What is a Solar Tracker?
A solar tracker is a device that directs a payload toward the sun. Payloads are typically solar panels, parabolic troughs, fresnel reflectors, lenses, or the mirrors of the heliostat. Because solar trackers follow the sun, they constantly have to change their orientation throughout the day so as to maximize energy capture.
In photovoltaic systems, solar trackers help minimize the angle of incidence, which is the angle that a ray of light makes with a line perpendicular to the surface, between the incoming light and the panel. Minimizing the angle increases the amount of energy the installation produces. Furthermore, in standard photovoltaic applications, it was predicted back in 2008-2009 that solar trackers could be used in at least 85% of commercial installations greater than one megawatt (MW) from 2009 to 2012.
Concentrated solar photovoltaics and concentrated solar thermal have optics that directly accept sunlight, so the solar trackers in these systems have to be angled correctly to collect energy. All concentrated solar systems have trackers because the systems can’t produce energy unless directed properly toward the sun.
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. According to data from Wood Mackenzie/GTM Research, global solar tracker shipments hit a record of 14.5 gigawatts (GW) in 2017. This shows a growth of 32% year-over-year, with similar or greater growth projected as large-scale solar deployment continues to accelerate.
Basic Concept of Solar Trackers
Sunlight has two components: the first is the “direct beam” that carries about 90% of the solar energy, and the second is the “diffuse sunlight” that carries the remainder. The diffuse portion is the blue sky on a clear day and is the larger proportion of the total on cloudy days. Since 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, it is not possible for this to happen all the time because the weather is constantly changing. So, there will be times that a day will be cloudy, and on days like that, 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. Additionally, the reflectance, which is averaged across all polarizations, is approximately constant for angles of incidence up to around 50°, beyond which reflectance degrades rapidly.
Direct Power Lost (%) Due to Misalignment (angle i) Where Lost = 1 – cost (i)
For example, solar trackers that have accuracies of ±5° can deliver greater 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 usually used in non-concentrating PV applications.
Factors That Affect Solar Trackers
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. In other words, 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 that was formalized in 1760 by Lambert’s cosine law. This cosine law describes that the observed brightness of an object is proportional to the cosine of the angle of incidence of the light illuminating it.
Not all of the light that is intercepted is transmitted into the panel; a little is reflected at its surface. The amount reflected is influenced by both the refractive index of the surface material and the angle of incidence of the incoming light. Additionally, the amount reflected also differs, depending on the polarization of the incoming light. Incoming light is a mixture of all polarizations. Averaged over all polarizations, the reflective losses are approximately constant up to angles of incidence up to around 50°, beyond which it degrades rapidly.
Daily East-West Motion of the Sun
The sun travels through 360° east to west every day. But from the perspective of any fixed location, the visible portion is 180° during an average half-day period — this is more in spring and summer, but less in fall and winter. Local horizon effects reduce this a little bit, making the effective motion about 150°.
A solar panel in fixed orientation between the dawn and sunset extremes will see a motion of 75° to either side, thus losing over 75% of the energy in the morning and evening. Rotating the panels to the east and west can help recapture those losses. A solar tracker that only attempts to compensate for the east-west movement of the sun is also known as a single-axis tracker.
Seasonal North-South Motion of the Sun
In addition to the fact that the sun moves from east to west every day, it also moves through 46° north and south during a year. This is due to the tilt of the Earth’s axis. The same set of panels that are set at the midpoint between the two local extremes will then see the sun move 23° on either side. In other words, 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° will lose up to 33% in winter.
A tracker that accounts for both the daily and seasonal motions is called the dual-axis tracker. This kind of tracker continually faces the sun because it can move in two different directions. Generally speaking, the losses that occur because of the seasonal angle changes is complicated by changes in the length of the day, thus increasing collection in the summer in northern or southern latitudes. This instance proves that collection is more inclined in the summer. So, if the panels are tilted closer to the average summer angles, the total yearly losses are reduced compared to a system that is tilted at the spring or fall equinox angle (which is similar to the site’s latitude).
There is a substantial argument within the solar industry on whether the small difference in the yearly collection between single- and dual-axis trackers makes the added complexity of a dual-axis tracker valuable. A recent review of actual production statistics from southern Ontario has suggested that the difference was about 4% in total, which was far less than the added costs of the dual-axis systems. This does not compare favorably with the 24-32% improvement between a fixed array and a single-axis tracker.
The factors mentioned above assume a uniform likelihood of cloud cover at different times of day or year. However, in different climate zones, cloud cover can vary with seasons, thus affecting the averaged performance figures as described above. On the other hand, another example is that in an area where cloud cover on average builds up during the day, there can be particular benefits in collecting morning sun.
The distance that sunlight has to travel through the atmosphere increases as the sun approaches the horizon since the sunlight has to travel diagonally through the atmosphere. As the path length through the atmosphere increases, the solar intensity reaching the collector decreases. This increasing path length is also known as the air mass (AM) 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 the 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. Additionally, even when the sun is only 15° above the horizon, the solar intensity can still be around 60% of its maximum value, around 50% at 10° and 25% at only 5° above the horizon. Thus, solar trackers can deliver benefits by collecting the significant energy that is available when the sun is close to the horizon.
