You may have heard the legend of how in 212 BC, Archimedes defended the port city of Syracuse against the invading Romans by setting their ships afire with the help of mirrors that concentrated the sun’s light. It sounds a bit implausible…
However, maybe you’ve heard of Comte Buffon — the guy who figured out how to compute the number pi by dropping needles on the floor. According to Michael Lahans, Buffon also did an experiment to see if Archmedes’ idea was practical. He got a lot of mirrors, each 8 × 10 inches in size, adjusted to focus their light at a distance of 150 feet. And according to Lahans:
The array turned out to be a formidable weapon. At 66 feet 40 mirrors ignited a creosoted plank and at 150 feet, 128 mirrors ignited a pine plank instantly. In another experiment 45 mirrors melted six pounds of tin at 20 feet.
Should we believe this? I don’t know. Some calculations could probably settle it. Or you could try the experiment yourself. If you do, tell us how it goes.
But there are also non-military uses of concentrated solar power. For example, the new power plant named after Archimedes, located in Sicily, fairly near Syracuse:
• Archimede website.
• Archimede solar power plant, Wikipedia.
It started operations on July 14th of this year. It produces 5 megawatts of electricity, enough for 4,500 families. That’s not much compared to the 1 gigawatt from a typical coal- or gas-powered plant. But it’s an interesting experiment.
It consists of about 50 parabolic trough mirrors, each 100 meters long, with a total area of around 30,000 square meters. They concentrate sunlight onto 5,400 metres of pipe. This pipe carries molten salts — potassium nitrate and sodium nitrate — at a temperature of up to 550 °C. This goes on to produce steam, which powers an electrical generator.
The news is the use of molten salt instead of oil to carry the heat. Molten salt works at higher temperatures than oils, which only go up to about 390° C. So, the system is more efficient. The higher temperature also lets you use steam turbines of the sort already common in gas-fired power plants. That could make it easier to replace conventional power plants with solar ones.
The project is being run by Enel, Europe’s third-largest energy provider. It was developed with the help of ENEA, an Italian agency that deals with new technologies, energy and sustainable economic development. At the Guardian, Carlo Ombello writes:
So why hasn’t this technology come before? There are both political and technical issues behind this. Let’s start with politics. The concept dates back to 2001, when Italian nuclear physicist and Nobel prize winner Carlo Rubbia, ENEA’s President at the time, first started Research & Development on molten salt technology in Italy. Rubbia has been a preminent CSP [concentrated solar power] advocate for a long time, and was forced to leave ENEA in 2005 after strong disagreements with the Italian Government and its lack of convincing R&D policies. He then moved to CIEMAT, the Spanish equivalent of ENEA. Under his guidance, Spain has now become world leader in the CSP industry. Luckily for the Italian industry, the Archimede project was not abandoned and ENEA continued its development till completion.
There are also various technical reasons that have prevented an earlier development of this new technology. Salts tend to solidify at temperatures around 220°C, which is a serious issue for the continuous operation of a plant. ENEA and Archimede Solar Energy, a private company focusing on receiver pipes, developed several patents in order to improve the pipes’ ability to absorbe heat, and the parabolic mirrors’ reflectivity, therefore maximising the heat transfer to the fluid carrier. The result of these and several other technological improvements is a top-notch world’s first power plant with a price tag of around 60 million euros. It’s a hefty price for a 5 MW power plant, even compared to other CSP plants, but there is overwhelming scope for a massive roll-out of this new technology at utility scale in sunny regions like Northern Africa, the Middle East, Australia, the US.
The last sentence is probably a reference to DESERTEC. We’ll have to talk about that sometime, too.
If you know anything about Archimede, or DESERTEC, or concentrated solar power, or you have any questions, let us know!