Fraunhofer Institute for Systems and Innovation Research ISI
Fraunhofer Institute for Systems and Innovation Research ISI

Project

ReStoreFlex: Linking Renewables, Storage and System Flexibility in District Heating

ReStoreFlex will improve TES deployment in district heating networks by identifying optimal TES solutions for flexible, resilient, multi-source networks combining renewable and excess heat sources, while analysing the system value of TES in different energy scenarios and addressing the policy, regulatory, and market conditions needed for large-scale implementation.

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District heating networks must transform from fossil-fuel dependence to flexible, multi-source systems to achieve climate neutrality and energy security. Recent price volatility and supply disruptions have shown how vulnerable fossil-dependent heat supply is. Thermal energy storage (TES) directly addresses this challenge: by decoupling heat generation from demand, TES allows operators to shift production to periods of low-cost renewable electricity, maintain supply during grid stress, and reduce exposure to volatile fuel prices.

Despite the technical maturity of TES, solid evidence on where, how, and at what scale TES investment delivers the greatest system value is lacking at local, European, and global level. The project ReStoreFlex aims to closes this gap.

 

ReStoreFlex is an IEA DEC Annex XV project. It aims is to identify optimal thermal energy storage (TES) solutions for flexible, resilient, multi-source district heating networks on a local level, across Europe, and beyond.

The project pursues five key objectives:

  • Systematic compilation and validation of technical and economic parameters for established storage technologies (pit, tank, borehole, aquifer storage) as well as emerging solutions (e.g. cavern and two-zone storage), published as an open-access database.
  • Energy system modelling (Enertile) at hourly resolution across all EU Member States, jointly optimizing TES investment with electricity and hydrogen markets up to 2050, quantifying cost reductions, CO₂ savings, and flexibility contributions.
  • Four in-depth case studies of district heating networks (including Markham, Canada, and Qingdao, China) using detailed network optimization (IESopt).
  • Deriving statistically grounded transfer functions so that non-European countries can assess their own TES potential using publicly available data.
  • Policy analysis and decision guidelines covering regulatory frameworks, financial mechanisms, and market conditions for large-scale TES deployment.

Fraunhofer ISI is coordinating the project and works on the energy system modelling and the policy analysis. 

ReStoreFlex will deliver practical outputs for district heating operators, municipalities, policymakers, and researchers: an open-access TES parameter database, European scenario reports, case study results, a TES decision guideline, policy factsheets, and peer-reviewed publications.

Duration

1 Oct 2026 to 30 Sep 2028

Clients

  • IEA DHC

Partners

  • Fraunhofer ISI (coordination)
  • AIT
  • Ener-D