Physical Insights

An independent scientist’s observations on society, technology, energy, science and the environment. “Modern science has been a voyage into the unknown, with a lesson in humility waiting at every stop. Many passengers would rather have stayed home.” – Carl Sagan

Water desalination, energy use, and where that energy comes from.

with 2 comments

Here are some thoughts on water desalination, from the Friends of the Earth Australia. Personally, I don’t know why they aren’t actually concerned with the fossil fuel fired electricity generation instead of the desalination plant proposal – since the former is actually where the greenhouse gas pollution comes from.

2006 was Victoria’s worst recorded year for rainfall, yielding only 165 gigalitres of inflow to Melbourne’s catchments compared with the previous 10 years’ average of 453 Gl. In a panic response, the government opted for what had previously been an option of last resort – a desalination plant.

The proposed desalination plant will produce 150 Gl water per year, upgradable to 200 Gl. Melbourne currently uses approximately 380 Gl per year. At present, 450 Gl of urban storm water and 150 Gl of treated waste water runs into Melbourne’s bays and Bass Strait. Independent expert water authorities confirm that at least half of the storm water, and most of the treated water can be easily collected and reused, at less economic and environmental cost than the proposed water factory.

The desalination plant will require a massive 90 megawatts of power (120 MW if upgraded as proposed). In real terms that means one million tonnes of CO2 per year, equivalent to 280,000 new cars driving our roads.

For the above to be true, the greenhouse gas emissions intensity of electricity generation would have to be as high as 1267 g/kWh. There is no fuel around that has carbon dioxide emissions intensity that high!

The most dirty, polluting coal fired generators generate about 1000 gCO2/ kWh.

Even in Australia, where the majority of electricity comes from coal combustion, there is some cleaner generation capacity in use – namely natural gas and hydroelectricity. Overall, the greenhouse gas intensity of Australian electricity is probably somewhere down around 800 gCO2/kWh. Still very high, but nowhere high enough to validate the numbers above!

Melbourne’s current water delivery comes at a low energy cost due to the passive system of catchments and the use of gravity from the catchment dams to the points of use. According to the 2005-06 Victorian Water Review, Melbourne Water’s average energy use for treatment and water delivery was 0.4 megajoule/kilolitre and the total urban weighted average across Victoria was 0.24 MJ/kL for 2005-06. In contrast the desalination plant’s power consumption will be approximately 19 MJ/kL assuming it is powered by brown coal.

That 19 MJ/kL energy requirement is completely independant of “assuming it is powered by brown coal” – it could be powered by wind turbines and it would still be 19 MJ/kL.

The Victorian government argues that the desalination plant will be ‘carbon neutral’. The current Victorian maximum wind power capacity is 134 MW. The desalination project will require 90-120 MW. To be true to federal emission reduction targets, all potentially available “green energy” should be used to satisfy current requirements, or for new demands that have no better alternatives. Desalination certainly does have preferable environmental and economical alternatives.

Recycling of stormwater or rainwater requires energy inputs too, of course.

Let me express this concern over water resources and access to water like this.

The total amount of water on Earth never changes – the water doesn’t just dissappear.

It’s simply that where human populations exist, especially dense populations, a localised region of “reduced water entropy” is needed, to provide clean, potable water to the population. There’s plenty of water on Earth – you just need to get clean fresh water, and get it delivered to where it’s needed. With our engineering and technology, and usually a little bit of an energy input, there are plenty of ways to accomplish this on the scale needed.

Written by Luke Weston

April 10, 2008 at 7:48 am

2 Responses

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  1. Roger Humphries and Keith Davies have been leading advocates of nuclear desalination and active in developing the technical basis for coupling nuclear power plants and desalination plants The system that they have developed called CANDESAL is a CANDU powered desalination plant that could supply a significant amount of fresh water at reasonable cost.

    Nuclear reactors have already been used for desalination on relatively small-scale projects; in total, over 150-reactor-years of operating experience with nuclear desalination have been accumulated worldwide. Eight nuclear reactors coupled to desalination projects are currently in operation in Japan. India has nearly completed a demonstration plant, and Pakistan has launched a similar project


    April 10, 2008 at 6:17 pm

  2. I hope people aren’t turned off the idea of nuclear desalination by the Thunderbirds episode “The Mighty Atom” – that fictional reactor was so dangerous it made the RBMK look like a PBMR!

    George Carty

    July 15, 2008 at 10:58 am

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