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Photovoltaic units or cells convert sunlight to electricity. A typical cell is made of silicon atoms (crystalline or amorphous), whose outermost electrons are easily knocked loose by incoming solar radiation. The flow of loose electrons (guided by electrostatic fields built into the impure silicon) creates an electric current; solar cells are therefore similar to transistors or computer chips in their structure. A typical cell generates about a half volt of current, so several dozen cells are wired together into a module to generate enough voltage to charge a 12 volt storage battery (for example). This technology has great promise, because of its flexibility and its use of a renewable resource (sunlight), but is still quite expensive; a 30 Watt panel runs about \$300.

The data were taken from a yearly report by BP (the former British Petroleum company), who obtained the data from an uncited source. Both individual photovoltaic cells and groups of cells (modules) are lumped together. The size of the yearly shipment (akin to the number of units manufactured) is measured in terms of peak generating capacity (megaWatts in this case) of cells and modules. Students should recall that the Watt is a measure of power (the capacity to produce power in this case), whereas Watt multiplied by time is a measure of energy (like the kiloWatt*hour or kWh).

The time series shows rather constant shipment throughout the 1980's followed by accelerated shipment in the 1990's. Overall the time series looks "exponential", but how well does an exponential model actually fit the data? Students can regress the data in different ways, and then use the exponential model to forecast photovoltaic production in the future. Will an exponential model over- or underestimate future production?

A number of questions can be asked about the nature of the data. Is photovoltaic production influenced by politics? Who controlled Congress and the White House during this period, and is their any correlation with photovoltaic shipments in the US? Why do photovoltaic shipments show exponential-like growth (what is the cause)? Students could think about the positive feedback loop associated with the cost of manufacturing a product; as costs decrease, the market expands, thereby decreasing unit costs (economy of scale).

Source: BP Amoco (soon to be just BP), Statistical Review of US Energy 2001, page 39.
http://www.bp.com/centres/energy/

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 Total Yearly US shipments of photovoltaic cells and modules source: BP Amoco Energy Report 2000, p. 39 capacity given in peak megawatts year mW 1982 7.0 1983 13.0 1984 10.0 1985 6.0 1986 6.6 1987 6.9 1988 10.0 1989 13.0 1990 13.8 1991 14.9 1992 15.6 1993 21.0 1994 26.1 1995 31.1 1996 35.5 1997 46.4 1998 50.6 1999 76.9
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