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Floating Solar Device Boils Water Without Mirrors (arstechnica.com)

Posted by BeauHD on from the blind-spot dept.

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."

6 of 80 comments (clear)

  1. Useful for desalination plants? by Jorgensen · · Score: 5, Insightful

    Can this be used in desalination plants? If so, it could provide fresh water - would make a big difference all around the world!

    1. Re:Useful for desalination plants? by queazocotal · · Score: 3, Insightful

      It could, you probably don't want to. Overall efficiency will be moderately low - you don't for a desalination plant want stuff floating on the water for long periods, it's too hard to anchor and make reliable.
      It's probably several times more efficient to have the plant on land, with 'conventional' solar collectors.
      Vacuum solar panels are surprisingly cheap now.

    2. Re:Useful for desalination plants? by hey! · · Score: 4, Insightful

      I should think not -- at least not in the way you're probably thinking.

      The device consists of a wicking layer topped by a light-absorbing layer. This boils water, which produces more or less pure steam. It also leaves the minerals from the water in the wicking layer. If you take distilled water directly away from the device and replace it with fresh seawater, those minerals will build up until the layer is no longer absorbent. On the other hand if all you want is the heat, you run the steam-distilled water through a heat exchanger and return it to the wicking layer, reconstituting the original water.

      So it'd probably wouldn't work to use this directly as a steam distiller. However you could use the heat you collect to run a separate steam distiller. That would be very inefficient, but the thing about "renewables" is that conversion efficiency is less important than low installation and operation cost, because you're not paying for your feedstock of energy; any sunshine you don't use would have been wasted anyway. So while it seems physically possible to use this device to power a desalinization plant, whether it makes economic sense depends on whether this is actually the cheapest way to run a plant.

      --
      Post may contain irony: discontinue use if experiencing mood swings, nausea or elevated blood pressure.
    3. Re:Useful for desalination plants? by kenwd0elq · · Score: 3, Informative

      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.

  2. Re:Steam generation under one sun by drakyri · · Score: 4, Informative

    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.

  3. Old hat? by enz · · Score: 3

    Solar thermal collectors that can boil water have existed for decades. You don't need concentration for that, simply use a black absorber plate with pipes under a glass plate and put a vacuum between the absorber and the glass. Selective absorbers or glasses will improve the efficiency.

    There are two problems with it: The generated steam will have a low temperature not much above boiling water. Most industrial applications need process steam of a much higher temperature (although there are some exceptions).

    There are also problems with the two-phase flow of the steam-water mixture in the pipes, which is generally unstable and difficult to control. Technically it is much easier to concentrate sunlight, use an oil with a high boiling point as the carrier liquid and use it to generate the steam in a seperate unit.