Solar photovoltaic panels are an excellent way to harness the sun's energy to produce electricity. Whether they're part of an off-grid system or roof-mounted in a grid-connected setup, they offer a wide range of power outputs, from small-scale milliwatts to large-scale megawatts, thanks to the modular nature of these panels.
Photovoltaic Shading
Photovoltaic cells are semiconductor devices that directly convert sunlight into electricity. These cells are linked together to form a solar array, generating power when exposed to direct sunlight.
In theory, all panels in a solar array should experience the same conditions. However, environmental factors like temperature, humidity, positioning, and sunlight intensity can affect the overall performance and reliability of a PV system. Among these factors, shading—caused by trees, buildings, or debris like leaves—can significantly degrade the system's performance. This shading can be either partial or full and will lead to a decrease in the output power of the affected panel or array.
Series-Connected Solar Cells
Solar panels consist of interconnected crystalline silicon cells, making them vulnerable to shading. Typically, these cells are connected in series to produce a higher voltage while maintaining the same current throughout the panel.
When sunlight evenly strikes the surface of the solar panel, each cell produces the same voltage, usually around 0.5 volts. For example, in full sun, a 2-watt PV cell can produce a current of about 4 amperes (0.5V x 4A = 2W). However, if one of these cells is shaded, it stops producing electricity and starts acting like a resistive load, significantly reducing the panel's total output. In a series string of three 0.5V cells exposed to 1kW/m² of sunlight, the total voltage would be 1.5V, and the power output would be 6 watts.
Shading Impact on Photovoltaic Cells
If one of the cells in the series becomes shaded, the overall output of the panel decreases drastically. The shaded cell stops producing power and behaves like a semiconductor resistance, which causes a dramatic drop in energy production. This mismatch between shaded and non-shaded cells leads to power from the unshaded cells being dissipated in the shaded cell, causing it to heat up and potentially suffer damage over time.
When a cell is shaded, its generated current decreases, forcing the unshaded cells to compensate by increasing their voltage. This causes the shaded cell to become reverse biased, meaning a negative voltage is applied across its terminals. In this reverse-biased state, the shaded cell consumes power rather than generating it.
Bypass Diodes
To protect solar cells, panels, or even entire arrays from the negative effects of shading, bypass diodes are connected across each PV cell in a series string. These diodes are installed externally and in reverse parallel with each PV cell to provide an alternate path for current when a cell is shaded. This setup helps prevent reverse bias voltage from accumulating across the shaded cell, reducing the power dissipated by it.
For example, if three PV cells are connected in series, bypass diodes can be placed across each of the three cells. These diodes are reverse-biased in normal conditions, with the cathode connected to the positive side of the cell and the anode to the negative side. As long as all cells receive full sunlight, the diodes remain reverse biased and inactive, allowing the series string to produce full power.
However, if one cell becomes shaded, the corresponding bypass diode becomes forward biased, turning it "on" and allowing current to flow through the diode instead of the shaded cell. This maintains the current flow through the remaining unshaded cells, allowing the system to continue generating power, albeit at a reduced level.
The main benefit of using bypass diodes is that they prevent high reverse voltages, which can create hot spots and damage the cells. Once the shading is removed, the bypass diodes stop conducting, and the solar cells return to their normal operation.
Bypass Diode Integration
While adding bypass diodes across individual PV cells may be impractical due to cost and complexity, manufacturers typically install bypass diodes across groups of cells (usually between 16 and 24) within the junction box of a solar module. For example, a 50-watt solar panel with 36-40 cells may require only two bypass diodes to provide sufficient protection.
Schottky diodes are the most common type of bypass diode used in solar panels. These diodes typically have current ratings between 1 and 60 amperes and voltage ratings of up to 45 volts, making them suitable for 12V or 24V battery-charging solar panels.
In conclusion, bypass diodes are essential components in solar panels, ensuring that shaded cells do not negatively impact the overall performance of the array. By providing an alternate path for current, bypass diodes protect shaded cells from damage and allow the rest of the array to continue operating efficiently.