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The future of heat storage is here


The Idea

50% of our energy demand is "heat". Not a big problem in summer, the main question is how to supply heat in winter. Heating up large quantities of water in summer, to be used in winter is probably the most straightforward approach to resolve this seasonal problem. Simple, safe and environmental friendly. Large pit storages or underground thermal energy storages are known for many years in different variants. Very high seasonal storage efficiencies exceeding 90% have been reported.

However, the space required is considerable, and this space is usually not available close to urban locations. 

Here we kick in: Bubble in the lake storages (BILS) will offer a solution for cities close to standing waters.

The seasonal storage is placed in the lake and connected to the district heating grid of the city. Charging in summer when there is enough heat, use in winter when we need the heat. This makes sense if the storage volume is in the range of some millions of liters for district heating. BILS is nothing for tiny single family homes.   

Heat sources will be solar thermal collector fields or PV driven heat pumps. Both technologies could potentially provide also solutions for the "urban heat islands" problem.

We decided to publish all our ideas and results immediately, so that the project can also profit form your ideas and concerns. That is why we show it all, here and now. The aim is not to make a business, but to demonstrate feasability and to provide a solution for a relevant problem. Please use the contact form if you can help us with good ideas, if you have concerns or if you want to tell us something else.


The project and the experiments

BILS is a research project sponsored by the SOUR Call 1-2021 of the Swiss Federal Office of Energy SFOE.

The BILS project aims at demonstrating the feasibility of large sensible heat storages placed in lakes or other waters. BILS are made of flexible skins (e.g. plastics, textiles and similar) enclosing a hot liquid (e.g. water) placed in waters (e.g. lakes). BILS can be submerged or surface floating, fixed in place or mobile. BILS can consist of a single chamber or of several communicating compartments. The project includes investigations on reducing thermal losses and also the development of energy management strategies such as for charging and discharging or for active and passive stratification. In the frame of this project, all these options are investigated to develop a new solution for seasonal heat storage. Environmental aspects will be considered, but our first focus is proofing the technological feasibility. 

In the frame of the research project we will work with samples in the range of less than 20 m³. It is however clear that true BILS must have volumes of hundreds of m³.  


The Technology

First the technology seems to be a but weired but basically simple. Thinking more about it and discussing with friends and experts, opens many concerns but also many new ideas related with insulation, materials, static behaviour, buoyancy, environmental problems, legal aspects, just to name a few.


The Potential

The potential of BILS in district heating systems is huge. To give an example, only 2% of the Lake Zurich would be sufficient to cover the total heat demand of the city of Zurich. Many smaller cities with a roughly estimated number of 1.5 - 2 Mio inhabitants are located ashore of similar or much bigger Swiss lakes.


The Contacts

SPF Testing_edited_edited.png

The BILS consortium unites the expertise of SPF Institute for Solar technology and Luft und Laune

If you have questions, remarks or good ideas to share, please let us know. 

Thank you, we will react swiftly

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