~ desalinate with <5% the energy & equipment ~ ---cross-post!
TL;DR — We need to stop droughts, especially as climate change makes them worse. Desalination is nice, capturing clean water from the sea, yet it requires expensive equipment and lots and lots of energy. We can, instead, accelerate evaporation OUT into the air above our heads! We won’t be ‘capturing’ that clean water — it just rains-down miles away. Perfect, because we don’t need pipes and pumps to transport it! Using this sort of design, we can end droughts and re-take the land captured by deserts in recent decades, cheaply and quickly.
Imagine you have a bowl of salty sea water, with a tiny hole at the bottom. The sea water is slowly dribbling out of that hole, into another bowl beneath, to then get pumped back up to the top. At that dribbling spout, you hang a loose, fuzzy length of yarn, straight down. The water is held against that yarn by surface tension, scuttling down along its surface.
As the wind blows past your wet-yarn-wick, it will cause evaporation from that surface, adding humidity to the air, to come back down as rain further downwind. (Place the evaporators geographically so that the wind carries the water to the general spot you plan to use.) With wide troughs, thousands of yarns packed per square yard, stacked a dozen feet high in layers, you can pump vast amounts of water into the air.
The cost of pumping sea water up onto those troughs, only a few dozen meters above sea-level, is less than 1 Megajoule per ton of water. A Megajoule generated on-site from solar, only pumping when the sun shines, costs about 1 or 2 cents, for a ton of water to evaporate and return as rain on farms and snowpack in the mountains. For anyone in Ag, this translates to “$12 per acre-foot of irrigation, with no pumping or well-drilling, maintenance, or ditches… because it just rains.” That sounds darn good to me!
Compare that to the top-of-the-line desalination plants, spending 10 to 20 Megajoules per ton of fresh water produced! If you want to irrigate with that, you need to pump it to the site, with the associated barrier to entry and lag to scaling, maintenance and risk, while the water itself still costs you $240 per acre-foot! Some regions of California have seen peak prices at $2,000 an acre-foot, which is crippling. At $12 a pop, we can get green again — in flora and finances.
Getting into Details
Salt, first — as you evaporate and regurgitate the water in those troughs, it becomes brine, and you pump that out. It’ll only be a fraction of the original volume, and it’ll be chock-full of lithium, magnesium, and potassium, so you might as well make money selling those to make batteries and feed chemical industries. The sodium salt, unfortunately, must be gently dripped back across the wide ocean, OR we can just pile it up to make a giant salt pyramid somewhere nearby. I favor the latter option, because it’s much cheaper and it would still take ages to accumulate a pyramid that would be so large as to be an ‘obstruction’ — plus, the ocean really won’t notice the salt-loss at all, being miles deep. It’d also be rad to ski down and take four-wheelers up it — there are tourist revenue streams for such a unique oddity!
Coastline, next — you do need to pump the sea-water short distances, and up only short elevations, to make it cheap. So, coastlines would need tall, layered racks of troughs, stretching inland as far as elevation allows, to spread water deep to the continent. Another option, with a higher capital cost and maintenance, yet which would allow you to use this option in cases where the coastline is too steep or precious to locals: buoys floating just off-shore, where the winds are still strong!
Adding to the coastline problems: you must pick coasts where the winds pull from the sea into the dry target-region. California’s central valley is a prime example. So are Libya and Israel, able to water deep into the dry lands. Yet, the regions receiving rain from such coasts are often a different country! That’s the biggest barrier to development, and it will be difficult to get those countries to form a neighborly arrangement, until other countries have demonstrated evaporation’s efficacy and value. Once other successes pile-up, the incentive to stop squabbling becomes larger — in order to cash-in on the opportunity.
Power is an issue, too — the best way to power the evaporators is with solar concentrators along that same coastline. Solar can extract hundreds of watts per square meter, which means you only use a thin ribbon of solar to pump for a wide, many-layered swath of evaporator-troughs and yarns. It’ll be the most expensive capital, yet it’s also a small percentage of the total area and equipment. And, by positioning the solar *up-wind* from the evaporators, you are guaranteed many days of clear skies, without your own clouds disrupting power-supplies!
The Big Picture
If these evaporators desalinate and transport irrigation water dozens of times cheaper, hundreds of miles inland, that makes it worth-while — profitable! — to convert many disregarded dry regions into crop land and pasture. That takes pressure off deforestation, much of which happens for the purpose of grazing lands. The more we green the deserts, the more native land we can return to the biome.
Further, we’ve been losing agricultural land, accelerated especially by desertification. This would reverse that trend in many locations, preserving what we do use. Rains are also acceptable for municipal water supplies, precious in regions without steady fresh-water reserves for drinking. And, the rain’s humidity stabilizes the wild temperature-swings of deserts’ day and night, protecting plants and making the region more comfortable and desirable for people to live there. (aka ‘real estate values’)
There is also a social dividend to the work: being the pioneer of it, as well as sharing rain with your neighbors. It can even be a diplomatic shield: who would attack one so useful, especially when that same diligent rainmaker could turn the water off? Evaporators are worth much more than lettuce!