Sun-Powered Device Sterilizes with Steam

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Two big issues in the developing world are how to sterilize medical instruments and getting clean water. A team at Rice University has solved both problems with a solar-powered sanitizer built with a combination of off-the-shelf parts and nanotechnology.

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If this can be mass-produced it would make a huge difference for medical teams that travel to remote areas. When a group of doctors goes to a place with no facilities, a big part of the weight they carry around is chemicals to sterilize instruments. A piece of equipment that’s easily built and requires little maintenance would make that unnecessary.

It’s a simple set-up: a parabolic mirror or Fresnel lens like the ones on overhead projectors concentrates solar heat onto a chamber full of water and nanoparticles of carbon and metal. The particles absorb the solar energy and re-emit it as heat. It’s efficient because the the shape of particles gives them a lot of surface area, which increases the amount of  heat they transfer to the water; without them the water would still get hot but it would take longer and more solar energy would be lost as the water re-radiated the heat.

The water hits temperatures of 284 Fahrenheit, hotter than if it were boiled to 212 degrees. The heated water, now steam, then goes through simple plastic pipes to a pressure vessel — essentially a pressure cooker used for cooking. That superheated steam sterilizes anything inside the vessel. The nanoparticles don’t make it into the steam as they are too heavy, so they remain behind in the first chamber.

When the water cools down, it’s collected and sent back to the chamber where it was originally heated. It can be collected for other uses — it’s simply sterile, distilled water at that point — or left in to be boiled again. The invention is outlined in the current issue of the Proceedings of the National Academy of Sciences.

Naomi Halas, a professor of electrical and computer engineering and chemistry who led the team and is one of the co-authors of the paper, told Discovery News that the sanitizer has been tested with ordinary tap water. They haven’t tested it with really heavily contaminated water yet, but the high temperatures would kill even some viruses and spores, which ordinary boiling doesn’t.

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Since it is made with simple parts, it doesn’t require sophisticated tools to maintain. And while the water needs to be topped up occasionally, if the system is operated as a closed loop it can run for many cycles with no need for refills.

Besides surgical instruments, it could also process waste. Halas noted that many rural areas in developing nations have a huge problem with water-borne disease because there is no way to process the waste in latrines, which are often simply trenches in the ground. Attaching a superheated steam generator to a waste tank would make an efficient, small-scale sewage treatment plant.

Credit: Proceedings of the National Academy of Sciences