Choosing the inclination of photovoltaic modules in triangular structures

Determining the inclination angle for photovoltaic modules installed on roofs with triangular structures or in ground plants is not as simple a task as in the case of modules installed on a single flat surface. That's what we're going to talk about in this article.

We know that for each location there is an optimal inclination angle for photovoltaic modules, which provides the best performance in energy generation throughout the year.

When there is no concern about shading, determining the best inclination angle is a relatively simple problem, which can be solved by a geometric analysis of the positions of the Earth and the Sun, which results in the famous recommendation of an inclination close to the latitude of the location. installation.

However, when modules are installed in rows, such as in slab installations or in ground plants, tilting the modules can produce shadows on the rear modules (Figure 2).

Therefore, the optimal slope angle for a location may not be the optimal angle for a given project. In ground-floor plants and horizontal roofs (or slabs) we then have the problem of shading that the rows of modules cause among themselves. Maximum light capture is now not only related to the inclination and orientation of the modules, but also to the spacing between the rows and their construction characteristics.

How to determine the angle of inclination and distance between rows

The answer to the best slope and appropriate distance between rows will be different for each location and for each specific project, which makes it impossible for structure manufacturers to present a table of standardized slopes and distances – or even a simple formula that allows you to solve this problem. problem directly.

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In soil plants, the distance between rows must be determined primarily based on the need for circulation. Minimum spacings of around 3 meters are necessary to allow the circulation of machines for cleaning modules and removing vegetation.

This spacing is also necessary to prevent the rows from causing shadows on each other. Once the appropriate spacing has been determined, the inclination angle must be determined, as we will see below. In photovoltaic systems mounted on triangular structures (on slabs or roofs), determining the spacing is not so obvious. The natural choice would be a large spacing, which would avoid the occurrence of shadows.

However, on flat slabs (or metal roofs with a low slope), the available area is limited. A large spacing between rows is not desirable due to the limitation of the number of panels in the available area. It is normal to find triangular structures with spacings of 1 meter or less.

And here we have a problem again: with a small spacing, can we tilt the modules at any desired angle? The most practical way to quantify the shading between rows and decide the spacing and its inclination is through studies in specific software, such as PVSyst and SOLergo.

In this article, we will cover the use of PVSyst software to support decisions about the distance and inclination of photovoltaic modules.

Determining the tilt angle with the PVsyst

As there is no simple formula or standardized table for determining the slope and distance between rows, the photovoltaic designer's best option in these cases is to use a design tool.

Guidance Tool

Figure 3 shows the button to access the PVSyst “Orientation” tool. In this tool we have several possible module constructions. Let's use the “Unlimited Sheds” option, shown in Figure 4.

Determining the best slope and row distance is an iterative process. The search for the best result requires a compromise between these two variables (slope and distance). Normally, distance is the first variable that the designer chooses depending on the most convenient distance for each project (plant or slab), as previously mentioned.

Then, using the PVSyst design tool, the best inclination angle for that condition is determined. In the “Unlimited Sheds” option of the “Orientation” tool, we must first fill in the constructive data for the rows: number of sheds (tables or rows), distance between sheds, height of the sheds (which depends on the size of the photovoltaic modules and the number of rows within the same table), lower deadband height and upper deadband height (normally zero).

To take into account the electrical effects of shading, that is, the activation of bypass diodes caused by mutual shading, we must check the box “Use electrical effect in simul”. The “Nb. modules in width” represents how many modules “in height” we have on the table. For example: in a row or table with 40 modules organized in two rows of 20, this number is 2.

Then, after the data is filled in, we must enter the graphical view by clicking on “See optimization”, as shown in Figure 5. The graph in Figure 5 is very important in the process of choosing the inclination angle of the photovoltaic module. This graph shows the solar energy capture gain of the inclined module in relation to capture in the horizontal position.

The numerical scale on the Y axis (vertical) shows the percentage gain (or loss) of energy in relation to installing the modules horizontally. The value “1.00” in the graph corresponds to the horizontal position. A value like “1.10” (see Figure 5) shows that we have 10% more light capture compared to the situation when the module is installed horizontally.

We observe three curves in Figure 5: green, black and orange. The first curve (green) represents the amount of light energy that the modules would receive if they were infinitely separated, that is, without considering mutual shading between the rows. This would be the case of installing the modules in a single flat area, without forming rows.

The black line shows the gain in capturing light energy from the inclined module, already considering the effect of mutual shading, however, disregarding the electrical effects, that is, the activation of the diodes bypass. The orange curve, which is the most important curve for sizing, represents the gain in energy capture of the photovoltaic modules, taking into account the effect of mutual shading between rows and also the activation of the diodes. bypass.

The black and orange curves are very close and in some cases, when the distance between the rows is large, they may appear overlapping. The information “Losses due to shadow” (see Figure 5) indicates the percentage loss of light energy (not generated electrical energy!) depending on the chosen geometry (spacing between rows and inclination).

In this first example (Figure 5), we have a loss of 2.8%. The percentages in the blue boxes represent the “GCR – Ground Coverage Ratio”, which is the relative measure of area occupied by the modules. A GCR of 100% means that in the top view of the solar plant we would not “see the ground” behind the modules – that is, the ground would be completely covered by modules installed horizontally.

Figures 6 and 7 below show the behavior of mutual shadows with varying spacing between modules. It is important to note that in the PVSyst “Orientation” tool, the distance is the measurement between the beginning of one structure and the beginning of another – and not the distance (or corridor) between the rows. We always need to remember this, as this measure can cause some confusion.

Let's interpret the figures above. First look at Figure 6, in which the rows are 5 meters apart. The slope is the same in both cases, but in Figure 6 we observe a large loss, of 6.8%. This loss is shown on the graph, indicated as the distance between the green and black lines.

If we consider the orange line (electrical effect) the loss is even greater. It is definitely not a good choice to keep photovoltaic modules tilted at this angle (30 degrees, in the example) so close together. Now let's look at Figure 7, in which we maintain the 30 degree angle, but with greater spacing.

The fact of spacing the modules (at the same angle) reduces the effect of shadows between them. We see that the black and orange lines are almost overlapping and are approaching the green line. And the loss is only 2.2%. In other words, in this chosen geometry, 2.2% of the energy captured is lost compared to a situation in which the modules would all be on the same surface, not organized in rows, with the same inclination of 30 degrees.

General rule for the project

When analyzing a shed-type structure in PVSyst, two approaches must be used:

1. For a given slope, the distance that provides the least loss must be chosen;
2. For a given distance between the sheds, the angle that provides the least loss must be chosen.

In the two cases above, the general rule is to always seek, during graphic analysis, to position the purple ball on top of the black and orange lines. Ideally the black and orange lines are very close or even overlapping.

By positioning the purple ball on top of the black line (or orange, if they are overlapping), the designer can find the best inclination of the modules for that project, already considering the effect of shadows caused between the different rows. Take a closer look at Figure 8 to understand the positioning of the purple ball.

In this example the modules are inclined at 30 degrees and the shading loss is 2.1%. The black line does not appear as it is overlapped with the orange line. This normally happens when the spacing between the rows is large enough that the electrical effect of the shadows is negligible.

By reading the graph in Figure 8, we see that the best installation angle, considering that the distance between the rows remains unchanged, would be approximately 20 degrees. This point corresponds to the top of the black and orange lines.

In the same example, keeping the distance between the sheds unchanged, we opted for a 20-degree slope. The result is shown in Figure 9, where we see the purple ball positioned on top of the orange line. In this case, we have the best possible inclination for this project, considering the distance between the sheds (or rows) that was chosen by the designer.