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Concentrating Solar Power solar thermal desalination

Published on February 18th, 2014 | by Tina Casey

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Tiny Solar Thermal Power Plant Solves Gigantic Salt Problem

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February 18th, 2014 by
 
We’ve been getting all excited about Ivanpah and Crescent Dunes, two new gigantic concentrating solar thermal projects in California and Nevada. Now let’s turn our attention to a third solar thermal plant at the Panoche Water and Drainage District in California.

This one, from a company called WaterFX, is different in terms of scale (miniature compared to the first two), solar energy collection (its concentrators are troughs, not flat panels), and thermal platform (it uses a form of mineral oil, not molten salt).

solar thermal desalination

Solar thermal energy collector (cropped) courtesy of WaterFX.

Solar Power Desalination Solves Part Of the Problem…

The plant is actually a modular, integrated system that WaterFX calls the Aqua4™ Concentrated Solar Still, and it’s designed to solve a problem bedeviling the water desalination field: what to do with all the leftover salty brine after you’ve extracted all the fresh water?

That problem is bound to keep growing as already-stressed global water resources keep shrinking, forcing farmers and other users to tap formerly overlooked sources including seawater, brackish groundwater, and various forms of wastewater.

One key element in the problem is that advanced desalination typically uses a high-pressure process called reverse osmosis, which requires a tremendous amount of energy, adding to the global carbon burden aside from adding to costs.

The energy factor can be partly solved by using solar power for desalination, along with other forms of clean, renewable energy.


However, that still leaves the salt problem. According to WaterFX, typical reverse osmosis operations only recover about half their input in the form of freshwater. The other half is a saltwater brine that needs to be further processed or transported offsite for environmentally responsible disposal.

To get reverse osmosis from the 50 percent recovery rate to a more concentrated byproduct would help ease disposal costs but that would require more pressure, which requires more energy, making the whole system less cost-effective.

The resource recovery route also involves additional expense, since the highly diluted brine would have to undergo additional steps to achieve a concentration that makes extraction cost-effective.

…And Here’s A Solution For The Other Part

One Aqua4 module is designed to produce about 65,000 gallons of freshwater daily, with a relatively small footprint of just 160 x 40 feet.

The system basically consists of common off-the-shelf components including a 400 kW trough-shaped solar thermal collector that focuses energy on a pipe through which mineral oil runs.

The heated oil goes to a heat pump to ramp up efficiency, which goes to a distillation system that yields a freshwater condensate and a concentrated brine.

The system also includes a thermal energy storage component so it can continue to operate at night, but that’s not what caught our attention.

What did catch our eye is the potential for resource recovery from spent brine, helping to make the operation cost-effective.

Here’s how it works on the agricultural drainage water of Central Valley, which is typically discarded as an unsuable byproduct of irrigation. The salinity level for drainage water in that region can range higher than the content of seawater.

The solar power plant provides emission-free energy that can be cheaper than fossil fuels, helping to keep energy costs down.

The distillation part of the process achieves a recovery rate of more than 93 percent while working far more quickly than natural evaporation and simple stills.

The concentration of salt in this solution is more than 20 percent by weight, enabling a cost-effective solids extraction process. The idea is that since you’re dealing with far smaller volumes of brine, you don’t need the kind of high-volume equipment (and energy input) that would be required when the recovery rate is only 50 percent.

Now, About That Salt…

Right now the project is in phase one, with the equipment humming along nicely and plans afoot for expansion.

Phase 2 is where the rubber will hit the road in terms of resource recovery. Due for completion this fall, phase 2 is expected to demonstrate that resource recovery can from the brine can be managed with minimal environmental impacts, if any.

The recoverable products include gypsum and calcium compounds that are widely used in the building industry for drywall, plaster, and cement.

Also present in the brine from this particular drainage area are magnesium salts, which are used in the medical industry, selenium (a health supplement), nitrates (fertilizer), and boron, best known for its use in bleach and pyrotechnics among many other uses.

Boron is also coming into its own in high-efficiency electronics and cutting edge solar technology, and for the record, selenium is also used in electronics as well as glass making, so altogether the region could be looking at a new high-value, job-creating industry in tandem with its agriculture base.

WaterFX also points out that the re-use of local groundwater reduces carbon emissions related to long-distance transportation of freshwater supplies, and it could also help alleviate water resource competition issues  that are sure to intensify unless more sustainable solutions are adopted.

