In Germany, we have every year about 1000 kWh/m² sunshine (equivalent of 100 l diesel / m²) but most of it is available in summer when we do not need heating. By storing this thermal energy (TES), we are able to overcome the winter with what we have collected in summer. The easiest medium is liquid water with its high specific heat (water 4182 J/kgK compared to concrete 800 J/kgK). A typical method is working with a large pit, covered with thermal insulation (PTES) although other methods are also available. A nice introduction is given in the following paper.

Newly installed PTES with polymeric cover

Like described, large difficulties are present with the thermal insulation in the cover. Indeed, polymeric foams absorb large quantities of humidity while also spontaneous deforming is present due to the high water temperature and concentration of rain water on the cover. For that reason, the temperature of the water is limited to 75-80°C.

To improve efficiency, it makes sense to use cellular glass and 95°C water to increase the thermal energy content of the PTES. However, to protect the stainless heat exchangers and to avoid biological contamination, the water is kept of pH=9, which stimulates the corrosion of soda lime glass and so cellular glass. For that reason, the cellular glass should be protected against this corrosion. Stainless steel foil has a large corrosion resistance in hot alkaline water and has a matching thermal expansion coefficient with (cellular) glass. GLAPOR proposed a modular solution on the ISEC 2022 conference in Graz, Austria with the following poster.

By working with a modular system, we are able to assemble larger covers than possible today (100 x 100m) and a temperature up to 99°C is possible with these materials. On top of the cover, we can use typical flat roofing technology with polyurea and tapered solution to drain/guide the rain water. Moreover, a second function is possible with this solution, which can be a photovoltaic system, a large greenhouse (in situ heating), a swimming pool (in situ heating), … . This second function will be logic because most PTES will be located near cities, where the land is very expensive. This is certainly not possible with the polymeric solution like shown hereunder. The dramatic situation hereunder can never happen with a cellular glass cover.