Evaluation of the efficiency and economic benefit of energy conversion processes and technologies requires a scientifically based analysis. The hierarchically structured exergetic analysis provides a detailed characterization of complex technical systems. By defining corresponding evaluation coefficients. the exergetic efficiency can be assessed for units within the whole system. Based on this exergetic analysis, a thermoeconomic evaluation method is developed. A cost function is defined for all units, subsystems and the total plant, so that the cost flow in the system can be calculated. Three dimensionless coefficients, the Pauer factor, the loss coefficient and the cost factor, enable pinpointing cost intensive process units, allocating cost in cases of co-production and gaining insight for future design improvements. The methodology is demonstrated by a biomass gasification plant producing electricity, heat and cold. (C) 2004 Elsevier Ltd. All rights reserved.