Description

The transition to a sustainable global energy system is one of the biggest challenges mankind has ever faced. Among photovoltaic technologies, the Concentrating photovoltaics (CPV) can play a key role in this transition, owing to the possibility to reach system efficiency beyond 30%, which cannot be achieved by any other PV technology. The potentiality of CPV systems to reach efficiency value higher than 30% is related to the utilisation of the solar cell multi-junction (MJ) technology originally developed for space application. MJ solar cells exploit a combination of different semiconductor materials, having complementary light absorption properties in order to utilise more efficiently the solar spectrum. In this way heat losses are reduced and higher efficiency are reached. CPV have high potential to achieve cost reduction for solar-generated electricity, since it is possible to decrease, in function of the concentration factor, the area of expensive MJ solar cells, by using cheaper optical elements concentrating the light. As a matter of fact, CPV have raised in the past 2 years an enormous interest, largely as a result of the silicon crisis blocking the downward trend of the cost of silicon flat plate modules, comprising more expensive silicon cells. However, in spite of the increased interest on CPV, a large penetration of this technology in the photovoltaic market is still missing: while crystalline silicon based module industry has reached a production volume of about 1000 MWp/year, the production of CPV system lies far back in the <1MWp range. There are several reasons that have hindered a large-scale commercialisation of CPV systems, and they are related, mainly, to the following items:

• Large variety of the CPV system typology
• Cost/Watt
• Reliability and lack of international norm and standardisation on CPV
• Lack of initiatives for supporting the industrialisation of CPV
  

CPV offers such a large variety of technical possibilities: CPV ranges for low concentration (C< 100 sun), trough medium (100 < C < 300 sun) up to high concentration (C> 300 suns) systems, with single or two axis tracker, with reflective elements (mirror) or refractive elements (lenses). This large variety of solutions has scattered the efforts in different technological path avoiding a fast development towards industrial application. Nowadays, however, there is a clear tendency to focalise the attention on two groups of systems : -Point focus systems -Dense array systems In Point focus (PF) systems each solar cell of the PV concentrating module is installed under a dedicated optic, usually Fresnel lens or non–imaging optic elements. Very small area solar cells can be utilised (1-2 mm2), thus assuring a thermal management by means of passive cooling. A well known Point focus technology is that developed by the Russian Academy of Science IOFFE Physico-Technical Institute together with the Fraunhofer Institute (ISE). In Dense array (DA) systems the solar light is focalised by using just one large optical element, usually a spherical-like mirror, on an arrays of cells, usually connected in series, one beside the other. A well known dense array system is produced by Solar System in Australia. At high level of concentration, thermal management of dense array is of paramount importance. Therefore active cooling is typically required for these CPV systems. Dense array systems use reflective optics, therefore they do not suffer from chromatic aberration (a typical problem found in point
focus system, which make use of refractive optics). On the other hand, any shape error in the mirror geometry is more important than a similar defect on a refractive optic: the reflected light beam from a mirror with a shape error of alfa degree will impinge the target with a deviation of 2*alfa degree; in other words the reflected beam will have a chance to go out of target and non uniform illumination of the cells will appear. Both CPV systems present advantages and disadvantages: therefore, so far, any of the aforementioned concentrating systems has been demonstrated superior to the other. For this reason, it is important to deepen in Europe the development of both technological paths. In the APOLLON Project research and development activities will be proposed on both PF and DA systems. In particular, to reduce one the typical disadvantages of the Dense array systems, that is, the complexity of the thermal management, the concept of the spectrum splitting of the solar light will be adopted. This Consortium will propose “Mirror Based Spectrum Splitting System (MBS3)” which will make use of mirror and diachromic optics, concentrating the solar radiation onto two separate dense array panels of photovoltaic cells, respectively made of “low gap” and “high gap” semiconductor material.