Photovoltaic systems now have energy pay back times as low as 1.7 years!

Photovoltaic module production has shown a tremendous increase, with shipments of PV modules accelerating from approx. 80 MWp in 1995 to approx. 1700 MWp in 2005. With such impressive growth figure, and in view of the very large potential of photovoltaics as a renewable energy source, an analysis of present and future environmental performances of photovoltaic systems becomes increasingly important. This is typically carried out using the method of Life Cycle Assessment* (LCA).

LCA studies on PV have been published but these were mainly based on PV technologies from the late '80s. Since then, the PV industry has implemented many innovations in the field of solar cell and module technology. An update was therefore urgently needed. The Copernicus Institute (University of Utrecht) and ECN Solar Energy (Petten) recently concluded a LCA study on crystalline silicon PV. Three mainstream Si technologies were analyzed: mono-, multi- and ribbon-crystalline silicon. Together, these PV technologies covered approx. 94 percent of the 2004 world market. The results of this study are thus representative for most of today's PV technology.

The study was carried out in the framework of the European Integrated Project CrystalClear, which aims to improve solar cells based on crystalline silicon. It attracted much enthusiasm since they were the result of a unique collaboration with 11 PV companies in Europe and USA. For the first time, up-to-date details about materials and processing steps were used over the whole production chain of PV systems.

Figure 1: Production for three types of crystalline silicon solar cells and the "system boundary" of the new Life Cycle Assessment study. Note that in the "ribbon" technology silicon wafers are pulled directly from molten silicon, without sawing, thus avoiding significant material losses.

The present LCA study focused on the determination of the Energy Payback Time and lifecycle greenhouse gas emissions for today's PV systems. These parameters are particularly important since the energy demand for cell and module production is found to be the dominant factor for the environmental impact of crystalline silicon PV.

The energy payback times were calculated for grid-connected roof-top installed PV-systems in two regions, South- and Middle Europe with annual solar irradiation of 1700 and 1000 kWh per square meter, respectively. The calculated EPBT's range from between 1.7 and 4.6 years, the exact figure depending on the annual irradiation and the type of silicon technology. This is already much smaller than the lifetime of PV modules, which is 30 years or more.

In figure 2 the life cycle CO2 emissions for various energy technologies are compared (in g-CO2/kWh produced). As expected, PV performs very well in comparison with fossil fuel based technologies, but further improvements are still needed to compare PV with, e.g., wind energy.

Figure 2: Greenhouse gas emissions of PV systems based on three silicon technologies compared to a number of other energy technologies. N.B. The emission from a coal-fired power plant (1000 g/kWh) exceeds the Y-axis maximum!
Sources: Coal, CC gas, nuclear, biomass and wind data derived from the Ecoinvent database

The environmental performance of PV can be further improved by increasing the module efficiency to 16 percent, reducing the wafer thickness down to 150 µm, and by switching to a new production process for polysilicon (based on Fluidized Bed Reactor technology). For this particular case, an EPBT of approx. 1.0 year and a life cycle CO2 emission of 20 g/kWh was calculated for multicrystalline Si PV module when installed in S-Europe. In view of the present state-of-the-art on crystalline silicon PV, we can expect such performance within the next 3 years.

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Mariska J. de Wild-Scholten
ECN Solar Energy

Erik Alsema
Copernicus Institute, University of Utrecht

* In a Life Cycle Assessment study the environmental impacts of a product are evaluated by making an inventory of all energy and material inputs, as well as the emissions to the environment. Typically, a “cradle-to-grave” approach is used, i.e. the complete life cycle of a product from resource mining to waste treatment is taken into consideration. For PV systems the product use phase has negligible impacts and recycling processes only exist at pilot scale. Therefore these two stages have been omitted from the present study.
The Energy Pay-Back Time indicates the number of years the PV system has to generate electricity in order to compensate the energy invested during production of the system components. The Life-Cycle Greenhouse Gas Emission of a PV system is calculated by determining the total emission of greenhouse gases over the PV system's life cycle (i.e. mainly from component production) and dividing this by the total amount of electricity generated by the PV system over its life time. The life cycle greenhouse gas emission indicator can be compared between energy technologies to determine their potential contribution to greenhouse gas mitigation. The greenhouse gas emission of the present electricity supply system in the Netherlands is 570 g/kWh.