Environmental Impacts of Photovoltaics

Unlike fossil fuel based technologies, solar power does not lead to any harmful emissions during operation, but the production of the panels leads to some amount of pollution. This is often referred to as the energy input to output ratio. In some analysis, if the energy input to produce it is higher than the output it produces it can be considered environmentally more harmful than beneficial. Also, placement of photovoltaics affects the environment. If they are located where photosynthesizing plants would normally grow, they simply substitute one potentially renewable resource (biomass) for another. It should be noted, however, that the biomass cycle converts solar radiation energy to electrical energy with significantly less efficiency than photovoltaic cells alone. And if they are placed on the sides of buildings (such as in Manchester) or fences, or rooftops (as long as plants would not normally be placed there), or in the desert they are purely additive to the renewable power base.

Greenhouse gases

Life cycle greenhouse gas emissions are now in the range of 25-32 g/kWh and this could decrease to 15 g/kWh in the future.^[76] For comparison, a combined cycle gas-fired power plant emits some 400 g/kWh and a coal-fired power plant 915 g/kWh and with carbon capture and storage some 200 g/kWh. Only nuclear power and wind are better, emitting 6-25 g/kWh and 11g/kWh on average. Using renewable energy sources in manufacturing and transportation would further drop photovoltaic emissions.

Cadmium

One issue that has often raised concerns is the use of cadmium in Cadmium telluride (CdTe) modules (CdTe is only used in a few types of PV panels). Cadmium in its metallic form is a toxic substance that has the tendency to accumulate in ecological food chains. The amount of cadmium used in thin-film PV modules is relatively small (5-10 g/m²) and with proper emission control techniques in place the cadmium emissions from module production can be almost zero. Current PV technologies lead to cadmium emissions of 0.3-0.9 microgram/kWh over the whole life-cycle.^[76] Most of these emissions actually arise through the use of coal power for the manufacturing of the modules, and coal and lignite combustion leads to much higher emissions of cadmium. Life-cycle cadmium emissions from coal is 3.1 microgram/kWh, lignite 6.2, and natural gas 0.2 microgram/kWh.

Note that if electricity produced by photovoltaic panels were used to manufacture the modules instead of electricity from burning coal, cadmium emissions from coal power usage in the manufacturing process could be entirely eliminated.

Energy Payback Time and Energy Returned on Energy Invested

The energy payback time is the time required to produce an amount of energy as great as what was consumed during production. The energy payback time is determined from a life cycle analysis of energy.

Another key indicator of environmental performance, tightly related to the energy payback time, is the ratio of electricity generated divided by the energy required to build and maintain the equipment. This ratio is called the energy returned on energy invested (EROEI). Of course, little is gained if it takes as much energy to produce the modules as they produce in their lifetimes. This should not be confused with the economic return on investment, which varies according to local energy prices, subsidies available and metering techniques.

Life-cycle analyses of the energy intensity of typical solar photovoltaic technologies in present use today find that the typical energy payback time at present is around 7 years. Mounting and installation of the system adds a further 1 to 4 years, depending upon whether it is on a roof or in an open field. This gives a total energy payback time for a PV system of 8 to 11 years.^[77]

Future PV panels that use thin films of crystalline silicon or other materials will have greatly reduced energy payback times. Such panels will be required if cost targets for large-scale production are to be met. The expected energy payback time will be in the vicinity of two years.

Thin film technologies now have energy pay-back times in the range of 1-1.5 years (S.Europe).^[76] With lifetimes of such systems of at least 30 years, the EROEI is in the range of 10 to 30. They thus generate enough energy over their lifetimes to reproduce themselves many times (6-31 reproductions, the EROEI is a bit lower) depending on what type of material, balance of system (or BOS), and the geographic location of the system.^[78]

References

^ ^a ^b ^c Alsema, E.A.; Wild - Scholten, M.J. de; Fthenakis, V.M. Environmental impacts of PV electricity generation - a critical comparison of energy supply options ECN, September 2006; 7p. Presented at the 21st European Photovoltaic Solar Energy Conference and Exhibition, Dresden, Germany, 4-8 September 2006.

^ Andrew Blakers and Klaus Weber, “The Energy Intensity of Photovoltaic Systems”, Centre for Sustainable Energy Systems, Australian National University, 2000.

^ Joshua Pearce and Andrew Lau, “Net Energy Analysis For Sustainable Energy Production From Silicon Based Solar Cells”, Proceedings of American Society of Mechanical Engineers Solar 2002: Sunrise on the Reliable Energy Economy, editor R. Campbell-Howe, 2002.

This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Photovoltaics"