Land Water and Critical Materials Requirements

No land or water resources are required for operation of the system (Table 4), which is installed on existing structures and uses rainwater for cleaning. The only critical material for crystalline-silicon PV modules is high-purity silicon . Silicon is one of the most abundant elements in the earth's crust, so the issue is not availability but the cost of purification. High-purity silicon is typically produced as either pellets or chunks of fine-grained polycrystalline silicon and is commonly known as "polysilicon feedstock."

Table 4. Resource requirements.

Indicator Name

Base Year 1997

Units

2000

2005

2010

2020

2030

Land

ha/MW

0

0

0

0

0

0

ha

0

0

0

0

0

0

High Purity Silicon

MT/MW

6.9

5

4

3

2

1

Water

m3

0

0

0

0

0

0

The availability of polysilicon feedstock is currently an issue for the crystalline-silicon photovoltaic industry, so it s availability to meet future large markets needs to be addressed [29,30]. The crystalline-silicon photovoltaic industr y used approximately 1,000 MT of polysilicon feedstock in 1995. It obtains most of this material as off-specificatio n material from the elec tronic-grade polysilicon feedstock industry. The quantity of silicon consumed by the photovoltaic industry is about 10% of the total electronic-grade polysilicon feedstock production. The price and availability of this material is affected by the business cycle of the semiconductor electronics industry. For example, there was exces s capacity in the electronic-grade polysilicon feedstock industry between the years 1985 and 1993 - so that the exces s feedstock from the electronic-grade silicon industry was both plentiful and inexpensive. Due to the phenomenal growth rate of the semiconductor electronics industry over the past three years, demand for electronic-grade silicon no w exceeds supply - which has led to the present situation of a tight polysilicon feedstock supply for the photovoltai c industry. Again illustrating the business-cycle nature of the polysilicon feedstock supply, one industry observer note s that announced capacity additions in the electronic-grade polysilicon industry, coupled with the more stringen t specifications for advanced integrated-circuit production, are likely to lead to a doubling of the quantity of exces s silicon available to the photovoltaic industry within the next five years [30]. The average growth rate of electronic -grade polysilicon fee dstock between 1975 and 1995 was around 10%, while the average growth rate of the photovoltaic industry is projected to be around 20%. Hence, the photovoltaic industry will become too large to use excess polysilicon feedstock from the electronic-grade polysilicon feedstock industry at some point in the future using current technology.

To meet large future markets, the crystalline-silicon photovoltaics industry will need to develop its own source o f polysilicon feedstock. The European study projected that using current technology, a photovoltaic-grade polysilico n feedstock could be produced for about $20/kg [24]. There are R&D programs that are attempting to develop technologies to reduce this cost further [31]. Present wire-saw technology can slice silicon wafers on 400-^m centers, which corresponds to about 7 g/W for 15%-efficient cells with 90% manufacturing yield. At $20/kg, the 7 g/ W corresponds to $0.14/W. This figure will not limit the industry through the year 2010. By the year 2010, ne w crystallin e-silicon photovoltaic technologies that use much less silicon per watt are anticipated to become widel y availabl e. For example, ribbon and sheet crystalline-silicon technologies, which can have effective silicon thicknesse s between 100 and 200 ^m, are just becoming commercially available. The thin-layer crystalline-silicon film cells tha t are currently under development have thicknesses between 10 and 50 ^m, and might be available after the year 2010 .

Using the previous assumptions of 15%-efficient modules and 90% manufacturing yield, the polysilicon usage and cost for these technologies are summarized in Table 5.

Table 5. Projected silicon feedstock usage and cost for various crystalline-silicon photovoltai c

Technology

Thickness

Usage

Cost

Cost

^m

g/W

$/Wp

$/m2

Wire Saw

400

6.9

0.138

20.70

Ribbon

200

3.5

0.069

10.35

Sheet

100

1.7

0.035

5.25

Thin-layer

50

0.9

0.017

2.55

Thin-layer

10

0.2

0.003

0.45

Note: Calculations assume a module efficiency of 15%, a manufacturing yield of 90%, and a polysilicon feedstock cost of $20/kg.

Note: Calculations assume a module efficiency of 15%, a manufacturing yield of 90%, and a polysilicon feedstock cost of $20/kg.

This analysis shows that the cost impact of the polysilicon feedstock is progressively less for the advanced technologies available in the future. Based on the anticipated establishment of a polysilicon feedstock production for photovoltaics at around $20/kg and the technology improvements available in crystalline-silicon photovoltaics, polysilicon feedstock is not considered a fundamental issue limiting continued crystalline-silicon photovoltaic industry expansion. However, as with any developing bu siness requiring large capital expenditures, there may be periods of difficulty until a dedicated photovoltaic-grade silicon supply is established. Of course, the emergence of thin-film technologies in future year s may also obviate polysilicon feedstock limits on PV module production. Critical material issues associated with thin-film PV production are reviewed in a companion report [12].

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