Multi-objective optimization of regenerative ORC system integrated with thermoelectric generators for low-temperature waste heat recovery

Abstract

Three novel geothermal-based organic Rankine cycle (ORC) systems are proposed to enhance the efficiency and for waste heat recovery purpose. The proposed systems are modeled based on a basic ORC system (concept 1), an ORC system with an internal heat exchanger (concept 2), and a regenerative ORC system (concept 3). Accordingly, two thermoelectric generators (TEG) are introduced into the systems to exploit the waste heat of the system. The condenser is replaced with a TEG unit while the other TEG unit is used to recover the waste heat of the reinjected geothermal fluid. A comprehensive numerical investigation is conducted to compare the proposed systems from the thermodynamic and thermo-economic points of view. Furthermore, the most favorable proposed system is considered for optimization purposes and multi-objective optimization using the genetic algorithm is performed. Accordingly, a Pareto frontier is extracted consisting of the optimal solutions along with the scatter distribution of the critical parameters. The results indicate that the third proposed concept holds the highest exergy and energy efficiency among the studied systems. Also, the concept concludes the highest power output from the employed TEG units. The results further show that the first concept has the lowest associated cost with the system and the smallest payback period and at the same time higher net power generation. At the baseline of the systems, the exergy efficiencies are 59.39%, 57.92%, and 60.27%, and the total product costs are 25.8 $/GJ, 28.79 $/GJ, and 31.27 $/GJ for the first, the second, and the third proposed concepts, respectively. Results of the multi-objective optimization of the third concept reveal that the superheater temperature difference should be kept at higher values while the optimal range of the figure of merit is between 0.9 to 1.6. The proposed systems can be designed practically to exploit the geothermal heat source and recover the waste heat of the system.