George Ni
To boil water using the sun, we usually burn fossil fuels with hundreds of millions of years old solar energy extracted from the ground at great expense. It’s a bit Rube-Goldbergian. We’re lucky that the sun’s heat isn’t strong enough to boil the oceans (or us), but extracting the sun’s energy on a large scale is tricky.
The usual solution, as many children with a magnifying glass already know, is to concentrate sunlight and increase its intensity. For example, solar thermal plants use huge arrays of mirrors to concentrate sunlight and generate electricity. All that extra equipment gets pretty expensive, especially when you need the mirrors to track the sun’s position in the sky.
So how can we engineer in a different way? In the past, researchers have made clever designs to focus concentration heat generated by lower intensity sunlight in small amounts of water. This heat consequently created higher local temperatures. While they managed to boil water using this method, they were unable to completely bypass the optical concentration.
But in a new paper, researchers from MIT and the Masdar Institute of Science and Technology, led by George Ni, describe a prototype design that boils water under ambient sunlight.
At the center of their floating solar device is a “selective absorber” – a material that both absorbs the solar portion of the electromagnetic spectrum well and gives little back as infrared heat energy. To do this, the researchers turn to a blue-black commercial coating commonly used in solar water heaters. The rest of the puzzle involves further minimizing heat loss from that absorber, either through convection from the air above or through heat conduction in the water below the floating prototype.
The construction of the device is surprisingly simple. At the bottom is a thick puck of polystyrene foam with a diameter of 10 centimeters. This isolates the heating effect of the water and allows the whole to float. A cotton wick occupies a hole drilled through the foam, which is spread and pinned down by a square of thin fabric at the top. This ensures that the collected solar heat is concentrated in a minimal volume of water.
The selective absorber covers a disc of copper that sits on top of the fabric. Slits in the copper allow water vapor from the wick to pass through. And the crowning glory of this technological achievement? Bubble wrap. It insulates the top of the absorber, with slots cut through the plastic to let the water vapor out.
Tests in the lab and on the MIT rooftop showed that under ambient sunlight, the absorber heated up to 100 degrees Celsius in about five minutes and started to make steam. That’s a first.
But it probably won’t be the last. The researchers used computer modeling to look for factors they could optimize, and they calculated that the device should make steam even at about half the full intensity of direct sunlight. With that much wiggle room, they say a cheaper, less effective sorbent could cut costs even more. The current design should only cost about $6 per square foot to make, and the researchers believe they can bring that down to $2 per square foot. At that price, they estimate that you can produce steam for about five percent of the cost of a system that needs to concentrate sunlight.
Steam produced cheaply and easily could become a popular way to generate electricity. It can also be used to heat buildings, for industrial applications, or even to boil seawater or wastewater to distill pure, clean water, just as the hydrological cycle generates rainwater.
A floating plate will not work for every application, and a fully functioning product will of course require some additional engineering. But as Wen Shang and Tao Deng of Shanghai Jiao Tong University write in a summary accompanying the paper: “With its innovative thermal concentration approach, this work certainly represents an important step forward in the development of interfacial solar steam generation, paving the way for its use in large-scale industrial applications that avoid the use of expensive optical concentrators.”
Nature Energy2016. DOI: 10.1038/NENERGY.2016.126, 10.1038/NENERGY.2016.133 (About DOIs).