Molten salt as heat transfer fluid in parabolic trough power plants
An international consortium led by the DLR Institute of Solar Research, will build and operate a solar thermal molten salt parabolic trough test facility in Évora (Portugal). In October 2021, the plant could be filled with hot salt for the first time. Companies and research institutions will test liquid salt as a heat transfer medium here under realistic conditions.
Molten salt: lower costs and higher efficiency
The aim of the research project was to examine the efficiency and reliability of parabolic trough power plants with molten salt as heat transfer medium.
To date, commercial parabolic trough power plants use thermal oils for receiving and transferring the high-temperature heat generated from solar radiation. Compared to thermal oil, the advantage of salt is its lower procurement cost and the higher maximum temperature to which the salt can be heated.
"A key advantage of the molten salt is its good resistance at high temperatures," says Project Manager, Dr. Michael Wittmann from DLR "For the thermal oil a temperature of 400 degrees Celsius is the limit. But the salt withstands temperatures above 500 degrees in continuous use. Depending on the salt mixture upper process temperatures of up to 560 degrees are possible."
In an innovatively designed once-through steam generator, the salt transfers its energy to a connected water-steam cycle. The elevated steam parameters compared to state-of-the-art technology allow higher efficiencies of the power plant unit. In addition, the once-through principle allows supercritical steam parameters for commercial application.
Salt can be used in parabolic trough power plants not only as a heat transfer medium. Commercial solar power plants already use huge tanks with molten salt as thermal heat storage. Such power plants run with a two-circuit system with synthetic oil in the collector field and liquid salt in the storage system. Where salt is used as storage and also as carrier medium, a two-circuit system is no longer necessary. This leads to a reduction in system complexity and investment
Luftaufnahme der Bauarbeiten im August 2018
The project partners optimize the system for operation with salt
The components and the overall system of the new test facility in Évora are specially adapted to the use of molten salt as heat transfer medium.
The biggest challenge of using salt is its high melting respectively solidifying temperature between 120 and 240 degrees Celsius. Solidification in the branched pipes of the solar field leads to stand-still and may cause damages. In order to prevent any solidification, an adequate design, an adapted operating concept and appropriate safety equipment is needed.
DLR participated in the planning and conceptual design of the test facility as well as the qualification of the collector field. DLR researchers accompany the experimental operation also scientifically.
The use of molten salt makes high demands on the design of the collector. TSK Flagsol Engineering adapted the design of its HelioTrough® collector accordingly and will demonstrate its applicability. Due to high concentration factors the collector is especially suited for being operated at high temperatures and hence for the use of molten salt. Eltherm developed the safety relevant solar field impedance heating system and is as well responsible for its construction and proof of operation. Rioglass continues to develop its receivers for the use of liquid salt at high temperatures and demonstrates the stability of the optical quality of the receivers in liquid salt operation. Steinmüller Engineering installed and tests on the new facility its once-through steam generator, heated with molten salt. Yara developed the required salt process technology and demonstrates the suitability of its low-melting, so-called ternary salt. A team of engineers of the South African energy supplier Eskom is responsible for the smooth operation of the plant.
The University of Évora as the owner of the Évora Molten Salt Platform supports the erection and operation of the infrastructure with dedicated operational and scientific staff.
The research project "High Performance Solar 2 (HPS2)" is funded by the Federal Ministry of Economics and accompanied by the Project Management Jülich (PTJ).
DLR Institute for Solar Research
University of Evora