Solar panels are devices, that convert solar energy into electricity and can already be seen on many rooftops and fields around the cities. As solar energy is considered a renewable energy source (even though the sun will eventually burn out), its use, along with, for example, wind energy, has become very popular in recent years. One significant advantage of using solar energy compared to burning fossil fuels is obviously the absence of pollution. Additionally, since the sun will continue to shine for a very long time (about five billion years), this energy source is practically infinite from a human perspective.
In this blog post, we take a closer look, at how solar panels work. We'll explore the main component of a solar panel - the solar cell, which makes up the structure of solar panels, how it's constructed, and what is the photovoltaic effect. Then, we briefly examine the essential components that come with solar panels, such as inverters and batteries, how efficient solar panels are, and what the optimal tilt angle for solar panels should be, based on the geographical latitude and season.
In a nutshell, solar panels work by harnessing the energy from sunlight and converting it into electricity through the photovoltaic effect. When solar cells absorb photons, they release electrons and generate a flow of electric current. A charge is stored in a battery through the accumulation of electrons during sunlight exposure, and it is converted into alternating current (AC) using inverters for integration with electrical systems.
The basic construction of a solar cell
Understanding at how one solar cell produces electricity, makes us understand, how do solar panels work. Solar cells are the basic building blocks of solar panels and are typically made from semiconductor materials such as silicon. They are also called photovoltaic cells and operate based on the principle of the p-n junction. A p-n junction is an interface between two different semiconductor materials, typically n-type (negative) and p-type (positive) silicon.
Creation of the p-n junction
To create a solar cell, a thin layer of phosphorus-doped (n-type) silicon is placed on top of a thicker layer of boron-doped (p-type) silicon. The donor element, such as phosphorus, has five valence electrons. When it replaces a silicon atom in the crystal lattice, it creates an excess electron. These extra electrons become the majority carriers in the n-type region. The acceptor element, such as boron, has three valence electrons. When it replaces a silicon atom in the crystal lattice, it creates a "hole" or a vacancy in the electron structure. These holes become the majority carriers in the p-type region.
The diffusion of electrons and holes results in the transfer of charge across the p-n junction. Free electrons and holes recombine, creating a region, where there are no free electrons or holes. This transfer of charge creates positive ions in the boundary of the n-type region (due to the loss of electrons) and negative ions in the boundary of the p-type region (due to the loss of holes). The presence of these immobile ions creates an electric field that opposes the further diffusion of charge carriers.
Creating current in solar cell
When light from the Sun, reaches the depletion zone, it excites electrons in the depletion zone, providing them with energy to break free from their atomic bonds. As a result, electron-hole pairs are generated within the depletion zone. The electrons move to the n-side of the junction, while the holes move to the p-side, due to the internal electric field created by the p-n junction.
The separation of electrons and holes results in a flow of charge carriers within the solar cell. Electrons move through the n-type region and toward the front metal contact, while holes move through the p-type region and towards the back metal contact. This movement of charge constitutes an electric current. The metal contacts on the front and back surfaces of the solar cell provide a pathway for this current to flow through an external circuit.
Each solar cell has a relatively low power output (1-5 W). By connecting multiple cells in series and parallel within the panel, we can achieve higher voltage and current levels, allowing the panel to capture more sunlight and produce a greater electrical output. For instance, if each solar cell in a 60-cell solar panel has an output of around 4 watts, the total power output of the panel would be approximately 240 watts (60 cells x 4 watts/cell). Similarly, a 72-cell solar panel with the same output per cell would have a total power output of around 288 watts (72 cells x 4 watts/cell).
Converting direct current to alternating current
All solar panels produce direct current (DC), which means that the direction of the current does not change over time, and the movement of electrons is unidirectional. However, most of the electrical devices in our homes use alternating current (AC) for their operation, where the direction of electron flow changes periodically over time. For example, in Europe, the frequency of AC power is 50 Hz, while in the United States, it is 60 Hz (meaning the direction of electron flow alternates 50 or 60 times per second). Therefore, it is necessary to convert the DC produced by solar panels into AC.
