The oil crises of the 1970s and the recent
surge in oil prices has rudely awakened the world to the fact that
the earth's fossil fuel resources are not inexhaustible and come
with a big price tag. The oil crises caused a reactivation of research
into photovoltaic cells, the first of which had been developed a
century earlier. Today, the question of post?nuclear energy is coupled
with the depletion of fossil fuel resources. Making solar energy
competitive has once again become a priority for Europe. The Hercules
project must be viewed in this context, in particular as in 1997,
when it was launched, it was estimated that the European share in
photovoltaic power generated worldwide was only 25%. The results
obtained have opened up unprecedented perspectives, so much so that
an American company has already begun using them under licence.
Higher efficiency, lower cost
In trying to improve photovoltaic performance one can focus on
a number of parameters. Naturally, the first of these is the intrinsic
output of solar cells. The Hercules partners have opted for recently
developed gallium arsenide solar cells which have a higher output
than silicon solar cells but have the disadvantage of being much
more expensive. This is where a second parameter comes in: the concentration
of the sun's rays. Using special devices whose efficiency is measured
by the concentration factor, the electricity generation of the cells
can be substantially raised while at the same time reducing the
size of photovoltaic panels and therefore the number of cells required.
The aim of the project was in fact to design a new type of device
with a concentration factor of 1000.
In order to attain this, the researchers chose very small cells
(1 mm²), which were much easier to cool down. Antonio Luque
of the Solar Energy Institute of Madrid and coordinator of the project
explains that a large number of cells per square metre were thus
obtained. On the other hand, it was necessary to ensure that this
did not lead to an excessive increase in assembly costs. The solution
to the problem was provided by the light?emitting diodes (LED) industry.
The German partner in the project, Vishay, transferred to photovoltaic
cells technologies used for LEDs. In the process, problems of heat
extraction in the solar cells were solved because of their similarity
to the problems encountered in high?output LED diodes.
A thousand suns on one cell
The concentrator is of an entirely innovative design. Conventional
devices, generally made up of lenses, necessitate extreme precision
of optical calibration and the sun?tracking system in order to obtain
strong concentrations of the sun's rays. The apparatus designed
under the Hercules project makes these requirements much more flexible.
Even when the concentrator is not accurately turned towards the
sun but deviates from this by an angle which may be as much as 1.5°degress,
the system will still continue to function. This means that manufacturing
constraints are significantly reduced and, concomitantly, also the
Developed by the Solar Energy Institute of Madrid, this new device
consists mainly of a plastic lens whose rear surface (not directly
exposed to the sun) is coated with a reflective layer. The incident
rays are thus refracted on the front of the lens (aimed at the sun)
and then reflected on to the rear surface before being returned
forward again. This to and fro movement is consistent with the "total
internal reflection" mechanism resulting from the difference
between the optical densities of the media traversed. The rays then
hit the photovoltaic cell, whose position in the concentrator has
been calculated for optimum effect.
Towards industrial production
Manuel Sanchez, scientific officer for the project at the DG for
Research, points out that one of the sources of the success of the
project has been the very multidisciplinary character of the partnership.
It included specialists in materials, photovoltaics and optoelectronics.
The basic design was developed by the Solar Energy Institute of
Madrid, which also designed the optical system. The solar cells
were the fruit of collaboration between this Institute, the IOFFE
Institute of St Petersburg ? well known for its excellence in high?performance
cells for applications in space ? and the Belgian Energies Nouvelles
et Environnement (ENE) company. Viatcheslav Andreev, Director of
the IOFFE Institute, is particularly pleased with this collaboration.
He notes that the Hercules project has enabled his Institute to
transfer their experience on to terrestrial applications while at
the same time offering it prospects on the European market.
The German Vishay company carried out feasibility studies for the
various components and their integration. Prototype moulds for the
concentrator had been studied by the Russian PROTECHN company. Its
Deputy Director Sergey K. Savelyev notes that the partnership gave
them the opportunity to turn their attention to the solar energy
sector, which was entirely new for the company, and work on technology
likely to be useful in other optical devices.
Antonio Luque points out that shortly after completion of the project
last year, the output of the systems composed of the cells and the
concentrator attained 26% for a concentration factor equal to 1
000. He adds that this is the highest output ever achieved with
photovoltaic cells for such high concentrations.
In order to consolidate this success, a new European consortium
has been set up, coordinated by the Spanish Isofoton company, which
is already marketing photovoltaic cells in some 50 countries. This
new consortium will be responsible for manufacturing complete solar
panels. In a parallel development, the Solar Energy Institute of
Madrid has been contacted by an American manufacturer of high-performance
silicon cells to design a concentrator geared to this technology.
The cost of solar power per kilowatt is thus becoming competitive.