Solar Cell Efficiency
Expectedly, the underlying power conversion efficiency of a solar photovoltaic cell has a major influence on the end result, regardless of whether tracking is employed or not. Essentially speaking, solar cell efficiency refers to the portion of energy in the form of sunlight that can be converted via photovoltaics into electricity by the solar cell.
The efficiency of the solar cells used in a photovoltaic system, combined with latitude and climate, determines the annual energy out of the system. Additionally, there are several factors that affect a cell’s conversion efficiency value. Some of these factors include reflectance efficiency, charge carrier separation efficiency, charge carrier collection efficiency, and conduction efficiency values. Because these parameters can be difficult to measure directly, other parameters are measured instead, such as quantum efficiency, open-circuit voltage (VOC) ratio, and § Fill factor.
Of particular relevance to the benefits of solar tracking are the following:
- Molecular structure. Much research is aimed at developing surface materials to guide the maximum amount of energy down into the cell and minimize reflective losses.
- Temperature. Photovoltaic solar cell efficiency decreases with increasing temperature, at the rate of about 0.4%/°C. For example, 20% higher efficiency will happen at 10°C in the early morning or winter as compared with 60°C in the heat of the day or summer. Thus, solar trackers can deliver additional benefit by collecting early morning and winter energy when the cells are operating at their highest efficiency.
The Importance of Solar Trackers
In order to keep the collector at the focus point, solar trackers for concentrating collectors must employ high accuracy tracking. On the other hand, solar trackers for non-concentrating flat-panel do not need high accuracy tracking for a variety of reasons. To begin with, there is only under 10% loss even at 25° misalignment, and the reflectance is consistent even to around 50° misalignment.
Moreover, the benefits of tracking non-concentrating flat-panel collectors flow from the following:
- Power loss degrades rapidly beyond 30° misalignment.
- Significant power is available even when the sun is very close to the horizon, such as around 60% of full power at 15° above the horizon, around 50% at 10°, and even 25% at only 5° above the horizon. High latitudes and/or during the winter months are of particular relevance.
- 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 to effectively capture early morning and winter sun, tracking is required.
What is a Solar Collector?
As the name suggests, a solar collector is a device that collects and/or concentrates solar radiation from the sun. This device is mainly used for active solar heating and allows for the heating of water for personal use. Usually, solar collectors are mounted on the roof and must be very sturdy as they are exposed to a variety of different weather conditions.
The use of these solar collectors provides an alternative for traditional domestic water heating using a water heater, thus potentially reducing energy costs over time. Just like in domestic settings, a large number of these collectors can be combined in an array and used to generate electricity in solar thermal power plants.
Types of Solar Collectors
There are a lot of different types of solar collectors, but all of them are constructed with the same basic premise in mind. Generally speaking, there is some material that is used to collect and focus energy from the sun and use it to heat water. The simplest of these devices make use of a black material surrounding the pipes that water flows through. This black material absorbs the solar radiation very well, and as the material heats up the water, it surrounds.
Even though the usual design of solar collectors is simple, they can still get very complex at times. Absorber plates can be used if a high-temperature increase is not necessary, but generally, the devices that use reflective materials to focus sunlight result in a greater temperature increase.
Flat Plate Collectors
Flat plate collectors are simply metal boxes that have some sort of transparent glazing as a cover on top of a dark-colored absorber plate. The sides and bottom of this kind of collector are typically covered with insulation to minimize heat losses to other parts of the collector.
Solar radiation passes through the transparent glazing material and hits the absorber plate. The plate then heats up before transferring the heat to either water or air that is held between the glazing and absorber plate. Sometimes, these absorber plates are painted with special coatings that are designed to absorb and retain heat better than traditional black paint. These plates are usually made out of metal that is a good conductor, such as copper or aluminum.
Evacuated Tube Collectors
Evacuated tube collectors utilize a series of evacuated tubes to heat water for use. These tubes use a vacuum, or evacuated space, to capture the sun’s energy while at the same time minimizing the loss of heat to the surroundings. They have an inner metal tube that acts as the absorber plate, and it is connected to a heat pipe to carry the heat that is collected from the sun to the water.
This heat pipe is essentially a pipe where the fluid contents are under particular pressure. At this pressure, the “hot” end of the pipe has boiling liquid in it while the “cold” end has condensing vapor. This enables thermal energy to move more efficiently from one end of the pipe to the other. And once the heat from the sun moves from the hot end of the heat pipe to the condensing end, the thermal energy is transported into the water that is being heated for use.
Line Focus Collectors
Line focus collectors, oftentimes known as parabolic troughs, utilize highly reflective materials to collect and concentrate the heat energy from solar radiation. These collectors are made up of parabolically shaped reflective sections that are connected into a long trough. A pipe that carries water is placed in the center of this trough so that sunlight collected by the reflective material is focused on the pipe, heating the contents.
Additionally, line focus collectors are very high-powered collectors. Because of this, they are generally used to generate steam for solar thermal power plants and are not used in residential applications. These troughs can be extremely effective in generating heat from the sun, particularly those that can pivot and track the sun in the sky to ensure maximum sunlight collection.
Point Focus Collectors
Point focus collectors are large parabolic dishes that are composed of some reflective material that focuses the sun’s energy onto a single point. The heat from these collectors is typically used for driving Stirling engines. Although very effective at collecting sunlight, point focus collectors must actively track the sun across the sky to be valuable. These dishes can work alone or be combined into an array to gather even more energy from the sun.
