Recycling heat for a ‘385% efficient’ solar desalinator

From the pv magazine global site

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 is 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%.

(Solar-to-vapor conversion efficiency is “the ratio of total vaporization enthalpy to total solar energy input, [which] for most previous studies has been limited to below 100% as the vaporization enthalpy is lost to the ambient environment.”)

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.

Commercial applications

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.