Sparks

The genius of Archimedes

 By Luca Longo

Today, let us take you back to a discovery that was made some 2229 years ago. Legend has it that Archimedes defended the city of Syracuse (Sicily) from the fleet of the Roman general Marcus Claudius Marcellus using rays of solar energy generated by hundreds of soldiers equipped with polished bronze shields. Deployed along the coast, they are said to have directed the rays of the Sun onto the Roman ships that besieged the city, concentrating them to such an extent that the ships eventually caught fire. A couple of millennia later, somebody had the bright idea of using the same trick to generate thermal energy…

We don’t know if the legend is true or not. What is certainly true is that Archimedes invented numerous weapons that the Syracusan army used to resist the Roman attacks for some two years. Now, though, students at MIT have shown that it really is possible to set fire to a reconstruction of a Roman ship by concentrating the light reflected by 300 polished bronze shields onto it. Since scientists never discard an idea simply because it is a bit eccentric, a couple of millennia later somebody thought that it might be possible to use the same trick to generate thermal energy. To cut a long story short, of all the projects developed using this technology we will focus on the thermodynamic solar power plant that bears the name of Archimedes himself. Designed by Italian Nobel laureate Carlo Rubbia and located in Priolo Gargallo, Sicily, the Archimede Power Plant is made up of 30,000 square metres of parabolic mirrors that concentrate the rays of the Sun into their geometric focus.
The sunlight is concentrated into over 5,000 metres of tubes or receivers that contain a heat-transfer fluid that is heated to a high temperature by the concentrated rays. The fluid reaches 550 °C, so it is not possible to use water as it would immediately become steam. Instead, special mixtures of molten salt that turn liquid at high temperatures are used. These, in turn, are used to heat industrial buildings or homes or are pumped to a heat exchange where they boil water to produce steam to run turbines for electricity generation.

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Archimedes’ burning mirrors

Ideas and limitations

Similar technologies have been developed all around the world. Lots of researchers have experimented with various mixes of salts or diathermic oils. Engineers have unleashed their imaginations as well, producing a variety of geometries ranging from simple parabolas to fields of mirrors that focus light to the top of a tower, and even rows and rows of linear parabolic mirrors. Unfortunately, there is no getting away from three serious limitations that make this technology inefficient for generating thermal energy or electricity.
The first is purely an engineering issue. The plants use glass mirrors made reflective by metal deposits (which by the way, are another Italian invention, having first been made in Venice in the 14th century). These have the drawback of being heavy, but since a solar concentration plant needs to track the Sun from dawn until dusk, the mirrors have to be kept moving for the entire day.
The second problem relates to the heat-transfer fluid. You need something that has a high thermal capacity and doesn’t decompose at working temperatures between 300 and 550 °C. The salt mixtures currently being used do work well, but the problem is that they cool down overnight or when it is cloudy. Below 270 °C, the salts become more and more viscous and it’s difficult to pump them along the tubes, but if the temperature falls below their melting point the salts solidify and turn into a ceramic block, which breaks the entire plant. For this reason, solar concentration plants not only don’t produce energy at night or when it’s cloudy, but actually consume it because the heat-transfer fluid has to be kept at a very high temperature. For example, the Archimede plant is paired with a gas-powered thermal plant that has to spring into action when the Sun isn’t shining. It speaks volumes that when the Archimede plant was launched, the 500 cubic metres of salts to be used had to be heated for about a month.

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The Archimede Power Plant project by Carlo Rubbia in Priolo Gargallo, Syracuse, Italy

The third problem is to do with the receivers, which need to have two opposing characteristics: they have to be an excellent absorber of heat (to absorb all the solar rays reflected by the mirrors) and a useless emitter of heat, so as to keep the heat inside the tube. We know that it is easy to make materials that absorb heat well (like saucepans) or that isolate something from heat (bricks), but it is not so easy to invent materials that have both characteristics at the same time.

The Italian solution

Once again, all three problems have been addressed and resolved by Italian ingenuity. Eni Renewable Energy and Environmental R&D Center has created a much more efficient prototype solar concentration plant. First of all ­– thanks to a partnership with Milan Politecnico and MIT – the heavy metal and glass mirrors that are curved under heat treatment have been replaced by light, thin and reflective polymer-based films. This has not only reduced physical weight but also investment costs, because it also made it possible to simplify the design of the entire mirror system and the mechanisms that point them in the right direction. Construction, too, has been simplified, because the system now needs standard mechanical components that are easy to find and can be handled by non-specialist labourers. This in turn provides a major boost to local business, even in industrially underdeveloped areas. The result is that the cost of the entire plant is halved and the cost of the mirrors falls by 75%.
The Research Center in Novara has also created innovative heat-transfer fluids made of ternary and quaternary salt compounds that remain liquid even when they cool down to 90-140 °C. This means that it’s no longer necessary to keep the system at 300-350 °C overnight and it can be heated much less, with obvious savings and lower fossil fuel consumption.
Lastly, Eni R&D Center in Novara has also invented a new type of coating for the receiver tube, using a combination of four innovative metaloceramic layers to build coatings that are excellent absorbers (95% absorbancy) of heat but also terrible dissipators (emissivity: 7% at 400 °C). In practice, the heat that goes into the tube does not come back out.
So, through the combination of these three innovations, researchers at Eni Renewable Energy and Environmental R&D Center have solved the three biggest problems that made solar concentration technology inefficient and economically unattractive. The first plants based on these technologies are now under construction in Gela (Sicily) and Assemini (Sardinia) and we hope to see them up and running soon.

READ MORE: Beyond the solar panel by Amanda Saint

about the author
Luca Longo