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14-07-2015

Taking Energy to Save Energy

Taking Energy to Save Energy
PV cells are the basic unit of conversion of sunlight into electricity and this process is named photoelectric effect.

Solar Photovoltaic Technology

These cells are made of two semiconductor plaques: n-type and p-type (usually silicon or thin-film); that absorb the sunlight that excites the electrons contained into one of the plaques to go to the other creating an electric current used by an external load such as a light bulb.


Solar PV panels and its carbon footprint

Generation of electricity from PV does not pollute nor produces green house gases (GHG) emissions directly although there is an associated carbon footprint. CO2 emissions, identifiable through a life cycle assessment, and special material requirements for its production can be used to determine the Energy payback time and answer the question: how long does it take for a PV system to become energy neutral?


Energy Payback for PV

A study performed by the US Department of Energy estimates a payback time of 3.5 years considering available materials and technology.


Available technologies

The two semiconductor plaques in a solar PV are made out mainly of three materials with different characteristics:

  • Silicon (Si) - It is the most popular material for solar cells. To be useful for this purpose it must be refined to a 99.99999% purity
  • Polycrystalline thin films - Copper indium diselenide (CIS), cadmium telluride (CdTe) and thin-film silicon. The thin-film use much less material and its active area is much more than thick-film (100-300 µm rather than 1-10µm).
  • Single-crystalline thin films - Higher efficiency materials such as gallium arsenide (GaAs). Gallium is a rarest metal than gold as it is a by-product of smelting aluminium and zinc. These cells can be moulded to wide range of designs
* thin-film terms comes from the method used to deposit the film not the thinness: thin film cells are deposited in very thin, consecutive layers of atoms, molecules or ions.


CO2 Emissions

The average amount of GHG emissions over its lifetime ranges between 15 to 72 gCO2eq/kWh (grams of carbon dioxide equivalent per kilowatt-hour of electricity produced). This range is so broad because there are many ways on how to measure it and each of them depend on a great diversity of variables such as: energy pay-back time (EPBT), efficiency of the module, site of installation (latitude, climate) and country’s mixture electricity grid used to material processing amongst others.


Material Requirements

To have a more accurate idea of the complete carbon footprint the manufacturing process can give us a hint. Silicon is the main material used and transforming it (purification and crystallisation) is an energy-intensive activity that can account for up to 60% of the total energy used to produce them. Aluminium, copper and glass complete the list of major components together with the materials used for the semiconductors.


Sources
  1. Fukurozaki S., Zilles R., Sauer I. Energy Payback Time and CO2 Emissions of 1.2 kWp Photovoltaic Roof-Top System in Brazil http://www.ijsgce.com/uploadfile/2012/1019/20121019125331523.pdf
  2. US Department of Energy, Energy Efficiency and Renewable Energy http://www.nrel.gov/docs/fy04osti/35489.pdf
  3. Scientific American: El futuro de la energia II. Temas 76
  4. Khaligh A., Onar O., Energy Harvesting. CRC Press 2010