The most efficient way to collect solar energy for space heating purposes is to do it at low to medium temperature, between 20 and 80C. Storage in this range is done since centuries with water.

Water is a good heat storage medium. One m3 of water can store 70 kWh between 20 and 80 C, that is the production of about 20 m2 of good flat plate solar collectors during a bright sunny day.

Keeping this energy in a water tank for a few days can easily be achieved by insulating the container with classical insulation material 10 to 12 cm thick.

The sensible heat capacity (ability to store heat in a sensible form) of water is one of the highest. On top of this interesting physical property, water has four other very important advantages over other material: It is non toxic, it is ubiquous, it is cheap, it is a fluid.
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To store cold energy, say around 0C, water is also a very good storage material! Using its latent heat from liquid to solid (ice), water can store 91.6 kWh par m3 ! This, combines with the total reversibility of the process, makes clearly water the first choice when cold storage is needed for cooling purposes in building.

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Advanced concepts

The density that can be theoretically achieved is shown on the vertical axis where as the horizontal axis shows the temperature needed.

Water our best in class of today is still a low dense material compared to the other solutions and noticeably chemical reactions, where the storage volume for a seasonal store in a low energy house could be not bigger than 1 m3 compared to the 34 m3 of water we need today.

The IEA Solar Heating and Cooling Programme decided to investigate these 4 alternatives during 2003 to 2007 within Task 32 “Advanced storage concepts for solar and low energy buildings”.