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About the Author

Tina Casey specializes in military and corporate sustainability, advanced technology, emerging materials, biofuels, and water and wastewater issues. Tina’s articles are reposted frequently on Reuters, Scientific American, and many other sites. Views expressed are her own. Follow her on Twitter @TinaMCasey and Google+.



  • venusenthusiast

    I Know this sounds CRAZY but has anyone ever heard of GIVING?
    the salts are a waste product and have little or no economic value so why not GIVE the salts to manufacturers that will use them instead of DUMPING them?
    GIVING ?????? that’s CRAZY ! ! ! !

  • Quincy Dukes

    Tina, you write several times that the waste that’s produced from WaterFX’s method will be easier to process for resource recovery, and that the recovery process will yield salable quantities of heavy metals (like selenium) and salt.

    This part seems to me to be a little fuzzy.

    Not the resource recovery part. I know that will be proven (or not) during the second phase of these trials.

    But the downstream salability and usability of the recovered heavy metals and salt.

    Who exactly is going to want to buy the salt? It’s not an in-demand commodity. There’s more than enough salt worldwide so that any additional source isn’t going to find a ready market.

    That brings up the unwelcome possibility that WaterFX will recover tons of salt from their desalination process that they cannot do anything with.

    In that case, tons and tons of salt will need to be stored or disposed of, and as we know, that’s going to be a real expense, and possibly an ecological problem if the salts are put back into the ground or transported to the ocean and dumped there.

    • Bob_Wallace

      Putting the salt into the ocean will not be a problem.

      Remember, we’re adding a lot of fresh water to the ocean from melting glaciers.

  • logeshs

    True, Solar helps us various fields, it will be safe and effective technique for generating electric power and the size is not at all the matter, efficiency is the only thing we have to think.

    http://www.efficaenergy.com/solar-madurai.html

  • lerwickjones

    hi

  • jburt56

    I wonder if osmotic pressure could be used as an energy storage method as opposed to elevated temperature approaches? Check this link from Norway–

    http://news.nationalgeographic.com/news/energy/2013/01/130107-osmotic-energy-norway/

    The advantage is you wouldn’t have to worry about heat leakage.

  • disqus_dERuDbbP3d

    It’s a lot simpler really and requires little power harvesting, once you have the pipes from the ocean to the desert built, the sun does all the work. have a constant flow of water to make a gigantic lake at least the size of lake MI and more rain clouds will be created. You just have to catch the rain. not only does it give us fresh water, it will reverse the desert:)

  • wideEyedPupil

    God this is badly written I though as I read it on another syndicated site. Look who it turns out the author is.

    • CsabaU

      And your comment is NOT that impressive!

      • juxx0r

        Yeah, but does anyone know how it solves the gigantic salt problem?

      • wideEyedPupil

        Point was made well, CT deleted it. Winning.

        • Bob_Wallace

          Make points, not attacks.

          • wideEyedPupil

            Ok fair enough. point was I read three badly written pieces on interwebs lately and since they were on syndicated sites I didn’t know author in each case. I’ve never made such observations as comments before but I go to the site of origin (CT) and it turns out they are all by the same author. I’m struck by this coincidence. Maybe it’s just me or some kind of cosmic joke on me. Or I don’t know how to write a well structured article. Or it’s a ridiculously small sample from an author who is a veritable mcDonalds mass producer of words from per processed press rele

          • wideEyedPupil

            Discus is broke on iOS so I can’t finish editing that comment. …pre-processed press released materials.

  • saurdigger

    This has the potential to be a very good thing.

    Nice to see that (at the moment, info from their website) they’re approaching it in a fairly open source manner.

    I’d still worry about stockpiled precipitated compounds with no market value, but since it would’ve all been going in the ground/sea anyway… IDK.

  • Will

    its the combination that gives most profit.
    Solar and heat pump, Solar and EV electric car.
    Wind turbine and Solar. Solar and desalination.
    Wind turbine desalination, and so on.
    the combination of renewables gives double profit.
    good news.

    • http://zacharyshahan.com/ Zachary Shahan

      We need to keep repeating that. Takes some time for that idea to catch on with most people.

      • Sean Gonzalez

        OR we need to have this run on the mass media for a week and something, with interviews of the developers, etc.

        We know the truth, but we need to get it out there and to the businesses.

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