Wind and solar energy have huge advantages. Fuel costs are zero. Their pollution footprint is miniscule compared to fossil fuels. Wind turbines and solar panels are significantly more reliable than large coal or nuclear plants, in the sense that they very rarely fail to produce power when the wind blows or the sun shines. On the other hand, they do not necessarily produce power when we want to use power. Their output is variable, and not entirely predictable.
Unlike most products we consume, almost all the electric power we consume must be pushed into the wires somewhere at the same time we are drawing it out. Exceptions are battery-powered devices. Our electric grid would be much more robust if we could connect batteries to it in strategic places on the grid. Currently, a number of hydroelectric facilities, especially in Norway, store energy by pumping water back uphill when surplus energy is available.
California has mandated a large increase in grid-tied energy storage by 2020, and much of the renewable energy is coming from solar plants in the desert, far away from hydroelectric plants that could be used for pumped-storage. So one startup company has figured out a way store energy by trundling rocks uphill on railcars to absorb surplus energy, and letting the cars trundle back down when energy is needed. The overall efficiency of this scheme appears better than pumped hydro.
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