How Is Photovoltaic Energy Generated?

Photovoltaic energy (PV) is the conversion of sunlight into electricity using semiconducting materials. The term photovoltaic comes from “photo” meaning light and “voltaic” meaning electricity.

The photovoltaic effect was first discovered in 1839 by French physicist Edmond Becquerel. However, it was not until the 1950s that PV technology became viable for practical use. The space industry was an early adopter of PV technology to power satellites and spacecraft.

Today, PV has become one of the most important renewable energy sources. It offers a sustainable and clean way to generate electricity without emissions, noise or pollution. Solar PV systems can be installed on rooftops or ground-mounted to provide electricity directly to homes, businesses and utilities.

The importance and benefits of PV include:

Renewable – PV harness the infinite power of the sun.
Clean – Solar PV generates zero emissions.

Distributed – PV systems can be installed almost anywhere.
Low maintenance – PV panels require little upkeep.
Price reductions – Costs of PV have dropped dramatically.

Energy independence – PV allows creation of your own power.

How Solar Cells Work

Solar cells generate electricity through the photoelectric effect, which was first discovered by French physicist Edmond Becquerel in 1839. When light hits certain materials like silicon, the photons in the light knock electrons loose from the material’s atoms. These free electrons then flow through the material producing an electric current.

The basic structure of a solar cell consists of two layers of semiconductor material, usually silicon. On the top is a layer with an abundance of electrons (n-type silicon), and on the bottom is a layer with an abundance of openings to accept electrons (p-type silicon). When sunlight hits the solar cell, the photons strike the top layer of electrons and knock them loose, allowing them to flow through an external circuit and produce electricity.

The most common type of solar cells use crystalline silicon in a rigid flat plate design. Polycrystalline silicon cells are made up of small silicon crystals, while monocrystalline silicon cells are made from a single silicon crystal. Thin-film solar cells use cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS) instead of silicon. Thin-film cells are lighter and more flexible than crystalline silicon cells, but tend to be less efficient.

Types of Solar Cells

There are three main types of solar cells used in photovoltaic systems:

Crystalline Silicon Solar Cells

The most common type of solar cell, crystalline silicon cells are made from silicon ingots sliced into wafers. They come in two main types – monocrystalline and polycrystalline. Monocrystalline cells are made from a single cylindrical silicon crystal, giving them a uniform appearance and high efficiency but higher production costs. Polycrystalline cells are made from many silicon crystal fragments melted together, which results in lower efficiency but also lower costs.

Thin Film Solar Cells

Thin film solar cells are made by depositing photovoltaic material in one or more thin layers onto a substrate like glass, plastic or metal. They are less efficient than crystalline cells but can be easier and cheaper to manufacture. Common thin film materials include cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon (a-Si).

Emerging and Novel Solar Cells

Some novel solar cell technologies that are still under development include multi-junction cells, quantum dot cells, dye-sensitized solar cells, perovskite cells and organic photovoltaic cells. While less mature, many of these emerging technologies aim for lower costs and greater efficiencies.

PV Cell Construction

Photovoltaic cells are made up of layers of semiconductor materials that generate electricity from sunlight through the photovoltaic effect. Here are the main layers that make up a typical solar cell:

Top Layer – Anti-Reflective Coating

The top layer is an anti-reflective coating that allows sunlight to pass through to the semiconductor material underneath. This coating reduces the amount of sunlight that is reflected off the cell, improving efficiency.

Upper Layer – N-Type Semiconductor

This layer is typically made of phosphorus-doped silicon. The addition of phosphorus gives this layer an abundance of free electrons. When sunlight hits the solar cell, the energy knocks electrons loose in this layer.

Middle Layer – P-N Junction

This layer separates the n-type and p-type semiconductors and is critical to the photovoltaic effect. The p-n junction generates an electric field that forces electrons to flow in one direction, creating current.

Lower Layer – P-Type Semiconductor

This layer is typically made of boron-doped silicon, giving it an abundance of openings for electrons. The electrons from the n-type layer enter the empty spaces in the p-type layer, generating an electrical current.

Rear Layer – Conductive Plate

The rear layer collects the electrons as they flow through the cell. This conductive plate connects to the wire circuit to transmit power out of the cell.

By stacking these varied layers of materials, solar cells can convert sunlight into electricity through the photovoltaic effect.

Connecting Solar Cells

Solar PV panels are made up of many individual solar cells wired together. These cells can be connected in different configurations to optimize voltage and current output.

Series vs. Parallel

Connecting cells in series increases the voltage, while connecting in parallel increases the current. In a series connection, the positive terminal of one cell is connected to the negative terminal of the next cell. This sums the voltage of each cell together for a higher overall voltage.

In a parallel connection, all the positive terminals are connected together and all the negative terminals are connected together. This adds the current of each cell together for a higher total current. However, the voltage stays the same.

Bypass Diodes

When cells are connected in series, if one cell is shaded it can reduce the current flowing through all the other cells. To prevent this loss, bypass diodes are connected across each cell to provide an alternate current path around any shaded cells.

Components of a PV System

The main components that make up a PV system include:

Solar Panels

Solar photovoltaic panels are composed of many solar cells that convert sunlight into electricity. The solar cells are made of semiconductor materials like silicon. When sunlight hits the panels, electrons are knocked loose from the atoms in the semiconductor material, creating an electric current.

Charge Controllers

Charge controllers manage the power going into and out of the battery bank. They prevent the batteries from overcharging by regulating how much electricity flows from the solar panels into the battery bank. Charge controllers also prevent the batteries from discharging back through the solar panels at night.

