Autoclaves used to sterilize medical instruments tools typically require a steady supply of 125°C pressurized steam. This usually comes from electrical or fuel-powered boilers. But in many rural areas and places decimated by natural or man-made disasters, power can be unreliable or unavailable, and fuel is expensive.
To bridge that gap, a researcher team at the Massachusetts Institute of Technology, working with engineers at the Indian Institute of Technology, have devised a machine that generates the needed steam using just solar power. The device requires a two-meter2 solar collector to power a typical small-clinic autoclave. A prototype of the device was successfully tested in Mumbai, India.
The key to the device is its use of an optically transparent aerogel. It is essentially a lightweight foam made of silica and consists mostly of air. The ultra-light material provides effective thermal insulation, reducing the heat loss by a factor of ten.
The aerogel is bonded onto the top of an off-the-shelf device for using solar power to heat water. It consists of a copper plate with a heat-absorbing black coating bonded to a set of pipes on its bottom side. As the sun heats the plate, water flows through the pipes and picks up that heat. But the MIT added a transparent insulating layer on top of it along with polished aluminum mirrors on each side of the plate, directing more sunlight at the plate. This lets the device create high-temperature steam instead of just hot water.
Water is gravity-fed from the tank into the plate. Eventually, steam then rises to the top of the device and gets fed through another pipe which carries pressurized steam to the autoclave. The steam must be maintained for 30 min. to ensure proper sterilization.
“Much of the developing world has limited availability of reliable electricity or affordable fuel,” explains MIT Professor Evelyn Wang. “So we saw this as an opportunity to potentially create a low-cost, passive, solar-driven device that generates the steam necessary for autoclaving or medical sterilization.”
In the Mumbai tests, even though the sky was hazy and cloudy—providing only 70% of the sun available on a clear day—the device was able to generate the saturated steam needed for sterilization in30 minutes.
The test was carried out an a scaled down version with only a 0.25-meter2 collector, but it showed that steam production rate enough that 1 to 3 meters2 would be enough to power a benchtop autoclave similar to the types used in a doctor’s office, according to the team.
The limiting factor for practical deployment of the new devices seems to be the availability of the aerogel. One company founded by one of the persons on the research team is trying to scale up the production of transparent material for use in thermally efficient windows. But so far, the material is only made in small amounts using expensive lab -grade drying machines. That means widespread use of the solar sterilizer is likely still a few years off.
The device, or others like it, could be used for other purposes. For example, many food and beverage processes rely on high-temperature steam typically from fossil-fuel powered boilers. Passive solar devices to deliver steam would eliminate fuel costs, making them an attractive option in many industries, they say.