WINDOW: More Sustainable Heat Provision in the Netherlands Through Subsurface Heat Storage

The WINDOW project focuses on the subsurface storage of heat. The implementation of subsurface heat storage at temperatures above 40°C is however still very limited in the Netherlands. In the WINDOW programme we aim to remove technical, legal and commercial obstacles, so that subsurface heat storage can be implemented as a proven technology in 2025. Heat storage contributes to reducing costs in the heat transition and ensures an optimal exploitation of sustainable heat sources.

Simplified operation of Subsurface Heat Storage: during the provision of heat, groundwater is extracted from the hot well; this heat is delivered to the heating network and the cooled water is then stored in the warm well. The opposite occurs during heat storage: groundwater is extracted from the warm well, heated up, and stored in the hot well. Source: Martin Bloemendal, 2018. PhD thesis, ‘The hidden side of cities’,

Subsurface heat storage: opportunity and challenge in the heat transition

The storage of heat in the subsurface is an important component of the heat transition and constitutes one of the cheapest forms of energy storage. It is important that the summer heat surpluses (from geothermal energy, industrial residual heat, solar or wind energy) can be stored for use in the winter.

Subsurface heat storage at temperatures above 40°C is however still only implemented to a very limited degree in the Netherlands. There remain important challenges, with regard to the technology, the effects, the robustness of the business case, and the relevant legal framework. Knowledge development, knowledge sharing and experience are required for a faster and responsible implementation of subsurface heat storage in the Netherlands.

Subsurface heat storage enables the optimal use of sustainable heat sources, such as geothermal and residual heat.

Further developing subsurface heat storage into proven technology

The objective of the WINDOW programme is to remove technical, legal and commercial obstacles to subsurface heat storage, and to gain a better understanding of its effects, with a view to the responsible implementation of the technology to promote the sustainability of the heat provision in the Netherlands. We are doing this through exploratory investigations and by supporting (the preparation of) demonstration projects in the country, and sharing and applying the resulting knowledge and experience in the joint development of generic tools, in close collaboration with stakeholders and policy-makers. The ultimate objective is to have subsurface heat storage implemented as a proven technology in 2025. The results will contribute to reducing the costs of collective systems at the system level, and to an optimal exploitation of the renewable heat sources.


Model simulation of a Subsurface Heat Storage system, with 1 hot and 2 warm wells after 10 years. The temperature contours show the dispersion of heat in the subsurface. Source: WINDOW fase 1 rapport C2: prestaties en thermische effecten.

Model simulation of a hot well, cross-section of the subsurface.

Phased approach, ‘learning by doing’ with supporting research

The project consists of four work packages:

  1. ‘Learning by doing’ from exploration to demonstration project
  2. Generic research: Potential & Integration
  3. Generic research: Technology & Effects
  4. Authorisations/legal assessment framework

The WINDOW project is divided into two phases; all of the work packages continue through the two phases.

WINDOW phase 1 (2019/2020) is a TKI project (TKI Urban Energy, TKI New Gas and TKI Water Technology). In phase 1, the first seven promising sites were selected on the basis of 21 quick scans. Exploratory investigations were then carried out on these promising sites, in collaboration with all the participating stakeholders (work package A: ‘learning by doing’). WINDOW phase 1 was successfully completed at the end of 2020. Sixteen reports were produced, and are available on Projectpagina WINDOW fase 1 or the WarmingUP website under the link ‘WINDOW fase 1 rapporten’.

The WINDOW programme draws on the previous studies in the field of subsurface heat storage, including for instance the development of an HTO system at Koppert Cress, the European HEATSTORE project and the Power-to-X (SPX) project. The WINDOW research makes optimal use of knowledge and experience from existing heat storage systems and of ongoing research, and no subjects are doubly researched.

At the moment, the WINDOW phase 2 (2020/2022) research programme is underway as theme 5 within the innovative sustainable heating collective ‘WarmingUP’. In phase 2, more detailed research is being carried out in work package A (‘learning by doing’) at the Rotterdam, Leeuwarden and Delft sites. Following a go/no go moment, the design for demonstration projects is being elaborated, including finalised decision-shaping for investors and for the competent authority. The generic research of work packages B, C and D is continuing in phase 2.