A Purdue University engineer is saving NASA
millions of dollars by devising a method to test a new type of solar-power
system on Earth instead of in the ultra-expensive environment of space.

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.

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 test in the laboratory for less than $100,000,”
said Shripad Revankar, an associate professor of nuclear engineering at
Purdue. He will present a paper detailing the work on Sunday (6/10),
during the 35th National Heat Transfer Conference in Anaheim, Calif. 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 research paper was written by
Revankar; Patrick George, an engineer at the NASA Glenn Research Center,
in Cleveland; and Purdue graduate student Travis Croy.

During daylight hours, portions of the experimental satellite system
exposed to solar radiation in the vacuum of space would reach 800 degrees
Celsius, or more than 1,400 degrees Fahrenheit. Central to the solar-power
system is a “phase-change” material that is liquid under high temperature,
which then 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.

“If a void is against the wall of a capsule, heat cannot be transferred
efficiently,” Revankar said.

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.

“We are investigating various geometries,” Revankar said. “NASA wants
to optimize the design so that it has excellent heat transfer

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.

Revankar also uses phase-change materials that melt at low temperature,
which are much 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

“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 said.

Experiments conducted on Earth also are more thorough because they are
not limited to the short duration of a space shuttle flight.

The research is funded by the National Aeronautics and Space Administration.


High res, http://news.uns.purdue.edu/UNS/images/revankar.solar.jpeg (1.35MB)]

Shripad Revankar, an associate professor of nuclear engineering at Purdue
University, displays the plastic “capsules” he has designed to test an
experimental solar-power system for space vehicles. Revankar has designed
the experiments so they can be conducted on Earth, making them far less
expensive than research carried out on the space shuttle. (Purdue News
Service Photo by David Umberger)


Void distribution in phase-change material capsule of solar latent heat
energy storage system

Shripad T. Revankar and Travis Croy, School of Nuclear Engineering, Purdue
University, and Patrick J. George, Space Flight Project Branch, NASA Glenn
Research Center

Thermal energy storage (TES) systems employing ncapsulated phase-change
material (PCM) are considered for space-based heat exchangers. The presence
of concentrated shrinkage voids in PCM can cause serious problems when one
attempts to melt the solidified PCM for the next thermal cycle. Experiments
were performed, and the void formation phenomena with spherical and torus
shape capsules were studied. The initial void growth, distribution and the
total void in the capsule were photographically studied from transparent
capsules using cyclohexane and hexadecane as PCM materials. Heat transfer
characteristics and associated design characteristics of the capsule were