NeX said:
that car seems to be slightly different to what i was thinking of, that is an electric car being powered by electricity generated by hydrogen.
I was just looking at reports on engine efficiency, and car engines are typically it seems around 32% thermal efficient. That is to say, for every KW of heat produced by burning the fuel in combustion only 32% is of any use.
i'm on about when you use hydrogen insted of petrol.
The problem with this should be immediatly obvious from the comment above. Why keep to IC technology when its so inefficient.
using hydrogen to generate electricity seems a bit pointless to me cos that is another state that it has to change into which is where energy is lost, if it was already electricity in the first place before it was turned into hydrogen then why not store it in batteries?
Because hydrogen is a much more compact and light weight source of stored energy when compared to a lead acid battery. Take a typical Micra battery 12.5v @ 45Ah = 540watts/hr or 1944000 Joules of stored energy!!! you can see how tiny that now is!! and to have enough stored for any use requires lots of batterys = expensive + very heavy and not at all really suited for the job.
Hydrogen however has an energy content when reacting with oxygen of 140,000,000 Joules per kilo. One Joule is 1W/Second. So 140,000,000/3600 (1hr) = 38.8 Kw/hr - compared to 0.54 KW/hr of the battery.
I dont know the exact weight of a Micra car battery. But I shall guess at 5KG. Now, assuming a Micra battery is fully charged, it stores as I said approx 540w/hr or:
1944000 Joules.
compare this to the equilivent weight of hydrogen =
700000000 Joules, hmm just a bit of difference then!
Also, I just had a look at some automotive motors and high efficiency ones are around 93% efficient. Fuel cells it seems vary from 30-90% efficient. So you can see that dumping the IC engine and using fuel cells with electric motors is a much more efficient method.
This is very simplified, 5kg of hydrogen at 1atm would take up a whole lot of space, compress it to 100atm its much more sensible, however you now need to make a nice and very strong bottle to do so. Which does have a weight penalty. And then there is the motor and fuel cell. I would imagine that modern examples of fuel cell technology are probably weight wise pretty comparable or perhaps just a bit more than a typical IC engine. Though to be honest thats a pure guess on my part.
Ed
I also found this if you want a real laugh!!!!!!
http://www.opposingdigits.com/watercar/