Comparative analysis of different cost allocation methodologies in LCA for cogeneration systems

Abstract

When analysing multi-product systems, the allocation of resources and waste in a Life Cycle Assessment presents a difficulty. This can be solved through thermoeconomic approaches. Thus, the objective of the present work was to carry out a comparative analysis between traditional methods, recommended by ISO 14000, and thermoeconomic methods in the Life Cycle Assessment of a cogeneration system composed of a gas microturbine and lithium bromide–water absorption systems. First, the system's total emissions were calculated, and then the allocation of resources was carried out. This allocation was achieved in three ways: traditional allocation, thermoeconomic allocation, and allocation through exergoenvironmental analysis. To carry out the last two allocations, the exergetic costs were calculated considering the physical structure and the productive structure of the production flows (using four different exergy breakdown models,). And with the result of the exergetic costs, the environmental impacts (environmental costs) were calculated through the thermoeconomic allocation and the exergoenvironmental analysis. The environmental impacts (environmental costs) found with allocations that use thermoeconomics were compared with those obtained by the Life Cycle Assessment with the traditional allocation (energetic and exergetic). Thus, it was found that the results obtained by the energy allocation and the allocation that uses the thermoeconomic model of total exergy have the highest environmental impact over the cold production. It was also found that there is no difference between using an exergetic allocation or thermoeconomic allocation if the system does not have many dissipative components, such as valves, and there is no need to know the influence of the components on the final environmental impact since both perform the same calculations with very few modifications. It was concluded that, when performing a thermoeconomic analysis on a system with valves, one can use any of the models studied, depending on how detailed one wants the results of their environmental impact to be. On the other hand, if the system does not contain many dissipative elements or one does not need a detailed result, one should just make the traditional exergy allocation. Moreover, if it is necessary to know which component has the greatest environmental impact, the exergoenvironmental analysis should be used. Thus, this work contributes to a compilation of all the thermodynamic models used to date, and how they can be applied in a Life Cycle Assessment to generate more detailed results.