BILS are either floating or submerged. Both approaches have their pro's and con's. We are investigating both ideas, with a slight preference for submerged. Most of the materials used to build our model have a density slightly higher than water, so they are submerging. As soon as the bubble is heated up and/or there are some air bubbles trapped in the storage it will lift upwards and surface. Just adding weight to the storage or anchoring somewhere seems to be possible for a small BILS like ours. For a Mio litre BILS this is becoming challenging as the needed forces will probably destroy the skin.
Currently our preferred approach is the use of saline water inside the storage.
The difference between the two pictures? The same temperatures and the same volume, but saline water in the submerged BILS. This works fine. By adding a few kg of salt, buoyancy is compensated and the BILS is sinking. In this case the BILS is at about 20°C and the pool at 7.5°C, meaning that about 0.25% NaCl (or 5kg) will be enough to submerge the BILS. In the experiment, about 12 kg were needed for the simple reasons that for technical reasons we did not manage to get out all the air bubbles from the storage.
More to come