a b s t r a c tThe effect of initial microstructure on alloy 617 creep behaviour has been investigated at 850°C and 950°C. The solution treated material shows non-classical creep behaviour at both temperatures with a strain rate drop at the beginning of the tests followed by a creep rate increase to a plateau before the onset of the tertiary creep. The intragranular secondary carbides which precipitate early at test temperature are responsible of the strong initial hardening effect by pinning the dislocations. This effect is overpassed during the thermo mechanical ageing of the alloy which induces growth of these carbides. Prior 1000 h thermal ageing at the temperature test totally removes the strain rate drop and reduces the lifetime. The intragranular microstructure has evolved thanks to the prior thermal ageing before the creep tests. Microstructural examinations also show the presence of grain boundary migration and recrystallization in the material during creep tests of the as received and aged materials. Preliminary cold work treatment highly reduces the strain rate of Inconel 617 and enhances the lifetime at 850°C while the opposite is observed at 950°C.

The POLYPHEM project is a research and innovation action funded by the European Union's H2020 program. The project started in April 2018 and will end in March 2022. It is implemented by a European consortium of 4 research centers and 5 industrial partners. The aim of this project is both to increase the flexibility and improve the performance of small solar tower power plants. The POLYPHEM concept consists in implementing a combined cycle formed by a solarized micro gas-turbine and a Rankine organic cycle machine, with an integrated thermal storage device between the two cycles. The need for cooling is minimal. Developed from a technology already patented by CNRS and CEA, the pressurized air solar receiver is integrated in the micro-turbine cycle. The thermal efficiency targeted for the receiver is 80% with a cost of 400 €/kW. The innovative thermal storage uses thermal oil and a single thermocline tank with a technical concrete filler material. The main expected impact of this project is to enhance the competitiveness of low-carbon energy production systems through the technology developed. The expected progress is a better fitting of electricity generation to variable local needs and an overall conversion efficiency of solar energy into electricity of 18% for an investment cost of less than 5 €/W with low environmental impact. By 2030, the cost of electricity production targeted by the POLYPHEM technology is 165 €/MWh for an annual direct normal irradiation of 2600 kWh/m 2 /year (North Africa and Middle East) and 209 €/MWh under 2050 kWh/m 2 /year (Southern Europe). In addition to decentralized power generation, other applications are considered for the deployment of this technology used in poly-generation: industrial heat production, solar heating and cooling, desalination of seawater or brackish water. A prototype plant of 60 kWel with a thermal storage of 1300 kWh is designed, built and installed on the site of the experimental solar tower of Themis in Targasonne (France). The objective of the project is to validate the technical choices under test conditions representative of actual operating conditions. THE CONCEPT OF POLYPHEMAs described in Fig.1, the overall concept underpinning the POLYPHEM project is to bridge together two thermodynamic cycles with an intermediate storage system to make a combined cycle with unique flexibility. In this way, three innovative ideas bear the POLYPHEM concept: Coupling a small-scale organic Rankine cycle (ORC) with an open Brayton cycle (µGT) to make a novel kind of combined cycle.  Integrating a low-cost thermal energy storage (TES) between both cycles.SolarPACES 2018 AIP Conf. Proc. 2126, 030022-1-030022-7; https://doi.

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