Reply #25: The cost of LN2 "fuel" and energy converson [View All]

Liquid Nitrogen Manufacture

The cost of the LN2 "fuel" is expected to be reasonable. The primary expense for producing LN2 is the energy cost for compression of air.9 Cryogenic separation of nitrogen from other condensables in air typically requires only a very small fraction of the total energy,10 so the ideal work to manufacture LN2 from air is very nearly that for using nitrogen as a feedstock. This work is exactly the reversible work obtainable from an ideal cryo-engine, 769 kJ/kg. The actual work required in a modern LN2 plant is 2.0-2.5 times the minimum, or 1540-1920 kJ/kg.11 Assuming an industrial electric rate for interruptible power of 5¢/kW-h, the energy cost would amount to 2.6¢/kg-LN2, in accord with delivery prices of LN2 in large quantities. Marketing the other commercially important components of air will help offset the LN2 production costs. Since the equipment needed for air liquefaction can be powered solely by electricity, it is conceivable to decentralize the "fuel" manufacturing process and to place small scale production facilities at the LN2 dispensing sites. A cost-benefit analysis is needed to determine the smallest air liquefaction machinery that can be used to produce LN2 in an economical manner.

The basic idea of the LN2 propulsion system is to utilize the atmosphere as a heat source and a cryogen as a heat sink in a thermal power cycle. This is in contrast to typical thermal engines which utilize an energy source at temperature significantly above ambient and use the atmosphere as a heat sink. In both cases the efficiency of conversion of thermal energy of the source to work (W) is limited by the Carnot efficiency, h = W/Qh = 1 - Tl/Th, where Qh is heat input, Tl is the sink temperature, and Th is the temperature of the heat source. By using liquid nitrogen as the cryomobile energy sink (Tl = 77 K) this ideal thermal efficiency is impressively high (74%) with an atmospheric heat source at Th = 300 K. The key issues are the ability to design a practical energy conversion system that can take advantage of this high efficiency and the available energy of the cryogen while still being cost competitive with alternative EVs.

http://www.aa.washington.edu/AERP/CRYOCAR/CryoCar.htm