Tag Archives: performance

Optimizing for Time of Use – Trackers vs Fixed Tilt

Most power companies have a tiered rate structure for electricity that makes energy generated during peak demand hours more valuable. In the northern hemisphere, you almost always get the maximum annual energy generation for a PV system by orienting the system due south (unless you have strange weather patterns, like cloudy mornings and sunny afternoons but that’s not typical). But if energy generated in the late afternoon/early evening (typical peak time) is worth significantly more the the power company, you can easily get a net benefit from altering the system azimuth to capture more energy during a specific time of day even though the result is a lower annual energy yield. You’re essentially trading annual energy for energy captured at a specific time of day.

With this in mind, I recently ran a performance analysis using actual time of use data for a large scale project, and the results were somewhat surprising.

First, the plots of annual energy vs. system azimuth (180 degrees is due south) for horizontal single axis trackers and fixed tilt PV systems:

Fraction of annual energy delivered versus system azimuth angle for single axis tracker and fixed tilt PV systems.

Fraction of annual energy delivered versus system azimuth angle for single axis tracker and fixed tilt PV systems.

Both systems deliver their respective maximum annual energy when the system azimuth is 180 degrees, but the overall effect of the azimuth rotation is quite small, even for large rotations of 20 degrees or so. There’s also some skew to the data due to the weather at the particular location I’m using. I should also point out that the single axis tracking system produces about 18% more energy at this location.

If we take the Time of Use factor (a value ranging from 0.6 to 2.5 depending on season and time of day) and multiply it by the hourly system output for each hour over the course of a year, we get some interesting results:

Annual Energy scaled by the Time of Use factor versus system azimuth angle.

Annual Energy scaled by the Time of Use factor versus system azimuth angle.

First, the fixed tilt system will deliver its greatest number of dollars per year when installed with an azimuth of 200 degrees (20 degrees West of South). Second, the single axis tracking system benefits very little from an azimuth rotation, and what little benefit there is occurs in the opposite direction (East of South). This is because the tracking system’s panels are already pointed to the West in the late afternoon, in a great position to capture sunlight during the period of peak demand. It’s a little counterintuitive, but rotating the tracker axis (the North-South axis) away from the sun actually points the panels more directly at the afternoon sun.

So to summarize, the tracking system produces 18% more annual energy, and doesn’t need any site-specific optimization of the install to maximize the benefit based on the Time of Use factors.