Solar Grade Silicon from Agricultural By-Products
In this project, Mayaterials developed a low cost, low energy and low temperature method of purifying rice hull ash to high purity (5-6Ns) and converting it by carbothermal reduction to solar grade quality silicon (Si pv ) using a self-designed and built electric arc furnace (EAF). Outside evaluation of our process by an independent engineering firm confirms that our technology greatly lowers estimated operating expenses (OPEX) to $\$$5/kg and capital expenses (CAPEX) to $\$$24/kg for Si pv production, which is well below best-in-class plants using a Siemens process approach (OPEX of ≈$\$$14/kg and CAPEX of $\$$87/kg, respectively). The primary limiting factor in the widespread use of photovoltaic (PV) cells is the high cost of manufacturing, compared to more traditional sources to reach 6 g Si pv /watt (with averages closer to 8+g/watt). In 2008, the spot price of Si pv rose to $\$$450/kg. While prices have since dropped to a more reasonable $\$$25/kg; this low price level is not sustainable, meaning the longer-term price will likely return to $\$$35/kg. The 6-8 g Si/watt implies that the Si pv used in a module will cost $\$$0.21-0.28/watt for the best producers (45% of the cost of a traditional solar panel), a major improvement from the cost/wafer driven by the $\$$50/kg Si costs of early 2011, but still a major hindrance in fulfilling DOE goal of lowering the cost of solar energy below $\$$1/watt. The solar cell industry has grown by 40% yearly for the past eight years, increasing the demand for Si pv . As such, future solar silicon price spikes are expected in the next few years. Although industry has invested billions of dollars to meet this ever-increasing demand, the technology to produce Si pv remains largely unchanged requiring the energy intensive, and chlorine dependent Siemens process or variations thereof. While huge improvements have been made, current state-of-the-art industrial plant still use 65 kWh/kg of silicon purified. Our technology offers a key distinction to other technologies as it starts one step upstream from all other Si pv production efforts. Our process starts by producing high purity SiO 2 /C feedstocks from which Si pv can be produced in a single, chlorine free, final EAF step. Specifically, our unique technology, and the resultant SiO 2 /C product can serve as high purity feedstocks to existing metallurgical silicon (Si met ) producers, allowing them to generate Si pv with existing US manufacturing infrastructure, reducing the overall capital and commissioning schedule. Our low energy, low CAPEX and OPEX process purifies the silica and carbon present in rice hull ash (RHA) at low temperatures (< 200°C) to produce high purity (5-6 Ns) feedstock for production of Si pv using furnaces similar to those used to produce Si met . During the course of this project we partnered with Wadham Energy LP (Wadham), who burns 220k ton of rice hulls (RH)/yr generating 200 GWh of electricity/yr and >30k ton/yr RHA. The power generation step produces much more energy (42 kWh/kg of final silicon produced) than required to purify the RHA (5 kWh/kg of Si pv , compared to 65 kWh/kg noted above. Biogenic silica offers three very important foundations for producing high purity silicon. First, wastes from silica accumulating plants, such as rice, corn, many grasses, algae and grains, contain very reactive, amorphous silica from which impurities are easily removed. Second, plants take up only a limited set of, and minimal quantities of the heavy metals present in nature, meaning fewer minerals must be removed. Third, biomass combustion generates a product with intrinsic residual carbon, mixed at nanometer length scales with the SiO 2 . RHA is 80-90 wt% high surface area (20 m 2 /g), amorphous SiO 2 with some simple mineral content mixed intimately with 5-15 wt% carbon. The mineral content is easily removed by low cost, acid washes using Mayaterials IP, leading to purified rice hull ash (RHAclean) at up to 6N purity. This highly reactive silica is partially extracted from RHAclean at 200 C in an environmentally benign process to adjust SiO 2 :C ratios to those needed in EAF processing to Si pv . EAF processing with silica depleted rice hull ash (RHASD), with nanometer scale carbon/silica mixing, reacts up to 10x faster than in traditional EAF processing because the physical distances over which the reactions occur are measured in nm vs cm. We have focused on demonstrating the efficiency in existing furnace technologies, meaning our success offers the potential to convert some portion of existing US furnace infrastructure (for Si met ) to high purity silicon production. The linkage of our process to the existing infrastructure of the U.S. silicon manufacturing industry, already a world leader, is unique compared to all other initiatives trying to produce Si pv . Purifying the silica/carbon mixture before EAF conversion to Si pv greatly reduces CAPEX and OPEX costs, reducing the final solar energy cost by $\$$0.18-0.24/watt.