Hydrogen fuel cells and Carnot's efficiency

[ironizer]

I'm trying to figure this out:

 

If you were to (hypothetically, under ideal conditions) reform a fossil fuel, say gasoline, into hydrogen, and use that in a fuel cell and through a motor to produce mechanical work--- would you be able to produce more work than by using the fuel in a heat engine that operates at its maximum theoretical efficiency (i.e. the carnot engine) ???

 

Remember, the electric motor, fuel cell, reformer and such can be calculated at their highest theoretical efficiencies.

[thedarkshade]

I'm trying to figure this out:

 

If you were to (hypothetically, under ideal conditions) reform a fossil fuel, say gasoline, into hydrogen, and use that in a fuel cell and through a motor to produce mechanical work--- would you be able to produce more work than by using the fuel in a heat engine that operates at its maximum theoretical efficiency (i.e. the carnot engine) ???

 

Remember, the electric motor, fuel cell, reformer and such can be calculated at their highest theoretical efficiencies.

I don't think you'd be able to produce more work. I mean, to reform gasoline into hydrogen you'd need to spend some energy, and if we calculate that too, I think we'd loose more energy till we get H more than H would give to us! Because just about 20% of the energy that H releases can be used, the other part goes into heat!

[ironizer]

In earth-like conditions, a perfect heat engine peaks at about 70% efficiency.

So if you were to ideally reform a fossil fuel and run it in a 100% efficient electric motor, the total efficiency of the latter system would be less than 70%?

[thedarkshade]

In earth-like conditions, a perfect heat engine peaks at about 70% efficiency.

Well in ideal conditions the things behave different, but so far I have not heard of a fuel whose released energy has been used 100%, not even close! I mean, in some countries of the world (many) coal is used as a fuel to produce electricity, But only 20% of the energy that coal releases is used to produce electricity while the rest is wasted (goes into heat and other stuff) and yet the pollution that this method of producing electricity causes is very dangerous! But the reason that they use coal, is because coal is "cheap dirt"!

So if you were to ideally reform a fossil fuel and run it in a 100% efficient electric motor, the total efficiency of the latter system would be less than 70%?

If you use 100% of the energy released from any fuel (which is impossible), there is no reason why the efficiency would be 70%. But using 100% of the energy released from any kind of fuel is something that so far has not happened yet!

[swansont]

In earth-like conditions, a perfect heat engine peaks at about 70% efficiency.

So if you were to ideally reform a fossil fuel and run it in a 100% efficient electric motor, the total efficiency of the latter system would be less than 70%?

 

Reforming the fuel will not be 100% efficient, nor will the fuel cell, nor will the motor. You have to account for all of these steps.

[CaptainPanic]

A fuel cell is nothing but a complicated system of a battery + electric engine.

 

There is no Carnot cycle in the fuel cell + electric engine. Theoretically, it can exceed the Carnot efficiency. Whether it is done in practice, I don't know, but I bet it's a hell of a job to beat the 70% efficiency that was mentioned earlier.

 

Thinking theoretically (everything is easier on paper), I don't see why you could not reach a very high efficiency, if you're somehow able to overcome all internal friction problems (this includes friction in the engine, and also diffusion problems, i.e. mass transfer, in the fuel cell). Turning hydrocarbons into hydrogen can also be done. One way of maximizing the hydrogen output is to "water gas shift": H2O + CO --> H2 + CO2. This makes sure that you oxidize all carbon as much as possible, and leave all hydrogen unoxidized. Of course, you need to gasify the fossil fuel first (gasify = heat up with no oxygen present, basically turning everything into gas molecules, mostly CO and H2 - definitely in the don't try this at home category, CO is highly toxic and it's all explosive).

 

However, the water gas shift reaction is an equilibrium reaction, and it does not go to 100% H2 + CO2. This means you need a separation and a recycle (costs energy). The fuel cell does produce a lot of heat, and to overcome the internal diffusion problems, they are heated to high temperatures. This costs energy. And finally, electric engines are efficient, but not 100%.

[Roger]

IIRC the 1st law of thermodynamics comes into play here.

 

I prefer get my H2 from water.

 

Electricity does work, its used to get H2 from H20, the 1st law IIRC says you cant get the energy back after doing work. SO you might as well use the electricity in an electric car.

[swansont]

IIRC the 1st law of thermodynamics comes into play here.

 

I prefer get my H2 from water.

 

Electricity does work, its used to get H2 from H20, the 1st law IIRC says you cant get the energy back after doing work. SO you might as well use the electricity in an electric car.

 

But gasifying a fossil fuel is not the reverse of the combustion reaction, whereas electrolysis of water and subsequent combustion is. i.e. fossil fuels are stored energy, water is not.