Solar Power » Technologies

Concentrating Photovoltaics (CPV)

Principle

Principle of Concentrating PV
In Concentrating Photovoltaics (CPV), a large area of sunlight is focused onto the solar cell with the help of an optical device. By concentrating sunlight onto a small area, this technology has three competitive advantages:

  • Requires less photovoltaic material to capture the same sunlight as non-concentrating pv.
  • Makes the use of high-efficiency but expensive multi-junction cells economically viable due to smaller space requirements.
  • The optical system comprises standard materials, manufactured in proven processes. Thus, it is less dependant on the immature silicon supply chain. Moreover, optics are less expensive than cells.

Concentrating light, however, requires direct sunlight rather than diffuse light, limiting this technology to clear, sunny locations. It also means that, in most instances, tracking is required.

Despite having been researched since the 1970s, it has only now entered the solar electricity sector as a viable alternative. Being a young technology, there is no single dominant design.

The most common classification of CPV- modules is by the degree of concentration, which is expressed in number of "suns". E.g. "3x" means that the intensity of the light that hits the photovoltaic material is 3 times than it would be without concentration.

  Low concentration Medium concentration High concentration
Degree of concentration 2 - 10 10 - 100 > 100
Tracking? No tracking necessary 1-axis tracking sufficient Dual axis tracking required
Cooling No cooling required Passive cooling sufficient Active cooling reuqired in most instances.
Photovoltaic Material High- quality silicon   Multi-junction cells


Concentration

Here are some examples of concentrator technologies and examples for both line and point concentrators. Although there might be differences in execution or materials used, most designs will follow one of those concepts..

Fresnel Lens

Fresnel Lens

A Fresnel lens, named after the French physicist, comprises several sections with different angles, thus reducing weight and thickness in comparison to a standard lens. With a Fresnel lens, it is possible to achieve short focal lenght and large aperture while keeping the lens leight.

Fresnel lenses can be constructed

  • in a shape of a circle to provide a point focus with concentration ratios of around 500, or
  • in cylindrical shape to provide line focus with lower concentration ratios.

With the high concentration ratio in a Fresnel point lens, it is possible to use a multi-junction photovoltaic cell with maximum efficiency. In a line concentrator, it is more common to use high efficiency silicon.

Parabolic Mirror

Parabolic Mirrors

Here, all incoming parallel light is reflected by the collector (the first mirror) through a focal point onto a second mirror. This second mirror, which is much smaller, is also a parabolic mirror with the same focal point. It reflects the light beams to the middle of the first parabolic mirror where it hits the solar cell.

The advantage of this configuration is that it does not require any optical lenses. However, losses will occur in both mirrors. SolFocus has achieved a concentration ratio of 500 in point concentrator- shape with dual axis- tracking.

CPV with reflectors

Reflectors

Low concentration photovoltaic modules use mirrors to concentrate sunlight onto a solar cell. Often, these mirrors are manufactured with silicone-covered metal. This technique lowers the reflection losses by effectively providing a second internal mirror.

The angle of the mirrors depends on the inclination angle and latitude as well as the module design, but is typically fixed. The concentration ratios achieved range from 1.5 - 2.5.

Low concentration cells are usually made from monocrystalline silicon. No cooling is required.

The largest low-concentration photovoltaic plant in the world is Sevilla PV with modules from three companies: Artesa, Isofoton and Solartec.

CPV with luminescent film

Luminescent Concentrators

In a luminescent concentrator, light is refracted in a luminescent film, and then being channelled towards the photovoltaic material. This is a very promising technology, as it does not require optical lenses or mirrors. Moreover, it also works with diffuse light and hence does not need tracking. The concentration factor is around 3.

There are various developments going on. For instance, Covalent are using an organic material for the film, whilst Prism Solar use holographic film.

Furthermore, this concentrator does not need any cooling, as the film could be constructed such that wavelenghts that can not be converted by the solar cell would just pass thru. Hence, unwanted wavelenghts would be removed.

Cooling for concentrating PV

Cooling

Most concentrating pv systems require cooling.

Passive Cooling: Here, the cell is placed on a cladded cermaic substrate with high thermal conductivity. The ceramic also provides electrical isolation.

Active Cooling: Typically, liquid metal is used as a cooling fluid, capable of cooling from 1,700°C to 100°C.

Εxamples

Low-concentration modules

Low-concentration pv with mirrors Low-concentration pv linear fresnel Luminescent concentrator
Low-concentration pv modules using mirrors without further tracking. Linear Fresnel concentrator devices by Entech Solar. Holographic concentrator by Prism Solar achieving concentration ratios of around 3.

High-concentration modules

Fresnel point concentrator module Parabolic mirror concentrator  
High concentration 300x "Diamond Power" series by EnFocus has been specifically developed for rooftop installations, including dual-axis tracking. Parabolic mirrors achieve 500x concentration in devices developed by SolFocus.  

 

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