Thermoeconomic analysis of improved exhaust waste heat recovery system for natural gas engine based on Vortex Tube heat booster and supercritical CO2 Brayton cycle

Abstract Polygeneration has gained increased attention since it embraces efficient energy utilization while contributing to alleviate environmental pollution. The fossil-fuel dependence of trigeneration/cogeneration systems presses for maximizing energy conversion rates. Waste Heat Recovery (WHR) from natural gas engine exhaust represents a significant opportunity to improve fuel utilization. This study presents a thermoeconomic analysis of a novel WHR system consisting of a Vortex Tube heat booster coupled to the exhaust stream of a 2000 kW natural gas engine that drives a Supercritical CO2 Brayton Cycle (SCBC). SCBC operates under two different configurations, namely Regenerative and Recompression. Results indicate that implementing the Vortex Tube (or Ranque-Hilsch tube) increases energy and exergy efficiencies up to around 1.85% and reduces exergy destruction by approximately 4–8%. The Recompression SCBC features higher thermal and exergy efficiencies, while the Regenerative SCBC results in higher power output. Economic indicators reveal that the energy production cost of the Recompression configuration is significantly lower than that of the Regenerative. The turbine features the highest share of total equipment cost, with around 40–50%, followed by the heater and regenerators. The proposed WHR system can potentially recover up to around 7.8% of the total engine power output and provide an electricity price lower than 0.3 $·kWh−1 with a payback period in the range of 8–12 years. Heat exchangers represent an opportunity to improve thermoeconomic performance since they significantly increase exergy costing and limit overall efficiency. Incorporating high thermal conductivity materials that fulfill safety requirements is vital for improving heat exchanger's performance and developing a cost-effective WHR system.