... I've recently changed my opinion to favoring solar heavily. Specifically, solar to various hydrocarbons.
What you're talking about is storage of energy, to smooth out unevenness in demand vs. supply. Sounds like you're focusing on storing in the form of gasoline, for powering cars at a later time.
But using solar energy directly as electricity has a lot of growth remaining before storage even starts to even become an issue. And when it does, you need to look at the full picture:
- Demand shifting can further defer the need for storage. For example, a relatively new trend in air conditioning for large buildings is to manufacture ice at night, which is then used to help cool the air during the day. This takes advantage of lower electric rates at night due to the excess supply from fossil sources, as well as improved efficiency in the ice making itself, due to cooler air temps. But when the installed base of solar becomes large enough that the cheapest rates are in the daytime on sunny days, you could easily switch the ice-making to those times. Of course, excess supply from wind sources at night could often keep the cheapest rates where they are now.
- By the time the installed solar base is large enough to need to worry about storage, the electric car fleet will be substantial. Most of these cars will be commuter cars, so that means that workplaces will need to have charging available. But that seems a relatively minor infrastructure improvement. And again, wind energy will usually be providing cheap rates at night as well.
- Battery technology is is getting a lot of investment attention these days, and in the next decade we're probably going to see the installation of very large flow-type batteries or similar on the grid. These are already very useful for smoothing out demand/supply fluctuations, and protecting against cascading failures. They will be one of the most efficient means of storing excess renewable energy of all kinds.
Only after all these options are at capacity would we start talking about storage by way of driving chemical reactions. And there, yes, making gasoline might make sense, because long-haul trucking and airplanes really need that energy density. But then again, by the time we get there, battery storage may be very competitive -- especially when you take the inefficiency of manufacturing gasoline from scratch into account.
And if you're talking about making gasoline or some such from solar in the near term, for use in the current fleet of cars, there are still problems. True, you'll be supplanting fossil-carbon use from oil, but you'd most likely be able to supplant many times the amount of fossil carbon from coal and natural gas if you put that solar energy into the grid. Even generating hydrogen loses 75% of the energy, if you're converting that H2 back into electricity. The other options above give you far more bang for your solar-investment buck.
Also, generating gasoline from solar only serves to boost the lifetime of the gas-based transportation industry, and all the ill health effects that go with it. It would soak up a lot of the innovative energy/investment that could otherwise go to building a clean infrastructure.