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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. 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. However, 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. For comparison, a combined cycle gas-fired power plant emits some 400 g/kWh and a coal-fired power plant with carbon capture and storage some 200 g/kWh. Nuclear power emits 25 g/kWh on average; only wind power is better with a mere 11 g/kWh.
Cadmium : One issue that has often raised concerns is the use of cadmium in Cadmium telluride (CdTe) modules (not all PV technologies use CdTe). 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 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. 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.
Energy return on investment : A key indicator of environmental performance is the ratio of electricity generated divided by the energy required to build and maintain the equipment. Of course, little is gained if it takes as much energy to produce the modules as they produce in their lifetimes. This ratio is called the energy return on investment (EROI) This should not be confused with the economic return on investment, which varies according to local energy prices, subsidies available and metering techniques. A related concept is the energy pay-back time, i.e. the time required to produce an amount of energy as great as what was consumed during production.
Crystalline silicon PV systems presently have energy pay-back times of 1.5-2 years for South-European locations and 2.7-3.5 years for Middle-European locations. For silicon technology clear prospects for a reduction of energy input exist, and an energy pay-back of 1 year may be possible within a few years. Thin film technologies now have energy pay-back times in the range of 1-1.5 years (S.Europe). With lifetimes of such systems of at least 30 years, the EROI is in the range of 10 to 30.
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