Performance of CaCl₂-reactor for application in ammonia-salt based thermal transformers

Thermochemical reactions, such as calcium chloride reacting with ammonia to form calcium chloride complexes, are attractive for application in heat pumps as they produce more heat per kg adsorbed sorbate but also adsorb a considerable higher amount of sorbate per kg of sorbent, compared to adsorbents such as zeolites. These benefits, however, come with together with a number of challenges. Firstly, the salts have poor thermal conductivity so the path from heat exchanger to the sorbent must be limited in order to allow for short cycling times and therefore high power density. Secondly, the salts typically swell and shrink upon (de)sorption, thereby easily losing their contact with the heat exchanger. To overcome these two problems, the salts are often placed in a matrix, such as expanded natural graphite (ENG) or zeolites. This paper shows the performance of a reactor containing approximately 1 kg of CaCl₂ placed in a 1 kg ENG matrix. Its performance in terms of adsorption/desorption rates, heat input and output as a function of temperature and pressure gradients and under typical heat pump and transformer conditions is shown and compared with model calculations. The parameters used in the model calculations have been obtained from literature or independently measured using apparatus such as Rubotherm microbalance and a large temperature jump setup. The results show that material properties measured on small samples do not easily yield a proper description of the sorbent performance on kW-scale reactors. Some suggestions are made to improve future model description and experiments.