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Concentrated Photovoltaics (CPV)

Amongst the various different photovoltaic technologies, the latest at present is concentrated photovoltaic, commonly known as CPV.

The great scientific and commercial interest that this technology has provoked is mainly thanks to the high yield offered by these installations in terms of energy produced particularly in areas of high irradiation. but also because of its great potential for reducing costs when compared not only with other solar technologies but also with other sources of renewable energy.  These characteristics could make CPV the most widely used solar energy vector for large generating plants in the future.

Solar Added Value has a high degree of technical specialization in the understanding of CPV technology, and its function principles of which are described within here.

 

The functioning principles of CPV technology

Concentrated photovoltaic modules are capable of collecting solar radiation and increasing its power density (measured in W/m2) by the use of lenses in order to focus it on a much smaller solar cell than the one in the collector entrance area.

The interest in this kind of technology stems from the fact that high efficiency solar cells (> 40%) are already commercially available. Whilst expensive this allows the impact of the overall cost of the system to be reduced by use of the optical devices mentioned and capable of intensifying up to 1300 times the irradiation (W/m²) coming from the sun. Thus, allowing the semi-conductor area required to be minimized, as shown in the diagram.

Concentrated Photovoltaics principle by use of a refractive lens.

Concentrated Photovoltaics principle by use of a refractive lens.

The increase in energy intensity created on the cells by the use of the optical elements inevitably leads to the narrowing of the numerical aperture of the rays of light which means that only direct sunlight can be used – with an approximate tolerance of ±1º. Therefore, double axis tracking systems are essential in the BOS.

 

Basic characteristic elements of CPV systems

Type of cells

CPV modules normally use multi-junction (MJ) cells. These cells are actually comprised of several solar cells, called sub-cells, connected in series and made of different semi-conductor materials. The advantage of this configuration is that it allows for greater use of energy from the photons present in the solar spectrum, thus providing the system with a higher degree of conversion of light into DC electricity.

Moreover MJ cells can be designed and specifically calibrated for a determined spectrum in order to make the photovoltaic conversion even more efficient. Currently, the most used MJ cells are the  triple-junction ones which normally offer an efficiency rate conversion of more than 40%, much greater than any other commercially known PV technologies and as shown in the graph.

Efficiency rates reached by photovoltaic cells as per type. Source: NREL, USA.

Efficiency rates reached by photovoltaic cells as per type.
Source: NREL, USA.

 

Use of optical elements

Lenses used in the CPV industry can be either reflexives (mirrors) or refractive (Fresnel lenses) and these elements are usually referred to as “primary lens”.

Different SOE designs mounted on MJ cells.

Different SOE designs mounted on MJ cells.

At the same time, the system usually also has a second optical element (secondary lens or SOE) which is attached  onto the cell. Its mission, amongst others, being to increase the numerical aperture of the set whilst at the same time ensuring a uniform distribution of the energy from the cell, which prevents the generating of hot spots on it.

 

Double axis tracking elements.

As previously explained, CPV systems only use solar resources coming from direct sunlight (the one with greatest energy), which means that their modules require the installation of double axis tracking collectors.

For a conventional CPV module, the value of the numerical aperture usually ranges between 0.7º and 1.2º. This value must be taken into account when designing or selecting the kind of tracker to be used. Currently there are sufficient precise tracking devices to guarantee this requirement, allowing the modules to be correctly orientated even with heavy loads.

Double axis tracker and tracking device with heavy load CPV modules.

Double axis tracker and tracking device with heavy load CPV modules.

Independently from the type of cell, it is evident that most of the characteristic elements defining CPV technology (optics, trackers, tracking devices and motorization, etc.) have already been used in other sectors, such as the car industry, IT or the aerospace industry, which means that it is relatively simple to bring about significant improvements by optimising the way in which they are used together. The fact is that there are important R&D centres researching into the optimization of this technology. In addition there are important companies already making CPV installations worldwide. These facts allow us to conclude that CPV technology will continue to acquire particular importance in the next few decades in the electrical sector and more precisely in the sector of renewable energies.