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Storing renewables in soil mounds via water balloons

Researchers from the Aarhus University in Denmark have conceived an energy storage technology to store large amounts of power from renewable energy sources such as wind and solar.

The scientists said the new technology is similar to pumped-hydro storage, as it uses water as a storage medium and responds to the same basic mechanism, but explained that it can be used in a flat country such as Denmark, where no hydropower plants have ever been built. The new tech uses surplus power from wind and photovoltaics to pump water from a reservoir into giant underground water balloons.

According to AquaNamic, which is a Danish startup partnering on the project, a full-scale balloon may reach a size of 330 m x 330 m and, when buried under thousands of cubic meters of soil, can be raised up to 14 meters when the balloon is filled up with water in the charging phase. This balloon would reportedly have a storage capacity of 230 MWh. In the discharging phase, a valve opens and the pressure of the soil pushes the water out of the balloon and through a turbine to generate power. According to the scientists, this system has an efficiency of around 85%, which they say is in line with most pumped-hydro stations.

AquaNamic and the Aarhus University are currently planning to build a 10 m x 10 m demonstrator and have secured DKK 4.9 million ($674.147) from the Energy Technology Development and Demonstration Program. “We’re about to begin analyzing, designing and testing selected critical technologies related to the membrane and to the construction of the ‘movable hill' that will form the terrain part of the battery,” said researcher Kenny Sørensen. “Naturally, we’ll have a strong focus on abrasion testing for the membrane, and we’ll need to develop a specially designed test rig to carry out lifetime tests for representative membrane solutions.”

One of the crucial aspects of the demonstrator will be assessing the extent of possible energy losses. “We want to retain as much energy in the system as possible, and this is a complex process with such a large system, which in principle will fill up every night, when the turbines are spinning and the world is sleeping, and then empty every day, when the energy is needed,” Sørensen further explained. “But every time the soil moves, the system is deformed, and these deformations contribute to the energy loss. They’re called plastic deformations. Our job is to optimize the system, using advanced calculation models.”

The new technology is also being developed in partnership with Danish wind energy giant Vestas and renewable energy developer European Energy.

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