molecules are in an excited state on the initial formation and then transition to a ground state; the transition releasing thermal radiation. When burning in air, the temperature is roughly 2000 °C (the same as natural gas). Historically, carbon compounds have been the most practical carriers of energy, as hydrogen and carbon combined are more volumetrically dense, although hydrogen itself has three times the specific energy (energy per unit mass) as methane or gasoline. Although hydrogen is the lightest element and thus has a slightly higher propensity to leak from older natural gas pipes such as those made from iron, leakage from plastic (polyethylene PE100) pipes is expected to be very low at about 0.001%. The reason that steam–methane reforming has traditionally been favoured over electrolysis is that whereas methane reforming directly uses natural gas as a source of energy, electrolysis requires electrical energy for this. When the cost of producing electrical energy (via wind turbines and solar PV) falls below the cost of natural gas, electrolysis will become cheaper than SMR. Before natural gas became generally available, in some cases tonnage hydrogen was produced for industry by water electrolysis, although not for use as a fuel. In a plant located at Risdon, Tasmania, Australia, designed to produce 60,963 tonne per annum of ammonium sulfate fertilizer, ammonia was synthesised by reacting nitrogen and hydrogen. The nitrogen was obtained by distilling liquid air and the hydrogen was made by electrolysing water. The design hydrogen production rate was 2,790 tonne per annum and the design ammonia production rate was 15,714 tonne per annum. Production commenced in late 1956 and the plant ran until 1986. Mercury arc rectifiers were used. Some ammonia production continued until 1993. The electricity for electrolysis was generated by hydro-electric power stations. Uses Hydrogen fuel can provide motive power for liquid-propellant rockets, cars, trucks, trains, boats and airplanes, portable fuel cell applications or stationary fuel cell applications, which can power an electric motor. Hydrogen is considered as the primary sustainable source of renewable energy and is "highly required for advanced energy conversion systems." The problems of using hydrogen fuel in cars arise from hydrogen being difficult to store in either a high pressure tank or a cryogenic tank. Alternative storage media such as within complex metal hydrides are in development. In general, batteries are more suitable for vehicles the size of cars or smaller, but hydrogen may be better