A piston or turbine driven by liquid nitrogen that has been demonstrated since 1902 but never reached production, and the physics explains why.
A liquid nitrogen engine uses the pressure from warming liquid nitrogen to drive a piston or turbine, drawing heat from the air. The tailpipe emits only nitrogen, but low energy density, liquefaction losses and safety risks have kept it from commercial production.
A liquid nitrogen engine runs on liquid nitrogen held in an insulated tank, turning the cold fluid into pressurised gas by drawing heat from the surrounding air. Vehicles using the principle have been demonstrated since 1902, but none is used commercially. The appeal is a clean tailpipe, and the difficulty is the energy needed to make the fuel in the first place.
How does a liquid nitrogen engine work?
A liquid nitrogen engine stores the fuel in an insulated tank and warms it in a heat exchanger, using heat drawn from ambient air. The resulting pressurised gas drives a piston or rotary motor. Some designs heat the nitrogen inside the cylinder, while others flash-expand it in a turbine.
The engine is a heat engine in the thermodynamic sense, even though the fuel is colder than the surroundings. The hot reservoir is the room-temperature air used to boil the nitrogen, and the conversion efficiency can be worked out from the laws of thermodynamics. It extracts energy from the temperature difference between the warm air, which acts as the hot reservoir, and the cold nitrogen. The Carnot efficiency equation applies to it, as it does to every heat engine.
In the Dearman engine the nitrogen is combined with a heat exchange fluid inside the cylinder, so the heat transfer happens within the engine itself. A separate turbine design takes a different route. It sprays liquid nitrogen into the high-pressure section, where it flash-expands and mixes with pressurised air to create a fast gas stream.
The exhaust is only nitrogen, but the energy spent making the liquid has to be counted too.
Where did the idea come from?
The idea is more than a century old. A vehicle called Liquid Air was demonstrated in 1902, and the concept remains a demonstration rather than a commercial product. In 2008 the US Patent Office granted a patent on a liquid nitrogen turbine engine, but the system had not been shown to power generators above 1 kW, although higher output may be possible.
In June 2016 trials were due to begin in London on the J. Sainsbury's fleet of food delivery vehicles. The plan was to use a Dearman engine to power cargo cooling while the main engine was off, because delivery lorries mostly carry a second diesel engine for that task.
The concept can also be part of a hybrid. A hybrid liquid nitrogen-electric system can pair battery propulsion with a fuel tank that recharges the batteries, and regenerative braking can be added. A nitrogen system can also share a powertrain with a petrol or diesel unit in a combined cycle, where recovered heat from one drives the other.
How is liquid nitrogen made and stored?
Liquid nitrogen is made by cryogenic or reversed Stirling engine coolers that liquefy the nitrogen in air. The coolers can run on electricity or on direct mechanical power from hydro or wind turbines. The liquid is then held in insulated containers, because heat from the surroundings boils it away.
Pipelines are a poor fit for the job. Long-distance lines would need costly insulation, so distant energy sources would be expensive to use. Petroleum, by contrast, can be moved at ambient temperatures. Liquid nitrogen consumption is in essence production run in reverse, with the nitrogen boiled off and returned to the atmosphere.
Storage tanks must meet pressure vessel standards such as ISO 11439. They can be made from steel, aluminium, carbon fibre or Kevlar. Fibre materials are considerably lighter than metal but generally more expensive, while metal tanks cope with many pressure cycles but need checks for corrosion.
Liquid nitrogen is commonly transported in insulated tanks of up to 50 litres at atmospheric pressure, and those tanks are not subject to inspection. Very large tanks are sometimes pressurised to less than 25 psi to help move the liquid at the point of use.
What are the emissions and environmental claims?
A liquid nitrogen vehicle's exhaust is nitrogen, which makes up most of the air, so the tailpipe produces no localised pollution. The emissions move to the plant that makes the liquid, where controls may be more effective and less costly. Energy is still needed to liquefy the nitrogen, so the overall benefit depends on where that energy comes from.
