Geothermal
exchangers , Baldur Gunnarsson of Tæknisetur explains that the “ results for the amorphous coating show great potential for silica scaling removal and by using the nanoporous coating on the working fluid side shows an increase in heat transfer coefficient .” David of GREIN Research expands further : “ The heat exchanger plates are where you need coatings with high thermal stability which our chrome coating can provide ( up to 750 degrees C , which is beyond the requirement in geothermal power plants ). But , we also considered other components of the geothermal system , such as the turbine blades or sensitive parts which might come into contact with more corrosive environments .” The researchers believe that the coatings will be just as useful in these applications as for the heat exchangers . Application of the coatings will require further testing , as this will involve hard to reach areas such as interior surfaces of pipework – physical vapour deposition techniques , for example , are difficult but not impossible to perform . Further analysis is also needed for cost predictions , although the team expects overall cost will be reduced as the coatings will provide superior protection , meaning lower cost materials can be used rather than austenitic stainless steels . As with all research projects , the team did encounter some issues , such as delamination . “ We saw delamination especially because the samples we produced initially for our first screening were mirror-finished ,” David explained . “ So we saw some peeling in certain cases , but that ’ s a problem that can be solved by using a better adhesion layer . In this case , we used a chromium adhesion layer , but there are other elements that can be used , titanium for example . So , if you do run into that problem where you ’ re getting too much tension in the coating , the tension becomes too much for the adhesion and becomes an engineering problem generally which can be solved by using proper adhesion layers beneath the coating . We ’ ve not used fracture mechanics , but we do standard scratch tests and adhesion tests to assess the coatings . We are working to improve on this .” Regardless of how early these test results are , the GeoHex team is confident about the durability of its coatings . “ The testing was only 200 hours for the long term testing , which is not a lot of time . So it would be good to test further for longer periods ,” said Baldur . “ We saw some indication of corrosion in some of the coatings , thermal sprayed aluminium and nickel phosperous , but most did not show any signs of corrosion . So I can ’ t yet commit to an exact number for lifetime assessment . But I assume that at one point for the heat exchanger , you would want to take disassemble the heat exchanger and clean it so we would have to assess the forces you can apply to
GeoHex global consortium
Iceland – ON ; University of Iceland ;
ICETEC ; Grein Research Norway – Flowphys AS UK – TWI Ltd ; Technovative Solutions ; University of Leicester France – CEA ; ENOGIA Italy – Spike Renewables Romania – Polytechnic University of Bucharest
Philippines – Quantum Leap
the coating without damaging it . In general , we believe these coatings will have a long lifetime .”
Current status By way of a conclusion , GeoHex is confident that amorphous metal coatings show a great potential for corrosion protection , but it can be challenging to find alloy compositions that form a stable amorphous structure . Combinatorial magnetron sputtering and X-ray analysis can be used to map out the phase space of TaSiM ( M = Al , Cr , Fe , Ti ) alloys and identify amorphous compositions . In a paper published in the ‘ Journal of Noncrystalline Solids ’, representatives from GREIN Research and the University of Iceland demonstrate how amorphous coatings are obtained at atomic percentages above 10-15 % of each constituent ; TaSiAl coatings are stable when annealed in air up to and including 550 ° C whereas TaSiFe , TaSiCr and TaSiTi remain amorphous up to and including 750 ° C . The research also shows that Ta35Si15Cr50 is almost unchanged at that temperature , and has a stable surface oxide shell less than 20 nm in thickness at 650 ° C . The stability of these materials at high temperatures means that they could be suitable for use as anti-corrosion coatings in high temperature applications , such as in geothermal .
Find out more
You can find out more about the GeoHex project through an interview with project leaders in Heat Exchanger World March 2022 issue : https :// heat-exchanger-world . com / featured-story-the-geohex-projectanticorrosive-coatings-for-geothermal-heatexchangers /
Learn more from the project ’ s website : https :// www . geohexproject . eu /
Recent paper produced by representatives from Grein Research and the University of Iceland , ‘ Structural stability and oxidation resistance of amorphous TaSi-based ternary alloy coatings ’: https :// www . sciencedirect . com / science / article / pii / S2590159123000353 ? via % 3Dihub
18 Heat Exchanger World May 2023 www . heat-exchanger-world . com