Introduction to Solar Trackers

In the global energy industry there is always the push for more efficient energy production, which is measured by the LCOE (levelised cost of electricity). Solar energy can already provide the lowest LCOE for most parts of the world in most applications, but there are more breakthroughs on the horizon for solar PV plants to enable them produce electricity even more efficiently.

One of the areas that has provided great gains in efficiency over the last few years, and also holds the promise of more breakthroughs in years to come, is in the area of solar trackers. In this article we will define solar tracking systems and explain why they are useful, we will discuss the applications and best practices as it applies to solar PV plants, and we will look at the cost versus yield to understand how it can reduce your LCOE.

What are solar trackers?

A solar tracker is any device that tracks the movement of the sun across the sky. These have many applications, but for the energy industry they are most commonly used for solar projects, both CSP and PV. CSP is concentrated solar power, this is technology that uses mirrors or reflective panels to concentrate the rays of the sun at a certain point which will become superheated and can be used to generate electricity. Trackers for PV orient the photo voltaic panels to continually receive the most direct irradiation from the sun. Since the power production from PV systems depends on the irradiation received by the panels, such a tracking system is very helpful in maximizing their output.

The trackers required for CSP are much more sensitive and exacting than the trackers required for PV—often within a tenth of a degree of accuracy, while those for PV panels can be less exacting.

The image above is an example of concentrates solar plant with trackers.

Solar trackers use various electrical components to orient the panels to receive the most direct sunlight possible in order to maximize the solar irradiance on the panel. The components of solar trackers include actuators, motors, and sensors and other components. These devices are able to then orient the panels or reflectors to face the sun at the most direct angle possible, thereby receiving the most irradiation possible and increasing the performance and efficiency of the solar plant.

In order to understand solar trackers better, let us take a moment and discuss how the sun moves in the sky. First of all, we should address that we know that the sun does not actually move relative to the earth, and it is the earth that is rotating and moving around the sun. However for ease of discussion we will be speaking from the point of view of a fixed location on earth where it appears the sun is moving in the sky.

In this solar journey, the sun has two types of movements: there is the daily east to west movement, and there is the annual movement in the north and south direction. This is the reason trackers can be categorized as single axis trackers, or dual axis trackers. Single axis trackers allow for the panels to rotate along an axis that tracks the sun’s daily movement from east to west. And the dual axis trackers have another axis of tracking for the annual north-south of the sun.

If you are familiar with solar installations you should know that panels are usually installed at a tilt to provide optimal irradiation. The direction and angle of this tilt is based on the location of the site, primarily based on the longitude of the installation. Installations in the northern hemisphere should be tilted facing south, and installations in the southern hemisphere should be tilted facing north. The degree of tilt varies according to longitude, generally the longitude of the installation site is a good reference for the angle of tilt. For example a fixed tilt installation in Sacramento, California, which is at 38.5 degrees North longitude could be facing south and tilted at approximately 38.5 degrees.

A single axis tracker in this scenario would keep the same tilt, but it would have an axis of movement that would allow the panels to rotate to face to the east early in the morning, and would follow the sun in the sky during the day to face west in the evening, and then east again the following morning, all while keeping the same north-south tilt.

The above photos are from Nextracker.

This is a good system and provides additional yields which will be discussed later in this article.

But lets return to the movement of the sun for a moment, because the single axis tracker only accounts for the east west movement of the sun, and not the north-south movement.

The north-south movement of the sun happens during the course of the year, so that if you plot the sun’s location on the summer solstice of June 21 at 12:00 noon, and then again at the winter solstice of December 21 at 12:00 noon you will notice a change in the location of the sun.

These images show the path of the sun on the day of June 21st, and then again the path of the sun on December 21st. You can see the sun is significantly higher during the summer months which provides more intense irradiation and also higher temperatures. And for the winter months the sun is much lower in the sky providing less direct irradiance and lower temperatures. (Please note both of these images are examples from the northern hemisphere).

During different times of the year the optimum north-south tilt will vary. In a fixed tilt installation or a single axis tracking installation the panels are positioned to try to get the optimum irradiance throughout the year, not taking into account this annual movement of the sun.

Therefore, if a panel were to have optimal solar tracking there would need to be two axises of movement, one to track the daily movement of the sun and another to track the annual movement of the sun.

The chart below shows an example of the optimal north-south tilt throughout the year for various locations of longitude so you can see how the optimal north-south tilt changes with the seasons.

Dual axis trackers are a way to provide additional power production from your solar system by allowing the panels to adjust their north-south angle as well so that they can face the sun more directly across all seasons of the year.

Generally single axis trackers are arranged in long rows with the panels in portrait orientation. They are commonly arranged 2 in portrait (2P) or 3 in portrait (3P).

Dual axis trackers are generally mounted on center poles. Often the second axis of movement is achieved but rotating the poles effectively allowing the panels to track the north south movement of the sun.

