Modeling and simulation of an absorption heat transformer employing ZnCl2–LiCl–H2O

This dissertation presents the modelling, simulation, and performance assessment of a single-stage Absorption Heat Transformer (AHT) aimed at upgrading industrial waste heat to higher process temperatures. The study is motivated by the global need to improve energy efficiency and reduce greenhouse gas emissions through the recovery and reuse of low and medium temperature heat. A comprehensive literature review on Chemical Heat Pumps (CHPs) and AHTs was conducted to identify the main working pairs and their limitations, followed by a systematic screening and thermodynamic validation of alternative aqueous salt solutions performed using Aspen Plus. Among several candidates, the ternary mixture H2O–LiCl–ZnCl2 was selected as the most promising working solution, combining favorable properties such as solubility, low vapor pressure, and good chemical stability. In particular, this work developed a detailed design of the AHT, including all the main components: generator, condenser, evaporator, absorber, and heat exchangers. Parametric analyses were carried out to identify the operating conditions that maximize the process performance indicators, namely the Coefficient of Performance (COP), the Exergetic Coefficient of Performance (ECOP), Gross Temperature Lift (GTL) and the Flow Ratio (FR). The model achieved a COP of 0.47, an ECOP of 0.59, and a GTL of 20 °C, values consistent with those reported in the literature for similar systems. An environmental comparison with a mechanical heat pump and a natural-gas boiler demonstrated the potential of the proposed AHT to reduce CO2 emissions by more than 99.9% with respect to the natural-gas boiler, confirming its suitability as a sustainable technology for industrial waste-heat recovery and energy decarbonization.