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By increasing the concentration
factor of solar radiation it is possible to increase electricity
production per cell considerably and thus reduce the surface area
of the photovoltaic panels. As a result of incorporating these innovative
devices, the concentrator developed by the Hercules project marks
significant progress.
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An
inexhaustible source of energy coupled with prohibitive production costs:
that is the contradiction of solar energy. The Hercules(1)
project, one of a number of research projects in this field, has sought
to resolve this through the efforts of an international and multidisciplinary
partnership bringing together materials, photovoltaic and optoelectronics
specialists from the worlds of industry and research. It was the Institute
of Solar Energy in Madrid that came up with the basic technological concept
and developed the optical system. The institute worked to produce the solar
cells with OFFE in St Petersburg - known for its excellence in producing
high-performance photovoltaic cells for use in space - and a Belgian company,
Energies nouvelles et environnement (ENE). A German company, Vishay, produced
the feasibility study for the various elements and their integration, while
a Russian company, Protechn, worked on the prototype moulds for the concentrator.
First, the
partners had to decide on the materials for the photovoltaic cells. Gallium
arsenide (GaAs) was chosen for its high output, despite being more expensive
than the alternative, silicon. To offset the cost problem, the researchers
focused their attention on a second parameter: concentrating the solar
radiation. Using special optics, whose effectiveness is measured in terms
of a concentration factor, the electricity production of the cells can
be increased considerably and the surface area of the photovoltaic panels
reduced accordingly.
Mini-cells
The aim was to develop a device with a concentration factor of around
1000. For this, very small (1mm2) cells, which are much easier to cool,
were used. 'On the other hand, we had to be sure that this high density
of cells per square metre did not result in an excessive increase in the
assembly cost,' explains Antonio Luque of Madrid's Institute of Solar
Energy, the project coordinator. The solution was provided by the Light-Emitting
Diode (LED) industry(2). 'Our German partner, Vishay,
transferred LED technology to the photovoltaic cells and, due to the similar
problems associated with cooling powerful LEDs, this helped solve the
problem of cooling the cells.'
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The
concentrator remains efficient even if it is not pointed precisely
at the sun. A deviation of up to 1.5 degrees is acceptable.
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Concentration
by reflection
The concentrator uses a totally new concept. Conventional devices, usually
consisting of lenses, require extremely precise optical settings and sun
tracking systems to achieve a high radiation concentration. The device
designed under the Hercules project is considerably less demanding in
this respect. The concentrator can be off target by as much as 1.5 degrees,
but the system will still continue to function. This means a significant
reduction in the production constraints and thus lower production costs.
This new
device also includes a plastic lens, whose back surface, which is not
directly exposed to the sun, is covered with a reflective layer. The incident
radiation is therefore refracted to the front surface of the lens (which
is aimed at the sun), then reflected to the back surface before returning
once more to the front surface. This reflecting back and forth is a mechanism
known as 'total internal reflection' and is the result of the difference
between the refractive indices of the media through which the radiation
passes. The radiation then strikes the photovoltaic cell, whose position
inside the concentrator has been calculated for optimal performance.
On track
for industrial production
'On completion of the project, the output of systems consisting of cells
and their concentrator was as high as 26% with a concentration factor
of 1000,' explains Mr Luque. 'That is the highest output ever achieved
with photovoltaic cells for such high concentrations.'
A new European
consortium coordinated by the Spanish company Isofoton (which markets
photovoltaic cells in about 50 countries) is now beginning to manufacture
complete solar panels. What is more, Madrid's Institute of Solar Energy
has been contacted by a US manufacturer of very efficient silicon cells
to develop a concentrator tailored to this technology. The cost of solar
kilowatts is thus becoming competitive.
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Energies
in synergy
How
can the use of renewable energy resources be optimised? How can
wind and solar resources, which are naturally intermittent, be used
to meet the needs of consumers at a given moment? How can the use
of various energy supplies - both renewable and traditional - be
planned to guarantee security of supply?
In
1999, an integrated management unit prototype, developed by the
Care(1) project, was set up on Crete. This
Greek island has 18 traditional electricity production centres plus
eight wind farms. The Care system combines modules for wind force
and direction forecasting, electricity demand forecasting, and dynamic
safety evaluation. Through real-time optimisation of the use of
resources, this management unit provides network operating officials
with a reliable set of decision-making tools.
"'The
development of each system component was a challenge in itself,"
emphasises Nikos Hatziargyriou, project coordinator. 'But all these
individual elements also had to be integrated into a common environment
and the final system designed in a way to make it easy to use for
the operators.' One of the keys to Care's success was to involve
the users at every stage in the development process. The screen
can continuously display the current or historic network status
in terms of electricity demand, share of wind power, etc., as well
as forecasts for the hours to come. On this basis Care then proposes
strategies to assist the operator.
The
initial evaluations are encouraging with a margin of error of just
5% for electricity demand forecasts and savings of around 3% in
the quantity of fuel used on Crete every day.
(1)
Advanced control advice for power systems with large scale integration
of renewable energy sources (Care) - a project launched under the
non-nuclear energy programme (JOULE).
Contact
Nikos
Hatziargyriou
National Technical University
of Athens
Fax : +30 1 772 36 59
nh@power.ece.ntua.gr
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(1) Ultra-compact, high-flux, GaAs-cell PV concentrator
- a project launched under the non-nuclear energy programme (JOULE)
(2) LED (Light-Emitting Diode) technology is widely used for indicator
lamps on common electronic appliances.
Contact |
Antonio Luque (coordinator)
Institute of Solar Energy
Technical University of Madrid,
Fax: +34 91 544 6341
luque@ies-def.upm.es
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