Energy

Creating renewable energy storage out of hot air

Creating renewable energy stor...
Old mines could get a second lease of life as energy storage caverns if a compressed air storage system proposal by EU scientists gets the green light
Old mines could get a second lease of life as energy storage caverns if a compressed air storage system proposal by EU scientists gets the green light
View 2 Images
With this AA-CAES system, heat from the compressed air is stored in rocks so it doesn't go to waste
1/2
With this AA-CAES system, heat from the compressed air is stored in rocks so it doesn't go to waste
Old mines could get a second lease of life as energy storage caverns if a compressed air storage system proposal by EU scientists gets the green light
2/2
Old mines could get a second lease of life as energy storage caverns if a compressed air storage system proposal by EU scientists gets the green light

The rapid growth of solar arrays and wind farms might sound like a win for the environment, but storing renewable sources of energy efficiently on the grid remains a challenge for energy providers. EU scientists are turning to a cheap and plentiful natural resource for the answer: air.

While flow batteries and various other smart grid technologies have emerged in recent years to address the problem of renewable energy storage, a number of scientists are revisiting a decades-old method: compressed air energy storage (CAES). It involves using surplus electricity (either from a power plant or renewable energy) to compress air, which is then stored in an underground cavern. When power is needed, the air is released through a gas turbine that generates electricity. On paper, it is an attractive solution: it is, theoretically, one of the cheapest ways to store massive amounts of energy and compared to hydropower reservoirs, it doesn't require facilities to be situated in the mountains, which not every country has.

However the process of compressing air results in the production of heat, just like what happens to a bicycle pump when it heats up while filling your tires. The problem here is what to do with the heat that's generated. At present, there are only two CAES plants in the world – the Huntorf plant in Germany and the McIntosh plant in Alabama, USA, which have peak capacities of 290 MW and 226 MW respectively. At both facilities, the heat is released as a by-product of the compression process as neither has the infrastructure to process and store it. Apart from losing a potential source of energy, what this also means is that the air needs to be heated up again, often with natural gas, to run the turbines that generate electricity. This reduces the energy efficiency of the system.

To address this issue, scientists working on the EU-funded RICAS 2020 project are turning to advanced adiabatic CAES (AA-CAES) to develop a design concept that enables heat storage.

A key component of this project is a section that enables the heat to be siphoned off during the compression process. As shown in the illustration below, after the air leaves the compressor, it passes through a cavern filled with crushed rocks, which absorb its heat. The cold air then makes its way down to the main cavern where it is stored. When it is needed, it makes its way back through the crushed rocks, where it is reheated, before being discharged, thus eliminating the need to burn fuel.

With this AA-CAES system, heat from the compressed air is stored in rocks so it doesn't go to waste
With this AA-CAES system, heat from the compressed air is stored in rocks so it doesn't go to waste

Giovanni Perillo, a materials scientist at SINTEF who is one of the project managers on the project, believes this technology has the potential to offer a better alternative to batteries due to its longer lifetime and lower capital cost per kWh of stored energy. Also, compared to existing storage sites, which have an efficiency rate of 45 to 55 percent and produce only half of the energy used in the air compression process, this new system could raise the rate to as much as 70 to 80 percent.

"The more of the heat of compression that the air has retained when it is released from the store, the more work it can perform as it passes through the gas turbine," he says. "And we think that we will be able to conserve more of that heat than current storage technology can, thus increasing the net efficiency of the storage facilities."

Another advantage that this AA-CAES technology offers is that it is not limited to certain geological formations. While the facilities do have to be located in places where large hollow spaces already exist, the researchers say these could also include disused caverns and mines. The RICAS 2020 project, for example, is being conducted in an abandoned mine in Austria.

That said, for the project to move past the design phase, which would be a first for an AA-CAES project, it has to meet a few conditions. These include developing a sealing membrane that can withstand the cavern's high temperatures and pressure. Based on the results and cost analyses, a decision on whether to move forward with a pilot plant will then be made.

Despite being around for decades, compressed air has yet to realize its potential in the renewable energy sector. This is due to a number of reasons, says Matthias Finkenrath, a professor of energy engineering at Germany's Kempten University of Applied Science, who cites technological challenges, low energy prices and uncertainties in the energy market as factors that have caused plans for large-scale facilities to stall or be abandoned. One example is the US$400 million Iowa Stored Energy Park, which was scrapped in 2011 after studies showed that the state's sandstone aquifers weren't suitable for CAES.

