From hot sand to liquid air – 5 early stages energy storage solutions that could help underpin the electric economy

After centuries of dominance, fossil fuels are set to lose their spot at the top of the energy podium.

According to Jeremy Oppenheim, Partner at systems change company Systemiq, “We are hitting peak oil. Over the next 20 to 30 years there will be a rewiring of the economy on the basis of clean electrons and electrification. This will change everything.”

This rewiring is already well underway. In the decade between 2010 and 2020, solar and wind together went from producing 1.7% of the world’s electricity to 9.3%¹, a rise that far outstripped industry expectations. With renewable power now outcompeting fossil fuels on price, this rapid growth looks set to continue.

Missing from these impressive figures, though, is the vital question of how to store all this energy over long periods of time. Unlike fossil fuels, renewable generation cannot simply be turned on and off at will – for renewables to successfully kick fossil fuels into touch, we must have a way to store the excess energy generated on blustery and sunny days, ready for days when the wind is resting or the sun reluctant to shine.

Read also: Energy – crisis or opportunity?

Rapid scale-up needed

Since 2010, as electric vehicles (EVs) have gone from niche to mainstream, the cost of lithium-ion batteries has fallen 90%². This sharp drop is now enabling the mass adoption of batteries in the power grid³. But while lithium-ion batteries are widely accepted as the best solution for short-duration storage (under 4 hours of continuous discharge) there remains heated debate about the best way to store electricity at low cost over days, weeks and even months, with long-term storage having an essential role to play in minimising the system-level costs of the switch to renewable energy, ensuring the profitability of renewables projects⁴.

By far the most widely-used long-duration energy storage solution today is pumped-storage hydroelectricity (PHES), which accounts for 99% of large-scale storage. In PHES, excess energy is used to pump water to a reservoir at elevation and then, when power is needed, the water is released to flow downhill through hydro-electric turbines. PHES has constraints however – it is difficult to install in an urban context, lacks economic efficiency at small scale⁵, and in flat landscapes it simply won’t work.

For energy storage to match the growth of renewable production, rapid scale-up of new long-duration storage methods is needed. Here, we take a look at five early-stage technologies that could one day help to underpin a new economy powered by near-limitless zero-carbon renewable energy.

1. Green hydrogen

Green hydrogen – hydrogen produced via the renewable-powered electrolysis of water – is already set to play an essential role in decarbonising heavy industries. Until recently, burning fossil fuels had been the only way to reach the high temperatures needed for many manufacturing processes. Green hydrogen, with its high energy-density and zero emissions, is now offering a carbon-free alternative.

Hydrogen could also, it is hoped, provide long-term energy storage for use outside of heavy industries. Hydrogen has a significantly better energy to weight ratio than chemical batteries and can be transported from high-renewable capacity areas to places where renewable infrastructure is either not viable or not yet fully developed. There, the renewably-produced hydrogen can be used to power turbines or converted directly to electricity via hydrogen fuel cells.

In January of this year, Norwegian state-owned Equinor announced plans to build a hydrogen pipeline from Norway to Germany. While the pipeline will begin operation transporting so-called ‘blue’ hydrogen – hydrogen produced using natural gas combined with carbon capture technology – the firm will later switch to ‘green’ hydrogen produced using renewables.

Green hydrogen brings with it the benefit of being able to store energy for near-indefinite periods. It is, however, the least efficient of all viable storage solutions, with a round-trip efficiency of just 30% of ‘energy-in’ being recoverable as ‘energy-out’⁶.

Read also: Green hydrogen – the key to decarbonising heavy industry

2. (Very) hot sand

At first glance, the municipal swimming pool in Kankaanpää, Finland, looks like any other public pool. It is, however, unique – the only one in the world to take its heating from a new commercial energy storage solution: hot sand.

Finnish start-up Polar Night Energy’s novel technology uses excess renewable energy to heat sand – any sand will do, “someone else’s dirt could be our heat storage medium,” the company says – to upwards of 500 degrees Celsius. Held in large, thermally insulated silos, the sand can retain heat for several months. When power is needed, air is passed through pipes that run through the silos – the air can then be used to heat water for district heating systems, or to produce steam to run turbines.

One major advantage of the system is its flexibility, with silos sized to fit the users’ needs, and capable of being built in most locations, above or below ground. The company founders estimate that a silo 40 metres in diameter and 25 metres high would be sufficient to store and balance the renewable supply for a population of 35,000 people.

Polar Night is now in talks for larger installations across Finland, and hopes to expand internationally in 2024. As they do, the technology may come under scrutiny due to its reliance on sand, the world’s most mined material. To feed the construction and digital communications industries as much as 50 billion tonnes of sand and gravel are extracted from deserts, riverbeds and beaches each year, often in lower and middle-income nations. In places this trade is unsustainable, damaging ecologies and local livelihoods. To avoid contributing to this trade, Polar Night say they prefer to use sand that is not suitable for construction.

3. Gravity power

Scotland-based Gravitricity aims to harness the power of gravity with their proprietary GraviStore technology. GraviStore uses renewable energy to winch weights of up to 12,000 tonnes, equivalent to 60 Boeing 747 jets, to the top of disused mine shafts or purpose-built towers – the weights are later allowed to fall, driving turbines as they go. The firm promises a long life-cycle of up to 50 years, with no degradation across time, and long-term storage at a cost well below that of centralised lithium-ion batteries.

Following a successful pilot project in Edinburgh, Gravitricity has now signed a memorandum of understanding with Czech state enterprise DIAMO for re-purposing the disused Darkov coal mine into the first non-hydro-electric large-scale gravity battery in Europe. Once up and running, the Darkov mine will store enough energy to support 16,000 homes.

The firm has grander plans, however. Around the world they have identified 14,000 disused mine shafts that could be swiftly turned over to gravity-based energy storage, many of them supporting renewable power rollout in emerging markets and developing economies. They are also working on a first ever “multi-weight” system that could lead to storage capacity at least twelve times greater than that at the Darkov project.

4. Liquid air

In late 2022, following success at their demonstration plant near Manchester, England, UK-based Highview Power announced plans to build the world’s first commercial-scale liquid air energy storage (LAES) plant⁷. Due to be completed by the end of 2024, the new site will store sufficient energy to power 600,000 homes for one hour.