Furthermore, point focus collectors and similar apparatuses can be used as well to concentrate solar energy for use with concentrated photovoltaics. In this case, instead of producing heat, the sun’s energy is converted directly into electricity with high-efficiency photovoltaic cells that are designed specifically to harness concentrated solar energy.
Non-Tracking Fixed Mount
Residential and small-capacity commercial or industrial rooftop solar panels and solar water heater panels are typically fixed, often flush-mounted on an appropriately facing pitched roof. The advantages of fixed mounts over trackers are as follows:
- Mechanical Advantages: simple to manufacture, lower installation and maintenance costs.
- Wind-loading: it is easier and cheaper to provision a sturdy mount. All mounts other than fixed flush-mounted panels must be carefully designed having regard to wind-loading due to greater exposure.
- Indirect Light: approximately 10% of the incident solar radiation is diffuse light, which is available at any angle of misalignment with the sun.
- Tolerance to Misalignment: effective collection area for a flat-panel is relatively insensitive to quite high levels of misalignment with the sun. For example, even a 25° misalignment reduces the direct solar energy collected by less than 10%.
Fixed mounts are generally used in conjunction with non-concentrating systems. However, an important class of non-tracking concentrating collectors, which are particularly valuable in the third world, are portable solar cookers. These utilize relatively low levels of concentration, usually around 2 to 8 suns and are manually aligned.
Even though a fixed flat-panel can be set to collect a high proportion of available noon-time energy, significant power is also available in the early mornings and late afternoons when the misalignment with a fixed panel becomes excessive to collect a reasonable proportion of the available energy. For example, even when the sun is only 10° above the horizon, the available energy can be around half the noon-time energy levels (or even greater, depending on latitude, season, and atmospheric conditions).
Therefore, the primary benefit of a tracking system is to collect solar energy for the longest period of the day, and with the most accurate alignment as the sun’s position shifts with the seasons.
Additionally, the greater the level of concentration employed, the more important accurate tracking becomes. This is because the proportion of energy derived from direct radiation is higher, and the region where that concentrated energy is focused becomes smaller.
Fixed Collector/Moving Mirror
A lot of solar collectors cannot be moved. A prominent example is a high-temperature collector where the energy is recovered as hot liquid or gas (like steam). Other examples are direct heating and lighting of buildings and fixed in-built solar cookers, such as Scheffler reflectors. In cases like this, it is necessary to employ a moving mirror so that, no matter where the sun is positioned in the sky, the sun’s rays are still redirected onto the collector.
Because of the complicated motion of the sun across the sky and the level of precision required to correctly aim the sun’s rays onto the target, a heliostat — which is a device that includes a mirror that turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun’s apparent motions in the sky — generally employs a dual-axis tracking system, with at least one axis mechanized. In other applications, mirrors may be flat or concave.
Solar trackers can be grouped into classes by the number and orientation of the tracker’s axes. In comparison to a fixed collector mount, a single-axis tracker increases annual output by approximately 30%, and a dual-axis tracker an additional 10-20%.
Furthermore, photovoltaic trackers can be classified into two types: standard photovoltaic (PV) trackers and concentrated photovoltaic (CPV) trackers. Each of these tracker types can be further categorized by the number and orientation of their axes, their actuation architecture and drive type, their intended applications, their vertical supports, and foundation.
Floating islands of solar panels are being installed on reservoirs and lakes in the Netherlands, China, the U.K., and Japan. The sun-tracking system that is controlling the direction of the panels operates automatically according to the time of year, and so it changes position by means of ropes that are attached to buoys.
Floating Ground Mount
Solar trackers can be built using a “floating” foundation, which sits on the ground without the need for invasive concrete foundations. Instead of placing the tracker on concrete foundations, the tracker is placed on a gravel pan that can be filled with a variety of materials, like sand or gravel, to secure the tracker to the ground.
Motion Free Optical Tracking
Additionally, solar trackers can be built without the need for mechanical tracking equipment. This kind of solar tracker is called the motion-free optical tracking. For over the past few decades now, there has been some series of advancements in this technology.
Non-Concentrating Photovoltaic Trackers
Photovoltaic panels accept both direct and diffuse light from the sky. And so, the panels on standard photovoltaic trackers gather both the available direct and diffuse light. The tracking functionality in standard photovoltaic trackers is utilized to minimize the angle of incidence between the incoming light and the photovoltaic panel. As a result, this increases the amount of energy that is gathered from the direct component of the incoming sunlight.
The physics behind standard photovoltaic (PV) trackers works with all standard photovoltaic module technologies. These include all types of crystalline silicon panels (either mono-Si or multi-Si), and all types of thin-film panels (amorphous silicon, CdTe, CIGS, and microcrystalline).
Concentrator Photovoltaic (CPV) Trackers
The optics in CPV modules accept only the direct component of the incoming light, and so, they must be oriented appropriately to maximize the energy collected. In low concentration applications, a portion of the diffuse light from the sky can also be captured. The tracking functionality in CPV modules is used to orient the optics such that the incoming light is focused on a photovoltaic collector.
CPV modules that concentrate on one dimension must be tracked normally to the sun in one axis. Likewise, CPV modules that concentrate on two dimensions must be tracked normally to the sun in two axes.
The physics behind CPV optics requires that tracking accuracy increase as the systems concentration ratio increases. However, for a given concentration, non-imaging optics offer the widest possible acceptance angles, which may be used to reduce tracking accuracy.
