California electricity grid tests ‘green’ technology

If a cloud goes over one of the large-scale solar farms east of the Coachella Valley — or one of the desert’s thousands of rooftop installations — Jim Blatchford is going to know about it and have a pretty good idea of its impact on the electric grid.

Blatchford is manager of short-term forecasting at the California Independent System Operator (ISO), the nonprofit corporation that keeps an even flow of power pumping through the state’s transmission lines.

He is also the agency’s resident expert on the complex and increasingly critical technology of predicting solar power output, with its sudden peaks and drops, so it can be smoothly integrated into the ISO’s electricity supply.

“If we’re expecting 250 megawatts, and a cloud comes over, it may drop in an hour to 200 megawatts,” he said. “We’re trying to forecast for these clouds coming over or forecast what kind of day it’s going to be.”

With California leading the nation in solar installations, the state’s grid has become a testing ground for the technologies that will allow it to handle a power load made up of 33 percent renewables by 2020 and even more in the future.

The state has about 5.3 gigawatts of solar power online, with rooftop installations accounting for more than 2 gigawatts of the total. By 2020, that total is expected to hit 12 gigawatts. A gigawatt is one billion watts.

The ISO already uses complex computer systems to balance power supplies and demand on a real-time basis for millions of consumers across the state. It began using newly developed programs to forecast the output of large-scale solar plants in 2011, Blatchford said.

It is also pioneering systems that will give it a better picture of the power being generated by rooftop installations, what is called “behind the meter” electricity that it cannot track as it does the energy from power plants.

“We’re trying to figure out how to measure that,” he said. “We’ve had an economic shift and with the boom in solar power going on, we’re trying to figure out how much there is, how much reserves do I have to carry. What happens if all of a sudden there is a frequency collapse because all of these rooftop panels could go out.”

Mike Taylor, director of research at the Solar Electric Power Association (SEPA), an industry trade group in Washington, D.C., sees intermittent solar as an additional layer of risk for utilities and grid operators, such as the ISO.

The development of forecasting technology by a small but growing number of companies is part of a shift in how utilities and grid operators are thinking about solar, he said. A recent report on solar forecasting from the association listed 17 companies across the country working on or already providing forecasting services.

Effective forecasting can have a major impact on transmission and utility costs, reducing the need to rely on expensive back-up power from natural gas “peaker” plants, the report said. The extra costs for integrating renewables were estimated at between 14 cents and 67 cents per megawatt-hour versus $28-$29 per megawatt-hour for fuel costs.

The report also singled out the ISO’s forecasting initiatives as one of handful of leading-edge case studies in the nation.

“Utilities have long treated solar as load reduction,” Taylor said. “It influences load; it’s not something they’ve treated as a generation source when they do resource planning.

“Now we’ve reached a level of penetration where we have to change that perspective. It’s starting to matter more,” he said.

While California utilities and the ISO are more familiar with wind forecasts, solar forecasts are more complicated, said Jan Kleissl, associate director of the Center for Energy Research at UC San Diego.

“To forecast better, we sort of have to put an unusual emphasis on clouds. Wind people are more interested in wind speed, but with solar forecasting, you absolutely need to have a good cloud forecast,” said Kleissl, who has worked with the ISO.

Using “an especially good camera that takes images of the sky and roads and mountains, you figure out where the clouds are and where they’re moving,” he said.

Even small wispy clouds can have an impact on solar output for some solar plants, Blatchford said.

Predicting the flow

For large-scale projects, the ISO combines computer modeling with information from on-site weather stations, Blatchford said. The data goes to a forecasting service to produce power output predictions for the 24 hours ahead and ongoing hourly forecasts for each day.

Under new federal regulations aimed at improving forecasting and integration for intermittent solar and wind power, the ISO will switch May 1 to a system providing almost real-time forecasts every 15 minutes.

“We call it a persistence forecast,” Blatchford said. “What happened in the last 15 minutes is what will happen in the next (15 minutes). You can adjust for the trends you’re seeing. We can see that cloud coming, the speed and size of it. In the next 15 minutes, you’re not going to have power, and we can factor that in.”

This information is available to utilities, including Southern California Edison, which can use it to help balance power supplies on the grid.

Beyond the forecast information from the ISO, both Edison and the Imperial Irrigation District said they continue to evaluate the available systems for predicting solar output, but have yet to buy or use one.

Experts such as Taylor and Kleissl say the amount of solar power a utility has to have on line to make forecasting cost effective may vary from company to company.

The ISO’s pilot program for forecasting rooftop output uses systems developed by Clean Power Research of San Francisco. The company’s technology combines satellite weather data, available down to the square kilometer, with technical information on about 170,000 individual installations culled from California’s solar rebate program, said Jeff Ressler, president of Clean Power’s software group.

To qualify for rebates, homeowners have had to submit the specifications for their systems, including individual components such as panels and inverters.

The Clean Power program also allows utilities or grid operators to group rooftop installations in a specific area into “fleets” — industry-talk for groups of generation plants.

“When you have that fleet definition and have the ability to forecast, that enables utilities and grid operators to manage and think about the resources in ways they’re used to,” Ressler said.

The ISO now has about a year of rooftop forecasting data from Clean Power that it’s evaluating for accuracy and figuring out options for integrating it into its existing forecasting systems, Blatchford said.

“We can kind of look from the ground up and space down,” he said. “The better the forecasting, the more visibility for behind the meter. The governor wants to put in another 12 megawatts; visibility is the key. We’re working hard to make sure we get that.”

K Kaufmann covers energy and green technology for The Desert Sun. She can be reached at (760) 778-4622, k.kaufmann@thedesertsun.com or on Twitter @kkaufmann.

For more information

■ California Independent System Operator: www.caiso.com

■ The Solar Electric Power Association: www.solarelectricpower.org

■ Clean Power Research: www.cleanpower.com