High-performance pumps for challenging applications in solar systems — When it comes to using solar energy, new heat transfer media are in high demand. Among the most promising candidates are in fact molten salts. But the handling of these new thermal medias quickly proves more difficult than it appears...
The Kyoto Agreement of 1997, the Paris Agreement in 2015, and the phase out of nuclear energy: Climate protection and, as a result, the need for alternative sources are on the agenda worldwide. The energy industry is focusing on renewable energies such as wind, solar or hydro-power. Since the early 1990s, solar power has been part of the global energy landscape, and the trend is rising: Already in 2015, more than 250 billion kilowatt hours were generated worldwide from photovoltaics alone. But the manufacturers of solar energy plant systems are also facing a constant surge of new challenges as well as a continuous demand for innovation in order to be able to serve the growing market.
As a result, new technologies have risen to the challenge: As the demand for solar energy is constantly growing, also concentrated solar power systems are being further developed and optimised in order to generate more energy and to use it efficiently. In addition to traditional thermal fluids like oil and gas, new heat transfer media such as molten salts have been tested over the past twenty years with great success. Today, many plant operators are already fully relying on thermally highly efficient molten salts as a heat transfer medium and, above all, as a heat accumulator for solar power plants. But the high medium temperatures poses certain challenges for pump manufacturers.
Molten salt is produced when salt is heated above its melting point, which — depending on the individual substance or mixture — occurs at temperatures of between 150 °C and 600 °C. Eutectic mixtures of alkali and alkaline earth salts are excellent substitutes for oil and gas as a heat transfer medium, because they can be used at temperatures above 400 °C. Even temperatures of up to 600 °C pose no problem.
It may sound surprising, but the idea to use salt in thermal processes has a long history: The use of molten salt as an economical alternative to common heat transfer medium dates back to the 1950s. Initially, molten salts were mainly used in chemical plants, for the thermochemical treatment of metals, in hardening processes or for thermochemical cleaning.
Pumping Molten Salts: A Demanding Task
There are three sustainable reasons for using molten salts in solar energy systems: Their good thermal transfer properties, high temperature tolerance and low viscosity. Yet, using molten salts as a heat transfer medium also presents component manufacturers with challenges, especially due to the high temperatures that the systems must be able to withstand. Reliable and economic pumping thus becomes extremely demanding and requires a great deal of know-how in the design. Enter Rheinhütte Pumpen in Wiesbaden/Germany: The company is a specialist for corrosion proof and wear resistant materials and for the construction of highly durable molten salt pumps.
Depending on the application, the company manufactures pumps made of metal, plastic or ceramics. Metal system components have demonstrated their suitability for pumping molten salts in many applications. The company’s GVSO, a vertical submerged centrifugal pump, is specially designed for use in solar power systems. For applications with medium temperatures of up to 470 °C, the pump is manufactured from ferritic heat-resistant steel, for higher temperatures, it is made from high-quality austenitic stainless steel.
In addition to its temperature tolerance, the GVSO pump is also ideally suited for demanding media as well as for use in solar energy due to its individual design options, offering maximum design flexibility: It is available in 38 sizes with single or multi-stage design and can be used with an immersion depth of 0.5 m to 18 m. Up to twelve intermediate bearings are used in long pumps which are lubricated by the molten salts themselves. Rheinhütte always designs each GVSO individually to serve the respective application of the customer. For example, the pump can be adapted to meet the desired pressure temperature conditions (the nominal pressure of the GVSO is up to 40 bar). With regard to the maximum operating depth, the discharge head (up to 200 m) and the details of the pipeline and tank, Rheinhütte designs the pumps of the GVSO series to customer specifications and to perfectly match the respective application.
“Molten salt is a challenging medium that requires a comprehensive materials know-how of constructors in the development of pumps — for solar energy systems, for example. For solar energy systems that work with salt as a heat carrier or storage, our experts develop tailor-made solutions,” explains Michael Stock, Application Manager Solar at Rheinhütte Pumpen.
How to Set up a Pump for Solar Applications
In solar power plants, the molten salt often fulfills the role of a so-called heat accumulator: It absorbs the solar energy in the form of heat and is then temporarily stored in heat storage tanks. There, the vertical centrifugal pump of the GVSO series is used to continue to pump off the heated molten salt, which can reach temperatures of up to 600 °C. Due to the sliding bearing principle, the molten salts also serve as a lubricating medium, which is why, even in intermittent operation, the GVSO has low bearing wear. In solar power applications, pumps with a length of at least 14 m are usually used. The manufacture and integration of such long pumps is a “speciality” of Rheinhütte — and a tasty solution to tackle the challenge of pumping liquid salts.
* * The author is a freelance journalist.