Most of the information in this message is contained in a PDF file that was attached to a previous post by me. I will refer to the parallel/series wiring
of solar panels
in the context of the instalattion that I performed in my sailing boat
about 7 months ago.
I used three panels
in series. They can produce a maximum voltage (see the table in the other post) of 3 x 23.94 = 71.82 volts. Most MPPT regulators do not admit such a high voltage input. Fortunately the controller that I have used (TR-2210RN) allows an input voltage of 100 volts. By connecting panels in series the current
is smaller than it would be if they were in parallel and the section of electrical cables
connecting the panels to the controller may be smaller. In my case I used a section of 4 mm2. The distance of the panels to the batteries was about 7 meters. So I have a total cable length of 14 meters. The section of the cable is critical, since the voltage drop along the cable will be responsible for a loss of power. If the panels were connected in parallel we could use a MPPT controller with a lower input voltage (less expensive). In that case, the current
would be 3 times higher and, for the same loss of power, I would need to increase the cable size from 4 mm2 to 12 mm2. That would be more expensive and, moreover, would make the job of inserting the cable through the 1 inch stainless steel
tube of my bimini
into a very difficult task. The controller cable to the batteries carries a larger current, but it has very short lenght. I used a cable with a 10 mm2 section. I used a 30 A fuse in the positive cable near the battery.
I really think that my choice of a serial
connection is fine. I leave here my reasoning and I hope I am not mistaken in my calculations. I hope this post will be useful to clarify some aspects of solar
. In the following picture it is shown the I-V curve for the 75 W solar panel that I am using.
I point out 4 possible operating points along the curve. Point A is the point of operation when there is no load connected to the panel (open circuit). The electric
current will be zero and the voltage 23.9 V. Obviously, the power to the load is zero. Point B refers to the case of an electronic device (MPPT controller) as the load to the solar panel. The voltage is 19.4 V, the current is 3.8 A and the power supplied to the controller is 73.7 W. Point C is the operating point if we connect the panel directly to the battery (or if you use a non-MPPT controller). The voltage supplied by the panel will be imposed by the battery where we assumed a normal battery voltage of 12.8 V. Using the curve, I believe that the current is about 4 A, which means that the power supplied by the panel is 51.2 W. These two numbers, 73.7 and 51.2 show the improvement when using the MPPT controller.
Before comparing the series to parallel wiring, let me consider the cable connecting the panels to the controller. In my case the distance between the panels and the controller is about 7 meters long. I used a photovoltaic cable with a cross section of 4mm2. This is a very flexible cable, well protected from the environment
, and with a total outer diameter of 6 mm. If we evaluate the ohmic resistance of a copper wire with a length of 14 m and a section of 4 mm2 we will get 60 mili-ohms. I will use this value to estimate the power loss along this critical cable. Later I will discuss the effect of shading on the panels and I will disagree with the opinion that parallel wiring offers significant advantages over the serial
Taking all of the three panels as a single identity (... Thevenin ...) the I-V curve of the set is very similar to that of the first image in the case of a single panel. We only need to change the numbers as labels. Assuming that the panels are used with an MPPT controller, I only comment about point B (the point of maximum power transfer). In the parallel case, the output voltage would be 19.4 V, but the output current would be 11.4 A. In the series case, the output voltage would be 58.2 V and the output current would remain 3.8 A. Note that, in the case of series wiring, the MPPT controller must support an input voltage of about 60 V. Not all controllers on the market support this "superior" voltage.
The voltage drop across a resistor R carrying a current I is R x I. In the case of the parallel connection (I = 11.4 A) the voltage drop is about 0.68 V. Since the voltage available at the panel is 19.4 V, the voltage drop means a loss of power of 3.5%. In Watts, the power loss is 7.8 W. For the series connection, the voltage drop is 0.23 V. However, the voltage available on the panel is 58.2 V, which means a loss of 0.4% or 0.87 W in terms of watts.
Following is my view of shading on solar panels
. I will assume a panel with 30 cells capable of supplying the 12V 5A. I am going to refer to 3 images
. The first image (Fig 1.A) represents the panel in normal operation. There are zonal protection shunt diodes (bypass diodes), as shown in the image. The diodes are reverse biased and do not consume any power. Suppose now that the panel is shaded as shown in the next image (Fig. 1.B).
The shaded cells can be considered as open circuits. The panel continues to operate but with a lower output voltage of 7.4 V. If this panel is connected in series with other panels, there will be no problem. By contrast, we do not want to connect this panel in parallel with other 12V panels. In Fig.2, I represent the mounting of the panels in series. One panel is shaded, so that the total output voltage is 31.4 V instead of 36 V.
In Fig.3, I represent what I would use if I had to connect the panels in parallel. I'm not entirely sure if I'm correct. First, I would use 3 external diodes. Electrical
engineers do not like voltage sources connected in parallel. By contrast, the current sources should be connected in parallel. The question is: should we consider the panel as a voltage or current source? or as a "mixed" source? I will not try to answer. External diodes consume 3 W power which is not good. And the shadded panel is completely off. It does not contribute to any current to the load. Instead of 15 A flowing into the load we will only have 10 A as if only had 2 panels.