Thermo-economic optimization of a Rankine cycle used for waste-heat recovery in biogas cogeneration plants

Abstract

In a typical biogas cogeneration plant, a part of the exhaust gas energy is used for heating buildings within the plant. However, a certain amount of this energy remains unused. This study examined the utilization of this waste-heat through the Rankine cycle and organic Rankine cycle. A multiobjective thermo-economic optimization procedure of a waste-heat recovery unit installed at the exit of an engine before the engine cooling fluid–exhaust gas heat exchanger is proposed herein. The optimization procedure includes sizing all heat exchangers in the waste-heat recovery unit based on the measured Colburn and friction factors. The thermo-economic optimization was performed to investigate the maximal power output and the minimal payback period while preserving the function of the heating system. The procedure was applied to a biogas power plant with two engines (mass flow rate of the exhaust gases from each engine was 1.77 kg/s at a temperature of 410 °C). The electrical efficiency of this system was 42.1% and the measured overall yearly energetic efficiency was 66.7%. Optimization shows that the waste-heat recovery unit based on Rankine cycle is more economical than the unit based on the organic Rankine cycle using toluene. The electrical efficiency of the entire power plant increased by 2.97% and the payback period of the investment was 6.8 years, while the Levelized Cost of Electricity was 0.0419 $/kWh. The proposed method could be used to analyze the investment profitability of waste-heat utilization in other cogeneration plants.