Investigation on Floating Lid Construction, pit Water Storage, Ottrupgaard, Denmark

At Ottrupgaard a pit water storage of 1,500 m3 and a lid area of about 700 m2 is built for seasonal storage of a solar collector field of 560 m2. The lid price is the largest component of a pit water store with a cost share of about 57%, more precisely 1,163 Dkr./m2. Due to the large share in price the development of lid constructions is crucial for the development of pit water storage and seasonal storage, as it seems that the development of solar collectors will not have a breakthrough in the near future.The Ottrupgaard lid design is basically a sandwich element construction of PUR-foam between two metallic covers.

The elements are joint in situ by special steel profiles. A two-step sealing with silicone mass and bitumen-tape is applied to tighten the construction.To ensure a proper lid design, two test lids of 1.5x1.5 metres were tested at the Department of Buildings and Energy under ambient conditions floating on hot water.

The test lids were examined for tightness by a number of means. The results showed critical construction errors of the first lid design. A redesigned lid showed acceptable results, but also some water penetration into the lid insulation. The entered water gathers on the colder side of the construction where it does no harm.

Anyway the worst case of hot water lying at the bottom of the insulation is examined by experiments. The experiments proof that the water will penetrate into the PUR-foam in time. It is not to say from the experiments if the PUR-foam cells are damaged due to this penetration. Anyway the heat resistance of the insulation material decreases with increased presence of water leading to larger heat losses through the lid, which is undesirable.

More work has to be done on such subjects to understand the effects of hot water on insulation materials and thereby to ensure proper lid constructions.In situ examinations of the lid show considerable heat losses through the lid borders. The concept can and must be improved to avoid such heat losses.The overall experience is that the construction is too expensive and complicated but can be optimized technically as well as economically.

The process of handling, joining the sandwich elements and tightening the construction is much too demanding and should be changed. The concept as such is working well if these problems are solved.To ensure proper lid design and monitoring, measurement methods of finding moisture and damp in highly insulated constructions plus conductive heat transport are to be found.Although there is no applicable lid design after this first project phase, the project has brought the lid design a step ahead.

The project has disclosed a finite number of ways to go on and find final solutions.