Audi is using electrolysis but there are ways to split water directly with heat: http://en.wikipedia.org/wiki/Water_splitting
In Germany, there is excess energy in the system when the sun is shining/wind blowing, even with many conventional plants shut off. It goes to the point, where negative energy prices are traded on the exchange markets.
But with renewals, you can't guarantee or augment the capacity you have at a certain time.
With the artifical fuels, we can store practically unlimited amounts of energy when it is available. The efficiency loss is a drawback. But it really doesn't matter much.