What is pumped hydro and how will it change Borumba Dam?

Kilometres of huge underground tunnels, mountain caverns as big as the Gabba and six new dams up to 1.4km long are just the start of the $14.2bn project powering up Queensland.

The game-changing, $14.2bn, 2000MW Borumba pumped hydro project which will transform the Mary Valley into a major hub of Qld electricity has been greeted locally with a mixture of fear and excitement.

Excitement has been expressed for the job opportunities around the transition to renewable energy but Valley residents say they are also anxious about how the project will be delivered and how it will change the region.

What exactly is pumped hydro and how will it transform and fit in with the landscape around Lake Borumba?

What is the timeline for project milestones?

How will it work in with the electricity grid, as it both supplies and extracts energy for its pumps.

We take a deep dive into the planning and engineering, putting questions to civil engineer and Qld Hydro project director Leah McKenzie.

Simple diagram of how pumped hydro works.

What is pumped hydro?

In essence, pumped hydro is the ability to capture electricity from the force of water flowing from uphill to downhill and then pumping it back up to collect energy again as it comes back down.

While the first pumped hydro apparatus can be traced back to the early 1900s in Switzerland, it wasn’t until reversible turbines were invented in 1920, that it became a feasible source of energy.

These reversible turbines could both collect energy from water’s downward force and pump it back up to be used again.

A model at a Qld Hydro community briefing in Imbil shows the reversible turbines underground and the generators. Picture: Christine Schindler

Pumped hydro requires two reservoirs of water at different elevations connected by tunnels running through reversible turbines.

“The water drops vertically, the pressure changes and spins the turbines and then that creates the energy,” Ms McKenzie explained.

A model at a Qld Hydro community briefing in Imbil demonstrates how pumped hydro works. Picture: Christine Schindler

The water then slows down through the tail race tunnel and flows into the lower reservoir, to be pumped up again at a later time.

The energy collected from the turbine then goes on its own journey through the power transformers into transmission lines, onto the electricity grid and powers appliances, machines, lights in homes, businesses, parks, roads, etc.

“An electron is produced when you create energy, and that electron will move through the electricity grid, much like water, moving through pipes in the water system,” Ms McKenzie said.

“It’ll flow typically to the closest demand source, and then when we have excess supply, that flows further within the grid.”

Civil Engineer and Qld Hydro Project Director Leah McKenzie is leading the Borumba pumped hydro project in the Mary Valley. Picture: Supplied

The demand source, for example, could be anyone charging their phone in Widgee to switching on the lights at Brisbane’s Suncorp Stadium.

The International Hydro Association estimated in 2019 there was more than 158GW of worldwide pumped hydro capacity.

There are more than 100 pumped hydro projects under construction, and by 2030, world hydro capacity will almost double to 240GW, it said.

Today, pumped hydro accounts for 97 per cent of the world’s energy storage.

Transmission lines cross Mary Valley Rd near Imbil and Borumba Dam. Picture: Christine Schindler

The Queensland government has identified pumped hydro as a reliable renewable energy source in transitioning away from fossil fuels.

This however, means investing billions of dollars in new infrastructure to reshape Queensland’s energy infrastructure along with figuring out where to build associated and controversial transmission lines. 

Borumba pumped hydro is expected to cost $14.2bn, with $6bn committed this financial year.

Department of Energy and Public Works has said it would be the “biggest single investment in Queensland’s energy infrastructure in decades”.

How will pumped hydro infrastructure change Borumba Dam?

A map of the proposed Borumba pumped hydro project.

While the state government has given the financial green-light for the project, moving it into the exploratory works phase on July 18, 2023, it is still subject to environmental approvals.

During the exploratory works, the first of two 1.8km long D-shaped profile access tunnels, 8.5m high and 8.5m wide, roughly the width of three lanes of traffic (~3m/lane) will be built through a drill and blast method.

These tunnel will be used for maintenance, access and running cables from the underground power stations to overground transmission lines.

The construction of the first tunnel during exploratory works will give a more comprehensive understanding of the types of rock and soil around Borumba.

Qld Hydro is in the exploratory phase of transforming Borumba Dam into a pumped hydro powerhouse. Picture: Christine Schindler

“The geotechnical information we get from exploratory works informs how the design is progressed forward,” Ms McKenzie said.

“So what are the ground conditions like: What‘s the rock like? How strong is the rock? What’s the profile of the rock? Does it change as we go along?

“Also, is there underground streams, and what sort of water are we going to encounter?

“There’s also inherently stresses within the ground, so it’s also us getting an understanding of what’s the natural flow of those stresses, and how can we design to suit those stresses, so we’re not going against them,” she said.

The exploratory works phase is expected to be completed by 2025, construction to begin in 2026, and first power penciled in for 2030.

Timeline of Borumba Dam pumped hydro project.

