The thermal insulation
One of the major challenges for BILS is the thermal insulation to reduce thermal losses. For conventional storages, there are different insulation materials available such as foams, mineral wools or even more sophisticated vacuum insulation materials. These materials seem to be unsuited for underwater applications. Either they lose insulation properties when wetted, they are compressed so that the insulation property is lost or they induce considerable buoyancy applying unwanted uplifting forces on the BILS.
A suitable insulation material for BILS should not be affected by humidity, should be flexible, should be pressure resistant, should have similar density as water and of course should be durable, safe and low cost. Quite a challenge.
The main drivers to select an insulatiion material are "density close to densiyt of water" and low thermal conductivity λ.
Plotting material data for denisty ρ versus themal conductivity λ shows that there is a relation and that there are vertain limitations when selecting insulation materials, Considering all the listed restrictions, the number of materials that could be taken into account is very small.
Data taken from: The Harmonisation of Thermal Properties of Building Materials
BRE Publication BEPAC Research Report
J A Clarke (1), P P Yaneske (1) & A A Pinney (2)
(1) Energy Simulation Research Unit, Department of Architecture and Building Science, University of Strathclyde,
(2) Building Research Establishment, Watford.
A deeper analysis of the potential insulation materials shows that there are materials with density similar to water and thermal conductivity down to approximately 0,1 W/mK. Conventional thermal insulations used for example in the building sector are usually found in a range of approximately 0.02 W/mK.
The insulation materials listed in the figure above can be divided into four classes "impermeable materials", "inorganic porous materials", "organic hygroscopic materials" and "non-hygroscopic materials":
Impermeable materials
Inorganic porous materials
non-hygroscopic materials
organic-hygroscopic materials
In the four graphics above, the red square indicated the range where water is found (different temperatures and saline concentrations).
A deeper analysis of these insulation materials shows, that there are materials with density similar to water and thermal conductivity down to approximately 0,1 W/mK. It is therefore evident that insulation materials for BILS will have higher thermal conductivities than those used for storages on land.
The following materials could be considered
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Natural wood, Wood fibres, Hardboard, Chipboards
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Bitumen, Bitumen + Wood type materials, asphalt
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Brick, Concrete, Masonry, Plaster, Gypsum
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Rubbers and similar organic materials
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Other types of plastics and synthetic materials.
Several materials such as asbestos or lubricating oils are excluded for environmental reasons.
Wood based materials seem to be unsuitable in a permanently wet environment and are not considered for the time being.
Bricks, concrete, and similar stiff materials are probably better suited for solid built storage concepts.
Rubbers and other organic materials, plastics and synthetic materials will be evaluated in a later stage.
One material that is missing in all these lists is Water. The thermal conductivity of water is about λ=0.6 W/mK which is rather high compared to the listed materials and even worse compared with conventional insulation materials like mineral wool (λ=0.03 W/mK). But this high conductivity can be compensated with material thickness. Increase the thickness of the water insulation to 2m to reach the same thermal resistance as 10cm of mineral wool. Not reasonable for a small storage, but probably acceptable if the storage has a diameter of 20m to add 2m of insulation.
To profit from the mediocre thermal conductivity of water, it has to be immobilized, meaning that convection has to be avoided. Different options to immobilize water (IW) will be investigated and tested. See the Bubble insulation experiments (BINS)