Grid-scale energy storage, long the province of utility and government pilot projects, is primed for a push into the harsh world of energy economics pretty soon. We’ve seen a good deal of news on this front recently, including California’s decision last week to commit to becoming the first major economy in the world to mandate grid storage targets, with a required 1.3 gigawatts of energy storage by 2020.

That’s quite a lot, considering that even the biggest grid storage vendors, such as AES Energy Storage, are just now passing the 100 megawatts of installed capacity mark. While efforts like California’s may be necessary to create the regulatory and economic frameworks to jumpstart widespread grid storage development, it’s worth asking where worldwide grid storage capacity stands today, and how California’s goals stack up against those figures.

Luckily, the Department of Energy has been tracking global energy storage projects and putting them into a public database. This week, it announced that the running tally now includes some 420 projects in 34 countries around the world, adding up to 123 gigawatts of energy storage.

Before you get too excited about that figure, bear in mind that almost all of it consists of pumped hydro projects -- the decades-old technology of pumping water uphill into reservoirs, then letting it flow to spin turbines when grid demand is peaking. That's not a new technology, but it's limited in terms of geography, as well as by the tens of billions of dollars and decades-long construction schedules that are required to build it.

Certainly California's new energy storage mandate, which is aimed at incorporating intermittent solar and wind energy into the grid, doesn't allow big pumped hydro to fill its quotas. Instead it's asking for range of technologies, including multiple battery technologies (lithium-ion, advanced lead-acid, flow batteries, and more), as well as flywheels, compressed air energy storage, and thermal energy storage.

These systems range widely in the duration of their storage capacity, from hours-long backup systems meant to shift peak demand or store solar and wind power for use later in the day, to those with only minutes of energy capacity meant for fast grid-balancing or solar and wind intermittency management.

DOE's database does have a sizable amount of information on these fronts, breaking down more than 150 battery-based projects, as well as more than 50 thermal energy storage projects, in terms of rated power, duration, ownership model, grid interconnection type, and other key details.

On the downside, it lists several potential storage technologies, such as hydrogen energy storage, superconducting magnetic energy storage, and gravel energy storage, that don’t actually have any projects logged so far.

Here’s a breakdown of the global numbers by technology category -- along with some caveats that come with reporting on data that’s provided by individual parties to the DOE and is being vetted as it comes in.


According to DOE’s database, the combined capacity of battery-based grid storage either built or in planning stands at 480 megawatts across 156 projects around the world. That includes everything from small-scale, distributed backup power projects to multi-megawatt installations like the wind power battery systems installed by NGK Insulators in Japan, by Xtreme Power at Duke Energy’s 36-megawatt, 24-megawatt-hour Notrees project in Texas, or by A123 Systems at AES’s 32-megawatt Laurel Mountain project in West Virginia.

But the projects that are actually up and running add up to just under 300 megawatts, a figure arrived at by adding up the projects that are listed as “operational ” in the database, plus a few other projects that are known to be operational, but are not yet listed as such. Another 131 megawatts of projects are in the “contracted” and “announced” status category, while another 84 megawatts are in the “under construction” category.

And then there are several projects that have since been taken offline, such as Xtreme Power’s 15-megawatt Kahuku wind power storage project on Hawaii’s Oahu Island, which saw the warehouse in which the batteries were housed burn to the ground in 2012, or several of NGK’s Japanese sodium-sulfur battery installations that were shut down after reports of individual projects catching fire in 2011.

Battery fires are certainly noteworthy, and an indication of the kind of bad press that can linger long after problems have been addressed. Both Xtreme and NGK continue to operate many megawatts of battery projects, though NGK did shut down a number of its projects in 2011, only to restart them early this year.

Thermal Energy Storage

This category includes a host of technologies that don’t store electrical energy as batteries do. Instead, they use cheap off-peak electricity to chill water, make ice, or cool another storage medium, and then use that stored cold to help air conditioners or HVAC plants use far less electricity during the heat of the day, when grid power demand is at its peak.

There’s a huge amount of thermal energy storage out there, according to DOE’s database -- some 886 megawatts of it, to be exact. The biggest combined figures come from Ice Energy, the startup that’s aggregating thousands of its Ice Bear rooftop thermal storage AC units for California customers such as Redding Electric, Glendale Water and Power and Southern California Edison.

But there are also plenty of thermal storage projects that have been around for decades, ranging from megawatt-scale college campuses and municipal district cooling systems, to building-scale systems being installed by CALMAC for customers from New York City to Bangalore, India, or by DN Tanks for hospitals, military bases and university campuses across the Sun Belt.

The other, very different form of thermal storage in this category is molten salt storage for the massive solar-thermal projects being built in the U.S. Southwest and across the world in Spain, India and Israel. Solar-thermal developers such as BrightSource Energy, SolarReserve, Abengoa and others have contributed only about 788 kilowatts in such energy storage potential to date, according to DOE’s database -- but that number could grow, if competing methods for storing the sun’s heat can prove their commercial viability. (One other solar-thermal project in Morocco plans to store waste heat from a nearby cement plant at a scale of some 650 kilowatts.)

Compressed Air Energy Storage

By far the biggest share of this storage category’s combined 1.02 gigawatts of capacity is made up of the two CAES projects that have been around for decades: the 290-megawatt plant in Huntorf, Germany, built in 1978, and the 110-megawatt McIntosh, Alabama plant, built in 1991. Both use underground caverns to store compressed air, then release that air to help drive natural-gas-fired turbines at greater efficiency.

Finding massive underground caves or salt domes close to gas-fired power plants isn’t easy, though right now two other such projects are being proposed. One 317-megawatt project from Dresser-Rand and Apex Compressed Air Energy Storage is to be sited in Texas, and another 300-megawatt project is being studied by California’s Pacific Gas & Electric as part of a DOE smart grid stimulus grant. Both are expected to cost hundreds of millions of dollars, but to yield long-term energy storage at prices that no other storage technology besides pumped hydro can match.

Beyond that, DOE’s database contains several projects seeking to take CAES aboveground, including SustainX’s 1.5-megawatt project in New Hampshire and Highview Power Storage’s 350-kilowatt, 2.5-megawatt-hour pilot project that will use super-cooled liquid air as a storage medium.