It is often said that “pumps are the workhorses of industry” in that they are used in nearly all processes and must withstand a diverse range of extreme operating conditions, including exposure to corrosive or abrasive chemicals, cryogenic temperatures and more. And as new technologies are pushing limits of what processes can achieve, pumps are evolving alongside these innovations.
One such example is the increased use of molten-salt technologies for energy-storage applications, which enable users to optimize their deployment of renewable power sources, like wind and solar by storing electricity when it is cheapest and most abundant. In the molten-salt energy-storage system developed by Hyme Energy ApS (Copenhagen, Denmark), for example, excess renewable energy is used to heat salts to temperatures around 600oC, and retain this energy in a hot storage tank. The hot salt is then pumped to a heat exchanger that produces steam at around 250oC, providing a usable source of industrial heat and energy.
The extremely high temperatures used in these systems demand robust pumps that are designed for the challenge. Sulzer Ltd. (Winterthur, Switzerland) recently announced an expansion of its partnership with Hyme Energy to demonstrate performance of its state-of-the-art high-temperature pump designs for energy-storage systems using molten salt technology. In this interview, Benoît Martin, Advance Engineering Manager at Sulzer describes the thought process behind pumps that must operate in such a uniquely challenging environment.
Hyme’s system stores energy from renewable sources in molten salts to enable industrial heat decarbonization (Source: Sulzer)
Specialized pump considerations
Pumps used in molten-salt applications obviously must go above and beyond the capabilities of traditional process pumps. because as temperatures rise to the temperatures required to melt the salts, the molten media becomes more corrosive and abrasive, which leads to material challenges with sealing and large temperature differentials.
As with any pump, seals are a crucial element, and they are made significantly more important in extreme applications. “Molten salt pumps need effective seals for both liquid and gas along the length of the driveshaft. Specific designs are dependent on each application and the most effective solutions are based on a combination of engineering excellence and experience. The exact type of medium being used and the design operating temperature both have crucial importance,” notes Martin.
Determining the proper materials of construction for all pump components that will withstand high levels of corrosion, abrasion and temperature swings is also an area that requires inventive thinking. “Although these pumps will operate continuously at elevated temperatures, they must still be assembled and maintained at room temperature. Long-shaft pump designs need to accommodate thermal growth during installation and impeller clearances need to be established once the operating temperature has stabilized. At the same time, the materials must have sufficient resistance to corrosivity, which is determined by the type of salt, as well as the operating environment,” Martin says.
Building upon the current state-of-the-art
In order to gather the proper information to make critical design decisions on materials, sealing specifications and application-specific constraints, digital tools and simulations are crucial. “A computerized model of the whole pump, including the drive motor, needs to be carefully assessed to ensure that the effects of the thermal coefficients of all the components are understood and accounted for. The huge variations in temperature distribution have a significant effect on the mechanical design of the pump. The theoretical model helps the engineers to understand this distribution and to develop both materials and a physical design for the pump. This includes examining thermal stress and weld seam evaluations to ensure consistent, reliable performance from the pump,” explains Martin.
Heat transfer within the system is another essential factor that must be examined to ensure that any pump components that are not in contact with the molten salt do not overheat, such as the electric motor and the top bearing. According to Martin, Sulzer uses a sophisticated thermal-modeling technique to ensure the pump design will match with this application’s next-generation process requirements. This work has led to refinements of the cooling system that will be important in creating a pump with optimum reliability and a long service life.
Next steps
Sulzer’s experience with these advanced pumps will directly support the commercialization of Hyme’s technology. Following a successful demonstration plant project in Esbjerg, Denmark in 2024, Hyme Energy is currently working with Arla, a large European dairy producer, to seek E.U. funding for a 200-MW energy-storage facility in Denmark. The project would involve installation of a molten-salt energy-storage system within Arla’s milk-powder facility in Holstebro, Denmark, reportedly enabling up to a 100% reduction in CO2 emissions from process heat at the site.