At 11:28 AM 2/10/5, <[EMAIL PROTECTED]> wrote: >How much energy is expended producing liquid N2, and how would this >potential resource compare to equivalent alternative fuel source energy >carriers.
Liquifaction is the main problem. It occurs at less than 50 percent Carnot efficiency. See: <http://www.phys.edu/~cordonez/imece01.pdf> Hydrogen formation from either methane or water can be more than 60 percent efficient, so LN2 or liquid air comes out a loser on the supply end, and would be a net polluter. The source of pollution might be moved to more convenient spot, but the sum of pollution per mile driven is more than doubled vs burning directly in the car the original fuel that provides the energy for liquifaction (unless the energy source is nuclear). As Mike Carroll points out LN2 is not an energy source. Like hydrogen, it can't be mined. Hydrogen is not an energy source. These things are merely energy storage mediums, like batteries. There is a net energy and pollution *cost* to obtain these things. The principle advantage to LN2 is that it is much easier to store, and the energy density is high compared to hydrogen or batteries. The EV1 batteries stored at 110 kJ/kg, LN2 stores 570 kJ/kg, of which about 200 kJ/kg is heat of vaporization. The high percentage of heat stored in heat of vaporization tells one right off that simply vaporizing LN2 in a heat exchanger with ambient air, as done at UW or UNT is inefficient. A Sterling engine would be a better means of achieving the vaporization, because this would not waste the 200 kJ/kg by cooling the atmosphere with it. Interestingly, an LN2 engine would run less efficiently in arctic or cold weather, in which heat engines run better. A good all around approach might be to combine a heat source, e.g. hydrogen, with an LN2 or liquid air cold source to drive a sterling engine in addition to the LN2 vapor driven turbine. This would also provide a backup power mode to get to a LN2 station. To obtain a decent power to weight ratio, this kind of engine might best run at constant speed and thus would work best as part of a hydbrid system. To obtain an efficient vehicle breaking energy must be recoverable and storable, and LN2 can't do this. The capital and operating cost for a hybrid system this complex may be a problem. I think a significant problem is the lack of a home liquifaction capability. One of the nice things about the EV1 was the ability to charge it at home. This problem can probably be solved for a liquid air vehicle. The problem of liabilities due to cryogenic product safety are another matter entirely. It is not likely we will see home hydrogen refueling, though that is also a technical possibility. The main problem with using hydrogen, other than the energy and pollution costs of obtaining it, is safe and effective storge. LN2 beats H2 hands down on this at the moment, but maybe not for long. Carbon nanotube storage or alternative storage methods are likely to show up soon due to the large amount of money being spent on development now. A CF driven hybrid would be the ultimate vehicle, but a few technical problems remain there too. Regards, Horace Heffner
