A liquid nitrogen vehicle is powered by liquid nitrogen, 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 are not used commercially. One such vehicle, Liquid Air was demonstrated in 1902. Liquid nitrogen propulsion may 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 be used in conjunction with this system. 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. Delivery lorries have second smaller diesel engines to power cooling when the main engine is off.
Liquid nitrogen is generated by cryogenic or reversed Stirling engine coolers that liquefy the main component of air, nitrogen. The cooler can be powered by electricity or through direct mechanical work from hydro or wind turbines. Liquid nitrogen is stored in insulated containers; the insulation reduces heat flow into the stored nitrogen. 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 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 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. Liquid nitrogen production is an energy-intensive process. Practical refrigeration plants producing a few tons/day of liquid nitrogen operate at about 50% of Carnot efficiency. Surplus liquid nitrogen is produced as a byproduct in the production of liquid oxygen. 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 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; this compares with 100–250 W·h/kg for a lithium-ion battery and 3,000 W·h/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, a 350-litre insulated onboard storage vessel is required. A practical volume, but a noticeable increase over the typical 50-litre 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.
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. Additional heat exchangers, complexity, efficiency loss, expense, would be required to enable frost free operation; however efficient the insulation on the nitrogen fuel tank, there will 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 makes up 78 per cent of air, such a change would be difficult to detect. Cryogenic liquids are hazardous. Liquid nitrogen can cause frostbite and can make some materials 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 vaporized. In an incident in 2006 at Texas A&M University, the pressure-relief d
North Sea is a 1938 documentary film produced by Alberto Cavalcanti under the auspices of the GPO Film Unit and directed by Harry Watt. The film makers challenged the conventions of documentary, casting non-professionals, as they had in their previous film The Saving of Bill Blewitt. In the same style, North Sea employed minimal narration and relies on action and characterisation to tell its story; the John Gillman, a deep-sea castle-class trawler is damaged during a storm in the North Sea and seeks help from the Wick coastguard. After a struggle against the elements, the dangers are overcome, the ship returns safely to harbour; the film was based on an incident in 1937 when an Aberdeen trawler got into distress and was saved through their radio distress calls. The portrayal of real fishermen and their lifestyle is the film's strength; the men are able to speak out and act themselves, rather than have their lives interpreted by professional middle-class actors. The'cast' did use some scripted dialogue devised by Watt, like many from the Documentary Film Movement, came from a middle-class background.
As a consequence and characterisation appears inauthentic and unconvincing. North Sea was the GPO Unit's second dramatised documentary, following from The Saving of Bill Blewitt, it provided a second role for the real-life Bill Blewitt, the Cornish postman who had proved a star turn in the first film. North Sea proved popular and was to lead to a number of feature-length dramatised documentaries during the 1940s, including Target for Tonight, Coastal Command, Fires Were Started, Western Approaches. Cavalcanti and Watt went on to work within the feature film industry. Bill Blewitt North Sea on IMDb North Sea at the BFI