World's largest pit thermal energy storage – pre-insulated stainless steel pipe systems for Hyvinkää, Finland

The project is developed by Hyvinkään Lämpövoima Oy, the municipal district heating utility of Hyvinkää, a city of approximately 47,000 inhabitants north of Helsinki. The storage concept uses a large insulated basin filled with clean drinking water as the storage medium. Thermal energy, sourced from local industrial waste heat and renewable-powered electric boilers, is charged into the water during summer months and discharged into the district heating network during winter, when demand is highest. At full operation, the system achieves a single-charge capacity of 18,000 MWh and a maximum discharge and charge power of 60 MW. The total investment amounts to €33 million, with €7.7 million in state support awarded by Finland's Ministry of Economic Affairs and Employment under its clean transition investment programme. Phase 1 of the project is being executed by L&T Teollisuuspalvelut Oy as main contractor.

ISOPLUS Finland produced part of the pre-insulated pipe systems and fittings for the underground pipe systems beneath the heat storage basin, forming the critical buried connection to the district heating network. Given the operating temperature range of 40–90°C and a system design pressure of PN10, all carrier pipes are manufactured in stainless steel to EN 1.4404 specification. 

The casing pipes are dimensioned to the wall thickness standard of PN16 pressure pipes, resulting in casing wall thicknesses of 57.2–81.7 mm – a specification that demanded precise engineering and close coordination with the customer. The product scope included straight pipe sections in DN400 and DN500, standard 90° elbows in both sizes, and large-dimension anchor elbows in DN400 and DN600. 

The anchor elbows in particular required extensive planning and design work before production could begin. Deliveries were tightly coordinated with the customer's strict construction schedule, ensuring timely availability of components for each phase of the build. 

Once fully operational in autumn 2027, the storage facility is expected to reduce CO2 emissions in Hyvinkää's district heating network by approximately 4,400 tonnes per year, making it a landmark reference for climate-neutral heat supply in Northern Europe.

• World's largest pit thermal energy storage (PTES) of its kind – first of its kind in Finland
• Basin volume: 380,000 m³ of clean drinking water
• Cover dimensions: 225 × 165 m, basin depth: 20 m (cover area equivalent to approx. 5 football fields)
• Operating temperature range: 40–90°C
• Single-charge capacity: 18,000 MWh
• Annual storage capacity: approx. 90,000–100,000 MWh (≈ 25% of Hyvinkää's annual district heating demand)
• Maximum discharge/charge power: 60 MW
• CO2 reduction: approx. 4,400 tonnes/year
• ISOPLUS scope: stainless steel pre-insulated pipe systems and fittings, DN400–DN600, system pressure PN10, casing walls dimensioned to PN16 standard
• Total project investment: €33 million (incl. €7.7 million state grant)
• Completion: autumn 2027