Themis solar tower

General Info
Central receiver (power tower)
Research & Development
Construction start date: 
Start date: 
April, 1983
Companies involved
General comments:

The THEMIS solar tower, built at the end of seventies to test at a 10 MWth scale the production of electricity from concentrated solar energy, is being refurbished and upgraded under PROMES-CNRS direction to study next generation high performance solar tower components and thermodynamic or thermochemical cycles at industrial prototype scale. The Pyrénées Orientales department council, the facility owner, organizes the site as an experimental platform for different solar technology for instance electricity production with PV cells on half of the original heliostats. The PROMES-CNRS operated plant provides industrial grade infrastructure, such as an external elevator to bring up to 25 metric tons loads to the focus. Up-to-date equipment is being installed: protected and secured network, water and air fluids, workshop... A new high performance high precision heliostats tracking system is being qualified to allow receiver temperatures beyond 900°C, still with low power consumption and low maintenance to accurately demonstrate real-world industrial-grade operating cost.

The THEMIS solar tower offers two experimental areas:

  • For the PEGASE project, the original large scale area is being prepared:
    • Power up to 4,6 MWth using all 107 PROMES-CNRS operated heliostats
    • Up to 3600 kW/m2 peak flux
    • 6,5m x 5m aperture
  • A new experimental area, "Mini Pegase" is being prepared for components qualification such as air solar receivers or other experiments:
    • Power up to 400 kWth with selected heliostats
    • Up to 600 kW/m2 peak flux
    • Hot pressurized air @ 8 bar, 1 kg/sec, 300 °C

Different instrumentation systems are available on site:

  • Solar flux qualification system with a 7 x 7 m target moveable near experimental areas
  • Weather (T°, P, Hr) and Solar radiations (DNI, DHI, GHI) continuous acquisition



The first THEMIS solar plant, was an experimental solar facility which produced power between 1983 and 1986, and then closed in part due to the difficulty of managing the cooling system, and in part due to a lack of political and financial support. Construction started in 1979 at a cost of 300 million French francs (about 45 million euros), and was operated and managed by Électricité de France (EDF).

The plant went into hibernation for more than twenty years, and turned into a scientific facility of the CERN, and the Commissariat à l'énergie atomique focusing on astrophysics, with an open air Cherenkov Telescope, measuring gamma rays hitting the atmosphere (see IACT).

In 2004, a rehabilitation program was devised by the General Council of the Pyrénées-Orientales, to produce power, and create a research and development centre on solar energy with the Centre national de la recherche scientifique (CNRS), and Tecsol, a local engineering office.

THEMIS still has its 201 sun-following devices called heliostats, most of them still equipped with mirrors (53.70 m² each) covering an area of 11,800 m² in total, able to send the solar energy towards the hot spot at the top of the 104 m central tower, where the boiler was originally placed (which is now part of the exposition outside the Odeillo solar furnace). The heliostats are, however, not operational.

The three rows of heliostats furthest away from the tower are having their mirrors replaced by photovoltaic cells. Of these cells, some will follow the sun while others will not, so as to measure the difference in efficiency.

The rehabilitation project will repair half of the heliostats in order to develop a power of 1 MW, thanks to a gas turbine installed at the top of the tower, and will replace the other half of the mirrors with solar cells.

It had a power output of 2 MW in 1983. It was based on an array of 201 mirrors which heated a boiler (a cavity lined with coolant tubes) at the top of a 100 m tower where the coolant (molten salts) carried the thermal energy to a vapour generator, itself powering an electric turbine. The molten salts were potassium nitrate (53%), sodium nitrite (40%) and sodium nitrate (7%). The coolant entry temperature was 250 °C and the exit temperature 450 °C. The vapour produced in the generator was at a pressure of 50 bar and a temperature of 430 °C.

Documents and links:
Themis solar tower Themis solar tower