A Low-Temperature Heat Storage System Utilizing Mixtures of Magnesium Salt Hydrates and Ammonium Nitrate

The heat storage system based on mixtures of salt hydrates and anhydrous salts desjribed in previous articles has been applied to Mg(NO/sub 3/)/sub 2/.6H/sub 2/O and MgCI/sub 2/. 6H/sub 2/O (alone or in eutectic mixtures), added with NH/sub 4/NO/sub 3/. Calorimetric measurements showed that in the temperature range 25-60/sup 0/C, Mg(NO/sub 3/). 6H/sub 2/O allows a Thermal Energy Storage (TES) capacity of about 56 kcal/kg (equivalent to 86 kcal/liter), which is slightly lower than the values previously recorded with ammonium alum. When the maximum temperature was lowered to 55/sup 0/ and to 50/sup 0/C, in order to be closer to the peak-efficiency of commercial flat-plate collectors, the best results were obtained, respectively, with Mg(NO/sub 3/)/sub 2/ .6H/sub 2/O and with the eutectic mixture Mg(NO/sub 3/)/sub 2/. 6H/sub 2/O - MgCI/sub 2/. 6H/sub 2/O, both added with NH/sub 4/NO/sub 3/. The mixtures of the aforementioned magnesium salts and ammonium nitrate do not show any significant efficiency reduction after many thermal cycles, and at the present stage of the research, they may be considered the most suitable for obtaining heat storages in the temperature range of commercial solar collectors.