Solar power, like all renewable technologies, is not free from adverse environmental impacts. The most comprehensive life-cycle assessment of these impacts in the UK was carried out last year by consultancy Environmental Resources Management on behalf of the DTI's Energy Technology Support Unit (ETSU).
ERM used life-cycle assessment (LCA) techniques to examine impacts from the production, use and disposal of PV systems - and to compare them with those from conventional power generation. Overall, it found, "PV is a more environmentally benign option than conventional means of generating electricity for urban areas, and could be adopted on a large scale if the environmental imperative becomes critical over the next decade."
The report focuses on conventional silicon and the more recently developed thin film cadmium telluride (CdTe) PV technologies, both of which were assumed to have an average lifespan of 20 years. All energy consumed in the manufacturing process was assumed to be produced by a generating mix comprising 65% coal, 1.5% gas, 22% nuclear and 9% oil. This mix was typical of the UK in the early 1990s. The strong growth in gas-fired generation means that in practice, emissions associated with production of PV cells - and those displaced by their operation - would now be significantly lower.
In almost all cases, the energy consumed in raw material extraction and cell manufacture dominated the overall environmental burdens of the various PV technologies. Purification of silicon is a particularly energy intensive process. As a result, silicon cells scored significantly worse than CdTe on resource depletion, global warming, human toxicity, acidification and eutrophication - even though the CdTe cells have a lower photovoltaic efficiency of 10%.
However, all PV systems were shown to have much lower releases to the environment from their fuel cycle than conventional power sources (see table ). The finding even holds true for cadmium emissions from CdTe cell manufacture.
According to ERM, the PV industry anticipates an increase in installed capacity in the UK to 50MWp by 2005. If this were to be achieved through CdTe technology, CO2 emissions would be reduced by 32,000 tonnes in each year of the panels' 20-year life. Savings from silicon-based technology would be marginally lower.
An alternative way of expressing the environmental benefits of PV is the "energy ratio" (see graph ). For example, CdTe cells generate about 15 times more energy during their life than is used in their manufacture - provided the electrodeposition process used by BP Solar is used rather than energy-intensive vacuum deposition.
The energy ratio of silicon cells is less impressive, but ERM says it could be improved significantly by a move to thin film technologies, more efficient production processes, reduced raw material inputs and less reliance on energy-intensive aluminium in PV modules. ERM expects that all these steps will occur as PV suppliers look to reduce manufacturing costs, and that more efficient thin film silicon cells - not as yet commercially available - will become the favoured economic option in the medium term.
However, ERM concluded that CdTe technologies suffer from a "significant environmental drawback" - their reliance on cadmium, a toxic metal. Even assuming 100% recycling of defunct CdTe modules, the technology has a somewhat higher ecotoxicity score than silicon-based systems. Total mass releases of cadmium would be less than from conventional generation - but are more likely to end up in watercourses where the metal is highly toxic to aquatic life.
If end-of-life CdTe cells are disposed of to landfill rather than being recycled, the ecotoxicity score increases by almost two orders of magnitude because of the potential production of contaminated leachate.
ERM notes that the PV industry intends to recycle CdTe cells in industrialised nations. However, it warns that the benefits of PV electrification in developing countries "may be offset by the unmanaged disposal of these cells to rubbish tips and eventual releases of cadmium into local water tables." Risks to human health may also arise in industrialised countries if fires in buildings clad with CdTe modules result in releases of airborne cadmium particles.
Dipesh Shah, BP Solar's Managing Director, takes issue with the ERM study on the cadmium issue. "Our cells pass Californian [leaching] tests for household waste disposal," he says. "To talk generally of CdTe cells as having a problem with cadmium toxicity is therefore not appropriate." However, Professor Bob Hill of the Newcastle Photovoltaics Application Centre says that many leachability tests on CdTe cells assume that the cell housing will remain unbroken in a landfill.