SUNFLOWER

Project objectives

Concentrated Solar Power (CSP) can be a relevant renewable energy sector that can provide cheap energy storage in order to enable higher shares of non-dispatchable renewables, by stabilizing the electricity grid. CSP plants using air as heat transfer fluid (HTF) can have high conversion efficiency, low cost of thermal energy storage as well as the potential of efficient electricity storage integration when combing a CSP power plant with compressed air energy storage (CAES).
The strategic development line of this project is therefore the analysis and optimization of novel air-based hybrid CSP-CAES plants, focusing on the open volumetric receiver (OVAR). The interaction of the material properties, the structure itself and the requirements of the preparation technique determine the performance of absorbers for OVAR. Therefore, the SUNFLOWER project targets the following interconnected specific objectives:

Optimization of raw material processing

Due to the well-known outstanding high-temperature properties of Iron-Chromium-Aluminium-alloys (FeCrAl) and Silicon Carbide (SiC), those two materials will be in focus for the preparation of absorber structures. However, the processing of the raw materials needs to be improved with respect to the environmental impact.

Optimization of absorber structure design

General objective will be the defining of the system-integration layout and design as well as the supporting of the pre-engineering work. The first innovative approach is to develop tailored static volumetric absorber structures for the novel power cycle architecture combining CSP technology with CAES. A second innovative approach is the evaluation of a novel active high-flux OVAR concept, introducing an active rotation of the absorber, which allows up to 1000 suns mean concentration ratio (≈1 MW/m² mean flux), pushing the peak flux up to about 4.5 MW/m². The present project will push this novel rotating absorber technology a step further (at TRL 4).

Absorber material improvement

The chemical composition of the materials must be optimized to increase the durability under OVAR application conditions. The FeCrAl alloy must be improved regarding the performance during the later operation: surface passivation by an Alumina-layer realized by the addition of Aluminium, enabling rapid temperature changes by the addition of Yttrium and enhanced temperature stability due to the addition of Nickel.

For the SiC the densification of the microstructure during the sintering must be improved. This will be done by an adjusted particle size distributions and the optimized combination of process and sintering aids.

Absorber fabrication improvement

Open cellular structures with high surface area (high functional porosity) and low strut thickness, as required for absorber structures, are still at a low technology readiness level (TRL3). Aim is the improvement of the preparation technology – Powder Bed Fusion – Electron Beam Melting for FeCrAl and replica technique for SiC – to reach a high densification and defect-free structures with high mechanical strength and oxidation resistance.

Testing and overall assessment

To validate the developed absorber design and the improved material properties, intensive tests under application-related conditions are needed. Lab-scale tests in the solar simulator with 1-3 kW thermal at CIEMAT and up to design operating temperatures of 900 °C will complement the numerical evaluation. The objective is to evaluate the performance of the recent development experimentally and use the generated knowledge for a life-cycle- and performance assessment.