Chapter 9- Solar Tracker

7/27/2019 Chapter 9- Solar Tracker

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Study of Solar Module & Its Efficient Use in Bangladesh

96Solar Tracker

Chapter 09

SOLAR TRACKER

09.1 Introduction

A solar tracker [19] is a generic term used to describe devices that orient variouspayloads toward the sun. Payloads can be photovoltaic panels, reflectors, lenses orother optical devices.

In standard photovoltaic (PV) applications trackers are used to minimize the angle ofincidence between the incoming light and a photovoltaic panel. This increases theamount of energy produced from a fixed amount of installed power generating

capacity. In standard photovoltaic applications, it is estimated that trackers are used inat least 85% of commercial installations greater than 1MW from 2009 to 2012.[1][2]

In concentrated photovoltaic (CPV) and concentrated solar thermal (CSP) applicationstrackers are used to enable the optical components in the CPV and CSP systems. Theoptics in concentrated solar applications accepts the direct component of sunlight lightand therefore must be oriented appropriately to collect energy. Tracking systems arefound in all concentrator applications because systems do not produce energy unlessoriented toward the sun.

09.2 Photovoltaic Tracker Classification

Photovoltaic trackers can be classified into two types: Standard Photovoltaic (PV)Trackers and Concentrated Photovoltaic (CPV) Trackers. Each of these tracker types canbe further categorized by the number and orientation of their axes, their actuationarchitecture and drive type, their intended applications, their vertical supports andfoundation type.

Standard Photovoltaic (PV) Trackers

Photovoltaic panels accept both direct and diffuse light from the sky. The panels on aStandard Photovoltaic Trackers always gather the available diffuse light. The trackingfunctionality in Standard Photovoltaic Trackers is used to minimize the angle ofincidence between incoming light and the photovoltaic panel. This increases theamount of energy gathered from the direct component of the incoming light.


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Accuracy Requirements

In standard photovoltaic systems, the energy contributed by the direct beam drops offwith the cosine of the angle between the incoming light and the panel. Thus trackersthat have accuracies of 5 can deliver greater than 99.6% of the energy delivered by

the direct beam and 100% of the diffuse light. As a result, high accuracy tracking is nottypically used.

Technologies Supported

The physics behind Standard Photovoltaic (PV) Trackers works with all standardphotovoltaic module technologies. These include all types of crystalline silicon panels(monocrystalline, multicrystalline, polycrystalline) and all types of thin film panels(amorphous silicon, CdTe, CIGS, microcrystalline).

Concentrated Photovoltaic (CPV) Module Trackers

The optics in CPV modules accept the direct component of the incoming light andtherefore must be oriented appropriately to maximize the energy collected. In lowconcentration applications a portion of the diffuse light from the sky can also becaptured. The tracking functionality in CPV modules is used to orient the optics suchthat the incoming light is focused to a photovoltaic collector.CPV modules that concentrate in one dimension must be tracked normal to the sun inone axis. CPV modules that concentrate in two dimensions must be tracked normal tothe sun in two axes.

Accuracy Requirements

The physics behind CPV optics requires that tracking accuracy increase as the systemsconcentration ratio increases. In typical high concentration systems tracking accuracymust be in the 0.1 range to deliver approximately 90% of the rated power output.[3][4]

In low concentration systems, tracking accuracy must be in the 2.0 range to deliver90% of the rated power output. As a result, high accuracy tracking systems are typicallyused.

Technologies Supported

Concentrated Photovoltaic Trackers are used with refractive and reflective basedconcentrator systems. There is a range of emerging photovoltaic cell technologies usedin these systems. These range from crystalline silicon based photovoltaic receivers togermanium based triple junction receivers.


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98Solar Tracker

There could be another type of tracker classification [17]:

Panel tracking

This is where the panels are on a mount that follows the sun. The most common

is the Zomeworks. These optimize output by keeping the panels very close to 90degrees angle for maximum sunlight. These typically give one about a 15%increase in winter and up to a 40%. Increase in summer.

Maximum power Point Tracking

This is electronic tracking, and has nothing to do with moving the panels.Instead, the controller looks at the output of the panel and compares it to thebattery voltage. It then figures out what is the absolute best power that the panelcan put out. It takes this and converts it to best voltage to get maximum AMPS

into the battery. Most important MPPTs are around 95-97% efficient in theconversion. One can get typically 30-40% power gain in winter and 10-20% insummer.

09.3 Tracker Types

Photovoltaic trackers can be grouped into classes by the number and orientation of thetrackers axes. Compared to a fixed mount, a single axis tracker increases annual outputby approximately 30%, and a dual axis tracker an additional 6%.[5][6][7]

Single Axis Trackers

Single axis trackers have one degree of freedom that acts as an axis of rotation. The axisof rotation of single axis trackers is typically aligned along a true North meridian. It ispossible to align them in any cardinal direction with advanced tracking algorithms.

