MEET THE TEAM: Discover work package 1 with CEA
Discover the people and work done behind POLYPHEM in our blog articles series!
For this first episode, meet Sébastien Chomette from CEA (France) who presents work package 1 dealing with the POLYPHEM new concept and manufacturing process of high temperature solar receiver in Concentrated Solar Power Plant.
Sébastien Chomette is a Material Science Engineer and Project Manager on metal diffusion bonding technology at the CEA-Grenoble. He has an engineer degree and a PhD in material science and worked 10 years on metal microstructure, mechanical properties, and diffusion bonding technology.
State of the art of CSP solar receiver and new hybrid concept
One of the most important equipments of a Concentrated Solar Power plant (CSP) is the solar receiver. This apparatus is the part of the Power plant that enables to transfer concentrated solar flux coming from the mirror field to the other parts of the installation thanks to a heat transfer fluid or particles (Figure 1).
Regardless of the fluid used, the most commonly used concept concerning the solar receiver is the tubular one. It consists in aligning one or more rows of tubes perpendicularly to the flux, which heats the tube surface and then the fluid. The conception could be directly in front of the solar flux using “simple” shapes (Figure 2) or with more complex ones inside cavities, to trap a maximum solar flux (Figure 3). These concepts are quite simple to manufacture and quite cheap, but they are not the most efficient ones.
The second concept of receiver widely used is the volumetric apparatus. It consists in equipment made with an insolated face composed of complex shapes to trap a maximum concentrated solar flux and to transfer it as efficiently as possible to the heat transfer fluid. However, these kinds of concepts are more difficult to manufacture and are more expensive.
Therefore, the challenge in solar receiver technology is now to ally the advantages of the tubular concepts (quite simple design) and the volumetric ones (enhanced performance) to build an advanced hybrid receiver conception.
Concept and research activities on POLYPHEM solar receiver
The POLYPHEM solar receiver is based on a previous modular high temperature concept developed by CNRS-PROMES and CEA using air as heat transfer fluid. This concept consists in nickel (Ni)-based alloy parts for the surface in contact with air at high temperatures (inner tubes, outer surface, and manifolds) and copper alloy between the tubes and outer surface (Figure 5). Twisted tape inserts are put inside the tube to enhance thermal exchange inside the tubes.
Copper plays a role of a heat sink that helps to conduct the heat from the insolated surface of the active part of the receiver to the different rows of tubes. The receiver is assembled by diffusion bonding, a solid phase soldering process at high temperature and pressure without metal fusion giving good thermal and mechanical properties for nickel-based alloy and copper junctions. Due to all the conception, it can then be considered as a tubular and volumetric hybrid concept of the solar receiver.
In the POLYPHEM project, one of the main objectives of the project is to obtain the main compromise between efficiency and cost. Among those objectives, the main task of CEA is to adapt and optimize the initial CNRS/CEA receiver conception to fit with the up-cycle requirements of the power plant, i.e. working conditions of the micro gas turbine (µGT) connected.
In parallel, CEA works on the alloy used for the inner tubes, the outer surface, and the manifolds in contact with high temperature air (up to 1000°C on the insolated surface). A benchmark of potential metallic alloys that could enhance the Ni-based alloy 600, used in the previous design is done using several criteria, such as high temperature environmental resistance, mechanical properties but also availability and cost. Then, diffusion-bonding tests are done using separately the two best candidates to determine the mechanical properties of the junctions. The hybrid concept manufacturing process is then adapted depending on the best alloy and its mechanical properties.
Material selection, characterization, and design optimization
The initial list of candidate metallic alloys contained 2 iron (Fe)-based alloys and 10 Ni-based alloys. At the end of the benchmark, the Ni-based alloy called 230 was the top-ranked one, the material used in aerospace, chemical industry, and energy engineering. However, the alloy 600, used to manufacture the previous receiver with the CNRS/CEA design, was in third place. Consequently, both alloys 230 and 600 were selected for further tests (Figure 6).
To do so, small diffusion bonded mock-ups were manufactured with the assembly process expected for the POLYPHEM receiver to evaluate the mechanical properties of interfaces between two parts of alloy 230 and the same for the other one. The results showed that, for both alloys, the diffusion bonded junctions do not show sufficient mechanical properties at high temperatures. This is because the diffusion bonding process is done under 1050°C (slightly below copper fusion temperature, 1083°C). Conventionally the diffusion bonding process for Ni-based alloys is done above 1150°C to obtain the best properties. Consequently, Ni-based parts in the receiver must be reinforced using conventional welding techniques, the diffusion bonding process goal is only made to join copper parts together and copper parts and Ni-based ones. The final alloy selection led to choose alloy 230 for the outer surface of the receiver (sustaining the highest temperature) and alloy 600 for the inner tubes (cheaper alloy, only for oxidation resistance).
In parallel, thermohydraulic simulations were done to optimize the receiver design according to the µGT specifications. Several configurations were tested modifying the number of tubes per row, the number of rows of tubes, and the copper thickness between rows of tubes to reach the target. All these calculations lead to a final design of the POLYPHEM receiver with an active surface of 1.2m² composed of 4 independent modules connected with simple tubular manifolds (Figure 7). Each module is composed of 7 rows of tubes and each row contains 27 tubes (number of tubes for the receiver = 756).
In conclusion, the main research tasks on the solar receiver in the POLYPHEM project led to a constitutive high temperature alloy selection and an optimized design of the apparatus. The current task is now to use these results to manufacture the scale 1 solar receiver that will be operated at the end of the project.