Preheat

Sorption heat storage

Adsorption is a process that occurs when a liquid or gas (called adsorptive), accumulates on the surface of a solid or liquid (adsorbent), forming a molecular or atomic film (adsorbate). It is different from absorption, where a substance diffuses into a liquid or solid to form a "solution". The term sorption encompasses both processes. Desorption is the term used for the reverse process.

In order to maximize the storage density, water is the most suitable adsorptive due to its exceptionally high heat of condensation per unit volume of the bulk liquid (676 kWh/m³ at room temperature).

Two different designs of adsorption heat storages are possible: open systems which operate at normal atmosphere pressure, using the humidity of the air as adsorptive, and closed systems which operate with a pure adsorptive atmosphere and thus have to be kept evacuated.

For both, an adsorbent is needed that meets the specific temperature requirements: adsorption processes are highly temperature dependent. Until today, sorption materials are used and optimised for many other applications (molecular sieves, air cleaning etc.).  Thus, materials optimized for thermal applications are still quite expensive.

Silicagels, metal hydrates or zeolithes (among others) are tested for possible adsorption heat storages.