How to Wire Solar Panels in Series
Welcome to this informative article.
After learning in the previous article how to wire two or more solar panels in parallel, in this page we will teach you how to wire them in series and obtain an increase of the voltage at the output, keeping the rated current unchanged.
We will also explain the difference between a series connection of two or more identical solar panels and a series connection of two or more solar panels with different technical characteristics. Finally, we will provide you with valid and practical tips to get an efficient system and to avoid the dreaded hot-spot effect that can occur during partial cloudiness in the sky or shading on our string of panels.
What is a solar panel and a solar cell?
Well, to better understand the series connection, let's start with some theory on the solar panel! A solar panel (formally known as PV module) is an optoelectronic device made from multiple solar cells normally wired in series. Here in Italy the best selling panel is the 230Wp 32V panel, that is composed of 60 polycrystalline solar cells wired in series.
A solar cell, or photovoltaic cell, is an element that has the ability to convert the sun's rays into electrical energy. This phenomenon is known by the name of photovoltaic effect. The solar cells that we mainly find on the market are made of semi-crystalline material (silicon) and have a black or blue color.
Series connection of two identical solar panels
If we have two or more solar panels with equal current and power, and we want to increase the voltage, the choice falls on the series connection.
By connecting multiple solar panels in series, we increase the system voltage. In a solar power system, the higher the voltage and the lower the energy losses along the cables. To know the maximum system voltage, we usually just need to turn the panel and read the label, where the value is reported.
After these clarifications, let's see how the series connection takes place. It's very simple. As clearly visible in the picture, it is sufficient to wire the positive pole of one panel to the negative pole of the other and at the output we will find a doubling of the voltage. Considering the example in the figure, two 5A 12V panels wired in series produce a voltage of 24V and a current of 5A. The current remains unchanged.
In parallel to each panel we have added a diode, called bypass diode (not to be confused with the blocking diode). This diode has a particular function, which we will explain later.
What happens in case of shading?
To have an excellent performance from our solar power system, it's important that the panels do not shade each other, that they have the same angle and that they are positioned away from possible causes of shading such as trees, poles or various protrusions. Of course, in case of shading due to climatic conditions, there's not much that can be done. Knowing the behavior of the entire string in case of shading is essential to avoid drastic yield drops of the electricity production.
As previously explained, a solar panel is composed of many solar cells wired in series. If a part of the panel is darkened, in that part a high resistance is formed which hinders the circulation of the current. In the worst scenario, instead of producing energy, the shaded solar cells absorb it, becoming loads. The current that flows through them, due to the joule effect, causes an increase in temperature and this increase in temperature can even cause fires or the dissolution of the welds (hot-spot effect).
It is therefore clear how the presence of a small shadow may turn out to be a serious loss of energy on the entire system. To avoid or reduce this problem, some solar panel manufacturers have divided the panel into various sections made up of a certain number of cells and a bypass diode has been inserted in each section. This diode has the function of excluding the section that is shaded, so that this section does not adversely affect the whole panel.
When designing the solar power system, it is therefore very important to choose solar panels that have bypass diodes. The more bypass diodes are into the panel, the more sections exist. But what if the shadow covers an entire panel while the other panels remain fully exposed to the sun? Well, here it's up to us to insert a bypass diode in parallel to each panel. In this way, if a panel is shaded, it will be excluded by means of the bypass diode and will not negatively affect the production of the other panels connected in series. In a grid-connected PV system, the fundamental role of tracking the maximum power point (MPPT) is played by the grid-tie inverter; while in an off-grid solar power system the role is played by the MPPT solar charge controller.
Choosing the correct bypass diode
The choice of the bypass diode have to be based on two considerations: the correct protection of the solar string in case of shading and the low power dissipation on the diode itself. It is therefore important to choose particular diodes, called Schottky diodes, which can safely withstand the current of the panels and which have a very low threshold voltage. The lower the threshold voltage, the lower the dissipation of solar power on the diode.
Series connection of two solar panels with different current
If we have two or more solar panels with the same voltage but with different current, it is NOT possible to wire them in series. Nonetheless it is possible to wire them in parallel. The parallel connection allows to increase the current, keeping the same voltage. For more information, visit the page how to wire solar panels in parallel.
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