About US photovoltaic shipments:
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.