## Life cycle analysis

Nieves Espinosa Martinez

With regard to energy systems, many countries have inducted new policies to widespread the installation of new renewable source plants in order to reach the Kyoto protocol benchmarks. Many projects focusing at the reduction of emissions are being planned to fulfil the more and more restrictive environmental laws, and often a specific mention to photovoltaic plants is reported.

Life cycle assessment (LCA) is a method for analyzing various aspects involved with the development of a product and its potential effect throughout the life of the product. The application of LCA is useful to compare different products and systems, or different materials production or recycling methods. LCA can be used as a tool to detect potential for improvements with the objective to minimize impact on human health, environment and resource depletion.

The realization of the assessment under ISO standards ISO 14040, International Organisation for Standardisation (ISO), Geneva, Switzerland, 1997 includes the definition of goal and scope, inventory analysis, impact assessment and interpretation of results as shown in Figure 1. The goal and scope describes the underlying question (objective), the system, its boundaries and the definition of a functional unit. The flows of pollutants, materials, resources are gathered in the inventory analysis phase. These elementary flows (emissions, resource consumption, etc.) are characterized and aggregated for various environmental problems in impact assessment stage and finally conclusions are drawn in interpretation step. The framework of LCA methodology is shown in Figure 1.

Such a life cycle analysis gains accuracy only by having complete detail of the entire process. The OPV case has been one example of technology ecodesign since 2010. The validity of the results resides in having primary data on the inputs and outputs in the fabrication of organic solar modules. A complete material inventory of all the needs is the basis to build up the energy inventory, and from there to estimate the environmental impacts as illustrated in Figure 2.

Figure 2. Life cycle stages of a product and evaluation of relevant impacts associated with LCA inputs and releases.

### The embodied energy parameter

One of the central parameters used to view and compare an energy technology is how efficient it is at converting the source energy into the form of energy that we would like to use (i.e., the conversion of sunlight, wind or wave energy into electrical energy).

Energy is needed to create energy systems in the extraction and processing of raw materials, in the manufacture of finished products and components, in the construction phase, and in the transport of materials/products to site. We can draw boundaries and consider some or all life cycle stages, and assess the inputs and outputs that only cross that boundaries; this is at the heart of LCA (Figure 2).

Using the energy embedded, aka cumulative energy demand, as a parameter in the context of polymer solar cells brings a new dimension from where to look at them. It helps to generate a framework for future decision-making and it is highly appropriate for evaluating the energy investment made in an energy system. So we are in a position to compare the efficacy of a given technology in terms of how quickly it conquers back the energy invested in its making. Polymer solar cells outperform all other PV technologies even on a laboratory scale described. This measure is known as the energy pay-back time (EPBT).

$$EPBT = \frac{{\sum {{E_{inputs}}} }}{{{E_{gen}}/year}}$$

The inverse number of EPBT is the energy return factor (ERF), which expresses the number of times the system pays back the energy invested in it. There are embedded in this parameter a series of considerations, like the isolation, the performance ratio, the degradation and the inclusion of the rest of components of the installation –inverter, cabling, structure, etc.

DOI:10.1016/j.solmat.2010.08.020

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