Solar electricity is created by using Photovoltaic (PV)
technologyby converting solar energy into solar electricity from sunlight. Photovoltaic systems
use sunlight to power ordinary electrical equipment,
for example, household appliances, computers and
lighting. The photovoltaic (PV) process converts
free solar energy - the most abundant energy source
on the planet - directly into solar power. Note
that this is not the familiar "passive" or Solar electricity thermal technology
used for space heating and hot water production.
A PV cell consists of two or more thin layers of
semi-conducting material, most commonly silicon.
When the silicon is exposed to light, electrical
charges are generated and this can be conducted
away by metal contacts as direct current (DC).
The electrical output from a single cell is small,
so multiple cells are connected together and encapsulated
(usually behind glass) to form a module (sometimes
referred to as a "panel"). The PV module
is the principle building block of a PV system
and any number of modules can be connected together
to give the desired electrical output.
PV
equipment has no moving parts and as a result
requires minimal maintenance. It generates solar electricity
without producing emissions of greenhouse or any
other gases, and its operation is virtually silent.
What
is PV power used for?
PV
systems supply solar electricity to many applications
in the UK, ranging from systems supplying power
to city buildings (which are also connected to
the normal local solar power network) to systems
supplying power to garden lights or to remote
telecom relay stations.
The main area of interest in the UK today is grid
connect PV systems. These systems are connected
to the local solar electricity network. This means that
during the day, the solar electricity generated by the
PV system can either be used immediately (which
is normal for systems installed on offices and
other commercial buildings), or can be sold to
one of the electricity supply companies (which
is more common for domestic systems where the
occupier may be out during the day). In the evening,
when the electrical system is unable to provide the
electricity required, power can be bought back
from the network. In effect, the grid is acting
as a Solar electricity energy storage system, which means the PV
system does not need to include battery storage.
Grid
connect PV systems are often integrated into
buildings. PV technology is ideally suited
to use on buildings, providing pollution and noise-free
solar power without using extra space. The use
of photovoltaics on buildings has grown substantially
in the UK over the last few years, with many impressive
examples already in operation.
PV systems can be incorporated into buildings
in various ways. Sloping rooftops are an ideal
site, where modules can simply be mounted using
frames. Photovoltaic systems can also be incorporated
into the actual building fabric, for example PV
roof tiles are now available which can be fitted
as would standard tiles. In addition, PV can also
be incorporated as building facades, canopies
and sky lights amongst many other applications.
Stand-alone
photovoltaic systems have been used for many
years in the UK to supply solar electricity to applications
where grid solar power supplies are unavailable or difficult
to connect to. Examples include monitoring stations,
radio repeater stations, telephone kiosks and
street lighting. There is also a substantial market
for PV technology in the leisure industry, with
battery chargers for boats and caravans, as well
as for powering garden equipment such as solar
electricity fountains. These systems normally use batteries
to store the solar power, if larger amounts are required they can be combined with another
source of power - a biomass generator, a wind
turbine or diesel generator to form a hybrid power
supply system.
PV technology is also widely used in the developing
world. The technology is particularly suited
here, where electricity grids are unreliable or
non-existent, with remote locations often making
PV power supply the most economic option. In addition,
many developing countries have high solar radiation
levels year round.
Types of PV Cell
Monocrystalline
Silicon Cells:
Made using cells saw-cut from a single cylindrical
crystal of silicon, this is the most efficient
of the photovoltaic (PV) technologies. The
principle advantage of monocrystalline cells
are their high efficiencies, typically around
15%, although the manufacturing process required
to produce monocrystalline silicon is complicated,
resulting in slightly higher costs than other
technologies.
Multicrystalline
Silicon Cells:
Made from cells cut from an ingot of melted
and recrystallised silicon. In the manufacturing
process, molten silicon is cast into ingots
of polycrystalline silicon, these ingots are
then saw-cut into very thin wafers and assembled
into complete cells. Multicrystalline cells
are cheaper to produce than monocrystalline
ones, due to the simpler manufacturing process.
However, they tend to be slightly less efficient,
with average efficiencies of around 12%.,
creating a granular texture
Thick-film
Silicon:
Another multicrystalline technology where
the silicon is deposited in a continuous process
onto a base material giving a fine grained,
sparkling appearance. Like all crystalline
PV, this is encapsulated in a transparent
insulating polymer with a tempered glass cover
and usually bound into a strong aluminium
frame.
Amorphous
Silicon:
Amorphous silicon cells are composed of silicon
atoms in a thin homogenous layer rather than
a crystal structure. Amorphous silicon absorbs
light more effectively than crystalline silicon,
so the cells can be thinner. For this reason,
amorphous silicon is also known as a "thin
film" PV technology. Amorphous silicon
can be deposited on a wide range of substrates,
both rigid and flexible, which makes it ideal
for curved surfaces and "fold-away"
modules. Amorphous cells are, however, less
efficient than crystalline based cells, with
typical efficiencies of around 6%, but they
are easier and therefore cheaper to produce.
Their low cost makes them ideally suited for
many applications where high efficiency is
not required and low cost is important.
Other Thin Films:
A number of other promising materials such
as cadmium telluride (CdTe) and copper indium
diselenide (CIS) are now being used for PV
modules. The attraction of these technologies
is that they can be manufactured by relatively
inexpensive industrial processes, certainly
in comparison to crystalline silicon technologies,
yet they typically offer higher module efficiencies
than amorphous silicon. New technologies based
on the photosynthesis process are not yet
on the market.
Typical
PV System Configuration
The
components typically required in a grid-connected
PV system are illustrated below.
The PV array consists
of a number of individual photovoltaic modules
connected together to give the required power
with a suitable current and voltage output. Typical
modules have a rated power output of around 75
- 120 Watts peak (Wp) each. A typical domestic
system of 1.5 - 2 kWp may therefore comprise some
12 - 24 modules covering an area of between 12
- 40 m2, depending on the technology used and
the orientation of the array with respect to the
sun.
Most
PV modules deliver direct current (DC) electricity
at 12 volts (V), whereas most common household
appliances in the UK run off alternating current
(AC) at 230 V. An inverter is used to convert
the low voltage DC to higher voltage AC. Numerous
types of inverter are available, but not all are
suitable for use when feeding power back into
the UK mains supply. Good suppliers and installers
of grid-connect PV systems will be able to offer
advice on suitability of commonly available models.
Other
components in a typical grid-connected PV
system are the array mounting structure and the
various cables and switches needed to ensure that
the PV generator can be isolated both from the
building and from the mains. Again, good suppliers
and installers of grid-connect PV systems will
be able to offer advice on these aspects of the
PV system.
Finally,
a meter will be required to ensure that
the system owner can be credited for any PV power
fed into the mains supply.
Suppliers
will normally offer a 12 months warranty on the
system, together with 2 years on the inverter
and a performance warranty of 10 - 25 years on
the modules.
Source: Complete Guide To Solar electricity Systems
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