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.

在典型的沼气热电联产厂中，一部分废气能量用于给厂内的建筑物供暖。但是，一定量的这种能量仍未使用。这项研究检查了朗肯循环和有机朗肯循环中这种废热的利用。本文提出了在发动机冷却液-废气热交换器之前安装在发动机出口处的废热回收单元的多目标热经济优化程序。优化程序包括根据测得的Colburn和摩擦系数确定废热回收单元中所有热交换器的大小。进行热经济优化，以研究最大功率输出和最小投资回收期，同时保留加热系统的功能。该程序应用于具有两个发动机的沼气发电厂（在410°C的温度下，每个发动机的废气质量流量为1.77 kg / s）。该系统的电效率为42.1％，测得的年度总能量效率为66.7％。优化表明，基于兰金循环的废热回收装置比基于甲苯的有机兰金循环的装置更经济。整个电厂的电力效率提高了2.97％，投资回收期为6.8年，而平均电力成本为0.0419美元/千瓦时。该方法可用于分析其他热电厂的余热利用投资收益。该系统的电效率为42.1％，测得的年度总能量效率为66.7％。优化表明，基于兰金循环的废热回收装置比基于甲苯的有机兰金循环的装置更经济。整个电厂的电力效率提高了2.97％，投资回收期为6.8年，而平均电力成本为0.0419美元/千瓦时。该方法可用于分析其他热电厂的余热利用投资收益。该系统的电效率为42.1％，测得的年度总能量效率为66.7％。优化表明，基于兰金循环的废热回收装置比基于甲苯的有机兰金循环的装置更经济。整个电厂的电力效率提高了2.97％，投资回收期为6.8年，而平均电力成本为0.0419美元/千瓦时。该方法可用于分析其他热电联产厂余热利用的投资收益率。优化表明，基于兰金循环的废热回收装置比基于甲苯的有机兰金循环的装置更经济。整个电厂的电力效率提高了2.97％，投资回收期为6.8年，而平均电力成本为0.0419美元/千瓦时。该方法可用于分析其他热电厂的余热利用投资收益。优化表明，基于兰金循环的废热回收装置比基于甲苯的有机兰金循环的装置更经济。整个电厂的电力效率提高了2.97％，投资回收期为6.8年，而平均电力成本为0.0419美元/千瓦时。该方法可用于分析其他热电厂的余热利用投资收益。