- Detailed UNESCO report on desalination technologies present and future. Good summary of the technologies on page 35 of the pdf. Page 10 says, "The research is focused on reducing the energy requirements for seawater desalination from the current benchmark of 3.5 kWh/m3 to the theoretical minimum of 0.8 kWh/m3". Existing plants have power consumption as low as 1.25kWh/m3 (p. 17)
- They raise an interesting point on page 43 that synthetic biology may create cheap membranes; plant tissues have channels that can separate ions from water.
A 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 desalinationIt'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 = 0.0125 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 less than 1.75kWh/m3 if we are to be realistic. Microbial desalination or forward osmosis would solve the problem at a stroke. 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.
Another way of looking at it is that every person who depends on desalination for water adds about another 10.5 watts to their energy needs: if they use 0.125m3 (125 liters) a day, and efficiency is 2kWh/m3, they need 250 watt-hours per day (per 24 hours), which is about 10.5W. It's nothing extravagant; it's like having another lightbulb in your house.
The places that need desalination all have good solar resources. If the technology needs 2kWh/m3, a plant desalinating 1 million cubic meters a day (a bit bigger than the world's current largest according to Wikipedia) would need 2 million kWh a day. Photovoltaic cells that are 20% efficient would need to receive 10 million kWh a day. In very sunny places, insolation is over 4kWh per square meter per day. Thus, 2,500,000m2 would be required.