That said, these setbacks have not stopped dimmed its appeal as scientists and those in the industry look for ways to improve the storage of off-peak generated renewable energy on the grid. Compared to the chemical compounds in cell-based battery systems, air costs nothing, won't degrade and is all around us. With places such as Hawaii launching initiatives to rely on renewable sources for the majority of its energy needs, compressed air storage could make wind power as dependable as energy generated by fossil fuels if scientists can figure out how to harness it and overcome the various geological challenges.

"Compressed air stores of the sort that this project is aiming for, could provide significantly lower costs and greatly improve storage capacity compared to for example batteries," says Finkenrath. "If this project leads to storage plants being established under a wide range of geological conditions, that in itself would be an important step forward.

"If the project partners are also successful with their plans for efficient heat storage, the use of compressed air for energy storage could be on the edge of a breakthrough."

Source: SINTEF

7 comments
notarichman
decades ago i suggested to a local power company to use old mines in Northern Idaho for compressed air energy storage. their reply was "we can't seal the mines." mines usually have multiple accesses for safety, ventilation, and moving ore and tailings. However, why can't the seal be located far enough inside the accesses that the rock is strong enough to contain the pressure. Why depend upon super high pressure instead of vast volume? Northern Idaho has mines with miles of adits & shafts that are not being used. Windmills don't use high air pressure to run. Higher pressure would certainly be more efficient though.
Daishi
There may be a lot of relevant prior research on suitable storage from the natural gas industry. Natural gas is typically stored under ground under very high pressure. 86% of of underground natural gas storage is done so in depleted reservoirs. This is what EIA.gov has to say about it: "Most existing natural gas storage in the United States is in depleted natural gas or oil fields that are close to consumption centers. Conversion of a field from production to storage duty takes advantage of existing wells, gathering systems, and pipeline connections. Depleted oil and natural gas reservoirs are the most commonly used underground storage sites because of their wide availability." So it seems like depleted reservoirs that are not being used for natural gas storage would be perfect for this and already contain much of the required infrastructure.
habakak
...and compared to hydropower reservoirs, it doesn't require facilities to be situated in the mountains, which not every country has.... No. But for the proposed solution you have to be near an abandoned mine or cavern. So it's 6 of the one and half a dozen of the other.
...air costs nothing, won't degrade and is all around us.... So is just about everything else, it's just not in a useful form. Even if air is free, you still have to spend millions to build this 'plant' with the compressors, turbines, etc. Sunshine is free too, but we have to spend millions to build solar farms to harness it. Batteries can be deployed almost anywhere and with cost rapidly falling and energy densities improving fast, grid-scale storage is becoming affordable in many locations in the world. In less than a decade it will be affordable enough almost anywhere in the world. And it is just in time as we are starting to generate so much renewable energy that we simply need cheap storage to be able to switch over to a carbon-neutral society.
Off course all avenues should be pursued, but this approach clearly has the same type of limitations as pumped hydro. The economics of grid-scale batteries are changing fast though.
Richard D. McDowell
Talk about Rube Goldberg !
apprenticeearthwiz
Renewable energy infrastructure cost are already lower than fossil fuel infrastructure costs and getting cheaper. Of course storage is the key and we need to try everything. Currently battery storage is already highly competitive with FF infrastructure costs and getting better and cheaper, so other forms of storage will have to compete with that. I hope they can, we would all be better off and end the fossil fuel age even sooner.
Bob Stuart
This is one of the better proposals I've seen. Another possibility is to use the heat of compression for process heat or district heating. Then, re-expand it in stages, with heat exchangers to bring it up to ambient temperature between stages. This can show net gains. They get better if you can take advantage of daily swings in temperature.
Sascha
there's an awesome startup in CA that does cools and reheats the air but has advanced past the stone-age technologically! They found a water spray cooling tech (ok, discovered in the 1800's I think) and applied it to their compression cycle, they recover the heated water and store it, then use it to heat the expanding air for the power cycle. they also plan to use abandoned mines to store the air.