In a glass of its own

Engineers from the University of Sydney, Australia, have developed a solar-powered oven for sterilising medical waste that can work in cloudy or even rainy conditions.

The ingenious autoclave, nicknamed Prometheus, recently scooped top prize in an international competition to develop affordable technologies for treating medical waste in developing countries.

The competition, run by US campaigning group Health Care Without Harm, was established to find an alternative to the widespread, unregulated disposal of medical waste in poor rural communities, which has caused pollution in some areas.

Prometheus consists of a sterilisation chamber connected to a set of long water-filled copper U-tubes, which are encased in evacuated solar collector tubes – highly efficient collectors of thermal energy.

Their optically sensitive surface allows visible light and UV radiation to pass in but not out. Radiation is absorbed as heat, and the vacuum between the glass layers acts as a thermal insulator.

The tubes can operate in cloud and rain because the vacuum doesn’t allow heat loss due to convection or conduction Solar rays strike the collectors and the energy gathered is used to heat the water.

This establishes a thermosiphon effect in which water is heated until it evaporates, and the resulting steam is superheated. The whole system reaches equilibrium at between 121 degrees C and 134 degrees C, a suitable temperature for steam sterilisation.

Praising the invention, medical waste expert and competition judge Dr Jorge Emmanuel said that the autoclave can be built using local materials, operated with little or no electricity, and doesn’t require highly skilled labour.

For example, the collector tubes are cheap and available all over the world. Two prototypes have already been built by the Australian team. The larger consists of 15 collector tubes and takes around an hour to process 14 litres, while the smaller device, with one tube, processes 1.5 litres in around 40 minutes.

For each device the team has figured out clever ways of controlling the temperature.

The heat supply to the pressure chamber can be controlled with a small micro-controller and actuator. An electric thermocouple wire runs from the chamber to the controller delivering information about the operating temperatures. When the optimum temperature is reached the heat supply is cut off to avoid overheating.

For the larger device this is accomplished by using a motor to tilt the evacuated tubes in the opposite direction: halting the thermosiphon effect.

In the smaller device, the heat supply is cut off by programming a motor to pull a retractable cover over the evacuated tube and parabolic mirror. This is powered by a 12V battery that is kept constantly charged by a small solar panel.

Commenting on Prometheus’ victory, team spokesman Rhys Hardwick Jones said: ‘I went into engineering because I want to help people. So to get this opportunity to make a difference is pretty cool.’