How Do Photovoltaic Panels Work?
Photovoltaics (PV) refers to technology that converts sunlight directly into electricity using semiconducting materials. The basic unit used for this energy conversion is known as a photovoltaic cell, or solar cell. PV has a long history beginning in 1839 when the photovoltaic effect was first observed by French physicist Edmond Becquerel. For over a century PV technology was primarily used in special applications such as powering satellites. But in recent decades, driven by concerns over climate change and energy security, photovoltaics have seen rapidly growing popularity as a sustainable renewable energy source.
Today, PV is used in a wide variety of commercial and residential applications such as solar panels on rooftops, building facades, and solar farms. With photovoltaic module prices dropping dramatically and efficiency increasing, PV capacity has expanded at unprecedented rates. Between 2012 and 2018, total global PV capacity grew over 600% making it the fastest growing energy source. Many experts expect PV to play a critical role in reducing fossil fuel dependence and mitigating greenhouse gas emissions.
What is a Photovoltaic Cell?
A photovoltaic (PV) cell is a specialized semiconductor device that converts sunlight directly into electricity. PV cells are made from materials that exhibit the photovoltaic effect, meaning when sunlight shines on the cell it generates an electrical current.
The most common material used in PV cells is silicon. Silicon atoms are chemically treated to form a large crystalline structure. The crystalline silicon is then sliced into thin wafers to produce the basic PV cell. Other materials like gallium arsenide and cadmium telluride are also used in some PV cells.
When sunlight shines on a PV cell, photons from the sunlight are absorbed by the semiconductor material. This frees electrons from the atoms, allowing them to flow through the material and produce electricity. The PV cell has electric fields formed by the addition of phosphorus and boron that provide direction for the current flow.
On the surface of the cell, metal contacts collect the current. Wires are attached to the positive and negative sides to carry the electricity for external use. An antireflective coating helps reduce light reflections off the surface and improves absorption.
How Solar Energy is Converted to Electricity
Solar panels convert sunlight into electricity through the photoelectric effect. When photons from sunlight strike the solar cell, they transfer their energy to the electrons in the atoms of the semiconductor material, causing the electrons to break free from their atomic bonds. These free electrons are then captured by the built-in electric field of the solar cell, generating an electric current.
Specifically, solar cells are made of semiconductor materials like silicon that have a unique atomic structure. Sunlight knocks electrons loose in this semiconductor, allowing them to flow freely. The PV cell has an electric field that acts to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current that we can then harness to power electrical devices.
The photoelectric effect generates electron flow in three key steps:
1. Photons from sunlight hit the solar panel and are absorbed by the semiconductor material, such as silicon.
2. The energy from the photons knocks electrons loose from their atomic bonds, freeing them to move around.
3. The electrons migrate from the p-type to the n-type semiconductor layers, creating an electrical current. This electron flow provides the electricity that powers our devices.
By tuning the properties of the semiconductor materials, solar cell manufacturers optimize the panels to efficiently convert the maximum amount of solar energy into electric current. The photoelectric effect enables this direct conversion of light into electrical energy.
Photovoltaic Panel Configuration
Photovoltaic panels are wired together in configurations to produce the desired voltage and current output. The two main wiring configurations are series and parallel.
In a series configuration, the panels are daisy chained together, connecting the positive terminal of one panel to the negative terminal of the next panel. This sums the voltage of each panel while maintaining the same current flow through the string of panels. Series wiring is used when a higher voltage is needed from the system.
In a parallel configuration, the positive and negative terminals of each panel are connected together in parallel. This sums the current outputs of each panel while maintaining the same voltage. Parallel wiring is used when a higher current is needed from the system.
Most photovoltaic systems use a combination of series and parallel wiring to obtain the desired voltage and current. Large systems may have multiple series strings connected in parallel.
Bypass diodes are also incorporated into photovoltaic panels to maximize power output. These diodes bypass any panels that are shaded or malfunctioning so the remaining panels can continue to generate electricity.
Measuring Solar Panel Power
The power output of a photovoltaic panel is measured in watts or kilowatts. A watt is a unit of power that measures the rate of energy conversion or transfer. For photovoltaic panels, it refers to how much solar energy is converted into electrical energy.
Kilowatts are equal to 1000 watts, so a solar panel rated at 250 watts would be 0.25 kilowatts. Typical residential solar panels range from 250-400 watts each. To find the total power output of a solar array, you multiply the wattage of each panel by the number of panels.
In addition to wattage, two other measurements are used to evaluate solar panel performance – voltage and current. Voltage measures the electrical potential or pressure that causes current to flow. Current is the amount of electrical charge flowing through the circuit per second. Multiplying voltage and current gives you wattage (Power = Voltage x Current).
So when looking at solar panel specifications, you’ll see measurements for wattage, voltage (in volts), and current (in amps). These metrics help determine how much total power a panel can produce under optimal sunlight conditions. Tracking voltage and current output helps monitor solar panel efficiency and performance.
Efficiency and Performance Factors
The efficiency of a solar panel refers to how much of the sun’s energy striking the panel is converted into usable electricity. It is expressed as a percentage, with higher efficiency panels able to convert more of the available sunlight into electricity.
