Solar tracker
A solar tracker is a device that orients a payload toward the Sun. Payloads are usually solar panels, parabolic troughs, Fresnel reflectors, lenses, or the mirrors of a heliostat.
For flat-panel photovoltaic systems, trackers are used to minimize the angle of incidence between the incoming sunlight and a photovoltaic panel, sometimes known as the cosine error. Reducing this angle increases the amount of energy produced from a fixed amount of installed power-generating capacity.
As the pricing, reliability, and performance of single-axis trackers have improved, the systems have been installed in an increasing percentage of utility-scale projects. The global solar tracker market was 111 GW in 2024, 94 GW in 2023, 73 GW in 2022, and 14 gigawatts in 2017. In standard photovoltaic applications, it was predicted in 2008–2009 that trackers could be used in at least 85% of commercial installations greater than one megawatt from 2009 to 2012.
In concentrator photovoltaics and concentrated solar power applications, trackers are used to enable the optical components in the CPV and CSP systems. The optics in concentrated solar applications accept the direct component of sunlight light and therefore must be oriented appropriately to collect energy. Tracking systems are found in all concentrator applications because such systems collect the sun's energy with maximum efficiency when the optical axis is aligned with incident solar radiation.
Basic concept
Sunlight has two components: the "direct beam" that carries about 90% of the solar energy and the "diffuse sunlight" that carries the remainder – the diffuse portion is the blue sky on a clear day, and is a larger proportion of the total on cloudy days. As the majority of the energy is in the direct beam, maximizing collection requires the Sun to be visible to the panels for as long as possible. However, on cloudier days the ratio of direct vs. diffuse light can be as low as 60:40 or even lower.The energy contributed by the direct beam drops off with the cosine of the angle between the incoming light and the panel. In addition, the reflectance is approximately constant for angles of incidence up to around 50°, beyond which reflectance increases rapidly.
| Angle i | Hours | Loss |
| 0° | 0% | |
| 1° | 0.015% | |
| 3° | 0.14% | |
| 8° | 1% | |
| 15° | 1 | 3.4% |
| 23.4° | 8.3% | |
| 30° | 2 | 13.4% |
| 45° | 3 | 30% |
| 60° | 4 | >50% |
| 75° | 5 | >75% |
Notes
For example, trackers that have accuracies of ± 5° can capture more than 99.6% of the energy delivered by the direct beam plus 100% of the diffuse light. As a result, high-accuracy tracking is not typically used in non-concentrating PV applications.
The purpose of a tracking mechanism is to follow the Sun as it moves across the sky. In the following sections, in which each of the main factors are described in a little more detail, the complex path of the Sun is simplified by considering its daily east-west motion separately from its yearly north-south variation with the seasons of the year.
Solar energy intercepted
The amount of solar energy available for collection from the direct beam is the amount of light intercepted by the panel. This is given by the area of the panel multiplied by the cosine of the angle of incidence of the direct beam. Put another way, the energy intercepted is equivalent to the area of the shadow cast by the panel onto a surface perpendicular to the direct beam.This cosine relationship is very closely related to the observation formalized in 1760 by Lambert's cosine law. This describes that the observed brightness of an object is proportional to the cosine of the angle of incidence of the light illuminating it.
Reflective losses
Not all of the intercepted light is transmitted into the panel; some is reflected at its surface. The amount reflected depends on both the refractive index of the surface material and the angle of incidence of the incoming light. The amount reflected also differs depending on the polarization of the incoming light. Incoming sunlight is a mixture of all polarizations, with equal amounts in direct sunlight. Averaged over all polarizations, the reflective losses are approximately constant at angles of incidence up to around 50°, beyond which they increase rapidly. See for example the accompanying graph, appropriate for glass.Solar panels are often coated with an anti-reflective coating, which is one or more thin layers of substances with refractive indices intermediate between those of silicon and air. This causes destructive interference in the reflected light, diminishing the reflected amount. Photovoltaic manufacturers have been working to decrease reflectance with improved anti-reflective coatings and with textured glass.
