Moving photovoltaic systems are installations whose orientation relative to the direction of the sun's rays changes as the sun's movement changes, or whose position changes as the objects on which they are installed change. Mobile PV systems are classified into two main groups:

• tracking systems [1], [2],
• mobile systems [3], [4].

Tracking systems

Photovoltaic tracking systems are those systems in which the plane of the panels rotates so that the sun's rays fall perpendicular to the plane of the panel to increase the amount of energy absorbed. Trailing systems produce up to \( 40\% \) more electricity compared to stationary systems [5], [6].
There are two types of tracking installations. The first group includes ground-based installations, in which the panels are placed on rotating poles called trackers. The second is floating installations, in which the planes of the panels rotate in the direction of the sun's current position.
Due to the number of planes in which the movement of photovoltaic panels takes place, the systems are divided into uniaxial and biaxial. Single-axis systems are installations with the ability to move panels from east to west only relative to one axis of rotation. Biaxial trackers allow simultaneous movement of panels in relation to two axes of rotation [7]. The dual-axis tracking systems move from east to west along with the ability to change the angle of the panels relative to the ground surface. The types of single-axis and dual-axis trackers are shown in the Fig. 1.

The working principle of both types of trackers is shown in the videos "Single Axial Solar Tracker" and "ASUN 2 Axis Solar Tracker".

Solar Trackers, Single Axial Solar Tracker, 22.01.2019 (accessed 02.12.2020). Available on YouTube: https://youtu.be/eJv39HPSLus.

Asun Trackers Pvt. Ltd, ASUN 2 Axis Solar Tracker (Two-axis photovoltaic system that follows the movement of the sun), 10.08.2019 (accessed 02.12.2020). Available on YouTube: https://youtu.be/NGvcINzsweQ.

The tracking systems are also classified according to the control system used, namely as follows [8]:

• open system (passive), open loop controlled,
• a closed system (active) in which the control is closed loop.

In an open system, control of the position of the tracker relative to the Sun is based on mathematical models. The models are based on data determining the position of the Sun at a given day and time for a given latitude, i.e. on the astronomical calendar.
In closed loop trackers, the position of the system is controlled by sensors e.g. photoresistors, i.e. resistors with a resistance depending on the light dependent resistor (LDR) illumination [9], [10]. Another way to control the position of the tracker is to find the maximum power point (MPP) [11]. It changes according to the irradiance and temperature. The control system with MPP works by changing the position of the installation in small intervals and comparing the resulting power values. Based on the result obtained, the installation moves to the position for which it receives the highest power.
In order to increase the efficiency of the tracking PV systems, concentrated photovoltaics (CPV) are also used [12]. The CPV technology is a new emerging technology for photovoltaic systems that allows solar radiation to be focused onto PV cells through the use of lenses or curved mirrors.

Ground tracking systems

A ground tracking installation consists of a set of photovoltaic modules placed on a frame located on a rotating pole. An example of a single-axis tracking installation is the 172 MW system located in Telangana, India ( Fig. 2 ) [13]. The entire installation consists of more than 520 000 panels made from Hareon Solar's multicrystalline silicon and covers more than 450 hectares. An example of a dual-axis tracking installation is shown in the figure Fig. 3.

Figure 2: Photovoltaic farm built with single-axis tracking systems in India. Aut. photo Vinaykumar8687, licensed under CC BY-SA 4.0, source: Wikimedia Commons.

Figure 3: Photovoltaic installation built with dual-axis tracking systems from Solar Tracker Polska. Photo used by permission of Solar Tracker Polska.

Photovoltaic tracking systems are also made with one of the latest Concentrating Photovoltaics (CPV) technologies using solar concentrators. The world's largest tracking biaxial photovoltaic farm made with CPV technology is the 138 MWp Golmud CPV Solar Park, which is located near the city of Golmud, China ( Fig. 4 ) [14].

Figure 4: Largest photovoltaic farm made of CPV bi-axial trackers with ray concentrators in Golmud, China with a capacity of 138 MWp. Aut. photo Vinaykumar8687, licensed under CC BY-SA 4.0, source: Wikimedia Commons.

Tracking systems located on the water

Photovoltaic installations installed on the water surface are the least common of all types of photovoltaic installations. In this type of installation, the panels are placed on a floating platform and follow the movement of the sun by rotating it around its axis. One way of making the platform with PV panels move is by twisting it thanks to a rolling wheel. Another way to put a floating installation into rotation is by using a fixed mooring and steering a cable. Another example of a floating tracking installation is the floating tracking cooling concentrator (FTCC) photovoltaic system. An FTCC installation consists of floating platforms on which photovoltaic panels are mounted horizontally and concentrators are mounted at an angle to the surface of the panels, as shown schematically in Fig. 5.

The FTCC system moves in a disorderly motion with the ripples of the water. The first installation of this type was performed in Colignola, Tuscany, in 2011; it is shown in the video "Italian engineers pioneer floating solar panels".

AFP News Agency, Italian engineers pioneer floating solar modules in Colignola, Tuscany, 22.02.2012 (accessed 02.12.2020). Available on YouTube: https://youtu.be/foGaExl84gw.