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Plasma spray coatings for geothermal applications

Wed, 19 August, 2020

The lifetime and efficiency of geothermal heat exchangers are adversely affected by the presence of geothermal fluids, hot gases, and humidity in varying temperature conditions. Generally, expensive heat exchanger materials are used in these systems due to the corrosion and scaling threat. The GeoHex project aims to develop cost- and energy-efficient heat transfer systems through coating and surface modification solutions. These coatings improve the heat transfer performance along with guarding the material against corrosion and scaling.

Boiling heat transfer is one of the main aspects that influence the heat transfer performance, and can be augmented by different active, passive and compound techniques such as flow disruption, surface roughness, out of plane mixing, fluid additives, channel curvature,  etc. Less complicated and with nominal energy requirements, passive techniques, especially surface modifications, are preferred. Modification of a surface for amplified boiling heat transfer can be achieved by altering its topography and chemistry. To alter the surface properties, coatings with micro- or nano-scale features are extensively favored because of their enhanced surface area, which increases the number of boiling nucleation sites, leading to the generation of a higher number of bubbles. Thermal spray, especially plasma spray, is the commonly used technique to obtain these coatings with desired features, providing the freedom of depositing any material on surfaces such as polymer, glass, metals, and fabrics. Being a time- and cost-efficient technique, plasma spray is already being used commercially. 

In plasma spray, the desired coating material could be in the form of powder, suspension, or solution precursor. Generally, liquid-based feedstocks are chosen over powder-based due to better flowability and interaction with the plasma. Once injected, the feedstock material is melted/semi-melted and accelerated towards the substrate with the help of a plasma source. These particles re-solidify and deposit on the substrate in the form of splats and a continuous stacking of splats forms the coating. Depending on the different parameters such as solid content in the suspension, particle size, precursor concentration, spray distance, solvent type, injection type (mechanical or atomized) and angle, plasma source (gas, water or combined), substrate characteristics (topography and curvature), etc., the coating properties can be varied. Deposited coatings display different micro- and nano- features such as columns, lamellae, pores, vertical cracks, etc., which make them suitable for different kinds of applications. For boiling heat transfer purposes, these features aid in increasing the heat transfer coefficient and critical heat flux.

Under the GeoHex project, we will perform the suspension and solution precursor-based plasma spray for different coatings such as TiO2, CuO, Fe3O4, and TiO2/Al2O3 composite coatings with varied concentration of Fe doping, which will improve the efficiency and lifetime of geothermal heat exchangers.