A few months ago, the Future Energy Hub compared the size of the proposed Lake Cethana pumped hydro energy storage project (PHES) to the size of Tesla Powerwall 2’s home-scale batteries. Feasibility work at Lake Cethana has now showed that the initial design option of 600MW and 11 hours can be scaled up to 750MW and up to 20 hours’ deep storage duration – so it’s time to update the numbers!

Here’s part of the original article, with the figures revised:

“To see just how big Tasmania’s pumped hydro storages are, let’s do a basic comparison between Lake Cethana pumped hydro and Tesla Powerwall 2’s.
Storage size is described by how quickly it discharges (in kW or MW) and how much energy it stores (in kWh or MWh). A Tesla Powerwall 2 discharges at a rate of 5kW and holds 13.5 kWh of usable energy, which means they last about 2.7 hours. They cost around $15,000 installed.

Lake Cethana pumped hydro has a proposed capacity of 750 MW – the equivalent of 150,000 Tesla Powerwall 2’s running at the same time. Lake Cethana will be able to supply electricity at that rate for up to 20 hours. The 150,000 Tesla Powerwall 2’s would need to be replaced 8 times over that period – meaning it would take 1,200,000 Tesla Powerwall 2’s to supply as much energy as Lake Cethana! The cost of Lake Cethana pumped hydro is estimated at around $1.5 billion, while the cost of 1,200,000 Tesla Powerwall 2’s is $18 billion.

In 2020, Tesla installed its 100,000th Powerwall worldwide.

What about big batteries? To last as long as Lake Cethana pumped hydro, it would take the equivalent of 5 of South Australia’s Hornsdale Power Reserves, which until recently was the biggest lithium ion battery in the world, running simultaneously, and replaced every 1.3 hours – that’s 75 batteries. Using Hornsdale’s construction costs, that would cost $12 billion (Hornsdale was built for $90 million and upgraded to 150MW/194MWh at a cost of $71 million).

Another difference between storing potential energy using pumped hydro vs storing chemical energy using batteries, is that the capacity of pumped hydro won’t change over time, while batteries eventually start losing their capacity to hold a charge”.

You can find out more about Tasmania’s shortlisted potential pumped hydro sites here

Image: Hydro Tasmania, Lake Cethana