Computer-aided molecular and system design of diffusion absorption solar-cooling systems
Creators: name: Harraz, Asmaa Aly Mohamed Hassan
Publisher: name: Imperial College London
Year: 2022

description: Diffusion absorption refrigeration (DAR) is an attractive thermally-powered refrigeration technology that can be powered using thermal solar collectors. DAR technology can address refrigeration security challenges, rising electricity costs and CO2 emissions. Thus, adopting DAR technology and exploring alternative environmentally friendly working fluids for solar-cooling applications is interesting. Nevertheless, DAR systems usually have a lower coefficient of performance (COP) than conventional refrigeration technologies, and commercially available DAR units usually use NH3–H2O mixtures which require temperatures higher than 150 °C to initiate cooling. An integrated computer-aided molecular design (CAMD) and system optimisation framework using the group-contribution equation of state based on the statistical associating fluid theory (SAFT-γ Mie) was devised to select optimum working fluids and DAR-unit designs simultaneously, which has not been implemented before. The CAMD-DAR framework was implemented to identify optimum working-fluid mixtures considering different organic families, exploring non-polar simple hydrocarbons and complex polar hydrocarbons. Working fluids for maximising cooling rates are not necessarily the same for minimising the specific purchase cost. Thus, multi-objective optimisation was considered to obtain a set of non-dominated design alternatives. Due to the particular importance of the dynamic performance of solar-powered DAR systems under given climatic conditions, a quasi-transient DAR model is considered to simulate the performance of solar-powered DAR systems with a specific interest in the start-up and shut-down phases. The quasi-transient DAR model helps assess optimum DAR systems identified by the CAMD-DAR framework. The CAMD-DAR framework identifies 2-C4H8–n-C8H18 as the optimum working fluid for maximum cooling. For ambient and cooling temperatures, respectively, Ta = 20 °C and Tco = 4 °C, A 250-W DAR unit charged with 2-C4H8–n-C8H18 can produce cooling up to 38 W corresponding to a specific purchase cost of cooling (SPC) of £ 15 per W. The CAMD-DAR framework suggests that non-polar organic working fluids enable higher cooling rates. Considering non- polar hydrocarbons, the CAMD-DAR framework identifies 2-C4H8–C2H5OH–He as the optimum working fluid for a wide range of cooling and ambient temperatures. This working fluid can produce maximum cooling rates up to 146 W for 440-W DAR system, corresponding to a SPC of £ 5.4 per W of cooling at ambient and cooling temperatures, respectively, of Ta = 20 °C and Tco = 4 °C. The CAMD-DAR framework suggests that olefins and branched refrigerants can produce higher cooling rates than saturated straight chains. The current CAMD-DAR framework identifies working fluid mixtures that can compete with the standard NH3–H2O system, thanks to the lower generator temperatures (&lt; 150 °C) required by these organic working fluids to activate the DAR cycle. The solar-DAR simulations for climate data of Alexandria, Egypt, confirm that a DAR design based on 2-C4H8–C2H5OH and powered by evacuated tube collectors (ETCs) can produce more cooling than other organic working fluids. Given the climate data of Alexandria, Egypt, the ETC-powered organic-based DAR system could produce up to 1939 h of cooling per year (68 kWhco of cooling per year). However, low electricity prices cannot encourage households to use solar-powered DAR systems for their very long payback time and negative levelised cost of savings, with a levelised cost of cooling of £ 1.93 per kWhco. The CAMD-DAR framework and dynamic DAR model results demonstrate the potential of this tool to introduce a new era of solar-powered cooling technologies for different applications.