To collect solar heat in a building windows are effective! In order to avoid high temperature swing, thermal mass must be however present inside the protected volume, i. e. inside the insulated envelope. Internal thermal mass can be used from 19 to 24C, barely more. Depth of penetration is however limited and all the thickness of a wall or slab cannot be used during a daily cycle. For concrete it has been shown that only the first 10 to 14 cm can be really used for a diurnal storage. In our example the necessary wall area becomes then 70 m2, free of carpets, paintings, furniture, wood or other thermal and radiation barriers. Any measure that can improve thermal mass in a building favours the comfort.

PCM in walls

To improve storage, several solutions of PCM (phase change materilal), embedded or not, into construction elements have been invented and tested since more than 30 years. Today 2 types of PCM

solutions for storing into of close to the structure of a building might see a future on the market: cool ceilings and PCM in microencapsulated balls inside a wall element.

PCM in microencapsulated polymers are now on the market. They can be added to plaster, gypsum or concrete to enhance the thermal capacity of a room, at a pre-defined temperature (22 or 24C). For renovation they provide a good alternative to new heavy walls.

Rock beds

Storage of air heated by a solar air system can be achieved in rock beds. The blocks of rock are between 1 to 10 cm diameter. Air is blown through the bed at low speed to heat up the rocks. Several solar houses have been built on this principle and works to satisfaction. It is not seasonal storage, and needs a air collecting and distributing system. Water solutions for storage are preferred because water collectors are more efficient than air collectors, and water can transfer much higher power rates than air.