Dark material offers a bright energy future

Concentrating Solar Power (CSP) systems use an array of reflectors to direct sunlight to a central receiver system where light is converted into heat, which is used to generate electricity. In the journal Scripta Materialia, researchers in Italy report the development of a dark form of aluminium oxide with potential to significantly improve the efficiency of solar receivers while greatly reducing their cost.

Basic physics theory deems that the efficiency of such systems increases as the working temperature increases, but achieving the highest possible working temperatures poses technical problems.

“The practicalities of increasing the temperature of the energy trapping cycle are challenging, mainly due to the bottleneck caused by the solar receiver material,” explains first author Elisa Sani of the Italian National Institute of Optics.

The best material tested and used in CSP systems to date is silicon carbide (SiC), but it is significantly more expensive than other materials that have been explored, such as ceramics composed of alumina (Al2O3) and zirconia (ZrO2). Until now, however, such alternatives to silicon carbide have suffered from deficiencies – in particular, low levels of absorption of sunlight.

Sani and her colleagues made a significant breakthrough by discovering that an uncommon form of alumina called dark alumina has much better properties as a receiver material than the white alumina that has been the focus of all other work that they are aware of.

“We have shined new light on an old material,” she says, explaining that simple techniques can generate dark alumina that is much better at absorbing sunlight. That key advantage is accompanied by several others, including high stability in air and exceptional strength, hardness and resistance to wear, while also being significantly cheaper and easier to produce.

The researchers prepared three forms of dark alumina and compared their performance relative to white alumina at temperatures as high as 1900 degrees Celsius. Unsurprisingly, as they are so dark as to be almost black throughout, the dark aluminas absorbed the sunlight energy to a substantially greater extent than the white alumina. But the differences were sufficiently impressive for Sani to describe them as “remarkable” – a more than fourfold increase for the best-performing material.

An additional feature of dark alumina, of potentially huge significance, is that it acts as a thermionic material, meaning that it emits electrons when heated.

“So, a fascinating further application could be in a hybrid CSP system simultaneously producing electricity by direct electron emission and also by the conventional indirect heat-exchange cycles,” Sani concludes.

“As an amateur gardener, I always have been fascinated by sunlight and it always amazes me to feel with my hands this huge amount of energy freely available to us,” she adds. Thanks to her research work, humanity may be better placed to use this free energy in the near future.

Article details:

Sani, E. et al: “Dark alumina for novel solar receivers,” Scripta Materialia (2020).

Scripta Materialia is part of the family of Acta Materialia Inc journals, which includes Acta Materialia, Scripta Materialia and the newly launched Materialia