It is always hard to pinpoint the moment when a long-term trend begun, or when it shifted gears into something different. The modern wind and solar power era begun decades ago, when electric-generating wind turbines and solar panels were first made commercially, but I think we've entered a new phase a few years ago when wind and solar started being price-competitive with fossil fuels (with subsidies at first, and now without) and this made the number of megawatts of installed capacity positively explode. Last year, we saw that solar installed capacity had increased by 18.7X since 2000 and wind was up even more, and since then the numbers are even higher.
But with all this intermittent power coming to the grid - when you have a lot of wind and solar spread out over large areas, the variations tend to even out to a certain extent, but it'll never be entirely stable - we are seeing increased demand for grid-scale energy storage. I think we're in the early days of this transition from a grid without much storage to one that has a significant amount of buffer to absorb shocks, making it more resilient and flexible.
I think that an early milestone for that new era was when Elon Musk announced his intention to build a battery 'Gigafactory' that would make as many batteries as the rest of the world combined at the time it was announced, and then Tesla announced the creation of a Tesla Energy division, making it not only an electric car maker, but also a player in the grid-storage world. While the home storage solution called "Powerwall" received most of the media attention, Tesla expects most of its batteries to be sold to large commercial and utility clients (pictured below), and this is what will make the biggest difference for the grid.
While Tesla might be the most visible player in this nascent industry, there are many others who are rapidly ramping up their efforts. This includes GE, who in the past few months announced 39 MWh of battery storage projects, including a single one for 30 MWh, its largest yet, located in California’s Imperial Valley, approximately 100 miles east of San Diego. "The facility will aid grid flexibility and increase reliability on the IID network by providing solar ramping, frequency regulation, power balancing and black start capability for an adjacent gas turbine," says GE.
Another promising technology is liquid-metal batteries, as described here. These are less mature than lithium-ion batteries, but it they work as we expect, and can be commercially produced at a competitive cost, they could handle massive amounts of energy storage for the grid and help change the game when it comes to the use of renewables.
Note: The photo at the top of this article shows Portland General Electric's 5-megawatt lithium-ion energy storage system at the utility's Salem Smart Power Center in South Salem, Oregon.