The problem of co-production in environmental accounting by emergy analysis

This paper deals with the application of emergy analysis to systems with co-productions. Emergy is a measure of how much work the biosphere has done in order to provide a product, in terms of solar energy joules. For this reason, especially by means of the emergy-based indices, emergy analysis is a one of the key functions for assessing sustainability. However the application of emergy analysis to processes with co-production can be misleading. Usually some inputs have to be added in order to obtain a 'useful' product from a by-product. This would lead to penalizing a co-production with respect to two (or more) independent processes, since the fact of having another output is neglected in calculations. To have a better comparison, we introduce the concept of joint transformity and the weighted average of the transformities. The same reasoning is applied to other emergy-based indices. These new indexes should be added to the set of the existing ones to enlarge the possibilities of application. A case study of a dairy farm in Puerto Rico was analyzed from this viewpoint, showing that co-production of milk and methane (or electricity) is more efficient and less impacting on the environment with respect to separate productions, while two separate processes would be more effective in using local resources. (C) 2000 Elsevier Science B.V.