Clean water is crucial not only for humans' direct use
but also for agriculture. Attention often focuses on drinking water,
but agriculture is far bigger.
Let's put it in numbers. How much drinkable water do you require
for drinking and for cooking? Perhaps a few litres per day per
person. In the UK, urban consumption of water is about 160
litres per day per person. And in developed countries, even if
they are being careful with water, agriculture requires about 340
litres per day per person. [Israel uses roughly 1000
million m3 of water per year for agriculture, and it has
a population of roughly 8 million. That's 340 litres per day per
Some lucky countries have plenty of rainfall, so this agricultural
requirement can be provided at very low cost.
But what if the water for agriculture must be produced
by desalinating sea-water?
of the reverse-osmosis facility at Jersey Water’s desalination
plant. The pump in the foreground, right, has a power of 355 kW and
shoves seawater at a pressure of 65 bar into 39 spiral-wound
membranes in the banks of blue horizontal tubes, left, delivering
1500 m3 per day of clean water. The clean water from this
facility has a total energy cost of 8 kWh per m3. From Sustainable
Energy - without the hot air
Standard reverse-osmosis facilities have an energy cost of 8 kWh per
m3, so an agricultural water requirement of 340 litres per
day per person implies an energy requirement of about 2.7 kWh per day
per person, if we had to make it all by desalination with today's
technology. To put that in UK terms, 2.7 kWh/d/p is roughly 17% of
the average UK electricity supply; delivering 2.7 kWh/d/p of
electricity to the UK would require roughly 7 extra nuclear power
stations the size of Sizewell B, or 13,500 2-MW wind turbines.
This is why the Longitude Prize Water Challenge
sets the goal of desalinizing water with significantly
less energy than today's technologies.
We are especially interested in approaches that could be low-enough
in cost not only at large scale but also when rolled out in small-scale facilities.
David MacKay FRS is a member of the
2014 Longitude Committee. He is
the Chief Scientific Advisor at the Department of Energy and Climate
Change, and Regius Professor of Engineering at the University of
Cambridge. He is well known as author of the popular science book,
Sustainable Energy —
without the hot air.