Liquid nitrogen economy

A liquid nitrogen (N₂ (l)) 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.

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Description

Currently, most road vehicles are powered by internal combustion engines burning fossil fuel. If transportation is to be sustainable over the long term, the fuel must be replaced by something else produced by renewable energy. The replacement should not be thought of as an energy source; it is a means of transferring and concentrating energy, a "currency".

Liquid nitrogen is generated by cryogenic or Stirling engine coolers that liquefy the main component of air, nitrogen (N₂). The cooler can be powered by renewable generated electricity or through direct mechanical work from a hydro or wind turbines.

Liquid nitrogen is distributed and stored in insulated containers. The insulation reduces heat flow into the stored nitrogen. Heat from the surrounding environment boils the liquid. 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.

Criticisms

The approach has been criticized on the following grounds, which can also be seen as the engineering challenges that must be overcome.

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 ^[1].

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 unrivalled success. In addition, a petroleum fuel is a primary energy source, not just an energy storage and transport medium.

The maximum energy density that can be realised from liquid nitrogen at atmospheric pressure is 213 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, 14 times the density of liquid nitrogen used at the Carnot efficiency ^[2].

For an isothermal expansion engine to have a range comparable to an internal combustion engine, a 350 litre (93 gallon) onboard storage vessel is required ^[2]. Add to that the fact the container would need to be insulated. A practical volume, but a noticeable increase over the typical 50 litre (13 gallon) 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 fuel 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 ^[2].

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 of accumulation of nitrogen gas. This is colourless and odourless, so difficult to detect. Although it is non-toxic, it can nevertheless cause unconsciousness and death because of the depleted levels of oxygen in the air.

See also

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

References

1. ^ J. Franz, C. A. Ordonez, A. Carlos, Cryogenic Heat Engines Made Using Electrocaloric Capacitors, American Physical Society, Texas Section Fall Meeting, October 4-6, 2001 Fort Worth, Texas Meeting ID: TSF01, abstract #EC.009, 10/2001.
2. ^ ^{a b c} C. Knowlen, A.T. Mattick, A.P. Bruckner and A. Hertzberg, "High Efficiency Conversion Systems for Liquid Nitrogen Automobiles", Society of Automotive Engineers Inc, 1988.

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.