Inverters

Inverters convert the DC electricity generated by solar panels and stored in batteries into AC electricity that can power home appliances and be fed back into the electrical grid. Grid-tied systems don’t always need batteries and use inverters to convert solar electricity directly into grid-compatible AC power.

Batteries

Batteries store excess solar electricity generated during the daytime for use at night or on cloudy days. Deep cycle lead-acid batteries or lithium-ion batteries are commonly used in off-grid PV systems.

Wiring

Electrical wires are needed to interconnect all the PV system components. Cables carry DC electricity from the solar panels to batteries and inverters. Additional AC wiring is used to connect inverters to appliances, switches and outlets.

Installing a PV System

Installing a photovoltaic (PV) system requires careful planning and execution. The main components that need to be addressed are the mounting structure, orientation, tilt angle, and wiring.

There are several mounting options for PV panels:

Rooftop mounting – Panels are mounted on top of the roof using racking brackets. This is the most common residential installation method.
Ground mounting – Panels are installed on top of poles anchored to concrete footings on the ground. This allows optimal orientation and tilt.

Tracking mounts – Panels are installed on motorized mounts that follow the sun to maximize energy capture throughout the day.

Orientation refers to the direction that the PV panels face. In the northern hemisphere, south-facing panels capture the most sunlight over the course of a day. Panels should point as close to true south as possible, though small deviations are acceptable.

The tilt angle reflects the angle at which the panels are mounted. For fixed mounts, the optimal tilt angle equals the latitude of the installation location. Tilt angles can be increased in winter or decreased in summer to boost energy production during those seasons.

Proper wiring requires connecting the solar panels in series strings to build up voltage. The strings are then connected in parallel to combine the currents. The DC output of the panels goes to an inverter which converts it to AC electricity compatible with the household electrical system.

Maintaining a PV System

A PV system needs some basic regular maintenance to keep it operating at maximum efficiency and producing the highest energy output possible. Here are some key maintenance tasks homeowners should be aware of:

Cleaning Solar Panels

Dirt, dust, bird droppings, leaves, and other debris can accumulate on PV panels, blocking sunlight from reaching the cells and reducing energy production. Panels should be cleaned at least a couple times per year using a soft brush and mild soap and water. Avoid abrasive cleaners or brushes that could scratch the panel surface. Panels can be gently rinsed with a garden hose if needed.

Checking Electrical Connections

All wiring connections between panels, inverters, batteries, and other system components should be visually inspected for damage or loose connections periodically. Tighten any loose connections and repair or replace any frayed or damaged wires. Proper electrical connections are vital for safety and optimal performance.

Monitoring Performance

Most PV systems have monitoring software that tracks energy production. Review this regularly to identify any decreases in output that could indicate a maintenance issue. Compare production to the specifications you expect for your system. Troubleshoot and resolve any problems if you notice a dip in performance. Keeping daily logs of energy output can help spot trends.

Efficiency and Output

The amount of power generated by a solar PV system depends on several factors related to efficiency and system capacity.

Factors Affecting Efficiency

The conversion efficiency of a solar cell refers to how much of the sun’s energy striking the cell is converted into usable electricity. The theoretical maximum efficiency of a traditional crystalline silicon solar cell is around 33%. In real-world conditions, average module efficiency ranges from 15-20%.

Several factors affect a solar panel’s efficiency:

Cell material and quality of manufacturing – Monocrystalline silicon cells tend to have higher efficiency ratings than polycrystalline.
Temperature – Solar cell efficiency decreases as temperature increases. Using solar panels with integrated back cooling can improve temperature-related efficiency losses.
Shading and dirt – Even minor shading or dirt on a solar panel can drastically reduce power output. Regular cleaning helps maximize efficiency.

Angle of irradiance – The angle at which sunlight strikes a panel impacts efficiency. Solar tracking systems allow panels to follow the sun for optimal alignment.

Capacity Factor

The capacity factor of a solar PV system refers to its actual power output over time compared to its potential output if operated at full rated capacity continuously. The capacity factor is affected by solar resource availability and system efficiency.

Typical capacity factors for solar PV systems range from 15-30%, depending on the location and system design. Areas with more annual sunlight and consistently sunny weather tend to have higher capacity factors.

Degradation

Photovoltaic systems experience gradual degradation over time, causing a decrease in power generation capacity. Typical degradation rates are around 0.5-1% annually.

Some key factors that affect degradation include:

UV exposure – solar panel materials degrade over time when exposed to UV rays.

Thermal cycling – fluctuations in temperature cause expansion and contraction that stresses materials.
Moisture ingress – water leakage into the panel can corrode connections and components.
Soiling – accumulated dirt on panels reduces light absorption.

Regular maintenance and using high-quality solar components can minimize degradation rates and extend system lifetime.

Future of Solar PV

The future looks bright for solar photovoltaic technology. Efforts are underway to continue improving the efficiency and lowering the costs of solar PV systems.

Some areas of research and development focused on improving efficiency include using new materials, multi-junction cells, concentrating PV, and thin-film technologies. These can potentially push solar cell efficiency from 20-25% up to 30-40%.

Emerging technologies like perovskite solar cells, organic PV, and quantum dot PV offer promising new options as well. These innovations could lead to flexible, lightweight, high-efficiency solar panels.

With the cost of solar already dropping dramatically in the past decade, continued technological improvements and economies of scale promise to make PV even more affordable and accessible.

Solar PV is one of the fastest growing renewable energy sources worldwide. Total global capacity is projected to reach over 2700 gigawatts by 2030, a threefold increase from 2020. As solar electricity reaches grid parity in more locations, this growth trend is likely to continue.

With solar PV becoming cheaper and more efficient than ever, its future role in electricity generation looks increasingly significant. In the coming decades, solar PV promises to play a major part in the transition to sustainable energy.