Improved exergoeconomic analysis of a retrofitted natural gas-based cogeneration system

Waste heat recovery has been commonly accepted as an efficient, clean, reliable, and promising approach to increase the utilization efficiency of primary energy and reduce the pollutant emissions. A RNGCS (retrofitted natural gas-based cogeneration system), in which domestic hot water is heated by the waste heat of condensing steam and exhaust gas, is presented to replace the ONGCS (original natural gas-based cogeneration system), in which the domestic hot water is heated by bleeding steam. The RNGCS and ONGCS are modeled and simulated using Cycle-Tempo. An IECA (improved exergoeconomic analysis) incorporating waste exergy cost allocation and a TECA (traditional exergoeconomic analysis) without considering waste exergy cost allocation are applied to these two cogeneration systems to achieve the unit exergoeconomic cost of products. The IECA shows a reasonable result that the unit exergoeconomic cost of power, steam, and domestic hot water of the RNGCS are lower than those of the ONGCS. However, the TECA shows an unreasonable result that the unit exergoeconomic cost of domestic hot water generated from waste heat recovering in the RNGCS is much higher than that of the domestic water generated from high-grade bleeding steam heating in the ONGCS. The comparison proved that the IECA is more general and more reasonable for the presented types of waste heat recovery projects. In addition, economic analysis and sensitivity analysis based on IECA are also performed to show the superiority of the RNGCS.