As a temporary measure, why don't we use renewables with a fallback to fossil fuels? During the day, solar could meet demand and at night, we use coal/gas/whatever.
I think there's a lot of inefficiency in only operating coal plants during some times of day. That was one of the major complaints that utilities in California used for crediting users with residential solar for their power generation. It wasn't actually worth the market rate to the utility if they had to run the plant anyways. I don't have any independent data on this, unfortunately.
There are interesting initiatives around the country to curb peaks in usage, however. For example, my parents are signed up for a service where they collect bill credits as they minimize power usage during a pre-announced hour of the week which exceeds normal capacity and would require dirtier and more expensive power plants to operate.
