Heat insulating duvet for solar heating plant storage

Year: 2015

Location: Vojens and Gram

Country: Denmark

Volume: 21.000 m³ expanded clay

Owner Builder: Vojens and Gram Fjernvarmeværk (district heating plants)

Contractor: Chr. Johannesen Eftf. A/S

Designer: Rambøll Danmark A/S

Problem / Solution

Two huge solar heating plants have been constructed. The plants collect and store the solar heat of summer. In order to save the heat for winter, during which the heat is to be extracted again, an insulating cover consisting of specially developed expanded clay has been constructed. The heating systems contain a total of 322.000 m³ of water and store the energy from 114.000 m² of solar cells. The Vojens storage is the world’s largest of its kind.


The storage is a 14 meter deep basin filled with water. It took 5 month to fill it with a pumping capacity of 50.000 liter water pr. hour.
On top of that a 70 cm heat insulating cover was established. Two membranes were installed with expanded clay in between. Blowing capacity of the blowing trucks is 1 m³ per minute so filling in 21.000 m³ of expanded clay took some time. Using solar heating is expected to reduce the consumers heating expenses with 10 – 15 %. The reduction of CO2 is calculated to be 6.000 tons per year.

Why expanded clay?

The advantages of applying expanded clay is that this material – in addition to being heat-insulating – is flexible and very easy to build in. The expanded clay was transported by truck loads and blown through hoses connected to the truck. In this project, up to 90 m of hose has been used to blow the expanded clay to the exact place of application.