Construction of the upper reservoir

Qld Hydro has identified a piece of land 350m higher than the current reservoir that has capacity for 70GL of water, or 24 hours of energy storage.

In order to create this upper reservoir, five saddle dams need to be built, the largest of these will be 1.4km long and 40m high.

The saddle dams will act as a reinforcing structure alongside the mountain ridgelines, connecting the peaks together in order to hold in the body of water.

Map of Borumba Dam pumped hydro project.

The main dam wall will be 100m in height from toe to top with a concrete spillway next to it.

The land surrounding the upper reservoir has been owned by the state government for the past 40 years, it is a primarily forested area used for cattle grazing.

The upper reservoir will have vertical twin pressure shafts, 9m in diametre with a 330m drop.

“The pressure shafts, for example, will be a steel bind pipe, because of the pressure running through them, they need to be very strong,” Ms McKenzie said.

“Once that reservoir is full, we pretty much press a button, the water drops down from the upper reservoir into the turbines, spins the turbines and produces energy.”

Underground caverns

Within the mountain face, 400m underground and between the upper and lower reservoir will sit two massive caverns connected via tunnels, expecting rock face, Qld Hydro will use a drill and blast method to build the caverns.

One cavern, estimated to be roughly the size of The Gabba Stadium in Brisbane (250m long, 35m wide, and 60m high), will be home to six reversible turbine pumps, with a capacity of 330MW each.

Model depicting possible underground caverns with transformers and turbines. Model only, not to scale or size. Picture: Christine Schindler

The next cavern will be slightly smaller (235m long, 30m wide, and 30m high) and will be home to six three-phase 500kV transformers.

Ms McKenzie said having the power station underground meant there would be more efficiency with the power generation and typically less environmental impacts.

Once the energy is captured within the turbines, it runs through the transformers, through one of the access tunnels as mentioned above, into the transmission lines and finally into the electricity grid.

Upgrading the lower reservoir

The lower reservoir is planned to increase to almost five times its current capacity. A new dam wall is to be built 300m downstream from the original one.

This dam will reach 20m higher than the one in place and increase Borumba’s water capacity from 46GL to 220GL.

“This provides reliability to run the hydro scheme, but also the reliability for the water allocations,” Ms McKenzie said.

“As we increase the height of the dam wall, you’re going to get increasing reliability because we can catch more water.”

The current Borumba dam wall. Another one is planned to be built 300m down stream (behind camera view) and will be 20m taller than this one. Saturday August 5, 2023. Picture: Christine Schindler

This will mean the reservoir will sit at a higher level, inundating the forested areas and the surrounding floodplains.

The land around the lower reservoir is owned by Qld Hydro and other state agencies. The reservoir will be open to the public during construction, except for integral parts of the project due to safety reasons.

Once complete, the reservoir will be open to the public as per the current conditions.

Pumping the water back up

Reversible turbines work in an either/or manner - either collecting energy as the water drains from the upper reservoir to the lower reservoir or pumping it back up to refill and reuse again.

The practice will be to recharge or refill the reservoir at peak energy supply and low demand.

“Typically, hydro power is used in the evenings when the sun’s not shining and your wind and solar drops off,” Ms McKenzie said.

“But everyone comes home from work and school and everyone goes, ‘Let’s cook dinner, let’s do the washing.’ So the power demand increases.”

Powerlines cross Mary Valley Rd near Imbil and Borumba Dam. Picture: Christine Schindler

“That’s why you need hydro to come into the mix, so you’ve got reliability in the network.”

During the day, when wind and solar energy is being captured there is less demand and an excess supply of it.

Qld Hydro intends to pump and refill the upper reservoir, relying on the excess of renewables expected to be around by 2030 and established energy use.

“You’ve got some (wind and solar) on the grid already, but there’s a lot that will happen between now and then in terms of the other renewable energies,” Ms McKenzie said.

“As an example rooftop solar from people’s houses as well, there’s a significant volume of that within Queensland.

“Even looking at the energy market now, you create more during the day than what the demand is, so there’s an increased amount that’s not used.

“We’ll be looking to use that surplus energy to pump during the day, and then when we’re ready at night, to generate what wind and solar aren’t generating.”

About Qld Hydro

Established in September 2022 by the Qld government, Qld Hydro is a publicly owned entity focused on securing future energy store, set up for the purpose of keeping pumped hydro assets within the hands of the Australian public.

Qld Hydro estimates 2000 or more jobs will be generated at peak construction of the Borumba Dam with an array of packages already available for tender.

For years Borumba Dam has been acknowledged as a potential pumped energy storage site, and it was in mid-2021 when plans started to progress towards reality with the project growing each year.

Qld Hydro has stated a commitment “to ensuring communities are at the centre of the energy transformation and we are listening, and will continue to listen, to local communities about the benefits that are important to them.”

This statement comes despite some saying the community consultation is just about crossing t’s and dotting i’s along with a population who are cautious and mistrustful towards ambitious government plans for the area.