Europe’s revised Energy Performance of Buildings Directive (EPBD) places renewed emphasis on reliable data, system transparency, and integrated planning to decarbonise the built environment. Yet while much of the discussion centres on individual buildings, full implementation depends just as much on the performance of the energy systems supplying them. In countries where district heating plays a major role, this systemic dimension becomes decisive.

Large-scale thermal energy storage — particularly Pit Thermal Energy Storage (PTES) — has emerged as a key enabler of renewable district heating. However, its deployment remains constrained not by technology but by fragmented data, inconsistent evaluation methods, and limited comparability across projects. The EU-funded project, TREASURE, addresses this gap by combining real-life demonstration with structured data generation, financial benchmarking, and coordinated knowledge exchange.

District heating networks are complex socio-technical systems linking buildings, production plants, storage facilities, markets, and regulatory frameworks. To meet EPBD requirements — including renewable integration, improved efficiency, and transparency — decision-makers need robust, system-level data. Yet large thermal storage is often assessed through narrow feasibility assumptions that overlook its full network value.

TREASURE responds by demonstrating PTES at scale while systematically collecting operational, economic, and organisational data from projects across Europe. Its aim is not only to validate technology, but to build a shared evidence base capable of informing planning and investment decisions aligned with EPBD objectives.

The Høje Taastrup PTES in Denmark illustrates the systemic role of storage. With a volume of 70,000 m³ and an energy capacity of approximately 3,300 MWh, the installation functions as a weekly buffer within the Greater Copenhagen district heating system. It operates at 30 MW charging and discharging capacity and can be fully filled or emptied in about 110 hours.

The total investment reached €12.04 million. Roughly 17% was covered by a national innovation subsidy, while the remaining €10.7 million was financed through a long-term municipal-guaranteed loan. Annual revenues are estimated at €1.05 million, with operating costs of around €0.16 million per year. The result is a simple payback of 12 years and an internal rate of return of 7.5% over 20 years, with a lifetime exceeding two decades.

Crucially, modelling before commissioning quantified where the value is created: 56% stems from avoided peak-load production, 28% from improved CHP optimisation responding to electricity prices, and 16% from better utilisation of waste-to-energy plants. These are measurable system effects that directly influence the carbon intensity and cost of heat delivered to buildings. Without such quantified modelling, these benefits would remain invisible in EPBD-related performance assessments.

A similar approach can be observed in Hechingen, Germany, where an 18,000 m³ PTES is integrated into a renewable heating system combining 7,600 m² of solar thermal collectors, multiple heat pumps, geothermal boreholes reaching 172 metres, and buffer storage. The overall system investment is estimated at €17.5 million.

Under Germany’s BEW programme for efficient heating networks, around 40% of eligible capital expenditure can be subsidised. Operational support is also provided for renewable heat generation, including 1 cent per kilowatt-hour for solar thermal heat and up to 6.4 cents per kilowatt-hour for geothermal heat over ten years. These quantified support levels are critical for ensuring that renewable district heating systems — and the buildings connected to them — remain cost-effective while meeting decarbonisation targets.

Beyond technical performance, TREASURE highlights how financing structures shape scalability. Public district heating companies typically access long-term debt at interest rates between 2% and 4%, with maturities exceeding 30 years and leverage ratios reaching up to 100% of capital expenditure after subsidies. Private developers often face higher rates of 4.5–6%, shorter tenors of 10–20 years, and lower leverage of 50–80%, requiring greater equity contributions.

These structural differences significantly influence the levelised cost of heat. Without transparent benchmarking of financing conditions, comparing decarbonisation pathways — and their implications for building energy costs — becomes difficult.

European funding instruments illustrate both opportunity and complexity. The Innovation Fund, with a €4 billion allocation in its upcoming call, can cover up to 60% of eligible project costs, though success rates remain around 25%. The Modernisation Fund, with €17.5 billion allocated for 2021–2030, can cover up to 100% of eligible costs for priority projects that align with national climate plans. The European Regional Development Fund allocates approximately €200 billion across cohesion priorities, with co-financing rates reaching up to 85% in less developed regions. Access to these instruments depends heavily on robust technical and financial evidence — precisely the type of data TREASURE consolidates.

The Polish demonstrator in Lidzbark Warmiński further illustrates both potential and constraints. A fully integrated renewable heating installation combining heat pumps, photovoltaic generation, and borehole storage was financed through a pre-commercial procurement scheme with 100% public funding, amounting to approximately 39.5 million PLN (about €9.37 million). The project required a minimum renewable share of 80% and underwent rigorous quantitative evaluation. However, national rules limit standalone financing for large thermal storage and cap public aid intensity at 45% for large enterprises, demonstrating that regulatory frameworks must evolve alongside technological progress.

The EPBD increasingly promotes zero-emission buildings, renewable integration, and district-level strategies. Yet building performance cannot be assessed in isolation from the systems supplying heat. Large thermal energy storage measurably reduces peak fossil generation, enhances renewable utilisation, and increases flexibility. When quantified in megawatt-hours shifted, megawatts of avoided peak capacity, and euros of system optimisation, storage becomes a tangible contributor to building decarbonisation pathways.

Through collaboration with IEA Energy Storage Task 45, joint expert webinars, structured regulatory exchanges, and cooperation with 17 satellite initiatives across Europe, TREASURE builds a harmonised evidence ecosystem. By documenting investment volumes, subsidy intensities, operational revenues, and risk allocation models, it strengthens comparability and reduces uncertainty.

As Europe moves from legislative ambition to practical EPBD implementation, system-level data becomes indispensable. Large-scale thermal energy storage has proven its technical feasibility and economic viability. The challenge now is to ensure that this growing body of evidence is systematically embedded into planning, regulation, and investment decisions – turning data into action for Europe’s buildings and cities.