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Engineers use nanoparticles to boost solar thermal’s heat absorption

07/04/2011
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Mechanical engineers at Arizona State University are experimenting with graphite as a cheap way of augmenting the efficiency of solar thermal systems by improving the heat-absorbing properties of liquid used for generating steam.

“We estimate that this could mean up to $3.5 million dollars per year more revenue for a 100-megawatt solar power plant,” said Robert Taylor, a graduate student in mechanical engineering at Arizona State University.

The team from Arizona State University turned their attention to the solar thermal collectors, an essential part of a solar thermal power system, also known as concentrating solar power systems. The collectors focus sunlight to heat liquid that makes steam which drives a turbine to generate power.

“The big limitation of PV panels is that they can use only a fraction of the sunlight that hits them, and the rest just turns into heat, which actually hurts the performance of the panels,” said Mr. Taylor. In contrast, solar thermal collects and uses the heat which would be lost by PV.

To further increase the efficiency of solar collectors, Mr. Taylor and his colleagues mixed in nanoparticles of graphite into the heat-transfer oils used in solar thermal power plants.

The researchers used graphite for the simple reason that it’s black and as such has great light absorption qualities. It does not reflect light and can make the oils capable of absorbing even more heat.

Under laboratory conditions, Mr. Taylor and his colleagues found that the nanoparticles increased heat-collection efficiency by up to 10 percent.

Another advantage to using graphite nanoparticles is that it is a common material that costs relatively low. According to Mr. Taylor, at less than $1 per gram, some 100 grams of nanoparticles provide the same heat-collecting surface area as an entire football field.

“It might also be possible to filter out nanoparticles of soot, which have similar absorbing potential, from coal power plants for use in solar [thermal] systems,” added Mr. Taylor.

“I think that idea is particularly attractive: using a pollutant to harvest clean, green solar energy,” he said.

An article detailing the study and findings appears in the Journal of Renewable and Sustainable Energy.

The heat transfer fluids in a solar thermal system can be as simple as water or a compound of various oils and other fluids. But being a good heat absorber is important.

Italian solar thermal supply company Archimede Solar Energy uses molten salts – a mixture of sodium nitrates and potassium – as the heat transfer fluid for their solar thermal systems.

Archimede is said to be the only producer to use molten salt as heat transfer fluid. The first concentrated solar thermal power plant to use the technology was inaugurated on July 2010 in Syracuse, Sicily.

Archimede Solar Energy is a joint venture with Siemens Energy. The Archimede solar thermal plant is owned and operated by Italian energy provider Enel and has a generation capacity of 5 MW.

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