Silicon

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Purity Levels

• Metallurgical grade silicon is 98% (one nine?) [1]
• Solar grade silicon is 6-8N [2]
• Microchip silicon (Electronic Grade Silicon) is nine nines pure, whereas power transistor silicon is 6 nines. [3]. Solar grade silicon is same purity as transistor grade - 6 nines. [4].
• 3 nines pure by acid leaching [5]
• Czochralski silicon is 11 nines pure [6]

Production Processes

• Siemens process involves chemical vapor deposition of silanes, with metallurgical grade silicon as the feedstock. [7]
• Fluidized bed reactor - similar feedstocks as Siemens, but is continuous, thus capable of lower energy use
• 3 possible processes: upgraded metallurgical, Siemens, Fluidized Bed - [8]
• Upgraded metallurgical silicon requires about 25% less energy than standard (Siemens)
• Fluidized bed reactor method uses 45-80 kWhr/kg of solar grade silicon produced [9]
• Siemens process takes 70-90 kWhr/kg [10]
• What about zone refining directly from metallurgical grade silicon? [11]

Purity Levels

• Microchip silicon is nine nines pure, whereas power transistor silicon is 6 nines. [12]

PV Panel Embodied Energy

• See this reference for more detail [13]
• And this discusses poly si vs mono si [14]
• embodied energy of 165-375 kWhr/kg [15]
  • Good news - 65% of all energy goes into the wafers, as opposed to everything else (glass + All + EVA) according to same link
  • Similar for amorphous silicon [16]
• Compare to steel at 20 MJ/kg [17], and 200MJ/kg more than this for aluminum [18]
• How many grams of silicon are needed for 1kW of PV panels? 2 kg [19], based on 2g/W.
• 5% of weight of PV is silicon [20]. 76% is glass, 10% is aluminum, and 10% is EVA.
• Embodied energy of EVA is about 80 mega joules per kilogram [21]

Total for 1kW of PV

• 600 kWhr for 2 kg of silicon
• 3kWhr per kg for glass, or 90 kWhr for 30 kg glass. [22]
• About 40 kWhr per my of EVA/aluminum x 8 kg = 320 kwhr
• Total 1010 kWhr
  • if the low estimate of 165 kilowatt hours above is true and this is 65% of the energy required then we have around 300 kilowatt hours of energy for wafers, and total around 500 kWhr for 1 kW of new PV panels - meaning energy payback of around 1/4 year!
• The case here would be easy for open sector PV production
• 365*6kWhr~2100 kWhr/yr
• Payback is thus 1/2 yr with only the above.
• This matches results from Fraunhofer 2021, close enough - .7-1.4 year payback time. [23]

Links

• DIY Glacial Acetic Acid
• Wikipedia says 585 kWhr/m2 of PV panels? [24]