High, medium and low latitudes?

[julmou]

Good morning,

I was reading this part of the Help about tracking planes: https://www.pvsyst.com/help/near_shadings_tracking.htm

The PV projects I work with are in Morocco, Spain, France, Germany, Scandinavia (Sweden, Finland) or Australia, so across a wide range of latitudes.

1. When there is mention of low, medium or high latitudes in the Help (see screenshot below), I would be very happy to know how you define that? Is high latitude anything >50°? >60°? What about medium and low latitudes (useful for Australia or Africa)?
2. Is there a rule of thumb for pitch or GCR as you get higher or lower in latitude (for sheds and one-axis trackers)? Like the higher the latitude, the less irradiance, so the more ground we want to cover with collectors, therefore the lower the pitch. Or we could argue the opposite way: the higher the latitude, the lower the sun's height, so the more mutual shading, therefore the lower the pitch should be.

Thanks in advance for your help,

Julien

[Michele Oliosi]

There are a few studies on the subject. Here is an example after some searching (behind paywall but you can access the plots)

https://www.sciencedirect.com/science/article/abs/pii/S0306261916307085
The results of the study seem surprisingly good for vertical tracking strategies. I didn't look into the details (please do) but cost and shadings may need to be taken into account and probably will modulate the results. In any case, for simple vertical tracking, the threshold from which it is interesting reads 30°. However, I wouldn't be surprised if this limit is increased to more when considering all practical factor. NS axis or tilted NS axis are more interesting below that threshold latitude.

[André Mermoud]

These Latitude denominations don't have a well-defined definition of course. These are just for an evaluation  of the suitability of some tracking modes according to the geographic situation. By Low latitude, we think close to tropical or sub-tropical  (say below 30°), medium are latitude of medium Europe  (35 to 45°) and high latitudes higher than 50°. 

Concerning the GCR of shed-like systems: you should essentially consider the mutual shadings: In low latitudes, the sun is very high in the sky, and the mutual shadings are low: you can diminish the pitch.  At high latitudes, the sun is lower on the horizon, so that the mutual shadings become important, and especially in winter. 

For Tracking systems (N/S axis), the mutual shadings arise in the morning and the evening. dependning on the sun's height (profile angle). This is rather similar at low and medium latitudes.  At  high latitudes, the sun is low on the horizon and runs over very large azimuts (in summer). Therefore a vertical axis is more suitable.

The advantage of tracking systems is mainly due to the beam component. Therefore tracking systems are suited for very sunny climates.

You have tool in PVsyst for easily evaluate the performance of such systems  (unlimited sheds or trackers).

 

 

 

 

 

 

 

[julmou]

@Michele Oliosi This is an interesting study indeed, although I'm a bit dubious of some of the results. It seems the E-W axis is always outclassing the  N-S axis all the way to latitude 55°, and with N-S only performing slightly better than fixed sheds... which is really strange! I mean, as you point out in the PVsyst documentation regarding E-W horizontal trackers: "This configuration is here for completeness, but is indeed not suited for PV systems." -> it only has a range of motion between the lowest and highest sun's height, which isn't a big angle, whereas the N-S axis has a broader range of motion, -60° to +60°, from east to west. Even when testing some cases in PVsyst, the N-S is always better than the E-W. Besides I don't know what the IEW system (Inclined East-West) would be, it's usually the N-S that can be tilted/inclined. Do you think the author might have mis-named between the two? and EW is actually a N-S axis tracking from east to west?

Regarding the vertical axis, the results are more believable, I tested some situations and the yield is actually really good and better than N-S trackers (even with irrad. optimization + backtracking). It is surprising though that we don't really see many PV plants with vertical axis trackers, the foundation + rotating system must cost quite a lot.

I attached further below the main graph from the study where I wrote the system's name on the curves themselves, it makes easier to read.

 

Edited May 11, 2022 by julmou

[julmou]

@André MermoudThank you, this is really helpful. I work mostly with Australia where the latitude goes from 12° S in Darwin, 27° in Brisbane, 34° in Sydney, to 38° S in Melbourne, or even down to 43° S in Hobart, Tasmania or in New Zealand. I am trying to get a method or a rule of thumb where I could quickly adapt a similar system from one latitude to the other by adjusting the pitch.

