Heliostat Tracking Techniques and Control Mechanism

1.1      Solar Tracker

The solar tracker is a device and a mechanism for orienting the solar concentrator in such a way that it reflects maximum sunlight on the receiver directly from the sun and the deviation angle is minimum throughout the tracking time. A highly efficient Solar tracking mechanism drastically enhances the efficiency of the solar concentrator since it can collect more sun light as compared to a passive mechanism.

Solar trackers are implemented using Astronomical equations, light dependent resistors, sun pointing sensors etc. coupled with other devices such as Stepper motor, Dc Motors, Servo motors, gas filled piston etc.  

1.1.1    Dual Axis Trackers

The dual axis tracker has both horizontal and vertical axis and hence can track the position of the sun and its apparent motion anywhere in the sky throughout the day. Dual axis tracker track the sun through both East to West and North to South, which gives it an added benefit of capturing more sunlight directly from the sun, which increases the added power output and the theoretical efficiency by approximately 40%.

It can be implemented with the help of sun pointing sensors or light dependent resistors, astronomical data or predicted trajectory of sun throughout the day.

1.1.1.1  Disadvantage

Dual axis tracking comes with greater complexity for the system. More motors are to be used to dual track which increases the difficulty level. It is also argued that it can be less reliable considering greater number of devices used in this mechanism which increases the maintenance and down time.

1.1      Tracking Mode Selection

 There are different automatic sun tracking methods according to different solar concentrator. These methods are usually sorted into active tracking methods, passive tracking method and combination of both.

1.2      Open loop Tracking

Open loop systems track the sun without physically following the sun via sensors (although sensors may be used for calibration). These systems typically employ electronic logic which controls device motors or actuators to follow the sun based on a mathematical formula.

1.2.1    Timed Tracker

A timer is used to track the sun throughout the day. A little increment in the time keeps the tracker facing the sun. Such mechanism is highly susceptible to inaccuracies and errors. It does not take into account the seasonal variations through the year. 

1.2.2    Altitude/Azimuth Angle tracker

It uses astronomical data or sun position tracking mechanism to find out the altitude and azimuth angles of sun, with the help of which an efficient mechanism can be devised of tracking the sun efficiently through the day. 

1.3      Open loop tracking with open loop motor control

In this mechanism the tracker is made to move on a predictable path of sun using motors. This mechanism is easy to implement and doesn’t add much to the complexity of the system.

1.3.1    Disadvantage

It has low power output efficiency due to inaccuracies in astronomical data. It doesn’t take into account the seasonal variations generally and if they are made to be considered, it will increase the complexity of the program.

1.4     Open loop tracking with closed loop motor control

This mechanism also uses the astronomical data for tracking the position of sun or relative altitude and azimuth angles. The real difference is the implementation of the actuators and motors. In this technique the motor has a continuous mechanism which tracks the sun throughout the year which moves the tracker to specific angles. It increases the tracking efficiency and the power output of the system.

1.4.1    Disadvantage

It increases the cost which is undesirable.

1.5      Open loop Mechanism with light sensors

This is a hybrid technique of astronomical data acquisition and light dependent sensors. It helps remove the infectivity of the system in cloudy weather by using the light sensing mechanism. It greatly increases the efficiency and power output of the tracking system.

1.6 Closed Loop Tracking

Closed loop tracking also uses the astronomical data for finding the position of sun and the relative altitude and azimuth angles. The difference between open loop tracking and closed loop tracking is that it has a feedback mechanism for obtaining the estimation of error or error bound. Analyzing the error estimation, the efficiency can be further increased.

It increases the complexity of implementation. Moreover the cost of the sensors used for tracking sun is quite large.