A “Brilliant” Idea to Improve Desalination.

Yesterday someone I admire called me out for pessimism. While the line between climate pessimism and realism is blurry to me, I know I didn’t say anything useful in yesterday’s post.

Today I’ll try to say something useful.

I’ve girded myself with an ungodly volume of coffee. As happens when I do that, my brain’s gone nuclear and birthed a slew of “brilliant” ideas, in quotes because my coffee-thoughts often later seem profoundly stupid in the harsh light of sobriety. Coffee dresses my thoughts in a misleading costume of incredibleness.

Once, after a little Mary Jane, someone I know convinced himself that he could experience time as a fourth spatial dimension; he thought he was perceiving directly the equivalence of space and time as specified in Einstein’s theory of relativity.

That’s the kind of thing that coffee does to me. So I present today’s idea with the caveat that I’m edging toward insane and my judgment’s shot.

My subject is desalination. I’ve been thinking about water because if you hang out with climate change thinkers as I do, water comes up all the time. I’ve heard someone say “Water is the new oil.” at least a dozen times in the last two weeks. Partly it’s because I’ve just moved to Eastern Wisconsin, which has awakened to the fact that it’s sitting on a gold mine in the form of Lake Michigan (Also overheard: “Milwaukee is the Silicon Valley of Water.”)

We know humanity has water supply problems, and climate change is expected to make it worse. So desalination is becoming more important.

I’m interested in solar desalination in particular, because it’s cheap, can help the poor, and emits no carbon. I have an idea about out how to do it better; specifically, how to improve one type of low-cost desalinator: the solar still.

I was reading an article on solar stills because that’s what passes for fun when all you think about is climate change:

Performance analysis of solar stills based on various factors affecting the productivity—A review

Here’s a pic of a typical setup:

Sunlight passes into the still through an angled glass cover, heats a reservoir filled with water, the water evaporates, condenses on the glass cover, and slides down to a collection trough. Salt and impurities are left behind in the reservoir.

The article claims that the temperature difference between the cover plate and the reservoir is key:

The yield of a solar still mainly depends on the difference between water and glass cover temperatures. The temperature difference between water and glass are acting as a driving force of the distillation process.

There follows some ways to increase the aforementioned temperature difference to boost yield.  As I read, an epiphany formed.

All the designs discussed in the article share a common feature: light passes through a glass cover on its way to the water.  Examples:

Reading through these, my first thought was: the temperature difference between plate and water in these setups must be limited by the fact that the sun passes through the glass on its way to the water.

Here’s my idea: prevent light from passing through the glass plate, and instead divert the light around it with a solar concentrator (which is like a satellite dish with reflective walls), to heat the water reservoir from sides and bottom. The plate itself can be painted white or made of a reflective material, and perhaps can be insulated. Here’s my amateur cutaway diagram:

Some of the other tricks described in the article should be used here. Specifically:

  1. The reservoir should either be painted black, or should be transparent and the water should contain black gravel, to help it absorb light.
  2. Sponges should be placed at the surface of the water in the reservoir, so that water creeps up into them by capillary action. This will increase the surface area of the water and increase the rate of evaporation.

Maybe my coffee’s wearing off, because I’ve thought of two potential drawbacks:

  1. The solar concentrator will boost cost. However, there are many places where solar concentrators are being deployed as solar cookers. If my device could be fit to these, then the system might be deployed cheaply.
  2. Because the reservoir has to fit in a solar concentrator, it might be difficult to make it as wide as it is in other designs. This could limit yield by limiting the surface area of the water, another factor which influences evaporation rate. There may be ways to design around the issue. I don’t know.

Finally, in a former life I used to work with optimization algorithms, like genetic algorithms. The physics of solar stills may be simple enough that genetic algorithms could be used to further refine the designs. Maybe. I don’t know because I’m not an expert, but it strikes me as possible.

This post dedicated to Anna.

-From the Sea

Posted March 30, 2011 in Smashing Ideas | 5 Comments on A “Brilliant” Idea to Improve Desalination.

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Post a Comment

  1. vivzizi says:

    possibly you have co inveted the “solarball” solar water purifier.

    http://www.greenoptimistic.com/2011/03/22/solarball-water-purifier/

    http://todaygadgetnews.com/tech-news/new-solarball-prototype-filters-3-liters-of-water-with-sun-park-2.html

  2. Bobo says:

    I like much. Plus seawater contains lithium which can go to batteries and photovoltaics production. No need for mining in Afghanistan.

  3. Nick B. says:

    Thx!

  4. Craig says:

    Check out this impressive solar desalinization setup. It may give you some clues on improving your idea. http://blogs.ei.columbia.edu/2011/02/18/seawater-greenhouses-produce-tomatoes-in-the-desert/

  5. Nick Bentley says:

    Thanks Craig. That’s fascinating, and thought it’s beyond anything I was thinking of, the general principle of using seawater to create the right temperature differentials might be applied to solar stills as well. I’ll think about how to do it. Thanks for stopping by.

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