Long-term storage (days to weeks) can support a reliable system with lots of variable renewable energy. What options are there for this?
A variety of longer-term energy storage options are under development, and investors are pouring R&D resources into alternative chemistry batteries, hydrogen and other so-called “power-to-gas” options. Current generations of lithium-ion chemistry batteries are typically installed today to provide about 4 hours of discharge at full capacity. They do not enjoy economies of scale, so, if you need 8-hours of discharge, it will still cost twice as much as a 4-hour system. Alternative chemistries including redox flow batteries, zinc-ion batteries, sodium-sulfur, ammonia, and a large number of others are under development.
Hydrogen has recently become a very hot subject again as planners envision it in several roles in a decarbonized economy of the future. First, it can be used to help decarbonize sectors of the economy that are difficult to electrify or shift to zero carbon fuels. Hydrogen burns without releasing greenhouse gas emissions, so it can be used in industrial processes like cement or steel manufacturing if costs continue to decline. Today, however, natural gas is far cheaper in most countries, so hydrogen cost reductions will be needed. Second, hydrogen can be a source of energy storage, and could be used to generate electricity when VRE is not available for long periods of time. The power-to-gas concept rests on using low-cost VRE in an electrolyzer to produce so-called green hydrogen gas. Blue hydrogen can also be produced from fossil fuels, where the carbon dioxide is captured and potentially stored or used for other processes.
Finally, other longer-term storage options exist including pumped storage hydropower, concentrating solar power, and compressed air energy storage.