There are several common implementations of single axis trackers. These includeHorizontal Single Axis Trackers, Vertical Single Axis Trackers, and Tilted Single AxisTrackers. The orientation of the module with respect to the tracker axis is important

when modeling performance.


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Horizontal Single Axis Tracker (HSAT)

Fig 45: Ray Tracker GC200 Horizontal Single Axis Tracker in California

The axis of rotation for Horizontal Single Axis Tracker is horizontal with respect to theground. The posts at either end of the axis of rotation of a Horizontal Single AxisTracker can be shared between trackers to lower the installation cost.

Field layouts with Horizontal Single Axis Trackers are very flexible. The simplegeometry means that keeping the entire axis of rotation parallel to one another is all thatis required for appropriately positioning the trackers with respect to one another.

In addition, with backtracking, they can be packed at any density without shading.

Horizontal Trackers typically have the face of the module oriented parallel to the axis ofrotation. As a module tracks, it sweeps a cylinder that is rotationally symmetric aroundthe axis of rotation.

Fig 46: Wattsun HZ-Series Linear Axis Tracker in South Korea. These trackers use ahorizontal axis.


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Several manufacturers can deliver single axis horizontal trackers. In these, a longhorizontal tube is supported on bearings mounted upon pylons or frames. The axis ofthe tube is on a North-South line. Panels are mounted upon the tube, and the tube willrotate on its axis to track the apparent motion of the sun through the day.

Vertical Single Axis Tracker (VSAT)

The axis of rotation for Vertical Single Axis Trackers is vertical with respect to theground. These trackers rotate from East to West over the course of the day.

Field layouts must consider shading to avoid unnecessary energy losses and to optimizeland utilization. Also optimization for dense packing is limited due to the nature of theshading over the course of a year.

Vertical Single Axis Trackers typically have the face of the module oriented at an anglewith respect to the axis of rotation. As a module tracks, it sweeps a cone that is

rotationally symmetric around the axis of rotate.

Tilted Single Axis Tracker (TSAT)

Fig 47: Single axis Sun Power T20 trackers, with roughly 20 degree tilt, at Nellis AirForce Base, in Nevada, USA. The arrays form part of the Nellis Solar Power Plant andwas designed and built by Sun Power Corporation. Credit: U.S. Air Force photo bySenior Airman Larry E. Reid Jr.

All trackers with axes of rotation between horizontal and vertical are considered TiltedSingle Axis Trackers. Tracker tilt angles are often limited to reduce the wind profile anddecrease the elevated ends height off the ground.

Field layouts must consider shading to avoid unnecessary losses and to optimize landutilization.

With backtracking, they can be packed without shading perpendicular to their axis ofrotation at any density. However, the packing parallel to their axis of rotation is limitedby the tilt angle and the latitude.


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Tilted Single Axis Trackers typically have the face of the module oriented parallel to theaxis of rotation. As a module tracks, it sweeps a cylinder that is rotationally symmetricaround the axis of rotation.

Dual Axis Trackers

Dual axis trackers have two degrees of freedom that act as axes of rotation. These axesare typically normal to one another. The axis that is fixed with respect to the ground canbe considered a primary axis. The axis that is referenced to the primary axis can beconsidered a secondary axis.

There are several common implementations of dual axis trackers. They are classified bythe orientation of their primary axes with respect to the ground. Two commonimplementations are Tip - Tilt trackers and Azimuth-Altitude trackers.

The orientation of the module with respect to the tracker axis is important whenmodeling performance. Dual Axis Trackers typically have modules oriented parallel tothe secondary axis of rotation.

Tip Tilt Dual Axis Tracker (TTDAT)

A Tip Tilt Dual Axis Tracker has its primary axis horizontal to the ground. Thesecondary axis is then typically normal to the primary axis. The posts at either end ofthe primary axis of rotation of a Tip Tilt Dual Axis Tracker can be shared betweentrackers to lower installation costs.

Field layouts with Tip Tilt Dual Axis Trackers are very flexible. The simple geometrymeans that keeping the axes of rotation parallel to one another is all that is required forappropriately positioning the trackers with respect to one another.

In addition, with backtracking, they can be packed without shading at any density. Theaxes of rotation of Tip Tilt Dual Axis Trackers are typically aligned either along a trueNorth meridian or an east west line of latitude. It is possible to align them in anycardinal direction with advanced tracking algorithms.


