Evaluation of sustainable useful index (SUI) by fuzzy approach for energy producing processes

The present paper proposes a fuzzy procedure for evaluating and scoring the energy sustainability of a process. The fuzzy procedure permits to handle Energy Return on Investment (EROI) and Energy Payback Time (EPT) values as characterizing parameters for the energy sustainability evaluation. The procedure is articulated in three steps: a first screening among many processes is performed by using the Energy Sustainability Index (ESI), which takes into account the total Produced Energy referred respect to the direct spent energy (heat and electricity). The second step is the Analogical Model, i.e. the quantification of each chemicals, materials and energy to estimate the Useful Energy with a Life Cycle approach, taking into account all the Indirect Energy terms. The third step is the evaluation of two parameters: EROI and EPT, considering all the involved energy flows. Therefore, a fuzzy combination of EROI and EPT permits to evaluate a Sustainable Useful Index (SUI). This procedure is useful for local planning, for allocating financing resources for new energy processes exploitation and for selecting the most sustainable choice among several research programs. Here two cases-studies are reported and compared to highlight the approach: the Anaerobic Digestion producing CH4 using Organic Refuses and maize as Energy Crop.