MIT bubble wrap design for low-cost floating solar thermal devices
MIT scientists have made a "key step forward" in the creation of low-cost, scalable, interfacial solar steam generation by using bubble wrap. It provides crucial insulation to a system which focuses the sun's light on a "minute" volume of water, generating steam in a few minutes. This interfacial steam generator model currently costs around $6 per square metre but the team behind it believes it can reduce that cost to $2, a fraction of expensive optical concentrators, and with potential applications in the large-scale industrial sector.

To boil water using the Sun, we typically burn fossil fuels carrying several-hundred-million-year-old solar energy that was extracted from underground at great expense. It’s kind of Rube-Goldbergian. We’re fortunate that the Sun’s heat isn’t strong enough to boil the oceans (or us), but extracting the Sun’s energy at a significant scale is tricky.

The usual solution, as many magnifying-glass-toting children already know, is to concentrate sunlight and increase its intensity. Solar thermal plants, for example, use massive arrays of mirrors to focus sunlight and generate electricity. All that extra equipment gets pretty expensive—especially if you need the mirrors to track the Sun’s position across the sky.

So how do we engineer another way? In the past, researchers made clever designs to concentrate the heat generated by lower-intensity sunlight into small volumes of water. This heat consequently created higher localized temperatures. While they managed to boil water with this method, they weren’t able to ditch optical concentration completely.

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.

Central to their floating solar device is a “selective absorber”—a material that both absorbs the solar portion of the electromagnetic spectrum well and emits little back as infrared heat energy. For this, the researchers turn to a blue-black commercial coating commonly used in solar water heating panels. The rest of the puzzle involves further minimizing heat loss from that absorber, either through convection of the air above it or conduction of heat into the water below the floating prototype.

The construction of the device is surprisingly simple. At the bottom, there is a thick, 10-centimeter-diameter puck of polystyrene foam. That insulates the heating action from the water and makes the whole thing float. A cotton wick occupies a hole drilled through the foam, which is splayed and pinned down by a square of thin fabric on the top side. This ensures that the collected solar heat is being focused into a minute volume of water.

Advertisement

The selective absorber coats a disc of copper that sits on top of the fabric. Slots cut in the copper allow water vapor from the wick to pass through. And the crowning piece of this technological achievement? Bubble wrap. It insulates the top side of the absorber, with slots cut through the plastic to let the water vapor out.

Tests in the lab and on the MIT roof showed that, under ambient sunlight, the absorber warmed up to 100 degrees Celsius in about five minutes and started making steam. That’s a first.

But it’s probably not a 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 of direct sunlight’s full intensity. With that much wiggle room, they say that a cheaper, less effective absorber material could bring the cost down even more. The current design should only cost about $6 per square meter to make, and the researchers think they could reduce that to $2 per square meter. At that price, they estimate you could produce steam for about five percent of the cost of a system that has to concentrate sunlight.

Steam that is cheaply and simply produced could become a popular way to generate electricity. It could also be used to heat buildings, for industrial applications, or even to boil seawater or wastewater to distill pure, clean water—just as the hydrologic cycle generates rainwater.

A floating sheet isn’t going to work for every application, and a fully functioning product is obviously going to take some additional engineering. But as Wen Shang and Tao Deng from Shanghai Jiao Tong University write in a summary accompanying the paper, “With its innovative thermal concentration approach, this work certainly represents a key step forward in the development of interfacial solar steam generation, paving the way to its use in large-scale industrial applications that avoid the use of expensive optical concentrators.”

Nature Energy, 2016. DOI: 10.1038/NENERGY.2016.126, 10.1038/NENERGY.2016.133 (About DOIs).