In an advance that may lead to affordable solar-powered sanitation and water purification for use in remote areas, researchers have demonstrated that shining light on aqueous suspensions of nanoparticles quickly generates copious amounts of steam without boiling the water, bypassing energy-intensive bulk water heating (ACS Nano, DOI: 10.1021/nn304948h). The finding may also lead to increased energy efficiency in electricity generation.
Researchers have known for some years that irradiating solutions of light-absorbing nanoparticles with laser light, for example, causes dramatic heating of the particle surfaces. Much of the work in that area has been devoted to improving the thermal conductivity of the liquids with the aim of developing applications that exploit light-to-heat energy conversion.
In the new study, which was conducted at Rice University, researchers follow a different course. The group, which includes graduate student Oara Neumann and electrical engineering and physics professors Naomi J. Halas and Peter Nordlander, is aiming to capitalize on the energy advantage of light-driven evaporation.
The team prepared aqueous solutions of nanosized particles with silica cores and gold shells and separate solutions of carbon nanoparticles. Both types of particles absorb a broad spectrum of solar light. The solutions were held in transparent tubes and partially immersed in an ice bath in a test apparatus fitted with pressure and temperature sensors.
The researchers report that within five seconds of exposing the core-shell particles to sunlight, the vapor pressure above the solution rose as a result of steam generation. For the carbon nanoparticles, the time delay was roughly 20 seconds. Once the process is under way, steam generation occurs at similar rates for both solutions, they say. In a related demonstration, the team showed that water-ethanol mixtures can be quickly separated via sunlight-driven distillation, resulting in 99% pure ethanol.
Evaporation is stimulated by light absorption, which excites surface electrons and causes a rapid temperature rise in the nanoscale vicinity of the particles, the team explains. The sudden heating envelops the particles in nanosized steam shells that can coalesce to form bubbles, which then move to the liquid-air interface and escape from the liquid.
The group finds that 82% of the absorbed sunlight goes directly to generating steam. They also report an overall sunlight-to-steam energy efficiency of 24% for their unoptimized process.
“This study is unique in that it demonstrates real-life applications utilizing ‘free’ solar energy for something potentially very useful,” says physics professor Alexander Govorov of Ohio University. The advantages of using solar-heated nanoparticles for applications such as distillation should certainly be studied further, he adds.