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Using mines for pumped storage of energy

The most cost-effective renewable energy options, wind and solar photo-voltaic, produce power intermittently; in the first case, when the wind blows; in the second case, when the sun shines.

pumped-hydro energy storage uses electricity when it is cheap (when there is lots of electricity being generated and less being consumed) to pump water from a low storage to a high storage. When the price of electricity goes up (because less is being generated and/or more is being consumed), the water flows through turbines back into the low storage, generating electricity as it goes.

Written 2015/10/08, modified 2016/09/04 – ©
Contact: email daveclarkecb@yahoo.com
 
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For a pumped-hydro energy storage system to be viable you need a high reservoir and a low reservoir, each of which can hold a substantial volume of water; the required reservoir size will depend on the desired energy-storage capacity. Unlike in conventional hydro-power, in pumped-hydro the same water can be used over and over again.

The amount of energy that can be stored is proportional to the product of the altitude difference times the volume of the storage: that is, either a big volume with a moderate altitude difference between the reservoirs, or a smaller volume and a big altitude difference.

So, what is needed?

  1. A mine, at least part of which is no longer used;
  2. A site with either a good wind resource or a good solar resource;
  3. Either a connection to a regional grid or at least a local micro-grid;
  4. A site for a surface water storage.
The mine would need:
  1. A large depth-volume product (if not great depth then great volume);
  2. A substantial volume of open workings beneath the water table;
  3. Stability (either underground workings in hard, stable rock, or an open cut that will not slump disastrously.
Tailings dams are unlikely to be of much use because:
  1. They would tend to be nearly full of tailings at the end of the mine's life;
  2. They would be large and shallow, while a smaller, deeper dam would be better in order to reduce evaporation losses.
As an example of the amount of pumped-hydro that might be required to fill-in for the intermittent generation of a wind farm consider the following. A moderate sized wind farm can generate 100MW. So a day's generation at full power would be 2400MWh. From Energy Units we see that 1GL of water falling 100m would generate up to 270MWh (at 100% efficiency), so we'd need around ten times this to generate 2400MWh (or 2.4GWh). (I believe industrial hydro-turbines can be about 85%-90% efficient.) I worked in the hydrogeology (underground water) field for about 30 years, but I had very little experience in mine dewatering.



Proposed projects

One Australian mine that is being considered for pumped-hydro energy storage is the open-cut Kidston gold mine in Queensland. This was discussed in Renew Economy on 2015/10/08.

Another, in Canada, is the Marmora project in Ontario, with an expected capacity of 400MW for up to 5 hours. Its designed average head is 140m.




In my state, South Australia?

Perhaps at Olympic Dam?

It seems that around 9 million tonnes of ore have been mined annually for at least 10 years; say 90 million tonnes total. This would have occupied a space of perhaps 25 million cubic metres. Supposing that this space is still open, it could contain 25GL of water. The depth of the ore body is up to 2km.

For the sake of calculations I will suppose that only 10% of this space is available for use in a pumped-hydro project, and that the available space is 500m below the surface. From Energy Units we see that lifting 1GL over 100m involves 270MWh of energy. So, lifting 2.5GL 500m would involve 3.4GWh, a very substantial amount of energy.

Olympic Dam is connected to the eastern Australian power grid and it has an excellent solar power resource, so the pumped-hydro could be combined with a major solar power station.

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