Liquid nitrogen engine

Liquid nitrogen can also serve as a direct coolant for refrigerators, electrical equipment and air conditioning units.

A heat engine runs by extracting thermal energy from the temperature difference between a hot and a cold reservoir; in the case of the liquid nitrogen engine, the "hot" reservoir is the air in the ambient ("room temperature") surroundings, which is used to boil the nitrogen.

The tanks to store the liquid nitrogen must be designed to safety standards appropriate for a pressure vessel, such as ISO 11439.

[8] The storage tank may be made of: The fiber materials are considerably lighter than metals but generally more expensive.

Metal tanks can withstand a large number of pressure cycles, but must be checked for corrosion periodically.

Liquid nitrogen, LN2, is commonly transported in insulated tanks, up to 50 litres, at atmospheric pressure.

Currently practical refrigeration plants producing a few tons/day of liquid nitrogen operate at about 50% of Carnot efficiency.

The convenient storage of petroleum fuels combined with its low cost has led to an unrivaled success.

The energy density—derived from nitrogen's isobaric heat of vaporization and specific heat in gaseous state—that can theoretically be realised from liquid nitrogen at atmospheric pressure and 27 °C ambient temperature is about 213 watt-hours per kilogram (W·h/kg), while typically only 97 W·h/kg can be achieved under realistic circumstances.

The addition of more complex power cycles would reduce this requirement and help enable frost free operation.

In a humid environment, frost formation will prevent heat flow and thus represents an engineering challenge.

Additional heat exchangers, weight, complexity, efficiency loss, and expense, would be required to enable frost free operation.

If a vehicle is stored in a poorly ventilated space, there is some risk that leaking nitrogen could reduce the oxygen concentration in the air and cause asphyxiation.

Since nitrogen is a colorless and odourless gas that already makes up 78 per cent of air, such a change would be difficult to detect.

Liquid oxygen can spontaneously and violently react with organic chemicals, including petroleum products like asphalt.

In an incident in 2006 at Texas A&M University, the pressure-relief devices of a tank of liquid nitrogen were sealed with brass plugs.