Hydrogen as an Alternative Fuel
The perfect alternative fuel?
Hydrogen is the most abundant element on Earth, a fundamental building block of nearly everything. Hydrogen fusion is the reaction that ‘drives’ the sun, making sunlight itself hydrogen-powered.
Seventy-five per cent of everything around you is made up of hydrogen, which exists in combination with other elements. Water is two atoms of hydrogen and one of oxygen. By mass it is 11.2 per cent hydrogen and 88.8 per cent oxygen.
Shifting from fossil fuels to hydrogen energy has far-flung implications – in theory. Emissions would be zero. Hydrogen is renewable, and it is – literally – everywhere. So hydrogen would remove national dependencies on oil-rich (and often unfriendly) countries, as well as slashing our greenhouse footprint.
There are a few problems, however…
HYDROGEN: THE BASICS
Hydrogen can exist as a gas (H2). It’s stable, but it will burn in air if the concentration is between four and 75 per cent and there’s a spark. If the concentration is right – 29 per cent – the temperature will reach a maximum of 2318 degrees C.
Petrol is an energy-dense fuel. Hydrogen is better, with one kilogram of hydrogen gas containing the same combustion energy as 2.8 kilograms of petrol. If one kilo of petrol takes you 10km, one kilo of hydrogen could hypothetically take you 28, everything else being equal.
Problems, start here: Petrol’s a liquid; hydrogen’s a gas. Per litre, petrol contains more energy. Four times more energy. Result? Severely reduced cruising range for the same tank size – even when stored as a compressed gas at a massive 5000psi.
The obvious answer is to liquefy the hydrogen, like the propane in your barbecue’s gas cylinder. (Liquids are denser than gasses; problem solved.) However, hydrogen requires much (potentially dirty) energy to liquefy, and is technically very difficult to maintain in that state. In fact, liquid hydrogen boils at minus 253 degrees C, just 20 degrees above absolute zero.
To store liquid hydrogen, special ‘cryonic’ storage tanks are needed, which maintain low temperatures for a long time. Commercially available 100-litre cryonic tanks offer super-insulation from up to 30 aluminium foil layers separated by plastic. Even so, evaporation is still around one per cent per day. It’s hard stuff to transport, store and decant – the infrastructure is not in place, and won’t be for decades.
WHERE DO YOU GET IT?
Hydrogen gets really dirty: Most hydrogen gas is made from natural gas (methane), a single carbon atom with four hydrogens attached. Basically the process strips off the hydrogens (two hydrogen atoms make one molecule of hydrogen gas, hence ‘H2’). The waste carbon is emitted as greenhouse-problematic CO2. Also, the energy used to drive the process is usually unclean coal-fired electricity.
To produce hydrogen gas, the hydrogen atoms may be stripped from any source hydrocarbon, and even from coal. The more complex the source hydrocarbon, the higher the proportion of emitted CO2.
Hydrogen gas can also be made by electrolysing water. Pump electricity into water and it splits into hydrogen and oxygen gasses – like a fuel cell on full ‘reverse’. This is not clean if the upstream electricity is dirty (from coal) but it would be if zero-emissions (‘green’) electricity sources, like hydro, tidal or wind were used.
Theoretically, hydrogen can also be made from biomass. Environmental engineers at Pennsylvania State University have developed a prototype process to treat waste water with microbes on its way into a fuel cell. They break down the organic matter, which emits hydrogen.
When Jim Lovell radioed the immortal words “Houston, we have a problem” on 13 April 1970 he announced the world’s biggest fuel cell-related transport problem. The Apollo 13 spacecraft derived electricity from Pratt and Whitney hydrogen fuel cells, which drank hydrogen and oxygen from cryonic storage tanks. The waste product – water – kept the astronauts hydrated. Unfortunately, one of the cryonic oxygen tanks exploded two days into the mission.
Meanwhile, back on Earth, hydrogen gas will explode in air if a spark as small as 20 micro-joules is emitted. A static electricity discharge between you and the car would do it. Don’t get too worked up over it – the same danger is present with petrol, but since hydrogen diffuses into air more quickly the danger period is shorter.
Hydrogen can be burnt in internal combustion engines and gas turbines. Plenty of car companies have developed hydrogen-burning prototypes. As early as 1988 in the former Soviet Union a modified Tupolev-154 airliner was flying on three jet engines fuelled by hydrogen.
Burning hydrogen produces far fewer pollutants than fossil fuels. Some trace pollutants are usually the result of residual engine oil getting burnt. The biggest problem is oxides of nitrogen (NOx), which increase dramatically as combustion temperature elevates. (Air is 80 per cent nitrogen, and a little of it gets unstable at high temperature.) Maintaining acceptable air-hydrogen ratios keeps combustion temperatures, and NOx levels, low.
Fuel cells are somewhat like chemical powered electric generators, except that they have no moving parts. Chemicals – hydrogen and oxygen – flow in, and electricity and water are produced.
The most promising fuel cell is the Polymer Exchange Membrane (PEM), sometimes called a Proton Exchange Membrane (same initials). It’s merely a positive and negative surface separated by a membrane that allows positive charge to flow only one way. Hydrogen gas goes in on the negative side, oxygen on the positive, and in the presence of an electrolyte and a catalyst, 0.7 volts of electricity comes out. Join enough units together in a nicely styled plastic casting, and you get enough DC electricity for useful work. (An electrolyte is a liquid that conducts electricity, and a catalyst is a high-priced metal like platinum that nudges the reaction into action.)
Fuel cells are efficient, converting around 80 per cent of the energy locked in the hydrogen to electricity. Coupled to an inverter and electric motor (for motive power), also 80 per cent efficient, the overall efficiency is very high – about 64 per cent.
The best way to run a fuel cell is on bottled hydrogen and bottled oxygen. If air is substituted for oxygen, it makes filling up at the servo easier (one cryonic gas only) and vehicle design simpler (one fuel tank, not two), however doing so reduces efficiency substantially.
Hydrogen fuel cells have zero undesirable emissions. Like those in Apollo 13, all they emit is water good enough to drink.
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