Steam is a key ingredient in a wide range of industrial and commercial processes—including electricity generation, water purification, alcohol distillation, and medical equipment sterilization.

Generating that steam, however, typically requires vast amounts of energy to heat and eventually boil water or another fluid. Now researchers at Rice University have found a shortcut. Using light-absorbing nanoparticles suspended in water, the group was able to turn the water molecules surrounding the nanoparticles into steam while scarcely raising the temperature of the remaining water. The trick could dramatically reduce the cost of many steam-reliant processes.

The Rice team used a Fresnel lens to focus sunlight on a small tube of water containing high concentrations of nanoparticles suspended in the fluid. The water, which had been cooled to near freezing, began generating steam within five to 20 seconds, depending on the type of nanoparticles used. Changes in temperature, pressure, and mass revealed that 82 percent of the sunlight absorbed by the nanoparticles went directly to generating steam while only 18 percent went to heating water.

“It’s a new way to make steam without boiling water,” says Naomi Halas, director of the Laboratory for Nanophotonics at Rice University. Halas says that the work “opens up a lot of interesting doors in terms of what you can use steam for.”

The new technique could, for instance, lead to inexpensive steam-generation devices for small-scale water purification, sterilization of medical instruments, and sewage treatment in developing countries with limited resources and infrastructure.

The use of nanoparticles to increase heat transfer in water and other fluids has been well studied, but few researchers have looked at using the particles to absorb light and generate steam.

In the current study, Halas and colleagues used nanoparticles optimized to absorb the widest possible spectrum of sunlight. When light hits the particles, their temperature quickly rises to well above 100 °C, the boiling point of water, causing surrounding water molecules to vaporize.

Precisely how the particles and water molecules interact remains somewhat of a mystery. Conventional heat-transfer models suggest that the absorbed sunlight should dissipate into the surrounding fluid before causing any water to boil. “There seems to be some nanoscale thermal barrier, because it’s clearly making steam like crazy,” Halas says.

The system devised by Halas and colleagues exhibited an efficiency of 24 percent in converting sunlight to steam.

Todd Otanicar, a mechanical engineer at the University of Tulsa who was not involved in the current study, says the findings could have significant implications for large-scale solar thermal energy generation. Solar thermal power stations typically use concentrated sunlight to heat a fluid such as oil, which is then used to heat water to generate steam. Otanicar estimates that by generating steam directly with nanoparticles in water, such a system could see an increased efficiency of 3 to 5 percent and a cost savings of 10 percent because a less complex design could be used.

Otanicar cautions that durability—the ability of nanoparticles to repeatedly absorb sunlight and generate steam—still has to be proved, but adds that the 24 percent efficiency achieved in the current study is encouraging. “It’s just the beginning for optimizing this approach,” he says.