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SOLAR GENERATION V -
2008
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The solar potential
There is more than enough solar radiation available around
the world to satisfy the demand for solar power systems. The
proportion of the sun’s rays that reaches the earth’s
surface can satisfy global energy consumption 10,000 times
over. On average, each square metre of land is exposed to
enough sunlight to receive 1,700 kWh of energy every year.
The statistical information base for the solar energy
resource is very solid. The US National Solar Radiation
database, for example, has logged 30 years of solar
radiation and supplementary meteorological data from 237
sites in the USA. European solar radiation data from 566
sites is published and assessed by the European Joint
Research Centre (JRC) (http://re.jrc.ec.europa.eu/pvgis).
The greater the available solar resource at a given
location, the larger the quantity of electricity generated.
Subtropical regions offer a better resource than more
temperate latitudes. The average energy received in Europe
is about 1,000 kWh per square metre per year, for example.
This compares with 1,800 kWh in the Middle East.
Figure 1.2 shows the estimated potential energy output from
solar PV generators in different parts of the world. The
calculation takes into account the average efficiency of
modules and converters, as well as the correct angle to the
sun required at different latitudes.
 A comparison between Figures 1.1 and 1.2 shows that only a
certain part of solar radiation can be used to generate
electricity. However, unlike with conventional energy
sources, there is no waste of energy through efficiency
losses, as sunlight cannot be wasted. It has been calculated
that if 0.71% of the European land mass was covered with PV
modules, this would meet Europe’s entire electricity
consumption. Furthermore, International Energy Agency (IEA)
calculations show that if only 4% of the world’s very dry
desert areas were used for PV installations, this would meet
the whole world’s total primary energy demand. Considering
the vast areas of unused space (roofs, building surfaces,
fallow land, deserts etc) the potential is almost
inexhaustible.
What is photovoltaic
energy?
‘‘Photovoltaic’ is a marriage of two words: ‘photo’, meaning
light, and ‘voltaic’, meaning electricity. Photovoltaic
technology, the term used to describe the hardware that
converts solar energy into usable power, generates
electricity from light.
At the heart of photovoltaic (PV) technology is a
semi-conductor material which can be adapted to release
electrons, the negatively charged particles that form the
basis of electricity. The most common semi-conductor
material used in photovoltaic cells is silicon, an
 element
most commonly found in sand. There is no limitation to its
availability as a raw material; silicon is the second most
abundant material in the earth’s mass.
All PV cells have two layers of semi-conductors, one
positively charged and one negatively charged. When light
shines on the semi-conductor, the electric field across the
junction between these two layers causes electricity to
flow, generating DC (direct current). The greater the
intensity of the light, the greater the flow of electricity.
A photovoltaic system therefore does not need bright
sunlight in order to operate. It can also generate
electricity on cloudy days. Due to the reflection of
sunlight, days with slight cloud can even result in higher
energy yields than days with a completely cloudless sky.
Generating energy through solar PV is quite different from
how a solar thermal system works, where the sun’s rays are
used to generate heat, usually for hot water in a house,
swimming pool etc.
The advantages of PV technology:
 The fuel is free.
There are no moving parts to wear out, break down or
replace.
Only minimal maintenance is required to keep the system
running.
The systems are modular and can be quickly installed
anywhere.
It produces no noise, harmful emissions or polluting
gases.
PV technology
The most important parts of a PV system are the cells which
form the basic building blocks of the unit, collecting the
sun’s light, the modules which bring together large numbers
of cells into a unit, and, in some situations, the inverters
used to convert the electricity generated into a form
suitable for everyday use.
PV cells and modules
PV cells are generally made either from crystalline silicon,
sliced from ingots or castings or from grown ribbons, or
thin film, deposited in thin layers on a low-cost backing.
Most cell production (90% in 2007) has so far involved the
former, whilst future plans have a strong focus on the
latter. Thin film technology based on silicon and other
materials is expected to gain a much larger share of the PV
market. This technology offers several advantages, such as
low material consumption, low weight and a smooth
appearance.
Crystalline silicon
Crystalline silicon is still the mainstay of most PV
modules. Although in some technical parameters it is not the
ideal material for solar cells, it has the benefit of being
widely available, well understood and uses the same
technology developed for the electronics industry.
Efficiencies of more than 20% have been obtained with
silicon cells already in mass production. This means that
20% of the incoming insolation can be transferred into
electricity.
 As well as the efficiency of the solar cells, their
thickness is also an important factor. Wafers - very thin
slices of silicon - are the basis for crystalline solar
cells. Thinner wafers mean less silicon needed per solar
cell and therefore lower cost. The average thickness of
wafers has been reduced from 0.32 mm in 2003 to 0.17 mm in
2008. Over the same period, the average efficiency has
increased from 14% to 16%. By 2010, the aim is to reduce
wafer thickness to 0.15 mm whilst increasing efficiency to
an average of 16.5%.
During wafer production, a significant amount of valuable
silicon is lost as sawing slurry. Ribbon sheet technology
represents an alternative approach. This avoids sawing loss
by producing thin crystalline silicon layers using a range
of techniques, such as pulling thin layers from the melt, or
melting powdered silicon into a substrate. As sawing
procedures, and the material losses linked to them, are
avoided, the demand for silicon per watt of capacity can be
reduced significantly.
