Introduction
The rapid growth of electricity demand in Indonesia has led to the extensive operation of gas engine power plants (PLTMG) due to their flexibility and relatively high efficiency. Nevertheless, a significant fraction of the fuel energy, typically around 25–30%, is rejected to the environment as exhaust gas waste heat. The exhaust gas temperature of gas engines commonly ranges from 350 to 450°C, indicating a considerable potential for waste heat recovery (WHR). Organic Rankine Cycle (ORC) technology has been widely recognized as an effective solution for converting low- to medium-temperature waste heat into useful electrical power.
Previous studies have demonstrated the feasibility of ORC integration with internal combustion engines. However, most investigations focus on generic operating conditions or rely on simplified thermodynamic models. Moreover, studies that simultaneously integrate energy, exergy, and environmental (3E) analyses using real operational data remain limited, particularly for gas engine power plants. Therefore, this study aims to evaluate the energetic, exergetic, and environmental performance of an ORC system integrated with an existing PLTMG using a comprehensive thermodynamic simulation framework.
Materials and Methods
A thermodynamic model of the ORC system was developed using Aspen HYSYS, which enables accurate representation of multiphase flows and heat exchanger behavior. Real operational data from a gas engine power plant were used as boundary conditions, including exhaust gas temperature and mass flow rate. The ORC configuration consists of an evaporator, turbine, condenser, and pump operating under steady-state conditions.
Six working fluids were investigated: R600 (butane), R600a (isobutane), R601 (pentane), R601a (isopentane), R134a, and R744 (CO₂). All simulations were conducted at a constant working fluid mass flow rate of 10 kg/s. The system performance was evaluated using energy and exergy analyses based on the First and Second Laws of Thermodynamics. Environmental performance was assessed by estimating fuel savings and the corresponding reduction in CO₂ emissions resulting from the additional power generated by the ORC system.
Results and Discussion
The simulation results indicate that the selection of working fluid has a pronounced impact on ORC performance. The turbine power output varies significantly among the investigated fluids, ranging from 139.5 kW for R744 (CO₂) to 1331 kW for R601 (pentane). Hydrocarbon-based working fluids generally exhibit superior performance due to better thermodynamic matching with the exhaust gas heat source.
The highest thermal efficiency of the ORC system is achieved using R601, reaching 21.89%, followed by R601a with 19.29%. In contrast, R134a and R744 demonstrate relatively low thermal efficiencies of 6.56% and 3.30%, respectively. From an exergetic perspective, R600 (butane) provides the highest exergy efficiency at 17.36%, indicating lower irreversibilities during energy conversion processes.
In terms of environmental performance, the integration of the ORC system results in a substantial reduction in fuel consumption and associated CO₂ emissions. The maximum CO₂ reduction is obtained with R601, reaching approximately 0.19 kg CO₂ per Nm³ of natural gas, while R744 exhibits the lowest reduction at around 0.02 kg CO₂ per Nm³. These findings confirm that higher turbine power output directly translates into greater fuel savings and emission reductions.
The novelty of this study lies in the application of a comprehensive 3E analysis using Aspen HYSYS combined with real operational data from a gas engine power plant in Indonesia, as well as the systematic comparison of hydrocarbon and CO₂-based working fluids under identical operating conditions.
Conclusions
This study demonstrates that ORC-based waste heat recovery is a technically feasible and effective approach for enhancing the performance of gas engine power plants. The results show that ORC integration can generate additional net power ranging from approximately 140 kW to 1331 kW, depending on the selected working fluid. Among the investigated fluids, R601 (pentane) provides the best overall performance in terms of thermal efficiency and CO₂ emission reduction, while R600 (butane) achieves the highest exergy efficiency. Although CO₂ (R744) offers advantages in terms of safety and environmental compatibility, its thermodynamic performance is comparatively lower under the investigated conditions. Overall, the findings highlight the critical role of working fluid selection and demonstrate the effectiveness of a 3E-based evaluation framework for ORC applications in gas engine power plants.
References
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