It seems safe to say that, in the public mind, the de facto standard for energy storage has become the lithium-ion battery. This is in part due to its ubiquity in smartphones and electric vehicles, which is, in turn, due to the lightweight nature, lifecycle and thermal resistance of these energy storage devices.
Nearly every market research report has the lithium-ion battery market on track to grow remarkably in the near-term, with increase rates ranging from over 11 percent to 17.5 percent by 2025. However, lithium-ion batteries are not without their issues, not least of which is the relative supply of the materials needed to manufacture them.
As renewable energy supplies grow in popularity, it will become increasingly necessary to turn to alternative energy storage solutions—such as gravitational energy storage—that take these other factors into account. Here we look at gravity-based batteries as both an established and emerging technology for grid energy storage.
Constraints of Lithium Batteries
According to Achieving the Paris Climate Agreement Goals, which analyzed the UN’s special report on 1.5° global warming and attempted to lay out strategies to meet the goals of the Paris Climate Agreement, lithium and cobalt supplies are not sufficient to achieve the demand that will be generated by the renewable energy and transport sectors. The authors suggest that metal recycling infrastructure, particularly for these rare and vital materials, will need to be developed.
Image courtesy of Achieving the Paris Climate Agreement Goals.
Not only are the relevant materials--lithium and cobalt, for example--difficult to procure and recycle, some parties also have ethical concerns about how they are currently sourced. With smart engineering, these gravity-based solutions may allow for energy storage that avoids these problems altogether.
Gravity Batteries
Gravity energy storage relies on the potential energy of an object due to its height relative to another object and could be key for intermittent power sources, like solar and wind. The basic concept is that excess energy captured from something like a solar array is used to lift a heavy object up. When there is not enough sunlight for direct power use, the heavy object is dropped down, converting the gravitational potential energy into electricity via generator.
Invented in the early 1600s, Cuckoo clocks exhibit early principles of gravity storage. Mechanical energy via human hand lifts weights to store energy that powers the clock. As the weighs fall, the energy is driven to the clock mechanism. (Image courtesy of The Tick Tock Shop.)
Though the public may be more familiar with lithium-ion batteries, gravity energy storage is actually the largest form of grid power storage in the world. Pumped hydroelectric energy storage (PHES) accounts for 95 percent of the all tracked storage globally at over 184 GW of capacity installed, with the U.S. alone hosting 25 PHES sites.
How PHES works is that during off-peak electricity usage, power is used to pump water uphill into a higher elevation reservoir. When demand exceeds the amount of electricity generated, the water is released downhill, with the force of gravity used to spin turbines that create more electricity.
The problem with PHES is that there’s not always a mountain into which these reservoirs can be built. Additionally, PHES sites achieve only 70 to 80 percent efficiency.
Alternative Gravity Batteries
For this reason, a number of new businesses have emerged looking to develop alternatives to PHES. One of the most well-known is a Swiss, venture-backed startup called Energy Vault. The startup envisions the use of a six-armed, 120-meter-tall crane to store energy in the form of a stacked tower of heavy concrete or composite blocks. Excess energy generated by an electrical grid causes the blocks to be lifted into the air and, when demand exceeds what is generated, the blocks are dropped to power an electric generator.
While the concept itself is an old one, Energy Vault has written software meant to enhance the lifting and dropping patterns for optimal power storage and delivery. The use of cameras attached to each crane arm trolley allows the software to determine which block to pick up or drop. Once the crane has created a tower of blocks around itself, it has been “charged,” with the tower capable of storing 20 megawatt hours (MWh)—enough to power 2,000 Swiss homes a day.
An additional advantage these blocks have over PHES is that concrete is much denser than water and can, therefore, store more energy than the same volume of water. The system is able to achieve about 85 percent efficiency, which is just under the 90 percent offered by lithium-ion batteries.
Energy Vault’s prototype is a much smaller version of the proposed 120-meter-tall plant, standing at just 20 meters and featuring only a single arm. However, the company was said to begin taking orders in 2019, and—according to the CEO of Energy Vault, Robert Piconi—once the startup has built its 10th 35-MWh plant, the cost of delivering electricity would be dropped to around $150 per kWh.
This competes with lithium-ion batteries, which currently cost about $280 to $350 per kWh, but could be scaled to provide power at $100 per kWh. Additionally, the startup suggests that an Energy Vault plant could run for 30 years with minimal maintenance, as opposed to lithium-ion batteries that degrade over time and live for only about 20 years. However, unlike lithium-ion batteries that can be shipped anywhere, block-lifting crane plants need to be built on-site.
Other Alternatives
Energy Vault is not the only company developing gravity storage solutions. Heindl Energy is a German company aiming to pump water beneath a rock piston, causing a rock to be lifted during excess power generation and dropped in times of need. The firm estimates energy cost to reach around $200 per kWh, depending on the size of each power plant site. Heindl’s proposal depends on large-scale underground excavation to develop its gravity storage plants.
In October 2019, a startup called Gravitricity raised £750,000 in a crowdfunding campaign to develop a concept that uses winches, generators, and 500- to 5000-ton weights within existing mineshafts or new, purpose-built shafts as sites for gravitational electricity generation. The company aims to build a 250-kW demonstrator by 2020 with a full-scale prototype using a disused UK mine in 2021 or 2020.
Another firm called ARES (“advanced rail energy storage”) is creating a method for storage energy using trains carrying weights up inclines ranging from six to 25 degrees from a lower trainyard to an upper trainyard. The company is looking into short duration 50 to 200 MW storage lasting from four to eight hours as well as 200 MW grid-scale and 3 GW regional-scale systems.
So far, ARES has developed a small-scale demonstrator that used a six-ton rail vehicle that will, if the company is successful, become an R&D site. Now, it is in the process of developing a system that will connect to the Nye County grid in Nevada and deliver 50 MW of power. The county has already approved of the project, and the company is in the process of obtaining the proper permits. Construction was slated for fall 2018, but no new updates have been provided by ARES.
Alternatives to the Alternatives
Outside of gravitational batteries, there are projects underway that rely on other technologies to store electricity. The Stored Energy at Sea system harnesses pressure from under the ocean to store electricity, with the pressure of the sea water replacing the need for higher and lower reservoirs. Compressed air energy storage stores energy in the form of compressed air, releasing it when demand is high to power a gas turbine.
Cryogenic energy storage has already been deployed at a small scale, with Highview Power Storage’s plant delivering up to 300 kW of power and storing up to 2.5 MWh, enough to power 16 houses for eight hours.
There are also extremely small-scale energy solutions like the GravityLight, which is exactly what it sounds like: an LED lamp powered by a weight, which comes in the form of a ballast bag that, when filled with 20 pounds of weight, drives an electric motor and generates 20 minutes of electricity.
It’s likely that more than one type of battery will be used to store all of the grid’s electricity, but we may end up seeing one achieve dominance over others overall—which one will likely depend on the cost, efficiency and feasibility. It’s also important to note that, as with any startup, we may see some of these firms fizzle up. So, as always, we should bet on more than one horse in the race.