Using more than about 30% renewable energy in the electric system requires major reengineering of the grid. At least according to conventional wisdom. Now, it appears, overbuilding solar capacity helps feed more than that into the existing grid.
The sun makes no electricity at night. It produces less in cloudy conditions than in full sunshine. The wind makes no electricity in calm conditions. Wind speed varies from time to time, and along with it how much electricity comes from a wind farm.
Utilities can now transform power from these sources into something firm and capable of providing power as needed. This “firming” of intermittent power makes renewables function much as traditional power plants always have.
Being able to rely on renewable energy depends in part on the ability to store it. But energy storage alone can’t make it workable.
Storage is becoming less expensive, but so are wind and solar power. Overbuilding generating capacity turns out to be more economical than storage. That is, it’s cheaper to build more solar and wind farms than to build storage capacity.
Utilities have always had to balance the supply of electricity with demand. They must reduce the output of a generator from time to time from what it would otherwise produce. The technical term is curtailment. Typically, the operators of the grid impose curtailment on the operators of an electric generator.
At first, with solar power being relatively expensive, utilities had to use all they got and curtail something else. That marginalized renewable energy. Now, overbuilding solar capacity makes both technological and economic sense.
A model for overbuilding and curtailment in Minnesota
The Walmart in Ponce, Puerto Rico is one of five Walmart facilities on the island equipped with solar panels.
A study published in the journal Solar Energy dismissed the idea that utilities should avoid curtailment except as a last resort. The same research forms the basis of the Solar Potential Analysis Report issued by Minnesota Solar Pathways.
Minnesota, a not especially sunny state, aims to get 10% of its electricity from solar power by 2025 and 70% from combined solar and wind by 2050.
The report recommends a grid strategy that allows overbuilding solar capacity and curtailing 20% to 40% of it at times. This tactic overcomes the intermittency of solar and wind. And it does so for less than current grid costs. The model requires storage capacity but minimizes it. Overbuilding and curtailment proved more economical.
Here’s the idea: build enough solar infrastructure to meet demand in the winter or other times of low solar resources. Then reduce their contribution to the grid whenever they produce excess electricity.
David Shaffer, executive director of the Minnesota Solar Energy Industries Association, has a caveat. It’s impossible to build overcapacity without some way to finance it.
Minnesota law now has solar projects built on the understanding that utilities will buy all they produce. Under a curtailment strategy, the utility would buy the solar energy it needs and curtail the rest.
Shaffer argues that would make it hard to finance solar projects.
California’s recent experience with curtailment
Photographer’s caption: The rails that will hold the solar panels in place are up, and we are wrapping up at the end of Day 1 of installing the panels on a low-income house in Oakland, California.
California intends to have 60% renewable energy on its grid by 2030, compared with 34% in 2018. By 2045, it intends to abandon fossil fuels completely. That plan can’t succeed if electric rates skyrocket in the process. The state already has some of the highest rates in the country.
Traditionally, utilities use baseline plants to supply most of the power. Baseline plants run constantly. When demand exceeds what these plants can supply, the utilities turn on peaking plants for a while.
Peaking plants are less efficient and more expensive to run than baseline plants. They make electricity more expensive at peak periods. With overbuilding and curtailment, utilities can bring more panels online instead.
In the last week of May 2019, California set two solar energy records. It had more solar power flowing through its electric grid than ever before. And it curtailed more solar power than ever before.
At about 1:00 PM on May 27, solar farm operators shut off almost 40% of the state’s installed solar capacity. The move set a record total of almost 4,700 megawatts.
California’s array of solar farms and grid-tied rooftop solar panels often generates more electricity than the grid can use. And that week, it did so at the same time the grid used a record amount of solar power.
In 2018, curtailment of solar generation overall amounted to less than 2%. This year it may reach 4%. Curtailment becomes a waste instead of a strategy if it approaches 10% overall.
California can take additional steps to reach its renewable energy goals. Build large storage projects, for example. It can also share electricity with neighboring states. Perhaps most important, it can structure rates to encourage customers to use electricity at the most opportune times of the day.
How not to waste renewable energy not needed by the grid
Solar panels over a parking lot at Arizona State University
Many utilities use a time-of-use rate structure. They charge more during times of peak demand. This structure encourages customers to shift energy usage to off-peak times.
Under California’s curtailment plan, consumers need to use more power when the sun is shining brightest. Peak demand for air conditioning happens then. That’s when fossil-fuel-based utilities need their customers to hold off. It looks like overbuilding and curtailment fit better with people’s habits.
So if the electric system relies on overbuilding solar capacity, what will happen all that electricity?
Curtailment doesn’t have to mean wasting renewable energy, however. Electric vehicles are becoming more and more common. Development of hydrogen fuel cells also continues. And it takes electricity to produce the hydrogen.
Theoretically, curtailed solar energy grid can fuel these cars. But drivers need to fuel them at the times when excess generation reaches its peak. Little infrastructure yet exists for fueling electric vehicles. So it ought to be possible to design a system to take advantage of overbuilding and curtailment.
Automakers have already developed a vehicle to grid communication standard. Electricity can flow in either direction between the car and the grid as necessary. A critical mass of electric vehicles, in other words, can help the grid balance the supply of solar power with the demand.
What about solar parking lots?
Shading pavement with solar panels and parking cars under them seems like an environmental no brainer. Parked cars don’t become ovens in the shade. Technological barriers don’t hinder solar parking lots. Financial and regulatory hurdles appear to be more significant.
Overbuilding solar capacity and curtailing the output seems counterintuitive. We only need to work out the necessary business models. They will help us move from fossil fuels to renewable energy entirely.