Daily east-west motion of the Sun
The Sun travels through 360° east to west per day, but from the perspective of any fixed location, the visible portion is 180° during an average half-day period. Local horizon effects reduce this somewhat, making the effective motion about 150°. A solar panel in a fixed orientation between the dawn and sunset extremes will see a motion of 75° to either side, and thus, according to the table above, will lose over 75% of the energy in the morning and evening. Rotating the panels to the east and west can help recapture those losses. A tracker that only attempts to compensate for the east-west movement of the Sun is known as a single-axis tracker.Seasonal north-south motion of the Sun
Due to the tilt of the Earth's axis, the Sun also moves through 46° north and south during a year. The same set of panels set at the midpoint between the two local extremes will thus see the Sun move 23° on either side. Thus according to the above table, an optimally aligned single-axis tracker will only lose 8.3% at the summer and winter seasonal extremes, or around 5% averaged over a year. Conversely a vertically- or horizontally-aligned single-axis tracker will lose considerably more as a result of these seasonal variations in the Sun's path. For example, a vertical tracker at a site at 60° latitude will lose up to 40% of the available energy in summer, while a horizontal tracker located at 25° latitude will lose up to 33% in winter.A tracker that accounts for both the daily and seasonal motions is known as a dual-axis tracker. Generally speaking, the losses due to seasonal angle changes are complicated by changes in the length of the day, increasing collection in the summer in northern or southern latitudes. This biases collection toward the summer, so if the panels are tilted closer to the average summer angles, the total yearly losses are reduced compared to a system tilted at the spring/fall equinox angle.
There is considerable argument within the industry about whether the small difference in yearly collection between single- and dual-axis trackers makes the added complexity of a two-axis tracker worthwhile. A recent review of actual production statistics from southern Ontario suggested the difference was about 4% in total, which was far less than the added costs of the dual-axis systems. This compares unfavorably with the 24–32% improvement between a fixed-array and single-axis tracker.
Other factors
Clouds
The above models assume uniform likelihood of cloud cover at different times of day or year. In different climate zones cloud cover can vary with seasons, affecting the averaged performance figures described above. Alternatively, for example in an area where cloud cover on average builds up during the day, there can be particular benefits in collecting morning sun.Atmosphere
The distance that sunlight travels through the atmosphere increases as the sun approaches the horizon, as the sunlight travels diagonally through the atmosphere. As the path length through the atmosphere increases, the solar intensity reaching the collector decreases. This increasing path length is referred to as the air mass or air mass coefficient, where AM0 is at the top of the atmosphere, AM1 refers to the direct vertical path down to sea-level with Sun overhead, and AM greater than 1 refers to diagonal paths as the Sun approaches the horizon.Even though the sun may not feel particularly hot in the early mornings or during the winter months, the diagonal path through the atmosphere has a less than expected impact on the solar intensity. Even when the sun is only 15° above the horizon the solar intensity can be around 60% of its maximum value, around 50% at 10° and 25% at only 5° above the horizon. Therefore, if trackers can follow the Sun from horizon to horizon, then their solar panels can collect a significant amount of energy.
Solar cell efficiency
The underlying power conversion efficiency of a photovoltaic cell has a major influence on the end result, regardless of whether tracking is employed.Temperature
Photovoltaic solar cell efficiency decreases with increasing temperature, at the rate of about 0.4%/°C. For example, there is about 20% higher efficiency at 10 °C in early morning or winter than at 60 °C in the heat of the day or summer. Therefore, trackers can deliver additional benefit by collecting early morning and winter energy when the cells are operating at their highest efficiency.Summary
Trackers for concentrating collectors must employ high-accuracy tracking so as to keep the collector at the focus point.Trackers for non-concentrating flat-panel do not need high accuracy tracking:
- low power loss: under 10% loss even at 25° misalignment
- reflectance consistent even to around 50° misalignment
- diffuse sunlight contributes 10% independent of orientation, and a larger proportion on cloudy days.
- power loss rises rapidly beyond about 30° misalignment
- significant power is available even when the Sun is very close to the horizon, e.g. around 60% of full power at 15° above the horizon, around 50% at 10°, and even 25% at only 5° above the horizon – of particular relevance at high latitudes and/or during the winter months
- photovoltaic panels are around 20% more efficient in the cool of the early mornings as compared with during the heat of the day; similarly, they are more efficient in winter than summer – and effectively capturing early morning and winter sun requires tracking.