Grid Scale Energy Storage Deployment by Technology Type: Part 1

What's getting built in the next decade?

A lot of people think the 2020s are going to see a “clean energy transition” where as much as half of the world’s electricity will start coming from solar and wind power by 2030.

Is that true? And if so, what is it going to look like? And are there any underrated opportunities in the space?

Well, if you look at trends in solar and wind power generation, the world is absolutely on track for a clean energy transition this decade.

If you look at trends in deploying energy storage for grid scale applications, it looks like we’re behind.

Intermittent power sources like solar and wind need to be complemented with energy storage, because there’s demand for power even when the sun doesn’t shine and the wind doesn’t blow.

How much energy storage (in megawatt-hours) do you need for a given amount of solar or wind power (in megawatts)? Well, it’s a complicated function that depends on things like location, demand curves, type of power plant, and so on, but all the estimates I could find suggested that we’d need several terawatt-hours of storage globally by 2030 if half of electricity generated in 2030 was from solar and wind.

And the current slate of completed, in-progress, and planned projects reported in the news between 2020 and 2022 adds up to less than a terawatt-hour by 2030.

There’s been a lot of recent growth, but it doesn’t look like it’s going to be enough unless something changes.

So what could be helpful to change, to accelerate energy storage deployment in the 2020s?

In the last post I suggested that there were policy changes that could make it easier to build energy storage. (The big ones that US industry groups advocate are simplifying and speeding up the process of getting permits to build and connecting to the electric grid.)

Another angle on the strategy question is looking more in depth at individual energy storage technologies.

• Which types of storage are ready for deployment at all?

• Which types of energy storage are being deployed at the largest scale?

• Is there any type of energy storage that’s “underrated” and deserves more attention or investment?

Metrics and Meanings

A grid scale energy storage system (ESS) is usually reported as having a maximum power output (measured in megawatts, MW), and a storage capacity (measured in megawatt-hours, MWh.)

Power output, of power plants or energy storage systems, is the primary thing we care about from the point of view of electricity consumption. If you want to know “how much stuff, how many lightbulbs and air conditioners, can be run off of this energy source?” you care about power, and you’re looking at megawatts.

Storage capacity, measured in megawatt-hours, is a measure of how much energy can be stored in the batteries (or other energy storage technologies).

The whole point of having energy storage is to smooth out fluctuations in demand or supply. If you want to store solar energy during the day to release at night, you’ll be powering homes and businesses off of the ESS for several hours each night, so you need the ESS to be capable of outputting close to its maximum power for eight hours or more.

This is what “long duration storage” is all about. Typically, lithium-ion batteries can’t release their maximum power for more than 4 hours at a stretch. To get a solar plant to provide all the power for a municipality, it needs to be combined with enough energy storage (in MWh) to cover demand overnight, and also needs to have that storage in the form of technologies that can release enough power for long enough to last through the night.

Even longer duration storage, on the scale of months, can compensate for seasonal fluctuations in sun or wind. Those would need to be able to supply maximum power for months at a time, if the intermittent renewables are not enough to meet demand during the “off-season.”

Arithmetically, an ESS that can output 50 MW for 10 hours is producing 500 MWh of energy storage.

But an ESS that holds 500 MWh of energy storage and is rated for 50 MW is not necessarily able to output 50 MW for 10 hours at a stretch; how long the power can last will depend on the specifics of the technology.

So “how much energy storage do we need, for a given target amount of power generated by renewables?” is a complicated question.

What you actually have are output curves, as seen in this figure from Ziegler et al, 2019:

So, for instance, a baseload solar power system (leftmost column) needs to have enough storage to meet demand for about 12 hours per day, while it’s dark; so a purely solar baseload system that produces 50 MW at all times would need an ESS that’s rated for 50 MW and stores (almost) 600 MWh of power and can run for 12 hours at a time.

The area above zero and under the red or blue curves is the amount of energy storage required; as you can see, that’s higher for baseload systems than intermediate, peaker, or bipeaker systems, and much higher for solar than wind.

There’s not one single measurement that determines whether an ESS has “enough” storage to allow the electric system to produce a given power output; you need to know the ESS’s power output, its energy capacity, the duration of its max power output, and the shape of the power demand and supply curves.

