Gasoline Fuel Cells: Energy, Weight & Efficiency

[bbbl67]

I understand that there is some research going on into gasoline (i.e. petrol) fuel cells (as opposed to the more typical hydrogen or alcohol fuel cells). How much energy can be released through a gasoline fuel cell, as opposed to simply burning it in an internal combustion engine? Will more energy be released or less than combustion? Also how much weight do fuel cell apparatuses usually weigh? Compared to an IC engine of course. Just looking for ballpark figures.

Just wondering if fuel cells would be more practical power sources for electric engines than batteries? And if so, how much efficiency can be expected? The reason I'm asking about gasoline fuel cells is because the infrastructure for gasoline already exists throughout the world. I'm pretty convinced that an electric engine is miles above an IC engine in terms of both performance and efficiency, so having cars driven by electric motors is the way to go from here on in, just it's a matter of whether batteries or fuel cells should be the power source?

 

[anorlunda]

A fuel cell using a liquid hydrocarbon fuel has a great volume advantage over gaseous fuels. A 20 gallon tank with liquid fuel holds much more energy than 20 gallons of gas at modest pressures.

However, the actual efficiency of a fuel cell depends on many factors and IMO is too difficult to estimate without a lot more detail. A persisting problem in fuel cell design is not efficiency but lifetime. As the cell makes energy, the fuel leaves contaminants behind that foul the mechanisms. You would not be happy with an energy efficient fuel cell car if you had to buy a new fuel cell (costing $10000) every six months.

But try the chemistry yourself. If you removed all the hydrogen from gasoline, what are you left with as a waste product? How would you remove that waste from the cell? Then if you took all that hydrogen and combined it with oxygen in the fuel cell, how much energy is there to capture? When you have that answer, then just divide by 2 or 4 to account for real life imperfections and you have a back-of-the-envelope estimate of the efficiency.

 

[bbbl67]

A fuel cell using a liquid hydrocarbon fuel has a great volume advantage over gaseous fuels. A 20 gallon tank with liquid fuel holds much more energy than 20 gallons of gas at modest pressures.

However, the actual efficiency of a fuel cell depends on many factors and IMO is too difficult to estimate without a lot more detail. A persisting problem in fuel cell design is not efficiency but lifetime. As the cell makes energy, the fuel leaves contaminants behind that foul the mechanisms. You would not be happy with an energy efficient fuel cell car if you had to buy a new fuel cell (costing $10000) every six months.

But try the chemistry yourself. If you removed all the hydrogen from gasoline, what are you left with as a waste product? How would you remove that waste from the cell? Then if you took all that hydrogen and combined it with oxygen in the fuel cell, how much energy is there to capture? When you have that answer, then just divide by 2 or 4 to account for real life imperfections and you have a back-of-the-envelope estimate of the efficiency.

One thing I'm lost about is why would you need to remove the hydrogen from the gasoline? Couldn't you do a fuel-cell reaction with the whole molecule of petrol (or diesel, or kerosene, etc.)? I mean you'd end up with the same waste products as a regular air combustion of the fuel (carbon dioxide & water), but hopefully it would be a lot slower reaction, and therefore more efficient than internal combustion?

 

[anorlunda]

 

[russ_watters]

How much energy can be released through a gasoline fuel cell, as opposed to simply burning it in an internal combustion engine?

Well, gas engines tend to run at about 25% efficiency and have complicated drivetrains. Fuel cells run on the order of 60% efficiency. So figure on around triple the output.

 

[bbbl67]

hydrogen fuel cell schematic

Well, sure, that's the case for a hydrogen fuel cell, where just hydrogen and oxygen goes in, and out comes water. But in a gasoline fuel cell you'll have other reactions too, like carbon and oxygen, can't you extract electric potential from those reactions too, and not just the hydrogen and oxygen ones?

 

[bbbl67]

Well, gas engines tend to run at about 25% efficiency and have complicated drivetrains. Fuel cells run on the order of 60% efficiency. So figure on around triple the output.

Well, that's why I was asking this question. But there are cases where a certain type of reaction is much more efficient than another type of reaction, yet it produces less power in the end. So would this be the case with a fuel cell vs. internal combustion engine? For example, in an IC engine, it's harnessing the heat energy of the fuel reaction. But in a FC, it's harnessing the movement of electrons. Is heat a more abundant source of energy than electrical potential in these reactions?

 

[Bystander]

from those reactions too,

"Air-garbage" fuel cells are possible in principle; the practice is another matter. The carbon oxidation is also described as a hydrogen reformer/steam reformer https://en.wikipedia.org/wiki/Steam_reforming .

 

[bbbl67]

"Air-garbage" fuel cells are possible in principle; the practice is another matter. The carbon oxidation is also described as a hydrogen reformer/steam reformer https://en.wikipedia.org/wiki/Steam_reforming .

Can this be put into an ordinary sized passenger vehicle, or is this something that belongs in an industrial-scale refinery plant situation?

 

[Bystander]

It's industrial scale;

60% efficiency. So figure on around triple the output.

... , and divide by three for the hydrogen/steam reformer, and you're back to square one. Very attractive at first glance, and break-even at best when examined in detail.

 

[anorlunda]

Well, sure, that's the case for a hydrogen fuel cell, where just hydrogen and oxygen goes in, and out comes water. But in a gasoline fuel cell you'll have other reactions too, like carbon and oxygen, can't you extract electric potential from those reactions too, and not just the hydrogen and oxygen ones?

It's not impossible, but I haven't heard of any such fuel cell designs.

You have to consider the end products too. With a hydrogen cell, H combines with O to produce H2O (water) which can be drained onto the road. With other reactions, the end product might not be liquid or it may be polluting or toxic which means you would also need to carry a additional tank to hold the waste products. You also need a way to dispose of those wastes eventually.

I repeat what I said earlier, factors other than efficiency can dominate fuel cell practicality.