This is an interesting concept:

https://re100.eng.anu.edu.au/2024/10/08/Energy-storage-is-a-solved-problem

It combines the advantages of pumped hydro and batteries. Basically, hydro power is cheaper for storing large reserves of energy for long periods, while batteries are cheaper for storing and supplying a lot of power over short periods.

So, you build a large hydro power plant, with a lot of storage (a big dam), but skimp on the power generation side to save money. Then you combine it with batteries. So if you have to use the combined system for a week straight, you run the hydro generator constantly over that week, and use it to charge batteries during off-peak times of the day, so during peak consumption periods of the day you are using both the battery and the hydro generator to meet demand. This gives you a cheaper system than either technology used in isolation.

Not just pumped hydro .. can also couple it with this idea


https://www.abc.net.au/news/2023-05-21/old-mine-shafts-make-renewable-energy-green-gravity-port-kembla/102367724

https://www.youtube.com/watch?v=QUP6hEeulX0


Quote:

The researchers estimate that by employing UGES into abandoned mines, this can provide a global energy storage potential ranging from 7 to 70 TWh. The majority of this potential is concentrated in China, India, the United States, and Russia.

This is a new one. Compressed air energy.


Quote:

What is compressed air energy storage? A compressed-air energy storage facility is a bit like a pneumatic nail gun but on a much bigger scale, says Ben Clennell, a research scientist at the CSIRO, who has done work for Hydrostor in the past. During periods of high supply, renewable energy is used to pump atmospheric air, under pressure, into a storage container. It is then released during periods of high demand to drive a turbine and generate electricity. It's a bit like storing air in a compressed air cylinder," Dr Clennell says. Unlike a nail gun, the Broken Hill facility has several extra steps to improve efficiency. The process of compressing the air generates heat. Your reverse-cycle heater works on the same principle. This surplus heat is stored and then used to heat the stored air when it is decompressed. The Broken Hill facility also uses water for a process called "hydrostatic compensation". The compressed air is stored in a water-filled 280,000 cubic-metre underground cavern, 600 metres below the surface. "As the air goes into the underground cavern, it pushes water out of the cavern to a lake at the top," Dr Clennell said. To tap this stored energy, the process is reversed — water is allowed to flow from the surface reservoir back into the underground cavern, pushing air back to the surface where it's reheated and used to spin turbines that generate electricity. Hydrostor, the Canadian company behind the Silver City Energy Storage Centre, said it would use a disused underground mine site adjacent to the proposed cavern to access and excavate rock to create the cavern. "It [the rock] will hold water and air without having to line it or build a steel vessel because it's very high-density rock," said Martin Becker, from Hydrostor. Dr Clennell said big compressed-air energy storage facilities had to be underground. "You can store air in pressure vessels or pipes, or various other arrangements at the surface, but they would rapidly become very large if you're wanting to store hundreds of megawatt hours of electricity-equivalent power," he said.

https://www.abc.net.au/news/2025-12-05/compressed-air-energy-storage-explainer/105999452