In Depth: Salt transfer may be the hottest thing to hit solar

On a heavily industrialised stretch of Sicilian coast dominated by ageing oil refineries, the blinding trough-shaped mirrors of the Archimede solar project shine out.

Operator Enel named the plant after the “ burning mirrors” that the Greek scientist Archimedes is said to have used to set fire to Roman ships besieging the nearby city of Syracuse during the siege of 214-212 BC.

Although it looks similar to other big concentrating solar power (CSP) projects that use parabolic-trough technology, the 5MW experimental plant is on its way to shaking up the market through its use of molten salt instead of synthetic oil as a heat-transfer fluid.

The molten salt can be heated to 550°C, compared to 400°C for synthetic oil. This allows the plant to feed steam directly at 540°C, and a pressure of 100 bars, into a conventional steam turbine of the type used in the thermal-power industry, rather than a specialised turbine designed for lower temperatures.

A standard turbine has efficiency levels of 41.5-41.8%, compared with 35.6% for those used by other CSP plants, according to project manager Daniele Consoli.

A pipeline takes the steam directly to the adjacent Archimede combined-cycle plant — also operated by Enel, Italy’s biggest power company and parent group of renewables developer Enel Green Power.

“This is the first project directly integrated into the high-pressure stage of a generation plant,” says Consoli. Other hybrid plants, such as those being built by Spain’s Abengoa, feed the steam into a heat-recovery generator, where it is heated further by the exhaust of a gas-powered turbine.

“What we are doing here means that we can more effectively reduce the use of fossil fuels, or eliminate them entirely in stand-alone plants,” says Consoli.

Being able to use conventional steam turbines is likely to bring significant cost benefits, as well as greater efficiencies.

The extra heat also means that storage — which allows CSP plants to produce even when there is no sun — is more effective. The molten salt can feed directly into the hot storage tank at high temperatures, whereas in conventional plants, synthetic oil is used as the heat-transfer fluid and to heat molten salt in storage tanks.

This allows Enel to reach a temperature of 550°C in the hot tank and 290°C in the cold tank, which lets it produce steam for up to eight hours. “The transfer fluid is also the storage medium,” says Consoli, “so the system can be one third less the size of an oil-based plant.”

Archimede uses ultra-thin mirrors — 1mm versus 4mm for conventional mirrors — in its troughs, increasing efficiency. Crucially, they are shatter-proof, allowing the loops in the power plant to keep functioning even if a mirror has been damaged. All in all, Consoli estimates that in similar conditions of direct nominal radiation, Archimede is 20-25% more efficient than existing CSP plants.

But the technology, which Enel developed with Italy’s national energy research agency, is not without its challenges.

The molten salt it uses comprises 60% sodium nitrate and 40% potassium nitrate. It can reach high temperatures and has a lower viscosity than synthetic oil, but it needs to be kept at a minimum of 280°C. Any lower, and it crystallises.

To stop this happening, the system needs to take power from the grid at times of persistent low sunlight to run a heating system that stops the temperature falling too far. This happens “not very often”, given high direct radiation, but it does raise the plant’s operating costs.

A solution is in sight, however, and Enel is about to sign an agreement with an undisclosed company that will supply molten salt that will not crystallise unless the temperature falls below 140°C. Enel aims to test the new mixture from April next year.

“This will simplify operational procedure and improve the safety operation temperature, and then it is a matter of defining the best configuration,” says Consoli.

Enel is not the only company betting on molten salt.

In May, German engineering giant Siemens increased its stake in Archimede Solar Energy (ASE) — the company that made the 1,296 receiver tubes for Archimede — from 28% to 45%.

ASE, whose majority shareholder is Italy’s Angelantoni Industrie, is expanding its Massa Martana plant to produce 140,000 tubes a year, with completion expected in September.

In June, ASE signed an agreement with Japan’s Chiyoda to explore CSP opportunities in the Middle East and North Africa.

Ben Backwell in Priolo Gargallo, Italy

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