Here’s the bottom line of this post, for those who don’t want to wade into the weeds: major reliance on wind and solar electricity generation demands a lot of electrical energy storage—many times what is currently available. For reasons why, we look at “load following” in California in the summer of 2016.
California’s Energy Program
The state of California is pushing ahead rapidly to achieve a goal of 50% renewable electricity power production by 2030.
This makes sense in California, since its terrain and climate are highly adaptable to both wind and solar generation. In-state conventional hydroelectric—which in many places currently accounts for the largest fraction of renewable generation— is not included in California’s ambitious program. That also makes good sense, since worsening and recurring droughts make hydroelectric an iffy proposition in the state.
However, the 50% goal will only make sense with abundant energy storage capacity, little of which is currently available. The reason is the intermittency of wind and solar. Solar, obviously, does not generate power at night, and not much in cloudy conditions, and wind power depends on weather.
“Load Following” and Fossil Fuel
For a graphic illustration of the importance of energy storage, see the following article on “load following” in the summer of 2016 (up to September). “Load following” is the practice of juggling output from both constant and variable power sources in order to meet demand. For a graphic illustration, see:
Load Following with Natural Gas in California
One feature jumps out at you immediately from the graph: at the peak of generation in the afternoon (summer average), “imports” and “thermal” account for about twice as much energy as “other renewables” (i.e. mostly wind and solar, and excluding conventional hydro, which is the dark blue band at the bottom). The “imports” and “thermal” make up the difference between demand and other supply—that is, they follow the load.
Thermal Means Fossil
“Thermal” is the term for heating water to make steam that drives turbines. In practice, in the United States, it means the burning of fossil fuels, with natural gas overtaking coal as the primary source. Gas is all to the good climate-wise, since natural gas emits half as much carbon dioxide per unit of energy as does coal. (Nuclear is also thermal, but California is giving nuclear the boot despite its near-zero carbon emissions.) The “imports” come from sources outside the state, and since those sources are largely thermal as well, 44% of California’s total power came from natural gas, and 6% from coal, in 2015. That can be seen in this table from the California government:
Table of total electric power in California
At present, the annual fraction from renewable energy (21.9%) is less than half that from fossil fuels. Still, if you consider how quickly renewable generation is expanding in California, the long-term trajectory for wind and solar looks very promising.
That’s where load following and storage come in, explaining why the California goal is no higher than 50% renewables by 2030—and, in fact, has been set no higher than 50% thereafter. See this report from the California government:
California Renewable Tracking
The problem of unreliability
Why just 50% in California, with its abundant sun and wind? Let’s go back to intermittency and its dance partner, unreliability. The fractions in the graph are averages, and do not reflect how many instances there were when solar and wind were providing little or next to nothing.
The range of thermal input to the system—2.6 gigawatts at 9:00 a.m. June 12th and 25.6 GW at 5:00 p.m. on July 25th—gives you a sense of just how variable the contribution of solar plus wind can be. On July 25th at the time of peak demand, thermal accounted for roughly 70% of total production. It also gives you a sense of how well natural gas follows the load, since natural gas generation is easily ramped up or down (although not without some loss of efficiency). Only hydroelectric beats it in this respect.
Storage, Storage, and More Storage
To get reliable electricity along with low greenhouse gas emissions, we need to solve the problem of matching supply with demand—”load following”—and to solve it without burning fossil fuels. Natural gas is a lesser-of-evils bridge to a renewable future, but it still emits plenty of carbon dioxide. Absent nuclear, or a major increase in the contribution of hydro and/or geothermal, massive storage is the only way to compensate for the unreliability of wind and solar. The greater the reliance on wind and solar, the greater the reliance on storage.
The race is on to produce large-scale energy storage on many fronts. It’s a formidable technological challenge, and is also a potentially huge money-maker—here’s an area where capitalism shines at problem-solving. If we reject nuclear, let’s hope the race gives us some winners soon. More on that in another post.
– Mark