Here’s how to bring fresh water to all of those arid places you keep hearing about on the news. This solution is simple and cheap enough that it could be built from parts from any hardware store and affordable in any country with a drought problem. Basically, you use the sun’s energy to both pump water from the sea and convert it into freshwater. It seems like most of the world’s water has salt in it, so we should use some of that.
Say you have a large otherwise useless plot of land, like the Sahara. By using the Thermosiphon principle you could pipe water from the ocean to the places that need it. Here’s how it works:
Step 1: The General Idea
Hot air rises, the same goes for all fluids, such as water. Given an extensively long pipe from the ocean to where ever needs water; you can coax that water to move through the pipe using heat. If your desert is at a higher elevation than sea level, by applying heat to a portion of the pipe, the water inside becomes less dense and elevates. The colder water prior to the heated portion of pipe, not wanting to create a vacuum, then rushes in to fill the pipe left by the heated water. So as long as you have heat, water moves up and through the pipe. (Just use check valves to prevent the water from descending at night.)
Hypothetically, you could pump seawater an infinite distance, given an infinitely long heat source. Next comes that infinitely long heat source cheap enough to drag across the entire Outback, which could be constructed from soda cans and glass panes.
Step 2: Heat Source
Say your pipe was in a really hot, sunny place, like a desert, then you could easily use solar power to achieve water flow without using pumps. A solar trough could cheaply be used to generate the heat necessary for this pumping. Here’s how a solar trough works: take a shiny half cylinder and face it generally towards the sun. The parabolic shape reflects and focuses the sun’s rays towards a center point, in this case, our pipe. (Here's the Wikipedia article) This system is in use all over the world for industrial uses such as heat exchangers and can achieve extremely high temperatures.
Step 3: Modifications to a Solar Trough
Generally, these systems rotate to follow the sun, but that’s expensive, and poor countries need fresh water. One way to make this hotter is to combine the greenhouse effect in order to trap our solar heat in the trough. Ever get in your car on a hot day and wish you could somehow utilize the heat difference between outside and inside? By laying a sheet of glass atop the trough, we can trap that heat, making our trough more efficient. Remember, we don’t need to boil the water, just attain higher temperature than the water before it necessary to generate flow.
Another solution might be to distort the shape of the glass into a lens. By making the glass get thicker towards the middle, we can further focus the suns rays and trap heat. Just like roasting ants with a magnifying glass!
While this will not be as efficient as a rotating trough, the longer you extend such a trough, say for a mile or more, then the more thermal energy will be transferred into the water.
Step 4: Converting to Freshwater
Once the water reaches its final destination, you need to remove the salt. The ancient and cheapest solution is to use a solar trough. For example, if one were to stretch out a trough such as this picture of a solar still over some miles, the freshwater generated could be considerable enough for agriculture.
Here’s how this proposition works: There is a reservoir of saltwater in the bottom of the still. As the Sun beats down on the still, the rays generate heat which is once again trapped inside the glass. As the saltwater in the reservoir heats up, the still becomes humid inside and freshwater condenses on the ceiling of the still. Being as this still is slanted, the beads of moisture roll down the ceiling and eventually drop off into our reservoir of freshwater.
In this solution, we crisscross the incoming seawater from our heat-pump pipe across the roof of the still. This accomplishes two things: One, the incoming water is cooler than the still, thereby giving our heated moisture somewhere to condense(picture a cold glass of water on a hot day.) The second is that it preheats the incoming seawater (via the sun) before dumping the seawater into the trough, which we want to be really hot.
Step 5: Testing the System
I'm going to do some measuring to see the relationship between heat input and elevation of water through the pipes. Commenter bbiorn informed me that coffee makers work on the same principle as this system, called Thermosiphon.
Apparently, a system similar has been proposed here.
I propose using copper tubes, slightly angled from a tub of water, and a propane tank underneath the tube for heat. I'll prime the tube and install a check valve to keep water in the pipe prior to flow.
I'll probably blow myself up or burn down my back yard, but I'm heading to the hardware store now.
Step 6: Final Thoughts
Once we have a constant supply of fresh water into our desert, we can start growing some groundcover. Apparently once you have a bunch of plants in an area, clouds are invited and rain becomes more common place. Check out this thesis, ADRECS, and this one, Desert Rose. Growing plants in the desert causes the soil to become enriched because the plants deposit nitrates from the air into the soil and the plants decompose further enriching the soil. Eventually, the soil is good for growing everything.
Even before you get to that stage, you have sun, dirt, and freshwater to grow plants. Drip irrigation or a rotating irrigator could work here. Still don’t feel like using pumps or fossil fuels for this project? This thing (picture of Ox) runs on the plants grown and can be used for moving that rotating and pumping water through that irrigator (pic of irrigator).
Use this invention if you live somewhere such as a small island with no fresh water, Yemen, the Sonora Desert, Australia… You’re otherwise useless land now has value!