So, does it make more sense to measure the “amount” of energy storage deployed in MW, or MWh?

It looks like the answer is “we need both figures and more.”

What Are The Leading Grid-Scale Energy Storage Technologies of The 2020s?

I’ll return to my dataset of Energy Storage News articles to show the breakdown of projects that have been planned for the next decade, drawn from articles between January 1, 2020 and May 5, 2022.

The dataset includes energy storage projects that have been announced for a specific site and a specific scale (power rating in MW and/or capacity in MWh.). Not all of these planned projects are necessarily going to happen. Some are quite early in the planning phase. But I only include projects that are far enough along in planning that they have an intended site to build on; that excludes announcements by companies or governments setting targets that they want to add so many megawatts of grid-scale storage by 2030.

The dataset reports 78,389 MW of total power output in grid-scale energy storage projects with expected or actual completion dates between 2020 and 2030.

Here’s the planned power output broken down by energy storage technology:

Most of that comes from lithium-ion batteries and pumped hydro storage. While the vast majority of existing grid-scale energy storage is pumped hydro, recent and near-future energy storage projects are mostly lithium-ion batteries.

If you count by number of projects rather than by total power output, lithium-ion is even more dominant; 78% of projects in the dataset were lithium-ion battery systems, and only 6% were pumped hydro projects. Pumped hydro storage systems are generally very large, while lithium-ion battery energy storage can scale down all the way down to Powerwalls that store only enough energy for a single solar-powered house.

If you look at total capacity by energy storage tech, some surprises come up. Green hydrogen and thermal hydro suddenly are a big part of the world’s total planned energy storage!

This is entirely due to two very large capacity storage projects: the Advanced Clean Energy Storage Project in Utah, which will store 300 GWh of “green hydrogen” in giant underground caverns and use it to run an 850 MW power plant, and the Vantaa Energy Cavern Thermal Energy Storage project in Finland, which will store 90 GWh of hot water in giant underground caverns and use it to heat homes in winter.

Are these two huge underground storage projects irrelevant outliers, or could building more such projects help the world meet its energy storage needs?

And what about more exotic, less common energy storage technologies, like compressed air, vanadium redox flow, thermal molten salt, or others?

Let’s go through technology by technology, in decreasing order of total power deployed.

Lithium-Ion Batteries

• Planned Deployment: 267 projects, totaling 54 GW and 108 GWh

• Cost: NREL reports a total system cost of $345/kWh in 2020

• Duration: 4 hours at max power

• Round Trip Efficiency: 95%

• Cycle Life: 3000 cycles at 80% discharge

• Energy Density: about 100-200 Wh/kg, depending on specific chemistry

Lithium-ion batteries are the most common type of new grid-scale energy storage construction.

Leading battery manufacturers for grid-scale applications include Tesla, LG, CATL, and Saft.

Packaging batteries into energy storage systems (which involves a lot of software for managing and monitoring power output) is a separate industry; some leading BESS integrators include Fluence, Wartsila, Powin, and Flexgen.

Pumped Hydro Storage

• Planned Deployment: 22 projects, totaling 21 GW and 458 GWh

• Cost: NREL estimates $262/kWh in 2019

• Duration: 10 hrs, according to the EESI

• Round Trip Efficiency: 80%

• Cycle Life: n/a, but plants can stay operational for decades

• Energy Density: about 0.8 Wh/kg

Pumped hydro storage is a form of potential energy storage. Electricity drives water up to an elevated reservoir; to release energy, water flows downhill and spins a turbine.

Since reservoirs can be enormous, a lot of energy can be stored in a pumped hydro plant. And pumped hydro is cheaper and has longer discharge duration than lithium-ion batteries. Not to mention it won’t catch fire.

The primary downside to pumped hydro is it’s a huge construction project. It takes several years to build one, not all geographic sites are suitable, and it can be difficult to get permits to build.

Some newer designs, like closed-loop pumped hydro, require building on a hill but don’t require a river, which expands the range of possible sites and reduces environmental damage. RPlus Energies is one of the leading companies building closed-loop pumped hydro.

There’s been research recently into a version of pumped hydro that can be done at smaller scale, called GLIDES. Instead of putting the reservoir at the top of a hill, you put it in a pressurized vessel. This is a form of pumped hydro that you could build anywhere, even on flat ground.

