The basics of powering generations

20 August 2017

Hydro Tasmania is Australia’s largest generator of renewable energy.

Tasmanians are our owners, our most important customers, and the very people we were created to serve.

For more than a century, Tasmanians have relied on hydropower to grow the economy and support communities. We now employ more than 1,100 people, with the vast majority in Tasmania.

Hydro Tasmania also stands ready to increase generation and help make Tasmania the Battery of the Nation. We have the skills, experience and passion to help create an energy future that’s clean, reliable and affordable.

Our operations touch every part of the state, but sometimes aren’t well understood. We need to communicate more often and more simply about the basics of generating energy in Tasmania.

How we operate water storages is a good place to start.

The water level in Hydro Tasmania’s hydropower storages is currently just over 40 per cent full, which is very secure for this time of year.

In fact, despite a fairly dry autumn and early winter, storages are actually at their most secure level in five years (for this time of year), as we enter what’s usually the wettest part of the year. We comfortably exceeded our target of 30 per cent at the end of June, and we’ve already just exceeded our start-of-summer target of 40 per cent.

Some Tasmanians will hear that storage level of 40 per cent and know it’s secure and routine – just as a level in the mid-20s would be secure and routine towards the end of summer into autumn.

But many others may think “hang on – that’s barely a third full!” or “where’s the other 60 per cent?!” And those are understandable reactions.

So let me explain…

A 100 per cent storage level reflects the absolute maximum amount of water that a lake or lagoon can actually hold. But in reality, the water never gets anywhere near that level. Nor should it.

At 100 per cent full, the lake would be much larger than its ‘normal’ size and appearance. Boat ramps and fishing spots would be underwater, and you’d face the possibility of flooding with just a sprinkle of rain or a puff of wind.

Just as you don’t fill a bath to the very top, lakes aren’t meant to be totally full. Indeed, non-hydropower catchments are naturally kept in check by rivers and evaporation taking water away.

Natural factors aside, building up storages to very high levels would be extremely inefficient and uncommercial. That huge amount of unused water would represent a huge amount of clean hydropower that we’d failed to generate.

In reality, Hydro Tasmania’s overall storage level - the aggregate level across all our storages - hardly ever goes above 50 per cent or below about 25 per cent.

The overall level (known as Total Energy in Storage – TEIS) generally peaks in the 40 per cent range during the wettest part of winter and spring, and bottoms out in the 20s in the driest part of summer into autumn. Both levels are quite normal and secure. They just represent different extremes in the seasonal cycle.

While our basic systems and operations are simple enough in practice, we’ve often passed up the chance to explain them practically to Tasmanians. We’ve taken for granted that people understand the basics of a complex system.

We want to communicate better, so Tasmanians can monitor energy security for themselves. And we think the new High Reliability Level (HRL) endorsed by the Energy Security Taskforce is a good starting point (pictured).

Secure energy infographic

The HRL will boost transparency by giving Tasmanians a clear and moving picture of energy security.

It’ll appear as an extra line on hydropower storage graphs. At any moment in time, as long as the storage level is above the HRL, Tasmanians can be assured that even if Tasmania again experienced very dry weather combined with a six-month Basslink outage, we could still manage through it, maintain supply and recover securely.

The HRL will curve up and down through the seasons of the year (just as storages do), and change dynamically over time as our electricity generation options and understanding of rainfall patterns change.

For example, if Tasmania builds more wind farms or gets more efficiency from our hydropower system (that is - we can generate more power from the water available), the HRL will go down because we’ll have more on-island generation to call on if things get tricky. If we lost a power generation source, or Tasmania’s weather gets continually drier, the HRL would go up to reflect the need for more “water in the bank”.

Above the HRL is another line – the Prudent Storage Level (PSL). The gap between the two lines is effectively a “buffer zone” or early warning zone. That buffer will generally need to be bigger during the drier months than the wetter time of year.

There’ll be times when storages drop below the Prudent Storage Level – for example, during drier than usual seasonal conditions. That won’t be a big deal, or pose a risk. It’s just an early indication that we may need to take some remedial action – for example, generating from gas or importing more energy for a short time – to avoid dropping further towards the HRL.

Let me also explain why we manage different-sized storages in different ways.

Within our overall Total Energy in Storage (TEIS) figure are a few very big and slow-changing storages, like Great Lake and Lake Gordon, which are typically about a third “full”, and look normal at that level. It also includes lots of smaller lakes and lagoons in which levels rise and fall much more quickly.

As a general rule, we try to generate most hydropower from the many smaller storages, and keep the major lakes in reserve for a “not-so-rainy day”. In very wet conditions, we may generate especially hard from smaller storages to prevent them spilling, or at least limit the spill, because the water going in is “use it or lose it” and simply goes to waste if it’s spilled.

We’ve essentially managed storages this way for more than a century, while adjusting to weather trends.

More recently, assets like the Tamar Valley Power Station, the Basslink interconnector and Tasmanian wind farms give us extra flexibility and security. When water levels are strong, we generate more hydropower for Tasmanians (our core purpose). During the drier months, when we want to support storages, we can generate less hydropower and rely more on wind power, gas generation or Basslink imports from the mainland.

Of course, the Energy Supply Challenge of 2015-16 was a prominent and unprecedented exception to these operating norms - caused by the driest spring on record coinciding with the Basslink cable breaking-down for much longer than anyone had thought possible. That unprecedented co-incidence put more reliance on hydropower and extra pressure on our storages, which fell to a record low of 12.5 per cent in April 2016.

Hydro Tasmania has updated its planning assumptions accordingly, and supports the recommendations of the Energy Security Taskforce, which has been considering Tasmania’s future energy needs.

I encourage you to take a close look at the High Reliability Level concept unveiled by the Energy Security Taskforce – particularly if you’d value a simple, week-by-week method of keeping an eye on storage levels.

Meanwhile, Hydro Tasmania will keep looking to have a stronger conversation with the people of Tasmania.

Steve Davy
Chief Executive Officer – Hydro Tasmania


An edited version of this opinion-editorial first appeared in the Mercury newspaper on Saturday 19 August, 2017.


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