Photovoltaic (PV) technologies have shown remarkable progress recently in terms of annual production capacity and life cycle environmental performances, which necessitate timely updates of environmental indicators. Based on PV production data of [2004][2005][2006], this study presents the life-cycle greenhouse gas emissions, criteria pollutant emissions, and heavy metal emissions from four types of major commercial PV systems: multicrystalline silicon, monocrystalline silicon, ribbon silicon, and thin-film cadmium telluride. Life-cycle emissions were determined by employing average electricity mixtures in Europe and the United States during the materials and module production for each PV system. Among the current vintage of PV technologies, thin-film cadmium telluride (CdTe) PV emits the least amount of harmful air emissions as it requires the least amount of energy during the module production. However, the differences in the emissions between different PV technologies are very small in comparison to the emissions from conventional energy technologies that PV could displace. As a part of prospective analysis, the effect of PV breeder was investigated. Overall, all PV technologies generate far less life-cycle air emissions per GWh than conventional fossil-fuelbased electricity generation technologies. At least 89% of air emissions associated with electricity generation could be prevented if electricity from photovoltaics displaces electricity from the grid.

Life cycle assessments and external cost estimates of photovoltaics have been often based on old data that do not reflect the extensive technological progress made over the past decade. Our assessment uses current (2004-early 2005) manufacturing data, from twelve European and US photovoltaic companies, and establishes the Energy Payback Times (EPBT), Greenhouse Gas (GHG) emissions and external environmental costs of current commercial PV technologies. Estimates of external costs are about 70% lower than those in recent high-impact publications which were derived from the old data. Copyright # 2006 John Wiley & Sons, Ltd. INTRODUCTIONI t is well understood that production of energy by burning of fossil fuels generates a number of pollutants and carbon dioxide. What is less known is that any anthropogenic means of energy production, including solar, generate pollutants when their entire life cycle is accounted for. A life cycle starts from the mining and processing of materials that comprise solar cells, modules and balance of system, and ends to their final decommissioning, disposal and/or recycling. Costs associated with the environmental, health and societal impacts that are not included in the direct cost of electricity, are called external costs of electricity production. While societal external costs are difficult to quantify, external costs associated with environmental and health protection or damage have been quantified in monetary terms. Perhaps the most well-known effort to quantify environmental and health damages due to electricity production, is the European Union's series of ExternE (External Costs of Energy) projects. The ExternE methodology starts from emissions generated at specific sources and follows their impact to receptors through atmospheric dispersion and dose-response functions. In general, this type of environmental impact assessment is well accepted, although assumptions related to Broader Perspectives the monetary valuation of estimated impacts, especially green-house related impacts, are debateable. 'The ExternE methodology has been applied in a large number of European and national studies to give advice for environmental, energy and transport policies.' 1 Photovoltaic installations in Germany were presented in the latest ExternE report to the European Commission 1 as having 30% higher health impacts than natural gas and GHG emissions of 180 g CO 2 -eq./kWh which would be 10 times higher than those for the nuclear fuel cycle (Figure 1). These results were based on 15-years old PV systems and even older data on module production technology. z Also based on outdated PV technology data a life cycle-based comparison of energy technologies in Australia 2 showed that PV emits about 100 g CO 2 /kWh during its life cycle (Figure 1). The results from these two studies were widely circulated and especially the ExternE publication with its official status is likely to have influenced policy decisions with regard to energy technology. More recent (i.e., 2000) data are included in the Ecoinvent...

The use of polymer materials for photovoltaic applications is expected to have several advantages over current crystalline silicon technology. In this paper, we perform an environmental and economic assessment of polymer-based thin film modules with a glass substrate and modules with a flexible substrate and we compare our results with literature data for multicrystalline (mc-) silicon photovoltaics and other types of PV. The functional unit of this study is ‘25 years of electricity production by PV systems with a power of 1 watt-peak (Wp)’. Because the lifetime of polymer photovoltaics is at present much lower than of mc-silicon photovoltaics, we first compared the PV cells per watt-peak and next determined the minimum required lifetime of polymer PV to arrive at the same environmental impacts as mc-silicon PV. We found that per watt-peak of output power, the environmental impacts compared to mc-silicon are 20–60% lower for polymer PV systems with glass substrate and 80–95% lower for polymer PV with PET as substrate (flexible modules). Also in comparison with thin film CuInSe and thin film silicon, the impacts of polymer modules, per watt-peak, appeared to be lower. The costs per watt-peak of polymer PV modules with glass substrate are approximately 20% higher compared to mc-silicon photovoltaics. However, taking into account uncertainties, this might be an overestimation. For flexible modules, no cost data were available. If the efficiency and lifetime of polymer PV modules increases, both glass-based and flexible polymer PV could become an environment friendly and cheap alternative to mc-silicon P

Measured and modelled JV characteristics of crystalline silicon cells below one sun intensity have been investigated. First, the JV characteristics were measured between 3 and 1000 W/m 2 at 6 light levels for 41 industrially produced mono-and multi-crystalline cells from 8 manufacturers, and at 29 intensity levels for a single multi-crystalline silicon between 0.01 and 1000 W/m 2. Based on this experimental data, the accuracy of the following four modelling approaches was evaluated: (1) empirical fill factor expressions, (2) a purely empirical function, (3) the one-diode model and (4) the two-diode model. Results show that the fill factor expressions and the empirical function fail at low light intensities, but a new empirical equation that gives accurate fits could be derived. The accuracy of both diode models are very high. However, the accuracy depends considerably on the used diode model parameter sets. While comparing different methods to determine diode model parameter sets, the two-diode model is found to be preferred in principle: particularly its capability in accurately modelling V OC and efficiency with one and the same parameter set makes the two-diode model superior. The simulated energy yields of the 41 commercial cells as a function of irradiance intensity suggest unbiased shunt resistances larger than about 10 kO cm 2 may help to avoid low energy yields of cells used under predominantly low light intensities. Such cells with diode currents not larger than about 10 À9 A/cm 2 are excellent candidates for Product Integrated PV (PIPV) appliances.

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The energy requirements for the production of PV modules and BOS components are analyzed in order to evaluate the energy pay‐back time and the CO2 emissions of grid‐connected PV systems. Both c‐Si and thin film module technologies are investigated. Assuming an irradiation of 1700 kWh/m2/yr the energy pay‐back time was found to be 2·5–3 years for present‐day roof‐top installations and 3–4 years for multi‐megawatt, ground‐mounted systems. The specific CO2 emission of the rooftop systems was calculated as 50–60 g/kWh now and possibly 20–30 g/kWh in the future. This leads to the conclusion that in the longer term grid‐connected PV systems can contribute significantly to the mitigation of CO2 emissions. Copyright © 2000 John Wiley & Sons, Ltd.

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Together with a number of PV companies an extensive effort has been made to collect Life Cycle Inventory data that represents the current status of production technology for crystalline silicon modules. The new data covers all processes from silicon feedstock production to cell and module manufacturing. All commercial wafer technologies are covered, that is multi- and monocrystalline wafers as well as ribbon technology. The presented data should be representative for the technology status in 2004, although for monocrystalline Si crystallisation further improvement of the data quality is recommended. On the basis of the new data it is shown that PV systems on the basis of c-Si technology are in a good position to compete with other energy technologies. Energy Pay-Back Times of 1.7-2.7 yr are found for South-European locations, while life-cycle CO2 emission is in the 30-46 g/kWh range. Clear perspectives exist for further improvements with roughly 40-50%.