4. Environmental life cycle analysis and exergy efficiency of cooling systems

Life Cycle Analysis (LCA) is an important tool to analyze environmental problems associated with the production, use, and disposal of products or systems [14]. For every product produced within a system the total inflow and outflow of energy and materials are evaluated. The 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 indirect exergy, E\_ ind. When the ejector refrigeration system driven by solar energy is put into operation, it starts to deliver a product (cold in this case) with exergy, E\_ pr. By considering renewable 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 becomes E\_ net = E\_ pr � <sup>E</sup>\_ ind. The higher this value is for the three time periods defined above, the 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]. This in turn means that the net produced exergy E\_ net increases too. Thus, the evaluation of ηtr of an ejector, as presented in Section 3.5, and its subsequent maximization, may lead to the construction and operation of more sustainable solar driven refrigeration plants.

of two thermodynamic metrics: exergy produced and exergy consumed. Their ratio represents the exergy efficiency; the difference between exergy consumed and exergy produced equals the exergy losses within the process. The phenomenological significance of the transiting exergy and the way in which it can be computed for processes below and across T0 has been illustrated for the cases of an expansion valve, a cryo-expander, a vortex tube, an adiabatic compressor, and a monophasic ejector. The input-output exergy efficiency is not an appropri-

Exergy Flows Inside Expansion and Compression Devices Operating below and across Ambient Temperature

http://dx.doi.org/10.5772/intechopen.74041

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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

ate criterion for evaluation of these processes.

Research Center of Natural Resources Canada (RDCPJ451917-13).

Acknowledgements

Nomenclature

E\_ Exergy, (kW)

H\_ Enthalpy, (kW)

Q\_ Heat rate, (kW)

Greek symbols η Efficiency, (%) ∇ Consumption

Δ Production

D\_ Destroyed exergy, (kW)

e Specific exergy, (kJ/kg)

h Specific enthalpy, (kJ/kg)

ṁ (Total) Mass flowrate, (kg/s)

s Specific entropy, (kJ/kg K)

W\_ Mechanical power, (kW)

P Pressure, (MPa, kPa)

T Temperature, (K, C)

d Specific exergy losses, (kJ/kg)
