The primary types of renewable energies are biomass/biofuels, hydroelectricity, geothermal energy, solar energy and wind power. Among them, solar energy and wind power are undergoing very rapid growth, while the others have so far limited impact on the world economy of energy. Right now, solar and wind are the big players in renewable energy.

Solar and wind share a characteristic that is largely unique to the two of them (along with hydroectricity, where water is not always available for power generation): the amount of power they generate varies fairly unpredictably with the time of day (which is why they are called VREs, Variable Renewable Energy sources).

Jay Apt and Aimee Curtright, “The Spectrum of Power from Utility-Scale Wind Farms and Solar Photovoltaic Arrays.”

Some of the variation in solar energy generation is predictable (daytime and season cycles), while the rest of it (cloudiness) can only be estimated statistically (check the big gap between 1000 and 2000 seconds, along with the many missing spikes along the day).

Greentech Media, “20 PowerPoint Slides That Shook The Earth”

As for the variability of wind power, it is extreme, as the figure above shows.

But, as we discussed in Is Energy A Commodity?, a power grid manager has no choice but to exactly follow demand with supply. While some of this variability can be significantly diminished by spreading solar and wind plants across large areas (check John Farrell’s analysis), in the end the only way to guarantee predictable output for solar or wind plants is to pair them with other power generators that can be turned on and off at will.

Today, these “other” power generators typically are gas-powered plants, which are fairly cheap to build and can be turned on and off quickly. However, there is a cost to that…