25. October 2021
Research for more economical solar power

DLR and the Uni­ver­si­ty of Évo­ra are test­ing the use of molten salt in a so­lar pow­er plant

Project engineers check the surface temperatures of the pipelines
Project en­gi­neers check the sur­face tem­per­a­tures of the pipelines
Image 1/4, Credit: © DLR. All rights reserved

Project engineers check the surface temperatures of the pipelines

Project en­gi­neers check the sur­face tem­per­a­tures of the pipelines be­fore pump­ing the molten salt in­to the so­lar field.
Four HelioTrough® collector modules with a total length of 684 metres
Four He­lioTrough® col­lec­tor mod­ules with a to­tal length of 684 me­tres
Image 2/4, Credit: DLR (CC BY-NC-ND 3.0)

Four HelioTrough® collector modules with a total length of 684 metres

Four He­lioTrough® col­lec­tor mod­ules with a to­tal length of 684 me­tres pro­vide a to­tal ther­mal out­put of up to 3.5 megawatts. Re­ceiv­er tubes from project part­ner Ri­o­glass are used in the col­lec­tors, which can be op­er­at­ed with molten salt at up to 565 de­grees Cel­sius.
The hot molten salt flows from the collector field into a storage tank
The hot molten salt flows from the col­lec­tor field in­to a stor­age tank
Image 3/4, Credit: DLR (CC BY-NC-ND 3.0)

The hot molten salt flows from the collector field into a storage tank

The hot molten salt flows from the col­lec­tor field in­to a stor­age tank. With a tem­per­a­ture dif­fer­ence of 275 de­grees Cel­sius be­tween the hot and cold tanks, this demon­stra­tion plant has a stor­age ca­pac­i­ty of ap­prox­i­mate­ly six megawatt hours, which can be used for steam gen­er­a­tion at full pow­er for about three hours, even af­ter sun­set. Com­mer­cial so­lar pow­er plants use large salt stor­age tanks that en­able elec­tric­i­ty gen­er­a­tion 24 hours a day.
The steam generator provides steam at a pressure of up to 140 bar and delivers a thermal output of 1.6 megawatts
The steam gen­er­a­tor pro­vides steam at a pres­sure of up to 140 bar and de­liv­ers a ther­mal out­put of 1.6 megawatts
Image 4/4, Credit: DLR (CC BY-NC-ND 3.0)

The steam generator provides steam at a pressure of up to 140 bar and delivers a thermal output of 1.6 megawatts

The steam gen­er­a­tor pro­vides steam at a pres­sure of up to 140 bar and de­liv­ers a ther­mal out­put of 1.6 megawatts. The forced cir­cu­la­tion boil­er in­stalled by project part­ner Stein­müller En­gi­neer­ing makes it pos­si­ble to run very flex­i­ble load pro­files.
  • At the Évora Molten Salt Platform (EMSP), operation of a parabolic trough field using molten salt as the heat transfer medium has begun.
  • Solar power plants with molten salt can reach particularly high temperatures and thus achieve higher efficiencies.
  • Increasing the salt temperature increases the yield of electricity and thus lowers generation costs.
  • Focus: Energy, solar power plants, energy storage

Engineers from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have taken an important step towards using molten salt as a heat transfer medium in parabolic trough solar power plants. Together with the University of Évora and industrial partners, a team from the DLR Institute of Solar Research has for the first time begun operating the solar field of the Évora parabolic trough test plant in Portugal with molten salt. This innovative technology is helping to further reduce the costs of operating solar thermal power plants. With their integrated storage systems, solar thermal power plants are the only technology able to generate large amounts of power from solar energy around the clock.

Salt offers higher efficiency as a heat transfer medium than oil

Current state-of-the-art commercial parabolic trough power plants use a special thermal oil as the heat transfer medium. The oil absorbs concentrated solar radiation collected using mirrors, converts it into heat and transfers it via pipelines to a heat storage unit or a steam turbine to generate electricity. The heat storage tank, filled with molten salt, can hold the thermal energy at temperatures of up to 560 degrees Celsius for a period of 12 hours and release it again when the demand for electricity increases. The power plant needs heat exchangers to transfer the heat from the oil to the salt in the storage tank, but some energy is always lost during this transfer before it can later be converted into electricity. The maximum possible operating temperature of the oil used is approximately 400 degrees Celsius, which limits the efficiency of the energy conversion. Researchers and industry are therefore looking for ways to further increase the temperatures in solar power plants in order to lower the costs of electricity generation.

