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Solar Tracker Fact Sheet
Introduction | When to Use Trackers | Tracker compared to stationary system | Monthly Percent of Full Tracking | Azimuth Trackers | Site Selection | Wind Loading | Solar Tracking for Architects

Introductions

Solar trackers are machines for orienting solar systems toward the sun. They are used to maximize the amount of time solar devices are oriented toward the sun, to keep optical elements properly focused, and for control purposes. The purpose of a solar tracker is to get the highest possible output from costly photovoltaic panels.

Except for Zomeworks passive trackers, most trackers are electro-mechanical with controllers that use sensors to follow the sun. Computer controlled trackers are used in larger fields of trackers, but they are expensive and somewhat difficult to install and maintain.

There are many tracking types or geometries. The two most common are equatorial and azimuth either with or without elevation drive to give dual-axis tracking. The other tracking geometries are of interest to building integrated systems and large fields of solar collectors, but are not currently commercially available.

When to Use Trackers

Trackers are helpful at those times when the sun is shining but stationary panels are not producing. Usually this occurs in the morning or afternoon during the longer days of the year, but it can also occur in the middle of the day if your panels are mounted for winter use and it is summer or vice versa. But how much will a tracker help? The advertised 40% gain is a good year round estimate, however it is important to know that most of this gain occurs during the longer days. Trackers do not help much during the hours between 9 and 3, especially if you seasonally adjust your panels, and there aren't many hours outside 9-3 in the winter. Similarly, if trees or hills block the sun many of the hours outside the 9-3 window, a tracker will not have a chance to add as much to your daily total. To figure out if a tracker is worthwhile economically, figure the extra power it will produce and compare how much panels to produce the same extra power would cost. Tracker Compared to Stationary Tilted to Latitude (40degree) Percent Gain and Amps Gained with 10 Amp Panel  

      Monthly Percent of Full Tracking for Various Configurations  

  Note: In real life all will do better than the table especially if it is hazy and/or cloudy.

Azimuth Trackers

Azimuth trackers rotate around a vertical axis so that the tracker and PV panels are at the same azimuth angle as the sun. (Azimuth angle refers to the direction of the tracker that corresponds to the compass directions north, south, east, west.)

An azimuth tracker with the panels properly tilted captures over 96% of all sunshine per year with no adjustments. This percentage is about the same as an unadjusted equatorial tracker (most manufacturers) but the character of the output of the two geometries is different. An azimuth tracker is at its best in spring, fall, and winter and at high latitudes. In summer, when there is the greatest amount of sunshine, output is shaved from the mid-day peak so the output stays very constant throughout the day. If adjusted twice a year, a slight increase in effectiveness can be achieved in the summer, and the winter output is helped even more, capturing almost 100% of full tracking at the winter solstice.

Site Selection

Since the effectiveness of trackers is dependent on the availability of sun at difficult hours a good site selection is necessary.  This is best done with a site survey tool such as Solar Pathfinder, but a person knowledgeable about a site and/or the sun’s path can make a good estimate in most cases.  Although a computer is needed to make an really accurate assessment, this is not usually needed.  One is just after hours of sun.  It might surprise many that the sun is quite strong when even just 10 degrees above the horizon.  At 20 degrees above it is about 70% of maximum with clear skies.

Keep in mind that the output of a panel is limited to the output of the least productive cell so if one cell is shaded the whole panel is out.  Also realize that the sun reflected off the ground, roof or whatever can significantly increase the output of panels especially if light colored or reflective.

Some things to try and avoid are sites with very high winds and/or subject to blowing or falling debris, areas of large snow accumulation, sites exposed to hazards such as baseballs or rocks thrown by lawn mowers, and locations subject to vandalism or theft.   For a more thorough general discussion read the following.
  Sunny Facing South Is Best But Not Only Way

Planning a photovoltaic system is like planning a house or kitchen remodel in that you must talk with a lot of different people about various parts of the job. And it is what you want that is important. My point here is that there is no one right way. You can, of course, turn the job over to a professional, but if you want to be part of the process one of the people you must talk to is the "solar engineer" who can tell you what alternatives you have and what the performance of the options will be. This information sheet speaks in general terms about site selection. This decision involves whether or not to use a tracker so we talk about that also. More detailed engineering information will be found on other pages.

It is our opinion that single crystal glass panels are still the best type of photovoltaic panel to use so that is what this is written for but most of it applies equally well to any type of commercially available panel. Roof shingle panels, semi-transparent glazing panels, etc. are best treated elsewhere.

