Evaporation used for cheap, clean water

Creating potable water out of brackish, saline or contaminated water sources often requires large amounts of energy at a high price, but a world-first approach to solar evaporation developed by Australian researchers is about to make water purification more cost-effective.

Led by University of South Australia Associate Professor Haolan Xu, the research team has revised solar evaporation to successfully produce enough potable water per day for a family of four from one square meter of source water.

“In recent years, there has been a lot of attention on using solar evaporation to create fresh drinking water, but previous techniques have been too inefficient to be practically useful,” Xu said.

“We have overcome those inefficiencies, and our technology can now deliver enough fresh water to support many practical needs at a fraction of the cost of existing technologies like reverse osmosis.”

The technology involves a photothermal structure that sits on top of a water source, converting sunlight to heat and focusing energy on the surface to rapidly evaporate the uppermost portion of the water.

“Previously many of the experimental photothermal evaporators were basically two dimensional; they were just a flat surface, and they could lose 10-20% of solar energy to the bulk water and the surrounding environment,” Xu said.

“We have developed a technique that not only prevents any loss of solar energy, but actually draws additional energy from the bulk water and surrounding environment, meaning the system operates at 100% efficiency for the solar input and draws up to another 170% energy from the water and environment.”

Xu and his team developed a three-dimensional, fin-shaped evaporator that shifts surplus heat away from the evaporator’s top surfaces, thereby cooling the top surface and achieving zero energy loss during solar evaporation.

This technique keeps the surfaces of the evaporator at a lower temperature than the surrounding water and air, allowing additional energy to flow from the external environment into the evaporator.

“We are the first researchers in the world to extract energy from the bulk water during solar evaporation and use it for evaporation, and this has helped our process become efficient enough to deliver between 10 and 20 litres of fresh water per square metre per day,” Xu said.

Low cost, low maintenance

Xu said another benefit of the new design is that it is built from low-cost, everyday materials that are sustainable and easy to obtain.

“One of the main aims with our research was to deliver for practical applications, so the materials we used were just sourced from the hardware store or supermarket,” he said.

“The only exception is the photothermal materials, but even there we are using a very simple and cost-effective process, and the real advances we have made are with the system design and energy nexus optimisation, not the materials.”

Xu said the system is also easy to maintain, as it has been designed to prevent salt and other contaminants building up on the evaporator surface, making the technology perfect for water treatment in locations where high-maintenance desalination systems are operationally inappropriate.

“For instance, in remote communities with small populations, the infrastructure cost of systems like reverse osmosis is simply too great to ever justify, but our technique could deliver a very low-cost alternative that would be easy to set up and basically free to run,” Xu said.

“Also, because it is so simple and requires virtually no maintenance, there is no technical expertise needed to keep it running and upkeep costs are minimal.

“This technology really has the potential to provide a long-term clean water solution to people and communities who can’t afford other options, and these are the places such solutions are most needed.”

Xu said his team is currently exploring a range of other uses for the technology, including treating wastewater in industrial operations.

“There are a lot of potential ways to adapt the same technology, so we are really at the beginning of a very exciting journey,” he said.