A liquid nitrogen engine is powered by liquid nitrogen, which is stored in a tank. Traditional nitrogen engine designs work by heating the liquid nitrogen in a heat exchanger, extracting heat from the ambient air and using the resulting pressurized gas to operate a piston or rotary motor. Vehicles propelled by liquid nitrogen have been demonstrated, but are not used commercially. One such vehicle, Liquid Air, was demonstrated in 1902. Liquid nitrogen propulsion may also be incorporated in hybrid systems, e.g., battery electric propulsion and fuel tanks to recharge the batteries. This kind of system is called a hybrid liquid nitrogen-electric propulsion. Additionally, regenerative braking can also be used in conjunction with this system. One advantage of the liquid nitrogen vehicle is that the exhaust gas is simply nitrogen, a component of air, and thus it produces no localized air pollution in the tailpipe emissions. This does not make it completely pollution free, since energy had been required to liquify the nitrogen in the first place, but that liquification process can be remote from the vehicle operation, and could in principle be powered by a renewable energy or clean energy source.
Description Liquid nitrogen is generated by cryogenic or reversed Stirling engine coolers that liquefy the main component of air, nitrogen (N2). The cooler can be powered by electricity or through direct mechanical work from hydro or wind turbines. Liquid nitrogen is distributed and stored in insulated containers. The insulation reduces heat flow into the stored nitrogen; this is necessary because heat from the surrounding environment boils the liquid, which then transitions to a gaseous state. Reducing inflowing heat reduces the loss of liquid nitrogen in storage. The requirements of storage prevent the use of pipelines as a means of transport. Since long-distance pipelines would be costly due to the insulation requirements, it would be costly to use distant energy sources for production of liquid nitrogen. Petroleum reserves are typically a vast distance from consumption but can be transferred at ambient temperatures. Liquid nitrogen consumption is in essence production in reverse. The Stirling engine or cryogenic heat engine offers a way to power vehicles and a means to generate electricity. Liquid nitrogen can also serve as a direct coolant for refrigerators, electrical equipment and air conditioning units. The consumption of liquid nitrogen is in effect boiling and returning the nitrogen to the atmosphere. In the Dearman Engine the nitrogen is heated by combining it with the heat exchange fluid inside the cylinder of the engine. In 2008, the US Patent Office granted a patent on a liquid nitrogen powered turbine engine. The turbine flash-expands liquid nitrogen that is sprayed into the high-pressure section of the turbine, and the expanding gas is combined with incoming pressurized air to produce a high-velocity stream of gas that is ejected from the back of the turbine. The resulting gas stream can be used to drive generators or other devices. The system has not been demonstrated to power electric generators of greater than 1 kW, however higher output may be possible.
Carnot Cycle Although the liquid nitrogen is colder than the ambient temperature, the liquid nitrogen engine is nevertheless an example of a heat engine. 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. As such, the nitrogen engine is extracting energy from the thermal energy of the air, and the conversion efficiency with which it converts energy can be calculated from the laws of thermodynamics using Carnot efficiency equation, which applies to all heat engines.
Tanks The tanks to store the liquid nitrogen must be designed to safety standards appropriate for a pressure vessel, such as ISO 11439. The storage tank may be made of:
steel aluminium carbon fiber Kevlar other materials, or combinations of the above. 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. These tanks, being non-pressurized tanks, are not subject to inspection. Very large tanks for LN2 are sometimes pressurized to less than 25 psi to aid in transferring the liquid at point of use.
Liquid nitrogen vehicles A vehicle propelled by liquid nitrogen, the Liquid Air, was demonstrated in 1902. In June 2016 trials will begin in London, UK on supermarket J. Sainsbury's fleet of food delivery vehicles: using a Dearman nitrogen engine to provide power for the cooling of food cargo when the vehicle is stationary and the main engine is off. Currently delivery lorries mostly have second smaller diesel engines to power cooling when the main engine is off.
Emission output Like other non-combustion energy storage technologies, a liquid nitrogen vehicle displaces the emission source from the vehicle's tail pipe to the central electrical generating plant. Where emissions-free sources are available, net production of pollutants can be reduced. Emission control measures at a central generating plant may be more effective and less costly than treating the emissions of widely dispersed vehicles.
Advantages Liquid nitrogen vehicles are comparable in many ways to electric vehicles, but use liquid nitrogen to store the energy instead of batteries. Their potential advantages over other vehicles include:
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