Along with economic return on investment, PV technology must achieve an adequate energy return on investment in order to qualify as a truly green technology. PV panels are made using a number of different processes under differing conditions, so there is no
one standard for measuring the energy input required per panel. Also, recouping the energy investment in embodied energy in the panel will depend on the location where it is installed, with sunny locations being more favorable than cloudy ones. Thus, there is not yet consensus on the value of PV energy return on investment or energy payback at this time. For conventional silicon technology, estimates vary, with numbers as low as 3 years and as high as 8 years reported. A study of building-integrated photovoltaic (BIPV) technology found that the difference in energy payback was nearly 2 years for a sampling of U. S. cities using this technology, ranging from Phoenix (3.4 years) to Detroit (5.1 years). Low energy consumption in manufacturing may provide an advantage for thin-film PV technology, with payback as low as 2 years anticipated for this technology, once it matures.
Another question is whether or not the possible toxic emissions from manufacturing PV panels and the material throughput from their disposal at the end of the lifetime undercuts the environmental benefit of reducing CO2 emissions through their installation and use. As with EROI, there is no one standard for PV production, and emissions data from manufacturing tied directly to units of panel output are not easily obtained, so it would be difficult to make any definitive statement about emissions per unit of PV capacity. Also, the productive capacity of PV panels produced worldwide each year at present is small relative to total world electricity consumption. For example, at an average of 100 W per panel, the 275 MW of capacity produced in 2000 from Fig. 10-1 would represent approximately 2.75 million panels manufactured worldwide, which is a small number relative to annual output of other mass-produced consumer products such as laptops or motor vehicles.
It is therefore not current emissions so much as future emissions that are of interest, were PV technology to grow into a major player. Future emissions are, however, difficult to predict. Because the technology is changing rapidly, projections about future total emissions or materials requirements based on today’s technology are not likely to be accurate. As an alternative to projecting toxic emissions or solid waste from a robust global PV industry in the future, we instead estimate an order-of-magnitude number for the volume of panels that would be turned over each year in such a scenario in Example 10-7.
Example 10-7 Suppose that at some future time several decades from now, PV panels have become the dominant producer of electricity in the world, so that they produce as much as all sources combined do currently, namely, 1.66 x 1013 kWh/year, equivalent to 2800 kWh for every one of the approximately 6 billion human beings currently living. The remaining electricity at that future time, assuming that total demand has grown in line with increasing population and average per capita wealth, is produced from some other complementary electricity source, most likely one that can generate electricity on demand, since PV is intermittent. Suppose that each panel lasts on average 50 years before requiring replacement, and that the PV technology produces on average 1200 kWh/year per 1 kW of capacity. How many panels must the world roll over each year in order to maintain the required electric output from PV panels?
Solution Based on the ratio of output-to-capacity, the required total capacity for 1.66 x 1013 kWh/year is 1.39 x 1010 kW. At an average of 100 W per panel, this amount is equivalent to 139 billion panels installed. At a lifetime of 50 years, one-fiftieth of the panels are replaced each year, or 2.8 billion panels per year. This quantity is around 1000 times as many panels as are produced currently.
The result of Example 10-7 is crude, but it does give a sense of what a robust PV future might entail. Large-scale manufacturing of PV panels of this magnitude would create a challenge for preventing escape of hazardous materials into the environment, unless they have been completely eliminated from the product. Also, the large volume of panels would either require safe disposal or else dismantling and recycling, which would not be a trivial task.
The actual future impact of the PV industry is of course sensitive to the assumptions made. For example, new technologies might emerge that are much more durable than the 50-year-lifetime projected, reducing throughput per year. Also, the PV industry might not grow as large as shown. New technologies may also emerge that take advantage of the photovoltaic effect without requiring a panel of the dimensions currently used, which might reduce the amount of material throughput. In conclusion, there are many uncertainties about the future growth potential, environmental benefits, and environmental burden associated with the production, use, and disposal of PV technology. If anything can be said with certainty at this time, it is that R&D aimed at reducing energy and materials requirements (and especially toxic materials such as heavy metals) per unit of PV generating capacity can only help to ensure that this technology, which today has an environment-friendly reputation, will not in the future become an environmental liability.
Photovoltaic technologies consist of a family of devices that take advantage of the photovoltaic effect to transform solar energy into electricity, without using a thermal working fluid or a mechanical conversion device such as a turbine. A PV panel is made up of many PV cells wired together to produce a rated power output in full sun; multiple panels can be joined in a PV array to produce a total output per month or year that matches the required demand of the system owner.
A PV cell produces current over a range of voltages from 0 V to the open-circuit voltage VOC. Because the cell does not produce current at V = VOC, the maximum power output from the cell occurs at some combination of maximum current IM and maximum voltage VM, where VM < VOC. Engineers design cells to have a fill factor (FF), or ratio of actual maximum power to theoretically possible maximum power if the cell were to produce the short-circuit current I at VOC, as close as possible to FF = 1.
In response to global concern about CO2 and pollutant emissions from fossil fuel combustion, as well as government incentives that promote PV systems, PV sales worldwide have been growing rapidly in recent years. Relatively high levelized cost per kWh as well as uncertainties about supplies of key materials may limit future growth of PV, so research activities are currently focusing on technologies that produce cells with lower material requirements and at a lower cost per watt of capacity.