How to manage inputs from joint production processes in emergy accounting.

In life-cycle assessments it is a challenge to allocate resource use and environmental impact in processes with multiple outputs. This is especially the case when systems include agricultural products that in their production cannot be separated from each other. For emergy accounting, Bastianoni and Marchettini (2000) suggested how to calculate transformities and other indices for joint production systems.
Their proposals however, do not include how to manage inputs from joint production systems. Thus a practical method for making systems with inputs from joint production processes comparable with systems that do not depend on joint production processes is still lacking. As a consequence, a product relying on inputs from joint production processes appears to compete poorly with a similar product that does not have to account for by-products appearing upstream. This is counter to perceived benefits of integrated production systems.
Inspired by the ISO 14040 standard for LCA, this paper investigates two approaches to making systems comparable: system expansion and allocation. The methods are exemplified by comparing willow production for combined heat and power (CHP), where pig manure is used for nutrient application, with CHP based on fossil fuel. Implications of applying the different approaches to making the systems comparable are discussed. Using the first approach, the systems are expanded to generate the same services namely electricity, heat, pigs, and nutrients. Applying the second approach, the bioenergy system is reduced by allocating the emergy flows of pig production between pigs and manure. Since the allocation basis often significantly influences the outcome of an analysis, allocations based on energy content, weight, nutrient content, and market value, respectively, are compared to gain perspective.