1. Solar energy – an important player in the Energy Transition
With the foreseeable end of the fossil-fuel era on the horizon, human society is looking for reliable alternatives to provide energy. The use of green energy sources becomes essential for the sustainable development of our planet. Photovoltaics (PV), a technology in which light is converted directly into electricity, is one of the most promising candidates. PV is an appealing technology because it is a renewable, environmentally benign, and domestically secure energy source. Moreover, PV technology is modular and can be readily scaled to meet different demands. PV systems range in size from several kW residential systems to utility-scale multi-100MW, even GW installations. In 2017, more than 100GW PV modules have been installed worldwide, a more than 20% increase compared to 2016.
2. How far are we away from ‘green’ PV production?
Today and in the coming decades, the mainstream PV technology is and will be based on Silicon (Si). Though Si is the second most abundant chemical element in Earth crust after Oxygen, highly purified PV-grade Si is obtained today through energy intensive processes with Capex investment of 65-100$/kg, and production cost of ~15$/kg which accounts for up to 20% of the total module cost. In the relative Capex investment along the production value chain to producer PV modules, PV-Si has a 45% share, which is almost the sum of all the other steps.
Furthermore, the production of PV-grade Si consumes a considerable amount of energy, and also generates significant amount of CO2. A guideline is shown in the table below which compares Si production and other material industries such as production or purification of steel, Aluminum, Magnesium, and Titanium. It can be seen that the processing energy for obtaining PV Si is in average 80kWh/kg, which is higher than production of Steel and Aluminum, and on the same order of magnitude as Magnesium and Titanium.
The emission of CO2 for every kilogram of PV Si reaches an astonishingly high value of 50kg, which is the highest among all the other industries in the same table. In other words, for every MW of produced PV modules, 200 tons of CO2 is emitted. With the announced expansion plan of the PV module production in the next years, it is urgent and imperative to have a reliable technology to reduce the CO2 generation during renewable PV energy production.
3. Pain point for a virtuous PV
However, the core material for PV, highly pure Si suffers from the biggest value loss during the PV module production, mainly from two sources:
- During the PV wafer production process by multi-wire sawing of Si ingots, the amount of Si lost in the form of micro-metric powder, as part of the wafer sawing process, referred to as “sludge” or “kerf loss”, typically accounts for 40-50% of ingots’ total mass and ends up being disposed of as waste at a cost to the PV producers. For the reported 100 GW PV installed in 2017, more than 250kt of PV Si, or more than 3.7 B$ in value, was lost. These values will continue increasing by at least 10-15% p.a..
- As an alternative to the Siemens method, the FBR method replaces the vapor deposition reactor (Bell Jar Reactor) used in the Siemens process with a fluidized bed reactor in which the mono-silane (SiH4) is thermally decomposed in a stream containing small germs of Si. As the process progresses, Si germs grow, and the larger particles move to the lower part of the bed and are extracted as Si feedstock. However, since the particle size of Si obeys Gaussian distribution, about 10 to 15% of the final particles have a diameter of less than 40 μm. These Si particles are highly pure, but have little market value because their particles sizes are out of specifications.