Molten Salt Mixtures to Boost Solar Thermal Plant Efficiency

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.

 

To achieve this aim, Siemens plans to build a pilot facility in Portugal for testing application of molten salts as a heat transfer medium in parabolic power plants. The parabolic power plants employ concave parabolic mirrors, which focuses sunlight using an absorber tube. The tube allows the heat transfer medium to flow through it. Later, the heat gets transformed into power by a generator and a steam turbine. Heat transfer medium’s highest working temperature determines the thermal power process efficiency.

 

University of Evora in Portugal has been chosen for construction of the pilot facility. The plant will be equipped with a steam generator, pipework system, solar components and pumps to manage the temperatures and properties of heat transfer media and molten salts. Siemens’ scientists will use the pilot results for implementation at commercial plants with more than 50 MW of installed powers. Federal Ministry for the Environment, Germany is funding  this solar thermal pilot plant project.

 

Siemens Corporate Technology and Siemens Energy researchers work on a major goal to enhance properties and composition of molten salts. Molten salts will be used as an alternative for thermal oil. This boosts the working temperatures ranging from 400° to over 500° Celsius. High vapor pressure and extremely flammable property are major drawbacks of thermal oil, whereas molten salts have about zero vapor pressure and they are non-flammable. Hence, power plant operations will be safe without pressure. In addition, molten salt mixtures possess greater heat storage ability and they are economical for use.