In typical high concentration systems, tracking accuracy must be in the ±0.1° range to deliver approximately 90% of the rated power output. On the other hand, in low concentration systems, tracking accuracy must be in the ±2.0° range to deliver 90% of the rated power output. With this, it is then no wonder that high accuracy tracking systems are the more common ones.
Concentrated photovoltaic trackers are used with refractive and reflective based concentrator systems. There is a range of emerging photovoltaic cell technologies that are used in these systems. Some of the most popular photovoltaic cell technologies are the conventional ones, such as crystalline silicon-based photovoltaic receivers. But there are times that these technologies can also be germanium-based triple junction receivers.
What are Single-axis Trackers?
As the name suggests, single-axis solar trackers rotate on one axis moving back and forth in a single direction. That is why they have one degree of freedom that acts as an axis of rotation. The axis of rotation of single-axis trackers is usually aligned along a true north meridian. That said, it is possible to align them in any cardinal direction with advanced tracking algorithms.
There are several common implementations of single-axis trackers. These include horizontal single-axis trackers (HSAT), horizontal single-axis tracker with tilted modules (HTSAT), vertical single-axis trackers (VSAT), tilted single-axis trackers (TSAT), and polar aligned single-axis trackers (PSAT). The orientation of the module with respect to the tracker axis is important when modeling performance.
Horizontal Single-Axis Tracker (HSAT)
The axis of rotation for horizontal single-axis tracker (HSAT) is horizontal with respect to the ground. The posts at either end of the axis of rotation of an HSAT can be shared between trackers to lower the installation cost. This kind of solar tracker is perfect for low-latitude regions, and field layouts with HSATs are very flexible. This is because the simple geometry means that keeping all of the axes of rotation parallel to one another is all that is required for appropriately positioning the trackers with respect to one another.
Appropriate spacing can maximize the ratio of energy production to cost, this being dependent upon local terrain and shading conditions and the time-of-day value of the energy produced. Backtracking is one means of computing the disposition of panels. Horizontal trackers usually have the face of the module oriented parallel to the axis of rotation.
As a module tracks, it sweeps a cylinder that is rotationally symmetric around the axis of rotation. In single-axis horizontal trackers, a long and horizontal tube is supported on bearings that mounted upon pylons or frames. The axis of this tube is on a north-south line. Panels are then mounted upon the tube, and the tube will rotate on its axis to track the apparent motion of the sun throughout the day.
Horizontal Single-Axis Tracker with Tilted Modules (HTSAT)
In HSAT, the modules are mounted at 0°, but this is not the case for horizontal single-axis trackers with tilted modules (HTSAT). In HTSAT, the modules are installed at a certain tilt. It works on the same principle as HSAT, keeping the axis of tube horizontal in a north-south line, and rotates the solar modules east to west throughout the day. These trackers are typically suitable in high-latitude locations, but they do not take as much land space as consumed by vertical single-axis trackers (VSAT). As a result, it brings the advantages of VSAT in a horizontal tracker and minimizes the overall cost of a solar project.
Vertical Single-Axis Tracker (VSAT)
The axis of rotation for vertical single-axis trackers (VSAT) is vertical with respect to the ground. These trackers rotate from east to west over the course of the day. They are also more effective at high latitudes than are horizontal axis trackers. Field layouts must consider shading to avoid unnecessary energy losses and to optimize land utilization.
Aside from that, optimization for dense packing is also limited because of the nature of the shading over the course of a year. VSAT usually have the face of the module oriented at an angle with respect to the axis of rotation. As a module tracks, it sweeps a cone that is rotationally symmetric around the axis of rotation.
Tilted Single-Axis Tracker (TSAT)
All trackers with axes of rotation between horizontal and vertical are considered tilted single-axis trackers (TSAT). Tracker tilt angles are usually limited to reduce the wind profile and decrease the elevated end height. With backtracking, they can be packed without shading perpendicular to their axis of rotation at any density. However, the packaging parallel to their axes of rotation is limited by the tilt angle and the latitude. Additionally, TSATs usually have the face of the module oriented parallel to the axis of rotation. As a module tracks, it sweeps a cylinder that is rotationally symmetric around the axis of rotation.
What are Dual-axis Trackers?
Unlike single-axis trackers, dual-axis trackers continually face the sun because they can move in two different directions. As such, they have two degrees of freedom that act as axes of rotation. These axes are usually normal to one another. The axis that is fixed with respect to the ground can be considered a primary axis while the axis that is referenced to the primary axis can be considered a secondary axis.
There are several common implementations of dual-axis trackers. They are typically classified by the orientation of their primary axes with respect to the ground. Two common implementations are tip-tilt dual-axis trackers (TTDAT) and azimuth-altitude dual-axis trackers (AADAT).
The orientation of the module with respect to the tracker axis is crucial when modeling performance. Generally speaking, dual-axis trackers have modules oriented parallel to the secondary axis of rotation. Because of their ability to follow the sun vertically and horizontally, this kind of tracker allows for optimum solar energy levels. Regardless of where the sun is in the sky, dual-axis trackers are able to angle themselves to be in direct contact with the sun.
