By integrating a transparent, cloud-like aerogel with a solar heater, scientists can now efficiently trap solar energy to generate steam that is hot enough and at high enough pressure for sterilizing medical instruments even under hazy and partly cloudy weather. The solar thermal device, reported November 18 in the journal Joule, may help alleviate infection-related healthcare burden and facilitate the adoption of solar energy as a potential power source in resource-limited regions.
Almost like a pressure cooker with high temperature and high pressure, devices called autoclaves are used to cook germs at 121°C (249.8°F) and 205 kPa as a standard medical equipment sterilization procedure. However, due to the heat loss to the environment, traditional solar heaters have difficulties reaching temperatures above boiling point. In the new study, the research team incorporated a thermal insulating aerogel into the solar thermal device to reduce the escaping heat, allowing the device to generate 128°C (262.4°F) and 250 kPa steam for sterilization.
"When we were doing the field research in India, many healthcare providers said that they can operate an autoclave if there is reliable energy, but there are certain occasions where the electricity is unavailable," said first author Lin Zhao of Massachusetts Institute of Technology, who conducted a field study in Mumbai, India. "They are then forced to use maybe unsterilized equipment, or they have to use some rudimentary method like boiling water to try to sterilize things. A solar-powered device, in that case, would certainly be helpful to them."
Unlike other steam-generating solar thermal devices, which rely on solar tracking that requires operators to concentrate sunlight, the team's device is stationary, more user friendly, and energy efficient. In the field test, the aerogel-aided device generated steam at 100°C (212°F) with more than two times higher energy efficiency than other solar thermal devices reported in the lab. It also successfully performed standard sterilization under the Centers for Disease Control and Prevention's guidelines.
"As long as you have sunlight, you can use this device. It doesn't have to rely on well-built infrastructure. You can use it off-grid in remote areas," said Zhao. But like other solar-powered gadgets, it also has a limit--cloudy and rainy days. "It really depends on your location. If you're in a sunny area, our device may offer more benefits."
Zhao and his colleagues' device also has a lower price tag, and most of its components are commercially available in the solar water heater industry. Compared to benchtop autoclaves on the market, which can cost up to $3,000, the researchers' device costs about $35 per unit. Three to six units of the prototype device can sufficiently power a benchtop autoclave, demonstrating its potential as an inexpensive and accessible approach to reduce infection in remote and developing regions. The next step for the research team is to manufacture the aerogel in high quantity and quality.
"The real 'secret sauce' or the enabling factor is the transparent aerogel. Once we have the aerogel scaled up, we can think about providing the device to people who need it, such as channels like NGOs and other organizations," said Zhao. "We can also think about using the same technology for larger-scale applications. For example, food processing plants and chemical plants, they use boilers steam generators too."
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This work was supported by the MIT Tata Center for Technology + Design, IIT Bombay, and the US Department of Energy, Office of Science, Basic Energy Sciences.
Joule, Zhao et al.: "A passive high-temperature high-pressure solar steam generator for medical sterilization"
https://www.cell.com/joule/fulltext/S2542-4351(20)30496-7
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