top of page

The two basic concepts

After working for some months on this project, it is evident that two basic concepts have to be considered: Surface BILS and Submerged BILS. 


Surface BILS

The surface BILS is always visible and consists of a cylindrical enveloppe separating part of the surrounding water to bild the storage. The surface BILS is open-to-ambient, meaning that there is no control of pressure or buoyancy needed. The warm storage content is making the BILS naturally float at the surface.  


Submerged BILS

The submerged BILS needs to control buoyancy similar to submarines. Therefore the BILS probably must be water-tight. Depending on the temperature, the storage intends to sink or to float. But things are not easy, controlling buoyancy is challenging as our experiments have shown. 

Specific Pros

  • Simple setup

  • Easy accessibility for operation and maintenance

  • Water is the medium

Specific Pros

  • Invisible

  • No (surface) space used

  • Stable conditions and no UV load on materials

Specific Cons

  • Materials suffer from UV and other impacts

  • Visibility, spoiling the lakescape

  • People have to stay away for safety reasons.  

  • Obstruction of lake traffic

Specific Cons

  • Difficult accessibility for operation and maintenance 

  • Buoyance control. Several technical challenges

Specific Challenges

  • Rain, snow: Water should not accumulate on the top of the storage. Measures must be taken to avoid that (known problem from pit storages on land)   

  • Wind, storms: Wind and storm are inducing water movements close to the surface. The deeper, the calmer or the more constant are the currents on waters.

Specific Challenges

  • Buoyancy is depending on the temperature of the storage and the lake and therefroe has to be actively managed.  


Common Challenges

  • Thermal insulation  

  • Mooring

  • Operation, hydraulic connection to the user

  • Environmental impact

bottom of page