Thin film
Thin film modules are constructed by depositing extremely
thin layers of photosensitive materials onto a low-cost
backing such as glass, stainless steel or plastic. This
results in lower production costs compared to the more
material-intensive crystalline technology, a price advantage
which is currently counterbalanced by substantially lower
efficiency rates.
 Three types of thin film modules are commercially available
at the moment. These are manufactured from amorphous silicon
(a-Si), copper indium diselenide (CIS, CIGS) and cadmium
telluride (CdTe). All of these have active layers in the
thickness range of less than a few microns. This allows
higher automation once a certain production volume is
reached, whilst a more integrated approach is possible in
module construction. The process is less labour-intensive
compared to the assembly of crystalline modules, where
individual cells have to be interconnected.
A temporary shortage of silicon has also offered the
opportunity for increasing the market share of thin film
technologies. Several new companies are working on the
development of thin film production based on a roll-to-roll
approach. This means that a flexible substrate, for example
stainless steel, is coated with layers in a continuous
process. The successful implementation of such a production
method will offer opportunities for significantly higher
throughput in the factory and lower costs. EPIA expects a
growth in the thin film market share to reach about 20% of
the total production of PV modules by 2010.
Among the three commercially available thin film
technologies, a-Si is the most important in terms of
production and installation, with 5.2% of the total market
in 2007.
Multicrystalline thin film on glass (CSG) is a promising
thin film technology which is now entering industrial
production. Microcrystalline technology, in particular the
combination of amorphous silicon and microcrystalline
silicon (a-Si/m-Si), is another approach with encouraging
results.
Other cell types
Concentrator cells work by focusing light on to a small area
using an optic concentrator such as a Fresnel lens, with a
concentrating ratio of up to 1,000. The small area can then
be equipped with a material made from III-V compound
semi-conductors (multi-junction Gallium Arsenide type),
which have efficiencies of 30% and in laboratories of up to
40%. The two main drawbacks with concentrator systems are
that they cannot make use of diffuse sunlight and must
always be directed very precisely towards the sun with a
tracking system.
Modules
 Modules are clusters of PV cells incorporated into a unit,
usually by soldering them together under a sheet of glass.
They can be adapted in size to the proposed site, and
quickly installed. They are also robust, reliable and
weatherproof. Module producers usually guarantee a power
output of 80% of the nominal power even after 20-25 years.
When a PV installation is described as having a capacity of
3 kW, this refers to the output of the system under standard
testing conditions (STC), allowing comparisons between
different modules. In central Europe, a 3 kW rated solar
electricity system, with a module area of approximately 23
square metres (depending on technology, see Table 1.1),
would produce enough power to meet the electricity demand of
an energy-conscious household.
Inverters
Inverters are used to convert the direct current (DC) power
generated by a PV generator into alternating current (AC)
compatible with the local electricity distribution network.
This is essential for grid-connected PV systems. Inverters
are offered in a wide range of power classes, from a few
hundred watts through the most frequently used range of
several kW (3-6 kW) up to central inverters for large-scale
systems with 100 kW and above.
Components for stand-alone
PV Systems
 Stand-alone
(off-grid) PV systems require a battery, frequently of the
lead acid type, to store the energy for future use. New
high-quality batteries designed especially for solar
applications, with lifetimes of up to 15 years, are now
available. However, the lifetime of the battery strongly
depends on the battery management and the user’s behaviour.
The battery is connected to the PV array via a charge
controller. The charge controller protects the battery from
overcharging or discharging, and can also provide
information about the state of the system or enable metering
and pre-payment for the electricity used. If AC output is
needed, an inverter is required to convert the DC power from
the array.
Types of PV system
Grid-connected
This is the most popular type of solar PV system for homes
and businesses in the developed world. Connection to the
local electricity network allows any excess power produced
to be sold to the utility. Electricity is then imported from
the network outside daylight hours. An inverter is used to
convert the DC power produced by the system to AC power for
running normal electrical equipment.
In countries with a premium feed-in tariff, payment for the
electricity generated (see Part Six: Policy Drivers) is
considerably higher than the usual tariff paid by the
customer to the utility, so all the electricity produced is
often fed into the public grid and sold to the utility. This
is the situation in countries such as Germany or Spain.
Off-grid
Where no mains electricity is available, the system is
connected to a battery via a charge controller. This stores
the electricity generated for future use and acts as the
main power supply. An inverter can be used to provide AC
power, enabling the use of normal electrical appliances.
Typical off-grid applications are repeater stations for
mobile phones, electrification for remote areas (mountain
huts) or rural electrification in developing countries.
Rural electrification means either small solar home systems
covering basic electricity needs in a single household, or
larger solar mini-grids, which provide enough power for
several homes.
Hybrid system
A solar system can be combined with another source of power
- a biomass generator, a wind turbine or diesel generator -
to ensure a consistent supply of electricity. A hybrid
system can be grid-connected, stand-alone or grid-support.
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