This task is accomplished by inverters - devices that convert direct current to alternating current. Charge controllers are needed in solar panels to regulate the charging process and prevent battery overcharging. They optimize the energy flow from the solar panels to the battery, ensuring efficiency and extending battery life. Conversely, batteries are essential in solar panels to store excess electricity generated during sunny periods. Batteries provide a steady and continuous flow of power even when electricity consumption significantly increases. Additionally, the energy stored in batteries can be used during periods when there is no sunshine, and the solar panels are not generating electricity.
Efficiency of solar panels
One very important parameter of solar panels, in addition to their output power, is their efficiency. Efficiency indicates how much solar energy the solar panel converts into useful electrical energy (usually expressed as a percentage). It is a crucial parameter that determines the panel's performance and indicates how effectively it can harness solar energy.
Typically, solar panel efficiency falls within the range of 15% to 20%, which means they only convert about one-fifth of the solar energy they receive into electricity. While this may seem like a relatively low number, technology is advancing, and solar panels are becoming increasingly more efficient over time. As of the current moment (in 2023), the most efficient commercially available panels have an efficiency of around 23%.
The most crucial factor that determines efficiency is usually the technology used in manufacturing the solar cells and the quality of materials employed. The use of high-quality materials and precise manufacturing techniques can significantly enhance light absorption in solar cells, thereby increasing the efficiency of the panels.
The tilt angle of solar panels
The tilt angle plays a significant role in capturing the optimal amount of sunlight and maximizing the electricity generated.
The tilt angle of solar panels refers to their angle of elevation concerning the ground or the horizon. It influences the angle at which sunlight strikes the solar panel surface, impacting the amount of energy captured. While the latitude of the installation site serves as a starting point for determining the ideal tilt angle, it is essential to consider additional factors like seasonal variations and the intended use of the solar panel system.
For example, Estonia where I live, lies at approximately 60 degrees latitude north. The recommended tilt angle would be equal to the latitude itself. Hence, the ideal tilt angle for solar panels in Estonia would be 60 degrees. This fixed tilt angle aligns the panels to capture sunlight optimally throughout the year, providing a balance between summer and winter energy generation.
However, suppose one wishes to enhance energy capture during specific seasons. In that case, slight adjustments can be made. Adding 15 degrees to the latitude for the winter tilt angle (75 degrees) allows the panels to capture more sunlight during the low sun angle months, increasing winter energy production. Conversely, subtracting 15 degrees from the summer tilt angle (45 degrees) ensures optimal energy capture during the sunnier months.
In summary, how do solar panels work
Although about solar panels can be written at length, and the topic is broad and can go into great depth, we still received a concise overview of how solar panels work and produce electricity. We learned about the photovoltaic effect and how it enables a solar cell to generate an electric current: sunlight hits the solar cell, creating pairs of free electrons and holes, and a voltage is formed between the top and bottom electrically conductive contacts on the panel. The generated electric current is direct current (DC) and usually requires conversion to alternating current (AC) to be directed to the power grid or to activate various electrical devices, which primarily operate on AC. The conversion of DC to AC is achieved using an inverter, which, along with batteries and a charge controller, is typically an integral part of solar panel systems.
An important parameter that characterizes solar panels is their efficiency. The efficiency of a solar panel is usually determined by the manufacturing technology of solar cells, its precision, and the quality of materials used. While modern commercial solar panels can convert solar energy into electricity with about 20% efficiency, research, and development efforts to increase efficiency are ongoing worldwide, leading to even more efficient panels. Optimal electricity production and increased efficiency can also be achieved by correctly aligning solar panels with respect to the sun. Moreover, the tilt angle of solar panels holds great significance in capturing maximum sunlight exposure. By aligning solar panels based on latitude and making seasonal adjustments, energy production is optimized throughout the year.