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Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd may edit and has the right to reproduce in electronic form and communicate these letters. This also applies to submissions to "Ask SILICON CHIP" and "Circuit Notebook".

Desalination is a
waste of electricity

I have read a lot in SILICON CHIP about where the electrical power is to be generated to supply the desalination plants that governments believe we require. To me it seems just a waste of electricity. If you could see the amount of fresh water that goes past our place in the stormwater drain and straight into the bay to become salt water during an ordinary rain shower, you would be amazed.

A few years ago, we installed a rainwater tank to collect the water off the house and the garage. We need about 75mm of rain to fill the tank from empty. The tank size is 22,500 litres and is enough for two people in this area. We recently came out of the longest dry spell that anyone can remember, yet we did not run out of water.

In other words, we have not used any town water since we got this system up and running. It cost about $8000 to install and costs the following per year to operate:

(1) 3-5 micron sediment filter – $36.00

(2) 1 micron activated charcoal filter
    – $60.00

(3) 1 UV tube (for steriliser) – $150.00

The total is $246.00 plus the cost of electricity and maintenance which is not much. As you can see, we have top-quality water but it is not free.

I know that a lot of people on small allotments and in apartments etc would not be able to do this but there is a very large number who could. The trouble is that there is no incentive to put in such a system because we only save on the consumption part of our water rates.

R. A. Groves,
Coloola Cove, Qld.

Increased wind power
is more problematic

When I wrote to SILICON CHIP (Mailbag, January 2010) refuting the claim that wind farms could supply the entire power requirements of a desalination plant, I hoped to start discussion on the wider usefulness of wind energy. A contributor to Mailbag in the February 2010 issue, Kevin Shackleton, has responded by analysing the relevant AEMO data and providing some thought-provoking comments.

Kevin is correct in stating that the connection of several wind farms spread over a wide geographic region does result in some smoothing of the total output compared with that of any single wind farm. In saying that the summed output is more noisy, I was looking at it from the grid manager’s perspective. The output might be smoother but if the variance is not much improved, as indeed Kevin found, the absolute amplitude of the power spikes is larger, consistent with the larger total installed capacity.

Under this circumstance, the larger the proportion of wind generation in the grid, the more difficult its management becomes, because only the faster-acting forms of generation can be used to track and compensate for it.

As Kevin alludes, if there is sufficient spare hydro generation available, then this might be used to compensate for the variations in wind output (Snowy Hydro and larger Hydro Tas­mania stations are not included in the same “Non-Scheduled Generation” AEMO category as wind output). Hydro is used extensively to manage existing rapid changes in demand, so the extent of “spare” capacity available to balance wind generation variations is not easy to determine.

In this regard, I quote from the Australian Energy Regulator’s “State Of The Energy Market 2009” (page 14): “As the cheapest and most mature renewable energy technology, wind generation is likely to grow significantly under the expanded RET. But wind generation depends on prevailing weather conditions, and its intermittent nature poses challenges for power system reliability and security. In addition, momentary fluctuations in wind output create issues for maintaining power flows within the capacity limits of transmission infrastructure. To maintain reliability and security, standby capacity – in transmission and generation that can respond quickly to changing market conditions – is required. Peaking plant (such as open cycle gas turbines) typically provides standby generation capacity”.

Open cycle gas turbines (OCGTs) are then required to fill the bulk of the required balancing role. I think we can also presume from this quote that there is no “utility-scale” energy storage solution at present. The question that arises is: what amount of OCGT standby capacity is required?

I have looked at the variation over time in the total wind generation output. The installed total wind capacity, as Kevin has found, is at present a little over 1600MW. Above is a composite chart showing the variation in total wind generation (blue trace) during January 2010, together with the total grid demand (red trace). (Note that the righthand total demand scale is different to that used for the wind output. The latter is expressed as a percentage of total installed wind generation capacity.)

Apart from showing a somewhat smoother output than that from a single wind farm, the curve shows that the output swings between a peak of 77% of installed capacity and a minimum of 2%. It is interesting to note the number of occasions during the month when the demand is at maximum while wind output is at minimum. The present fleet of wind farms extend right across the grid in an east-west direction, so additional wind farms, being embedded in the same weather systems, will probably not add further smoothing and so the power spikes will grow with increased installed wind capacity.

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