The GreenStalk

Trackers are becoming rare in most solar projects.  The common wisdom, which just about anyone in the industry will say, is that dropping module prices make it difficult for trackers to compete.  I decided to model this, to confirm quantitatively that it is true.

Fundamentally, trackers improve the performance of solar plants by keeping the solar modules aligned with the sun.  At the highest level, there are two types of trackers: single-axis trackers (rows of modules pivoting along a bar), and dual-axis trackers, where a block of modules rotates in every direction.  (See pictures of examples below.)  In general, a single-axis tracker will increase a system’s energy production by about 25%, while a dual-axis tracker will increase production by about 35%.

Single-axis tracker

Dual-axis tracker

So why aren’t trackers used more often, if they can increase production?  Again, this can be shown with a simple model.  In this model, I am dramatically over-simplifying a solar system, to show only the installed cost per watt, versus the total energy production per watt.  The only variable that I’m changing is the system cost – and then adding a tracking system.

Single-axis tracker model

A tracker changes the system in two ways: it adds a fixed cost (in $/watt), and increases production by a percentage.  Given these assumptions, you can see that a dropping system cost makes trackers less attractive.  What’s interesting is that the drop in ROI actually accelerates as systems get cheaper:

Tracker ROI

There is some intuition for this.  Think of a tracker as a product which ‘extends’ the power production of a system.  But when the system gets cheap enough, it becomes more cost-effective to simply extend the system by installing more solar modules and ditching the tracker.

Finally, I want to explain why SunPower is still using trackers today (correctly).  First, they are more expensive, so they tend to be on the left side of the chart above.  But they also have the highest power density – so the tracker is actually slightly cheaper on a $/watt basis.  The same amount of tracking hardware moves more watts.  (I chose not to model the power density in the exercise above, because it made the model much more complex, and therefore more difficult to explain.)

End Notes & Disclaimers:

All of the costs here (including system and tracker cost) are approximate.  But again, the fundamental relationship (lower PV costs lead to lower tracker ROI) will be the same.

Also, a big omission here is the time value of money: I’m not discounting future production (since, of course, you pay for a system up front, while the production is later on).  This will make the trackers look worse across the board.  Like the module efficiency, the time value of money is something that adds a lot of complexity, but doesn’t change the point.

Finally, this entire discussion only applies to flat-plate solar modules.  For concentrating solar PV, tracking is an absolute requirement at any cost.