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Searching for solar’s feedstock
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Pure silicon ingot. (Courtesy: Saperaud)
Researchers at UC Berkeley, and the Lawrence Berkeley National Laboratory (LBNL) have begun a project that ultimately hopes to find a replacement for silicon in the solar industry. The use of silicon is proving to be a limiting factor in the solar industry reaching grid-parity. Their report was released through the American Chemical Society in the Environmental Science and Technology Journal and looked extensively into alternative feedstocks for the solar industry. The team of researchers began by surveying 23 promising materials before selecting nine that produced “a significant raw material cost reduction over traditional crystalline silicon”.
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In order for solar power to become competitive with oil, coal, and gas, it is necessary to lower the cost per kilowatt-hour either through improvements in the efficiency of conversion or by using less expensive materials to make the solar cells/films. Up until now, silicon cells have been the mainstay of the industry because of their higher efficiency yields. Pure silicon, though, is more expensive than the silicon blends or the alternative materials being used in thin film production.
There are numerous companies springing up that are bringing these alternative materials into the solar market, but the solar industry seems to be having trouble finding the right feedstock. Solar has been experiencing problems reaching cost-parity with fossil fuels because of the high prices associated with processing silicon specifically tailored to allow for high efficiency conversion rates. Up until the recent introduction of thin films, the solar industry was solely relying on feedstock from the same supply chain used in the semiconductor industry. This was causing numerous shortages in silicon supply and dramatic cost increases in an industry that was struggling to keep costs down in an effort to survive.
According to Eicke Weber, on of the researchers from UC Berkeley, “photovoltaics could grow much faster if researchers and manufacturers could further reduce the cost of solar cells and overcome the shortage in the high-quality, semiconductor-grade silicon used presently to make commercial solar cells". The silicon that is currently being used in large quantities in the solar industry is usually the high quality stuff because the monocrystalline structure produces cells with higher efficiencies. The more pure polysilicon is also used technical gadgetry.
Monocrystalline silicon “is pulled as a single crystal; the internal crystalline structure is completely homogenous”. This kind of silicon is used in all sorts of electronic devices from cell phones and computers to appliances and military hardware because its purity allows for more efficient transference of energy than lower grade silicon does. The single crystal structure of monocrystaline silicon, though, makes the process of producing it very expensive; for this reason, monocrystalline is less than an ideal source to support the solar power build-out on a global scale.
Polycrystalline silicon is made up of multiple silicon crystals all latched together; this kind of silicon has a visible grain. Up until recently, poly-Si has been used primarily for the manufacturing of solar cells, but innovations in the solar industry over the course of the past two years have been coming at a blistering pace. After nanotechnology entered solar’s space, it started to become apparent that silicon was not the only material that could be used to efficiently convert light from the sun into electricity.
The majority of the solar industry, until recently, has essentially operated in the shadow of the silicon industry. Firms negotiated multi-year silicon feedstock contracts as their primary way of securing stability. As more solar players entered the game, contracts became harder to come by and demand for silicon started to approach the levels of available supply. It was obvious that innovation was coming to the solar industry.
Enter First Solar, the first big solar player to jump into the thin film solar business; according to their website, FSLR uses “a thin layer of cadmium telluride semiconductor material to convert sunlight into electricity”. They were able to succeed initially, even though their efficiency rates were lower than silicon-based modules, because their feedstock costs were significantly lower than companies using crystalline technology. FSLR uses cadmium telluride (CdTe), but other thin film manufacturers are using different materials; Q-cells uses copper, indium, gallium, and selenide (CIGS) to produce solar modules. Nanosolar’s, another CIGS firm that has generated a lot of excitement in the solar industry for being able to manufacture film at a rate of 100 feet per minute, represents the hybrid solar industry that marries together nanotechnology with solar feedstocks; but that was all before the markets ground to a stop and the current solar module oversupply problem developed. The research still continues, but some analysts are now beginning to say that only a handful of the strongest solar companies will survive the recent glut.
Another solar manufacturing technique from another company, Trina Solar, uses upgraded metallurgical grade (UMG) silicon material to cut down costs of production while still delivering relatively comparable efficiencies. Using lower grade silicon in the manufacturing process is one way to cut down on costs, but it is important to also consider the losses in efficiency rates from using a less conductive material. This is what researchers at LBNL were working on and essentially discovered that when contemplating costs on a commercial scale over a decade long build-out period, sometimes it pays to put out a less expensive/less efficient solar module.
The report raises important concepts for the solar industry. The entire industry seems to be getting ready to make a giant leap forward. The introduction of the concept of engineering solar cells or film on a nano-level is allowing Science to once again come to the foreground of business strategies and government policies. According to Cyrus Wadia one of the researchers at LBNL, “because the sun is the Earth's most reliable and plentiful resource, solar definitely has potential, but current solar technology may not get us there in a timeframe that is meaningful, if at all. It's important to be optimistic, but when considering the practicalities of a solar-dominated energy system, we must turn our attention back to basic science research if we are to solve the problem".
The report essentially examines whether or not we are pursuing the use of the right materials for the solar industry; if it is going to be built-out globally, analyzing our current options is the prudent thing to do. Do we continue working on efficiencies with silicon; do we develop more UMG polysilicon; do we pursue cranking out thin film at nanospeed pace; or do we pursue altogether new feedtocks? In the conclusions of the report, “the team identified a large material extraction cost (cents/watt) gap between leading thin film materials and a number of unconventional solar cell candidates, including iron pyrite, copper sulfide, and copper oxide. They showed that iron pyrite is several orders of magnitude better than any alternative on important metrics of both cost and abundance. In the report, the team referenced some recent advances in nanoscale science to argue that the modest efficiency losses of unconventional solar cell materials would be offset by the potential for scaling up while saving significantly on materials costs.”
One of the lead researchers on the project said, “we must quickly consider alternative materials that are Earth-abundant, non-toxic, and cheap. These are the materials that can get us to our goals more rapidly”. Earth-abundant, non-toxic, and cheap; efficiency didn’t even make the list. It does not seem currently possible that one material is going to take over the solar market. Instead, perhaps the solar industry, over the next several years will come together as a patchwork of varying technologies until a standard can be agreed upon.
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