Soiling
-- the accumulation of dust and sand -- on solar power reflectors and
photovoltaic cells is one of the main efficiency drags for solar power plants,
capable of reducing reflectivity up to 50 percent in 14 days. Though plants can
perform manual cleaning and brushing with deionized water and detergent, this
labor-intensive routine significantly raises operating and maintenance costs
(O&M), which is reflected in the cost of solar energy for consumers.
Under
the sponsorship of the Department of Energy's Energy Efficiency and Renewable
Energy SunShot Concentrating Solar Power Program, Oak Ridge National Laboratory
is developing a low-cost, transparent, anti-soiling (or self-cleaning) coating
for solar reflectors to optimize energy efficiency while lowering O&M costs
and avoiding negative environmental impacts. The
coating -- which is being designed by members of the Energy and Transportation
Science Division, including Scott Hunter, Bart Smith, George Polyzos, and
Daniel Schaeffer -- is based on a superhydrophobic coating technology developed
at ORNL that has been shown to effectively repel water, viscous liquids, and
most solid particles. Unlike other superhydrophobic approaches that employ
high-cost vacuum deposition and chemical etching to nano-engineer desired
surfaces, ORNL's coatings are deposited by conventional painting and spraying
methods using a mixture of organics and particles. In addition to being
low-cost, these methods can be deployed easily in the field during repairs and
retro-fitting.
There
are, however, challenges to the successful development of such a transparent,
anti-soiling coating. First, the coating must be very superhydrophobic to
minimize the need for occasional cleaning, and it must have minimal (or even
zero) effect on the transmission and scattering of solar radiation between the
wavelengths of 250 to 3,000 nm. To meet these requirements, the coating must be
no more than a few hundred nanometers thick, and the embedded particles must be
considerably smaller. The extremely thin coating must also be durable under
environmental exposure, including UV radiation and sand erosion, and be
compliant according to the US Environmental Protection Agency Clean Air Act
emission standards -- which limits the selection and combination of particles
and organics that can be used effectively.
During
the first year of this project, researchers experimented with a variety of
Clean Air Act-compliant organics and silica particles of different sizes. They
arrived at a particular formulation combining organic compounds with silica
particles, which are dispersed in two sizes to enhance area coverage of
particles within the coating. The
anti-soiling coating exhibited excellent superhydrophobic properties, losing
less than 0.3% of transparency over the entire solar radiation wavelength
range. When exposed to several hundred hours of accelerated UV radiation and
one hundred hours of salt fog exposure, the coating exhibited no degradation in
superhydrophobic or optical transmission properties. Also, when glass slides
with the anti-soiling coating were exposed to sand and dust in a custom-made
wind tunnel, the particles did not adhere to the coated surface of the slides
-- showing great potential for its use in harsh environmental conditions.
In
addition to anti-soiling coating for solar applications, ORNL researchers are
using their superhydrophobicity expertise to develop anti-soiling cool roof
coatings, as well as anti-icing and anti-condensation coatings for air
conditioning and evaporative cooling applications, respectively. Going into
2014, the project has been funded for another year and will optimize the
coating and perform accelerated exposure tests, as well as begin development on
a scalable coating technique and perform small-scale field testing.
Source Science daily
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