Another way to group tracker technology, besides the axis and ability to move the panels in one or two directions, is based on the method of moving the panels.

There are passive systems that adjust their angle with the use of various compressed gasses or memory shape alloys that are sensitive to unbalanced irradiation on the top and bottom, and they will adjust themselves to face the sun directly and get balanced and maximum irradiation.

"This system does not involve mechanical drives to orient the panel towards the sun’s radiations. Instead it uses some low boiling point compressed gas fluid or shape memory alloys as actuators which on receiving unbalanced illumination, forces the panel to undergo some angular movement so as to re-establish equilibrium of irradiance by inducing thermal expansion in expansible gases or in on-shape memory alloys. When one side of the liquid gas receives more amount of heat energy than the other, then the gas expands and moves towards the other side of the tracker. It causes an unbalanced gravitational pull and forces the panel to tilt until a point of equal illumination is reached"

- https://www.sciencedirect.com/science/article/pii/S2352484719304780

Then there are active tracking systems:

"These systems use electrical drives and mechanical gear trains to orient the panels normal to the sun’s radiations. It uses sensors, motors and microprocessors for the tracking and are more accurate and efficient than the passive solar trackers. But on the other hand they are needed to be powered and consume energy"

- https://www.sciencedirect.com/science/article/pii/S2352484719304780

 Another way to group trackers is the method used to follow the sun. There are systems that track the sun based on algorithms that plot the trajectory of the sun based on historical and projected models. And there are others that use optical lenses to actually follow the sun, and then there are systems that use a combination of both.

And finally open loop trackers have no feedback system to perfect their movements, and closed loop trackers have feedback systems that allow them to adjust their movements more accurately. This allows for both coarse and fine adjustments to the movements.

Of all these types, electro optical based active trackers with closed loop feedback are the most popular.

What are the benefits of tracking systems?

Without a doubt trackers create increased energy generation in PV plants. Below is a graph of a study that shows the energy generation from a demonstration site that had three installations side by side to demonstrate the differences in energy yield between a fixt tilt installation, a single axis tracker installation, and a dual axis tracker installation. The four graphs represent a day in spring, summer, fall and winter.

As you can see there is a significant increase in power in all seasons with the tracking systems.

Generally the data across sites and manufacturers shows that a single axis tracking system will generate around 20-30% more energy than the equivalent system as fixed tilt. And the data shows that a dual axis tracker can add around 8-10% more power than the single axis tracker system.

However in order to fully understand if trackers make sense, we must look not only at the increased power production, but also at the costs and challenges associated with tracking systems. Solar trackers only make sense when the increased yield over fixed tilt installations is greater than the increased cost of the tracking system. This is ultimately reflected in the LCOE.

Costs and Challenges of Solar Tracking systems

The cost of solar tracking systems has fallen considerably over the past decade, mostly due to better data and better engineering, as well as increased overall demand for the systems.

The cost of solar trackers can vary by site and application, they certainly require extra effort in the design and planning stages compared to fixed tilt installations.

One reason for this is the increased wind thrust on tracking installations. Trackers are required to be installed higher than fixed tilt installations to allow for the movement of the panels and the mechanical equipment required for the movement. This increased height and the movement of the panels will catch the wind at various times of year, which means that it is more important to understand the wind speeds throughout the year at tracking installations, and to make sure that the racks are strong enough to withstand the thrust forces.

Another consideration is that as the panels track the sun, there will be increased shadows from the panels, especially in the morning and evening. Care must be taken so that the shadows from one row to not fall on the next row, thereby eliminating some of the benefit from the more direct irradiation. This increased shading leads to irradiance losses, but also electrical and performance losses in the form of string mismatch.

The above photo is from Nextracker

One consequence of this is that tracking installations require more space between rows to minimize this shading, therefore requiring more land area than fixed tilt installations.

Another challenge is the increased operation and maintenance costs. Adding motors, sensors and moving parts to an installation carries increased maintenance and operation costs that must be accounted for. And because the technology is progressing very rapidly we have not yet seen the full O&M costs over the lifetime of many of these systems.

Yet another challenge with using trackers is how to accurately predict the yield of the plant. Tracker manufacturers have sought to build better 3D models of their equipment and their algorithms to help designers and engineers predict the output more accurately, but there is still room for improvement on this front. The shortcomings of the predictive models are increased if the terrain is not completely flat. PVSyst and other solar modeling software do not yet accurately account for undulating terrain when it comes to trackers, and the shadows from one row to the next on undulating terrain is very complicated.

Despite all of these challenges, the solar tracker market is increasing rapidly. This is because the yield gains of single axis trackers are out pacing the costs of the single axis trackers.

