Concentrator systems are actually one method to increase the efficiency of photovoltaic installations. Concentrator photovoltaics (CPV) is based on the application of optical elements to focus sunlight onto a small solar cell. The development of concentrator photovoltaics began in 1976 in New Mexico (Sandia National Laboratories) with the one kilowatt Sandia I and Sandia II [1].

The introduction of an additional, expensive facility is justified when the installation is limited by free area. The use of a concentrator enables the energy flow to the cell or panel to be increased.

Concentrator photovoltaics achieve high efficiency, generate relatively low costs and therefore provide a quick payback. A major advantage of this type of installation is its scalability.
The classification of concentrator photovoltaics is based on the energy multiplicity measured by the solar concentration ratio, abbreviated as solar concentration ratio which determines the degree of concentration of the radiation. Several types of concentrator-cell systems can be distinguished according to efficiency ( Fig. 1 ): Low Concentrator Photovoltaic (LCPV), Medium Concentrator Photovoltaic (MCPV) and High Concentrator Photovoltaic (HCPV) [2], [3]. The fastest growing and most popular technologies are HCPV [4].

A quantitative measure of the concentrator is the geometric concentration factor of solar radiation CR.

It determines to what extent the solar radiation flux incident on the cell is increased. The geometric concentration factor is the ratio of the active area of the concentrator \( A_{C} \) over a surface of receiver \( A_{0} \).
Another measure used to describe concentrators is the effective concentration ratio ( 2 ). It is defined as the total amount of radiation absorbed by a surface.

In the equation \( C_{g} \) is the geometric concentration factor \( \eta \) of the solar concentrator efficiency.
As opposed to a conventional PV panel, the CPV receiver must be positioned in the direction of the direct normal irradiance. If the solar concentrator and CPV receiver are not aligned towards the sun, they will lose some of the incoming solar radiation. The maximum angle at which the incoming solar radiation can still be captured by the solar concentrator is called the angle of acceptance [5], [6]. A part of the concentrator system must therefore be a tracer. For two-dimensional concentrators, the reception angle can be different (for different directions). For a given receive angle \( \theta \), for a point-focused concentrator, the maximum possible concentration \( C_{max} \) is about

In the equation ( 3 ) \( n \) is the refractive index of the medium in which the receiver is located.
The world's largest operating CPV facility (138 MWp) was built by Suncore Photovoltaics and is located in the city of Golmud, China. Large facilities (19.9 MWp) of this type also include the Fort Churchill Solar Array in Yerington, Nevada.

The parabolic concentrators located in Harper Lake are presented below. The installation takes the form of an array of solar panels spread over 125 metres long. One row consists of 10 solar panels, each 12 metres long and with an aperture of 5.76 metres. The skeletal arrangement with monolithic glass reflective panels is shown on Fig. 2 [7].

Figure 2: Parabolic trough at Harper Lake in California (parabolic concentrator). Aut. photo by Z22, licensed under CC BY-SA 3.0, source: Wikipedia.

Research on concentrator techniques is conducted at Sandia National Laboratories, a US scientific research institute dedicated to scientific research and technology development in the energy field. They are featured in the film 'Parabolic Dish Stirling Engine at Sandia National Laboratories'.

Sandia National Labs, Parabolic Dish Stirling Engine at Sandia National Laboratories (Concentrator Sandia National Laboratories), 29.03.2010 (accessed 12.09.2020). Available on YouTube: https://youtu.be/AdCNnSdtU7U.