One promising way to raise temperatures in parabolic trough power plants is to use molten salt not only as a heat storage medium, but also as the heat transfer medium in the collector field. Depending on the composition of the molten salt, it can withstand significantly higher temperatures than thermal oil – up to 565 degrees Celsius. Another advantage is that the storage tanks can be filled directly with molten salt from the solar field – eliminating the need for a heat exchanger.

In order 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 parabolic trough test facility using molten salt as its heat transfer medium. The work started in 2016 and has taken place as part of the High Performance Solar 2 (HPS2) research project, which is funded by the German Federal Ministry for Economic Affairs and Energy (BMWi). The aim of the project is to demonstrate that parabolic trough power plants can be operated safely and economically with molten.

A technical challenge when using molten salt as a heat transfer fluid is that heating of all the pipelines is necessary. To prevent the molten salt from solidifying as the plant is filled, electrical trace heating must be used to preheat all salt-carrying components.

Successful initial filling and test operation of the system at 300 degrees Celsius

The collector modules of the HelioTrough® 2.0 generator from project partner TSK Flagsol, which are now filled with molten salt and connected to each other, provide a total thermal output of up to 3.5 megawatts across a total length of 684 metres. Currently, the plant operates with a ternary salt mixture from the project partner Yara, which has the advantage of a lower melting temperature compared to a binary salt solar salt mixture and can absorb heat up to a temperature of approximately 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 systems.

Starting from an initial temperature of 300 degrees Celsius, the engineers want to gradually increase the operating temperature up to 500 degrees Celsius. In the coming weeks, the other components of the salt circuit will be brought into operation in Évora. In addition to the two-tank storage system, this includes the steam generator and the measurement equipment.

"We are very satisfied with the way the first filling went. Our next goals are to gain operating experience, fill all further components with molten salt step by step and test regular operations and also critical operating scenarios," says Jana Stengler, head of the Fluid Systems Group at the DLR Institute of Solar Research, on the results of the initial testing.

The HPS2 plant is designed to also be operated with solar salt, a mixture of potassium nitrate and sodium nitrate, to achieve even higher temperatures of up to 565 degrees Celsius. Higher temperatures in the solar field allow for higher efficiencies in the conversion of solar energy into heat and heat into electricity, which lowers the cost of generating electricity.

"Power plants using the technology from HPS2 can be built more easily and operate more efficiently. This reduces electricity production costs by up to 10 percent," says Mark Schmitz from the project partner TSK Flagsol, underlining the importance of the project for future solar thermal power generation. "That is an enormous step for a single technical change. At the same time, it makes longer storage durations of 12 full-load hours and more economically achievable."

Sponsors and project participants

This research project is part of the 'High Performance Solar 2' research project funded by the Federal Ministry for Economic Affairs and Energy (Bundesministerium für Wirtschaft und Energie; BMWi). The project is being monitored by Project Management Jülich (Projektträger Jülich; PtJ).

In addition to DLR, partners TSK Flagsol, Yara, Rioglass, Steinmüller Engineering, eltherm and RWE are involved in the project. The University of Évora, as the owner of the Évora Molten Salt Platform (EMSP), is supporting the construction and operation of the plant infrastructure with operating personnel and research staff.

Contact
  • Michel Winand
    Cor­po­rate Com­mu­ni­ca­tions, Bonn, Cologne, Jülich, Rhein­bach and Sankt Au­gustin
    Ger­man Aerospace Cen­ter (DLR)

    Com­mu­ni­ca­tions and Me­dia Re­la­tions
    Telephone: +49 2203 601-2144
    Linder Höhe
    51147 Cologne
    Contact
  • Jana Stengler
    Team Lead­er Flu­id Sys­tems
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of So­lar Re­search
    So­lar High Tem­per­a­ture Tech­nolo­gies
    Linder Höhe
    51147 Cologne
    Contact
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