Recent studies in the United States, such as the Los Angeles 100% Renewable Energy Study (LA100), have made a case that 100% renewable power grids are feasible. With the Biden administration pledging to cut down the country’s greenhouse gas emissions by 50% by the end of 2030, the country is set to devising ways to build a critical infrastructure to produce and deploy clean technology. Add to that the plummeting wind and solar costs, and the U.S. seems to be poised to deploy significant volumes of renewables. Several U.S. cities, states, and corporations have already set their individual targets for 100% renewable energy.

In a fresh analysis, a team of 17 power systems experts from the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) and DOE’s Office of Energy Efficiency and Renewable Energy (EERE) has highlighted a critical aspect of the ambitious goal. Funded by EERE’s Office of Strategic Analysis, the research suggests that expanding this 100% renewable energy end-goal across the U.S. presents an equally expansive set of challenges – and the plausibility of doing so has been a topic of vigorous debate among the energy research community in recent years.

‘The Challenges of Achieving a 100% Renewable Electricity System in the United States’ offers important insights into the technical and economic challenges that would need to be overcome to achieve 100% renewable electric power across the U.S.

“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 on LA100,” said Paul Denholm, NREL’s Principal Energy Analyst and lead author of the paper.

The authors of the paper began with explaining two key aspects of a 100% renewable-powered grid: technology type and system boundary.

According to Denholm, 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. When it comes to defining the system boundary, the authors require the grid to physically operate with a 100% renewable energy supply at all times.

Rather than focusing solely on the end goal of a 100% renewable grid, the team looks at how the challenges of incorporating renewables change with increasing deployment. Robust 100% renewable solutions must consider how to optimally use existing power system assets, stressed the researchers.

Within this framework, the report organizes the techno-economic challenges of achieving 100% renewables across all timescales into two categories: 1) economically maintaining a balance of supply and demand (referred to as the Balance Challenge) and 2) designing technically reliable and stable grids using largely inverter-based resources like wind and solar (referred to as the Inverter Challenge).

The Balance Challenge

The team elaborates that the Balance Challenge boils down to making sure the power system can economically balance supply and demand at a variety of timescales – from the critical seconds-to-

minutes scale required to withstand unexpected outages to the seasonal scale that matches scheduled power plant outages and maintenance with periods of lower demand.

“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,” he added.

This chart from the paper conceptually illustrates the Balance Challenge in terms of how expected costs and challenges may change with the increasing deployment of renewables. At current levels, renewable energy is cost-competitive with traditional generation sources in many regions of the U.S. because the utility industry has been able to cost-effectively address the hourly and sub-hourly variability.

The researchers point out that beyond these levels, we reach the second zone, where studies have explored how the diurnal mismatch problem might be cost-effectively addressed to reach annual contributions in the range of 80% renewables. But beyond this point, in the third zone, the seasonal mismatch issue may require technologies that have yet to be deployed at large scale – so their 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, as opposed to the broader economic issues associated with the Balance Challenge.

The Inverter Challenge is all about issues associated with transitioning to a grid dominated by inverter-based resources (IBRs)—primarily wind and solar PV generation, along with battery storage.

Technology Needs

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% renewables at a national scale for all hours of the year.

The authors 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.

Realizing a high renewable electricity future for the United States will require more than just addressing the Balance and Inverter Challenges—including addressing resource access, environmental, market, and human behavior issues that themselves can affect the design and pace of getting to 100% renewable electricity.

“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.

The authors further pointed to a need to continuously re-examine the most effective pathway toward national emissions reduction and decarbonization goals – whether that is through 100% renewable electricity or another combination of low-carbon technologies.

The LA100 study, although not on a national scale, found that electrifying the vehicles and buildings sectors could lead to substantial improvements in air quality – and that realizing these benefits is principally a matter of achieving high energy efficiency and electrification, independent of any particular renewable energy pathway for the power sector. LA100 also found that technology restrictions result in higher costs when it comes to meeting the last 10% of electricity demand with renewable energy.

In September last year, the researchers at NREL had discovered new materials for high-efficient solar cells of the future, focusing on a carefully designed molecule that can efficiently split the energy given away by one photon into two excited states.

Mercom had earlier reported that researchers at the NREL recently conducted the first global assessment of various approaches used to manage solar PV modules at the end of their life spans.