- Mon Aug 19, 2013 7:13 pm
#27606
I found this posted on another forum, which seems to sum it up for me:
The concept is basically flawed when it comes to producing a vehicle fuel. The web is full of examples of people demonstrating hydrogen being produced by various pieces of apparatus, but the volume is always small. That part of it is true. You can produce hydrogen by cracking water molecules using electricity. There is nothing complex about the apparatus. It's easy to do. It's a basic high school physics or chemistry experiment.
The problem is that the amount of energy it takes to break a water molecule into its constituent parts (hydrogen and oxygen) is the same amount of energy that you get back if you recombine them by burning the hydrogen in oxygen (or air) to get water. Therefore, if you intend to run an internal combustion engine on hydrogen produced from water on board that vehicle using energy produced by that engine, you have a huge energy deficit. If it worked it would be an example of perpetual motion. There are far too many losses in the cycle.
A typical car petrol engine has an efficiency of about 25% to 30%. That's how much of the energy available in the fuel that is available to propel the car. The rest is lost as heat, friction (heat) in the transmission and the tyres, air resistance (more heat), and so on. The car's alternator, from which you would draw the electricity to produce the hydrogen, is probably about 60% efficient; so there are more losses there.
Those who claim to achieve improved fuel consumption using hydrogen produced on-board, and genuinely believe it, are probably trying very hard when they drive to obtain the best possible mileage figures. If you drive your existing petrol-only powered car in the same way, you'll also experience a marked improvement in economy.
Hydrogen powered cars may come on day. Car companies and gas companies have been working on it on and off for quite awhile. But when they do, the hydrogen will have to be produced industrially using power from the electricity grid, and then placed in a fuel tank in the car, at a filling station. If the grid-sourced electricity used to produce the hydrogen comes from burning coal, then the exercise is pointless.
The problem with using hydrogen as an automotive fuel is that it is very difficult to store in sufficient quantity. There are only 3 methods.
1. You compress it into a cylinder at high pressure.
A cylinder with a capacity of about 50 litres would only hold about 6m3 of gas, and it would be at a pressure of about 14,000kPa. That's a very heavy, cumbersome, inconveniently shaped cylinder to fit in a car to obtain very little driving range.
2. You could use a cylinder that contains a material that will adsorb hydrogen at a lower pressure, but they haven't found anything suitable yet. And you would still have the problem of very limited range.
3. You store it as a cryogenic liquid at close to absolute zero in a special (and bulky) vacuum insulated cylinder. That would give you a range similar to a petrol car but would require special filling stations, an awful lot of safety devices and systems, and a very expensive vehicle. BMW were working on this with BOC in Germany about 15 to 20 years ago. They had test cars on the road - but we still haven't seen anything practical.
