CE 2020 | GEOHEX: Advanced Materials and Performance Testing for Next-Generation Geothermal Heat Exchangers
GEOHEX is a European project aimed at revolutionizing heat-exchanger technologies used in geothermal plants by reducing costs and improving efficiency, corrosion resistance, and anti-scaling performance. The goal is to overcome the limitations of special alloys such as stainless steel and titanium—materials that are robust but expensive and often vulnerable to mineral deposition from geothermal brine. The GEOHEX strategy is based on the use of low-cost carbon steels coated with advanced nanocoatings capable of increasing thermal performance and reducing fouling phenomena.
Within the consortium, Spike played a central role in the design and construction of the three test platforms used to measure the actual effectiveness of innovative materials and coatings under real heat-transfer conditions, both in single-phase flow and during condensation and evaporation.
Why New Materials Are Needed for Geothermal Heat Exchangers
Heat exchangers are among the most critical and expensive components in a geothermal plant. High-temperature geofluids cause corrosion and scaling, leading to efficiency losses, higher maintenance costs, and frequent cleaning operations. Current materials with high chemical resistance, however, have low thermal conductivity and extremely high cost. GEOHEX addresses this challenge by developing nanostructured surfaces that increase the active heat-transfer area and control wettability, roughness, and fluid behavior during condensation and boiling.
Spike’s Contribution: Three Test Platforms to Validate Materials and Coatings
Spike designed and built three complete test rigs for single-phase heat transfer (WP2), condensation (WP3), and evaporation (WP4), all essential for characterizing the performance of the nanoporous surfaces developed within the project. These facilities were engineered to reproduce laboratory conditions equivalent to the operating environment of a geothermal plant, with accurate control of temperature, pressure, flow rate, and fluid quality.
The design involved developing customized heat exchangers, independent multiple fluid circuits (geothermal brine, chilled water, and organic fluids such as R134a), high-frequency data-acquisition systems, and optical components for the direct observation of condensation and bubble-nucleation phenomena.
Single-Phase Test Rig: Evaluating Thermal Efficiency with Real Geothermal Brine
The WP2 test bench recreates the operating conditions of an ORC preheater, handling brine up to 200 °C and organic fluids in counterflow. Spike developed a compact heat exchanger in 316 stainless steel with interchangeable 100 × 100 mm plates on which GEOHEX coatings are applied. The system enables the measurement of very small performance variations thanks to high-precision sensors and strong thermal stability; initial tests show consistent and repeatable heat-transfer coefficients.
Condensation Test Rig: Observing the Transition from Dropwise to Filmwise
The WP3 bench was designed to study condensation behavior on treated surfaces. Its cylindrical chamber with double observation windows allows for direct visualization of the process through a high-speed camera. The goal is to verify whether the nanocoatings promote dropwise condensation, which is significantly more efficient than filmwise. The geometry allows for both vertical and horizontal plate configurations, enabling researchers to correlate condensate dynamics with coating properties.
Boiling Test Rig: Studying Bubble Dynamics
In the WP4 bench, Spike developed a similar system to observe nucleate-boiling phenomena. R134a evaporates directly on the coated plate while the internal camera records bubble formation, detachment, and growth. These data make it possible to evaluate how coating microstructure influences nucleation mechanisms and the transition to film boiling—essential information for designing higher-performance materials for ORC evaporators.
A Complete Platform for Scientific and Industrial Testing
The test facilities developed by Spike form the experimental core of GEOHEX: they will allow partners to compare up to 70 different surfaces, evaluate stability, resistance to corrosion and scaling, and quantify the improvement in heat-transfer coefficients achieved through nanocoatings. Initial commissioning tests show excellent parameter stability and the ability of the platforms to generate reliable data to support further project developments.
GEOHEX and Spike: Advanced Materials for More Efficient and Cost-Effective Heat Exchangers
Spike’s contribution to the GEOHEX project demonstrates the importance of an integrated approach combining system engineering, experimental design, and surface technologies. The platforms developed enable the transition from materials research to measurable, repeatable results, accelerating industrial validation of new coatings. GEOHEX ultimately aims to make geothermal plants more competitive by reducing costs, improving ORC efficiency, and expanding the exploitation potential of low- and medium-enthalpy geothermal resources.
Design and development of three test facilities to evaluate heat transfer performances of advanced and low cost materials and coatings for geothermal application
76th Conference of the Italian Thermal Machines Engineering Association [ATI 2021], 15-17 September 2021, Rome, Italy