Molten salt and human sweat: the weird batteries that could store renewable energy

In the deserts of the United Arab Emirates a sprawling clean energy project, stretching across an area roughly the size of ۱۲,۶۰۰ football fields, will play host to a breakthrough allowing solar energy to power the equivalent of half a million homes through the night.

The Gulf state has been steadily combining ۵.۲GW of solar power capacity with ۱۹GWh of battery storage to create the largest battery scheme in the world.

Meanwhile, about ۷,۵۰۰ miles away, at the US’s National Renewable Energy Laboratory facility in Colorado, researchers and engineers are making some of the smallest batteries the world has ever seen. A fraction of the size of the grid-scale lithium-ion batteries used to store renewable energy, they have been designed to power electronic tags that will track the ۳in-long (about ۷.۵cm) young from salmon and eel species.

As traditional lithium-ion batteries push boundaries in size and scale, alternative battery types could provide the same vital role in harnessing low-carbon energy – but without the scramble for critical minerals such as lithium, cobalt and nickel that have raised concerns for environmental protection and vulnerable communities.

Unlike lithium-ion batteries, which have a finite number of charge/discharge cycles, many of these alternative battery technologies can be used indefinitely and recycled when the units reach the end of their ۲۰-year service life.

Already developers are turning to a large range of materials to store renewable electricity when it is available and use it to power everything from wearable technology to heating networks, factories and missiles.

Here are some of the energy storage innovations raising eyebrows.

Liquid air

Late last year, on the site of a former coal plant in Trafford, the former mayor of Greater Manchester marked the start of construction on a long-awaited energy storage project that could usher in the “re‑industrialisation” of the county.

Andy Burnham said the development of the Trafford green cluster, which includes the Carrington “liquid air” cryobattery, could mean “this decade is the most exciting since the Victorian period for Greater Manchester”.

In simple terms the Carrington project, developed by the British startup Highview Power, aims to capture renewable energy when it’s abundant and store it as liquid air. In theory, a “cryobattery” can store energy for hours, days or even weeks.

The science is more complicated: Carrington uses surplus renewables to cool air to -۱۹۶C, which reduces it to a liquid one-۷۰۰th of the volume. It can be kept in this state until renewable energy is scarce and market prices begin to rise. Then the liquid is allowed to become a gas once again, rapidly expanding through a turbine that generates electricity without the emissions typically associated with gas-fired power plants.

The project has faced a number of delays but once it begins operations, slated for the end of the year, it will deliver ۳۰۰MWh of storage and an output of ۵۰MW for six hours – or enough clean, renewable energy to power almost half a million homes.

Molten salt

While in Manchester energy is being stored at sub-zero temperatures, in the Nevada desert the opposite approach has been taken.

Here, ۱۰,۰۰۰ panes generate electricity for the Crescent Dunes scheme, which for the past ۱۰ years has harnessed the sun’s power to heat a reservoir of potassium and sodium nitrate to ۵۶۰C.

This temperature can be maintained for ۱۰ hours after the sun has set, before the stored thermal energy can be converted into electricity by using the heat to drive a traditional spinning turbine. The system effectively stores clean electricity as heat in a process known as molten salt storage.

These molten salt batteries provide the primary power sources for most modern guided missiles and nuclear weapons by acting as “reserve batteries” that hold energy until the pyrotechnic heat generated when the weapon is launched activates the release of energy.

In the ۲۰۲۰s Denmark proved molten salt storage technology could provide an answer to the dilemma of decarbonising heavy industry, too. The leading wind power nation unveiled a ۱GWh large-scale molten salt battery project earlier this year, which can store clean electricity for up to two weeks by heating the salts to approximately ۶۰۰C. When needed, the hot salt is circulated through a generator that produces high-temperature steam that can be used directly in industrial processes.

Sand

Much like renewable electricity can be stored by heating salt, clean energy can be stored in sand.

In the small town of Pornainen in southern Finland, thousands of tonnes of sand help store energy used to heat schools, libraries and town halls. This should allow the district to end the use of oil in its local heating network completely and reduce the use of wood chips by about ۶۰%.

The sand battery uses about ۲,۰۰۰ tonnes of crushed soapstone to store clean energy in the form of heat to provide ۱MW of thermal power and a storage capacity of ۱۰۰MWh.

In summer, the sand battery, which stands about ۱۳ metres tall and ۱۵ metres wide (۴۳ft x ۴۹ft), can cover almost a month’s heat demand in Pornainen – and in winter, close to a week. It is about ۱۰ times larger than an earlier version launched in the country in ۲۰۲۲, underscoring the potential of sand batteries to play a greater role in cutting emissions across the country.

Sweat tech

In Japan, researchers at the Tokyo University of Science are investigating how human sweat could be a viable energy source to power wearable technologies.

By treating the body as an energy source capable of providing continuous power, the researchers hope to solve the problem of how to fuel the sensors that underpin wearable devices without tiny batteries that make them bulky and need constant charging.

In response they have developed a thin wearable patch that generates electricity directly from human sweat. It uses an enzymatic biofuel cell to capture the chemical compounds – especially lactate – released in sweat and convert them into power. When sweat comes into contact with the cell, the enzymes embedded in the patch trigger a biochemical reaction, releasing electrons.

This means electricity can be generated without the need for any external power source – during everyday activities such as walking, exercising or running errands.