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In July 2019, the Australian government announced approval for the construction of the nation’s first compressed air energy storage (CAES) facility.
To be built by the Canadian company Hydrostor, the storage facility will repurpose an old zinc mine in South Australia and cost $30 million (€18.2 million). The facility will make use of surplus electricity generated by solar and wind energy installations to compress air in a storage facility underground. This compressed air can then be stored until it is needed to meet higher electricity demand when it is used to drive electricity-producing turbines.
The project will include the conversion of a disused mine into a large cavern capable of storing compressed air. Located 240 meters underground, the facility is based on an innovative design that is expected to achieve emissions-free storage of energy. South Australia has 47 percent of its energy production coming from renewable sources, and the CAES facility will allow the state to better access that clean energy when needed.
hen the facility is being charged, heat by-product from the compression process is gathered and stored. The high-pressure air then displaces water in the cavern, pushing it up to a surface reservoir. During the discharge phase, water would flow back to the cavern pushing air to where it can be reheated with the reserved thermal energy and then onto driving turbines to produce electricity.
Through these processes, a CAES can supply the dispatch-ability necessary to ensure the dependability of solar and wind power. A compressed air system is similar to pumped hydro energy storage, which stores energy by pumping water uphill and discharges energy by releasing the water back downhill to drive turbines. However, a CAES is more flexible with respect to location and topography. The Australian facility also has the added benefit of repurposing a former mining location.
The Issue of Heat Storage
One of the main issues with CAES is that when air is compressed, it gets hotter. Then, to discharge energy, the compressed air is expanded, which requires heating. Additionally, more air can be stored at cooler temperatures. A major focus of those developing CAES facilities is looking for ways to manage the heat produced during compression.
There are three major approaches to addressing heat generated during compression: adiabatic, diabetic, and isothermal storage.
Adiabatic storage involves keeping the heat generated during compression and reuses this heat during discharge. Even though the theoretical efficiency of this technique is 100 percent, the actual efficiency tends to be closer to 70 percent.
Adiabatic storage system dissipates generated heat into the surrounding atmosphere. During power discharge, the compressed air is normally heated with natural gas combustion. The expected efficiency of a diabatic storage system is approximately 70 percent.
An isothermal storage system uses heat exchangers to create an equilibrium between the internal and external temperatures, which causes heat to dissipate into the atmosphere as air is compressed. When the compressed air must be released, it is heated by thermal energy from the outside environment.
Maintaining Air Pressure
Another issue that any CAES facility must address is maintaining continuous pressure as air is released from storage. When the storage facility is full, air pressure is relatively high, but if the air is unregulated as it flows out, the turbine it drives will create less electricity as the storage facility depressurizes.
One tactic is to govern the flow of air so that it releases more air near the end of the discharge cycle, effectively countering a potential drop in pressure and ensuring a steady supply of power from the turbine. This approach also makes it easier to figure out the amount of power that can be produced from a volume of pressurized air.