Silicone oils - a key to success

By Sonia Martínez, Kai Schmitz and Mark Schmitz, June 2021

In 2012 TSK Flagsol, together with its partners Wacker Chemie and DLR, started to study silicone oils for use as a heat transfer fluid (HTF) in solar thermal power plants in order to substitute current hydrocarbons, with their maximum temperature around 395°C. Silicone oils are a mixture of various chains made of silicon, oxygen and hydrogen that are characterised by their chemical stability at high temperatures. These high temperatures lead to a higher efficiency of the steam turbines and a more efficient utilisation of the storage system. The R&D projects associated with this HTF development and their most relevant results were the following:

1. Si-HTF (2012-2015):

A characterisation of a selection of silicone-containing oils was carried out to develop the HTF HELISOL® 5A, which allows application temperatures of at least 425 °C. In addition, the thermal stability and physico-chemical parameters of this new fluid (vapour pressure, viscosity, conductivity, thermal capacity, etc.) were studied by carrying out the following research:

  • Long-term tempering tests in the temperature range of 400 to 465 °C in glass ampoules
  • Quantitative determination of volatile decomposition products, in particular hydrogen and a study of potentially hazardous exhaust gases.
  • Determination of physical properties.

In addition to improved physical properties, HELISOL® 5A has lower health and environmental risks. On this basis, TSK Flagsol has made a first estimate of the electricity generation costs that can be achieved with silicone oils, resulting in an advantage of approximately 4 %.

2. SITEF (SOLAR-ERA.NET project 2016-2017):

This project demonstrated the applicability of the HELISOL® 5A in interaction with the associated parabolic trough components at temperatures up to 425 °C. For this purpose, a commercial-scale parabolic trough loop was upgraded by TSK engineer Sonia Martinez to operate at these conditions. Within the framework of the project, a technology readiness level of TRL=7 was achieved and the following tests were carried out:

  • Demonstration of the HELISOL® 5A functionality at 425 °C for 480 h
  • Simulation of an overheating scenario, at 450 °C for 50 h.

The trials were successful in demonstrating the feasibility of operating a solar field at a temperature of 425°C, with much less hydrogen production (which is very damaging to the receiver tubes) than the regular HTF.

3. SIMON (2017-2020):

This project demonstrated the applicability of an HTF with improved properties, including a treatment/recycling process and conducting the following research:

  • Demonstration of fluid functionality at 425 °C for 480 h
  • Simulation of an overheating scenario at 450 °C for 50 h
  • Realistic release and leakage testing with HELISOL® 5A and risk analysis

The results were used by scientists to predict the behaviour of the HTF for a typical plant lifetime of more than 20 years. The outcome was that no treatment (e.g. in a ullage system) would be necessary. TSK Flagsol has used this project as a vehicle to develop a portfolio of basic engineering documents for a commercial project, thus being prepared for future proposals.


Wacker Chemie is currently adjusting the physical properties for different fields of application. TSK Flagsol is participating in the standardisation process of the solar power plant fluid application. TSK Flagsol has made calculations for a 60 MW CSP plant in Crete with both, the standard hydrocarbon HTF and the silicone oil, with the result that the HELISOL version would generate 3-5% more electricity than the standard HTF, with approximately the same investment. This is equivalent to a cost reduction from 315 million euros to 300 million euros, i.e. a profit margin of 15 million euros. Therefore, the know-how developed within these projects provides TSK with an international leadership position with the inclusion of this technology.

Prometeo demo loop
Prometeo demo loop at the Plataforma Solar de Almería, which was refurbished by TSK engineer Sonia Martinez to withstand the higher temperatures. She has also been the project manager for the R&D projects SITEF and SIMON