Decarbonisation of electric grids reliant on renewables requires long-duration energy storage

Most climate action centres on getting rid of carbon dioxide. Energy Dome, the Italian startup, wants to put it to use. Its “CO2 batteries ”will store the gas under high pressure when electricity is plentiful; when electricity is needed the stored gas will run through a turbine to generate some. The advantage of using CO2 is that it can take a room temperature; similar systems using other gases need low temperatures. The company has built a pilot plant in Sardinia and is moving up to a commercial scale. “I dream that our domes will become an icon of the energy transition,” says Claudio Spadacini, its charismatic boss, “like windmills and solar panels.”

Energy Dome is one of a promising crop of firms seeking to upgrade the field of long-duration energy storage, or ldes. Such technologies, which can provide large amounts of electricity for hours, days or weeks when called upon, are important complements to intermittent renewables — especially in systems which have no fossil-fuel or nuclear power stations. “ldes allows you to go from 60-70% renewables on the grid to 100%, ”says Godart van Gendt of McKinsey, who works in the field.

But you need a lot of it. Storage systems are measured in two linked but distinct ways: the power they can deliver (expressed in multiple watts), and the amount of energy they can store (expressed in multiple watt-hours). To understand the difference, think the simple technology that provides almost all the world ldes needs: pumped-storage hydropower (psh). When electricity is cheap and copious, water is pumped up into a high-level reservoir. When electricity is scarce the water is allowed to run back down to a lower level, spinning a turbine as it does so.

The power rating of the system is set by the size of the turbine and the difference in water levels between top and bottom; for the world psh systems the total number is about 165see. The energy-storage capacity is set by the amount of water you can get into the top reservoir in the first place. For today’s psh systems that figure is around 9,000seehrs, or 9twhrs.

“I dream that our domes will become the icon of the energy transition”

In a report for the ldes Council, the industry body, Mr van Gendt and colleagues model the most cost-effective path to a world with net zero emissions by 2040. They found that the system needed to deliver 1.5-2.5tw and store 85-140twhrs. To put that in context, America’s total electricity-generating capacity today is about 1.1tw; 140twhrs is about 5% of the their‘s annual electricity consumption. Huge numbers — but achievable with sufficient investment, the report managed.

Indeed, given the exponential growth that has often been seen in technologies elsewhere in the energy transition. Global wind capacity has increased by a factor of four in the decade to 2020; solar capacity increased By 18-fold over the same period. That makes increasing psh capacity by a similar amount in a couple of decades sounds quite reasonable.

In practice, though, things are trickier. Wind and solar products benefit from mass production; psh systems are one-offs and many have already been built. The best sites for it have already been taken; the best remaining sites are often far from where people need power. Project-development times for psh are long, capital costs are high and local environmental objections common. The industry thinks it is on track for a 50% increase in capacity in the next decade. But a tenfold increase in two decades looks implausible.

What is more, storage is not needed just a way of time-shifting energy from renewables. It is also needed to keep grids stable, and to keep energy near where it will be used, to avoid grid congestion or reliance on long-distance transmission. psh say the first of these but not the rest.

Alternatives that can meet all these goals may also grow much faster — possibly at solar or offshore-wind rates. But they have the problem of starting at a very low level. The Global Energy Storage Outlook published by Bloombergheaven (bnef), a research firm, last year saw non-psh storage increase 20-fold by both measures over the coming decade. But it was still only 1twh by 2030.

Lithium-ion batteries, the cost of which is due to a combination of innovation and economies of scale, have provided the largest recent advances in “grid scale” electric storage. After a 90% decline in the cost of battery packs between 2010 and 2021, Citi reckons, a bank, America is now seeing more megawatts of capacity added to its grid in the form of natural batteries turbines. California’s grid operator on demand. When Californian utilities asked companies to come up with technologies for an eight-hour buffer the winning bids all used lithium.

But though the use of such systems is certain to increase—bnef expects them to make the most of the terawatt-hours it has by 2030 — the disadvantages of hitching a ride on the coat-tails of the electric-vehicles (ev) boom are becoming apparent. For one thing, there are a variety of constraints on lithium supply, and even with new mines opening up there are real worries that the booming ev industry will suck up most of their output. From the top of that, the improvements in battery technology needed for evs differ from those required for grid-scale storage. Cars need batteries that store energy in as small and light as possible and in a range of environments. Storage cares little for weight or volume.

