FRACTALS:
SYNTHESIZED COATINGS
CREATE A HABITAT FOR ENERGY
PRODUCING PHOTONS
The cost difference between fossil
fuel and solar high capacity power
plants can be measured in cents
per unit of energy. A Virginia Tech
mechanical engineer believes he’s
found a way to bridge that gap by
catching the sun with fractals.
“Coal and natural gas electricity
generation costs about 5 cents a
kilowatt hour compared to about 10
cents for current solar thermal plants,”
said Professor Ranga Pitchumani,
the George R. Goodson Professor of
Mechanical Engineering. “With better
receiver coatings, we can increase the
operating temperatures and make
up that nickel and get solar-powered
electricity down to cost parity at 5
cents per kilowatt hour.”
This savings can be made thanks to
fractals, objects whose measure varies
depending on the scale used for the
measurement. They are multiscale,
self-similar, hierarchical structures that
are abundant in natural constructs such
as coastlines, snowflakes or biological
structures. This is unlike Euclidean
objects such as a straight line, whose
length is the same measured with a
meter stick or a foot ruler. A meter
stick measuring the coastline of Rhode
Island will come up with a different
answer than a one-inch stick which will
better resolve the irregularities of the
16
coastline. A one-millimeter stick will
come up with an even different, and
larger, number.
As Pitchumani, explains, “There
are two ways to convert sun’s energy
into electricity. Solar cells using the
photovoltaic effect work by discharging
electrons, by virtue of impact by
packets of energy in sunlight called
photons; the electrons go through
a circuit to provide electricity. The
second way is by converting the energy
of photons to heat.” It is the latter
that Pitchumani and his group in the
Advanced Materials and Technologies
Laboratory are focused on.
The problem Pitchumani and his
graduate student Rahul Jain were
confronted with was how to increase
the amount of sun’s energy a receiver
can capture and convert to heat at
higher temperatures.
Unlike rooftop photovoltaic
solar cells, a solar thermal power
plant requires acres of mirrors that
aim the sun’s reflection at a receiver
which amasses the photons and turns
them into heat. The energy is used to
heat a fluid (water or molten salt) to
generate steam to run a turbine, in
the same way a fossil-fired plant would
operate. The hotter the steam entering
the turbine, the more efficiently the
turbine can generate electricity. And