PV trackers can be classified into active trackers and passive trackers on the basis of their drive systems. Most common are the active solar trackers, while less common are the passive solar trackers. As a result of their different designs, their performance and cost also differ.
Active solar trackers
The words active and passive occur commonly in electrical engineering. Those devices which must be provided with source of energy for their full performance are classed as active. Those which do not need any external source of energy are called passive. Thus, active trackers must be provided with energy for their actuators to move. That energy may even be part of the energy harvested by the PV system they are driving. The actuator systems may consist of motors and other elaborate mechanical devices.
Passive Solar Trackers
Passive solar trackers will also track the sun, but they do it without requiring any external energy source, at least from us. How can they move the payload without energy? They do not! They too get their energy from the sun, but not the one we have harvested. Most often, the sun’s heat is used to cause expansion of a fixed mass of material, eg a low boiling point gas. The expansion causes a mechanical movement of an actuator. A pair or more of actuators are positioned so that they expand differentially depending of the orientation of the payload tracker with respect to the sun. Differential expansion causes the tracker to move closer to the direction of the sun. If it derives its power from the sun how does it move at sunrise and sunset when there is virtually no solar energy to drive this dumb device? It is manually retained in a near vertical position with a self-releasing tie-down at night so that at daybreak when it gets sufficient differential energy, it can move down into position and then keep tracking. Passive trackers are OK for simple PV systems but unfit for concentration type PV systems which demand high aiming accuracy. Instead of gas expansion systems, actuators made of shape memory alloys (SMA), have also been used.
A holographic solar tracker is a possibility which has been well analyzed but not put in practice except at laboratory level. This type of tracker will be a still tracker. Light from different angles will be handled by different holograms and brought to the solar cell. Since no movement is involved, there is no need for power supply and hence this is a form of a passive tracker.
Comparing Active trackers and Passive Trackers
Active trackers and passive trackers can be compared in a sentence. A passive tracker (or any other device) in front of its active counterpart is somewhat like a simple person compared to an intelligent technical expert. Now which one better is a very arguable point.
A little interesting anecdote can also throw some light on this issue. An old engineering professor is very fond of telling his students of a verse written by a very famous Indian Muslim poet called Ghalib of 19th century India. Ghalib said, “My earthen drinking bowl is better than the precious crystal bowl in which Jamshed (a legendary Persian King) drank!” Why, explained Ghalib in the next line, “Because I can buy another one from the market easily if my bowl breaks. And Jamshed could not!” The verses make sense today, although poor Poet Ghalib knew nothing about life-cycle costs and availability issues taught to engineers these days.
Now back to the comparison:
Complexity-The simple passive tracker is of simple construction and requires fewer parts. The active tracker is much more complex, necessarily involving a power control circuit, sensors, computer elements, mechanical gears and electrical motors etc.
Reliability-The passive tracker has little which can go bad, except the gas seals. Use of shape memory alloys instead can avoid that risk also. Modern active trackers are also very reliable, but it is a natural fact that reliability is affected by component count. Reliability of complex systems demands a tighter design and hence, increased cost.
Aiming Accuracy- The passive tracker cannot employ very accurate positioning sensors which would need external power supply. Therefore, passive trackers cannot boast accuracy compared to active trackers. As a consequence, passive trackers are best suited to simple panel systems and cannot be used in concentrated PV systems.
Monitoring and Alarms- Powered systems can incorporate elaborate automated monitoring and warning systems. Passive trackers cannot do that.
Morning and Evening Performance- The passive tracking system may take quite some time in the morning before the irradiation is high enough for it to start operating. Similarly, the tracking will become ineffective in the evening when solar irradiation is decreased.
Low Temperature- Passive trackers may not work at low temperatures.
Wind- Active trackers have the ability to compensate for wind loading. The passive tracker may not. Active systems have the ability to shut down and lock-up in a safe condition during wind storms and resume operation automatically. The passive tracker must be manually tied down and then released brought to the correct position to resume operation.
Cost- The passive tracker is inexpensive, but the active tracker is much more costly.