So having decided to install a photovoltaic system, where are you going to put the panels? First let us urge you to be creative. The technology is there, though still expensive. It is up to the homeowner, architect, builder, and solar dealer to find creative ways to integrate panels with buildings, structures, and landscaping. What we want to work on here is where the panels need to be located. We divide this question into two groups, the simple cases and the complex with, of course, some in between. The average home owner can do a pretty good job siting PV panels in the simple situations especially with a little help such as we try to provide. But sometimes there is a tree right to the South, or a hill to the East, or you want to put the panels on a west facing roof or flat roof. In those situations it is best to get some expert advice and we will describe that process later.

Where to put the PV panels depends on whether or not you will use a tracker. And whether or not you are going to use a tracker (and what kind of tracker) depends on where you are going to put the panels. So starting with trackers, trackers are used to increase the output of expensive panels. They work best if you have lots of hours of sunshine at the site and if those hours occur during a part of the year or day that you need the power. We have lots of material to give you a better idea of how well trackers of various types will work, but generally they can add 20-40% output year around and can almost double the output of stationary panels during the summer in favorable locations. Other reasons to use trackers are that they provide a convenient mounting system and/or rack, they can be used to control the output of the PV system, they provide a more constant output over longer hours reducing other component costs, and they can provide flexibility. By the latter I mean that you do not have to decide to install the PV panels facing South or other direction at a fixed angle only to find out later that a different installation would work better. If you use a tracker you usually but not always want to find a site with the most hours of sun.

In the simple site selection case one can stand at or near the possible site and easily see that there is or is not good sun. For tracker use one usually wants the most hours of sun possible. For stationary installations modules mainly need a 6 hour window of sunshine. Usually this is facing south and occurs 9-3 sun time and the only decision is what tilt for the panels for optimal output according to the local weather and power usage. If the installation is on a roof top or other structure, the siting choices are pretty limited and the optimal solar location fairly easy to discern. If one is searching for the optimal solar site anywhere which usually means using a tracker, one is looking for total daily hours of sunshine when the power is needed.

In complex cases where the siting decision is not obvious even a solar professional will often need to use rather sophisticated tools. The first step in analyzing a complex situation is to do a site survey. Usually one uses a site survey tool such as Solar Pathfinder to determine monthly sunrise and sunset times (and any mid-day obstructions). Really one only needs half the year since the other half is exactly the same but since the calendar year does not correspond with the solar year all the months are sometimes recorded. With that information in hand one can count the hours of sunshine throughout the year for a rough estimate or better yet input that data into a solar computer program to get a more accurate and detailed answer. A new program has just come on the market at a reasonable price that has both weather data and most tracking options so one can get a really accurate estimate of how your various site and equipment options compare. If your installer-dealer does not have this ability Small Power Systems can do the simulation. You might be surprised what will work. Two cases that have surprised me recently are that a West facing roof works well in Eugene, Oregon and a site with horrible morning sun but great afternoon sun can use a tracker advantageously.

The results you get from either a simple visual survey or a more thorough analysis are often not definitive. There is a lot of gray area in solar installations. While there may be a theoretical optimal economic and engineering answer there can also be a lot alternatives that are not much different in how much power you get from dollar spent. That is what the solar engineer can tell you - what the alternatives are and how they will work. You can put panels on the north roof if that is where you want them, it is just going to cost more, but usually the alternatives are not that clear cut and considerations other than engineering play a role in the decision. For instance you might want to put solar panels on a Southwest facing roof or a tracker on a flat roof where it will be hidden even though you might have to add an extra module to get the same results as with the "ideal" installation. While the usual advice to put panels in a sunny location facing south is valid, alternatives can be surprisingly effective.
 

Wind Loading

The mechanical problem in designing a tracker is wind loading.  Wind loading increases as the square of the wind speed; therefore,  the force of the wind at 71 mph is one half what it is at 100 mph.  This is to say that the same tracker can carry twice the number of panels at 71 mph as it can at 100 mph.
 

Solar Tracking for Architects

Summary

As architects start to design buildings with photovoltaic arrays they should be aware that tracking can significantly increase the output of the costly modules. There are many tracking geometries that can be used for various design situations and that have different output characteristics. Most such schemes require that the modules be distributed so that the modules do not shade each other. Tracking requires that the modules receive sunshine early and late in the day to be effective. At mid and high latitudes the summer gain is signifi-cantly greater than the winter gain which typically is not more than 20%.

The full Solar Tracking for Architects article is available by clicking here.

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