrogen-fueled ICE vehicles as possibly a transitional technology. Ford is developing hydrogen ICE and FCEV technologies simultaneously.
BMW has worked on hydrogen ICE technology for more than 20 years and is testing about 20 prototype vehicles. The most recent model, based on the 7-Series sedan, is called the 750hL. It can run about 220 miles on hydrogen and another 370 miles on gasoline. This dual-fuel capability is a must until hydrogen-refueling stations become common.
|The driving range of Ford's hydrogen-fueled P2000 H2ICE depends on the pressure at which hydrogen is stored.|
According to BMW, the process works like this: Electricity, possibly generated from solar power, splits water into hydrogen and oxygen. Oxygen is released into the atmosphere, while hydrogen is liquefied and stored at low temperatures. During combustion, hydrogen combines with oxygen to form water. The resulting energy powers the vehicle, while the water returns to the environment. Harmful emissions are virtually eliminated.
Cooling hydrogen to -423°F shrinks it to a thousandths of its uncompressed volume. Seventy layers of aluminum and fiberglass sheets between the exterior and interior vehicle walls keep the liquid hydrogen at extremely low temperatures. Beyond that, the car gets electricity from a fuel-cell battery that converts hydrogen into electric current. Because it has several cells in a series, the battery supplies enough power to keep the climate-control system running even when the car is stationary.
Ford Motor Co. has been testing its prototype hydrogen ICE vehicle for about two years. The P2000 with a hydrogen-fueled ICE (H2ICE) is based on a stretched Ford Contour. Range depends on the pressure at which hydrogen is stored -- 160 miles with 5,000-psi tanks to 270 miles with 10,000-psi tanks. To attain low-emissions-vehicle status in terms of NOx without a catalytic converter, the P2000 cruises at an air-to-fuel ratio of 86:1, which is lean compared to hydrogen's stoichiometric ratio of 34.2:1. In terms of carbon-based emissions, the P2000 meets the super-ultra-low-emissions-vehicle (SULEV) standard, considering the only carbon-based emissions come from lubricating oil.
Ford claims an 18% improvement in fuel economy for the P2000 compared with similar vehicles using gasoline with an equal energy content. In the next generation of test vehicles, Ford plans to use turbocharging to offset the power penalty of lean combustion and to add after-treatment of NOx to attain SULEV status. The relative improvement in fuel economy is expected to rise to 25%.
|Ford's P2000 H2ICE meets the SULEV standard because the only carbon-based emissions result from lubricating oil.|
When will it happen?
If there was enough demand, hydrogen-fueled ICE vehicles could be put into large-scale production within a few years because they represent a relatively small leap in technology. However, demand assumes cost-competitiveness of hydrogen-fueled vehicles compared to today's vehicles, cost-competitiveness of hydrogen compared to gasoline, and the existence of a refueling infrastructure. These conditions could come about as a result of a drastic change in the world's petroleum supply or through government action, perhaps anticipating a change in the petroleum supply.
Estimates of the current cost of 2.2 lb of hydrogen (regarded as the equivalent of a gallon of gasoline) range from $1.25 to $4.66. (This price should be compared to the cost of gasoline excluding taxes.) Even a 25% improvement in fuel economy may not offset the penalty cost of this price. Government incentives may help put a limited number of hydrogen-fueled vehicles on the road. However, a mass market would need either a substantial reduction in the cost of hydrogen or a substantial increase in the cost of gasoline. Hydrogen-fueled ICE vehicles are technically feasible, but the economic aspect of supplying hydrogen remains a major hurdle.