7.1 Photovoltaic modules
A photovoltaic cell is a basic component of a photovoltaic system. A single photovoltaic cell typically produces between 1 and 2 watts of power, which is insufficient for most applications. For higher voltages or currents, photovoltaic cells are connected in series or parallel to form a photovoltaic panel (PV panel, sometimes called a photovoltaic module). PV panels which are commercially available range in size from 0.3 to 2 \( m^{2} \) depending on their power. The power output of such panels is expressed in watts of peak power (Wp - maximum power in watts), defined as the power they deliver under standard conditions (STC); this is typically between 30 Wp and 600 Wp. In practice, PV panels rarely operate under standard conditions, so it is useful to have current-voltage I(U) characteristics over a wide range of operating conditions. PV panels are encapsulated to protect them from corrosion, moisture, pollution and atmospheric influences. The enclosures must be durable as the life expectancy of PV panels is expected to be at least 20-30 years.
Each PV panel consists of photovoltaic cells. Traditional photovoltaic cells are usually 156 mm x 156 mm in size, connected in series and depending on the type, there are usually 60 or 72 cells in a PV panel. Photovoltaic cells are made in monocrystalline, polycrystalline, amorphous and thin-film technologies. The differences between them are due to the type of material used for their production. A silicon cell gives a voltage of about 0.6 V, which when the cells are connected in series gives about 36 V, and since each photovoltaic cell gives off about 9 A of current, the maximum power from a PV panel is about 300 W. The power depends on the technology used to make the photovoltaic cell. There are already emerging technologies and manufacturers announcing PV panels with 500 watts or 600 watts or more. For example, Canadian Solar announces the production of 665 W PV panels made of 132 monocrystalline PERC cells, with a side length of 210 mm, with an efficiency reaching \( 21.4\% \) [1].
Photovoltaic panels produce direct current. The current intensity at the output of the panel strictly depends on the insolation. The current intensity can be increased by connecting PV panels in parallel in a photovoltaic system.
The voltage received from the PV panel depends little on the level of insolation, while it can be adjusted by series and parallel connection of photovoltaic cells in the PV panel. Photovoltaic panels can operate at 12 V or 24 V in an isolated installation and at 240 V or more when connected to the utility grid.
The construction of a photovoltaic panel can be described as a "sandwich" construction, which is one that consists of layers of materials and components applied sequentially. The manufacturing of a PV panel starts with the top layer, i.e., glass; it can be said that the PV panel is assembled from top to bottom. So, the whole process will be described from the top layer of the PV panel to its bottom layer.
From the top, i.e., from the solar side, the PV panel is protected from mechanical damage by tempered glass with a thickness of 3.2 mm or 4 mm. This glass also reduces the amount of reflected sunlight, limiting energy losses that reduce the PV panel's power output. Polishing the surface of the glass, coating it with an anti-reflective layer, or using surface texturing can help to reduce reflection.
On the other hand, silicon cells ( Fig. 1 ) are covered with an anti-reflective layer and a very thin metal mesh and, together with the current-carrying busbars, are protected by an EVA film to form a hermetic shield. The manufacturing process is shown in the video "Solar cell manufacturing and solar panel production by Suntech".
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On the front side of conventional photovoltaic cells, a negative electrode is placed, which are thin horizontal paths (fingers). The fingers continuously collect the charges generated on the surface of the photovoltaic cell, which are then collected by wider vertical paths (busbars). The positive electrode is located at the back of the cell. From the busbars, current flows to a copper ribbon that connects the negative electrode of one cell to the positive electrode of another. The ribbons thus allow the generated photocurrent to be transported from the area of one photovoltaic cell to the next cells that make up the panel. Ten years ago, all PV panels were built with photovoltaic cells containing 2 busbars. Today, most PV panels are based on photovoltaic cell designs containing at least 5 busbars. Increasing the number of busbars improves the efficiency of the photovoltaic cells as well as their durability.
From below, the tightness of the PV panel is ensured by a special insulating film, the so-called backsheet, which also gives it a suitable color (usually white, black, or transparent). An aluminium frame is used to stiffen the whole structure. Another element is the junction box, from which two cables ending in plugs come out, connecting the panels in series.
In this junction box there are also bypass diodes [2]. Bypass diodes are an essential part of a photovoltaic panel, protecting and improving its operation. Photovoltaic cells are connected in series in the direction of conduction ( Fig. 2 ). If one cell were to stop conducting, the entire PV panel would be out of power production, but bypass diodes connected in the negative direction allow the non-working photovoltaic cell to be bypassed. They also reduce the risk of damaging a shaded PV cell - current flows in the opposite direction through a shaded (leaves, snow) cell, causing the PV cell to overheat significantly.
When several photovoltaic panels are connected in series to increase the voltage, the current flowing in the circuit will be equal to the current of the weakest element in the system. If one of the photovoltaic panels is shaded (e.g., by a chimney or a bay window), the power of the circuit will decrease dramatically. The bypass diodes, by taking the shaded PV panel out of the chain, will reduce the losses in the entire system. Current flows as per Fig. 3. Some boxes have a special disconnect switch that will disconnect a series or individual PV panel in the event of their fault.
Fig. 4 shows an example connection of three and ten bypass diodes, which puts that part of the PV panel out of operation when it is shaded.
Including bypass diodes in a PV panel divides the panel into 3 parts if three bypass diodes are included and into 10 parts if ten bypass diodes are included. This division allows each part to be used separately to produce electricity regardless of the state of the other components of the photovoltaic cell.
Tables specifying the characteristics of photovoltaic panels typically provide:
- two power values by two standards – STC (Standard Test Conditions) rated power which means that the test was conducted under AM1 insolation conditions 5, with a power of 1000 \( \frac{W}{m^{2}} \) at \( 25_{}^{o}\textrm{C} \) and power at the so-called real-world NOCT (Normal Operating Cell Temperature) insolation intensity with AM1 spectrum. 5 800 \( \frac{W}{m^{2}} \), wind speed \( 1\frac{m}{s} \) at an ambient temperature of \( 20_{}^{o}\textrm{C} \),
- efficiency is a quantity that describes the proportion of solar energy that can be converted into electricity; for mass-produced monocrystalline panels, it is up to \( 25\% \),
- photovoltaic module dimensions and weight – these vary and depend on the manufacturer; obviously the heavier the PV panel, the more weight it puts on the roof structure, and the larger it is, the more space it takes up,
- temperature coefficient allows you to determine how much power the module will achieve at a certain temperature; the lower the better,
- annual power loss – PV panels can lose efficiency over time; most manufacturers specify the efficiency of their PV panels at \( 80\% \) after 25 years; typically, PV panels lose \( 2-3\% \) of efficiency in the first year, and lose between \( 0.3\% \) to \( 0.6\% \) of efficiency annually thereafter,
- fill factor (FF) – can be said to check the quality of the cell
- dependence of PV panel efficiency on solar insolation magnitude,
- the ratio of efficiency under NOCT to STC conditions is also used, if it is 0.8 it means that the panel is usable.
These properties make it possible to evaluate the quality of the photovoltaic cells that make up the PV panel as well as the panel itself. PV panels are divided into three classes A, B and C. Class A PV panels should have an FF fill factor above 0.75. It should also be taken into account, that the insolation in Poland is usually between 200 \( \frac{W}{m^{2}} \) and 600 \( \frac{W}{m^{2}} \), and take this into account when planning a PV power plant.