Surgery and Healing in the Developing World - Dartmouth-Hitchcock

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76 Surgery and Healing in the Developing World
Figure 5. A number of modules are electrically connected to form an array. This array
supplies about 1 kW of power. (Photo courtesy NREL/PIX.)
silicon accounted for about 90% of all module production. The modules achieve
about 14% sunlight-to-electricity conversion efficiency.
Thin-film modules, on the other hand, are a newer approach to photovoltaics
which has generated much excitement. They often take the form of amorphous silicon
deposited on an inexpensive substrate, e.g., rigid glass plate or flexible stainless
steel ribbon. Production of commercial modules started in the mid-1980s, and some
of these products are now integrated into roofing shingles. Amorphous silicon modules
have efficiencies between 6% and 8%. In the late 1990s, two other thin-film
PV technologies started to receive much attention, namely, modules made from
cadmium telluride (CdTe) and modules made from copper indium gallium diselenide
(CIGS). Conversion efficiencies for modules made from these materials are about
8% and 10%, respectively. Because of maturity and performance, it is safe to say
that modules made from crystalline silicon wafers will remain the dominant PV
technology through at least the year 2012, and perhaps well beyond.
In 2003, crystalline silicon and thin-film modules were selling for about the same
retail price—US$ 3.00 to 3.50/watt. From the purchaser’s standpoint, the major
difference between the crystalline and thin-film options is found in the module warranties—and,
by implication, the likely reliability of the technologies. Because of the
maturity of the crystalline silicon technology, these modules now come with 25- to
30-year warranties. Crystalline silicon modules are very robust and are not expected
to suffer more than one percent total rated power degradation per year. Thus, a
25-year warranty assumes the module will achieve at least 75% maximum power
output after 25 years in the field. And this is probably a conservative estimate. On
the other hand, as of 2003, thin-film modules still have shorter warranties.
Balance of system equipment includes batteries for storing solar energy, a charge
controller to allow correct charging and discharging of the battery, a current inverter
for producing alternating current (if any AC appliances are in the clinic), circuit
breakers or fuses for safety, mounting hardware for securing the array, and electrical
wiring. All of these things are necessary to make the entire system work. The BOS is
independent of the particular module technology. It is determined by the required
performance and environment of the system. Some items in the BOS are not as