5.15.2 Embodied energy and a bit more pollution
The other environmental impact comes from embodied energy. In this respect bulk processing has a much higher impact than thin film because of the higher temperatures involved. The cells are designed to save energy, so energy used in their manufacture should be minimised. The energy used to make cells derives mostly from fossil fuels, and fossil fuels are responsible for greenhouse gas emissions and a fair amount of cadmium emission (principally from coal but also from oil). However, when the emissions resulting from the production of PV cells are compared to those for burning fossil fuels as a primary energy source, even systems based on bulk crystalline silicon are at least ten times better than fossil fuel generation per kW h of electricity produced.
The energy pay back periods depend on the amount of sunlight that a module is exposed to, but the figures quoted here are based on sensible positioning in a temperate location. For crystalline silicon modules it takes about three to four years to capture more energy than was consumed in making the units. They are usually guaranteed for ten years, and their life is expected to be around 30 years. The thin film technology-based cells, produced using lower-temperature manufacturing processes, can pay back the energy consumed in production after only two years, but the expected life of such cells is around 20 years. So, embodied energy is recouped when cells have been generating for only around 10% of their expected life.
The other components of photovoltaic systems (support structure, inverters, wiring and installation) will also have an associated embodied energy; some of this may be offset where the PV arrays also provide a roofing or cladding material.