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Alternative Fuels for Motor Vehicles
Hydrogen
After examining the short term prospects there are two other technologies that may, in the longer term, prove more successful. The first revolves around the most abundant element in the universe - Hydrogen.
The simplest and lightest fuel is hydrogen gas (H2). Hydrogen is in a gaseous state at atmospheric pressure and ambient temperatures. Hydrogen may contain low levels of carbon monoxide and carbon dioxide, depending on the source.
Hydrogen, chemical symbol H, is the simplest element on earth. An atom of hydrogen has only one proton and one electron. Hydrogen gas is a diatomic molecule - each molecule has two atoms of hydrogen (which is why pure hydrogen is commonly expressed as "H2"). Although abundant on earth as an element, hydrogen combines readily with other elements and is almost always found as part of some other substance, such as water, hydrocarbons, or alcohols.
So production of Hydrogen gas means that the element is split from other hydrogen bonded elements by various process technologies, including thermal (splitting Hydrocarbons such as Methane by heating) or electrolytic (water splitting using a variety of energy resources).
Because of it's abundance and it's almost zero environmental charge there is enormous scientific and engineering effort being put into the Hydrogen Fuel technology.
How does it work?
Two kinds of engines can use hydrogen as a fuel
- Those that have an internal combustion engine converted to use hydrogen and
- Those that are made up of a stack of fuel cells.
Internal combustion engines have powered cars since they first began to be developed more than 100 years ago. These engines can be converted to run on a variety of fuels, including hydrogen. When hydrogen burns the only by-product is water, not the polluting emissions given off by burning petrol and other fossil fuels. BMW successfully demonstrated this technology in 2006 with a 7 series prototype. The fact that no major changes need to be made to the basic internal combustion engine design is a major attraction.
The BMW750hL has a hybrid 12-cylinder combustion engine capable of running on either gasoline or hydrogen. The 5.4-liter engine has two independent, electronically controlled fuel induction systems. The hydrogen engine offers excellent torque and acceleration, while the specially insulated 140-litre tank for the liquid hydrogen provides a range of 300+ kilometers.
These are, as yet, just prototypes and as you might guess extremely expensive but they have proved the concept works.
Fuel cell engines However, most car makers think that fuel cells powering an electric motor offer a better alternative. Electric cars are hardly a new idea, but the need to recharge heavy stacks of batteries after relatively short journeys has stopped them becoming popular. Now fuel cells have made electric cars practical. Unlike batteries, which store electricity, fuel cells make electricity as they go. Recent developments in technology have greatly increased the amount of power that a stack of cells, small enough to fit under a cars bonnet, can provide. This has opened up the prospect of non-polluting electric cars with the levels of performance we expect from conventional vehicles.
Fuel cell technology sounds simple. The hydrogen fuel reacts with oxygen from the air to produce water and electricity, the reverse of the familiar electrolysis process that releases oxygen and hydrogen from water. In reality it is much more complicated.
The big advantage of a fuel cell engine over an internal combustion engine running on hydrogen is its greater efficiency. The same amount of hydrogen will take a fuel cell car at least twice as far as one with a converted internal combustion engine. This is because the power is used directly to produce motion of the vehicle via an electric motor instead of powering an engine, with its attendant power train resistance, and then powering the vehicle.
Honda have developed and launched a prototype Fuel Cell automobile called the Honda FCX. This employs a system that combines a fuel cell stack and large capacitor with onboard high-pressure hydrogen tanks. The Fuel Cells are located under the floor and the tanks under the rear seat. The purpose of the capacitor is twofold, firstly to provide power over and above the fuel stack for responsive acceleration and secondly to recover the energy released during deceleration and stores it along with power from the fuel cell stack. This results in greater fuel efficiency and a more natural feel on deceleration.
When stopped the auto idle stop system shuts off output from the fuel cell stack to reduce fuel consumption. Electricity required to operate the air conditioner and other components is supplied by the ultra-capacitor. On startup, the system immediately supplies the necessary power for smooth acceleration.
