How storage with compressed air could boost renewable energies

Decarbonizing the global power grids will not be easy, but it is necessary if we are to mitigate some of the worst effects of climate change. Since wind and solar energy are intermittent, the decarbonisation of the grid involves, among other things, storing renewable energy so that it can be used when the sun is not shining and the wind is not blowing.

Although daily storage covers gaps when the wind slows down or the sun goes down, on networks with more than 80% renewable energy, you will also want inter-seasonal storage. Indeed, the sun and the wind are seasonal and the energy demand is also seasonal. In the winter, people consume much more energy than in the spring because it is darker and colder.

Researchers at the University of Edinburgh and the University of Strathclyde believe that a potential step towards seasonal storage should be to identify large underground salt aquifers in which energy could be stored as compressed air. Saline aquifers are usually found under water; the compressed air displaces some of the water, which can be thrown away as it is not drinkable.

Mbadive tanks of compressed air could fill gaps between the seasons of highly renewable networks. The researchers suggest a system in which excess renewable energy is used to compress the air and pump it into a saline aquifer for several months. Then, when necessary in winter, the air is brought back and developed, feeding a turbine that returns electricity to the grid.

The cycle is comparable to the number of regions that store and use natural gas. That is, the excess natural gas is pumped into underground storage caverns where there are a few months left. In winter, when it takes gasoline to heat homes, these stores are exhausted. The difference here is that the stored energy is not chemical.

Researchers used existing geological mapping around the UK to find potential storage areas for carbon dioxide (CO₂), but their method of verifying geological repositories for compressed air storage could apply to submarine areas around the world.

They found that at least 77 to 96 terawatt hours (TWh) of electricity could be stored in aquifers off the British coast. (This represents about 160% of the UK's electricity consumption in January and February 2017. Combined with other forms of electricity, it would therefore be ample for a typical winter.)

An imperfect solution

Despite the strong theoretical underpinnings that underlie the development of vast subsea wells for storing compressed air, the researchers recognize that it is an imperfect solution to our problems of energy storage.

On the one hand, it is expensive. The researchers estimate that storage of compressed air in saline aquifers would cost between $ 0.42 and $ 4.71 per kilowatt hour (kWh).

In comparison, Lazard's 2018 Weighted Storage Cost (PDF) report found that the top of the line The cost of lithium-ion batteries for wholesale energy storage was about 298 USD / MWh, or about 0.30 USD / kWh. Of course, most utilitarian lithium-ion batteries currently on the world's networks are designed for short-term frequency response rather than for long-term applications such as compressed air energy storage systems. Long – term energy storage systems, at a slightly higher cost, could be viable if governments began to impose more and more renewable energy on the grid.

Another problem with this system lies in the fact that carbon neutral compressed air energy storage is not (yet) commercially proven. When conventional compressed air systems have to expand stored compressed air to inject additional energy into the grid, they do so by heating the air by burning fossil fuels. Highly advanced compressed air systems can be carbon neutral by recycling the heat extracted from the air when it is compressed, but these so-called "advanced adiabatic compressed air systems" only exist in form pilot projects and small-scale trials, as opposed to conventional systems. compressed air systems, used for decades.

Even though fossil fuels require air heating, compressed air energy storage emits less carbon per kWh than a natural gas plant (and many networks, particularly in the United States, use fast-start gas-fired plants to supplement renewable intermittence, so trading this for compressed air energy storage is a step in the right direction). Conventional energy storage by compressed air releases about 228 g of CO₂ per kWh, which represents "less than 388 grams of CO₂ per kWh reported for combined-cycle gas turbines used in gas-fired power plants, "notes the document.

For the moment, even conventional compressed air energy storage is rather rare; Only two commercial compressed air facilities are currently in operation: one on the network in Germany and one in Alabama. In their article, the researchers write that 8 TWh of compressed air storage potential could be built in the UK on land, in salt caves. The use of these terrestrial cavities for compressed air would only cost a fraction of what it would cost to build compressed air storage in saline aquifers. For this reason, the researchers recommend that compressed air plants "be initially developed on land to improve technology and reduce operational costs".

DO I: Nature Energy, 2019. https://doi.org/10.1038/s41560-018-0311-0 (About DOIs).