Liquid hydrogen
From Wikipedia, the free encyclopedia
Liquid hydrogen is the liquid state of the element hydrogen. It is a common liquid rocket fuel for rocket applications. In the aerospace industry, its name is often abbreviated to LH2 or LH2. Hydrogen is found naturally in the molecular H2 form, hence the H2 part of the name.
To exist as a liquid, H2 must be pressurized and cooled to a very low temperature, 20.27 K (−423.17 °F/−252.87°C).[1] One common method of obtaining liquid hydrogen involves a compressor resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a concentrated form of hydrogen storage. As in any gas, storing it as liquid takes less space than storing it as a gas at normal temperature and pressure. Once liquified it can be maintained as a liquid in pressurized and thermally insulated containers.
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[edit] Uses
In rocket engines, liquid hydrogen is frequently used as a coolant to cool the engine nozzle (regenerative cooling) and other parts before being mixed with the oxidizer (often liquid oxygen (LOX)) and burned. The resulting exhaust of such LH2 - LOX engines is very clean water with traces of ozone and hydrogen peroxide.
Liquified hydrogen can be used as a fuel in an internal combustion engine or fuel cell. Various concept hydrogen vehicles have been built using this form of hydrogen (see BMW H2R). Due to its similarity, builders can sometimes modify and share equipment with systems designed for LNG. However, because of the lower volumetric energy, the hydrogen volumes needed for combustion are large. Unless LH2 is injected instead of gas, hydrogen-fueled piston engines usually require larger fumigators. Unless direct injection is used, a severe gas-displacement effect also hampers maximum breathing and increases pumping losses.
Liquid hydrogen is also used to cool neutrons to be used in neutron scattering, since neutrons and hydrogen nuclei have similar masses, kinetic energy exchange per interaction is maximum (elastic collision).
[edit] Advantages
Hydrogen has one of the highest gravimetric energy densities of all available fuels, which means it has very high energy content per unit mass (143 MJ/kg, 40 percent more than other rocket fuels).[1].
As one of the lightest fuels available, one liter of hydrogen weighs only 0.07 kg. That is a density of 70.99 g/L (at 20 K).
Producing “zero emissions”, the byproducts of its combustion with oxygen alone are mainly water vapor.
[edit] Drawbacks
In terms of volumetric energy density, liquid hydrogen requires much more volume than other fuels to store the same amount of energy. Four liters of liquid hydrogen are needed to match the same energy content of one liter of gasoline.
Liquid Hydrogen requires complex storage technology such as the special thermally insulated containers and requires special handling common to all cryogenic substances. This is similar to, but more severe than Liquid oxygen.
Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away. (Typically it will evaporate at a rate of 1% per day.[2])
The low strength of the hydrogen-hydrogen bond results in low minimum ignition energy. For intermittent-combustion engines, this results in a low octane rating. In jet and rocket engines, which are typically ignited once per flight, this is an advantage, making the engine easy to start, and resistant to "flameout."
Hydrogen burns with a very high flame temperature. Typical piston engines burning hydrogen in ambient air (not simply oxygen) thus produce high amounts of NOx pollution.
Hydrogen will leak into the chemical structure of very simple containers and weaken them (see Hydrogen embrittlement).
