MEDIUM TEMPERATURE RANGE Al(OH)3/AlOOH CHEMICAL HEAT PUMP

Al(OH)3/AlOOH chemical heat pump shows operating temperature (150 - 250 °C) referable to medium temperature thermal energy storage (MTTES) and lower than similar but more investigated systems (e.g. magnesium or calcium hydroxides). However, involved processes are not so efficient, after the first thermal charge, without an appropriate use of adjuvants. Purpose of this work was therefore to evaluate the thermochemical behavior of -Al(OH)3 (known as bayerite) when it is subjected to partial dehydration, that occurs at 200-250 °C, becoming gamma-AlOOH (boehmite). Then, the feasibility of the reverse reaction, i.e. hydration of boehmite, was also studied, trying to increase the efficiency of this process, hampered by the great stability of the formed phase. In this regard, hybrid materials were prepared by adding exfoliated graphite and calcium chloride (CaCl2), acting on the specific surface area and giving a hygroscopic tendency, respectively. In both cases, improvements were made in terms of reacted fraction, compared with the pure material, in dehydration-hydration cycles.