Compressed Air

• Planned Deployment: 3 projects, totaling 1.1 GW and 8.8 GWh

• Cost: $119/kWh, according to the DOE

• Duration: 10-24 hours

• Round Trip Efficiency: 40-60%

• Cycle Life: n/a, but can last decades

• Energy Density: about 28 Wh/kg

Compressed air energy storage (CAES) has been around, but rarely used, since the 1970s. There are two CAES plants in the world, both decades old.

Energy is used to compress air and store it in a tank in a big underground cavern; letting the air flow out releases energy to spin a turbine and generate electricity.

Like pumped hydro, compressed air is long-duration, long-established, cheap energy storage that typically involves construction projects at epic scale requiring unique geological features in the site.

All the new planned CAES facilities are being built by a company called Hydrostor. They use thermal storage as well as compressed air to increase the efficiency of the cycle (they’re claiming 60% efficiency compared to the usual 40% in older CAES plants) and they build their own caverns, rather than needing to be sited in a place with naturally occurring caverns.

Other companies developing grid-scale CAES technology include Alacaes (which built a pilot plant in the Swiss Alps in 2016), Corre Energy (now building a project in the Netherlands), and Siemens Energy.

Green Hydrogen

• Planned Deployment: 1 project, totaling 840 MW and 300 GWh

• Cost: the DOE estimates $312/kWh

• Duration: 120 hours

• Round Trip Efficiency: 45%

• Cycle Life: n/a, but can last decades

• Energy Density: a whopping 34,000 Wh/kg

To use hydrogen as a grid-scale energy storage system, you first use electricity to electrolyze water into hydrogen and oxygen; then you store hydrogen in caverns underground; then you burn it in a gas power plant to get the electricity back out.

Just like pumped hydro and compressed air, hydrogen energy storage is a long-duration, relatively-low-cost energy storage method, whose limitations are its low efficiency and the difficulty of finding suitable sites for this large construction project.

As with compressed air, vast quantities of hydrogen can be stored cheaply for decades in underground caverns. But you have to find or dig those caverns.

Most of the cost of hydrogen storage systems isn’t the storage itself, but the cost of turning electricity into hydrogen (the electrolysis equipment) and of turning hydrogen back into electricity (the gas plant).

The one big planned hydrogen storage project, the Advanced Clean Energy Storage Project in Utah, is using hydrogen equipment from Mitsubishi Power, including “220 MW of electrolyzers, gas separators, rectifiers, medium voltage transformers, and distributed control system.”

Thermal Hydro

• Planned Deployment: 3 projects totaling 450 MW and 94 GWh

• Cost: an estimated $226/kWh

• Duration: 10 hours to several months

• Round Trip Efficiency: 32%

• Cycle Life: n/a

• Energy Density: unknown

Thermal hydro storage stores energy as the difference in temperature between two water reservoirs.

The biggest planned thermal hydro storage project is VECTES, in Finland, which will use renewable energy and waste heat to warm up water to 284 degrees Fahrenheit, and store millions of cubic meters of hot water in underground caverns to be used in place of natural gas for home heating. This is ultra-long duration energy storage — the water can stay warm for months and keep heating homes all winter.

Some other underground thermal energy storage technologies exist, mostly at pilot scale, and in some way or other storing hot water underground for months to be used for heating and cooling buildings.

Most existing underground energy storage designs don’t involve turning the stored heat into electricity, but in principle it could also be used to power electrical generators.

One example of thermal hydro storage that does turn stored heat into electricity is Raygen’s proprietary technology, which uses the heat from solar power to heat up one water reservoir, while the electricity from solar power is used to cool the other reservoir, and the energy gradient is used to spin a turbine. This technology can keep going at its maximum rated power output for 10 hours.

Like pumped hydro, compressed air, and hydrogen, thermal hydro storage is less expensive per kilowatt hour than lithium-ion batteries and has longer output duration. With sufficiently big reservoirs (like the Vantaa, Finland project) thermal hydro can store vast amounts of energy for months.

The disadvantages of thermal (hydro) storage are poor efficiency and difficulty finding suitable sites to build underground reservoirs.

Coming soon: Part 2, where we compare more types of energy storage technology!