WINDOW focuses on the subsurface storage of heat

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.

In WINDOW phase 1 (2019/2020), explorative studies were conducted for seven promising sites in collaboration with all the participating stakeholders (‘learning by doing’).


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 permits the optimal exploitation of renewable heat sources like geothermal energy 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.

Phased approach, ‘learning by doing’ with supporting research

The project consists of four work packages:

  1. ‘Learning by doing’ from exploration to demonstration project
    B. Generic research: Potential & Integration
    C. Generic research: Technology & Effects
    D. 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’). The exploration sites are spread throughout the Netherlands, and vary in project size, temperature level and geohydrological situation. A memorandum (in Dutch) was produced on the comparison and selection of the sites.

The exploratory investigations were carried out for the following sites:

Work packages B, C and D contribute to the exploration in work package A, while also developing generic insights. This generic research includes the following elements:

WINDOW phase 1 was successfully completed at the end of 2020. Sixteen reports were produced, and are available on the WarmingUP website under the link ‘WINDOW fase 1 rapporten’.

The WINDOW programme built on the results of 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 made optimal use of knowledge and experience from existing heat storage systems and of ongoing research, and no subjects were doubly researched.

The WINDOW research programme is being continued: WINDOW phase 2 (2020/2022) is being carried out 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.

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