• With sheds, basically what you're saying is "In low latitudes [...] you can diminish the pitch". The reciprocal of that would be to increase the pitch as we get higher in latitude, which I understand (sun lower on the horizon, mutual shadings accentuated, hence the need for more pitch). However, this seems to go against some of the advice I had gotten from the Help (Project Design > Plane Orientation > Optimizing sheds) where the last paragraph states:

  Quote

  With shed installations, choosing a rather low tilt is often a very good solution [...], with the following advantages: - The installable power is much greater on a given ground area.

  And from the Forum FAQ (Shadings and tracking > In sheds arrangement, which power can I install on a given area ?), where you mention:

  Quote

  Therefore when the area is limited, you are always advised to put your collectors as horizontal as possible

  With sheds, opting for a low tilt results in being able to have our PV rows closer to each other, with a lower pitch. So, I guess, where I'm confused is that on one hand, we are advised to cover more ground and put as many panels as possible, which means lowering the pitch, and on the other hand, we are advised to increase the pitch to reduce the mutual shadings as we get higher in latitudes. Could you help me understand how to compromise between the two? With sheds, as we get higher in latitudes, is it always better to steadily increase the pitch, or do we reach a point at some latitude where it makes more sense (as the irradiance gets lower) to put more rows and panels instead and compensate the reduction of irradiance due to the low latitude with a lot of PV coverage (with low tilt), which means lowering the pitch?

 

• With horizontal single-axis tracker (HSAT) with N/S axis, we actually have quite a large amount of these systems in Australia (sunny place -> good for trackers), most notably in a one-module-in-width, portrait arrangement. I tried to compare and see a trend for the pitch according to latitude with these same systems (all with backtracking):

Clare SF                                     -> lat. 19.8°, GHI 2020 kWh/m2, pitch 5.7m, GCR 34.4%

Hamilton SF                               ->  lat 20.5°, GHI 2016 kWh/m2, pitch 6.9m, GCR 28.4%

Degrussa Gold & Copper Mine ->       25.6°,        2244 kWh/m2,          5.2m,         37.7%

Agnew Hybrid Power Station    ->      28°S,         2159 kWh/m2,          5.3m,         37%

Moree SF                                   ->       29.6°,        1994 kWh/m2,          6.2m,         31.6%

Bungala SF                                ->       32.4°         1970 kWh/m2           5.1m           38.4%

Yatpool SF                                 ->       34.4°         1917 kWh/m2           5.4m          36.3%

Williamsdale SF                         ->       35.6°         1761 kWh/m2           7.5m           26.1%

 

There doesn't seem to be a real trend, neither according to latitude, nor to irradiation. I do use the Optimization Tool, but just like with sheds, basically the higher the pitch the better it is (not only less near shadings from the beam component, but also more irradiance from the diffuse and albedo, as we increase the pitch). And in reality, we can not increase the pitch indefinitely, so where do we set the bar? Again, for a given system (in this case HSAT N/S axis 1x module width portrait), shouldn't there be an easy rule, for setting the pitch parameter according to the latitude?

Sorry for the long post, and I really hope you can shed some light on this. Thank you,

Julien

[Michele Oliosi]

@julmou Yes the definitions seem to be the opposite. Many people use the term EW tracker when they mean a NS-axis tracker. It depends whether you think of the axis or the possible directions of the plane.

[julmou]

@André Mermoudany thought on my previous post addressed to you? :)

[julmou]

@André MermoudSorry to raise this again, would you mind helping me regarding my post from the 12 May (where I compare different tracker PV plants at different latitudes) ?

[André Mermoud]

The case of the trackers (as a function of the latitude) is quite different than the case of the sheds.  For trackers with N/S axis, at first sight, the "optimal" pitch should not be directly dependent on the latitude.

Now in your studies, you should define what final indicator you are considering,  what you want to optimize and which constraints you have. 

The indicator may be the total Energy yield, the system efficiency (with respect to the occupied ground), the price of the kWh, the total investment, the real seasonal electricity needs, etc. Each of these criteria will lead to different optimal decisions. These criteria should be your first choice.

After that, the optimization is multi-criteria. The conditions may be the price of the PV modules, or of the full system, the price of the occupied area, the investment,  the price of the sold electricity,  the grid power limitation, the real needs of energy, etc. 

As an example, if the terrain availability is not a problem, you can increase the pitch for limiting the mutual shading losses. If the module price is low, you can accept more shadings or misalignment (i.e. accept more losses). If the needs are mainly in winter, you may prefer higher tilts and larger pitch, etc...

Each situation is different. The only way of performing such studies is the simulation.