Desalination: Drinking Energy

Via Lightbucket, a look at desalination’s energy impact:

Very large scale desalination projects may be needed to meet the world’s future fresh water needs. There’s certainly enough seawater, but how much energy does it take?

In many areas, fresh water is getting used up faster than it’s being replenished, and demand for water is growing. We face a growing “water crisis” [1]. As we start running out of fresh water, there’s an obvious place to look for more. Just 2.5% of the Earth’s water is fresh water, the other 97.5% is saline [1]. Desalination could become a very large part of the world’s future water infrastructure. This raises several questions, but I’ll stick to just one of them here: How much energy is that going to take?

How much desalination capacity will be installed world-wide? I don’t know, my crystal ball’s clouded up. What I’ll do is calculate two sets of numbers to try to bracket the possible options, a “low” scenario which may be a plausible outcome, and an extreme upper bound “maximum” scenario.

How many people will there be? The world population will most probably flatten out and peak at around 9.2 billion soon after 2050 [2]. I’ll take this as the maximum world population. How much water do we use? The highest per capita fresh water abstractions among OECD nations are those of the U.S., at 1730 m3 per capita per year, the lowest are Denmark’s at 130 m3 per capita per year [3]. I’ll use these two values to bracket the range.

How much energy does desalination require? The most efficient (reverse osmosis based) desalination plants consume about 5 kWh of energy per cubic metre of fresh water produced [4]. The fundamental thermodynamic limit for desalinating seawater is 0.86 kWh m−3  [4], so there’s plenty of room for improvement. I’ll use the 5 kWh m−3 figure for the “maximum” scenario, and assume a two-fold improvement for the “low” scenario. For the “maximum” scenario I’ll assume that all fresh water must come from desalination (i.e. that no other source of fresh water is available), for the “low” scenario I’ll adopt the more plausible assumption that 10% of the world’s fresh water will come from desalination.

Table 1 summarises the two sets of assumptions, and the resulting calculations.
The “maximum” scenario takes the world population at its peak, assumes they all have U.S. levels of water consumption, and assumes that all that water comes from desalination. This is, of course, very unrealistic. It’s there to provide an upper bound to the set of possible outcomes. The “low” scenario goes for a more plausible possibility, with Denmark’s level of per capita water use, and 10% of the world’s water coming from desalination.

Table 1.  Energy requirements for global scale desalination
Peak world population (circa 2050) [2] 9.2 bn 9.2 bn
Water consumption [3] (a) (m3/capita/yr) 130 1730
Desalination energy [4] (b) (kWh m−3) 2.5 5.0
Fraction of water from desalination 10% 100%
Power for desalination (globally) (GW) 34 9100
Additional price of water per capita (c) (UKP/capita/yr) £3.25 £865
Mean electricity generation in 2005 (for comparison) was 2100 GW.
(a)  The “low” value is water abstraction for Denmark; “maximum” value is water abstraction for U.S.
(b)  The units are kilowatt hours of electricity per cubic metre of fresh water.
(c)  Energy cost assumes an electricity price of £0.10 per kWh.

The “low” scenario uses 34 GW of electrical power worldwide, to provide 10% of the world’s water needs by desalination. For comparison, global electricity generation in 2005 averaged 2100 GW [5]. That is, the entire world population can have 10% of its water needs met by desalination with less than 2% of the world’s present day electrical energy. The “maximum” scenario requires over four times the world’s electrical power generation, but this is an extreme upper bound.

How much would that cost? For the “low” scenario and an electricity cost of £0.10 per kWh, the cost of the energy for desalinated water is £3.25 per person per year. For the “maximum” scenario, it’s £865. The cost of desalination is sometimes described as “prohibitively expensive”. The need for water is absolute; if there’s no cheaper alternative, the word “prohibitive” isn’t particularly accurate.

I’ll cover the caveats first. Large scale desalination would require extensive infrastructure changes, such as aqueducts from coastal areas leading inland, and so on. I haven’t considered any of that here, I’ve just focussed on a single fundamental question, the energy requirement of this arrangement.

The energy requirement is large, but well within the range of existing energy infrastructure. The price is high, but in the same ballpark as existing utility costs. There are no fundamental showstoppers to desalination on a massive scale. Very large scale desalination is a viable way to extend fresh water resources. There’s a phrase about “eating oil” to describe agricultural fertilizer use. We may end up “drinking energy”.

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About This Blog And Its Author
As the scarcity of water and energy continues to grow, the linkage between these two critical resources will become more defined and even more acute in the months ahead.  This blog is committed to analyzing and referencing articles, reports, and interviews that can help unlock the nascent, complex and expanding linkages between water and energy -- The Watergy Nexus -- and will endeavor to provide a central clearinghouse for insightful articles and comments for all to consider.

Educated at Yale University (Bachelor of Arts - History) and Harvard (Master in Public Policy - International Development), Monty Simus has held a lifelong interest in environmental and conservation issues, primarily as they relate to freshwater scarcity, renewable energy, and national park policy.  Working from a water-scarce base in Las Vegas with his wife and son, he is the founder of Water Politics, an organization dedicated to the identification and analysis of geopolitical water issues arising from the world’s growing and vast water deficits, and is also a co-founder of SmartMarkets, an eco-preneurial venture that applies web 2.0 technology and online social networking innovations to motivate energy & water conservation.  He previously worked for an independent power producer in Central Asia; co-authored an article appearing in the Summer 2010 issue of the Tulane Environmental Law Journal, titled: “The Water Ethic: The Inexorable Birth Of A Certain Alienable Right”; and authored an article appearing in the inaugural issue of Johns Hopkins University's Global Water Magazine in July 2010 titled: “H2Own: The Water Ethic and an Equitable Market for the Exchange of Individual Water Efficiency Credits.”