Direct Energy, 2018a

130 6.6 Solar Cells
dot based materials [78]. Organic solar cells also fall into this category.
The active part of these solar cells is a thin, often 100-200 nm, layer of
an organic material [79]. One advantage of organic solar cells is that their
processing may not require as high of temperatures as the processing of
solar cells made from pn junctions of inorganic semiconductors [79].
6.6.3 Solar Cell Systems
Solar cells are used in a wide range of devices. Inexpensive lawn ornaments
with solar cells are available at hardware stores for less than a dollar. Small
photovoltaic devices used as optical sensors are equally inexpensive. On
the other extreme, solar cells power the NASA Mars rovers Spirit and
Opportunity as well as satellites orbiting the earth. Also, large arrays of
solar cells are used to generate electricity.
A typical solar cell produces around a watt of electrical power while
a typical house may require around 4 kW of power [73]. To produce the
necessary power, individual solar cells are connected together into modules,
and the modules are connected together into solar panels. In a typical installation
on the roof of a house, a panel may be composed of around 40
solar cells, and 10 or 20 panels may be mounted roof [73]. A typical solar
panel installation on the roof of a building has a number of components
in addition to the solar panel arrays. The additional components are often
referred to as the balance of the system, and they consist of batteries,
mounting or tracking hardware, solar concentrators, and power conditioners.
These components are illustrated in Fig. 6.16.
The mounting system is composed of the foundation, mechanical supports,
brackets, and wiring needed to physically mount and connect the
solar panel. Some solar panels are mounted in a xed position. Other
solar panels are mounted on systems that angle the panels towards the
sun. Some tracking systems rotate the panel around a single east-west
axis. Others have two axes. Two axis tracking systems are often used with
solar concentrators. A concentrator is a mirror or lens system designed to
capture more of the sun's light onto the panels.
Solar panel systems require batteries or some other energy storage mechanism
to provide electrical power at night, on cloudy days, and other times
when inadequate sunlight falls on the solar panels. Solar panels can last
30 years or more with only about 1% or 2% degradation per year. Also,
solar panels rarely need maintenance, and they cannot easily be repaired.
If a solar panel fails, the entire panel is replaced. However, batteries have
a typical lifetime of three to nine years, and they are often the rst part of
a solar panel system that needs replacement [73].