Siemens is building a molten-salt pilot plant on the grounds of the University of Evora in Portugal. The plant should be operating by early next year. The plant—part of a German research consortium including salt and chemicals giant K+S AG and the German Aerospace Center—will be used to drive down energy losses associated with both the highest and lowest temperatures that a commercial plant will experience.
At the high end, the losses come from heat that’s captured by the collector tubes and then dissipated before it can be delivered to the plant’s turbines. “The heat loss is an exponential curve, and it climbs very steeply at the higher temperatures,” explains Mürau. Siemens will seek to achieve the highest temperatures possible without going so high that these losses outweigh the gains from the hotter steam.
The low-end challenge stems from molten salt’s high freezing point. The mixture of molten potassium and sodium nitrate used in heat storage systems and in Enel’s demo plant freezes when it cools below 220 °C. Freezing is easy to prevent in centralized energy storage tanks, but presents a serious risk in kilometer-long stretches of collector tube. To counter the freezing threat, Enel’s plant maintains the salt in its tubes above 290 °C, using considerable heat that could otherwise be used to generate power. Mürau says Siemens is looking for a salt formulation with a 150 °C or lower freezing point, which would mean they’d have to use much less heat to prevent the tubes from freezing.
If Siemens’s efforts succeed, trough plants heating molten salt could reduce the cost of power generation by more than 10 percent compared to an oil plant, according to Mürau. (Estimates of current solar thermal costs vary between 13 to 20 cents per kilowatt-hour, which is still significantly higher than power generated by fossil fuels.) The cost reduction comes from both a several-percent increase in generation from turbines running on hotter steam, and a lower cost of construction.
However, some experts argue that the risk of freezing could still be a deal-killer for commercializing molten-salt-based plants. Thomas Mancini, program manager for Sandia National Laboratory’s concentrating solar-power program, says he remains “skeptical” of using molten salts in collector tubes given the inherent freezing threat. Mancini says that even at 100 °C (the temperature that boils water), there would be a significant risk of freezing.
But others in the industry are warming to molten salt’s potential. In January, for example, Colorado-based SkyFuel kicked off a $4.3-million R&D effort, supported by the U.S. Department of Energy, to scale up its metallic film-based trough mirrors for use with high-temperature collector tubes.