Operating in the intense glare of thousands of mirrors focusing the desert sun’s rays, solar absorbers in solar towers also have to cope with the “aggressive” environment of molten salt. To help improve their performance and offer a longer-lasting power source, researchers at the National University of Science and Technology in Russia and colleagues at the Central Metallurgical R&D Institute in Egypt developed porous composites using two materials.
Solar power plants using towers have arrays of revolving mirrors, known as heliostats, around the towers. The mirrors focus the sun beams on the absorber, which absorbs heat and in turn heats a special salt solution to 600°C. The salt heats water in an adjacent tank, producing steam to rotate the power plant’s turbines.
Operators traditionally use silicon carbide (SiC), a porous ceramic material with useful characteristics including high density, strength and resistance to oxidisation. It is also, however, sensitive to the aggressive environment of salt melts.
To tackle the issue and potentially add up to five years on to a tower’s lifetime, the researchers added aluminium nitride (AIN) to SiC. AIN has high thermal conductivity, a low coefficient of thermal expansion and high temperature resistance.
The resulting porous composites, containing up to 40% aluminium nitride, “significantly” exceeded the operation of traditional materials due to the formation of a solid solution at the grain boundaries of SiC. Along with high thermal conductivity and heat resistance, the composites also had a low coefficient of thermal expansion, significantly improving their performance.
"Solar energy requires durable materials without defects,” said Dr. Emad Ewais, head of the Egyptian research group, adding that the group is planning to test the material in solar installations.
The composites could also be useful in other high-temperature fields such as metallurgy and aerospace engineering, the researchers said.
The research was published in Renewable Energy.
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