A key milestone in the demonstration of molten salt as a heat transfer fluid in parabolic trough solar power plants has been reached by DLR engineers in Évora, Portugal. Together with researchers from the University of Évora and industrial partners, a team from the DLR Institute of Solar Research has started up the solar field of the Évora parabolic trough test facility with molten salt for the first time. The innovative technology is helping to further reduce the cost of solar thermal power plants. With their integrated storage systems, solar thermal power plants are the only technology that can generate large amounts of solar electricity around the clock.
Salt instead of oil as heat transfer medium for higher efficiencies
Commercial parabolic trough power plants at the current state of the technology use a special thermal oil as a heat transfer medium. The oil absorbs concentrated solar radiation from mirrors, converts it into heat and transfers it via pipelines to a heat storage tank or steam turbine for power generation. The storage tank, which is filled with molten salt, can hold heat at temperatures of up to 565 degrees Celsius for a period of 12 hours full load, for example, and release it again when demand for electricity increases. The power plant requires heat exchangers to transfer the heat from the oil to the storage tank. In this process, a part of the heat energy is lost for later conversion into electricity. In addition, the maximum possible operating temperature of the oil of around 400 degrees Celsius limits the efficiency of the energy conversion. Researchers and industry are therefore looking for ways to further increase temperatures in solar power plants in order to reduce the cost of electricity generation.
One promising path is to use molten salt not only as a heat storage medium but also as a heat transport medium in the collector field. Depending on the composition of the molten salt, it can absorb significantly higher temperatures than thermal oil of up to 565 degrees Celsius than thermal oil. Another advantage is that the storage tanks can be filled directly with the molten salt from the solar field and the previously required heat exchanger is no longer required.
To demonstrate this approach, the DLR Institute of Solar Research, together with the University of Évora and companies from Germany and Spain, has been building a solar thermal liquid salt parabolic trough test facility since 2016. The work took place as part of the HPS2 (High Performance Solar 2) research project, which is funded by the German Federal Ministry for Economic Affairs and Energy. The aim of the project is to demonstrate that parabolic trough power plants can be operated safely and economically using liquid salt as the heat transfer medium.
A technical challenge in the use of molten salt as a heat transfer fluid is the trace heating of all pipelines: To prevent the hot molten salt from freezing when the plant is filled, electrical trace heaters must preheat all salt-bearing components. An impedance heating system from project partner eltherm is used in the solar field.
First filling and test operation of the plant at 300 degrees Celsius successful
The HelioTrough® 2.0 generation collector modules from partner TSK Flagsol, which have now been filled with molten salt and interconnected, produce a total thermal output of up to 3.5 megawatts, with a total length of 684 meters. The collectors concentrate the solar radiation onto a glass-encased tube through which the molten salt flows. This so-called receiver is a development of the project partner Rioglass and is intended to demonstrate the stability of the optical quality even at high operating temperatures.
The plant currently operates with a so-called ternary salt mixture from the project partner YARA, which has the advantage of lower melting temperatures compared to the binary salt mixture Solar Salt, and can absorb heat up to a temperature of about 500 degrees Celsius. In addition to its use in solar thermal power plants for electricity generation, this salt mixture is also of interest for solar process heat supply.
Starting from a starting temperature of 300 degrees Celsius, the engineers intend to gradually increase the operating temperature to 500 degrees. In the coming weeks, the engineers in Évora will commission the other components of the salt cycle - in addition to the two-tank storage system, this also includes the steam generator, which was developed by the project partner Steinmüller Engineering, and the measurement equipment.
Dr. Jana Stengler, head of the Fluid Systems Group at the DLR Institute of Solar Research, on the results of the first test: "We are very satisfied with how the first filling went. Now the next goal is to gain operating experience in order to fill all further components with molten salt step by step, to test regular operation and also critical operating situations."
Dr. Mark Schmitz from project partner TSK Flagsol emphasises the importance of the project for future solar thermal power generation: "Power plants using the technology from HPS2 can be built more easily and operate more efficiently. This reduces electricity production costs by up to ten percent. This is an enormous step for a single technical change. At the same time, it makes longer storage durations of at least 12 full-load hours and more economically achievable."
Sponsors and project participants
The research project is part of the "High Performance Solar 2" research project (FKZ 0324097) funded by the German Federal Ministry for Economic Affairs and Energy. The project is supported by the project management organisation Jülich PtJ.
In addition to DLR, TSK Flagsol, YARA, Rioglass, Steinmüller Engineering, eltherm and RWE are involved in the project. The University of Évora, as owner of the Évora Molten Salt Platform (EMSP), is supporting the construction and operation of the plant infrastructure with operating personnel and scientific staff.