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Tuesday, February 4th, 2020

A solar umbrella device can enhance evaporation in lithium mining, reducing the amount of time needed and land required.

A device using solar energy can rapidly speed up the evaporation process for lithium mining ponds by more than 100%, lowering the environmental impact of wastewater disposal. According to researchers at the University of California, Berkeley, USA, the solar umbrella enhances evaporation by converting sunlight into infrared radiation range, where wastewater is strongly absorbing.

The team claimed that because the device has a non-contact nature, it is suited to treating a wide range of wastewaters without contamination. The use of commercial materials also makes the device low-cost and highly scalable for sustainable wastewater management.

‘In this work, we focused on evaporation ponds that are broadly used to dispose of wastewater from power plants, oil and gas operations, desalination plants or for lithium extraction,’ Lawrence Berkeley National Laboratory (LBNL) Department of Energy Postdoctoral Researcher, Akanksha Krishnakumar Menon, told Materials World.

Department Researcher, Ravi Prasher, added, ‘This is a big societal problem we are trying to solve. To either dispose of the wastewater or to extract a valuable salt like lithium, you would like to increase the evaporation rate dramatically and in a scalable manner. If we could do so, that could reduce the environmental impact by reducing the amount of land required.’

Tackling waterwaste with umbrella tech

Recognised as a major global risk by the World Economic Forum, there have been efforts in the past to address the waterwaste issue, many of which used the approach to increase the evaporation rate. The most prominent ones have consisted of structures that float on the surface of water to localise the heat, but the team highlighted that the porous nature of these structures exposes them to clogging of contaminants that need separating.

‘The floating structures comprise porous materials – think of it as a black sponge – so if the water it is floating on is very salty, those salts can clog the pores of the sponge over time and reduce the amount of water that can be evaporated,’ said Menon.

In response to this problem, the team developed the solar umbrella, termed for its capacity to shield the water surface from the sun. The photo-thermal device is a flat sheet that selectively absorbs solar energy on one side, emits mid-infrared energy on the other, and sits above the water in an evaporation pond like an umbrella. The device achieves zero-liquid discharge (ZLD), a process in which all the water is extracted from a waste stream such that the final product is a solid that can be disposed easily.

Menon described the three layers that comprise the device – a selective solar absorber being a composite of a ceramic and a metal substrate, copper or aluminium, and lastly a blackbody emitter, in this case using a high-temperature black paint.

‘The photo-thermal device converts incoming sunlight and photon energy into infrared heat and thermal energy,’ said Menon. ‘Each layer performs a specific function in this process – the top selective solar absorber layer has optical properties such that it absorbs 95% of the incoming sunlight and reflects only 5%.

The process begins by heating the device up as it absorbs the sunlight. This heat is then transferred to the second layer, which is a metal with a high thermal conductivity, so it transfers that heat to the third layer very effectively. ‘This last layer is a black material and so it emits the heat as infrared radiation to the water surface,’ said Menon. ‘Water strongly absorbs this heat at its surface, which in turn increases the temperature and this leads to a higher evaporation rate.’

According to the team, the device acts like a radiation transformer, which can absorb energy from sunlight in the range 400-1,500nm, and convert it 3,000nm or greater. This is achieved in the mid-infrared range.

Testing the waters

To test the device, the team ran lab-scale experiments using a solar simulator that mimicked the spectrum of sunlight. ‘We used an acrylic tank filled with saltwater to mimic wastewater typically found in evaporation ponds,’ Menon said. ‘We placed the solar umbrella above the water surface, non-contact, and recorded the mass loss of water over time as water evaporated from the tank. We also repeated the same test outdoor under natural sunlight to confirm the performance.’

The results showed the device enhanced the evaporation rate by more than 100% over natural evaporation. Due to the success of the prototype, the team believes that such solar umbrellas could be scaled for applications in desalination plants.

They also expect the device to be a highly scalable ZLD technology that does not require energy, and will now be performing a techno-economic analysis to understand the cost and compare it to other technologies.
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