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Floating Solar Device Boils Water Without Mirrors (arstechnica.com)
An anonymous reader quotes a report from Ars Technica: 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 photovoltaic 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. 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. The study has been published in two separate Nature articles: "Steam by thermal concentration" and "Steam generation under one sun enabled by a floating structure with thermal concentration."
A lot of similar systems use solar concentrators involving a concentrating lens or reflector, increasing the amount of illumination on the area of interest. It's conventional to refer to the amount of illumination in terms of multiples of normal solar radiation - so 2 suns, 10 suns, etc.
The surface is non-emitting in long wavelength infrared. This is likely due to a particular coating. You can obtain paints or do procedures like torching stainless to produce a low emissivity surface. This is common practice for solar heat collectors. Really the only thing special about this device is the controlled introduction of water so the heat delivery of the device is not overwhelmed by the volume of water.
Making steam seems quite plausible if you feel a shiny aluminum surface in the sun. The shiny surface is very hot even though the aluminum surface is very reflective. The ratio of absorption and emissivity determines whether things get hot in the sunlight. If aluminum only absorbs about 10% or less of the energy of the sun and gets that hot then something absorbing over 90% and emitting less than 5% will get much hotter than the shiny aluminum.
Oh, look, you've re-invented the "solar still", a WWII-era survival device that pilots would carry in their escape kits with the 1-man life raft. Inflate the balloon, add sea water, and drink distilled fresh water a few hours later. Dump the brine, add more sea water, and repeat. A few pilots survived for weeks in their rafts, eating sun-dried fish and drinking distilled water from their solar still balloons.