The most cost effective way of storing solar heat is water. Water is a cheap and convenient material and tanks ranging from 50 to 1000 l are built by millions each year for the HVAC market. Modern solar tank combines several specific features that improve the overall efficiency of storing solar heat: Few thermal bridges, Enhanced insulation, even vacuum insulation in the future, Improved bottom insulation, Siphon introductions pipes to avoid natural convection losses, Reduced number of connections to avoid thermal bridges, Stratification enhancers, Internal devices to reduce speed of inlet water not to disturb stratification, Large heat exchangers or mantle heat exchanger.

2. Storage in PCM in tanks (Phase Change Material)

The idea of using PCM in a storage tank has been investigated in the 80s with paraffin. Although it works, the advantage is nowadays not strategic since the solar collector have been much improved and are less dependant of the collecting temperature in the range 50 to 80C than they were. Parafin has also a major drawback, its flammability. A few manufacturers propose PCM material for storing solar heat on the market (see table). From 20 to 80C, there is some choice.

Even for 0C, one company manufactures polymer balls that encapsulate water for ice storage. The same idea could be applied to a class 50 or 60C material that would end in a ball storage acting with the solar fluid fluid like a rock bed does with air.

New ideas are to combine PCM material with water in a hybrid storage so that the top part of the storage tank would stay at a maximum of 60C. Some first results both theoretically and in laboratory were found within IEA Task 32. PCM materials were found difficult to caracterize (supercooling and hysteresis effect). The advantage of PCM in tanks for short term storage was not demonstrated since the temperature range at which a combitank operates is quite large. The optimum position and operating temperature of the PCM and the material choice itself still needs investigation.