Solar Academy Lesson 3: How solar panels work
A solar panel is made up of solar cells that are often bundled together in the panel into solar modules. A typical solar panel is made up of 60 or more individual solar cells. A solar cell is built like a sandwich. It has an upper layer and a lower layer just like slices of bread. Those layers are made of silicon, which is treated (referred to as doping) with other elements like boron and phosphorus that cause the silicon to either have too many electrons or too few of them. The solar cell produces electricity when light hits it because the energy from the light knocks electrons loose from the layer in the cell that has too many electrons. The result is that the electrical current flows from the cell
The photovoltaic effect
The photovoltaic effect describes the ability of some materials to emit electrons when exposed to light. Most solar cells are made primarily of silicon, but other materials are used as well. Materials like silicon are used because they are semiconductors. A semiconductor is a substance that shares some of the properties of metals, which conduct electricity, and some of the properties of insulating materials that don’t conduct electricity.
How semiconductors in solar cells work
The two layers of silicone in a solar cell are referred to as the n-layer and the p-layer. The n-layer has a negative electrical charge, the p-layer has a positive electrical charge. When sunlight enters the cell, the photons pass through the n-layer carrying their energy with them. The photons then give up their energy to electrons in the lower p-layer. Those electrons then use the energy given them by the photons to jump across into the n-layer. That results in the n-layer emitting those electrons into the circuit, producing electricity.
How solar cells work in a solar panel
The solar cells within a solar panel are wired together in series. This means that each solar cell raises the ultimate voltage output of the panel. A typical solar cell produces about 0.46 volts. But there are several different kinds of solar cells, so actual power output will vary according to the type of solar cells used to build the solar panel. A solar panel can be made up of 32, 36, 60, 72 or 96 individual solar cells. Thus:
- 32 cells = 14.72 volts
- 36 cells = 16.56 volts
- 60 cells = 27.60 volts
- 72 cells = 33.12 volts
- 96 cells = 44.16 volts
A solar panel’s power output can be determined by using this equation: P = V x I. Where P equals power, V equals voltage, and I equals current. Using the Hanwha Q 310 watt solar panel as an example.
- (V) Voltage = 32.78
- (I) Current = 9.31 Amps
- (P) wattage = 305 Watts
The negative effect of partial shade on a solar panel
Solar panels are highly affected by even a little shade. During partial shading, the output of a solar panel drops dramatically. This happens because the solar cells in a solar panel are wired together in series. If even one cell gets shade, that cell’s performance drops and it takes all the other cells down with it. Worse yet, in solar systems with a central inverter, if one panel’s power output goes down due to shading, it reduces the output of all the panels in the entire system!
Standard 60-cell panels are electrically connected as three sets of 20 cells each. When as few as one of those cells is shaded, it can shut off that entire 1/3rd of the panel. Small areas of partial shading from trees and roof obstructions can cause this sort of loss consistently.
Some panel manufacturers have started using 120 half-cells, rather than 60 full cells, in order to make their panels even more tolerant to shading. The six distinct circuits, rather than just three total circuits for the panel, mitigate half the shading losses while keeping the base electrical profile the same.
Solar panels also mitigate the loss of output caused by shading by connecting the cells in a solar panel together with bypass diodes. A bypass diode will allow the power output from non-shaded solar cells to bypass the shaded cell. Some output is still lost due to voltage drop, but overall power output is higher than it would be without the diode.
Module-level power electronics
Module-level power electronics (MLPEs) are electronic devices that are attached to individual solar panels to manage their power output. These devices are able to mitigate the loss caused by partial shading through a process called maximum power point tracking (MPPT). MPPT works by monitoring the output of the solar panels in the system and then adjusting the electrical load on the solar system to maintain that system’s best possible power output. There are two devices that provide MPPT.
A DC optimizer is a piece of equipment that’s connected to a solar panel to monitor and adjust the flow of voltage from the panel. Should the voltage drop, the DC optimizer will reduce the current output. That, in turn, will increase the amount of voltage being produced by the DC optimizer to match the voltage output of the other panels in the system. This prevents the partially-shaded panel from dragging down the power output of the other panels in the system.
For example, if a panel is partially shaded, a SolarEdge DC optimizer will reduce the current to keep the voltage at 380V-400V so the inverter operates consistently.
Solar panels with microinverters are less susceptible to output loss caused by shading. In a microinverter system, each panel has its own inverter. Therefore, if one panel’s output is reduced by shade, it has no effect on the other panels.
The last step: The inverter
Solar Panels produce DC power – the same type used in a 9-volt battery, just much more powerful! An inverter is required to change that DC power into the AC power used by the lights, appliances, and even battery chargers in a house. It does this by detecting the exact power profile coming from the utility and using a series of switches to mimic that same power profile. Once that power is output to the house, it is the same or higher quality than the power coming from the utility grid.
In recent years, improvements in solar inverters have allowed them to support the utility grid by making it more stable. Solar inverters can support low or high voltage when the utility grid calls outside the recommended limits. This grid-interactive support benefits the entire neighborhood with consistent, well-conditioned power.