Generally speaking, the cost of a single axis tracker system will be about 35% of the total cost of your solar panels. This translates to approximately 10-15% increased costs over a similar fixed tilt system. As mentioned earlier the yield gains of a single axis tracker can be 20-30% over an equivalent fixed tilt system, effectively reducing your LCOE by around 10-20%.

Dual axis trackers are much more expensive than single axis trackers. The dual axis trackers require more complex technology coupled with high-maintenance for motors and control systems result in high O&M expenditure for dual-axis trackers, as well as higher land requirements. These factors act as a major restraint for the wider adoption of dual-axis trackers.

The cost of a dual axis tracker can add as much as 50 percent to the overall cost of your PV plant over a fixed tilt installation. The yield gains of dual axis trackers are generally around 30-40%, which means the cost is higher than the yields which therefor makes dual axis trackers unattractive for most types of installation. Typically, dual axis trackers have been used on highly specialized installations: sometimes on farms for dual use of the land because the poles with the dual axis tracking can be high enough for crops, animal grazing beneath the panels, and farm equipment can also pass underneath the panels. Some residences in crowded urban areas where space is limited can use a dual axis tracker system on their roof. Or in very high longitudes far from the equator dual axis trackers may make sense.

 Tracker Market

These days single axis trackers are almost standard on utlility scale installations. Approximately 70% of utility scale installations in 2018 utilized single axis tracking technology. The vast majority of solar trackers are in ground mount single axis utility scale installations, which accounted for 85% of the trackers installed in 2018.

In 2018, the global solar tracker market demand was in excess of 25GW. Here is a chart showing the historical and projected demand of solar trackers through 2025 where the technology is expected to be in approaching of 150GW.

The total value of the global solar tracker market size was estimated at 2.35 billion USD in 2019, and 3.01 billion USD in 2020, and projected to reach 11.09 billion USD by 2025.

These figures show that tracking technology is not going away.

Opportunities and Potential for growth

There have been some recent advances in the tracker technology that continue to reduce the costs and increase the efficiency.

One of these is the so called “backtracking” algorithms. These are algorithms designed to predict and reduce the inter row shading. Tracker manufacturers have been working on these algorithms for decades and the data shows that they are continuing to make progress. Undulating terrain still presents a challenge for these algorithms and more developments are on the way.

Another recent advance is in the “stowing” of the panels. This is when the trackers are able to use data to put the panels in a flat or stowed position, There are two scenarios when this can be very advantageous. One is when the wind speeds are high and there is potential of damage to the panels and the racking systems. Some of the latest tracking systems can detect these dangerous wind conditions and put the panels into a flat position to reduce the risk of damage. Advanced versions of this technology can even detect differences in the risk per row and only stow the rows that are at risk to be damaged while the others can continue to face the sun.

The other advantageous scenario of this stowing mechanism is when the sun is hidden behind clouds. Studies have shown that when the sun is behind clouds the diffuse light energy becomes very high and the direct light energy becomes very low. This means that orienting the panels to the location of the sun behind the clouds results in decreased irradiation compared to a flat or stowed position. Latest tracking technology can account for this with sensors and intelligent computers that can put they panels into a flat position when the sun is hidden by clouds, and then return to direct tracking once the clouds are gone, leading to increased yields.

And another area of great potential in the tracking space is with the use of bifacial modules. Bifacial modules are PV panels that have the solar cells on the front and back of the panels. Studies have shown that using bifacial panels with single or dual axis trackers can increase the yields, and often the cost is low enough that the yields are greater than the costs, thereby resulting in a reduced LCOE.

A study by NREL showed that using single axis trackers with bifacial modules results in the lowest LCOE out of all other installation types for more than 90% of the land area of earth. Bifacial modules present their own challenges which I have discussed in another article. Relating to trackers, the challenge of the bifacial modules is around the yield projections, and therefore to the bankability and financial confidence of the yield of such installations.

To calculate the correct angle of the trackers to maximize the combined front and rear production of bifacial modules is very complicated. In addition to the challenges mentioned earlier of predicting the output of tracker systems (angles of tilt, undulating terrain, inter row shading, irradiance losses, electrical losses, etc.) the use of bifacial modules requires to factor in the albedo of the environment and how it changes throughout the year, increased mismatch losses, complex shadows cast by the racking system itself onto the ground and onto the panels. All of which makes the projections less accurate and therefore the extra yield from these systems is less bankable in the eyes of solar financiers. Breakthroughs in the modeling technology are necessary to boost this sector of the industry.

Conclusion

In conclusion, solar trackers are a feature of the solar industry that is here to stay. It has already achieved lower LCOEs and widespread adoption, especially in ground mount utility scale installations. As more data and better engineering continues to develop, we will see trackers being used in more applications including rooftop installations, and perhaps even floating solar installations. And with the development of tracking technology, the use of bifacial modules will continue to grow so that in a few years trackers with bifacial modules will be the standard of utility scale installations, and perhaps C&I installations as well.