Thanks to generous feed-in tariffs and other incentives, Germany’s wind and solar installations have soared over the past two decades to the point that renewables could cover half of the country’s electricity demand. But given technology and market conditions, a substantial portion of renewable power, primarily wind, is pushed to other European countries at peak times as baseload coal must stay online.

“They are donating all of this electricity they paid highly for in subsidies to other countries, often with negative prices,” said Jussi Heikkinen, director of growth and development for the Americas at Wärtsilä, a Finnish company that manufactures battery storage and flexible gas power plants.

One of the primary reasons Germany cannot use all of the renewable energy it produces is because the nation’s fleet of coal and nuclear plants is still needed to ensure power system resilience when the wind slows or clouds cover the sun.

“Coal plants can only ramp down to minimum load, which is typically 50 percent of full output, and that is where you stay,” said Heikkinen. “It takes days to stop and restart a coal plant, which means you can’t manage weather variations.”

An emerging problem the world over

California faces similar challenges because of its nearly 26 gigawatts of solar. “Something has to fill in the gaps as solar ramps up and down, and to cover the full load when the sun sets at night,” said Joseph Ferrari, general manager for utility market development at Wärtsilä North America.

Today, California utilities primarily rely on combined-cycle gas turbines to fill in the gaps when solar generation sags. While gas is far more flexible than coal, Ferrari notes that most combined-cycle gas turbines take between one to four hours to start and then must run for four to six hours before shutting down again. He added that instead of shutting down, many combined-cycle plants are ramped down to minimum load for extended periods, which is about 40 percent, and kept online, burning gas, waiting for when they are needed.

But increasingly, California and other places with high penetrations of renewables will need truly flexible gas power plants. That is because when inflexible combined-cycle plants run at minimum load, they use much more gas to produce a megawatt-hour of electricity than when on full output, thereby producing more CO2 emissions while adding fuel and maintenance costs for every stop-start cycle.

Ultimately, the challenge faced by California and Germany with high-level penetrations of renewables is one that many federal and local governments must grapple with as they pursue policies in an attempt to mitigate the most dangerous impacts of climate change. But 100 percent clean energy targets vary widely among the 13 states and 200+ cities and counties that have adopted them in the U.S., and even more so for the myriad targets that have been adopted globally.

However, these varying definitions impact everything from the costs of making the transition to the future reliability of the power system as a whole.

“Some ask for 100 percent renewables with no storage because wind and solar are getting so cheap you can overbuild and always have enough to go around,” said Ferrari. “Other scenarios allow for storage, hydro, nuclear and renewable fuels.”

Storage is not enough

Discussions of 100 percent clean energy pathways are a subject of intense debate and controversy given the varying assumptions about technology and pricing that inform the plans. Wärtsilä defines the goal of 100 percent decarbonization as a carbon-neutral power grid with a combination of generation resources — the vast majority of which would be wind and solar — as well as ample amounts of battery storage, demand response and flexible gas power, which will ultimately run on carbon-free renewable fuels.

While Ferrari agrees that storage will play an increasingly important role in any effort to achieve carbon-neutrality — particularly as the costs of energy storage continue to plummet — economic and technical challenges remain.

“If you look across storage [technologies such as] lithium-ion and pumped hydro, there are few commercial projects with a duration of more than 12 hours,” he said. “It’s not economical today.”

But technology is available today that makes it possible to quickly dispatch flexible gas whenever solar and wind generation ramps down, and even to provide power for long periods of time.

“These power plants can be at full power in five minutes or less, and once started you can shut down instantly and restart again five minutes later,” said Ferrari. “Flexible gas power allows you to absorb more renewables from hour to hour and day to day, and [it] allows...very rapid ramping. This enables minimization of use of fossil fuels and maximized reduction of emissions.”

Hawaii already has plans to achieve 100 percent clean energy by 2045. To meet that goal, the Hawaiian Electric Company is looking at flexible plants that can use renewable biofuels as a way to maintain system reliability under worst-case weather conditions.

“They did a comprehensive weather analysis based on Hawaii’s history and found that they had experienced a full week without almost any renewable generation,” said Ferrari. “The amount of storage needed to cover such extended unusual weather conditions would be off the charts and doesn’t make any economic sense.”

Flexible gas assets, however, can just be used when necessary and at at a far lower cost.

Renewable power-to-gas comes of age

Hawaii is not the only region exploring the power-to-gas concept. For example, the National Renewable Energy Laboratory and SoCalGas launched a new bioreactor in 2019 that turns renewable power into renewable methane.

Power-to-gas involves using excess renewable power to manufacture carbon-neutral synthetic methane. That methane can then be liquidized, transported using existing LNG carries, and fed into the existing natural-gas networks. This fuel is carbon-neutral and can be used in flexible gas generation in place of natural gas.

Historically, combined-cycle gas power plants were run continuously, with a focus on achieving the highest possible generation efficiency to reduce the cost of generation. In the future, gas power plants will only run when renewable output dips, and factors such as start and stop times become more important than the full output efficiency, said Ferrari. Utilities investing in inflexible gas generation today could find themselves with stranded assets as renewables penetration levels increase rapidly.

This outcome is exactly what Wärtsilä found in 2014 when it modeled the impacts of replacing most of California’s planned new-build inflexible gas power with truly flexible gas power. In the past decade, Wärtsilä has done comprehensive modeling on more than 70 power systems, from utility-level to the grids of entire countries. Even though the modeling was based on California’s then-33-percent renewable portfolio standard, the annual impact of the technology switch was estimated at $1 billion in cost savings and carbon reduction of 6 percent when many of the planned combined-cycle plants were replaced with flexible gas plants.

More recently, Wärtsilä modeled two approaches to achieve a 100 percent carbon-neutral system for a utility in the Southwest U.S. One approach included a massive build-out of wind, solar and storage, which was based on one of the politically favored paths forward. The other scenario modeled included flexible gas power along with renewables and storage, albeit with the condition that no fossil-based natural gas could be burned after 2040 — instead utilities would be using renewable methane.

Even with renewable methane likely having a higher price than natural gas in 2040, the model showed the net present value of investments in wind, solar and storage over the next 20 years to be 40 percent higher due to the amount of overbuilding required to account for all weather conditions, noted Ferrari.

“The path forward toward 100 percent decarbonized power systems is long and requires the right decisions now,” said Heikkinen. “Luckily, we can study what has already happened in nations and regions with high shares of renewable generation. This helps us avoid the mistakes made in the past when there was nobody to copy.”