And dispelling photovoltaic (PV) carbon footprint myths.
Researching the Of Silicon Metal, Solar Grade, and PV-1201 post, I noticed a slide similar to this one in the Globe Specialty Metals, Inc. (NASDAQ:GSM) March 2010 Investor Presentation. So I decided to revisit aspects of making silicon metal or metallurgical silicon (mg-Si) via the Carbothermic Reduction process described in Solar Grade Silicon roads lead to Ruše – Part 2. Silicon metal is the primary feedstock for almost all polysilicon and solar grade silicon production processes.
Carbon and even coal is good for something
In order to produce a single (1) metric ton (MT) of silicon metal, raw material inputs of 2.8 MT quartz, 1.4 MT coal, and 2.4 MT wood chips are required and represent a 6.6:1 ratio of process inputs to outputs. As Globe Specialy notes, “Proximity to high purity, low cost raw materials” near the submerged arc furnace facility are crucial and represent 40% of production costs.
“Winning the Global Race for Solar Silicon” by David Lynch in the November 2009 JOM issue has an excellent Figure 2, “Partial cross section of silicon submerged arc furnace and chemical reactions”, showing the intermediate reactions behind the separation of oxygen from quartz (silica or SiO2) through reduction with carbon at high temperatures to yield silicon and carbon monoxide (CO) in the overall reaction:
SiO2 + 2C = Si +2CO(g)
Generating the arc between the consumable carbon electrodes to heat the quartz and carbon mix above 2200 degrees C and enabling the reduction reaction is energy intensive contributing over one third or 36% to the silicon metal production cost. For historical and cost reasons, silicon metal production has been located near sources of stable low cost electricity such as hydroelectric power.
Keep in mind silicon metal was priced at about $2875 per MT or $2.85 per kilogram (kg) in North America on February 26, 2010, per MetalPrices.com.
In addition, about 1.4 MT of silicon metal are required to produce one (1) MT of polysilicon suitable for crystalline silicon PV. With a high estimate of 10 grams per Watt, one MT of polysilicon can be manufactured into at least 100 kWp (kiloWatt-peak) of crystalline silicon solar modules.
PV Carbon footprint
At the 1st International Conference on PV Module Recycling held in January 2010, Mariska de Wild-Scholten with ECN presented the “Life Cycle Assessment of Photovoltaics: from cradle to cradle”. As shown in the Carbon footprint slide, the silicon feedstock (both silicon metal and polysilicon production) used to manufacture crystalline silicon solar modules contributes less than four (4) grams of equivalent CO2 emissions per kiloWatt-hour (g CO2-eq/kWh) despite the creation of a carbon byproduct during the Carbothermic Reduction process. While crystalline silicon solar modules have carbon footprints ranging from 21 to 29 g CO2-eq/kWh including Balance of System (BOS) components and takeback and recycling, CdTe (cadmium telluride) thin film solar modules have the lowest carbon footprint at around 16 g CO2-eq/kWh.
The PV carbon footprints are ultra low when compared to oxymoronic “clean” coal technologies such as CCS (CO2 Capture and Storage) planned for commercialization by 2020.
“Timminco suspends solar-grade silicon production” was the late news reported by Reuters:
Timminco said its average selling price of solar-grade silicon in the fourth quarter fell to C$36 per kilogram from C$65 at the same time last year.
Polysilicon spot prices in the $50 to $55 per kg range have forced the Timminco move. Of course, every silicon PV manufacturer wants to purchase the mythical $45 per kg spot price polysilicon cited in BARRON’S “Solar Sector’s Long-Term Outlook Is Far From Sunny” by Eric J. Savitz.. I’ve observed a negative bias to polysilicon pricing but high quality polysilicon isn’t at $45 per kg yet. However, if the polysilicon price decline continues, higher cost new entrants may also be forced to shutdown their polysilicon production.