(259g) Fabrication of a Novel Carbon/Carbon Composite with Micro-Channels for Concentrated Solar Power Gas Receivers

Concentrated solar power (CSP) systems focus sunlight into a single receiver to transfer energy to a heat transfer fluid, which is later used to convert thermal energy into electricity. A recent U.S. Department of Energy (DoE) roadmap [2] has identified the use of super-critical carbon dioxide (sCO₂) Brayton-cycles as an essential adoption to further decrease generation costs of CSP systems. Considering that, recent developments aim to use sCO₂ directly as the heat transfer fluid in modular micro-channel gas receivers, showing the potential to achieve increased global efficiencies (up to 90% in simulated solar testing) [1]. These receivers are constructed from nickel-based alloys and are particularly susceptible to thermal stresses due to daily startup operations, which currently limits its operating temperatures to between 530 °C and 750 °C. In this DoE-funded work, the development of a novel carbon/carbon (C/C) composite with an embedded micro-channel network is investigated as a modular gas receiver for CSP systems. When compared to the traditional metal receivers, these composites are lightweight with comparable heat transfer coefficients while having much lower coefficients of thermal expansion, mitigating thermal fatigue problems and allowing for higher temperatures to be used (> 800 °C). Thus, this new material developed in this work has the potential of increasing efficiencies through the use of higher operating temperatures, contributing to the decrease of electricity generation costs.

The production of C/C composite modules with micro-channels starts with the fabrication of a prepreg, using PAN-based (polyacrylonitrile based) carbon fibers and a resorcinol-added phenolic resin. Curing is done in steps up to 150 °C and 160 psi using a combination of vacuum bag and an in-house designed autoclave. Carbonization of the prepreg is performed at 1000 °C under nitrogen atmosphere, which converts the cured phenolic resin matrix into a continuous carbonaceous matrix. During this heat treatment, mass loss from the resin decreases density and increases porosity. Thus, a new proposed way of performing CVI (chemical vapor infiltration) as the densification step is done with a mixture of methane and nitrogen at 1000 °C. After the desired density is achieved, graphitization is promoted by heat treating the specimen to 2200 °C under argon atmosphere. It has been shown that an increase in graphitization degree improves heat transfer compatibilities [3]. Lastly, a silicon carbide (SiC) coating using a commercially available precursor (SMP-10 Starfire Systems) is applied to prevent oxidation of the C/C composite.

Characterization of the fabricated composite is done by measurements of density at each fabrication stage. Scanning electron microscopy (SEM) was done to evaluate the morphology of the composite, where the presence of deposited carbon from the CVI method was identified. Energy dispersive spectroscopy (EDS) was performed to look into the SiC coating coverage. After graphitization, X-ray diffraction (XRD) analysis revealed the presence of graphite. The (002) peak characteristic for graphite became sharp and more symmetric after graphitization. These peaks were used to calculate the crystallite size, which was found to be 1.82 nm before and 4.55 nm after the heat treatment at 2200 °C, showing that the graphitization degree was greatly increased. The presence of SiC coating at the surface of the composite was also confirmed with XRD analysis. On-going work involves the continuation of the above-mentioned characterization techniques, as well as tensile stress/strain and pressure burst tests.

References

[1] Zada, K. R., Hyder, M. B., Drost, M. K. & Fronk, B. M. J. Sol. Energy Eng. 138, 61007 (2016).

[2] Mehos, M., Turchi, C., Vidal, J., Wagner, M., Ma, Z., Ho, C., Kolb, W., Andraka, C., & Kruizenga, A. (2017). Concentrating Solar Power Gen3 Demonstration Roadmap.

[3] Sedghi, A., and F. Golestani Fard. “The Effect of Graphitization on the Mechanical Properties of Twodimensional Carbon - Carbon Composites.” Materialwissenschaft Und Werkstofftechnik, vol. 28, no. 5, May 1997, pp. 236–40.