Limited resources of fossil energy carriers and increasing problems, like climate change, pollution and population growth, require new solutions to provide a global energy supply system. One of the most important questions is, which primary energy carriers can and should be used. To ensure a sustainable supply, unintentional impacts and interactions are necessarily to be considered. Thus, full examinations with regard to technology assessment (TA) have to be made. The problem of such sustainable feasibility studies is characterized by the aggregation of many criteria of different character to achieve a one-dimensional expression for evaluation. It is especially difficult to consider uncertain knowledge because it resists a description with classical mathematics. In this paper I present a fuzzy logic based assessment method which can be used as a decision tool. It allows the usage of aggregated indicators to consider any impact on arbitrary features. The method is applied to energy conversion technologies taking into account entire environmental aspects. If the assessment criteria are chosen for sustainability the result is a technology using renewable energy carriers.