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The world’s traditional economic model of “take, make, dispose” relies on immense quantities of cheap and accessible material resources and energy. This model is quickly reaching its physical limits. In response, the global economy is beginning to engage in new conversations centered on transitioning to a more sustainable system. The idea of a “circular economy” has emerged as a more attractive and practical alternative to the traditional linear model.

According to the Ellen MacArthur Foundation, “A circular economy is restorative and regenerative by design and aims to keep products, components and materials at their highest utility and value at all times.”

In layman’s terms, the circular economy is an industrial economy that produces minimal waste and pollution. In a circular economy, biological material inputs can reenter the biosphere safely, and technical material inputs will continue to be used without entering or harming the biosphere.

In late 2015, the European Commission adopted a “Circular Economy Package.” A group of companies pursuing circular economy strategies promoted the new model in Davos at the World Economic Forum this January, bringing it to the attention of investors and world leaders.

The solar industry contributes to sustainability goals, but solar manufacturers still need to improve their own processes. All actors in the solar supply chain must commit to sustainability. Improving solar production processes is a key part of the solution. In a circular economy, production processes don’t use more energy than they need to, and byproducts and waste products can be reused to make new things. Drawing inspiration from circular economy principles, the solar industry can change the way in which it reengineers one of its key components: silicon.

 

Silicon production

Silicon is the primary building block for the solar photovoltaic industry. It’s at the heart of more than 90% of today’s solar modules, and the vast majority of this material is created through what’s known as the Siemens process. This 50-year-old chemical method takes metallurgical grade silicon (MG-Si) and purifies it by removing boron, phosphorous, oxygen, carbon and other metals.

There are many reasons why the Siemens method is harmful and wasteful, contradicting the environmental values of the solar industry as a whole. The two main challenges that this traditional process presents are wasteful energy use and the involvement of toxic materials.

During the Siemens process, MG-Si is milled to a fine powder and reacted with hydrogen chloride, an extremely reactive and toxic gas. This chemical reaction results in the production of several gases, including tetrachlorosilane, which combusts on contact with air and must be distilled in order to remove the contaminating compounds. Once purified, this gas is decomposed in a reaction chamber to yield pure silicon. Another primary byproduct of the Siemens process is silicon tetrachloride (STC), a hazardous substance that is both toxic and combustible.

This lengthy and hazardous process produces “electronic grade” (EG) silicon, a high-purity silicon product best suited for use in semiconductors in consumer electronics but often used in solar PV modules.

In essence, the solar industry is accepting overly engineered silicon that creates a number of headaches for both solar manufacturers and the environment. Emissions and spending on energy in silicon plants are high because purifying silicon to EG standards encompasses four phase changes: solid to liquid, liquid to gas, gas to liquid and liquid to solid. This can use up to 125 kWh for a single kilogram of EG silicon.

One recently developed alternative to the Siemens process is more energy-efficient, but problems remain. In an attempt to reduce energy requirements, the chemical vapor deposition reactor of the Siemens process can be replaced with a fluidized bed reactor (FBR) process. In an FBR, trichlorosilane is thermally decomposed in a counterflow with small silicon particles (dust). Silicon then deposits onto the existing particles, causing the granules to grow until they reach the appropriate size and fall out of the reactor.

Although the use of FBR technology reduces the amount of energy needed for purification, it also adds considerable complications. The FBR process requires the use of toxic and explosive gases, which must be carefully handled at all times. The process also yields significant amounts of hazardous byproducts, including STC and hydrogen, which require costly transportation and disposal procedures.

There is much to gain from a new, improved process for silicon purification. For example, purification via traditional methods such as the Siemens process - and FBR, to a slightly lesser extent - is highly capital-intensive. These processes also require a considerable amount of energy, and that consumption contributes significantly to overall emissions from solar manufacturing. Further, the involvement of hazardous materials (such as trichlorosilane gas) can put the environment and workers at risk, and toxic byproducts require careful handling and transportation. All of these impose costs, which are passed down through the supply chain.

 

Process innovation

As customer demand and technology innovation continue to chip away at solar equipment prices, module manufacturers are experiencing ever-shrinking margins. Process innovation in silicon would provide a welcome cost reduction in the supply chain while improving environmental outcomes.

