Maximum power and corresponding efficiency of an irreversible blue heat engine for harnessing waste heat and salinity gradient energy

In this study, a novel irreversible cyclic model of a capacitive mixing blue heat engine mainly consisting of super capacitors,charging and discharging circuits, a heat source, as well as two water sources with given salt concentrations is established for harvesting salinity gradient energy and waste heat. Additionally, the effects of the charging voltage and ratio of the minimum to maximum surface electric charge density on the thermodynamic efficiency and power output of the cycle are discussed. The maximum power output of the cycle is calculated. The optimized ranges of efficiency and power output as well as the temperatures of two isothermal processes are determined. It is established that during the isoelectric quantity process, there is not only an increase in thermal voltage owing to the temperature difference, but also an increase in concentration voltage owing to the salinity gradient. Consequently, the blue heat engine can obtain higher energy conversion efficiency than a conventional heat engine. When the temperature ratio of the heat source to the heat sink is 1.233, the maximum efficiency can reach approximately36%. The results obtained can promote the application of capacitive mixing technology in real life, reducing the consumption of fossil fuels.