Composite tubes, such as glass fibre-reinforced polymer (GFRP), offer significant benefits in geothermal energy storage due to their corrosion resistance, long lifespan, and ability to be tailored for underground applications

Global composites manufacturer Exel Composites has recently completed a research and development project with geothermal technology specialist QHeat in this area, suppling composite tubes to store excess heat energy at the Lounavoima waste incineration plant in Salo, Finland. The project's simulations suggest the GFRP tubes can enable plant operators to store 14GWh of heat in underground wells two kilometres down in the granite bedrock.

Prior to installing a heat storage system, the Lounavoima plant released its excess heat into the air during the summer. When ambient temperatures fell to -20°C in the Finnish winter, its emissions increased because of the supplementary oil burners needed to supply heat at peak times. The energy saved by improving the plant's storage efficiency equates to the heating requirements of 700 detached houses per year.

"60% of all EU heat production comes from fossil fuels, and the remaining 40% is still mostly combustion, like natural gas or waste incineration," explains Erika Salmenvaara, CEO at QHeat. "In the context that half of global energy use is for heating, we see the environmental gains that electrification of heating can provide."

However, this process isn't perfect, Salmenvaara continues: "Electrification of heating puts significant strain on the world's ageing power transmission infrastructure, but innovative engineering can help there too. Grid-enhancing technologies are advancing rapidly and, besides, heat pumps deliver up to 600% more energy efficiency than using electricity directly."

Exel Composite's R&D department worked closely with QHeat's engineers to tailor the composite tubes to the job's specific environmental and performance requirements. The two teams went through several iterations, optimising for sizing, connection methods, and thermal insulation versus mechanical strength trade-offs. GFRP is well-suited to the pressure and temperature involved with underground applications. The partnership between the two companies focused heavily on the material's insulating capacity to ensure energy efficiency in heat transfer was as high as possible.

"GFRP offers significantly higher strength than plastics, which alone might provide good insulation but would not be strong enough for this application," adds Tiina Uotila, technical sales manager at Exel Composites. "The combination of corrosion resistance, structural strength and thermal insulation makes GFRP a very suitable choice here. In this case, the material provides both insulation and structural benefits without a trade-off."

"Heat preservation combined with the right mechanical performance was the biggest criterion to satisfy with these pipes," Uotila adds. "However, we collaborated with QHeat to understand how we could make them easier to assemble and install, and to make sure that we have a defined, sustainable end-of-life route."

Rather than going to landfill, the pipes will now be co-processed into cement, where the reinforcement replaces raw materials and matrix replaces fuels.

"I was impressed that Exel Composites' head designer came on site with us in Salo to see the first pipes installed," Salmenvaara says. "I also had the chance to visit Exel's Mäntyharju factory in Finland. Then I understood its expertise, design capabilities, and the quality of its products."

According to Uotila, "This first project is only the beginning. Geothermal energy has the potential to reduce the reliance on burning fossil fuels or biofuels for heating in many countries, offering a strong contribution to net zero goals."