Do you refer to the efficiency of fuel cells (which are made of not so eco friendly elements)? How about using hydrogen in combustion engines (like the BMW 7H), is efficiency higher? The problem with hybrids in general are batteries,
which are improving a lot, but still are not perfect. As the owner of a Prius I know how much these things are far from ideal.
Giovanni, apologies, I wasn't very clear. I wasn't talking about fuel cells. When I was referring to Hydrogen, I was talking about the ability to create Hydrogen chemically when energy is cheap and then to burn that Hydrogen to make energy when Energy is expensive
(this would often be within the same 24 hours). On the Prius front, Toyota have announced that the next version of the Prius will be a plug-in hybrid. This car will be able to serve as an energy store, able to take in and store energy when electricity is cheap,
and sell energy back to the grid when electricity is expensive. If only 10s of plug-in hybrids are sold, this will make no significant contribution. But if 100,000's of plug-in hybrids are sold, the stabilising effect on the grid could be crucial.
Regarding the problem of energy sources which produce a variable amount of energy during the day (Tom mentions wind which I think is a good example), could hydrogen be a good way of storing that energy for later use? Shouldn't be easier
to store hydrogen in pressure vessels than electric energy in aging batteries? Just a thought.
Giovanni, absolutely hydrogen is better than batteries as a store. Batteries are expensive, have a short usable life (8-10 years typically), and are not particularly environmentally friendly.
Hydrogen is not particularly efficient however either and while it may have its place, better options for now appear to be using cooling and heating as energy stores (think refrigeration plants, swimming pools, etc.).
Another interesting option will be the ability to use plug-in hybrids as an energy store. If there is a high uptake of plug-in hybrids, they could make a real difference for uptake of renewables. A real win-win.