Tip-Tilt Dual-Axis Tracker (TTDAT)
As the name suggests, a tip-tilt dual-axis tracker (TTDA) has the panel array mounted on the top of a pole. On top of the rotating bearing is a T- or H-shaped mechanism that provides vertical rotation of the panels and provides the main mounting points for the array. The posts at either end of the primary axis of rotation of a tip-tilt dual-axis tracker can be shared between trackers to lower installation costs.
Other TTDATs have a horizontal primary axis and a dependent orthogonal axis. The vertical azimuthal axis is fixed, and this allows for great flexibility of the payload connection to the ground-mounted equipment because there is no twisting of the cabling around the pole.
Field layouts with tip-tilt dual-axis trackers are very flexible. The simple geometry means that keeping the axes of rotation parallel to one another is all that is required for properly positioning the trackers with respect to one another. Normally, the trackers would have to be positioned at fairly low density in order to avid one tracker casting a shadow on others when the sun is low in the sky. Tip-tilt trackers can make up for this by tilting closer to horizontal to minimize up-sun shading, thus maximizing the total power being collected.
The axes of rotation of many tip-tilt dual-axis trackers are usually aligned either along a true north meridian or an east-west line of latitude. Additionally, given the unique capabilities of the tip-tilt configuration and the appropriate controller, totally automatic tracking is possible for use on portable platforms. The orientation of the tracker is of no importance and can be placed as needed.
Azimuth-Altitude Dual-Axis Tracker
An azimuth-altitude (or alt-azimuth) dual-axis tracker (AADAT) has its primary axis — the azimuth axis — vertical to the ground. Thus, the secondary axis, also known as the elevation axis, is typically normal to the primary axis. AADATs are similar to tip-tilt systems in terms of operation, but they differ in the way the array is rotated for daily tracking. Instead of rotating the array around the top of the pole, AADAT systems can use a large ring mounted on the ground with the array mounted on a series of rollers.
The primary advantage of this particular arrangement is that the weight of the array is distributed over a portion of the ring, as opposed to the single loading point of the pole in the TTDAT. Additionally, this arrangement allows AADAT to support much larger arrays. However, unlike the TTDAT, the AADAT system cannot be placed closer together than the diameter of the ring because doing so may reduce the system density, especially considering inter-tracker shading.
Solar Tracker Type Selection
The selection of solar tracker type relies on many factors. These factors include installation size, electric rates, government incentives, land constraints, latitude, and local weather.
Horizontal single-axis trackers are usually used in large distributed generation projects and utility-scale projects. The combination of energy improvement and lower product cost and lower installation complexity results in compelling economics in large deployments. Additionally, the strong afternoon performance is particularly desirable for large grid-tied photovoltaic systems so that production will match the peak demand time.
Horizontal single-axis trackers also add a substantial amount of productivity during the spring and summer seasons when the sun is high in the sky. The inherent robustness of their supporting structure and the simplicity of the mechanism also result in high reliability, which keeps maintenance costs low. And since the panels are horizontal, they can be compactly placed on the axle tube without danger of self-shading. Aside from that, they are readily accessible for cleaning as well.
Meanwhile, a vertical axis tracker pivots only about a vertical axle, with the panels either vertical, at a fixed, adjustable, or tracked elevation angle. The trackers with fixed or seasonally adjustable angles are perfect for high latitudes, where the apparent solar path is not particularly high, but which leads to long days in summer, with the sun traveling through a long arc.
Dual-axis trackers are usually used in smaller residential installations and locations with very high government feed-in tariffs.
What is a Multi-Mirror Concentrating PV?
This device makes use of multiple mirrors in a horizontal plane to reflect sunlight upward to a high-temperature photovoltaic or other system requiring concentrated solar power. Structural problems and expenses are significantly reduced since the mirrors are not greatly exposed to wind loads. Through the employment of a patented mechanism, only two drive systems are required for each device. Because of the configuration of the device, it is particularly suited for use on flat roofs and at lower latitudes.
A multiple mirror reflective system, combined with a central power tower, is employed at the Sierra SunTower, located in Lancaster, California. This generation plant is operated by eSolar, and it started operations on August 5, 2009. This particular system uses multiple heliostats in a north-south alignment, and it also uses pre-fabricated parts and construction as a way of decreasing startup and operating costs.
Types of Drives of Solar Trackers
Active trackers make use of motors and gear trains to perform solar tracking. They can also use microprocessors and sensors, date and time-based algorithms, or a combination of both to detect the position of the sun. In order to control and manage the movement of these massive structures, special slewing drives are designed and rigorously tested. The technologies used to direct the tracker are constantly evolving, and recent developments at Google and Eternegy have included the use of wire-ropes and winches to replace some of the more costly and more fragile components.
Counter-rotating slewing drives sandwiching a fixed angle support can be applied to create a “multi-axis” tracking method which eliminates rotation relative to longitudinal alignment. If placed on a column or pillar, this method will generate more electricity than fixed PV, and its PV array will never rotate into a parking lot drive lane. Additionally, it will also allow for maximum solar generation in virtually any parking lot lane or row orientation, including circular or curvilinear.
Moreover, active two-axis trackers are used to orient heliostats, which are movable mirrors that reflect sunlight toward the absorber of a central power station. As each mirror in a large field will have an individual orientation, these heliostats are controlled programmatically through a central computer system, which also allows the system to be shut down when necessary.
