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Address:
Glafo, PG Vejdes väg 15,
SE-351 96
Växjö
SWEDEN
Visitor's address:
Vejdes plats 3
Tel: +46 10 516 63 50
E-mail: info@glafo.se
You find
a map at the
"about Glafo" page.
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Project leader: Mikael
Ludvigsson
Project status: closed
A
FEASIBILITY STUDY - GLASS FOR SOLAR CELLS AND SOLAR COLLECTORS
The
objective of the work has been to perform a study of the literature
and carry out a market analysis of existing technology and production
in the solar energy sector.
BACKGROUND
Solar energy is an effectively infinite source of energy. If we can
develop technologies to capture and use energy from the sun, our future
energy needs will be almost completely solved. At present, there are
two main paths to follow for collecting energy from sunshine.
The first of these
is the thermal solar collector, which converts radiation to heat by
absorbing the sun's radiant energy and transferring the heat to a heating
system where it is needed. The second is that of the solar cell, which
converts the sun's photons to electrons.
SOLAR COLLECTORS
Solar
collectors (unglazed) - low temperature
The simplest possible thermal solar collector is that of the garden
hose lying on the ground in the garden. The water in it is heated by
the sun's rays. This simple model can be used, for example, to heat
a swimming pool during the summer.
Flat plate solar
collectors (glazed) - medium temperature
Flat plate solar collectors consist of an insulated case, with a glass
front. The bottom of the case consists of an absorber material that
absorbs as much of the solar energy as possible, converting it to heat,
which is carried away by a water piping system in contact with the absorber.
The glass front of the case helps to insulate the case and retain heat.
Vacuum solar
collectors - high temperature
Vacuum solar collectors consist of two concentric pipes. The outer pipe
is transparent, while the inner pipe is made of a high-absorption material.
Temperatures in the vacuum tube can reach 250 °C.
SOLAR CELLS
A solar cell
converts solar radiation to electricity by the photovoltaic effect.
A single solar cell is rarely sufficient to generate a useful quantity
of power. A solar module can consist of 36 solar cells, and delivers
power at 12 volts or more. A simple solar cell installation consists
of a solar cell module and a battery, which is charged by the module.
Silicon-based
solar cells
Silicon-based solar cells are the commonest type of cell on the market,
and have an efficiency of about 15 %. The underside of the cell has
a metal contact layer, while the upper side is covered by a mesh of
metallic contacts and a cover glass, which generally incorporates an
anti-reflection coating. The individual cell voltage is about 0,5 V,
and a common module can consist of 36 cells connected in series, to
give an output voltage of about 20 V. A standardised installation with
an area of 8 m2 has an output of 1 kW, and can produce about
850 kWh per year on average.
Thin-film solar
cells
Thin-film solar cells are preferable in terms of utilisation of materials,
as the amount of material required for a 1 m cell is very little. However,
the manufacturing process can be very energy-demanding in comparison
with the amount of material produced. The films are normally produced
by an appropriate vaporisation process, such as sputtering.
CIGS (copper indium
gallium diselenide - Cu[In,Ga]Se2)
Thin-film CIGS cells are composed of the elements copper, indium, gallium
and selenium. A module of these cells usually has an area of about 0,5
m2, but is not limited by the size of the substrate material.
The highest efficiency of CIGS cells is at present 20,3 %.
Cadmium telluride (CdTe)
Solar cells based on cadmium telluride are also thin-film cells. They
have reached an efficiency of 16,7 %. TCO stands for Transparent Conducting
Oxide, indicating that the glass substrate carries a transparent electrically
conducting coating of a metal oxide, which acts as a window for light
to pass through to the active semiconductor material beneath.
Multijunction
solar cells (gallium arsenide, germanium, gallium indium phosphide -
GaAs, Ge, GaInP2)
Multi-junction solar cells are based on several p-n junctions, each
of which is tuned to a different part of the solar spectrum. The manufacturing
process is complicated, involving the deposition of several different
layers by thin-film technology. A conversion efficiency of about 42
% has been achieved in the laboratory.
Grätzel
cells
Grätzel cells are based on semiconductor materials and work by
a photoelectrochemical process. The top of the cell consists of a glass
sheet, coated with a transparent electrically conducting metal oxide.
This electrical contact, the anode, carries a porous layer of titanium
dioxide (TiO2), which is saturated by a dye, ruthenium polypyridine.
These cells mimic nature's chlorophyll-based photosynthesis process.
Manufacturing cost is low, and the efficiency is about 7 %.
Future solar
cells
One of present lines of development is that of transparent solar cells,
which are generally applied to glass. Another development line is concerned
with flexible cells, as shown in the diagram below. The advantage of
transparent solar cells is that they could be incorporated in windows,
while flexible cells can be shaped to suit appropriate surfaces. However,
the efficiency of these technologies is still low, at about 1 %.
SUMMARY
Regardless of the particular technology, glass is a natural part of
any solar energy-based system. It protects solar cells from ambient
conditions, while transmitting a wider spectrum of radiation than any
other material can transmit. The combination of glass and transparent
electrically conducting metal oxides is a prerequisite for further technical
development. The performance of the glass can be improved by application
of functional coatings, toughening and lamination, to produce highly
resistant and efficient solar energy system.
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