HONG KONG (ResourceInvestor.com) -- First we will look at the two categories of physical geometry that the PV solar cell industry has selected:
- Silicon wafers, generally thick and heavy enough to be self supporting; and
- Thin films, deposited or created in various ways, of photovoltaic materials, made from 'doped' single materials such as (very thin) polycrystalline or amorphous silicon or from doped compounds of copper, cadmium, gallium, and indium, and a chalcogenide such as selenium or tellurium.
Silicon is the most common element in the earth's crust, that 70-km thick layer of mostly igneous material upon which we live. Essentially all of the silicon accessible to us is silicon dioxide (SiO2) or quartz, the very hard, very high melting point material, which when weathered and disintegrated by processes that take eons forms most of the sand on our beaches and in our deserts. When not so completely weathered, it forms the prime constituent of the minerals that make up most of the mountains.
The fact that silicon is the most common element in the earth's crust is often mistaken as evidence that silicon is an inexpensive material for technologies such as PV solar cells.
Nothing could be farther from the truth.
The basic form of silicon required to manufacture PV solar cells is high purity polycrystalline silicon. Even though today more than 80% of all PV solar cells are made from polycrystalline silicon, as of today this material is still very expensive to manufacture and process. If it weren't for the fact today that an even purer form of silicon, single crystal ultrapure silicon, is the most common substrate for integrated circuits, chips, for electronic devices, it is unlikely that any PV solar cells could be built from polycrystalline silicon wafers at costs anywhere near acceptable. This is because the mass production of polycrystalline high purity silicon would most likely have never been undertaken if it were solely for the production of PV solar cells built from polycrystalline silicon wafers. Neither would ultrapure single crystalline silicon, the basis of chip electronics, have ever been mass produced if the only demand for the greater expense had been for PV solar cells with higher efficiencies than could be made with less pure and polycrystalline silicon.
A fierce competition is now going on among various companies most of which have been in the specialized metallurgical grade silicon space either as producers or researchers. Metallurgical grade silicon is made in huge quantities as an additive for producing specialty steel alloys and alloys of aluminum and copper. The goal is to find a relatively inexpensive process for making UMGSI, upgraded metallurgical grade silicon, which could then be used for manufacturing polycrystalline wafers of sufficiently high purity for direct use in fabricating PV solar cells.
As with the lithium battery hoopla, a great many announcements of progress and even of success in mastering the mass production of UMGSI have been made in just the last 24 months. However, no one has announced the routine delivery of the oft stated industry target of as much as 1,200 tons per month per company which would be required of at least three or four suppliers in order to produce enough UMGSI to meet the supply needs for the polycrystalline, high-purity silicon-wafer-type PV cell producers to be able to meet the goal of manufacturing enough PV cells to produce the 26 gigawatt target that has been set for 2010-11.
In addition to being able to produce the tonnage of UMGSI it will be necessary for the new upgrade industry to be able to demonstrate that its costs can be in line to achieve the industry's holy grail of $1 per watt of installed, solar-energy conversion.
Silicon 'metal' or 'native' silicon is not found on the earth's surface; the silicon having combined with oxygen evolved in the primordial matter. However there is no possible shortage of metallurgical grade silicon 'metal' accessible to mankind because there is sufficient silicon oxide (quartz) and other silicate minerals in the lithosphere (the earth's rocky crust) that, so long as energy and reducing agents, coal, for example, or anthropogenic (manmade) hydrogen, are economically available it will be possible to produce crude (99%) silicon 'metal.' And, if iron ore is utilized in the process, so that it, along with the silicon oxide, is reduced then ferrosilicon (an alloy of iron and silicon used as an additive to introduce controlled amounts of silicon in specialty steel alloys) will also be available. This is true so long as the supplies of energy, the reducing agents, and to be fair, the ores of sufficient grade, remain available.
This situation must not be, but unfortunately often is, confused with the availability of silicon for the manufacturing of photovoltaic solar cells (PVSC). Silicon forms for that use, which can be ultrahigh purity single crystal or high purity polycrystalline silicon, are forms the manufacture of which is today energy and labor intensive. With just the currently proven purification and crystallization technologies considered, the production of these forms cannot be scaled up to meet the projected demands for PVSCs so that their economics can be competitive with existing methods of producing electricity.
This has caused the current ferment about UMGSI or upgraded metallurgical grade silicon. The hope is that the producers of metallurgical grade silicon (the silicon metal and ferrosilicon described above) can perfect add-on processes whereby they can 'upgrade' multi-ton charges of low purity silicon metal to the purity, at least, required to manufacture polycrystalline silicon solar cells.
