As the renewables revolution gathers pace, the role of energy storage for harnessing green power has never been more important. Ten breakthrough technologies – using gravity, concrete and even trees – claim they hold the key to revolutionising the energy ecosystem.

The rollout of wind and solar power is racing ahead at record levels as countries and companies try to hit challenging net zero targets to help avert climate catastrophe.

But while fossil fuel power stations can hit the on switch whenever needed to meet demand, the variable nature of the wind and sun means that green energy assets often generate too much (or too little) power at any given moment.

Energy storage is therefore garnering increasing attention as the perhaps underappreciated backbone of the green energy ecosystem – helping save excess power for when it is needed.

Lithium-ion batteries have become the kingpin of the energy storage ecosystem due to their energy density – meaning they can pack a huge amount of power into a small space.

But lithium-ion batteries suffer from issues around their Chinese-cornered supply chain, sustainability and sometimes make headlines for going up in flames.

Perhaps unsurprisingly for a technology first commercialised by Sony in 1991 to power Walkmans, they are not always suitable for backing up an electric grid as they do not fare well storing energy for more than eight hours.

Two other stalwarts of the energy storage ecosystem, hydro storage and green hydrogen, are well suited to ultra-long energy storage but require hugely time-consuming and costly buildouts. Hydro storage is also hindered by requiring specific mountainous geographies to support it.

This has given rise to an array of alternative and highly creative energy storage solutions that are looking to step into the space left by that established trio. Below is a (non-exhaustive) list of ten technologies that have recently made headlines.

1. Gravity

Perhaps one of the most creative ideas to emerge in the sector is storing excess green energy by using it to haul custom-made bricks upwards by crane and release them down again for discharge.

Storing kinetic energy like this works in a similar way to hydropower, but it is not limited by requiring mountainous geographies to make it work.

Swiss company Energy Vault is a leader in this space, having recently won what it said amounted to $1bn in orders in China with its custom-built facilities. Scottish start-up Gravitricity is meanwhile eyeing US mineshafts as a home for its own similar concept.

Energy Vault recently commissioned this gravity energy storage facility in China Photo: Energy Vault

2. ‘No-water’ hydropower

Another idea for unshackling the huge potential of hydropower from its geographical chains is being pioneered by a UK company that says its technology can turn even gently undulating hills into green batteries.

RheEnergise says it has achieved this by developing a system to pump a patented fluid uphill when energy is cheap. Because its fluid is two and a half times denser than water, the incline needed for the system to work is claimed to be two and a half times shallower.

The upside is that the system can be used in far more places – RheEnergise claims to have already identified almost one million suitable sites around the world. For now, the company has inked a deal to roll out its tech in the British countryside.

3. Compressed air (anywhere)

Compressed air is another long-standing energy storage technology that has been historically encumbered by geography – often relying on large salt caverns and depleted oil or gas reservoirs.

Canadian developer Hydrostor says it has solved the problem by developing custom-built caverns it can build anywhere. The company uses excess or off-peak energy to produce heated compressed air. It then extracts the heat and pumps the air into a cavern part filled with water, which is pushed to the surface.

When energy is needed, water is allowed to rush back down the shaft, forcing up the air, which is recombined with the stored heat to power a turbine.

Hydrostor has received backing from the likes of Goldman Sachs for its concept, and is developing projects in the US, UK, Canada and, most recently, Australia.

A Hydrostor diagram of how its technology works. Photo: Hydrostor

4. Concrete batteries

A team at the Massachusetts Institute of Technology in the US has recently made a breakthrough it said could create homes that are powered from their foundations and roads that charge electric vehicles as they speed along.

Using cement and carbon black, a highly conductive material that looks like very fine charcoal, the team found that they could create a supercapacitor to store electricity. The abundance of these materials means that the supercapacitors could be easily manufactured anywhere on Earth.

Because the material is so strong, it could be used as part of the concrete foundations of buildings, or in roads, say the researchers, turning them into batteries that can power everything from a kettle to a car.

5. Superheated bricks

Microsoft and oil giant Saudi Aramco have thrown their weight behind a California start-up that wants to help industry slash its emissions by storing excess renewable energy in superheated bricks.

Rondo Energy is pioneering a system that uses electric heating elements, like those in a toaster or oven, to superheat thousands of tons of bricks. When power is wanted, air flows up through the brick stack before being delivered to the end point as superheated air or steam.

Rondo has already partnered with a Thai conglomerate to expand the production capacity for its system, creating a facility it says would be “larger than any current battery manufacturing facility worldwide.”

6. Metal blocks

Another oil giant, Shell, is backing another thermal energy storage technology that can pack power into shoebox-sized blocks of metal alloy particles.

Excess energy is used to heat the alloy particles until they melt, while a matrix material remains solid and keeps the molten particles in place. When the blocks are allowed to cool, the heat they give off can be used to power a downstream turbine.

The technology is being pioneered by Australia’s MGA Thermal, although it has had a rocky start after a demonstration unit in the country dangerously overheated, sparking an evacuation of the surrounding area due to fears of an explosion.

MGA Thermal workers during the commissioning of an automated production line for its metal blocks. Photo: MGA Thermal

7. Tree power

One of the biggest names in energy storage, Sweden’s Northvolt, is developing sustainable batteries using lignin-based hard carbon produced from wood from Nordic forests.

Together with Finnish paper and pulp giant Stora Enso, Northvolt is aiming to create the world’s first industrial battery featuring anode sources entirely from European raw materials, lowering the carbon footprint and cost.

Lignin, a plant-derived polymer, makes up around 20–30% of trees. Stora Enso will provide the lignin from sustainably managed forests, while Northvolt plans to drive cell design, production process development and scale-up of the technology.

8. Iron-flow batteries

One of the next generation of battery technologies being developed, iron flow batteries circulate liquid electrolytes to charge and discharge electrons via a process called a redox reaction. Unlike in conventional batteries, the same electrolyte can be used on both the negative and positive sides of the equation.

One of the principal manufacturers of these batteries – US-based ESS Tech – says that this eliminates cross-contamination and degradation, meaning its batteries will last an expected 25 years. Conventional batteries typically last up to ten.

ESS, which has received backing from Bill Gates’ Breakthrough Energy Ventures (BEV) among others, claims its batteries are also safer than lithium-ion, reducing the need for safety equipment.

Lorries transporting ESS Tech's battery units. Photo: ESS Tech

9. Iron-air batteries

They also use iron. They are also backed by BEV. But the iron-air batteries being pioneered by Form Energy work on a very different principle – described by the US developer as reversible rusting”.

When charging, the application of an electrical current converts rust to iron and the battery – made up of cells including iron and air electrodes – breathes out oxygen. When discharging, the batteries breath in oxygen from the air and convert the iron back to rust.

Form Energy claims its batteries can store electricity for 100 hours at system costs competitive with legacy power plants. Its batteries were recently cleared for a demo at one of America’s largest solar plants.

10. Nickel-zinc batteries

One California start-up believes its technology can unlock the potential of a battery first invented by Thomas Edison, while also unleashing a “sleeping giant” of global energy storage capacity.

The breakthrough technology is a new electrode that Enzinc, the company commercialising it, claims can be dropped into factories producing the dependable but low-power lead acid batteries – converting them into production lines for far more energy dense nickel-zinc batteries.

Nickel-zinc batteries were developed by renowned US inventor Edison but never gained much traction as they would only last a few dozen cycles. Enzinc claims the new electrode solves this problem, resulting in a battery as powerful as lithium and as safe as lead.