A systematic thermodynamic performance assessment of a solar-driven double-effect absorption chiller integrated with absorption energy storage

Abstract

Sensible heat storage in the form of hot water is usually applied to solar-driven cooling systems to increase their cooling coverage. The main limitation of sensible heat storage is high thermal losses. A double-effect absorption chiller is operating in a typical generating temperature range of 150–180 °C and has a better performance compared to the single-effect type. Due to the high operating temperature range of the chiller, this study suggests the integration of an absorption energy storage (AES) on a double-effect chiller based on H2O-LiBr solution and driven by parabolic trough solar collectors. This is because in AES, heat is stored in the form of chemical potential and the issue of heat loss is minimal. The paper presents a model of the integrated system and its performance evaluation considering the undesirable solution crystallization phenomena under various operating conditions. The results show that, under certain operating conditions, there is a high risk of solution crystallization in the chiller during the charging operation when the solution distribution ratio is below 50 %. Moreover, the results indicate that when a solution with an initial lithium mass fraction of around 55 % in the strong tank is considered, there is a tendency for crystal formation in the solution storage tank and at some locations within the chiller. There is a potential risk of solution crystallization in the solution storage tank and at some locations within the chiller cycle when a solution with an initial LiBr mass fraction of 55 % and an initial mass of 64,000 kg is considered. Considering the weather data of Dhahran, the best-operating conditions are obtained at an initial solution mass of 64,000 kg in the storage tank, initial solution LiBr mass fraction of 0.5, and solution distribution ratio of 55 %. At these and other conditions detailed in the paper, the highest energy storage density is about 136.8 kWh/m3, with an average cooling effect of 1700 kW, overall solar system COP, and exergy efficiency of 0.985 and 0.067 respectively, during a fourteen-hour operation. The findings in this study provide useful information to researchers on sizing the storage unit considering the appropriate initial conditions to avoid solution crystallization.