Like other non-combustion energy storage systems, a liquid nitrogen vehicle moves the emissions source from the tailpipe to the central electrical generating plant. Treating emissions at one plant can be more effective and less costly than managing the output of widely dispersed vehicles. Where emission-free sources are available, net pollution can be reduced, and liquefaction could in principle be powered by renewable energy.
What are the advantages of the design?
Liquid nitrogen vehicles are comparable to electric vehicles, but they store energy in liquid nitrogen rather than in batteries. They avoid the transport of fuel, can be refuelled at rates comparable to liquid fuels, and are not constrained by the degradation problems of battery systems. Their tanks can also be recycled with less pollution than batteries.
Because the power comes off the electrical grid, the pollution from fuel transport is eliminated, and maintenance costs are said to be lower. A single source of emissions is also easier to control than millions of separate vehicles on the road.
What are the main drawbacks?
The chief drawback is inefficiency. Energy is spent liquefying nitrogen, and then the liquid's energy is turned back into motion with further losses. Liquid nitrogen is also absent from public refuelling stations, although welding gas suppliers already run distribution systems for it.
The same liquid also cools refrigerators, electrical equipment and air conditioning, so it has uses beyond the engine. Once used, it simply boils away and returns to the atmosphere.
Liquid nitrogen is produced as a by-product of liquid oxygen production, so surplus supply exists. Production itself is energy-intensive, and even practical refrigeration plants operate at about 50% of Carnot efficiency.
Why is energy density the main obstacle?
Phase-change energy stores hold far less energy per kilogram than chemical fuels. Theoretically, liquid nitrogen at atmospheric pressure and a 27 degree Celsius ambient can deliver about 213 Wh/kg, but realistically only about 97 Wh/kg is achieved. Lithium-ion batteries offer 100 to 250 Wh/kg, while petrol burned at 28% thermal efficiency gives about 3,000 Wh/kg.
| Energy store | Energy density (Wh/kg) | Note |
|---|---|---|
| Liquid nitrogen, theoretical | 213 | Atmospheric pressure, 27 degrees Celsius ambient |
| Liquid nitrogen, realistic | 97 | Achieved under realistic circumstances |
| Lithium-ion battery | 100 to 250 | Typical range |
| Petrol at 28% thermal efficiency | About 3,000 | Combustion engine |
The gap shapes range. An isothermal expansion engine would need a 350-litre insulated vessel to match the range of a combustion car, compared with a typical 50-litre gasoline tank. More complex power cycles reduce that requirement, but no commercially practical liquid nitrogen vehicle exists.
Gasoline stores about 14 times the energy per kilogram that liquid nitrogen can deliver at Carnot efficiency. That difference makes transport and storage far easier for petroleum fuels, which is a large part of their dominance.
What makes the engine difficult to build?
Cryogenic working fluids need heat exchangers that warm and cool them, and in humid air frost can block the heat flow. Engineers can add topping cycles to keep the exchanger above freezing, but each addition brings weight, complexity, efficiency loss and cost.
Frost-free operation therefore needs extra heat exchangers and more complex cycles. That is one of the main reasons the concept has stayed at the demonstration stage.
Is liquid nitrogen safe to handle?
Liquid nitrogen is hazardous if spilled. It can cause frostbite and make some materials extremely brittle. Because nitrogen is colourless and odourless and makes up 78 per cent of air, a leak in a poorly ventilated space could reduce oxygen to dangerous levels without warning.
Liquid nitrogen is colder than 90.2K, so oxygen from the atmosphere can condense. Liquid oxygen can react violently with organic chemicals, including petroleum products such as asphalt. Rapid vaporisation is also a hazard, because liquid nitrogen expands by a ratio of 1:694 when it turns to gas.
In 2006 at Texas A&M University, a tank's pressure-relief devices were sealed with brass plugs. The tank failed catastrophically and exploded, which shows how much force a sealed cryogenic vessel can release.
