Is 100% Renewable the Best Way to Decarbonize?

Scientists broadly agree that humans must cut greenhouse gas emissions to mitigate the effects of climate change. But they definitely do not agree on how to do that. An example of how extreme these disagreements can become is the debate between Professor Mark Jacobson of Stanford and Dr. Christopher Clack of Vibrant Clean Energy regarding the feasibility of a future grid consisting of 100 percent renewable power.

In 2015, Jacobson and colleagues published a paper in the Proceedings of the National Academy of Sciences (PNAS) arguing that a future grid powered solely by wind, water and solar (i.e., 100 percent renewables) was possible with a massive amount of energy storage and demand response. They also claimed “business costs” would be similar to one based on fossil fuels and “social costs” would be 40 percent cheaper when factoring in externalities of health and climate.

In 2017, Clack and colleagues published a rebuttal in PNAS calling the Jacobson paper “a poorly executed exploration of an interesting hypothesis” and claiming it contained modeling errors, implausible assumptions, and insufficient power system modeling. For example, Clack said Jacobson assumed a future amount of hydroelectric power 15 times higher than the actual installed capacity.

Mark Jacobson responded by suing PNAS and Chris Clack for $10 million. The lawsuit argued the Clack paper caused him “significant undue damage,” and claimed the supposed hydropower error was actually an assumption regarding future gains due to improved turbine efficiency. The lawsuit shocked some in the scientific community, who believed that such disagreements should be debated in the academic literature, not courts of law. In February Jacobson dropped the lawsuit.

Around the same time a new contribution by Shaner and co-authors reiterated the challenges of a 100 percent renewables grid. The authors analyzed 36 years of global, hourly weather data to model the variability of wind and solar resources over continental length scales. They concluded that an 80 percent renewables penetration would either require a US-wide transmission grid or enough storage to hold all of the electricity used in the US for twelve hours . And that’s for 80 percent renewables. To get to 100 percent renewables, the United States would need a combination of building more renewable generation than is theoretically needed (capacity overbuild), ways of controlling electrical loads such as demand response, and weeks worth of energy storage.

Altogether, these analyses indicate a grid powered by 100 percent renewables is theoretically possible, but would be very expensive. Is that the right way to go? Or do low-carbon technologies that can be dispatched anytime as needed, such as nuclear and fossil fuels with carbon capture and storage (CCS), have a place in a future grid?

Most of the recent news for nuclear and CCS has not been positive. Several nuclear plants are facing premature retirement because they cannot compete with cheaper sources of electricity like natural gas. The V.C. Summer nuclear plant project in South Carolina was halted due to cost overruns associated with the size and technical complexity of the project and the bankruptcy of contractor Westinghouse Electric, after 10 years and $9 billion that had already been spent. As a result, ratepayers are now paying an extra $27 per month for a large slab of concrete. And a large CCS project in Mississippi, the Kemper County plant, was completed three years late and $4 billion over budget. Although that plant was supposed to run off gasified coal, persistent technical issues caused plant owner Southern Company to switch to running off natural gas instead.

Meanwhile, a request for proposals from Xcel Energy in Colorado resulted in shockingly low costs for renewables, such as 1.8 cents per kilowatt-hour for wind power and 3.6 centers per kilowatt-hour for solar plus storage.

In the near-term, it’s no contest—renewables and energy storage technologies will be adopted more broadly thanks to their falling prices. The trickier question is the long-term. As renewable penetration increases, the economics and grid dynamics could shift in subtle ways making the dispatchability of nuclear and CCS more valuable than it is today. Jenkins and Thernstrom reviewed decarbonization scenarios and concluded that having nuclear and CCS in the mix “would significantly reduce the cost and technical challenges of deep decarbonization.”

Plus, not all recent nuclear and CCS news has been negative. New nuclear concepts such as small modular reactors could overcome some of the barriers that have plagued the deployment of traditional designs, though there are still concerns about cost and safety. Another large CCS project, the Petra Nova plant in Texas, opened on time and on budget. And a promising new technology to capture carbon dioxide, the Allam Cycle, is being built at the 50 megawatt NET Power plant in Texas.

In the meantime scientists are still working to understand the impact of 50–100 percent renewable penetrations on the grid, especially considering that in 2016 renewable resources only accounted for 15 percent of U.S. electricity generation. Nevertheless, the idea of 100 percent renewables is gaining traction. According to the Sierra Club there are five cities currently powered by 100 percent renewables. The state of Hawaii and 53 other cities have committed to reaching 100 percent renewable power.

The issue of finding the best path to a clean energy future will continue to evolve — hopefully without lawsuits.

Image: Molgreen, Wikimedia Commons (https://commons.wikimedia.org/wiki/File:20140823_xl_solaranlage-zw-ruedersdorf-fredersdorf8534.JPG)