Autoclaves, which are used to sterilize medical tools in hospitals, clinics, and medical and dental practices, require a constant supply of pressurized steam at a temperature of around 125 degrees Celsius. This is usually provided by electric or fuel-powered boilers, but in many rural areas, especially in developing countries, electricity can be unreliable or unavailable and fuel is expensive.
Now a team of researchers at MIT and the Indian Institute of Technology have found a way to passively generate the steam they need, using only the power of sunlight, without the need for fuel or electricity. The device, which a typical autoclave in small clinics would need a solar panel about two square meters (or yards) to power, could maintain safe, sterile equipment in remote locations at low cost. A prototype was successfully tested in Mumbai, India.
The system is described in Joule magazine in an article by MIT graduate Lin Zhao, MIT professor Evelyn Wang, MIT professor Gang Chen, and ten other people at MIT and IIT Bombay.
The key to the new system is the use of optically transparent airgel, a material that Wang and her co-workers have developed over the past few years. The material is essentially a light foam made of silica, the material of beach sand, and consists mainly of air. Light as it is, the material offers effective thermal insulation and reduces heat loss by ten times.
This transparent insulating material is glued to the top of what is essentially a commercially available system for generating solar hot water, which consists of a copper plate with a heat-absorbing black coating that is attached to a series of pipes on the underside. As the sun heats the plate, the water flowing through the pipes below absorbs this heat. With the transparent insulating layer on top and polished aluminum mirrors on each side of the panel to draw extra sunlight onto the panel, the system can generate high temperature steam instead of just hot water. The system uses gravity to move water from a tank into the plate. The steam then rises to the top of the housing and is discharged through another pipe that carries the pressurized steam to the autoclave. A constant supply of steam must be maintained for 30 minutes to achieve proper sterilization.
With much of the developing world facing limited availability of reliable electricity or affordable fuel, “we saw this as an opportunity to reflect on how we could possibly create an inexpensive, passive, solar-powered system for generating steam, given the conditions are for autoclaving or necessary for medical sterilization, “explains Wang, professor of mechanical engineering at Gail E. Kendall and head of the mechanical engineering department.
The opportunity to test the system in Mumbai was a bonus, as the city was “relevant and important” as a type of location that could benefit from such low-cost steam generation systems.
In the Mumbai tests, the device managed to generate the saturated steam required for sterilization for the required half hour, even though the sky was cloudy and cloudy and only had 70 percent sunlight compared to a sunny day.
The test was performed on a small unit that was only about a quarter square meter and about the size of a towel. It turned out, however, that the rate of steam production was sufficient to make a similar unit between 1 and 3 square feet would be enough to power a benchtop autoclave, such as would normally be used in a doctor’s office, says Zhao.
The main limiting factor in the practical use of such devices is the availability of the airgel material. A company founded by Elise Strobach PhD ’20, co-author of this paper, is already trying to increase the production of transparent airgel for use in windows with high thermal efficiency. So far, however, the material has only been produced in small quantities using relatively expensive laboratory-grade supercritical drying equipment, so the widespread adoption of such a sterilization system is likely to be several years away, according to the researchers.
Since the other components, with the exception of the airgel, are already widely available at low cost even in developing countries, such systems may ultimately be practical to manufacture and maintain in the areas in which they would be used. The parts needed for the quarter-square-foot prototype were less than $ 40, according to Zhao. A system sufficient for a typical small autoclave would likely cost $ 160 once the required airgel material is commercialized. “If we can supply airgel, the whole thing can be built on site with local suppliers,” he says.
The process could also be used for a variety of other purposes, the team said. For example, many food and beverage processing systems rely on high temperature steam typically provided by fossil fuel boilers. Passive solar-powered systems to deliver this vapor would reduce fuel costs and therefore could be an attractive option in many industries, they say.
Ultimately, such systems should be much less expensive than systems that concentrate sunlight ten times or more to generate steam, as they require expensive mirrors and mounts, contrary to the simplicity of this airgel-based approach.
“This is a significant advance,” said Ravi Prasher, professor of mechanical engineering at the University of California at Berkeley and assistant director at Lawrence Berkeley National Laboratory, who was not involved in the work. “The generation of high-temperature steam with high energy efficiency was a challenge. Here the authors achieved both.”
“The quality of the research is very high,” adds Prasher. “Access to passive sterilization techniques for low-income communities who do not have access to reliable electricity is a big deal, so the passive solar device developed by the MIT team is very important in this regard.”