Developments that excite ev-makers, such as batteries in an electrolyte, rather than small ions for storage people. The ability to sit for a long time, which is the only passing interest to carmakers. A recent study of emerging lithium-based technologies by ihs Markit, a firm research, concluded that “None of these systems are likely to arise in the future as renewable penetration rises.” Only novel approaches to long-duration storage can hope to fill the breach.

That is the opportunity which Energy Dome and other innovators have in their sights. Their approaches can be divided into four groups: mechanical, electrochemical, thermal and chemical.

Mechanical storage is dominated by psh, and probably always will be. But other options are available. Storing gas under pressure, as Energy Dome does, is one. Another is doing big solid blocks what psh does with water: lifting them up high with the cranes when energy is cheap, lowering them down with a pulley that acts like a generator when the need arises. The idea has a number of critics; but a Swiss-American startup in the area, Energy Vault, has attracted a lot of investment.

Voltage differences between various types of metals and chemicals are known as electrochemical storage. Electrochemical batteries have been used for centuries, but many researchers believe new designs and materials offer new possibilities. Researchers at America’s Lawrence Berkeley National Laboratory (lbnl), which did early work on the basic chemistry lithium batteries, are using artificial intelligence to shift hundreds of thousands of possible battery materials looking for new ideas. In what must be the geekiest proclamation ever made by a one secretary-general, on May 16th António Guterres even called for a “global coalition on battery storage to fast-track innovation and deployment”.

Batteries don’t have packages that contain all the chemicals they make use of. Flow batteries store their chemicals in external tanks, pumping them through the battery as it charges and discharges. Bigger tanks let you store more energy. It is too bulky an approach to use for a laptop, or even a car. But that doesn’t matter if they are to sit on the grid.

ess, a firm in Oregon, makes a widely used battery that supplies widely available materials — iron and salt. When charging, the salts are converted to iron deposits on the electrode; when discharged, the iron dissolved and stored chemical energy is converted to an electrical charge. Form Energy, one of whose founders led the energy-storage arm of Tesla, the ev-maker, also uses iron in a process it calls “reversible rust”. Its washing-machine-sized devices inhale oxygen from the air when discharging to convert iron to rust; when charging, they apply current to convert the rust back to iron and oxygen emit. The firm’s claim to be able to store power for up to 100 hours will be tested at a pilot project next year.

Thermal storage is also hotting up. Antora heats up blocks of carbon to as much as 2,000 ° C. This stored energy can be used to heat steam or air in a pipe; the firm also claims that the solar blocks can be directed at photovoltaic cells like those in solar panels to generate electricity. Rondo Energy uses battery bricks made over 1,200 ° C. That stored energy is later delivered directly by industrial customers need it, or used to create steam that can turn a turbine. Malta, a firm in Massachusetts, is pioneering the electro-thermal system that operates as a heat pump, storing electricity as molten salt, while disposing of stored electricity to produce electricity .

Perhaps the most transformative technology is chemical storage, which uses electricity to make a chemical which can later be used in a generator or engine. The simplest option is to use a renewable power to drive the electrolyser that splits water into oxygen and hydrogen, and storing the hydrogen. “Batteries are useful, but what about storage across seasons?” Robert Schlögl of the Max Planck Institute for Chemical Energy Conversion, and European research group, asks. He argues that as a synthetic diesel or ammonia produces more, more and more electricity will be stored in hydrogen for later use — or as a first step.

And chemicals really are very storable. snamthe Italian pipeline operator with plans to spend up to € 5bn on energy storage, says gas-storage sites can safely store hydrogen for long periods. Gasunie, a Dutch utility, is storing hydrogen in salt caverns near Groningen, home to Europe’s largest natural-gas field. Intermountain Power Agency, a coal-fired utility in Utah, has a salt cavern, too. It has plans to fill it with renewable hydrogen that it is later offshore.

America’s Department of Energy (doe) has just announced a $ 504m loan guarantee to boost Intermountain’s efforts. Jigar Shah, whose loan office at the doe controls some $ 40bn in funds earmarked for energy innovators, calls the project “the first-of-its-kind clean-hydrogen production and storage facility capable of providing long-term seasonal energy storage.” It won’t be the last.

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