Liquid nitrogen engine

A liquid nitrogen vehicle is powered by liquid nitrogen, which is stored in a tank. The engine works 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 engine.

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.

A liquid nitrogen economy is a hypothetical proposal for a future economy in which the primary form of energy storage and transport is liquid nitrogen. It is proposed as an alternative to liquid hydrogen in some transport modes and as a means of locally storing energy captured from renewable sources. An analysis of this concept provides insight into the physical limits of all energy conversion schemes.

Criticisms

Cost of production

Liquid nitrogen production is an energy-intensive process. Currently practical refrigeration plants producing a few tons/day of liquid nitrogen operate at about 50% of Carnot efficiency .

Energy density of liquid nitrogen

Any process that relies on a phase-change of a substance will have much lower energy densities than processes involving a chemical reaction in a substance, which in turn have lower energy densities than nuclear reactions. Liquid nitrogen as an energy store has a low energy density. Liquid hydrocarbon fuels by comparison have a high energy density. A high energy density makes the logistics of transport and storage more convenient. Convenience is an important factor in consumer acceptance. The convenient storage of petroleum fuels combined with its low cost has led to an unrivaled success. In addition, a petroleum fuel is a primary energy source, not just an energy storage and transport medium.

The energy density — derived from nitrogen's isobaric heat of vaporization and specific heat in gaseous state — that can be realised from liquid nitrogen at atmospheric pressure and zero degrees Celsius ambient temperature is about 97 watt-hours per kilogram (W-hr/kg). This compares with about 3,000 W-hr/kg for a gasoline combustion engine running at 28% thermal efficiency, 30 times the density of liquid nitrogen used at the Carnot efficiency .

For an isothermal expansion engine to have a range comparable to an internal combustion engine, an 350-litre (92 US gal) insulated onboard storage vessel is required . A practical volume, but a noticeable increase over the typical 50-litre (13 US gal) gasoline tank. The addition of more complex power cycles would reduce this requirement and help enable frost free operation. However, no commercially practical instances of liquid nitrogen use for vehicle propulsion exist.

Frost formation

Unlike internal combustion engines, using a cryogenic working fluid requires heat exchangers to warm and cool the working fluid. In a humid environment, frost formation will prevent heat flow and thus represents an engineering challenge. To prevent frost build up, multiple working fluids can be used. This adds topping cycles to ensure the heat exchanger does not fall below freezing. Additional heat exchangers, weight, complexity, efficiency loss, and expense, would be required to enable frost free operation .

Safety

However efficient the insulation on the nitrogen fuel tank, there will inevitably be losses by evaporation to the atmosphere. 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 % of air, such a change would be difficult to detect.

Cryogenic liquids are hazardous if spilled. Liquid nitrogen can cause frostbite and can make some materials extremely brittle.

As liquid N2 is colder than 90.2K, oxygen from the atmosphere can condense. Liquid oxygen can spontaneously and violently react with organic chemicals, including petroleum products like asphalt.

Since the liquid to gas expansion ratio of this substance is 1:694, a tremendous amount of force can be generated if liquid nitrogen is rapidly vaporized. 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. As a result, the tank failed catastrophically, and exploded.

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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.

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:

• Much like electrical vehicles, liquid nitrogen vehicles would ultimately be powered through the electrical grid. Which makes it easier to focus on reducing pollution from one source, as opposed to the millions of vehicles on the road.
• Transportation of the fuel would not be required due to drawing power off the electrical grid. This presents significant cost benefits. Pollution created during fuel transportation would be eliminated.
• Lower maintenance costs
• Liquid nitrogen tanks can be disposed of or recycled with less pollution than batteries.
• Liquid nitrogen vehicles are unconstrained by the degradation problems associated with current battery systems.
• The tank may be able to be refilled more often and in less time than batteries can be recharged, with re-fueling rates comparable to liquid fuels.

Disadvantages

The principal disadvantage is the inefficient use of primary energy. Energy is used to liquefy nitrogen, which in turn provides the energy to run the motor. Any conversion of energy results losses. For liquid nitrogen cars, electrical energy is lost during the liquefaction process of nitrogen.

Liquid nitrogen is not yet available in public refueling stations.

See also

• Energy storage
• Future energy development
• Hydrogen economy
• Lithium economy
• Methanol economy
• Solar power
• Vegetable oil economy
• Wind power

External links

• Liquid Nitrogen Economy - Similar overview with diagrams. (License GFDL)
• LN2 Vehicle 1 - A liquid nitrogen powered car using a Cryogenic Heat Engine at the University of North Texas.
• LN2 Vehicle 2- Another liquid nitrogen powered car at the University of Washington.
• WhisperGen - Domestic Stirling generators.
• Cryogenic Coolers - Small, rapid, compact cooling.
• Discussion on LN2 vehicle feasibility at How stuff works
• Thermodynamic Properties of various fuels - Tabulated data.

Further reading

• C. A. Ordonez, M. C. Plummer, R. F. Reidy "Cryogenic Heat Engines for Powering Zero Emission Vehicles", Proceedings of 2001 ASME International Mechanical Engineering Congress and Exposition, November 11–16, 2001, New York, NY.
• Kleppe J.A., Schneider R.N., “A Nitrogen Economy”, Winter Meeting ASEE, Honolulu, HI, December, 1974.
• Gordon J. Van Wylan and Richard F. Sontag, Fundamentals of Classical Thermodynamics SI Version 2nd Ed.