Scientists at the Massachusetts Institute of Technology (MIT) have developed a prototype solar-powered water desalinator which they say achieved solar-to-vapor efficiency of 385% through a multi-stage process where the heat released as water condensed was recycled, flowing into the next layer to power the next stage of evaporation.
Rather than using photovoltaics to power electrically-driven desalination – a method which has been used in large scale applications already – MIT’s process uses solar absorbers to gather heat from the sun and evaporate the saltwater.
A prototype on an MIT rooftop delivered water which exceeded local drinking water standards at a rate of 5.78 liters per hour, per square meter of solar collecting area. The university said that was more than double the previous record for water produced by passive solar desalination. By optimizing and adding further stages to the desalination process, the group estimates devices based on the concept could reach efficiencies as high as 800% – meaning that eight times as much energy as is initially collected from the sun would be available for the conversion of water into vapor.
A real no-briner
MIT said the device – described in the paper Ultrahigh efficiency desalination via a thermally-localized multistage solar still, published in Energy & Environmental Science – addresses concerns related to solar desalinators as it could operate in regions without a reliable electricity supply and doesn’t leave a build-up of concentrated brines to be disposed of. Instead, said the institution, the salt that accumulates during the day is carried back out of the system once the sun goes down.
With further innovation, the device could be built using low-cost, readily available materials and offer the potential for further cost optimization. Key to that is the separation of the solar absorber and wicking material, which in other systems were a single component requiring a highly specialized material. “This design provides more flexibility and permits the use of low-cost materials since it is possible to use any commercially available solar absorber – having no wicking ability – and any affordable capillary wick, regardless of their solar absorptance,” read the research paper.
The MIT group said the device offers potential applications in regions with limited infrastructure but plenty of sunshine and seawater. The researchers have considered the possibility of large scale systems and smaller, residential applications and estimate an installation large enough to serve the needs of a family could be built for around $100.
“One of the challenges in solar-still-based desalination has been low efficiency due to the loss of significant energy in condensation,” said Ravi Prasher, associate lab director at Lawrence Berkeley National Laboratory, who was not involved in the research. “By efficiently harvesting the condensation energy, the overall solar-to-vapor efficiency is dramatically improved … This increased efficiency will have an overall impact on reducing the cost of produced water.”
MIT says it will continue to experiment with the process, focusing on testing for durability and optimization with different materials and in various configurations, as well as on scaling up from the lab-sized device which achieved the results.
This article was amended on 12.02.2020 to further clarify what the claimed 385% efficiency refers to in this context.
This content is protected by copyright and may not be reused. If you want to cooperate with us and would like to reuse some of our content, please contact: email@example.com.
385% efficiency? Hoping for 800%. Sounds like the same BS artists that are selling toilet paper “3=7, 6=17”. Does 1 gallon of seawater give 3.85 gallons of drinking water? Does $100 invested spit out $385 cash? Explain your “math”!! Or is MIT claiming some sort of perpetual motion, free energy machine? Should sell big in Alaska—free drinking water and use the extra heat to warm your igloo.
Thanks a lot for your comment. While perpetual motion or a free money machine would certainly be nice, all that MIT is claiming here is that by reusing the heat in multiple stages they can achieve a ‘gain’ on the energy output compared to the sunlight that is input.
While it is a different process, an ordinary heat pump is 200-400% efficient so this not a leap of logic by any means. It certainly makes sense as you reuse energy through the process.
Sorry, Mark. But the concept remains largely unclear. This machine does not output any energy. Only water.
Did you mean that the new model is nearly 4 times more efficient, than a similar machine without heat recuperation?
Then, the header should say that: ‘nearly-4-times more efficient’; not ‘385% efficient’. The current wording is misleading/click-bait.
Thanks for your comment. I’m not sure how changing the header as you suggest would change the meaning at all. 385% efficiency is the figure reported by MIT in a peer-reviewed journal. You are correct though that my stating ‘energy output’ doesn’t really make sense, I should say ‘energy use’.
I think we all understood what you meant, but that really could be worded a lot better. Or a little bit of expanding on what metric they are claiming an 800% improvement in?
Thank you for the informative and kind response.
I would think that this system could be used to purify water from sources other than saltwater.
If such a unit that could supply enough water for a residential household from pond water could be made for $1K (not counting installation cost), I certainly would be very interested and I believe that a lot of other similar households would be interested as well.
It appears to be, at the core, a plain-Jane distillation system, plus a few tweaks to improve the production efficiency. At that rate, it would work great on a residential pond, sewage effluent, whatever.
This is similar in principle to a reflux distiller, only that it has a different geometry. An illustration would help a lot. The claimed 385% or 800% efficiency should have been contextualized, as to what was the basis? Was it generating more water than the ordinary pan evaporation? Was it compared with the usual setup for solar desalination given the amount of solar radiation?
I’d like to see these dispersed along the Atlantic coastal nations of Africa, like Western Sahara or Mauritania. These are arid places with little to no surface water of any significant utility; fresh water is provided via expensive, energy-intensive desalination plants. They’re pretty sunny, too. There are LOTS of places like this that could benefit from a solar purifying system.
Any news of the waste products being generated being put to good use?
Rather than put back into the ocean (making the Seas and oceans saltier) are there plans to filter out pollutants, salts, minerals, organic matter etc and finding a positive eco use for them, ie plastic waste to recycle, organic waste for compost or marine food, salts and minerals for domestic and commercial use or sale???
Hi to do 2 million ltrs per day id it possible and estimate cost for project in Eastern Cape South Africa .
By submitting this form you agree to pv magazine using your data for the purposes of publishing your comment.
Your personal data will only be disclosed or otherwise transmitted to third parties for the purposes of spam filtering or if this is necessary for technical maintenance of the website. Any other transfer to third parties will not take place unless this is justified on the basis of applicable data protection regulations or if pv magazine is legally obliged to do so.
You may revoke this consent at any time with effect for the future, in which case your personal data will be deleted immediately. Otherwise, your data will be deleted if pv magazine has processed your request or the purpose of data storage is fulfilled.
Further information on data privacy can be found in our Data Protection Policy.