General issues on photovoltaic technologies
Photovoltaic technology uses sunlight to generate electricity. The
basic energy producing unit is the photovoltaic cell. It is made of
semiconductor material which is sensitive to sunlight and it usually
has an area of 1–100 cm2. Individual cells are usually connected
together to a module with typical areas of 0.5–2 m2, and modules are
combined in an array to create a PV system which produces the needed
output voltage and current.
Peculiarities
Sunlight is a renewable energy source that could theoretically be
exploited to supply energy abundantly for an indefinite time into
the future. Completely free of cost, sunlight is widely available on
Earth regardless of geographical location. On the other hand, the
intermittency of sunlight causes the operation of photovoltaics to
rely directly on the time of day and weather. On a cloudy day or at
night, the power supply is diminished or cut off unless some other
source of electricity is used. Additionally, the power density of
sunlight is low (1 kW/m2 in clear conditions), so large-scale PV
electricity production requires either a large area covered with PV
modules or mirrors for concentrating sunlight on a smaller area.
The power generated by a PV module depends on the module technology
and on the intensity of sunlight. The power a module produces at a
given moment is proportional to the perpendicular sunlight intensity
on the module surface. Power is therefore reduced if conditions are
cloudy or if the angle of incidence of sunlight is large. In
general, the average power production of a PV system can be reliably
estimated on a monthly basis from previously measured meteorological
data. Shorter time intervals introduce uncertainty, but weather
forecasts can well be used to predict power production one day in
advance.
As a semiconductor device the PV module is quiet, static, and
solid-state. It requires little maintenance in order to operate: the
conversion of sunlight into electricity is a very reliable process
that does not produce any emissions. In addition, the module does
not include any moving parts that could wear out or break down.
However, the encapsulation of the PV module is critical to ensure
that the electrically active parts of the module are not harmed by
the surrounding environment. In normal operation the system feeding
the PV electricity to the network is more susceptible to damage than
the module itself. (EPIA and Greenpeace, 2004;Kurokawa, 2003;Ross
and Royer, 1999)
PV arrays can be built ranging from a few milliwatts up to several
megawatts due to their modular design. Existing arrays can always be
expanded to meet growing electricity demand, although the
electronics in the system may need updating. Easily adaptable and
lightweight, the PV panels can be installed virtually anywhere on
Earth without concerns for fuel transportation logistics.
Furthermore, PV modules can produce electricity at the point of
consumption, which reduces transmission losses and can improves
service reliability. (EPIA and Greenpeace, 2004;Kurokawa,
2003;PV-TRAC, 2005)
PV technology also offers an option for energy diversification. It can be
used to complement other energy sources, both traditional and renewable. In
particular, photovoltaic electricity is well suited for providing additional
electricity during peak demand, which in some regions coincides with the
sunniest hours of the day due to air conditioning. (Kurokawa, 2003;PV-TRAC,
2005)
At present, the major drawback of photovoltaic electricity is its high
price. When a lot of power is needed, photovoltaics is seldom
cost-effective. However, in small-scale consumption such as in residential
buildings, the competitiveness of PV electricity is improving. While the
initial investment into a PV system demands capital, operation and
maintenance require next to nothing. (Ross and Royer, 1999)
Environmental aspects
Photovoltaic technology provides clean energy: sunlight acts as the fuel and
there are no harmful emissions or polluting gases released during operation.
In particular, PV systems do not produce any CO2 emissions when operating,
so they can be used to cut greenhouse gas emissions. Apart from operation,
PV systems nevertheless carry the environmental weight of other stages in
their life cycle. (Alsema and Nieuwlaar, 1997;Battisti and Corrado,
2005;EPIA and Greenpeace, 2004)
The life cycle of photovoltaic systems consists of the manufacturing,
operating, decommissioning and recycling of the system. The life cycle of
the module can extend at its best over 40 years (Realini, 2003), whereas the
other components of the system last substantially less and thus require
replacement. Presently the components do not have a large impact on the
energy requirements of a grid-connected PV system compared to the
energy-intensive module production. (Alsema and Nieuwlaar, 1997)
More