All means of electricity generation, including photovoltaic (PV) systems, create polluting emissions when the entire life-cycle is taken into account. In the case of PV systems, those emissions are concentrated in the manufacturing stage. PV manufacturing is energy intensive, resulting in the emissions that accompany the use of standard grid electricity. The energy balance of a PV system is expressed by the Energy Pay-Back Time (EPBT), which is the time it takes for the PV system to generate the amount of energy equal to that used in its production.
A new paper by M. Vasilis, V. Fthenakis, H.C. Kim and E. Alsema, published in the January 2008 Environmental Science & Technology, finds yet again that PV technologies generate far less life-cycle atmospheric emissions per GWh than conventional fossil-fuel generation technologies. It states that at least 89% of the harmful emissions into the atmosphere could be prevented if conventional grid electricity was to be replaced by photovoltaic electricity. According to this paper, the EPBT of a PV system varies between 1 and 6 years. Two years ago, a comparable literature study by the Energy Bulletin reported EPBTs between 2 and 8 years.
The first part of the Environmental Science & Technology paper tackled the question of EPBT and Greenhouse Gas (GHG) emissions of PV systems. The larger the energy yield of the PV system, the faster the energy consumed during its manufacturing phase is gained back, so obviously the EPBT depends heavily upon the average insolation at a particular manufacturing site. The paper refers to four studies conducted on monochrystaline silicon PV panels in four different geographic regions:
The GHG emissions over the life-cycle of a PV panel are strongly related to the EPBT. They can mainly be allocated to the use of electrical energy during the manufacture of PV panels. Consequently, those emissions differ for the same PV panel according to the energy mix that is used for generating electricity in that particular location.
The findings in the Environmental Science & Technology paper were calculated with three different energy mixes and for four different types of PV panels: multichrystaline silicon (Multi-Si), monochrystaline silicon (mono-Si), ribbon silicon (ribbon-Si) and thin film cadmium telluride (CdTe). In the UCTE energy mix, the CO2 emissions vary between 21 g CO2-eq/kWh for the thin film CdTe to 43 g CO2-eq/kWh for Mono-Si.
The Thin Film CdTe panel clearly demonstrates the best results, but differences between PV systems are small in comparison with the difference of PV systems and conventional fossil-fuel based generation. The UCTE average CO2 emission for power generation is 470 g CO2-eq/kWh.
The study not only takes GHG emissions over the life-cycle into account, but heavy metal emissions as well. Heavy metals are emitted directly during the manufacturing process of PV systems, or via the use of grid electricity during the manufacturing process. Here again thin-film CdTe PV panels present the best results, even for cadmium emissions. This type of PV cell requires much less electrical energy for its manufacture, so it produces fewer heavy metal emissions attributed to the use of grid electricity. This lower energy consumption more than compensates for the higher direct cadmium emissions occurring during its manufacturing process.
The above conclusions describe the picture with state-of-technology over the last five years, but should not be interpreted as final. The trend in the environmental impact of PV manufacturing is decreasing even further and the energy efficiencies are increasing. As a result, the EPBT and the life-cycle environmental profile of PV panels can be expected to continue to improve in the upcoming years.
The paper also considered the future possibility of a 'PV breeder' scenario, in which a large part of the electrical energy used in PV manufacturing is generated by PV panels. Such a scenario would cut the current GHG emissions of PV life-cycles more or less in half.
A last consideration in the paper is that a future high penetration of PV energy on the grid would require altering the grid concept and structure. It is difficult to predict whether these changes would have a positive or a negative impact on the emissions, but it would in each case have to be taken into account in future life-cycle analyses of PV systems.Log in to post comments