Analytical Study of the Waste Heat for Energy Use in a Hygroscopic Cycle with High Lithium Bromide Concentration

Abstract

The current world energy context requires solutions to reduce energy demand and increase the energy efficiency of thermal power plants, which typically release heat into the ambient without a further useful purpose. Climate change, leading to higher temperatures and water scarcity, may make difficult heat rejection processes in power cycles. Hygroscopic Cycle Technology can become a relevant technology due to the increase in the cooling reflux temperature thanks to the incorporation of hygroscopic salts. This work analyzes the possibility of using the waste heat rejected from the cycle cooling reflux by developing a thermodynamic model with a lithium bromide-water (LiBr-H2O) mixture as the working fluid. The model, validated with experimental tests performed at a pilot plant, was used to optimize cycle operating conditions regarding waste heat recovery potential without substantially decreasing the cycle efficiency. The increase in LiBr mass concentration led to higher cooling reflux temperatures, allowing for easier heat rejection and an increase in enthalpy and the cooling reflux mass flow rate. The effect of condensing pressure was found to have a relatively low impact on the mass flow rate and the potential heat recovery per unit mass flow. An exergy analysis revealed a decrease in potential physical exergy recovery as LiBr concentration increases. Concentrations between 30 and 50% seem the most suitable ones for maximizing the power output of the cycle, while maintaining a high enough heat recovery potential, with values of 0.6 kJ/kg for the optimum concentration of 45%. Finally, prospective uses for the waste heat are proposed, considering that current applications of the technology rely on the use of biomass fuels from olive oil production waste.