Efficiency of reverse osmosis desalination

The exploding need for fresh water can only be satisfied by seawater desalination. So thanks to John Morgan for this Arts Technica piece based on a new Science paper The Future of Seawater Desalination: Energy, Technology, and the Environment. John reminds us how inefficient flash distillation is:

Any desalination process that avoids the phase change will be more efficient than distillation. It takes 5.4 times as much energy to turn water at 100 C into steam at 100 C as it does to heat the water from 0 C to 100 C. Even under low pressure, vaporization takes a lot of energy.

The bottom line is that state of the art reverse osmosis plants are within 25% of the realistic minimum energy for the pumping to maintain the membrane pressure:

The authors of the perspective point out that, because osmosis is a simple matter of thermodynamics, it’s possible to calculate exactly how efficient we can make the process. And, as it turns out, we’re really quite close as these things go. A state-of-the-art facility is now within a factor of two of the theoretical energy minimum, and only 25 percent higher than the realistic minimum for the current reverse osmosis process. In short, it’s going to be tough to squeeze too much more energy out of reverse osmosis, and we’re unlikely to find an alternative method of desalination that will provide a significant boost over that.

It should be obvious that nuclear is the preferred technology for generating the reliable zero-carbon electricity required. A useful reference is the ASIRC report for Victoria Overview of Treatment Processes for the Production of Fit for Purpose Water: Desalination and Membrane Technologies [PDF]. The report estimates energy requirements for seawater desalination at about 4KWh per cubic meter of product water. Excerpts:

(…) The projected annual shortfall in Melbourne’s water supply in the future is about 93 million m3. A plant producing this amount of desalinated seawater by reverse osmosis would require around 390 GWh or about a 1% increase in Victoria’s energy requirements. Greenhouse gas emissions are related to the amount of energy produced and the energy consumption for a plant requiring 390 GWh per year would therefore release approximately 540,000 tonnes of carbon dioxide per year if coal-fired power generation was utilised. Brackish water RO desalination has a lower energy use of 1.0-2.5 kWh/m3 compared to seawater RO desalination of 4.5-8.5 kWh/m3. These energy uses and associated greenhouse gas emissions are much lower than for other technologies such as multi-stage flash distillation where the total energy consumption is between 10.5-13 kWh/m3.

(…) Interest in using nuclear energy for producing potable water has been growing worldwide in the past decade. This has been motivated by a wide variety of reasons, inter alia, from economic competitiveness of nuclear energy to energy supply diversification, conservation of limited fossil fuel resources to environmental protection, and by nuclear technology in industrial development [IAEA, 2000].

Integrated nuclear desalination plants have been operating in Japan and Kazakhstan for many years. At Aktau in Kazakhstan, the liquid metal cooled fast reactor BN-350 has been operating as an energy source for a multipurpose energy complex since 1973, supplying electricity, potable water and heat to the local population and industries. The complex consists of a nuclear reactor, a gas and/or oil fired thermal power station and MED and MSF desalination units. The sea water is taken from the Caspian Sea. The nuclear desalination capacity was about 80,000 m3/d, however part of this capacity has now been decommissioned. In Japan, several nuclear power plants have seawater desalination systems using heat and/or electricity from the nuclear plant to produce feed water make- up for the steam generators and for on-site supply of potable water. MSF was initially employed, but MED and RO have been found to be more efficient. The individual desalination capacities range from about 1000 to 3000 m3/d [UIC, 2004].

2 thoughts on “Efficiency of reverse osmosis desalination

  1. “It should be obvious that nuclear is the preferred technology for generating the reliable zero-carbon electricity required. ”

    Not so obvious! The first thing to consider when producing energy with fission is that efficiency is not an important criteria. You have a huge amount of cheap process steam from the extraction steam system used to make low pressure turbines more efficient. KISS.

  2. Kit – sorry for the tardy reply. I think I understand your point. Is your view that, given a nuclear heat source, flash distillation wins on the life cycle economics?

    I wonder if there is any published analysis on this…

    I don’t have a dog in this hunt – other than a conviction that there is little risk of having too much nuclear heat available for desalination. I’m happy to drink either RO or distilled water:-)

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