Light-sensing trackers usually have two or more photosensors, like photodiodes, configured differentially so that they output a null when receiving the same light flux. Mechanically, they should be omnidirectional — or, in other words, flat — and are aimed at 90° apart. This will cause the steepest part of their cosine transfer functions to balance at the steepest part, which translates into maximum sensitivity.
Since the motors consume energy, one would want to use them only as necessary. That is why instead of a continuous motion, the heliostat is moved in discrete steps. Additionally, if the light is below some threshold, there would not be enough power generated to warrant reorientation. This is also true when there is not enough difference in light level from one direction to another, such as when clouds are passing overhead. Consideration must be made so as to keep the tracker from wasting energy during cloudy periods.
The most common passive trackers use a low boiling point compressed gas fluid that is driven to one side or the other (by solar heat creating gas pressure) so as to cause the tracker to move in response to an imbalance. Passive tracking is a non-precision orientation, and because of this, it is unsuitable for certain types of concentrating photovoltaic collectors. That said, it does work well for common photovoltaic panel types.
Passive trackers will have viscous dampers to prevent excessive motion in response to wind gusts. Shader or reflectors are used to reflect early morning sunlight to “wake up” the panel and tilt it toward the sun. This can take some hours, depending on the shading conditions. The time to do this can be greatly reduced by adding a self-releasing tiedown that positions the panel slightly past the zenith — this way, the fluid does not have to overcome gravity — and using the tiedown in the evening.
An emerging type of passive tracker for solar photovoltaic panels makes use of a hologram behind stripes of photovoltaic cells so that sunlight passes through the transparent part of the module and reflects on the hologram. This enables sunlight to hit the cell from behind, thus increasing the module’s efficiency. In addition to that, the panel does not have to move since the hologram always reflects sunlight from the correct angle towards the cells.
In some third world nations, drives have been replaced by operators who adjust the trackers. A few benefits of this include robustness, having staff available for maintenance and creating employment for the population in the vicinity of the site.
What are Rotating Buildings?
In Freiburg im Breisgau, Germany, Rofl Disch built the Heliotrope in 1996. The Heliotrope is an environmentally friendly residential building that is rotating with the sun and has an additional dual-axis photovoltaic sail on the roof. It produces four times the amount of energy that the building consumes, and it is the first building in the world to capture more energy than it uses.
Aside from that, another perfect example of a rotating building is the Gemini House. It is a cylindrical house in Austria that rotates in its entirety to track the sun, with vertical solar panels mounted on one side of the building, rotating independently, thus allowing control of the natural heating from the sun.
And, last but not least, the ReVolt House is a rotating, floating house that was designed by TU Delft students for the Solar Decathlon Europe competition in Madrid. The house was realized back in September 2012, and it has a close facade that turns itself towards the sun in summer to prevent the interior space from direct heat gains. Meanwhile, in the winter, the glass facade faces the sun to get direct sunlight in the house.
Advantages of Solar Trackers
Considering that solar trackers shift to follow the sun in the sky, this kind of solar device actually offers a slew of benefits. Some of their primary benefits are as follows:
The attractive point of solar panels with solar trackers is that they are significantly more efficient than the fixed solar panels. A dual-axis solar tracker may be as much as 40% more efficient than a fixed solar panel. And in addition to that, even single-axis trackers can provide a 25% or more boost to the solar power generation.
Every additional photon that is collected is more energy that can be offset from polluting electrical generators like coal-burning power. With that said, having a more efficient method of harnessing solar energy is a really compelling argument towards installing solar trackers.
Variety of Models
When it comes to solar tracking systems, there are many sizes, types, and models to choose from. Aside from single- and dual-axis solar trackers as well as active and passive tracking, there are even more subtypes and brands with different sizes and configurations.
Because of the fact that there is so much variety with solar trackers, it can be easy to work with solar installers to find a type of solar panel tracker that suits the needs of the site. If a site has some unusual circumstances that would need a specific size or height constraints, then that should be discussed with the solar installer. Through communication, solutions can be uncovered.
Solar panel tracking systems do not need much more space than a fixed solar panel. Usually, a solar tracking system will allow your solar panel to pivot within the same area that the fixed panel would fit into. In other words, there is no need for extra space for the movement of solar panels with solar trackers.
Moreover, with solar trackers, there is also no need to have many panels in order to reach your energy generation goals. What this means is that a user can utilize 30% less space, on average, when they are installing solar tracking solar panels, as opposed to fixed solar panels. This is because the energy generation goals can easily be reached if the solar panels have solar trackers.
Additionally, having that extra space can also mean all the difference if the site has limited space or if the space that gets optimum sunlight is limited.
Energy Cost Reduction
If a user is selling energy back to their utility under a Time of Use (TOU) agreement, they can often create even more savings in the summer months with a solar tracker. This is because TOU rate plans for solar power allow for utilities to purchase the power generated during the peak time of the day at a higher rate. And because the sun stays up well into “peak usage time” during long summer months, solar energy can still be harnessed to provide to the grid during these high-use times.
With enough solar panels and a good tracking system, more energy can even be generated that what the home draws. When that happens, the utility bill can be reduced to zero — or it can even begin drawing income from the utility provider.
Of course, this is still subject to the terms and conditions of the local electric company of the user. And these terms and conditions can vary quite a bit from provider to provider. Sill, it is an opportunity to recover some of the costs that are associated with installing solar tracking systems. And it is also an added benefit that an energy company can send you a check instead of a bill.
