Historically, the vast majority of the world’s power has been consumed as quickly as it is made, or it's wasted. But climate change has made governments interested in renewable energy, and renewable energy is variable—it can't be dispatched on demand. Or can it? As research into utility-sized batteries receives more attention, the economics of adding storage to a grid or wind farm are starting to make more sense.

But grid-tied energy storage is not new; it has just always been limited to whatever resources a local power producer had at the time. Much like electricity production itself, storage schemes differ regionally. Power companies will invest in batteries that make sense on a local level, whether it is pumped storage, compressed air, or lithium-ion cells.

Looking at the kinds of storage that already exist is instructive in helping us see where storage is going to go, too. Lots of the latest battery projects merely build on engineering that has been in service for decades. To better see our way forward, we collected a number of images and diagrams of the world’s biggest energy storage schemes.

Pumped storage

Pumped storage is possibly one of the oldest forms of modern grid-tied energy storage, and it certainly packs the most punch as far as megawatt-hours delivered.

The way it traditionally works is simple: the system has a bottom reservoir of water to draw from and a top reservoir that's topographically higher than the bottom reservoir. When there's not a lot of demand for electricity, you use that power to "charge" the battery by pumping water up to the top reservoir. When demand for electricity is high, that reservoir can be drained via a hydroelectric generator, back down to the bottom reservoir.

In the future, Germany is looking at using old coal mines for pumped storage, and some German researchers have been working on building giant concrete spheres that can function as pumped storage containers after they're placed on the ocean floor.

Compressed air energy storage

Compressed air energy storage, or CAES, is a lot like pumped hydro energy storage, except power producers use electricity during periods of low demand to pump ambient air into a storage container instead of water. When electricity is needed, the compressed air is allowed to expand and used to drive a turbine to generate power.

According to the Energy Storage Association, since air heats up as it's compressed, that heat has to be removed from the high-pressure air before it's stored. Then that heat has to be added back to the high-pressure air as it's released. This is done via a generator (usually a natural gas generator) or in a more environmentally friendly way using heat saved from the storage process in an adiabatic CAES system.

Although compressed air energy storage schemes have been discussed for decades, the expense of building storage facilities means there are only a handful of deployed systems and a slightly larger handful of test systems.

On the cutting edge, Canadian company Hydrostor is working to build bigger adiabatic compressed air systems in Ontario and Aruba.

Molten Salt Thermal Storage

Molten salt can retain heat for a long time, so it's generally found in solar thermal plants, where dozens or hundreds of heliostats (large mirrors) use the heat from sunlight to create energy. In some plants, sunlight is directed toward a large central thermal tower that heats up quickly and boils a working fluid inside. In other plants, pipes full of fluid run in front of parabolic mirrors, and the fluid heats up in those pipes. Either way, that heat can be used immediately to drive a steam turbine, or it can be transferred to molten salt, where the heat can be stored for hours. This helps solar plants extend their working hours and provide electricity well into the evening.

On the horizon, molten salt seems to have a clear future. Researchers have been looking into perfecting molten salt batteries for a variety of uses, and just recently, SolarReserve announced plans for a solar thermal plant in Chile that would run for 24 hours a day thanks to a massive molten salt storage area.

Some companies are dreaming up ways to use molten salt energy storage without the need for solar energy, too. Bloomberg recently reported on a molten salt energy storage scheme from Alphabet's X lab, which would use cheap electricity to heat up molten salt and cool antifreeze. When energy is needed, the process reverses to combine streams of hot and cold air that can push turbines.

Future systems may not use molten salt, either. Researchers from Georgia Tech recently built a ceramic pump that could move liquid metal at very high temperatures. Swapping super-hot liquid metal for molten salt could make this kind of energy storage more efficient.

Redox Flow Batteries

Redox flow batteries are huge batteries that charge and discharge through reduction-oxidation reactions (hence, redox).They usually involve giant shipping containers full of electrolytes, which flow into a common area and interact, often through a membrane, to create an electrical charge. Vanadium electrolytes have become common, although zinc, chlorine, and saltwater solutions have also been tried and proposed.

Although flow batteries are much lower density than the lithium-ion batteries most of us are familiar with, their drawbacks aren't disqualifiers in a grid-tied situation. Their unwieldy size and weight aren't a problem because utilities will never have to move them, and flow batteries generally have a long service life and few combustible materials in them, according to Sumitomo Electric, a Japanese technology company. Furthermore, you can always increase the capacity of flow batteries by simply adding more tanks.

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Sumitomo



Sumitomo Energy

There are few flow batteries currently on the grid, but there are several plans in the pipeline. The largest planned flow battery that we know of to date is being built by Chinese corporation Rongke on the Liaodong Peninsula. That battery will be a 200MW/800MWh system, expected to be completed by the end of 2018.

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