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It’s an up and down game: gravity as a source of energy
Several startups want to engineer alternatives to pumped storage and batteries. The basic principle is ancient

“Simple but revolutionary,” are the words used by Swissinfo to describe the ideas behind a gravity power plant. The principle dates back to the olden days: One stores energy by lifting weights and recovers it by lowering them. A historic example is the trebuchet, of which knowledge has existed since antiquity.

Researchers and businesspeople in various startups are working on using this principle for civilian applications. As with catapults a weight – in this case using excess electricity from solar and wind farms – is raised. When necessary – via a winch or turbine –  the falling weight drives generators, which convert the kinetic energy into electric power.

And it’s the very simplicity of this concept that proves to be revolutionary. After all, there are only two types of utility-scale energy storage to date: batteries and pumped storage power stations. And both of these technologies have their advantages, along with their typical drawbacks.

Alternatives made of steel and concrete

Developers of gravity power plants tout the pros, claiming that these stations overcome these problems while having all the advantages: Gravity power plants do not have the downside of lithium-ion batteries because they are largely made of steel and concrete, construction waste and rocks. This simplifies procurement and recycling. However, the latter is a non-issue in the beginning, as once they are erected, gravity storage systems can be charged and discharged without suffering the effects of wear common to Li-ion batteries for decades before having to replace individual components on a regular basis.

Moreover, they occupy less space while delivering the same output – especially when compared to damming lakes. Some gravitational storage technologies are virtually infinitely scalable, allowing them to adapt to local circumstances and needs. They come up against their limits in terms of profitability more than anything else.

Storage – a balancing act

While sharing the same basic principle, the technologies’ fields of application differ widely. The approach taken by the Swiss-US company Energy Vault entails storing energy by stacking composite blocks weighing 35 metric tons each around a six-boom crane. On unloading, the electric winches serve as generators, feeding the power produced back into the grid.

Also using winches, Gravitricity, based in Edinburgh, Scotland, seeks to store electricity in what is termed ‘potential energy.’ To this end, the engineers intend to suspend about 3,000 metric tons of concrete weights in mining shafts. They are raised for charging and lowered for discharging. Both concepts could deliver electricity in fractions of a second and could thus be used as primary balancing power for grid stabilisation. This would pit them against battery storage power stations.

Energy Vault’s concrete tower is said to be capable of storing as much as 80 megawatt hours (MWh) and discharging up to eight megawatts of continuous capacity. The company claims that the mine storage systems can store a maximum of 25 MWh and deliver up to ten megawatts of continuous electricity. This would be enough to supply 6,000 residents with power for about a day.

Animated function of the concrete energy storage

Hydroelectric power without dams

Taking things a whole dimension further are Gravity Energy and Heindl Energy. Their engineers speak of storage capacities in single to double-digit gigawatt territory, thus contending against pumped storage power plants – also using hydropower.

They share the same concept: Weighing in at several million metric tons, a huge cement or rock piston rests on a sheet of water within a cylinder. To store energy, electricity from the grid drives pumps which push up the piston using water pressure. When needed, the water is allowed to flow back, turning the pumps into turbines which drive the generators.

First pilot plant near completion

Energy Vault has progressed the farthest. Located in the vicinity of Lugano, Switzerland, its first storage tower has almost been completed and is set to be connected to the Swiss grid before year-end. According to the company, so far all of the other concepts only exist on paper. Gravitricity plans to build a small demonstration plant in the autumn of 2020. Gravity Energy AG hopes to get going in early 2021. A site has already been chosen in Edinburgh and the search for investors is underway.

Over the short term, all of these technologies are more expensive than batteries, which have become very affordable as they are now mass produced. But storage needs are not a snapshhot – they are part of a megatrend. And once today’s Li-ion batteries have long been scrapped, the systems in which they were installed will continue to raise and lower the same weights, operators say.

Proven technologies with defects
The two dominant utility-scale storage facilities to date are pumped storage power stations and battery storage systems. Lithium-ion batteries lead the way in battery technology. They discharge over 95 percent of the stored electricity at the push of a button and have become quite affordable. The only cheaper means of storing energy is to pump water from one basin to a higher one. It is recovered by allowing the water to drop back into the lower basin and drive the generators via turbines.
Both technologies are reliable, but also have some shortcomings. Over time, Li-ion batteries become less able to store electricity and they are problematic in terms of disposal and raw material sourcing. Pumped storage power plants are confined above all by geographic barriers, as they depend on a certain difference in elevation between the basins and the artificial lakes take up a lot of space. Therefore, it does not make sense to build damming lakes everywhere, and wherever this is done, one grossly interferes with the landscape.
Existing utility-scale and mega storage systems
Europe’s largest battery power plant is situated in Schleswig-Holstein, is rated at 48 megawatts (MW) and has a capacity of 50 megawatt hours (MWh). This completely depletes the Li-ion battery within just over an hour. Tesla’s megapack battery in Australia is over twice as big, boasting performance figures of 100 MW and 129 MWh.
Record figures posted by battery storage systems are bread and butter for pumped storage power plants. Most hydro storage facilities ar rated at between 50 and 1,000 MW. The latter figure equals the performance of a medium-sized nuclear power station. Most pumped storage power stations are capable of delivering this output over several hours. Even bigger systems have been set up elsewhere, with outputs of up to 3,000 MW and over 10,000 MWh in storage capacity.
This is why we need utility-scale storage

Wind and solar farms currently rank among the den lowest-cost power producers bar none. So far, however, conventional power stations have been required to cover demand whenever the wind and sun take a break. The only workaround is energy storage systems, which store surplus electricity temporarily in order to supply it when needed. The higher the share of volatile energy sources such as wind and sun in the electricity mix, the more storage capacity is required to guarantee a continuous supply of power.

Photo credit: © Energy Vault

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