A “metal sock” in the ground filled with hydrogen. Baking with burning sand. Huge weights move very, very slowly up and down old mine shafts. Is this the future of energy?

This menagerie of strange machines and heat-trapping craft is about to emerge across Europe as the continent looks for ways to store the excess energy produced by renewables. The UK, for example half a billion pounds of wind energy wasted in 2021 because it had nowhere to store it. Without such storage, electricity must be used the moment it is generated.

As wind energy continues to be lost across Europe, the EU is spending record amounts – billions of euros – on gas imports, as it becomes less dependent on fossil fuels from Russia.

“We are at a turning point,” said Dominic Walters, chief corporate affairs officer at Highview Power, a UK-based company working on a way to store energy in the form of liquid air. “Everything needs to be accelerated everywhere,” he adds, referring to the colorful array of energy storage projects currently in early stage development in Europe.

Proponents of alternative energy storage technologies argue that lithium-ion batteries will only get us so far. Their production depends on mining, they don’t have a very long life span and arguably they do not ideal for storing energy for more than a few hours.

“If we don’t find out soon how to stabilize Europe’s electricity grids, we will regret it,” says Jacopo Tosoni, head of policy at the European Association for Storage and Energy (EASE): “You are generally at risk power outage in 2030.”

There is now a battle to put in the necessary storage media so that energy can be kept ready and waiting for the times when it is needed.

The heating is on

In an industrial corner of Kankaanpää, Finland, a town of about 12,000 inhabitants, there is a seven meter high, dark gray silo full of sand. Sand that can store energy in the form of heat.

“Our year-round efficiency is about 90% for the system, so 10% loss, which is obviously pretty good,” said Tommi Eronen, CEO and co-founder of Polar Night Energy, an eight-person start-up that is established €1.25 million to date. Eronen described how the sand, heated to 600˚C using excess electricity, stays warm for months thanks to the insulation along the walls of the steel container. Tubes filled with hot air run through the sand to transfer heat in or out.

This sand battery is connected to a heat exchanger, says Eronen, so operators can transfer thermal energy to district heating systems or, in possible future versions of the technology, turbines for electricity generation.

Eronen explains that early versions of the company’s sand battery are relatively small-scale. The Kankaanpää unit offers 100 kW of heating capacity, or a capacity of 8 MWh, but Polar Night Energy is planning units of 100 MW and above, which could yield several GWh of juice in a day. Such units would be about eight meters high and 44 meters in diameter, says a spokesperson for Polar Night Energy.

Expect news about the delivery of a 2MW version as early as this spring, Eronen adds.

In the Netherlands, GroeneWarmte is working on a different kind of thermal energy storage called Ecovat, which uses water heated to temperatures of up to 95˚C instead of the much hotter sand chosen by Polar Night Energy. “It actually just stores water in a large underground tank,” says project engineer Marijn van den Heuvel. “It’s a really big thermos.”

However, a little more construction is required when setting up this system. The concrete “thermos” must be installed carefully in a huge cylindrical hole in the ground. But after that it can be covered and the storage works in the same way as Polar Night Energy’s design. The heat retained by the ship, for several months if necessary, would be transferred to district heating systems via heat exchangers. Van den Heuvel says that GroeneWarmte and his team of eight people are in talks with a Danish company about a possible first use of this technology.

These approaches are fairly new, but Highview Power is already building a 50 MW facility in Carrington, England, where energy is to be stored in the form of liquid air. The site will be a bewildering array of silos, pipes and platforms bundled together. It will consist of thermal and cold storage units and containers for the liquid air itself.

“We filter it so effectively that it’s clean air, that the air is liquefied, and then we freeze it cryogenically,” explains Walters, referring to the process by which air is cooled to almost -200˚C. By later heating this very cold, liquid air, it turns back into a gas and expands, and can be used to drive a turbine and feed electricity back into the grid. The system achieves an efficiency of 55-65%, which Highview says is comparable to other storage technologies. One of the benefits of this approach is that the technology should have a multi-decade lifespan, much longer than lithium-ion batteries, so governments may be able to plan around such infrastructure more easily.

Walters says the Carrington site should go live by the end of 2024. Currently, the 55-person company is raising a £400 million round of funding and planning a further 19 installations across the UK. It ultimately aims to deliver 4 GW, or 20%, of the UK’s projected energy storage needs by 2035.

Another storage method is lost

Perhaps the simplest concept currently vying for its place in the energy storage landscape of the future is the gravity battery. Most of us learned about “potential energy” in school. Surely there is no better illustration of that than a great weight, held aloft, just itching to give in to gravity and fall to the ground. By attaching cables to such a weight – literally tensioning it – it is possible to slow the descent to about a meter per second and use the traction it exerts to generate electricity via a turbine.

Gravitricity’s approach in this vein, to begin with, is to lower its weights hundreds of feet into disused mine shafts using a guide mechanism. The company, which employs 17 people, has so far raised £7.5 million to make its vision a reality.

“If it were to swing around you would very quickly collapse the shaft into itself, which is obviously not what we want,” explains commercial director Robin Lane. A single weight can provide 4-8 MW of power, he estimates, and can be calibrated to provide energy for a set period of time, say 15 minutes or an hour. Imagine a system where multiple weights are ready to descend, one after the other, in a carefully synchronized sequence so that electricity can be generated at a constant rate. Early commercial systems will use a combination of large weights totaling 1,000 tons.

Lane admits that this approach cannot yet compete with lithium-ion batteries on a cost-per-MW basis, but he argues that gravity batteries will eventually be commercially competitive. In addition, it should be possible to lift and lower weights over and over for years with little impact on the integrity of the system. Lithium-ion batteries, on the other hand, do stricter restrictions on cycling.

Another company, Energy Vault, which employs 150 people, is also pursuing gravity battery technology. It has raised about $410 million in funding to date.

Gravity is also exploring completely different ways of separating energy from old mine shafts, such as lining them with metal and converting them into hydrogen storage units.

“It’s a metal sock, which you lower into the shaft, and then you bury that metal sock with a mixture of ballast, concrete, and steel,” says Lane. It potentially makes it easier and cheaper to store hydrogen at high pressure than above ground, because the container can rely on the existing geology of the shaft for structural support. The hydrogen could come from electrolysers linked to wind farms and use excess energy to produce the gas from water.

To Tosoni, the diversity of storage projects emerging in Europe is encouraging given the anticipated energy demands countries will face in the coming years. But less important than choosing one technology over another is the funding and political strategies required to scale them up.

“The big problem is funding,” he says, noting the caution of some investors. Governments could help, he argues, by setting more ambitious targets for establishing energy storage facilities.

Eronen is generally optimistic about the future, noting that Polar Night Energy is entering a new funding round of 5-10 million euros. But it remains frustrating to witness the current energy crisis in Europe knowing that, even with the best will in the world, these systems are not quite ready for primetime.

“It feels so bad,” he says. “We see the crisis now and we can’t possibly help.”

According to EASE, the current rate of storage added each year in Europe, 1 GW, needs to explode to 14 GW per year if the continent is to reach the 200 GW total grid-scale storage capacity it is expected to need by 2030. .