Purdue engineer brings test method down to earth

A Purdue University engineer could save NASA millions of dollars with a new method to test a solar-power system on Earth instead of in space.

Shripad Revankar, an associate professor of nuclear engineering at Purdue believes the experimental power system would be ideal for satellites, including those in a geosynchronous orbit, which are exposed to a day-night cycle similar to the Earth’s.

The system would generate electricity during hours of darkness, when conventional solar-power systems rely on bulky rechargeable batteries.

Central to the solar-power system is a ‘phase-change’ material that freezes during hours of cold darkness but is liquid under high temperature. During daylight hours, portions of the experimental satellite system exposed to solar radiation in space would reach 800 degrees Celsius.

Because heat from the sun is required to melt the material, heat is released when the liquid freezes. The heat released by the freezing liquid can then be used to generate electricity by driving small steam turbines or thermoelectric units.

Because the systems generate at least three times more power than batteries of comparable size, they are seen as a possible alternative to conventional satellite solar-power systems that rely on batteries.

But a major obstacle is that bubble-like cavities form in the material as it freezes. The phase-change material is contained in a series of metal cells, called capsules. Gaps that form against the outer walls of the capsules interfere with the flow of heat from the freezing liquid to the rest of the system.

Revankar has found that using capsules of certain sizes and shapes might control voids.

He has found that the best shape and size for the vessels is a donut, or torus, about two inches wide. New satellite solar-power systems would contain a series of such donut-shaped capsules filled with a phase-change material.

He designed transparent capsules made of plastic, enabling researchers to see what is happening inside the vessels as the gaps form.

Revankar also uses phase-change materials that melt at low temperature, which are easier to work with than materials that melt at 800 degrees Celsius. One of the materials that he uses remains transparent while frozen, permitting researchers to take detailed photographs of gap formation.

‘We take pictures in the lab when they are freezing to see how many voids there are and how they are distributed inside,’ Revankar said. ‘This will tell us, for example, how many voids there are in the centre and how many migrate to the walls.’

Test results are then subjected to mathematical analysis, and the findings are used to create computer models that might enable engineers to design better capsules.

In contrast, experiments conducted on the space shuttle can not be analysed until the frozen phase-change material is returned to Earth and cut into slices for analysis.

Ultimately, researchers are trying to envisage the physical mechanisms involved in the formation and movement of cavities inside the capsules.

Because the weightless environment of space does not affect the formation of gaps in the capsules, the results of Earth-based experiments can be applied to space systems, concluded Revankar.