Back to the topThe drawbacks
Hydrogen has many advantages as a fuel for vehicles, but a big disadvantage is that it is difficult to store. This is because at normal temperatures hydrogen is a gas. The hydrogen must be packed tightly into a cars tank, otherwise a filling stop will be needed every few kilometres.The obvious solution is to strongly compress the hydrogen, or liquefy it. However, large amounts of energy are needed for this, an estimated 20 to 40 per cent of the energy content of the fuel. Also, tanks designed to hold hydrogen at extremely high pressures, or at temperatures approaching absolute zero, are heavy and expensive.
The special insulated tanks in the BMWs held 140 litres of hydrogen, enough to drive at least 300 kilometres. (That is a reasonable range, although a 95 litre tank of petrol would take the same cars twice as far.)
High cost and the large amount of energy needed to liquefy the fuel are likely to be the main problems with refuelling with liquid hydrogen. Filling up with compressed hydrogen gas will probably prove more practical, even though it may reduce the distance between fills. Cars could store the hydrogen in high pressure tanks similar to those used for compressed natural gas. Or, if current research proves successful, some high-tech alternatives could be employed. Scientists have found that various metals can absorb up to a thousand times their own volume of hydrogen gas. Specially treated carbon may also hold large amounts. These discoveries could shape the fuel tanks of the future
To summarise
Recent technological advances, particularly in fuel cell design, have made hydrogen-powered cars a practical proposition, and car makers expect to start mass-producing them within the next decade or so. Their power and acceleration should match those of todays petrol-powered vehicles, but they may have to be refuelled more often.The best ways to produce, distribute and store the hydrogen still have to be sorted out. In the short term fossil fuels may remain in demand as a hydrogen source. However, the idea that in the not too distant future most of us will be driving non-polluting cars fuelled by hydrogen from a clean, renewable source is no longer a flight of fantasy.
Back to the topPostscript
The "Hydrogen Economy" may not be all it's cracked up to be. A hydrogen economy is a hypothetical future economy in which energy, for mobile applications (vehicles, aircraft) and electrical grid load balancing (daily peak demand reserve), is stored as hydrogen.Since hydrogen is an energy transfer medium, not an energy source, it requires other fuels or energy sources to produce, and each of these has energy conversion efficiencies, which may pose limitations on their use in hydrogen manufacture, vs. more direct use. In addition, a hydrogen economy would impose high initial infrastructure costs associated with distribution and use, even if clean primary energy sources to make hydrogen were identified and utilized.
In addition it must be of concern that the main movers on this front appear to be the oil companies. The Bush administration in the USA has been working to ensure that the system used to produce hydrogen will be as fossil fuel-dependent -- and potentially as dirty -- as the one that fuels today's internal combustion engines. According to the administration's National Hydrogen Energy Roadmap, drafted last year in concert with the energy industry, up to 90 percent of all hydrogen will be refined from oil, natural gas, and other fossil fuels -- in a process using energy generated by burning oil, coal, and natural gas.
Such a system, experts say, would effectively eliminate most of the benefits offered by hydrogen. Although the fuel-cell cars themselves may emit nothing but water vapor, the process of producing the fuel cells from hydrocarbons will continue America's dependence on fossil fuels and leave behind carbon dioxide, the primary cause of global warming.
A study carried out by fuel cell expert Ulf Bossel and explained in PhysOrg.com that a hydrogen economy is a wasteful economy. The large amount of energy required to isolate hydrogen from natural compounds (water, natural gas, biomass), package the light gas by compression or liquefaction, transfer the energy carrier to the user, plus the energy lost when it is converted to useful electricity with fuel cells, leaves around 25% for practical use, an unacceptable value to run an economy in a sustainable future. Only niche applications like submarines and spacecraft might use hydrogen.