Photovoltaic (PV) emissions are often considered as some of the highest amongst renewable energy systems. This is due to the high levels of energy required for silicon processing in older technologies. Manufacturing techniques have dramatically improved over the years to reduce the energy required and new technologies such as thin film and CdSe with vastly lower embodied energies are becoming more established. In addition to this, as with any energy technology a whole array of factors impact the overall emissions of a system, the most significant of which are listed on this page.
Resource inputs and technology
Depending on the material PV is made from the emissions profiles vary substantially. Of the most established technologies organic panels seem to have the largest footprint with an average of 63.4 gCO2e/kWh, with crystalline silicon an average of 55.3 gCO2e/kWh and thin film technologies the lowest at 20.9 gCO2e/kWh. A few other less well established technologies have much lower values such as Cadmium selenide quantum-dot photovoltaics (CdSe QDPV) which has only 5gCO2e/kWh (value based on only one study (Şengül and Theis, 2011)) (Nugent and Sovacool, 2014)
It makes sense that the higher the solar irradiance the more energy a panel can generate during its lifetime and the lower its CO2e/kWh emissions will be (Nugent and Sovacool, 2014). However intense sunlight also degrades the panels at a faster rate and many sunshine rich desert locations have added problems of dust deposits on the panels surface which can prove troublesome to keep at bay.
As it stands in most EU countries at the moment there is rarely a period where PV electricity production outstrips demand. As such the requirements for storage is not as pressing as it will be when renewable technologies increase their share of the electricity mix. However the inclusion of storage systems can dramatically affect the overall emissions. Studies on this seem to be lacking and whilst it is clear the embodied energy in batteries must be considered, it is widely acknowledged storage technology is progressing in leaps and bounds and manufacturing techniques and resource management will improve.
Ground mounting a PV system can reduce the overall footprint required as can tracking devices despite the increase in electrical hardware (Nugent and Sovacool, 2014).
In general there is not a significant amount of information on the contribution the distances PV panels travel has on their lifecycle emissions (Nugent and Sovacool, 2014). The only study to consider this on its own put the figure at 6.3 gCO2e/kWh (Querini et al., 2012)
Sizing and Capacity
There is an economy of scale benefit from installation of large PV arrays. This is primarily thought to be down to efficiency gains in logistics and transportation (Nugent and Sovacool, 2014).
The emissions associated with manufacturing varies a great deal depending on the country of origin. A product manufactured in China could have double the total emissions of the same manufactured in Germany. In the case of crystalline silicon which requires significant thermal processing, the energy mix used can have a significant impact on the lifecycle emissions of the panels (Nugent and Sovacool, 2014).
The general trend unsruprisingly is that CO2e emissions are substantially reduced as the lifetime of the installation increases. A 5 year operating lifetime results in very large emissions of over 100 gCO2e/kWh, where as increasing to 20 years drops it to 17.5gCO2e/kWh (Nugent and Sovacool, 2014). PV systems have the advantage that they are almost maintenance free owing to their solid state nature.
An analysis of a PV roof installed in 1997 by the Centre for Alternative Technology (CAT) in 2010 (13 year old panels) found an average of 0.7% decrease per year in output, with the poorest recording an overall 20% drop (CAT, 2010). Modern panels can expect far better performance.