Engineering design in power generation or conversion has typically focused on minimizing exergy losses (entropy gains), or maximizing economic returns. A mathematical model for maximizing resource renewability is proposed. A life-cycle metric describing the replacement time frame of the primary exergy source is presented. When coupled, the two metrics together partially assess the overall sustainability of the design. Release of CO2 from fossil fuel combustion may require anywhere between 80 and 200 yr to sequester from the atmosphere by natural processes such as photosynthesis. Efforts in these areas have yielded some means of quantifying fuel sustainability, but they have more often led to illogical or incomprehensive results. This research aims at taking the next step toward developing a widely accepted and universally applicable metric for the thermodynamic renewability of a fuel. Future work will be directed towards application of the model proposed to the problem of bio-based transportation fuels, corn ethanol, and soy biodiesel in particular, as substitutes for petroleum-based resources. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 10/30/2005-11/4/2005).