How does a hydrogen powered car work and still conserve energy?

Question

The Hydrogen first needs to be separated from the oxgen in water, and this, due to the 'conservation of energy' should take up at least as much energy as is produced by the engine... does this mean that H2 cars aren't as green as they're made out to be?

Answer 1

First, the answer to the question as asked - If you make the hydrogen from natural gas (~97% methane), it is not extraordinarily clean or efficient. The only benefit from this is that it concentrates the "mess" in one place where, at least theoretically, it's easier to capture/contain/control. I've never met anyone that would tell you that this is how to realize the benefits of a hydrogen economy. The ideal scenario is one others here have described... you need to use renewable and clean energy not only to electrolyze the water, but also to compress the hydrogen to usuable vehicle pressures. Then you put it in a PEM fuel cell which generates electricity and water, and the cycle continues. If the energy you're using is renewable and clean, then the efficiencies involved are less of an issue. This is done currently only on small scales for economic reasons, but is certainly possible.
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Issues brought up by others...
Most H2 powered vehicles today run on compressed gas, but other options are out there. The driving range of vehicles varies enormously, largely dependent on the amount of space the manufacturer has been able to manage for the tank (sometimes at the expense of a trunk in a prototype). Several are in the 200-300 mile range, and tank pressures are being doubled in the next generation (2x far for the same size tank). The hydrogen does not all leak out of the tank.

Hydrogen is quite small and leaks easily, but can be contained and used safely with proper design. Gasoline has at least as many "dangerous" traits, including pooling under a vehicle and being toxic, neither of which is true of hydrogen. Hydrogen is non-toxic. It is also very light and diffuses away from a leak source quickly, making accumulation unlikely. There is an incredible amount of work going into using hydrogen safely in cars - it's been used safely in industry for years. The key, as with any fuel, is to know it's properties and treat it with respect.

H2 can be used in an internal combustion engine (with a small amount of emissions), or a fuel cell ("zero emission"). There is still work to be done for sure, but it's not as bad as it's being made out to be!

Answer 2

Hydrogen fuel cell cars are a somewhat joke. My theory is they are a bit of a scam to help the oil companies to survive. Anyway heres some titbits I uncovered whilst researching the various powertrain efficiencies of various vehicles

-To extract the hydrogen from a hydrocarbon source is very dirty. I calculated if hydrogen was soley produced this way then the CO2 emissions from would be 120+/-10g/km for a small hatchback. Although the emissions are obviously zero out of the car itself.

-It takes three times as much energy to extract hydrogen through electrolysis than the MAXIMUM POSSIBLE energy that can be extracted from it (this being a physical contraint, actually its closer to 1/5). Considering a fuel cell has a maximum efficiency of around 45%, then assuming you had a good electric motor with heat recovery at 95% efficiency, the process becomes 0.33x0.45x0.95=0.13% roughly, in best case! This ignores the efficiency of the electricity generation too! Considering hydrocarbons are overall about 30% over the whole stream thats pretty wastefull.

- The efficiency of the power train is roughly equal to petrol, slightly worse than deisel.

-The energy density of hydrogen at 200Pa is about 1/10 of that of petrol, this can be improved by liquifying it to about 1/4. This essentially means that the volume required to theoretically produce the same amount of energy is much greater for hydrogen than petrol. This is not mentioning the volatility of hydrogen, and the safety concerns that go along with it.

My opinion, wait for batteries to get better energy density, a fuel cell is essentially a middleman that wastes energy. The efficiency of the powertrain for an EV ran on batteries is the same/similar to its electric motor, the source of the electric is the only concern, produced on an industrial scale its pretty efficient. I worked out the CO2 emission of an electric vehicle on batteries these days would be 30g/km.

Answer 3

That depends on the energy source used for that separation. If solar or wind energy is used, then you have a "green' fuel. Hydrogen can be used as any other fuel, but with some changes if you want to do it right. Putting th gas line down into the air intake will work, but it has it's problems. There is a special injector made for this fuel, but right now it is fairly expensive.
An easier fuel would be methane derived from animal waste. It needs to be scrubbed of other gasses, but a lime water filter will take care of that. The only real problem then is to compress it. The only free transportation is still walking, or riding a bicycle. Everything else has some kind of problem to go with it.

Answer 4

James, you have got it. Hydrogen powered cars are a joke, and not just because they consume more power than they produce.

How are they going to store fuel in a moving vehicle?. Hydrogen has a habit of leaking from the tiniest cracks, cracks that would stop other gases except for helium. It can only be stored as a gas in the practical real world, which means it's energy density is pathetic compared to petrol/gasoline. It is also explosive when mixed with air over a range of about 5% to 74%, a far, far wider range than petrol/gasoline vapor, so it is also far more dangerous.

Far better to use an hydrogen rich fuel like methane as CNG -compressed natural gas or as the more carbon intensive but still hydrogen rich fuels like liquid petroleum gas or LPG, which can be stored as a liquid at moderate pressure and ambient temperature.

Answer 5

h2 cars are green if the energy which sperates the h2 came from green sources, like solar-panels or so.

one type of h2 car uses a pem-cell putting together h2 with ogygen from the air, and getting electrical power out of it.

the advantage is, that those cars emit nothing else than some water.

but as stated above, the technology is useless unless producing hydrogen does not mean burning fossile fuel or so

Answer 6

Making electricity and transferring it to batteries, and then using that energy to move people miles is much less polluting that just running a gas engine. A hybrid uses both electric and gas, often charging a battery with the gas engine. This is a pretty efficient use of energy. It does not, however, keep the pollution in industrial areas and away from cities, also the gas engine can still make noise, though, this is not a problem even with today's hybrids. Oh, so you want us to... not use cars? Well, that sounds all well and good to some people, I'm sure, but that would destroy... well... almost everything advanced, and much that isn't advanced. I'm sure it would lead to the death of millions of poor people. So, we are trying to make things more efficient with pollution in preferable areas and in concentration that make it more efficient to clean. We would like to keep the freedom of automobile use. But, yes, Reduce, Reuse, Recycle, does include REDUCE.

Answer 7

Firstly, nothing conserves energy! What you want there is perpetual motion, and the clever bods haven't got that one sussed out yet!

The whole idea of H2 cars, or electric cars, or whatever, is that you can manage the primary source of energy such that it is minimal impact, for example, creating all your Hydrogen from electricity generated by renewable / sustainable means. That way you avoid burning up fossil fuels etc.

So that's it! It isn't necessarily about the efficiency of H2 cars, it is more about the translation from the primary power source, be it solar, wind power, wave power, hydro-electric, into a clean, acceptable secondary power source.

Answer 8

Engines can be made to burn various kinds of fuel including hydrogen. The really non-green thing about a hydrogen powered is that hydrogen on an industrial scale is made from methane (nat gas).

Answer 9

That is part of it. The H atom is so small it will leak through anything ,and explode. If your car has a 10 gal tank it could wipe out a hole block. It would also produce NO2 ,due to the Burning temperature.

Answer 10

You're right and storage is a huge problem. With today's technology and that for the foreseeable future, a cars range is about 100 miles and because of leakage, even if you don't drive it, the tank will leak itself empty in about a week.