There are a few key factors that can affect the efficiency and overall performance of photovoltaic panels:
Cell type and materials: The type of photovoltaic cell technology and the materials used have a big impact on efficiency. Monocrystalline silicon cells tend to have the highest efficiencies, usually around 15-22%. Polycrystalline silicon cells are a bit lower, around 13-19%. Thin film technologies like cadmium telluride (CdTe) tend to have lower efficiencies of 7-13%. Improvements in materials and manufacturing continue to push efficiency higher.
Operating temperature: As panels get hotter, their efficiency drops. On very hot days, a solar panel may operate 20-25% less efficiently than on a cooler day. Proper solar panel installation to allow air circulation can help keep panels cooler.
Shading and dirt: Even small amounts of shading or dirt buildup on panels can have a big effect on electrical production. Shading just 5% of a panel’s surface can reduce power output by 50% or more. Keeping panels clean and free of any obstructions maximizes efficiency.
By understanding these factors, steps can be taken to optimize the performance and efficiency of photovoltaic systems.
Applications of Photovoltaic Panels
Photovoltaic panels have a wide range of applications and can be implemented in systems ranging from small scale to utility-level power generation. Some of the main applications include:
Off-Grid
Off-grid photovoltaic systems are not connected to an electricity distribution grid. They are often used in remote locations where grid power is inaccessible, such as on boats or RVs, remote cabins, and in developing regions without electricity infrastructure. Off-grid systems store solar energy in batteries for use at night or on cloudy days.
Residential
Residential solar refers to photovoltaic panels installed on people’s homes, either on the roof or ground-mounted. Excess solar energy can be fed back into the grid. Residential systems are sized to match the home’s electricity demand and help reduce reliance on grid power.
Commercial
Commercial photovoltaic systems are installed on the rooftops or grounds of businesses like warehouses, factories, offices, stores, etc. System sizes are scaled to match the facility’s electricity consumption. Commercial solar helps businesses reduce electricity expenses and meet sustainability goals.
Utility-Scale
Utility-scale photovoltaic power stations are large-scale solar farms that generate electricity for sale to utilities, which then distribute it to customers via the grid. They can range from a few megawatts to over 1 gigawatt in capacity. Utility-scale solar provides clean, renewable electricity to the grid that displaces fossil fuel generation.
Installation and Maintenance
Properly installing and maintaining photovoltaic panels is critical to ensuring optimal performance and longevity. There are several key factors to consider:
Mounting
Photovoltaic panels need to be securely mounted, most commonly on rooftops or ground mounts. Panels are mounted on racks attached to roof rafters or concrete footings. Proper mounting keeps the panels fixed in position to maximize solar exposure.
Positioning
Panels should be pointed true south in the northern hemisphere, or true north in the southern hemisphere, to optimize energy production. The tilt angle of the panels should match the latitude of the installation location.
Cleaning
Dirt, dust, bird droppings, and other debris can accumulate on the panels, reducing their efficiency. Regular cleaning helps maintain optimal performance. Panels can be gently wiped with a soft brush, microfiber cloth, or compressed air.
Lifetime
With proper installation and maintenance, photovoltaic panels typically last 25-30 years. Their efficiency slowly declines over time. Most solar panel warranties cover at least 80% output for 25 years.
Environmental Impact
Photovoltaic solar panels have a relatively small environmental impact compared to other forms of electricity generation. They produce no air pollution, water pollution or greenhouse gas emissions during operation. The majority of their impacts come from the manufacturing process.
The emissions generated during manufacturing depend on the energy sources used to produce the panels. Most emissions come from the energy used to purify silicon and other semiconductor materials. Producing solar panels in locations powered by renewable energy can greatly reduce manufacturing emissions.
There are some concerns around the toxicity of semiconductor materials like cadmium telluride used in thin film panels. However, any toxicity risk is contained within the panel itself. New recycling processes can recover over 90% of semiconductor materials for reuse in new panels.
At the end of their lifespan, solar panels can be fully recycled. Semiconductor materials, aluminum, glass and various metals can all be recovered and reused. Recycling reduces the number of panels ending up in landfills and reduces the need for new raw materials.
Overall, solar power emits 80-90% less greenhouse gases per kilowatt-hour compared to fossil fuels. As manufacturing techniques improve and the electric grid gets cleaner, these emissions will continue to fall. The environmental advantages of solar power will keep growing over time.
Future Outlook
The future is bright for photovoltaic panel technology. There are continuous efforts to improve the efficiency and lower the costs of solar panels. Some key areas of innovation include:
Improving Efficiency: Researchers are exploring new materials and manufacturing techniques to increase the efficiency of solar cells. Perovskites are emerging as promising new materials that are cheaper and easier to manufacture than traditional silicon cells. Tandem or multi-junction cells that combine multiple materials are also being developed to utilize more of the solar spectrum.
New Materials: Thin-film solar cells made from materials like cadmium telluride and copper indium gallium selenide allow for flexible, lightweight panels. Organic solar cells based on carbon-rich polymers and molecules are also being researched as a low-cost option. Quantum dot solar cells are an emerging technology using nanoscale semiconductor particles.
Growth Predictions: The solar industry has averaged impressive growth rates of over 20% annually over the past decade. With solar energy costs continuing to fall, photovoltaic capacity is forecasted to expand exponentially in the coming years. Solar power has the potential to generate 20-50% of global electricity by 2050.
In summary, photovoltaic technology is poised to scale up and become a major sustainable energy source worldwide. With ongoing improvements in efficiency, cost, materials and manufacturing, the future is bright for solar panels to play a leading role in the global transition to renewable energy.