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Fig 48: Point focus parabolic dish with Stirling system. The horizontally rotatingazimuth table mounts the vertical frames on each side which hold the elevationtrunnions for the dish and its integral engine/generator mount

Azimuth-Altitude Dual Axis Tracker (AADAT)

An Azimuth Altitude Dual Axis Tracker has its primary axis vertical to the ground.The secondary axis is then typically normal to the primary axis.

Field layouts must consider shading to avoid unnecessary energy losses and to optimizeland utilization. Also optimization for dense packing is limited due to the nature of theshading over the course of a year.

Fig 49: Azimuth-Altitude Dual Axis Tracker 2 axis solar tracker, Toledo, Spain

This mount is used as a large telescope mount owing to its structure and dimensions.One axis is a vertical pivot shaft or horizontal ring mount that allows the device to beswung to a compass point. The second axis is a horizontal elevation pivot mountedupon the azimuth platform. By using combinations of the two axis, any location in theupward hemisphere may be pointed. Such systems may be operated under computercontrol according to the expected solar orientation, or may use a tracking sensor tocontrol motor drives that orient the panels toward the sun. This type of mount is alsoused to orient parabolic reflectors that mount a Stirling engine to produce electricity atthe device. [8]


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09.4 Drive types

Active tracker

Active trackers use motors and gear trains to direct the tracker as commanded by acontroller responding to the solar direction.Active two-axis trackers are also used to orient heliostats - movable mirrors that reflectsunlight toward the absorber of a central power station. As each mirror in a large fieldwill have an individual orientation these are controlled programmatically through acentral computer system, which also allows the system to be shut down whennecessary.

Light-sensing trackers typically have two photo sensors, such as photodiodes,configured differentially so that they output a null when receiving the same light flux.Mechanically, they should be Omni directional (i.e. flat) and are aimed 90 degrees apart.This will cause the steepest part of their cosine transfer functions to balance at thesteepest part, which translates into maximum sensitivity.

Since the motors consume energy, one wants to use them only as necessary. So insteadof a continuous motion, the heliostat is moved in discrete steps. Also, if the light isbelow some threshold there would not be enough power generated to warrantreorientation. This is also true when there is not enough difference in light level fromone direction to another, such as when clouds are passing overhead. Considerationmust be made to keep the tracker from wasting energy during cloudy periods.

Passive tracker

Fig 50: Zomeworks passive tracker head in Spring/Summer tilt position with panels onlight blue rack pivoted to morning position against stop. Dark blue objects are hydraulicdampers.


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Passive trackers use a low boiling point compressed gas fluid that is driven to one sideor the other (by solar heat creating gas pressure) to cause the tracker to move inresponse to an imbalance. As this is a non-precision orientation it is unsuitable forcertain types of concentrating photovoltaic collectors but works fine for common PV

panel types. These will have viscous dampers to prevent excessive motion in responseto wind gusts. Shader/reflectors are used to reflect early morning sunlight to "wake up"the panel and tilt it toward the sun, which can take nearly an hour. The time to do thiscan be greatly reduced by adding a self-releasing tie down that positions the panelslightly past the zenith (so that the fluid does not have to overcome gravity) and usingthe tie down in the evening. (A slack-pulling spring will prevent release in windyovernight conditions.)

The term "passive tracker" is also used for photovoltaic modules that include ahologram behind stripes of photovoltaic cells. That way, sunlight passes through the

transparent part of the module and reflects on the hologram. This allows sunlight to hitthe cell from behind, thereby increasing the module's efficiency. Also, the module doesnot have to move since the hologram always reflects sunlight from the correct angletowards the cells.

Chronological tracker

A chronological tracker counteracts the Earth's rotation by turning at an equal rate asthe earth, but in the opposite direction. Actually the rates aren't quite equal, because as

the earth goes around the sun, the position of the sun changes with respect to the earthby 360 every year or 365.24 days. A chronological tracker is a very simple yetpotentially a very accurate solar tracker specifically for use with a polar mount.Thedrive method may be as simple as a gear motor that rotates at a very slow average rateof one revolution per day (15 degrees per hour). In theory the tracker may rotatecompletely, assuming there is enough clearance for a complete rotation, and assumingthat twisting wires are not an issue, such as with a solar concentrator or the tracker maybe reset each day to avoid these issues. Alternatively, an electronic controller may beused, with a real time clock that is used to infer the "solar time" (hour angle). Trackingadjustments can be made periodically or continuously.

09.5 Maintenance

Some solar trackers may operate most effectively with seasonal position adjustment andmost will need inspection and lubrication on an annual basis. As most trackers aremade from mild steel, maintenance of paint is typically required, and may be critical in


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highly corrosive environments, such as near saltwater or in polluted industriallocalities. In regions with extended summer dry seasons the periodic washing of thepanels may significantly increase performance at a critical demand time, particularly forgrid-tied systems.

Chapter 9- Solar Tracker

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