How to Correctly Simulate a Horizontal SAT w/ Fixed Tilt

[Paul G]

Hello,

In PVsyst, I am attempting to run a simulation for a saw-tooth type tracker (horizontal SAT w/ a fixed tilt). From my experience,

this should simply be done by selecting "Tracking tilted or horiz. N-S axis" and by choosing the appropriate axis tilt (In my case 10d tilt).

See figure below.

 

 

However, another company we are working with feels that running such a simulation will produce substantially less energy and is missing out

on potential annual energy production. They feel that an equatorial tracker at 10 deg is not a "true" equatorial tracker as the latitude of the site

is higher and does not have its axis of rotation parallel to the Earth's axis of rotation. They are insisting on running a "Tracking two axis,

Frame NS" simulation in PVsyst instead and setting the min/max frame angle to the same value (in this case 10 deg). See below.

 

 

If all other variables and loss assumptions are held constant, choosing the "dual axis" simulation while fixing the tilt angle shows ~30 MWh/yr

increase in energy production. They state that this is due to PVsyst assuming a smaller angle of incidence than the pseudo equatorial tracker

in the morning and evening parts of the day.

Can you provide an explanation on the assumptions PVsyst is making that is causing the significant difference in energy production between the

two simulations? And then based on these assumptions, which is the correct way to simulate a horizontal SAT w/ a fixed tilt?

Thank you.

Paul G.

[André Mermoud]

I don't know exactly what you mean by SAT

These tracking options correspond to different physical situations: in one case you have one-axis trackers, and in the second case dual-axis trackers. The dual-axis obviously gives a higher yield if you don't take the mutual shadings into account, as the PV modules are always perpendicular to the sun's rays. But it implies a more sophisticated (and costly) mechanics.

Now with mutual shadings (or backtracking) the performances highly depend on the geometry, the situation should be analyzed in detail using simulations with the exact system's configuration.

There are no special assumptions in PVsyst: at each time (each sun's position, taken in the middle of the hour during the simulation), the trackers have a given mechanical position and the transposition model is applied to this plane orientation.

[Paul G]

Hi Andre,

I apologize, when I referenced a SAT I meant a single-axis tracker.

My main question is though in your opinion and based on the program what would be the "correct" way in PVsyst IN which to simulate a single-axis horizontal tracker with a fixed tilt.

 

 

 

 

Option 1: Choosing a "Tracking tilt or horiz. N-S axis" simulation and choosing the tilt angle (10 deg)

Option 2: "Tracking two axis, N-S" and setting the max/min frame angle (both to 10 deg).

As I stated earlier, there is a significant difference in annual energy in MWh/yr between the two different simulations and would not want to guarantee potential increase in energy

production if it is not realistic assumption.

Paul

[André Mermoud]

Of course, if you had explained your acronym SAT and what you meant by a "saw-tooth type tracker", the answer would have been more accurate.

In this case indeed, you can use the Frame with North/south axis, and fix the tilt within the frame by defining "Min/Max tilt on the frame" = 10°.

Depending on the latitude and the chosen spacing, there may be mutual shadings from element to element in the tracker.

[Paul G]

Thank you for your response, but it still is not clear which is the correct way to run a simulation in PVsyst for a horizontal single-axis tracker w/ a

fixed tilt (saw-tooth type, SAT).

Option 1: Choosing a "Tracking tilt or horiz. N-S axis" simulation and choosing the tilt angle (10 deg)

Option 2: "Tracking two axis, N-S" and setting the max/min frame angle (both to 10 deg).

We understand that you can fix the tilt by defining the "max/min tilt of the frame" to 10 deg for a two axis system, but doing this results

in a different annual energy production in MWh/yr than running option 1 (option 2 results in ~7 % increase in energy production).

For this reason, we are trying to understand the difference in the algorithms that PVsyst is assuming between the (2) simulations, which must be

causing the 7% difference in energy production. That is a significant amount, especially when you might need to guarantee the annual energy

production in order to meet a PPA requirements as an engineering firm designing a project.

Also, the project site latitude is 35°15'16.63"N.

Thanks!

[André Mermoud]

I already answered: you have to use the tracking frame, with a fixed tilt for the tracker.

Now there is no differences in the algorithms, it is always the Transposition model (Hay or Perez).

But the "physical" plane orientation is different in both cases.