As the solar industry keeps on growing, technologies associated with it will also continue to improve. This means that the technology behind solar trackers will constantly face advancements. And when there is a rise in technological advancements and reliability in electronics and mechanics, long-term maintenance concerns for solar tracking systems will be significantly reduced.
Disadvantages of Solar Trackers
Even though solar trackers boast a lot of advantages, they also have their own disadvantages. And these drawbacks should be carefully considered first before coming to a decision of purchasing a solar tracking system. Some of the prominent disadvantages of solar trackers are the following.
Generally speaking, getting into solar generation is expensive. A fixed solar panel system already represents a significant outlay for most homeowners. With that said, it is no surprise that solar trackers are even more expensive than fixed panel systems.
If solar trackers add 25% to the cost and improve the output by 25%, the same performance can be achieved by making the system 25% larger. Additionally, a quality dual-axis solar tracker can nearly double the cost of the solar setup from a basic fixed panel system. And the cost only goes up faster than energy generation. A 100% more expensive unit will still only generate 30-40% more energy. This means that it will take longer to regain the expenses.
Solar tracking systems used to be very cost-effective back in the past when photovoltaic modules were still expensive. This is because of the fact that since solar modules were expensive, it was important to use solar tracking so as to minimize the number of panels used in a system with a given power output. But as panels get cheaper, the cost-effectiveness of tracking versus using a greater number of panels decreases. However, in off-grid installations where batteries store power for overnight use, a solar tracking system reduces the hours that stored energy is used, thus requiring less battery capacity. As the batteries themselves are costly — both traditional lead-acid stationary cells or newer lithium-ion batteries — then this cost need adds into the cost-effectiveness equation.
On the bright side, more affordable options for solar trackers do exist, so it’s possible for the costs of installing a solar tracking system not to be doubled. However, the initial outlay of money can still be a real barrier for some people. And aside from that, trading a significant cost increase for a moderate energy increase can put solar trackers out of reach for certain customers.
Installation and Site Preparation
In addition to more expensive purchasing costs for solar trackers, these systems can also be complicated to install. Though they do not take up much more space than fixed trackers, the moving base for the panels may require extra digging, grading, or running of additional wiring to fit on-site and stay secure through the daily movement of the panels.
Moreover, solar trackers are also not suitable for typical residential rooftop photovoltaic installations, especially those homes with pitched roofs. A flat, industrial roof may accommodate solar trackers, but still, trackers are much more likely to be suited for ground installation. The primary reason for this is simply because tracking requires that panels tilt or move, which means that the panels have to be offset a significant distance from the surface. This would require expensive racking and increase wind load. Additionally, such a setup would not make for a very aesthetically pleasing installation on residential rooftops.
Because of all these reasons, solar tracking is not used on residential rooftop installations and is unlikely to ever be used in such installations. This is especially true nowadays since the cost of photovoltaic modules continues to decrease, which makes increasing the number of modules for more power the more cost-effective option.
A solar tracking panel has many more moving parts than a fixed system. This means that long-term maintenance costs may be higher since there are more parts that can wear out or become defective.
Aside from the physical parts like actuators or hinges, most solar panels have internal circuitry that includes wires, chips, and sensors. All these can tell when the device should move, thus helping to maintain correct angles and balance. The electronic and physical parts represent an order of magnitude more complication than a simple fixed panel. This is because, with a fixed panel system, only very few components could fail or wear out. But with a solar panel tracking system, it is very likely that several parts will need regular care or replacement before they become worn and damage the operation of the system.
And maintenance is very important when it comes to solar tracking systems. Without properly maintaining all the working parts of a solar tracker, the initial investment will be wasted. Additionally, any energy savings as a malfunctioning solar tracker will keep the user from getting the solar power that they need.
Solar tracking can also cause some shading problems. As the panels move during the course of the day, it is likely that if the panels are located too close to one another, they may shade one another due to profile angle effects. For example, if there are several panels in a row from east to west, there will be no shading during solar noon. But in the afternoon, panels could be shaded by their west neighboring panel if they are sufficiently close.
As a result, this means that panels must be spaced sufficiently far to prevent shading in systems with tracking, which can reduce the available power from a given area during the peak sun hours. This is not really a big problem if there is sufficient land area to widely space the panels. But it will reduce output during certain hours of the day (e.g. around solar noon) compared to a fixed array.
Aside from shading problems, solar trackers, particularly single-axis tracking systems, are prone to become unstable already at relatively modest wind speeds (galloping). This is because of the torsional instability of single-axis solar tracking systems. Anti-galloping measures, such as automatic stowing and external dampers, must be implemented so as to help the system gain some stability.
Solar systems, in general, are great for the environment, but depending on where a site is located, the environment may not be ideal for a solar tracker.
For example, solar trackers can sometimes have difficulty with snowy conditions. A dual-axis tracker might be able to move more efficiently to heat up the panel and melt snow cover. However, the movement can make the panels somewhat less sturdy when covered in a thick, heavy layer of snow. In addition to that, the extra weight of snow may be a point of concern for the moving parts of the tracker, or it might also affect the delicate balance.
Furthermore, solar tracking may also break down more easily in areas near saltwater. The ocean air can be corrosive to the sensitive parts of solar tracking systems. Basically, the environment should be considered before choosing to install solar trackers.
