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Are Nitrogen Powered Cars The Future?
_Chainsaw writes: "The local paper ran this story about a retired engineer who has come up with the concept of nitrogen powered cars. The idea is that the pressure generated when liquid nitrogen is changed into a gas state via a heat exchanger could power a car. What are the pros and cons of this idea versus conventional gas-powered cars and the ideas of battery powered and fuel cell powered cars? Safety issues? In the event of an accident is being flash-frozen better than being burned to death or dissolved by battery acid? What is the environmental impact of letting tons of nitrogen a day escape into the atmosphere?" I wonder how easy it is to keep the nitrogen cold, too. It's interesting to consider what things will look like in 50 or 100 years, though. Will cars still be the dominant form of transport for Americans? Will Nitrogen-fueled trains zip from city to European city?
Pick up a handy can of keyboard dusting spray and imagine it's a can of liquid nitrogen. You don't have to keep it cold, you just have to maintain the pressure in the can to keep the nitrogen as a liquid (the difference between it being a gas or a liquid is a function of pressure and tempature). When the ambient pressure in the can drops (you pull the trigger on the can, opening the valve), the liquid inside the can expands into a gas (you may actually feel it "boiling" inside the can). Now imagine you're pointing this can of dusting spray at a small model windmill. The blades on the windmill turn from the air rushing by. This is what spins the turbines in the car, and makes the wheels go round. You may also notice that the can is by now getting pretty cold. The liquid nitrogen in the can (we're pretending, remember?) is absorbing the ambient heat as it expands (this is how your refrigerator works BTW). As the can gets colder and colder, the rate of the gas coming out will slow. If the can gets cold enough, the liquid nitrogen in the can will be able to remain a liquid at room pressure. So the trick with the car is to carry enough liquid nitrogen to make the trip worthwhile, and to heat it efficiently as it expands into a gas so that the expansion chamber doesn't turn into a giant frosty popsicle and stop working.
If we wanted to build clean, safe power generation facilities which are not dependent on the sun or wind, with a fuel source which will last for thousands of years, all we need is to start building the (already designed) LMFBR (Liquid Metal Fast Breeder) reactor. Argonne developed a particularly good design of this, the IFR (Integral Fast Reactor). These types of reactors are much better than current commercial reactors. First, they use fuel much more efficiently because they create more fissile material as they run, i.e. you can burn the U-238 as well as the U-235. The first nice effect of this are that you get much less high-level waste as you typically burn the fuel until it is no longer significantly radioactive. The second is that it means that our Uranium reserves will last for thousands of years even if we used this technology to provide all of our energy. The third very nice feature of the IFR and I believe LMFBRs in general is that it is inherently safe. If there is a cooling problem and the reactor core begins to overheat, the reaction will automatically stop because the coolant won't pass fast neutrons as efficiently at higher temperatures. Therefore if there is a pump failure or other coolant failure, the reactor automatically shuts down without any human or mechanical intervention.
The existence of technology like this, and its lack of political support make me frustrate me greatly. The public has an irrational fear of all things nuclear, and thus there is no political support for setting this up on a commercial basis. More people die every year from coal mining and lung diseases related to the burning of coal than died from the Chernobyl disaster, the largest nuclear power accident ever, yet no one ever talks about the "2000 coal disaster" or the "1999 coal disaster." Write your congressman and tell him that you want funding for the IFR project and that you want one built commercially.
The engineering behind nitrogen powered cars is pretty compelling, but there are some big drawbacks to using nitrogen as a vehicle fuel source. First and foremost is that nitrogen is very heavy. Imagine the weight of carrying a load of compressed liquid nitrogen, the necessary coolant to keep is liquid, the weight of the container, and the weight of the engine itself. Sure, nitrogen does deliver more power than batteries, but the power/weight ratio would seem to be prohibitive to me. Even factoring in the composite materials that would presumably be developed in the next 50 years, nitrogen just doesn't have the performance to justify the weight.
The fuel of the future, IMHO is hydrogen. Long maligned for the Hidenburg disaster, hydrogen is a fuel source with a lot of potential. (In fact, it was *not* hydrogen that caused the Hindenburg disaster. Static electricity discharged between sections of the outer skin covering, igniting the highly flammible weatherproofing compound that covered the skin of the Hindenburg. That's why the fire was described as a bright orange - a hydrogen fire would be virtually colorless. The myth of a hydrogen accident on the Hindenburg has attached a stigma to hydrogen that is based on poor evidence.)
The way a car hydrogen engine could be made safe is through the use of advanced carbon nanotube technology. These nanotubes can trap the hydrogen molecules, making them safe to transport. The holy grail of hydrogen/nanotube research is a nanotube that can hold 65% of its own weight in hydrogen. Above that figure, hydrogen fuel cells become economically feasible. Hydrogen is a clean burning fuel that provides a great deal of energy and can power a car with greater efficiency than electric motors or gasoline engines. If someone can create a nanotube storage system (and there have been rumors that breakthroughs are pending - they're at least up to 10% hydrogen/carbon) then hydrogen will become the fuel of choice for automobiles.
The specific heat of liquid nitrogen is pretty low. So it's not that dangerous in the event of an accident, although if you were completely bathed in it, it would kinda suck. I guess it's a matter of quantity...
But back in the physics department, we used to shoot each other with liquid nitrogen from squirt guns. Yow! That gets your attention.
Then there was Frank, who would actually swallow a few drops, and belch long musical selections.
As for transporting it, a thermos dewar is your best bet. Make it out of stainless steel, so you don't have shattering glass in an accident. I'm not sure how much you'll need to have in your tank to fuel the car though, so it may get big, bulky, and heavy.
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Heat cannot be used to do work. You will never be able to make a car that runs off of ambient heat energy without a net negative efficiency.
That's a fundamental concept of thermodynamics..one of the proposed "ends" of the universe is called "heat death." That is where all the matter and energy have been converted to heat, which can_not_ be converted to any other form of energy.
Power generation is dirty however you slice it (unless you use solar or hydro or some other "free" energy). Compressing gas so you can extract energy later by expanding it, is just another way of storing energy that you had to generate somewhere. Just like storing electricity in a battery directly.
The advantage over an internal combustion engine, is the huge gain in efficiency of using a big powerplant (where it's in the power company's best interest to spare no expense to make it more efficient, because it helps their bottom line in the long run) vs. millions of little engines designed for performance rather than efficiency. If everybody had a V8 in their backyard running their own generator rather than buying power from the power company, you could kiss clean air goodbye.
Just so we don't get stupid arguments like "if we can get energy from the air, why can't we just make an engine that runs on air?", the thermodynamic principle here is that a heat engine can produce work from a temperature difference, by letting the energy flow from hotter to cooler while "skimming" off part of the energy to transform into work. The percentage that it skims is the complement of the efficiency, which obviously can't be higher than 100%.
So a theoretically generalized heat engine doesn't care whether you carry around a fire or a block of ice, or (for that matter) if you skim along with one ski in a trough of cold water and another ski in a trough of hot water, as long as it's got access to a temperature difference. In practice, of course, you've got to design different engines if you want reasonable efficiency.
Incidentally, being from the frosty Northern near-state of Canada, I'd really rather not have a car without a toasty hot engine. While you can use liquid nitrogen to heat your cab, it's not terribly efficient. However, people who live in hot places might appreciate the cheap, efficient, and simple air-conditioning.
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