Technologies proven today for the manufacturing of the highest efficiency thin-film PVSCs utilize compounds, combinations, and/or alloys of the following chemical elements:
- Silicon, in high purity but amorphous form;
- Selenium; and
Amorphous silicon thin-film PVSCs are made from high purity silicon, laid down as a film on a conducting surface and then activated. They utilize much less silicon than solid wafer-based cells. Their production and utilization is not natural-resource limited. Neither is there any critical shortage of copper, which is today produced globally at an annual volume of 16 million tonnes.
The remaining critical natural resources for thin-film PVSCs are all minor metals, both historically in terms of limited use in the pre-electronic age, and in terms of annual global production even today:
Critical Minor Metal for
Thin Film PVSC Production Primary Metal Source Maximum Concentration Global (tonnes)
1. Cadmium zinc 0.3% 20,500
2. Indium zinc 100 PPM 510
3. Gallium aluminum, zinc 50 PPM 80
4. Selenium copper 150 PPM 1,700
5. Tellurium copper, lead, gold 15 PPM 200-800
The chemical elements listed above have in common something much more important than the fact that all of them, in various combinations, are the materials of choice for thin-film PVSC technologies. That is that they are all obtained as byproducts, there is no primary mine for any of them anywhere in the world nor will there ever be! Therefore in order to determine the risk of their supply being interrupted you must follow the market fundamentals of the primary 'major' metals the mining of which produces the PVSC thin-film critical elements as byproducts.
It is not possible to predict future volumes, much less increases in those volumes, of the PVSC thin-film critical elements without being able to predict future volumes and increases in those volumes of the primary metals from which the PVSC thin-film critical metals are produced, and only produced, as byproducts.
Furthermore it must be understood that the ongoing improvements in processes for recovering primary metals from lower grade ores must not always be considered by thin-film PVSC makers to be positive, because of the negative effects such processes may have on the recovery of byproducts. For example recent improvements for recovering copper from lower grade ores have actually increased serendipitously the recovery of molybdenum and rhenium, a byproduct and a byproduct of a byproduct, from those ores. Simultaneously, those improvements apparently decreased the recovery of tellurium and selenium! In this case, the economic value to the copper miner makes the increase in copper, molybdenum, and rhenium production far more valuable than the decrease in selenium and tellurium production.
Also, the volumes produced by primary miners of copper, lead, and zinc are subject to the volatility of the demand of the global market and to interruption due to political instability and due to resource nationalism (collectively these are known as 'country risks'). Therefore the supply forecasts for the PVSC thin-film critical elements are subject to these almost impossible to quantify risk factors also. It is most unlikely and not really practical or possible for a copper producer, for example, to run a huge volume of copper for inventory just so he will be able to obtain a small quantity of selenium and tellurium for a customer or even a market. The same holds true for cadmium, indium, and gallium if their supply requires a zinc or aluminum miner to inventory huge amounts of lead or aluminum. It is critical to understand this supply-chain-linkage between the PVSC thin-film critical metals and the primary mining industries which produce them as byproducts.
A good example of the problems that arise from a technology such as solar energy conversion arises when one considers a statement by the United States Geological Survey (USGS) in its 2008 Commodity survey of indium. In that document it says that "Thin-film copper indium gallium diselenide (CIGS) solar cells require approximately 50 tonnes of indium to produce 1 gigawatt of solar power."
Note that total U.S. electrical energy generation in 2007 was 4,159.5 gigawatts, of which 0.6 gigawatts were produced by solar energy conversion devices connected to a power grid.
Therefore, assuming that all of the new indium produced in the world could be utilized to produce CIGS thin-film PVSCs, the result would be the production of an additional 10 gigawatts of electricity annually. Although the first year would mean a multiplying of the contribution of 'solar' to the electricity supply by 20; the contribution would only double the second year when compared with the first year, and so on until by the 20th year the additional contribution yearly would be less than 5% of the total supply of solar generated electricity.
Clearly, even if the cost of solar energy conversion to electricity by thin-film PVSC made from copper indium gallium diselenide films were zero, the contribution to the total demand from this source would be of no value at all, due to the limitations of the supply of indium.
NOTE: much of this can also be said for gallium
For solar energy conversion to electricity by thin film PVSCs, the construction of which is critically dependent on a minor metal, the annual global supply of which is limited, the question to be asked is:
What is the value to the demand for electricity of adding a supply the sum total of which is miniscule compared to the total demand?
Wouldn't a simple conservation procedure such as having no personal usage of electricity beyond 60 watts an hour, for example, between midnight and 6:00 AM reduce the consumption of fossil fuels far, far more than the substitution by all of the non-silicon thin-film PVSCs that could be built, considering the raw material limitations, for fossil fuel produced electricity?
Doesn't the world have far better uses for what minor metals we have on the above list than building PVSCs?
NOTE: the sacrifice of forest space or agricultural space for the acre-feet necessary for PVSC fields to have a meaningful contribution is not much different from the alcohol/grain problem
Editor's note: This article is based on the author's recent presentation to the Minor Metals and Rare Earths Conference.