Ammonium Chloride (NH₄Cl)—Ammonia (NH₃): Sorption Characteristics for Heat Pump Applications

In a resorption heat pump, the adsorption and desorption reaction of ammonium chloride (NH4Cl) with ammonia (NH3) is of interest as a Low Temperature Salt (LTS). Reviewing previously published NH4Cl-NH3 equilibrium lines, ammonium chloride appears to offer useable working temperatures (50–70 °C) in the 10–15 bar pressure range during the adsorption reaction, and provides beneficial working conditions for the desorption reaction, when compared with alternative LTS candidates at atmospheric pressure. The NH4Cl-NH3 adsorption and desorption reactions, using a NH4Cl composite salt, have been evaluated under dynamic ‘real-world’ conditions in a Large Temperature Jump (LTJ) experimental testing rig; although there are concerns with mass transfer characteristics, the salt exhibits no hysteresis between the adsorption and desorption reactions, contrary to previous literature. The experimentally obtained equilibrium line values for the reaction enthalpy and entropy are 29,835 J/mol and 207 J/(mol∙K), respectively. Using a semi-empirical model, the NH4Cl composite salt has been successfully characterised, enabling the prediction of salt reaction behaviour. The model constants, A and n, identified are 4.5 and 5 for adsorption and 5 and 4 for desorption, with an overall salt active fraction (applicable to both reactions) of 0.98. Overall, the working equilibrium line and the dynamic performance of ammonium chloride has been investigated and the applicability of NH4Cl as a LTS for a resorption heat pump determined.