Solar Tracker Manufacturers
Very much like solar panels, solar trackers are very easily found since there are a lot of manufacturers and wholesalers all over the world that produce and sell them. The following are only some of the popular manufacturers and wholesalers of solar trackers.
Top Solar Tracker Manufacturers in China
- Xiamen Grace Solar Technology. Xiamen Grace Solar Technology Co., Ltd. is a high-tech enterprise that is dedicated to renewable energy investment, engineering PV power station service, and photovoltaic power station construction schemes.
- Jiangsu Aikang Industrial Group. Established in 2006, Jiangsu Aikang Industrial Group Co., Ltd., or Aikang Group for short, is a new energy comprehensive service group headquartered in Zhangjiagang City, Jiangsu Province. In 2016, the company ranked 325th in the list of China’s top 500 private enterprises and ranked 46th in the global new energy 500 list.
- Arctech Solar Holding. Arctech Solar is a solar manufacturing company that is headquartered at Kunshan, Jiangsu Province with an international division in Shanghai.
- Dalian CDS Solar Energy Technology. CDS Solar is a manufacturing company that focuses on bringing innovative solar ESS products and tracking devices to their clients.
- Clenergy Technology. Clenergy Technology started out as a boutique solar solutions provider, but the company has grown to become a passionate and globally renowned renewable energy company. As of right now, the company invests heavily in the research, development, and service of various solar products, particularly mounting systems, solar inverters, and solar trackers.
- Zhenjiang Dacheng New Energy. Established in November 2006, Zhenjiang Dacheng New Energy Co., Ltd. is a high-tech enterprise that integrates R&D, production, sales, and installation of photovoltaic products.
- Daqo Group. Daqo Group is a leading manufacturer of electrical, new energy, and rail transportation. The company’s primary goal is to develop and produce high and low voltage electrical equipment, intelligent components, rail transit equipment, polysilicon, solar cells, and other products.
- Qingdao Eternal Electronic. Qingdao Eternal Electronic Co., Ltd. is a manufacturing company that is deemed as a technology leader in solar tracking system solutions. As such, the company develops and produces state-of-the-art solar tracking systems and sustainable solar mounting brackets from hot-dip galvanized steel high-grade steel.
- Guangdong Xuke Solar Technology. Established in 2013, Guangdong Xuke Solar Technology, also known as Exco Solar, is a professional photovoltaic scaffold manufacturer. The main products that Exco Solar provides include household photovoltaic solar sheds, car shed photovoltaic support systems, tracking bracket systems, BIPV, and more.
- Yiteng New Energy. Yiteng New Energy, also known as Exten Solar, is a company that mainly covers one-stop PV for fixed bracket and photovoltaic tracking system design, site survey, professional testing, mechanics verification, product supply, installation guidance, and more.
Top Solar Trackers Manufacturers in India
- Amberroot Systems. Amberroot Systems was founded in 2009 with the goal of bringing solar energy closer to everyone in India. Some of the most popular products that this company provides include solar batteries, solar inverters, solar charge controllers, and of course, solar trackers.
- Ephysx Technologies. Ephysx Technologies is a leading design and manufacturing company that is focused on developing innovative products and solutions in the field of solar tracking, fleet telematics, smart metering, and IoT. Eventually, the company became the largest solar tracker manufacturer in India and the preferred partner to leading solar EPC players.
- Greenera Energy India. Greenera Energy India Pvt., Ltd. is a startup firm that is involved in the research, development, and commercialization of energy-efficient products and concentrating solar energy products. In particular, the company specializes in the design and manufacturing of concentrated solar power (CSP) units for industrial process heat (PH) and single-axis cost-effective solar trackers for photovoltaic systems.
- Hyquip. Founded in 2001, Hyquip Ltd. is a leading solutions provider for the material handling requirements across the verticals, such as steel, coal, power, cement, paper, food, sugar, pharmaceuticals, and chemical industries.
- InfiniteERCAM Solar Systems India. InfiniteERCAM Solar Systems India, a wholly and jointly owned subsidiary of InfiniteENERGY and EnergiaERCAM, has been established with the goal of supplying solar PV fixed mounting systems and tracker mounting systems.
- Kaizen Imperial. Kaizen Imperial is primarily a manufacturer and exporter of scientific products. The company offers a wide array of products, including soil instruments, oil and petroleum testing instruments, soil and plant testing instruments, laboratory instruments, hydrological instruments, meteorological instruments, and many more.
- Milman Thin Film Systems. Milman Thin Film Systems was established in 1996, and since then, the company has grown to become a major player in the state-of-the-art thin film equipment and plasma processes.
- Nordic (India) Solutions. Founded in 2010, Nordic (India) Solutions focuses on manufacturing, exporting, trading, and supplying a wide assortment of products, such as PV BOP components, panel junction boxes, solar inverters, solar batteries, solar trackers, and many more.
- Parco Engineers. Founded in 1996, Parco Engineers is primarily involved in various manufacturing activities of hot-dip galvanized iron and steel and earthing materials in association with other companies. But aside from that, they are also involved in the manufacturing of SolSun (solar PV mounting structures) and 11kv/33kv galvanized structures.
- SAG Steel. Established in 2009, SAG Steel Pvt., Ltd. is a prominent manufacturer, exporter, and supplier of a wide assortment of solar products. Some of the products that the company is known for include submersible solar pumps, surface solar pumps, poly house, solar pump accessories, solar tracking systems, and many more.
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