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Advanced material for cost-efficient and enhanced heat exchange performance for geothermal application

Periodic Reporting for period 2 - GeoHex (Advanced material for cost-efficient and enhanced heat exchange performance for geothermal application)

Période du rapport: 2021-05-01 au 2023-10-31

Heat exchangers (HXs) are one of the most critical components of geothermal power plants, particularly for organic Rankine cycle (ORC) based power plants, where the capital cost of heat exchangers accounts for a large proportion of capital costs. For example, HX capital costs can account for approximately 86% of plant costs when air cooled condensers are used.

ORC HXs, including superheaters, preheaters and evaporators have heat exchanger surfaces in contact with the geothermal brine and this can result in scaling and corrosion of the heat exchanger materials. The extent of scaling and corrosion will be different in different components, or areas of components, based on the local temperature and chemical composition of the geothermal fluid. Corrosion resistant alloys (CRAs) are often used in geothermal HXs to prevent corrosion, however these materials are expensive and have low thermal conductivity meaning that the size of HXs have to be increased. Corrosion resistant alloys are also prone to scaling, which can degrade performance over the lifetime of the HXs.

GeoHex focuses on improvements to the antiscaling and anticorrosion properties of heat exchanger materials, which will lead to smaller, more efficient and less costly HX systems. The project will also utilise carbon steel substrates, as one of the base materials for the HXs, which are lower in cost relative to CRAs, whilst also having higher thermal conductivities, thereby potentially improving the overall heat transfer performance of HXs. GeoHex will significantly reduce the cost of ORC power plant whilst lowering the environmental impact. The technology concept can be exploited to build cost efficient HXs for other industries, including solar thermal energy, heat pumps, absorption chillers and geothermal energy-based district heating and cooling systems. GeoHex enabled ORC plant, heat pumps and absorption chillers can also be used for waste heat recovery applications. Hence, GeoHex will significantly contribute to enhance energy security, decarbonise the economy and establish EU leadership on renewables.

The objectives of GeoHex will be to modify the surface of carbon steel and stainless steel substrates in order to enhance heat transfer, anti-scaling and anti-corrosion performance. Through modifying surfaces, on the working fluid side of the HX, by controlling the surface chemistry and nano-structure, GeoHex will significantly improve heat transfer performance of single phase and phase change heat transfer processes.

The overall objectives of the project are:
o Develop tools to characterise bubble droplet dynamics, using both numerical simulation and the development of an image processing algorithm
o Develop materials for 3 different heat transfer mechanisms used in heat exchangers:
Single phase heat transfer
Condensing surface
Boiling surface
o Develop a sustainability model for GeoHex using parametric lifecycle assessment (LCA) and cost model of the GeoHex materials (to be developed in this project) to identify the environmental and cost performance of the materials
o Develop a knowledge based engineering (KBE) tool combined with a multicriteria Decision-Support System (DSS) incorporating all the models and experimental results from the project.
o Demonstrate the scalability and manufacturability of six prototype GeoHex materials.
The project GeoHex has focused on improvements to the anti-scaling and anti-corrosion properties of heat exchanger materials, which will potentially lead to smaller, more efficient and less costly HX systems. The project has utilised carbon steel substrates, as one of the base materials for the HXs, which are lower in cost relative to corrosion resistant alloys (CRAs), whilst also having higher thermal conductivities, thereby potentially improving the overall heat transfer performance of HXs. GeoHex has also aimed to significantly reduce the cost of ORC power plant whilst lowering the environmental impact. The project methodology included modification of the surface of carbon steel and stainless steel substrates to enhance the heat transfer, anti-scaling and anti-corrosion performance. Through these modifications of surfaces, on the working fluid side of the HX, by controlling the surface chemistry and nano-structure, GeoHex has worked towards improvement of heat transfer performance of single phase and phase change heat transfer processes. To improve the anti-scaling and anti-corrosion properties, the brine side of the HX surface has been coated with Ni-P/Ni-P-PTFE duplex coating by electroless method or amorphous coatings.
GeoHex core technology concepts regarding the development of enhanced heat transfer surface are based on four key pillars:
• Nanoporous coating to increase effective heat transfer surface area,
• Oleophilic and bi-oleophilic surface to enhance boiling heat transfer coefficient (HTC) and critical heat flux (CHF),
• Hierarchical hi-mesh surface to enable sucking flow condensation,
• Super hydrophobicity and oleophobicity to promote dropwise condensation (DwC).
GeoHex has worked towards decarbonisation of EU energy system, bringing environmental, social and economic benefits, ensuring the EU leadership on renewables. The consortium has worked towards:
• Enhancement of heat transfer in HXs;
• Reduction in CAPEX and OPEX of geothermal ORC through development of novel materials;
• Improved environmental performance.
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