CAPÍTULO V PROPUESTA PARA LA REDUCCIÓN DE DESPERDICIO
5.4 Propuesta del Diagrama de Procesos Futuro
Another LCA is implemented to compare 5 different PV modules technologies in a large-scale PVGCS. The five PV module technologies are the most commercialized ones: m-Si, p-Si, a-Si, CdTe and CIS. In this example all the components of a PV system (PV modules, BOS components and the mounting system) are analysed. The procedure followed in this example as well as the resulting flow inventory will provide the basis for the integration and evaluation of the environmental aspect into the main model for the dimensioning of a large-scale PV system.
A guide published by International Energy Agency (V. Fthenakis et al., 2011) for LCA of PV system determines four main aspects that must be taken into consideration:
Technical characteristics related to PV systems. The life expectancy of PV components and systems is not the same. e.g. 20-30 years for PV modules or 10 year for AC/DC inverter.
Depending on the goal of the study, the irradiation collected by modules or their degradation can be important.
LCI/LCA modelling aspects. The appropriate system model depends on the goal of the LCA.
It can have a short-term prospective as for the choice of a PV electricity-supplier or comparisons between PV systems or electricity-generating technologies; or long-term
prospective as for comparison of future PV systems or of future electricity-generating technologies. The electricity mix must be considered carefully as well as the reference flow to enable comparisons. The reference flow can be expressed in kWh electricity produced (used for comparing PV technologies and modules), m² of module or kWp rated power.
Discuss and interpretation of results. Beside the impact indicators used in LCIA it may be helpful use another indicator as energy payback time (EPBT)
Reporting and communication. Some key parameters must be kept in mind : irradiance level and location; PV module efficiency; type of mounting system; components expected lifetime;
system boundaries; technical and modelling assumptions; LCA tool and database used.
Goal and scope definition 2.7.1
This LCA study aims at comparing the environmental impact associated with electricity production with different PVGCS configurations using the 5 most commercial PV module technologies. As in previous LCA study, the functional unit is fixed as energy demand. It concerns here the demand of 5 MWh that must be supplied by the PV power plant each year during 20 years. System boundaries are represented in Figure 2-32. They include the manufacturing of core infrastructures (modules, mounting system, cabling, and AC/DC inverter), the plant installation (excavation and mounting system) and the energy generation for a 20 year period (including component replacement).
Recycling processes of the different components of PVGCS are not included in this study due to lack of reliable information for all PV modules technologies evaluated. The different recycling processes currently implemented for PV modules and their implementation in LCA will be discussed in Chapter 6.
Technology assumptions, LCI and data collection 2.7.2
A yearly irradiation 1200 kWh/m2 on an inclined plane (30°) is considered. A fixed-mounting system with aluminium supports is used. A 10-years lifetime is considered for AC/DC inverter and 20-year life time is considered for the other components. The conversion efficiency of PV modules is constant over time and is based on the characteristic given by PV modules contained at Ecoinvet database.
energy generation installed PVGCS
mounting system BOS components
energy distribution
electric utility components recycling
process
PV module manufacturing
system boundaries
Figure 2-32 Boundaries of the system examined to compare different PV technologies
2. Life Cycle Assessment (LCA) for PV systems 67
The five PV modules are found in Ecoinvent database. The datasheets contain the input and output flows in order to calculate the total emission flows. The reason why these PV modules were used was the lack of data for material and energy flow found in the literature for manufacturing process of thin-film modules. Ecoinvent database is constantly under improvement to become a more accurate tool for create the LCI of total emission for a given process or product.
The 2.5kW inverter is selected from Ecoinvent database for the five PV installations. The number of PV modules and DC / AC inverters needed to supply the energy demanded during the evaluation period as well as the main features are summarized in Table 2-15. The calculation was made taking into account the amount of irradiation received, the PV module efficiency as well as the electrical characteristics of the DC / AC inverter.
