Solar thermal power plants that produce hotter steam can capture more solar energy. That’s why Siemens is exploring an upgrade for solar thermal technology to push its temperature limit 160 °C higher than current designs. The idea is to expand the use of molten salts, which many plants already use to store extra heat. If the idea proves viable, it will boost the plants’ steam temperature up to 540 °C—the maximum temperature that steam turbines can take.
Siemens’s new solar thermal plant design, like all large solar thermal power plants now operating, captures solar heat via trough-shaped rows of parabolic mirrors that focus sunlight on steel collector tubes. The design’s Achilles’ heel is the synthetic oil that flows through the tubes and conveys captured heat to the plants’ centralized generators: the synthetic oil breaks down above 390 °C, capping the plants’ design temperature.
Startups such as BrightSource, eSolar, and SolarReserve propose to evade synthetic oil’s temperature cap by building so-called power tower plants, which use fields of mirrors to focus sunlight on a central tower. But Siemens hopes to upgrade the trough design, swapping in heat-stable molten salt to collect heat from the troughs. The resulting design should not only be more efficient than today’s existing trough-based plants, but also cheaper to build. “A logical next step is to just replace the oil with salt,” says Peter Mürau, Siemens’s molten salt technology program manager.
The German engineering giant will actually be the second player to try to push molten salts through solar collector tubes. Last summer, the Italian utility Enel began running molten salt through a field of about 30,000 square meters of trough mirrors adjacent to its natural gas-fired power plant near Syracuse, Sicily. The salt exits the 5.4-kilometers of collector pipe at 565 °C, boosting the power plant’s output by 5 percent.
Enel’s plant uses collector tubes from Italy’s Archimede Solar Energy, the only producer of collector tubes designed to handle molten salts. Their collector tubes use a heat-stable metalloceramic coating to maximize heat absorption, as well as thicker titanium-stabilized steel pipes to resist bending at high temperatures. Paolo Martini, Archimede’s business development director, says the plant is operating well. Enel plans to build a 30-megawatt plant in Sicily.
Since 2009, Siemens has amassed a 45 percent stake in Archimede, but it has opted to go back to pilot-scale to optimize the molten-salt concept before offering commercial-scale plants to global clients. “We are convinced the technology itself will work. But a lot of work needs to be done to optimize the economics,” says Mürau.