Energy storage technology might be moving from a nice-to-have addition to solar and wind installations to a component that's necessary for project approval, if developments in California are any indication.

Solar developer BrightSource Energy and its utility partner Southern California Edison (SCE) may face rejection for two of five proposed power purchase agreements because they are too expensive and don't include a plan to store the power, reports GigaOM.

The California Public Utilities Commission (CPUC) sites energy storage in its recommendation for a "no" vote for BrightSource's planned Rio Mesa project, but three other projects that have  energy storage are likely to get the greenlight when they are voted on this month.

"These projects incorporate molten salt storage capacity which will allow SCE to optimize generation from these facilities based on changing system requirements. This unique attribute decreases renewable integration risk and provides more value for ratepayers," says CPUC staff in its recommendations report. 

The intermittent nature of solar and wind is challenging, because it means these projects can't send electricity to the grid as consistently or reliably as fossil-fuel or nuclear sources. Energy storage technology levels the playing field by helping generators store power so that demand can be better balanced.

BrightSource is no stranger to the potential of energy storage: its deal with SCE to add SolarPLUS energy storage technology to its project in the Mohave Desert is described as the "largest solar storage deal in the world." It will help the company forego building a 200 megawatt (MW) plant at that site. 

China's BYD Company built the world's largest battery energy storage for a utility scale combined wind and solar project in China's Zhangbei, Hebei Province. The $500 million battery bank (the size of football field) provides 36 megawatt-hours of storage for the associated 140 MW project. 

The Department of Energy (DOE) plans to invest $120 million in energy storage technology for renewable energy and sustainable transportation starting with $20 million this year.

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