3.1. Why tracking?
Solar tracking is a technology for orienting a solar collector, reflector, or photovoltaic panel towards the sun. As the sun moves across the sky, a tracking device makes sure that the solar collector automatically follows and maintains the optimum angle to receive the most of the solar radiation. Some solar concentrators hugely benefit from tracking, while some others do not. So, the tracking systems can be added with additional cost and certain trade-offs in system design only when it pays off.
The required accuracy of tracking varies with application. For example, concentrators, especially in solar cell applications, require a high degree of accuracy to ensure that the concentrated sunlight is directed precisely to the solar conversion element. Tracking the sun from east in the morning to west in the evening can increase the efficiency of a solar panel up to 45%, according to some manufacturers [Linak]. Precise tracking of the sun is achieved through systems with single or dual axis tracking.
Watch this introductory video (5:33), which provides an illustration to the benefits of sun tracking:
So, what types of systems should include tracking devices (a.k.a. trackers)?
First of all, the systems that specifically utilize the direct beam radiation benefit from tracking. In majority of concentrating solar power (CSP) systems, the optics accept only the beam radiation and therefore must be oriented appropriately to collect energy. Such systems will not produce power unless pointed at the sun. Tracking is required for heliostats in central receiver (solar tower) systems. CSP collectors require significant degree of accuracy of sun tracking.
In photovoltaic (PV) applications, tracking devices can be used to minimize the angle of incidence of incoming solar rays onto a PV panel. This increases the amount of energy produced per unit of installed power generating capacity. This increases the efficiency of the system and its cost-effectiveness, but, at the same time, tracking is not strictly required for regular flat panel PV as they accept both beam and diffuse radiation.
In concentrating photovoltaics (CPV), the optics requires beam radiation and therefore must be oriented appropriately to focus light on the PV collector to maximize the energy converted. CPV modules that concentrate in one dimension must be tracked normal to the sun in one axis. CPV modules that concentrate in two dimensions must be tracked normal to the sun in two axes [Solar Tracker from Wikipedia.org]. CPV modules require high degree of accuracy of sun tracking.
There are many types of solar trackers, which are different in costs, design complexity, and performance. But we can distinguish two basic classes of systems:
- Single axis trackers
The single axis solar trackers can either have a horizontal or a vertical axis. The horizontal axis is used in tropical regions where the sun gets very high at noon, but the days are short. The vertical type is used in high latitudes, where the sun does not get very high, but summer days can be very long. In concentrated solar power applications, single axis trackers are used with parabolic and linear Fresnel mirror designs.
- Dual axis trackers
The dual axis solar trackers have both a horizontal and a vertical axis and thus they can track the sun's apparent motion at any location. Dual axis tracking is commonly used for CSP applications, such as solar power towers and dish (Stirling engine) systems. Dual axis tracking is extremely important in solar tower applications due to the angle errors resulting from longer distances between the mirror and the central receiver located in the tower structure.
In more detail, these types of trackers will be studied in Section 3.3. of this lesson.
With tracking incorporated in the system design, the cost of the system is understandably higher compared to fixed tilt systems. According to the US DOE report [Barbose et al., 2013], "among projects completed in 2012, the capacity-weighted average installed price in US dollars was 3.3/W for systems with crystalline modules and fixed tilt, compared to 3.6/W for crystalline systems with tracking and 3.2/W for thin-film, fixed-tilt systems." Efforts are constantly made by manufacturers to lower the cost of the tracking systems, making them less complex, more compact, reliable, and easier to maintain. In spite of the additional costs, use of trackers is often a preferred option for utility-scale installations due to the significant boost to the system performance. Figure 3-1 shows the trend of increasing use of tracking systems in the U.S. utility-scale PV installations over the 2007–2017 decade. Cumulative tracking system installation reached 79% in 2017 (meaning that only 21% of large PV installations opt not to use trackers). These data include both one-axis and dual-axis tracking systems cumulatively, however there are many more one-axis trackers deplyed than dual-axis trackers.