Using Exergy-Based Metrics in Assessing Sustainability of Fossil-Fueled Thermal Energy Systems

This chapter examines the importance of exergy-based parameters like exergy efficiency, environmental compatibility, sustainability index, depletion number, and improvement potential of hydrocarbon fuel utilization. The main import of system exergy efficiency is relatively well-known. A hydrocarbon fuel environmental compatibility (ζ) evaluates the fuel exergy performance when its combustion gases emission abatement exergy is factored in. A fuel with low emission abatement exergy has a high environmental compatibility and, thus, high sustainability. Another metric is the depletion number, Dp. This measures the rate of fuel exergy destruction with respect to the fuel input exergy. Since fuel exergy flow is directly related to its material flow, its exergy destruction is similarly directly related to its material depletion. Hence, fuel utilization sustainability necessitates a low Dp. Dp indicates the fraction of input energy resources degraded through entropy creation, turning them into thermodynamic states of no useful energy values. The sustainability index is the reciprocal of Dp. The Improvement Potential (IP) is, mathematically, the product of the square of Dp and the fuel input exergy. When IP is high, it means the exergy losses are too high and there is a big room for exergy efficiency improvement.