Implementation

Single column structure or of parallel console type
One or two moving motors often acting as actuators.

These are usually DC motors, stepper motors or servo motors

One or more Light sensing devices
Autonomous or auxiliary energy supply. Often the collected energy is used to power the control system hardware and motors used for moving the collector.
Light following or moving according to a calendar
Continuous or step-wise movement to adjust collector position
Tracking all year or all year except winter.
For dual axis systems, orientation adjustment with the tilt angle adjustment [11].

Sun Tracking Algorithms

Closed Loop Sun Tracking Algorithms

Various Active sensors are used such as a CCD sensor or photodiode, which sense the position of the sun on a receiver using images. If the position changes, a feedback signal is sent to the controller. The dish is then moved to a position for maximum insolation absorbtion. If shading occurs on the sensors, the motors will return the dish to a position where the sensors are maximally illuminated. This tracking approach is more dynamic and does not require mathematical formulae or algorithms in its set-up process.

Advantages:

IThe closed loop system has high accuracy, particularly during fine weather. This method of system can be cost effective and save time and effort by not involving as much precise tracker alignment work [9]. If the dish is tampered with the system will self-adjust and if a system relocation is needed, it has mobility.

Disadvantages:

The closed loop system performance is easily affected by weather conditions and environmental factors such as shade and clouds. Disturbances and interference in the atmosphere have a significant effect on sun tracking.

Sensors calibration and tolerances affect maximum efficiency of sun alignment. The system is more power consuming as sensors need to be accounted for when drawing power.

Open Loop Sun Tracking Algorithms

Unlike closed loop, do not require any solar image for feedback. An open loop encoder will set the position of the collector at predetermined angles, from mathematical formulae/algorithms relating to time/date and geographical information. User input for all relevant parameters such as elevation angles and azimuth, is enabled with the system.

The programmed controller moves the collector using a motor to follow the sun correctly.

Advantages

This type of system is simpler and cheaper to set up than a closed loop system [10].

Less maintenance or calibration is needed compared to a closed loop approach.

The operating principle of the system eliminates any interference from outdoor disturbances, mechanical errors or other errors from the solar concentrator. Once up and running, the input program is very simple to use and compared to closed loop systems, less power consumption is needed, as no sensors are required.

Disadvantages

The initial set-up of the system is very time consuming. As it is very important to calculate the solar zenith and azimuth angles with as little error as possible, ultimate success of the system will rely on an extremely accurate set-up.

If enough system disturbances occur, the set-up may have to be performed again.

Hybrid Sun Tracking Algorithms

The hybrid system is a combination of both the closed and the open system. It has a higher accuracy because it combines both systems. It uses the open loop system when there is interference in the atmosphere and uses the closed loop to eliminate any errors that occur due to tolerances in the initial installation or calibration. The hybrid system uses both open and closed loops as needed to maintain the highest possible accuracy in sun tracking.

During the early morning or evening when there is cloud cover, open loop tracking is used.

Advantages:

This system offers the highest accuracy in sun tracking, and maximises operating efficiency by utilizing both closed and open loop systems.

Disadvantages:

This system is the least cost effective of the three methods. This method needs initial installation as an open loop system. The system draws more power due to the closed loop sensors.

Passive (mechanical) and Active (electrical) Trackers:

Active Solar Trackers

For our application, Major active trackers can be categorized as microprocessor/

electro-optical sensor based or PC controlled date and time based :

Electro-Optical/Micro Processor Tracking Control

This basis of this method uses electro optical sensors or light dependant resistors to physically measure illumination characteristics. The set -up is usually composed of at least one pair of anti-parallel connected photo-resistors or PV solar cells, which are, by equal intensity of illumination of both elements, electrically balanced so that there is either no or negligible control signal on a driving motor [12]. If there is a differential in the illumination levels at the electro-optical sensors, a control signal will be used to drive the motor and to orient the apparatus in such direction where illumination of electro-optical sensors become equal and balanced. Photodiodes can be used for more sensitive control.

PC controlled date and time based,

This basis of this method uses a PC to calculate the sun positions with respect to date/time and geographical information with mathematical formulae/algorithms. Signals are created for the system to control to adjust the position of the dish, using an electromotor.

Typically used in large scale collector systems, a single controller can control many arrays. The systems can calibrate themselves by using a current maximising search routine, programmed internally. This method of tracking is highly accurate but requires a lot of time and precision to implement.

Passive solar trackers are based on thermal expansion of a matter (usually Freon) or on shape memory alloys. Usually this kind of tracker is composed of couple of actuators working against each other which are, by equal illumination, balanced. By differential illumination of actuators, unbalanced forces are used for orientation of the apparatus in such direction where equal illumination of actuators and balance of forces is restored.