Early investigations indicate Lake Onslow meets six "critical requirements" necessary for a large-scale pumped hydro scheme — but it comes with a trade-off against environmental, social and cultural values.
In a recent report to the Cabinet, Energy and Resources Minister Dr Megan Woods said work — which falls under the $100 million NZ Battery Project, a Government scheme investigating solutions for New Zealand’s dry year electricity woes — was ongoing but any technical barriers to the development and operation of the proposed scheme had yet to be identified.
Findings determined Lake Onslow contained enough elevated water storage for the project’s upper reservoir and the surrounding land was strong enough to support the building of a dam.
The capacity for electricity generation was large, Dr Woods said.
"The various tunnel options could support up to 1500MW, with up to six turbines/pumps of 250MW each. For comparison, New Zealand’s largest hydro plant is Lake Manapouri, with installed capacity of 850MW. Huntly power station has a total installed capacity of 953MW."
Nearby existing national grid infrastructure ticked off another requirement, state-owned Transpower confirming a pumped hydro scheme could integrate into the national grid by connecting to nearby high-voltage lines.
The Clutha River was also sufficiently lower in altitude than Lake Onslow with enough water to pump from and had suitable tunnel and lower intake options, the report said.
"The feasibility study is narrowing down the most feasible intake and tunnel locations. There are some prospective locations for a lower water intake along the Clutha River/Mata-Au both upstream and downstream of the Roxburgh dam, each with their own practical and geological risks to manage."
Private interests and environmental challenges needed to be be managed as Lake Onslow was also an important cultural, social and landscape asset, Dr Woods said. Impact studies identifying how these might be affected by the development of a pumped hydro scheme were ongoing.
"Many of the features of Lake Onslow that present environmental and social challenges are the same features that make it technically attractive for developing a pumped hydro scheme, for example, an elevated basin, close to an existing water source. These challenges speak to inherent trade-offs necessary when developing large-scale hydro assets."
Lake Onslow was a "highly regarded" location for local anglers, with notably high stocks of brown trout, which were likely to deteriorate with the raising and varying of the lake level that came with a pumped hydro scheme and electrical infrastructure to support the scheme would visually affect the "naturalness and uniqueness" of the area’s landscape.
Raising the water levels at Lake Onslow to support pumped hydro would inundate private land, for which the Crown would need to compensate.
Background
The NZ Battery Project is investigating solutions to the "dry year problem" when hydro catchments are low and fossil fuels are burnt to generate electricity to cover shortfall.
- Lake Onslow is a man-made lake, 20km east of Roxburgh.
- The pumped hydro storage scheme proposal would raise the footprint of the lake from 700m above sea level to 760m.
- Pumped hydro technology involves transferring water between two reservoirs at different heights.
- The maximum depth of Lake Onslow at present is 10m-20m.
- A pumped hydro storage scheme at Lake Onslow is one option being investigated.