From the values shown in the Table 2-15 is noticed that the PV modules based on thin film technologies require a greater number of panels due to the low efficiency they have.
LCI of each technology under evaluation was performed from data displayed at the Table 2-15. The procedure followed was similar to the two last examples.
LCIA results and interpretation 2.7.3
IMPACT 2002+ method was applied for evaluating the environmental impacts. The characterization scores were obtained as in previous examples. The environmental assessment was carried out both in the main midpoint impact categories as in the four damage categories. The total score for each category was separate in order to compare the different elements that compose a PVGCS. Figure 2-33 shows the normalized results.
Looking at the total score for each of the five configurations in all categories shows that the highest environmental impacts in almost every category midpoint are obtained when a-Si PV modules are used. Only into the categories related with climate change and resources m-Si PV module configuration has the highest impacts.
A more detailed analysis, focusing into the components of a PVGCS, shows that the most influential process is PV module manufacturing for the total impact scores in all the categories. Among the five PV technologies under analysis, m-Si PV module leads in almost all the categories. CdTe PV modules had interesting results: it has the lowest total scores.
Table 2-15 Key features for LCA study
Module technology m-Si p-Si a-Si CdTe CIS Module efficiency (%) 14.00 13.2 6.45 9.00 10.00
Module surface (m²) 1.46 1.46 1.10 0.72 0.72
Module life expectancy
(years) 20 20 20 20 20
No. PV modules 22 24 58 62 69
No. AC/DC inverters 4 6 6 4 6
(a) Midpoint categories
(b) Damage categories
Figure 2-33 Normalized results for the five PV installation considering 1200 kWh/m2 yr of irradiation on an inclined plane (30°). IMPACT 2002+
The LCI as well as the procedure followed in this example will be used in the latter chapter when the evaluation of environmental impacts will be included and taken into consideration as criteria for the design of large-scale PV power plants.
2.8 Conclusion
The analysis of the manufacturing processes of the five main modules PV technology highlights that it is necessary to quantify the embodied primary energy required for their manufacture, especially with c-Si based technologies to guarantee the sustainable nature of a technology. An environmental assessment is required to confirm that, indeed, PV systems really help to reduce and / or prevent pollution. There are many techniques developed for environmental assessment and among them Life
2. Life Cycle Assessment (LCA) for PV systems 69
Cycle Assessment (LCA) which is particularly interesting for energy production. Three cases have been tackled with LCA. The results show that the PV modules are the elements of a PVGCS that contribute most to the overall environmental impacts. Another aspect to mention is the influence of the composition of the electricity mix in the assessment of environmental impacts generated by a PV module.
This discussion reinforces the need for a multi-criteria study that allows establishing a methodology that conciliates both the technical-economic and environmental criteria. The procedure of LCA applied in a PVGCS will serve to integrate the environment criterion into the proposed study. To our knowledge, this kind of approach has not yet been implemented. The classical LCA tools (SimaPro and other LCA software) are generally not flexible and do not exchange with other programs. From a practical viewpoint, a specific environmental module was designed from extraction of the dedicated EcoInvent database that is used for PV systems.
It must be highlighted that this kind of study has been extended to other kinds of solar systems to compare two heliostat configurations (autonomous and classical heliostats) for heliothermodynamic power plants for concentrated solar power. This research was conducted within the OSSOLEMIO project in collaboration with the Laboratoire Plasma et Conversion d'Energie (LAPLACE) in the framework of Alaric Maintenon’s PhD thesis (Montenon, 2013), under the supervision of Prof. Pascal Maussion.
The results indicate that even if variation between the two configurations is not so high at design stage, the electrical grid heliostat generates the most important impacts to the environment after 20 years of operation. The energy supplied for operating the grid-connected heliostat is the main element that affects the different categories analyzed in LCA. It also depends on the energy mix of the country in which the power station will be built. This work was presented at First International Conference on Renewable Energies and Vehicular Technology (REVET) (see Appendix A).