Impact of climate change on Hydro Tasmania’s Dams

20 08 2019

This is a guest post by Chris Harries, a consumate reader and follower of this blog. To my way of thinking, this shows yet again that renewables will not be able to power the future as we currently take for granted.

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Water inflows into Tasmania’s western river systems has been inexorably declining in recent decades. Furthermore, runoff is predicted to continue to decline in these catchments to the end of this century. This climate change trend has quite profound negative implications for Hydro Tasmania’s future business performance. A summary of these findings is attached – as extracted from Climate Futures for Tasmania CRC research document. It should be noted that the lowered water inflows are only partly caused by reduced rainfall. A bigger factor is soil dryness, caused by increased ambient temperatures. This factor reduces run-off more markedly, especially in the shoulder seasons (Autumn and Spring) Reduced runoff into the hydro-electric system can be notionally apportioned thus: 30% resulting from reduced rainfall as compared to 70% as a result of the soil dryness factor.


As a consequence of declining water runoff Hydro Tasmania officially downgraded the Long Term Average Energy Yield of its hydro system by over 10 percent in 2008. To graphically appreciate the scale of this, this equates to an equivalent loss of 130 MW of power generation capacity. To
replace that loss with new dam infrastructure would cost the business upward of $500 million. This downgrade was based on retrospective evidence from the previous 20 years performance data, showing that the performance of its whole system had been in decline, as shown in the


chart below. That time period was long enough for the business to accept the reality that this was an impact of climate change, not a temporal weather fluctuation issue.

Hydro Tasmania is fully aware that this trend in gradually lowered water inflows, is predicted to continue for the rest of this century.

This chart, showing electricity yield of the Tasmanian system, clearly shows the trend described above. Look at the horizontal bars. This information resulted in a downgrade of the system’s rated output by a factor of 10 percent.

Why soil dryness matters


Just as increasing soil dryness is causing dramatic changes to wildfire incidences in Tasmania, the very same condition is having dramatic impact on the state’s hydro-electric system. To understand this it is informative to compare Tasmania’s monthly rainfall with its river flows. From this chart we can see that Tasmania receives fairly even distribution of rainfall throughout the year.

By contrast the runoff into our river systems markedly peaks in winter months. The chart below shows a fairly typical pattern in this regard. Why is this so?

This phenomenon is almost entirely explained by the effect of soil dryness (temperature related). When soils become saturated, as they do in Winter, any rains that fall will instantly run off into streams and rivers. However, in warmer months when soils are dry a frontal shower may wet the soil surface temporarily and then evaporate without running off at all.


This hyper sensitivity – between soil dryness and water runoff – is resulting in rather dramatic consequences as climate change increases ambient temperatures, shrinking the mid-year band, above, where water flows are relied upon to replenish storages.

This drying trend is continuing


This year the Bureau of Meteorology published further clear data showing that these trends are continuing right to the present. The two charts below record a high level of deviation from historic conditions from the early 1970s to the present.

This data applies to the whole of Tasmania. The negative trend would be magnified further in the state’s western river catchments. It is perhaps a sobering thought that had the Franklin Dam being built it would have served no purpose at all other than to shore up declining system output.


Looking into the future

As we look to the future now, this double whammy (less precipitation + higher temperatures) has serious consequences for the bottom line of hydro-electric production and profitability.


Hydro Tasmania’s currently estimates that Tasmania is 90% self sufficient in electricity supply (from hydro + wind energy capacity). This estimate may indeed be a generous, top end figure since longer term climate trends become statistically valid only over considerable time. A few drought years can be seen as an aberration, accepting that weather fluctuates from year to year anyway. Longer term trends tend to be accepted only after following a good many years of data collection.


Continued modeling is being undertaken to further refine analysis of these climate change trends for Tasmania.


Why this may be the main driver behind the Battery of Nation project. It is worth putting these regressive energy losses into a practical context. The hard reality for Tasmania is that climate change induced energy losses from the Hydro system mean that 9,154 new 5kW rooftop solar systems would need to be added each year, just to compensate for climate change losses alone. This is three times the current installation rate of solar in Tasmania.


Alternatively, this would be equivalent to adding 6 new wind turbines (of typical capacity) each year to compensate for loss of hydro-electric output. That is, a major new wind farm, comprising sixty wind turbines, would have to be built each ten years just to stop us slipping backwards.


It should be noted here that the predicted decline in Long Term Average Yield of our power system affects base load supply. Hydro Tasmania can only supply energy to meet base load demand according to how much water goes into its dams.


From this we can see why the corporation is so keen to pursue its much vaunted Battery of the Nation project. Pumped-hydro technology is much less rainfall dependent because it stores energy by cycling the same water (generating electricity then pumping the same water back up). Hydro Tasmania’s ultimate expressed aim is to switch its entire hydro-electric system from base load energy production to peak load supply for the national market, seeing this in the interest of optimising its business bottom line.


References
Cooperative Research Centre: Water and catchments summary
‘Climate Futures’ reports for Tasmania
State government website
Hydro Tasmania Annual Report 2009
Entura website reference (mainly focuses on managing drought)


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6 responses

20 08 2019
Doone Wyborn

And of course the southward migration of rainfall belts will mean that the winter cold fronts will drop off into the southern ocean thus decreasing the wind resources as well.

20 08 2019
Perran

The claim, supported by the graph, that Tasmania’s rainfall is evenly distributed throughout the year is wrong. While Hobart’s rainfall might be evenly distributed throughout the year Tasmania’s west has strongly winter dominated rainfall. In the west of the state winter rainfall is about double that of summer. This is why runoff into hydro’s dams is much greater in the winter time.

Aside from that, I tend to agree with the overall premise of the essay.

21 08 2019
chrish618

Hi Perran, Yes, as a former Rosebery resident, the far West coast gets plenty of Winter rain. But not all hydro dams are in Queenstown. They are spread right across from from Poatina to the Mersey Forth to Lake Gordon.

Hydro Tas is under no illusion about the impact of increasing soil dryness.

To quote “The Climate Futures modelling indicates that long-term climate impacts on water resources in Tasmania are likely to reduce in flows and increase variability. This poses a significant risk for Hydro Tasmania. We have already de-rated our system’s long-term capacity to generate electricity due to recent climatic conditions of a reduction of average water in flows of over 10 per cent compared to the historical long-term average.”

21 08 2019
Lloyd

Have to agree with Perran on that one, as I sit here in Strahan with hatches firmly battened down.

21 08 2019
Bev

I must admit I don’t know anything about pumped hydro. I presume it works because the energy needed to pump the water back up to the dam doesn’t exceed the energy produced by the water falling under gravity. It sounds a bit like perpetual motion. It’s obviously unsustainable because population is still growing, dams silt up eventually and how many rivers are left available to be dammed. And all this is still only about electricity. The much more important liquid fuel decline is being ignored.

21 08 2019
chrish618

There’s a twenty percent loss in energy during the pumped-hydro cycle, Bev. This is due to frictional losses in water pumped. This energy loss figure is pretty well undisputed in the energy sector. But it’s what you would expect. As you say there’s no magical perpetual motion.

And yes, this story is about the issue of climate change on the electricity system. I have focussed heaps elsewhere on liquid and solid fuels.

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