Two steel tanks are used as seasonal thermal storage to hold a total of 25,000 liter of water with a maximum temperature of 90 ⁰C. The tanks are installed underground, covered with a first thick layer of Foamglas insulation followed by an XPS layer on the outside. The insulated tanks are placed completely inside an EPDM sealed bag to ensure the insulation remains dry and effective. Steel tanks were preferred above concrete tanks due to the 3D-thermal pressure caused by the stratification in the tank: a temperature difference of 60 ⁰C can occur over a height of 1 meter. Steel tanks do require cathodic protection on the inside, to prevent corrosion, but are solid and can resist high pressure.
The tanks will be loaded with the vacuum solar system that will be installed spring 2016.
The schematic of the thermal set up can be downloaded here.