Hydroelectricity storage – Expert Q&A

The government is looking at using pumped hydro storage to fill the gaps of a 100% renewable energy system.

But does New Zealand have the water resources for such a scheme? And what role should hydro power have in NZ’s clean energy goals?

The SMC put these questions to experts.

Dr Daniel Collins, hydrologist and climate change scientist, NIWA, comments:

How will increasingly common and severe dry years affect pumped hydro storage?

“Climate change will bring shifts in precipitation and river flows across the country, in some places bringing more water, in some less, and in others we may not even notice the difference. This is important for New Zealand’s hydropower generation. Increasing westerly winter airflow under climate change will likely bring more water to the headwaters of the major South Island hydropower schemes, which collectively dominate national generation, while North Island schemes may see little change or a slight decline.

“This would increase southern lake levels on average, reduce the severity of dry years, and shift the national dry season from winter to summer. This seasonal shift would be particularly helpful as electricity demand currently peaks in winter. However, projected increases in electricity demand far exceed additional generation due to increased flows.

“Storage of water in hydropower lakes acts like a battery, but the battery is only as big as the operating range of the lake. Nationally our hydropower schemes only have about three months’ worth of storage, so any additional storage – particularly longer than a single season – would help to avoid electricity crises.

“As New Zealand’s energy sector moves to decarbonise, hydroelectricity will increasingly need to focus on filling the gaps when the wind isn’t blowing or the sun shining, rather than just filling gaps during dry years. Again, storage between seasons would be invaluable, as well as simply between day and night.”

Could demand for irrigation affect the amount of water available for hydroelectricity?

“Irrigation and hydroelectricity compete for the same water in some places. This competition may increase over the century as irrigation demand rises to balance higher temperatures and shifts in rainfall.

“There would also be competition with growing municipal demand as the current situation with Auckland illustrates. At the same time, climate change would lead to increases or decreases in water availability around the country which may exacerbate competition or lessen it.”

Will there be ecological costs of expanding hydroelectricity?

“All hydroelectricity generation comes with an ecological cost, as does energy generation in general. With hydroelectricity, raising lake levels affects adjacent ecosystems while altered flows change the character, ecology, and other values of downstream rivers. Development of the Manapouri hydropower scheme, for example, played a large role in the rise of the environmental movement in New Zealand.

“But climate change also comes with ecological, social, and cultural costs, and shifting to renewable energy sources across the energy sector – transport included – is an important part of our climate stewardship. Deciding how to balance these costs and benefits will be no easy task.”

What role should hydroelectricity play in the clean energy transition?

“Increasing electricity demand, driven in large part by decarbonisation of New Zealand’s energy sector, is likely to rely heavily on expanded wind and solar generation. However, these are variable and spasmodic. The storage of water in lakes, which acts like a battery, will make hydroelectricity an important back-up when the wind isn’t blowing or the sun shining. Currently our national hydropower schemes only provide about three months’ worth of storage, so new pumped-storage schemes would help in this regard.

“New schemes associated with rivers fed by the Southern Alps would be particularly helpful, such as Lake Onslow in Otago, given the additional water these rivers are likely to receive under climate change.”

No conflict of interest.

Dr Geoffrey Pritchard, Senior Lecturer, Department of Statistics, University of Auckland, comments:

How will increasingly common and severe dry years affect pumped hydro storage?

“The inflows to South Island hydro lakes are strongly seasonal, with the lowest inflows in winter. This occurs because much winter precipitation in the Southern Alps falls as snow, and doesn’t reach the lakes until the spring thaw. A trend towards warmer winters in the region would tend to reduce this effect, and some modelling does suggest that a warmer global climate would have a favourable effect on the seasonal profile of inflows. But if so, this is a slowly evolving process that will take decades to play out.

“In the meantime, there is a mismatch between the demand for electricity (highest in winter) and hydro inflows (highest in summer). The controlled storage lakes (Tekapo, Pukaki, Hawea, and Manapouri) go some way towards ironing out the difference, but are too small to do so completely. (Lake Taupo is a different case: it enjoys high inflows in winter.)

“The limited water storage capacity in these lakes is most noticeable in a dry year: a period (not literally a year, typically more like three months) of low hydro inflows during which a large amount of electrical energy that might have been generated via hydropower must instead be produced another way, or its consumption foregone. The means of alternative generation must be one that is not already operating (else it would have no more to give when the dry year arrives), which creates a considerable economic problem: how to pay for something that is only rarely of any use?”

No conflict of interest.

Professor Justin Hodgkiss, Co-Director, MacDiarmid Institute, comments:

How will increasingly common and severe dry years affect pumped hydro storage?

“The key point about pumped storage is that the water is effectively cycled through a hydroelectricity station over and over again (with the input of other renewable energy sources like wind and solar).

“For this reason, pumped storage may help us mitigate the risk of dry years, which currently makes it challenging and expensive to reach 100% renewable electricity, and also increases the price of electricity.”

What role should hydroelectricity play in the clean energy transition?

“Hydroelectricity already delivers most of New Zealand’s electricity, and while our electricity supply is already >80% renewable, it only represents 5% of our total carbon footprint. The Interim Climate Change Committee found that the most cost effective way to decarbonise our economy is to start electrifying industries that use fossil fuels, for example coal-fired milk powder drying plants, along with transport. Electrifying industry and transport without increasing the cost or the carbon footprint of electricity for other consumers will require increased supply of renewable electricity.

“Since the most feasible sites for hydroelectricity are already being used, many hopes naturally rest on other renewable energy sources – particular solar and wind – to boost our energy supply. Those two sources suffer intermittency – they don’t generate electricity when the sun isn’t shining or the wind isn’t blowing. Over-reliance on wind and solar without storage to mitigate this intermittency can actually increase carbon emissions because gas powered electricity plants are then required to stabilise the electricity supply.

“This is where pumped hydro comes in – at times of abundant solar or wind power, the excess energy could be used to pump water uphill, ready to be released to generate hydroelectricity at a later time. A bit like a battery (which is another worthy alternative for distributed grid storage). Thus, pumped hydro might actually unlock the potential of other renewable energy sources in our energy mix.”

Conflict of interest statement: Justin Hodgkiss is Co-Director of the MacDiarmid Institute, which includes researchers working on renewable energy technologies like solar PVs and batteries.