How do Solar P.V. Panels Work?
Greetherm supplies Solar Photovoltaic (P.V.) and Solar Thermal Systems to the Irish Market. It’s fascinating how a simple flat sheet of silicon can harness the sun’s energy and generate electricity. So, how does it work?
The majority of Solar P.V. Panels available today are made from various forms of Silicon. This is the same Silicon used to manufacture computer chips, and many production processes are shared between the two. Silicon is classified as a semiconductor–it’s not a complete insulator like plastic, nor a conductor like copper. Its unique crystal structure grants it special properties. When exposed to an external energy source, such as heat, its ability to carry current, known as conductivity, increases. By utilising specific manufacturing techniques like doping, the conductivity and electricial characteristics of silicon can be precisley controlled.
Doping involves introducing other materials into the Silicon crystal to modify its current-carrying properties, making it more receptive to current flow. Positively doped (P-type) Silicon is adjusted to have a slight positive charge, while negatively doped (N-type) Silicon is endowed with a slight negative charge. When P-Type and N-Type Silicon are combined, they form what’s called a P-N Junction. This junction creates a natural barrier that allows current to flow in one direction only, once a certain switch-on voltage has been reached. Think of it as a damn in a river that permits water to flow over when the river level rises above the dam wall. Similarly, a P-N Junction in reverse blocks the backward flow of current.
P-N Junction and Diodes
The P-N Junction serves as the foundation for most modern electronics, with its most basic application found in a Diode–a device used to restrict the flow of electric current in one direction.
Semiconductors don’t behave like ordinary electrical devices; they possess unique characteristics. Once a diode “switches on,” its effective resistance drops significantly. This means that even a small increase in voltage leads to a rapid surge in current. This switching function makes semiconductors highly valuable in modern electronics, such as computer processors. Moreover, the “switch” need not be the voltage across the junction alone; it can be triggered by any energy source. For example, a Photodiode is a diode switched on by light energy.
But how do we harness this to generate electricity?
The Photoelectric Effect:
Light is a form of energy. When light strikes an object, it can dislodge electrons within the material, generating a small voltage. This phenomenon is known as the photoelectric effect. Some substances exhibit a greater propensity for the photoelectric effect than others. However, due to the nature of most materials, these voltages tend to dissipate quickly unless some form of barrier exists between the charges.
Enter the P-N Junction:
In a solar cell, the P-N Junction prevents the charge from dissipating within the cell, allowing them to accumulate and create a voltage across both terminals of the cell. This voltage is limited by the solar cell’s characteristics, including its size and the quality of the P-N Junction separating the charges. The lower the quality, the easier it is for charge to leak back across. Eventually, this leakage reaches the rate of charge production, establishing the Open Circuit Voltage of the panel.
However, if these terminal are connected with a cable, current will start to flow.
A Current Source
Under ideal conditions, a solar cell functions as a current source. When connected to any load, it strives to push a fixed current through that load, say 3 Amps. It adjusts the voltage to meet the requirements for that specific current. The amount of current the cell endeavors to deliver related to the amount of solar energy falling on it. Simply put, the more sunlight, the higher the current output. It’s that straightforward.
Nevertheless, there is a limit to this relationship. Remember the Open Circuit Voltage mentioned earlier?
If the resistance to current flow in the load is sufficiently high, the voltage required to drive current through the load becomes so high that more charge starts to dissipate within the panel. The voltage will gradually increase to the Open Circuit Voltage, while the delivered current will drop off sharply. This dissipation of energy causes the panel to heat up, which, in turn, somewhat reduces its performance by facilitating charge leakage, further limiting voltage and current.
The Power of the Panel
The I/V characteristic Curve for the Viridian Clearline range of Solar Panels offered by Greentherm is depicted below. It displays curves for the 250W, 300W, and 500W PV panel models. Notably the maximum operating current remains the same for all three panel models–the only change is the maximum voltage at which this current is supplied. Additionally, it’s evident that the open circuit voltage of the 500W panel is twice that of the 250W panel. This disparity arises due to how the panels are manufactured, with individual solar cells connected in series, combining their open circuit voltages.
Essentially, these lines represent the operational power of the panel for various loads. The delivered power through a load can be easily calculated from these graphs for any attracted resistive load. The Maximum Power Point of the panel, where it operates at peak efficiency, occurs when the voltage multiplied by the current (Vxl) is at its maximum. This occurs just as the current starts to decrease and is indicated on the graph with a red line. The lines signify the Maximum Power Voltage and Current shown on the datasheet. At this point, the panel achieve its rated power output, knows as Watts-peak (kWp or Wp)
Maximum Power Point Tracking (MPPT)
When a load is directly connected to the panel, it’s unlikely that the panel will operate at its maximum power point. For example, if a battery is connected directly to the panel for charging purposes, the system voltage will be regulated to match the battery voltage. The maximum current delivered will always be constrained by the panel’s characteristics–in the case of the aforementioned panel, approximately 8 Amps. With a 12 Volt battery connected, this would restrict the maximum power deliverable to the battery to less than 96W, which is less than half of the rated power output.
The same principle applies to other electrical loads. Attaching a simple resistive immersion heater or a basic charge controller to the PV panel will push it beyond its most efficient operating range. The same occurs if an individual panel is shaded within an array or experiences a malfunction–the output of the entire array is limited b the shaded panel’s maximum voltage or current capability, depending on how they are interconnected.
To optimise the performance of a Solar PV Panel, the load must be matched to the Maximum Power Point of the Panel.
However, the Maximum Power Point is not necessarily a fixed point on the graph; it varies with the amount of sunlight received by the panel. The maximum output current of the panel is directly influenced by the sunlight falling on it. Shading of individual panels or modules within the PV system also affects the maximum power point.
Consequently, it becomes essential to employ an electronic device–such as a charge controller or grid-tie inverter with Maximum Power Point Tracking capability. Devices equipped with MPPT capabilities can adjust their electrical characteristics to ensure that the connected Solar Panels operate close to their peak efficiency, regardless of the load connected to them.
At Greentherm, we offer charge controllers and inverters with MPPT capability.
For More Information:
Please feel free to contact us for more details on the design, specification, and installation of Solar PV systems. We are also available to provide a no-obligation quotation or consultation upon request.