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Subproject 7

Bubble in the lake storage (BILS)

Novelty of the solution of the subproject 

While very high seasonal storage efficiencies exceeding 90% have been reported for large pit storages, the required space for land is considerable. Given limited availability of land in Swiss urban areas, SP7 pursues the strategy of placing very big storages in lakes. A novelty is the use of a flexible skin instead of a solid structure. The main advantage is the possibility to build up (and remove again) very large storages at low costs and in a short time.

To the best of our knowledge, this approach is currently not pursued in any other laboratory. The general feasibility of BILS (bubble in the lake) is currently investigated by SPF in the context of a SWEET/SOUR project. TRL is between 3 and 4, but as a result of the ongoing project several relevant questions have been identified that need further clarification (e.g. How can a buoyancy be compensated? How and where can such large structures be assembled and be placed in the lake?). It is evident that BILS must be very large and storage sizes of less than several thousand m3 will not be useful or economical.

SP’s 7 aim is to investigate the technical and economic requirements for building BILS in a P&D project. The integration in a district heat network and the mode of operation are ignored here.

Objectives of the subproject 

Some of the relevant technical questions cannot be answered without building a real size storagedevice. The general objective is to provide a solid theoretical basis for the next steps, i.e. theconstruction of a P&D unit with a size of up to 100 m3, a perquisite to consider a market introduction.


The scientific goals of SP7 are to investigate the mechanical properties of a real-size BILS. So far, only basic calculations and experiments have been carried out with small samples. Based on these conceptual experiences, FEM and CFD simulations are required to characterize the dynamic mechanical interaction between the BILS and the surrounding lake with currents and flows and the buoyancy effect of the bubble. These simulations allow the quantification of the mechanical stress on the different parts relevant for the design of the mooring system.


The economic goals of are to quantify the expected costs and to compare with other storage solutions.


The societal goals are mainly linked to investigate the legal framework and boundary conditions of the concept (SP3).


Previous investigations show that thermal effects on the lake Temperature are insignificant. The most important questions relate to the materials used for the shell or shadow of the large structure. Other very relevant issues are related to the biological impact of the lake on the BILS such as Quagga mussel or algae.


Technical challenges are mainly related to the material selection for the shell, which is closely linked to the scientific simulation of the mechanical and thermal behaviour. Suitable geotextiles and linings are available for various similar applications. It remains unclear whether these materials can be used for large scale BILS. However, not only the material selection but also the on-site processing and installation is expected to be a major challenge.

Research partners

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