Acknowledgements

environmental burdens are associated by quantifying the energy and materials used, as well as the wastes released into the environment. The impact of these uses and releases on the environment is assessed. The multidimensional approach of LCA causes some problems when different substances need to be compared and general agreement is required. This problem may be avoided if exergy is used as a common quantity as proposed by Life Cycle Exergy Analysis [15]. The crucial idea behind this method is the distinction between renewable and non-renewable resources. In order to illustrate the method, let us consider three defined time periods within the life cycle of an ejector refrigeration system driven by solar energy [16]. At first, exergy is required during the construction stage to build the plant and put it into operation. During this period the spent exergy is stored in materials, such as metals, glass etc. For the second period, maintenance required for the system's operation takes place. Exergy necessary for this maintenance is evaluated. The third period is the clean-up stage, including the plant demolition and the recycling of materials. Exergy used for the clean-up is assessed. The exergy used for the construction, maintenance, and clean-up is assumed to originate from non-renewable resources and is named

ind. When the ejector refrigeration system driven by solar energy is put into oper-

ind. The higher this value is for the three time periods defined above, the

resources (solar in this case) as free, there will be a net exergy output from the plant until the plant is decommissioned. By considering the total life cycle of the plant the net produced exergy

more sustainable the system is, because the input of non-renewable resources will be paid back during the system's lifetime. The rise in exergy efficiency of an ejector calculated according to Eqs. (31) and (32) leads to an increase in efficiency of the solar driven refrigeration system [16].

an ejector, as presented in Section 3.5, and its subsequent maximization, may lead to the con-

The common feature of expansion processes operating below or across ambient temperature is the partial transformation of the mechanical exergy component into the thermal exergy component. Sub-ambient compression processes are characterized by the transformation of work into the mechanical exergy component and the partial destruction of the thermal exergy component below T0. In order to evaluate the efficiency of these transformations the calculations of the variation in mechanical and thermal exergy components are required. These calculations may be done in many different ways, for example the variation in eP depends on the chosen temperature conditions, while the variation in eT depends on the chosen pressure conditions. This multiplicity in the exergy variation evaluation leads to ambiguity in the exergy efficiency definition. The approach based on the exclusion of the "transiting flow" from thermo-mechanical inlet and outlet exergies of an analyzed process overcomes this difficulty. This improvement is possible because the transiting exergy is uniquely defined by a specific combination of the process intensive parameters, namely the inlet and outlet pressures and temperatures, as well as T0. The transiting exergy approach allows non-ambiguous evaluation

struction and operation of more sustainable solar driven refrigeration plants.

pr. By considering renewable

net increases too. Thus, the evaluation of ηtr of

ation, it starts to deliver a product (cold in this case) with exergy, E\_

indirect exergy, E\_

78 Energy Systems and Environment

5. Conclusion

net = E\_

pr � <sup>E</sup>\_

This in turn means that the net produced exergy E\_

becomes E\_

This project is a part of the Collaborative Research and Development (CRD) Grants Program at "Université de Sherbrooke". The authors acknowledge the support of the Natural Sciences and Engineering Research Council of Canada, Hydro Québec, Rio Tinto Alcan and CanmetENERGY Research Center of Natural Resources Canada (RDCPJ451917-13).
