A heat storage process by absorption is studied in this paper. It is devoted to solar domestic systems. Energy and exergy studies are performed on the ideal cycle, and prove the contribution of the solution crystallisation to the system storage density, with an improvement of 22%, without degradation of the exergetic efficiency of the process. A prototype has been built and tested in conditions compatible with domestic solar thermal collectors. The process has been proved successful for heat storage. The heat charging was more efficient than the discharging phase, with respective heat transferred in the range of 1–2 kW and 0.3–0.5 kW, in typical solar domestic conditions. Crystallisation has been observed, and will increase the storage density but discrepancies were found between the ideal solution and the global prototype crystallisation behaviour, possibly due to some impurities presence, corrosion products and a slow dissolution kinetic.