Talk:Fundamental resources/Water

Desalination energy analysis

• http://urila.tripod.com/desalination.htm - 0.66 kcal per liter is the theoretical minimum amount of energy needed
• http://www.eai.in/club/users/Nithya/blogs/1154 - This says 120 joules per gram (28.68 kcal/l) is needed after accounting for inefficiencies. Discusses some good ideas on using solar energy for desalination.
• http://www.businessweek.com/magazine/content/11_12/b4220041560310.htm - Robert McGinnis claims to have developed a method of desalination called 'forward osmosis' that uses 10% the energy of reverse osmosis. He "plans to start taking orders in late 2011". Promising if true. Here is a presentation from him at Yale where it says (page 18) that forward osmosis uses 0.24 kWh/m³
• http://www.sciencedirect.com/science/article/pii/S0960852410010114 - Recent trial-run of a microbial desalination cell. It successfully ran for 4 months, removing 99% of salt from water and constantly generating electricity.
• Detailed UNESCO report on desalination technologies present and future

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There's a good blog post here that does quantitative analysis of world energy requirements for desalination with the formula -

Population * water consumption per capita * energy needed for desalination * fraction of water that comes from desalination

It's difficult to see any scenario where it will require more than half a terawatt. For example -

20 billion * 0.2m³ /person/day * 2.2kWh/m³ * 0.5 = 8.8 billion kWh per day = 0.37 terawatts.

More realistic is-

10 billion * 0.15m³ /person/day * 2kWh/m³ * 0.1 = 300 million kWh per day = 0.125 terawatts.

The second term can be minimized with greywater and good system design (especially in field of the agronomics), the fourth term with rainwater harvesting. The third can and will be minimized with improving desalination technology. The blog post says 5kWh/m³ (4kcal/l) is needed for reverse osmosis. A blog comment quotes a reports saying that 2.2kWh/m³ (1.76kcal/l) has been achieved.

As with photovoltaics, there are several potential breakthrough technologies on the horizon promising to make desalination much more energy-efficient. If just one of them pays off, it is very easy to see how unlimited fresh water can be supplied. If we consider a scenario for, say, 2020, we must assume that water can be desalinated with 2kcal/l or less if we are to be realistic. The science behind microbial desalination cells seems solid and there is a very strong possibility that desalination will become an energy source rather than an energy drain.