Dec 12 2018
Engineers at MIT have developed a device that absorbs enough heat from the sun to boil water and generate “superheated” steam hotter than 100 °C, without the need for expensive optics.
In this experiment, the new steam-generating device was mounted over a basin of water, placed on a small table, and partially surrounded by a simple, transparent solar concentrator. The researchers measured the temperature of the steam produced over the course of the test day, October 21st, 2017. (Image credit: Courtesy of the researchers)
On a hot day, the structure will be able to passively pump out steam sufficiently hot to sterilize medical equipment, and also for use in cleaning and cooking. The steam may also be used to supply heat to industrial processes; conversely, it can be collected and condensed to synthesize desalinated, distilled drinking water.
Earlier, the researchers developed a sponge-like structure that floated in a container filled with water and converted the water absorbed by it into steam. However, a big reason to worry about is that contaminants in the water led to the degradation of the structure as time passed. The new device is developed to be suspended over the water, to prevent any probable contamination.
The thickness and size of the suspended device are comparable to that of a small digital tablet or e-reader, and its structure is similar to that of a sandwich: The top layer is made of a material that can efficiently absorb the heat from the sun, and the bottom layer efficiently liberates that heat to the water below. Upon reaching the boiling point (100 °C), the water releases steam that rises back up into the device; here, it is funneled through the middle layer, which is a foam-like material that further heats the steam over the boiling point, before pumping the steam out through a single tube.
It’s a completely passive system—you just leave it outside to absorb sunlight. You could scale this up to something that could be used in remote climates to generate enough drinking water for a family, or sterilize equipment for one operating room.
Thomas Cooper, Assistant Professor of Mechanical Engineering, York University.
Cooper headed the study as a postdoc at MIT. The outcomes of the study have been described in a paper published in Nature Communications on December 11th, 2018. The research includes researchers from the lab of Gang Chen, the Carl Richard Soderberg Professor of Power Engineering at MIT.
A clever combination
In 2014, for the first time, Chen’s team demonstrated a simple, solar-driven steam generator, which was in the form of a graphite-covered carbon foam floating on water. This structure absorbs and localizes the heat from the sun to the water’s surface (otherwise, the heat would penetrate down through the water). From that time, his team and others have made efforts to enhance the efficiency of the design using materials of varying solar-absorbing properties. However, nearly every device has been developed to float directly on water, and they have all faced the problem of contamination since their surfaces come into contact with salt and other impurities present in water.
The researchers decided to develop a device that, on the other hand, is suspended above water. The device is designed to absorb solar energy of short wavelength, which in turn heats up the device, making it reradiate the heat in the form of infrared radiation of longer wavelength to the water present below. It was interesting for the researchers when they observed that water more readily absorbs infrared wavelengths, rather than solar wavelengths, which would simply pass right through.
To form the top layer of the device, the researchers selected a metal-ceramic composite that is a highly efficient solar absorber. The bottom layer of the structure was coated with a material that efficiently and easily liberates infrared heat. A layer of reticulated carbon foam—essentially, a sponge-like material distributed with winding tunnels and pores—was sandwiched between these two materials, where the sandwiched layer retains the incoming heat from the sun and has the potential to further heat up the steam that rises back up through the foam. A small outlet tube was also attached to one end of the foam; steam can exit and can be easily collected through this tube.
Lastly, the device was placed over a basin of water and the entire setup was surrounded with a polymer enclosure to arrest the escape of heat.
It’s this clever engineering of different materials and how they’re arranged that allows us to achieve reasonably high efficiencies with this noncontact arrangement.
Thomas Cooper, Assistant Professor of Mechanical Engineering, York University.
Full steam ahead
First, the researchers tested the structure by performing experiments in the lab, with the help of a solar simulator that simulates the properties of natural sunlight at differing, controlled intensities. The structure was found to be able to heat a small basin of water to the boiling point and generate superheated steam, at 122 °C, under conditions that mimicked the sunlight produced on a clear, sunny day. Upon increasing the solar intensity by 1.7 times, the researchers found that the device generated even hotter steam, at 144 °C.
The device was tested on October 21st, 2017, on the roof of MIT’s Building 1, under ambient conditions. The day was bright and clear, and the researchers increased the intensity of the sun’s heat by building a simple solar concentrator—a curved mirror that helps collect and redirect increased sunlight onto the device, thereby increasing the incoming solar flux, akin to the way a magnifying glass is used to concentrate the solar beam to heat up a patch of pavement.
Using this added shielding, the structure was able to generate steam with a temperature over 146 °C in 3.5 hours. The researchers were able to generate steam from seawater in subsequent experiments, without contaminating the surface of the device with salt crystals. Through another set of experiments, they were also in a position to collect and condense the steam in a flask to synthesize pure, distilled water.
According to Chen, apart from overcoming the difficulties of contamination, the design of the device allows steam to be collected at a single point, as a concentrated stream; in contrast, earlier designs produced a more dilute spray.
This design really solves the fouling problem and the steam collection problem. Now we’re looking to make this more efficient and improve the system. There are different opportunities, and we’re looking at what are the best options to pursue.
Gang Chen, Carl Richard Soderberg Professor of Power Engineering, MIT.
This study was supported in part by MIT’s Abdul Latif Jameel Water and Food Systems Lab (J-WAFS), and from MIT’s S3TEC Center, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Basic Energy Sciences, and by an Early Postdoc Mobility Fellowship from the Swiss National Science Foundation.