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Better prospects for solar energy

Increasing the concentration factor of solar radiation makes it possible to considerably raise electricity generation and thereby reduce the size of photovoltaic panels and the number of cells. Thanks to innovative devices, the concentrator developed under the Hercules project is an important step in this development.

While solar energy has the enormous advantage of being available whenever needed, its use remains relatively costly. This situation is about to change thanks to improvements in the performance of photovoltaic systems. The Hercules project has demonstrated the feasibility of a new type of device which, using gallium arsenide cells, would bring the cost of solar energy in line with that of conventional forms of energy.

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 production costs.

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.

Ultra compact high flux GaAs-cell PV concentrator. Hercules


Non-nuclear energy (JOULE)

Antonio Luque (coordinator)
Solar Energy Institute
Polytechnic University of Madrid, IES-UPM
ETSI Telecominicacion. Ciudad Universitaria
28040 Madrid
Fax : +34 91 544 6341
E-mail :

- Solar Energy Institute, Polytechnic University of Madrid, Spain (coordinator)
- Institute physico-technical IOFFE, St Petersburg, Russia
- Progressive Technologies (PROTECHN), St Petersburg, Russia
- Vishay Semiconductors, Heilbronn, Germany
- Energies nouvelles et environnement (ENE), Brussels, Belgium

The concentrator remains efficient even if it is not exactly pointed towards the sun. It may have an angle deviating by up to 1.5 degrees.