An engineer at Purdue University (West Lafayette, IN) is saving NASA millions of dollars by devising a method to test a new type of solar-power system on Earth instead of in space. The experimental power system would work 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.
Because experiments conducted on the space shuttle can cost millions of dollars, the engineer devised a way to conduct experiments on Earth at a fraction of the cost. “We can perform the same tests as would be done in space in the laboratory for less than $100,000,” said Shripad Revankar, an associate professor of nuclear engineering at Purdue. During daylight hours, portions of the experimental satellite system exposed to solar radiation in the vacuum of space would reach 800 degrees C, or more than 1,400 degrees F. Central to the solar-power system is a phase-change material that is liquid under high temperature, but freezes during hours of cold darkness. 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 devices called thermoelectric units.
A phase-change solar energy system would be more compact and would store more energy than conventional systems that use rechargeable batteries. Such systems are used to provide energy for solar homes and other solar-power applications. 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.
However, engineers are trying to make the system more efficient so that it will be practical for space vehicles. But a major obstacle is that bubble-like cavities, or voids, form in the material as it freezes. “The problem is these materials shrink a lot when they freeze,” Revankar said. “That means you have a large gap.”
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. According to Revankar, if a void develops against the wall of a capsule, heat cannot be transferred efficiently. The repeated formation of gaps against the metal walls also can damage the capsules over time.
Revankar has found that voids might be controlled by using capsules of certain sizes and shapes. He has found, for example, 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.
Revankar has made it possible to conduct the experiments on Earth. He designed transparent capsules made of plastic, enabling researchers to see what is happening inside the vessels as the gaps form. His research also uses phase-change materials that melt at low temperature, which are much easier to work with than materials that melt at 800 degrees C. One of the materials, for example, 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 center 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't be analyzed until the frozen phase-change material is returned to Earth and cut into slices for analysis.
Ultimately, researchers are trying to figure out the physical mechanisms involved in the formation and movement of cavities inside the capsules. Because the formation of gaps in the capsules is not affected by the weightless environment of space, the results of Earth-based experiments can be applied to space systems, Revankar says.
The research funded by NASA was detailed this month in Anaheim, CA, during a presentation at the National Heat Transfer Conference. The conference is sponsored by the American Society of Mechanical Engineers, the American Institute of Aeronautics and Astronautics, and the American Institute of Chemical Engineers. The technical paper was written by Revankar; Patrick George, an engineer at the NASA Glenn Research Center, in Cleveland; and Purdue graduate student Travis Croy. For more details, email Revankar at [email protected].