Driven by the U.S. power sector's heightened focus on grid flexibility and the need to manage vast new volumes of variable renewable energy resources, a more dynamic class of power plant is emerging: wind, solar and fossil fuel facilities hybridized with battery storage.

Roughly 80 such power plants are operating and planned to come online across the United States in the next few years, totaling 11,628 MW, according to S&P Global Market Intelligence data. That includes 4,433 MW of generating capacity at 50 existing power plants backed by 367 MW of battery storage capacity, most of which came online in the past two years. In addition, project developers have proposed adding another 820 MW of batteries at 31 planned and operating power plants with 6,008 MW of generating capacity.

These recent and near-term hybrids are the vanguard of a trend that appears to be quickly gathering momentum. Current transmission interconnection queues in several states and recent bids in response to utility requests for proposals hint at a much larger pipeline taking shape. Moreover, the data does not include behind-the-meter storage tied to rooftop photovoltaic systems and other small-scale plants, which increasingly may be aggregated to offer wholesale power market services in the wake of a recent Federal Energy Regulatory Commission order.

Among the most recently completed projects is Florida Power & Light Co.'s new 4-MW storage system at its existing 74.5-MW Citrus Solar Energy Center in DeSoto County, Fla. Unveiled in February 2018, the NextEra Energy Inc. subsidiary's hybridized array harnesses sunlight for use later in the day, like other solar-plus-storage installations. Unlike others, however, it also includes so-called "DC-coupled" batteries that capture additional energy at peak sun by avoiding power losses in the conversion of direct current into alternating current for the grid.

Also announced in February, SunPower Corp. energized its 10-MW Redstone Arsenal Solar Project, which includes a 1-MW battery system. The project at the U.S. Army's Redstone Arsenal base in Alabama is designed to reduce the post's peak power demand charges and may become part of a future microgrid. SunPower is majority-owned by Total SA.

The two storage-backed solar projects followed E.ON SE's disclosure in January that it completed a pair of 10-MW battery storage systems at two existing wind facilities in Texas. Located at the 249-MW Pyron Wind Farm (Roscoe Phase 3) and 197-MW Inadale Wind Farm (Roscoe Phase 4), the Texas Waves storage projects "react quickly to fluctuations in the power grid and thus improve security of supply for customers," according to the German energy giant.

Edison International's Southern California Edison Co., meanwhile, is well on the way to hybridizing its entire gas peaker fleet with batteries. Since 2016, three of its five 49-MW natural gas peak power plants have added batteries. Two of the projects, at SoCalEd's Grapeland Generating Station (Peaker) and Center Substation Peaker, feature "hybrid electric gas turbine" systems from General Electric Co. The gas-battery hybrids use control software that enable them to operate as fully integrated facilities. Tesla Inc. has supplied two 10-MW storage units for the utility's Mira Loma Substation Peaker. Like most battery projects online and planned, SoCalEd's fleet relies on lithium-ion technology.

Overall, the batteries in the pipeline are supersized compared with those installed to date. Touted as the "largest battery storage facility in the world," AES Corp.'s gas-fired Alamitos Energy Center in Long Beach, Calif., includes plans for a 300-MW battery storage system. Canadian Solar Inc.'s RE Gaskell West project, a five-phase solar farm in Kern County, Calif., includes 125 MW of battery storage. Eversource Energy and Ørsted A/S have proposed 55 MW of batteries for their Bay State Offshore Wind project off the coast of Massachusetts. Pinnacle West Capital Corp. subsidiary Arizona Public Service Co. has tapped First Solar Inc. to supply power from a new 65 MW solar array with a 50-MW battery system.

Project developers are exploring facilities with even bigger batteries, U.S. grid operators' active interconnection requests show. As of Feb. 27, the California ISO's active queue contained 18 solar plants and three gas facilities with a combined 4,282 MW of battery storage. Eleven of those projects have a proposed storage capacity of 100 MW or greater. One solar project under study includes a massive 1,920-MW battery installation.

California is far from the only state with increasingly large-scale hybrid ambitions. In Arizona, seven solar-plus-storage systems, totaling nearly 3,350 MW, were active in APS' interconnection queue, as of Feb. 8, including a 2,000-MW project. Active requests in PJM Interconnection, as of Feb 27, include 10 battery-backed wind and solar facilities combining for almost 1,100 MW.

While projects in interconnection queues often are withdrawn rather than completed, major developers, utilities and independent system operators clearly are giving hybrid plants with ever larger batteries a closer look. With storage prices falling, economics is beginning to make a strong case. In response to a 2017 request for proposals, Xcel Energy Inc., for instance, collected bids for battery-backed wind and solar projects that were well below Lazard Ltd.'s recent levelized cost estimates for storage and conventional generation. Xcel received 11 bids for wind-plus-storage projects at an average price of $21/MWh, 87 bids for solar-plus-storage arrays at an average $36/MWh and seven bids for solar-wind-battery hybrids at an average $30.60/MWh, the utility revealed in December.

Federal tax reform and President Donald Trump's imposition of import tariffs on solar cells and panels, however, prompted Xcel to request new offers. The rebooted results may be disclosed as early as March 1, a utility spokesman said.