Page 54 - RSE - Results of the Apollon Project

p

tracking System

(ASSE, TECNALIA, RSE)

Brief Description of the State of the Art

CPV applications are defned by their great accuracy in the tracking process. Therefore an accurate movement
and control of the tracking structure is needed. For this reason, knowing the relative and the absolute position of the
tracker is of paramount importance. Information about the relative position of the tracker is necessary to quantify
the movement on each step. Conversely, parameters about the absolute position are needed to know the tracker
situation when the system falls down. For CPV tracking applications, encoders are the most used position sensor
devices. There are two kinds of encoders, incremental and absolute encoders. An incremental encoder produces a
series of square waves as it rotates. The number of square wave cycles produced per one turn of the shaft is called
the encoder resolution. Generally, incremental encoders provide more resolution at a lower cost than absolute
encoders. They also have a simpler interface because they have fewer output lines. However, they require an internal
position reference to determine the exact rotary position. By contrast to incremental encoders, absolute encoders
provide a “whole word” output with a unique code pattern representing each position. Therefore, absolute encoders
are used in applications where a device is inactive for long periods of time or moves at a slow rate, such as food
gate control, telescopes, cranes, valves, etcetera. They are also recommended in systems that must retain position
information through a power outage, like a CPV tracker in a power down situation.
In conclusion, both methods are valid. The frst one is cheaper in relation to the components but it could be less
effcient from the energy point of view.

Electronic Control Systems

Programmable Logic Controllers (PLC) applications are generally designed to meet the system needs, so for low
volume production, the cost of the PLC is low compared to the cost of the specifc design implemented on purpose.
Furthermore, PLC provides fexibility, robustness, and a user-friendly interface. However, in case of products with
high-production volumes, ad-hoc control systems are more proftable.

Actuators, Electric Motors

There is a large variety of solar tracker systems in the market and it is practically impossible to fnd two similar
systems. Current PV solar tracking systems have different tracking strategies involving different philosophies for
movement, systems operation, transmission, braking, and so on.
The most relevant parameters that determine when to use a given type of actuator are the PV tracking frame
area and the PV system-required precision. Most of the CPV solar trackers with electrical actuation analyzed use
a rotational type of actuator for azimuth tracking, while for elevation there are different variants. A great number
of them use linear actuators to move the tracking frame area. This is a very low cost method for tracking the sun,
because the high cost of the transmission system is eliminated. Furthermore, with this type of actuator it is very
easy to modify and to obtain high overall systems precision just by changing the actuator’s fxing point.
Other CPV tracking systems use rotational actuators with gear systems. This actuation improves on the
robustness of the linear one, enhancing the load capacity and reducing looseness. Some actuators of this kind are
able to actuate as a brake without consuming any energy.

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