The Siemens method has gone largely unmodified for the last few decades. Although EG silicon is well suited for the semiconductor industry, the solar industry does not require the same purity level in order for modules to work at full capacity. Only recently have processes emerged to produce silicon specifically for solar manufacturers. “Solar silicon” derived from a two-phase-change process is less expensive and less energy-intensive to produce than EG silicon.

Why optimize the process specifically for PV? Solar silicon takes a bite out of purification costs by producing a material whose performance is equivalent to that of the Siemens method. Solar silicon production only requires two phase changes: solid to liquid and liquid to solid. It also leverages a metallurgical process, rather than the chemical process utilized by the competition. This metallurgical process uses aluminum as a solvent to remove boron and phosphorus from MG-Si, which have an affinity for aluminum over silicon and, thus, migrate naturally with aluminum present. This combination of aluminum and silicon reduces the melting point of the alloy, enabling lower-temperature operations and dramatically reduced energy use.

To ensure a high-quality product, solar silicon production can use molten aluminum to dissolve the raw metallurgical silicon. By doing so at a lower temperature than other processes and reducing the number of phase changes, energy costs fall to just 20-30 kWh per kilogram of material produced. The silicon is then re-crystallized out of the solution by lowering the temperature of the solution. The solar silicon is subsequently refined, and any scrap of silicon left over is reintroduced back into earlier steps of the process for another round of purification.

This production process yields no hazardous byproducts. Instead, solar silicon production yields two aluminum-based byproducts: aluminum alloy and polyaluminum chloride. These byproducts are not wasted. They are then recirculated for use, providing valuable inputs to industries ranging from automotive and aerospace to wastewater treatment.

By reducing energy inputs and waste and successfully recirculating resources for further reuse, solar silicon production achieves a more sustainable method of material production. Overall, solar silicon production uses one-third of the energy, one-third of the capital costs and half of the cash production costs of EG silicon production. The process does not use high-risk substances and creates no hazardous byproducts.

Harkening back to a circular economy model, the metallurgical process for solar silicon reduces energy expenditure and does away with unnecessary and inefficient uses of precious natural resources. Optimized solar silicon production also eliminates the environmentally hazardous gas phase of the Siemens method. Altogether, this optimized method of solar silicon production reduces waste, safety risks and capital costs.

 

Quantifying impact

Overall, growing our solar capacity is key to reducing emissions in the global economy. The introduction of solar silicon in place of EG silicon makes solar even more efficient.

Consider this: One ton of coal burned in a power plant produces only 2,083 kWh of power and 2.29 tons of CO2. That energy is used up quickly, and the cycle of extraction and emissions continues. However, that same ton of coal could alternatively be used to make enough solar silicon (using the aforementioned process) for 1,600 Wp of solar modules. Over their 30-year life span, those modules will produce 69,907 kWh of clean power - much more than the ton of coal that was burned in a plant.

Manufacturing solar panels requires an initial energy investment, but the use of solar silicon brings that investment down. Currently, the emissions required to produce a single solar panel with EG silicon are such that it takes a year of production for a panel to pay for itself in terms of energy savings. Switching from EG silicon to solar silicon can bring down this payback period by several months. The use of solar silicon rather than EG silicon makes the emissions impact relative to coal even lower than it is today.

Solar manufacturers can further benefit our planet by producing modules in a way that is both energy- and resource-efficient. This is increasingly important as some stakeholder groups, such as the Silicon Valley Toxics Coalition, cast more light on the waste in solar supply chains.

As demand for solar grows and renewable energy becomes more mainstream, so, too, will demand for environmental responsibility. Fortunately for the solar industry, upstream innovations promise to reduce emissions while also improving profit margins for module manufacturers. Solar silicon is the next step for a maturing solar industry.

 

Terry Jester is a 37-year veteran of the solar industry, with leadership experience in the manufacturing and engineering of photovoltaics. She joined Silicor Materials Inc. in 2010 following her active involvement in the company as entrepreneur-in-residence at Hudson Clean Energy. Silicor Materials produces solar silicon through a proprietary purification process.