Researchers in the US have been taking a fresh look at the challenges of building a power grid based entirely on renewable energy, but have found a number of unanswered questions.
The team of 17 power systems experts from the U.S. Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) and DOE’s Office of Energy Efficiency and Renewable Energy (EERE) developed an analysis, published in the journal Joule, that looks at the technical challenges achieving 100 percent renewable energy across the country.
“Our paper offers perspective drawn from real-world experience in deploying variable renewables, the literature, and our team’s experience studying these issues in detail over the past two decades at a variety of scales—from our 2012 national-scale Renewable Electricity Futures Study to our 2021 work,” said Paul Denholm, NREL principal energy analyst and lead author of the paper. “While our focus here is on the US power system, many of the issues addressed and lessons learned apply more generally to other regions—and these are complex, multidisciplinary challenges that will require a lot of collaboration among the research community to solve.”
“Technology type essentially establishes the definition of the word renewable—which can vary based on the parameters of a research study or the priorities of a community setting a renewable target or policy,” said Denholm. “Here, we distinguish between two general types of technologies: what we call variable technologies that depend on short-term weather conditions and typically use inverters, like wind and solar photovoltaics [PV]; and those that are less—or not at all—variable and typically use traditional synchronous generators, including hydro, biomass, geothermal, and concentrating solar power.”
“Our emphasis is on questions we think can be addressed through technology development and engineering, but we recognize that other topics are critically important—from siting considerations, to energy equity concerns, to policy, regulatory, and market design challenges,” he said. “We want to clear a path for resolving the technical and economic issues so that we can better address other complex aspects of the power system transition.”
The variable grid
“Variable resources are just that—variable—so they inherently fluctuate across various timescales,” Denholm said. “There’s what we call a diurnal mismatch between the timing of peak demand and when solar and wind generation are highest during the day, which we see in phenomena like the duck curve. Beyond that, there’s a significant seasonal mismatch between wind, solar, and demand patterns that is even more challenging to address.”
At current levels, renewable energy is cost-competitive with traditional generation sources in many regions of the US because the utility industry has been able to cost-effectively address the hourly and sub-hourly variability.
The other balance challenges are between power produced in the day and at night, and in the different seasons. This seasonal mismatch issue may require technologies that have yet to be deployed at large scale, with the costs and requirements are unclear.
The Inverter Challenge
The Inverter Challenge is similar to the Balance Challenge in that they both involve balancing supply and demand on various timescales. But the Inverter Challenge is different in that concerns are narrowly focused on a set of specific engineering considerations of wind and solar PV generation along with battery storage.
“Inverter-based resources have very different characteristics compared to synchronous generators, including a lack of physically coupled inertial response and, historically, a limited ability to provide large amounts of current under fault conditions,” said Ben Kroposki, director of NREL’s Power Systems Engineering Centre and co-author of the paper. “So, as we rely more on inverter-based resources, they will need to provide services currently provided by synchronous generators—which may mean changes in the way the power system is controlled and protected.”
The paper explores both the Balance Challenge and the Inverter Challenge in detail—including the significant unanswered questions that remain when it comes to getting close to or achieving 100 percent renewables at a national scale for all hours of the year.
“There is no simple answer to how far we can increase renewable deployment before costs rise dramatically or reliability becomes compromised,” said Denholm. “As far as the ‘last few percent’ of the path to 100 percent, there is no consensus on a clear cost-effective pathway to address both the Balance Challenge and the Inverter Challenge at the national scale.
“Studies have found no specific technical threshold at which the grid ‘breaks,’ and we can’t just extrapolate from previous cost analyses because, when it comes to the future, there are many nonlinearities and unknown unknowns—things we don’t even know we don’t know yet,” he added.
The researchers say additional research is needed to evaluate the suite of technologies that can help ensure renewable supply matches demand patterns across all time periods—and that we will need significant engineering and design to transition the grid from one that is dependent on synchronous machines to one that is based on inverters.
“The unanswered questions in our paper provide a research agenda for the analysis, technology R&D, and engineering needed to achieve cost-effective 100% renewable systems,” said Dan Bilello, director of NREL’s Strategic Energy Analysis Center and co-author of the paper. “Not only do we need new tools and data sets to advance future studies, but we need more uniform terminology and facilitated interaction between researchers and research organizations, especially across different fields.”
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