It makes up the vast majority of all energy storage worldwide – but do you know how pumped hydro actually works?

With more and more wind and solar power in the electricity generation mix, there’s a growing need for energy storage. That’s because electricity generation from wind and solar power is variable – it’s dependent on the weather. But for power grids to function, operators need to be able to manage supply and demand in real time, with a continuous flow of electricity, in order to avoid shortages, overloads and outages.

Electricity generation from wind and solar power is variable, because these generators are dependent on the weather. But in order for power grids to function, operators need to be able to manage supply and demand in real time, with a continuous flow of electricity, in order to avoid shortages, overloads and outages.

This means that, as variable renewable energy makes up an increasingly large percentage of the electricity in Australia’s generation mix, there’s a growing need for large-scale storage to store excess power at times when wind and solar output exceeds demand, and inject that power into the grid at times when wind and solar output can’t meet demand, to provide 24/7 reliable power.

Whereas coal can be stockpiled, electricity itself cannot be stored – but it can be converted into other forms of energy, which can be stored and later reconverted to electricity when required. Batteries, for instance, don’t actually store electricity, but they do store chemicals that can be converted into electricity, through a process known as electrochemistry.

Similarly, excess wind and solar energy can be used to power the production of renewable hydrogen. This can be stored for long periods of time and converted back into electricity that can be dispatched when the market needs it, helping to firm the grid and smooth out peaks and troughs in supply.

What is pumped hydro?

Hydropower, which converts the energy of moving water into electricity, is one of the oldest energy technologies in existence.

Hydropower works by passing water from an upper reservoir down to a lower reservoir, usually a dam. On the way down, the water flows through a turbine. The force of the water causes the turbine’s blades to rotate. The spinning turbine, in turn, drives a generator connected to it, which converts the motion into electrical energy, or hydroelectricity.

Pumped hydro takes this process a step further. A plant using pumped hydro technology won’t just pass water down through a turbine. It’ll then pump that water back up to the upper reservoir through a pipe or tunnel, where it can be stored and used to generate energy again.

Pumped hydro storage consumes more electricity than it produces – it requires about 20 per cent more electricity to pump water uphill than is generated when the water comes back down. But timing is everything, and the key to pumped hydro’s usefulness is in when it consumes and produces energy.

At times when supply is plentiful and demand is low, electricity – either from the grid or from a nearby renewable energy source – is used to pump water from the lower reservoir into the upper reservoir.

The water is then held in the upper reservoir until energy is needed, either because wind and solar production has reduced or stopped or because demand has increased (or both). The water is then passed back through the turbine into the lower reservoir to produce hydroelectricity.

This essentially turns the upper reservoir into a giant battery, storing surplus energy until the grid requires it. Pumped hydro generators are capable of fast response times (from idle to full capacity in one or two minutes), so their hydroelectricity can be dispatched into the grid almost immediately whenever the operator requires it. Their generators can also be operated as synchronous condensers, providing grid stability services without consuming any significant quantity of water.

Today’s lithium-ion batteries are ideal for storing and releasing energy over short periods of time – say, for storing energy while the sun is shining during the day and releasing it when demand peaks in the early evening.

But pumped hydro is generally capable of storing larger amounts of energy for longer periods of time, making it a more cost-effective option for overnight and long-term storage – at least until bigger and better batteries are more economical.

Because of their heavy rotating generators, pumped hydro plants provide the grid with inertia. Inertia acts as a shock absorber, maintaining consistent frequency across the grid and preventing surges that can cause damage to connected electrical systems and equipment.

Presently, this inertia is provided by coal-fired and gas-fired power stations. But it’s not provided by generators that are connected to the grid via inverters, including solar panels, wind turbines and batteries, although new developments in grid-forming inverters may address this issue.

What’s the status of pumped hydro in Australia?

Pumped hydro accounts for about 97 per cent of all energy storage worldwide, but there are only three major pumped hydro systems connected to the grid in Australia at the moment.

The 1.6 GW national pumped hydro fleet includes the Wivenhoe Power Station in Queensland, and the Tumut and Shoalhaven stations in New South Wales. In May 2021, the Wivenhoe pumped hydro station ramped up quickly to generate 530 megawatts (MW) over a four-hour period, helping to meet demand after an unexpected outage.

The Borumba Dam pumped hydro project will be the first long duration pumped hydro to be built in Queensland. The project is currently undergoing early works onsite while Queensland Hydro seeks important regulatory approvals.

Ultimately, the more energy storage is connected to the grid – whether it’s in the form of pumped hydro, renewable hydrogen, batteries, or other emerging technologies – the more renewable energy resources will be unlocked, ensuring a steady and secure supply of power for homes and businesses around Queensland and Australia as our energy system evolves.