With the drought in the SE USA there is a lot of attention being paid to where we are going to get our water in the future. Switching to “clean” biofuels will only make the water shortages worse; since biofuels only trade oil for water (one gallon of corn ethanol conservatively requires the use of 5,000 gallons of water.)
This means unless we learn to control the weather in the next couple years, we are going to have to get more and more water from desalinization of sea water. The issue with that is pure sea water destroys Reverse Osmosis membranes very quickly and recent research has shown that water made by RO can contain dangerous levels of boron, and other ions smaller than sodium. So, RO water will still require ion exchange and remineralization before it can be used for agriculture or drinking.
Originally I was thinking it might be cheaper draw the very cold water from the bottom of the oceans up and freeze it to remove the salt, but looking at energy loses to freeze the water and purge the brine this is only a viable method for extremely cold areas of the world. However, Canada could make a fortune desalinating the fridge and already hyposaline waters of Hudson Bay and selling the water to the thirsty and land locked grain producing Midwestern USA via massive underground pipelines. (This might sound stupid now, but Hudson Bay is the closest any ocean gets to a large part of the USA. That makes it is the best source of water if we can’t get it from the ground, and at that point we would need water no matter the cost.)
For the rest of the world where it isn’t cold enough to freeze the salt out of the water, evaporative methods are the best bet. In the Middle East they burn crude oil to boil seawater to make fresh steam. We could burn coal and make fresh water but that would be a waste of coal and create lots of CO2, etc. We need renewable or at least clean evaporative methods to get fresh clean water. Many people have proposed elegant solutions like massive greenhouses, or induced draft evaporators, but elegant often means impractical. I believe that large scale desalination plants will look more like oil refineries then they will the Sydney Opera House. With that in mind I think the two best bets for large-scale evaporative desalination are using nuclear power to directly boil water or if electricity is more available (solar, wind, etc) large scale microwaves to boil the water, instead of resistive heating. We’ll come back nuclear desalination, so that we can explore the microwave-based method.
First off why microwaves, when submerged resistive coils would be more efficient? Microwaves can directly excite the water molecules, so water under pressure can be flowed through a microwave and only the water gets hot. There are no elements to be corroded by the salt or eroded by the pressure or bubble formation. There are no high power electrical connections to be made in wet and corrosive environments, just massive microwave sources, that sit on the pipes and covert electric into heat and rf energy. Then just like many unfortunate people who have seen super heated water flash to steam as the remove a container from a microwave oven, the water is flash boiled by reducing the pressure. The fresh water steam is condensed by boiling a secondary loop of low boiling point fluid, which turns a turbine and regenerates some of the power. Then all the waste heat is used in a counter current circulator to preheat the incoming seawater, and chill the out going fresh water. On a small scale this is a terrible idea, but on a large scale (for a small, hot and windy city) I think the economy of scale would make it viable.
The best solution I still think is nuclear (or coal) co-generation/desalination, since you get the water and the electricity. Instead of the reactor boiling the same already fresh water over and over and turning a turbine to make power, the reactor heat would boil seawater via a secondary loop. (Direct boiling of seawater would be very, very dangerous since neutron bombardment of the ions would make them very radioactive.) Then like before the steam would be condensed by boiling a low boiling point fluid in a quaternary loop which would turn the turbines. Then the quaternary fluid would be condensed by preheating the incoming seawater, to complete the loop. (Think of it, no more massive cooling towers, just massive reservoirs of warmish freshwater.) I think a 5 to 10% lose in generation capacity; to get millions of gallons a day of fresh water is a very worth while trade. I will admit that it won’t be easy or cheap since everything touching the seawater would have to be made of stainless steel, but what price should we place on ensuring a ready supply of drinking water. Ask Atlanta. They are only a couple hundred miles from all the water in the world and have among others a 2 GW coal fired generation plant, but they are worried about where they are going to get water next year.
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