How Is Generation Of Electricity Using Solar Photovoltaic Cells?

Photovoltaic (PV) cells, also known as solar cells, are devices that convert sunlight directly into electricity. PV cells are made of semiconductor materials that exhibit the photovoltaic effect – when sunlight hits the cell, the absorbed photons energize electrons inside the semiconductor material to create an electric current.

PV cells are typically interconnected and sealed together as modules that can range from a couple inches across to several feet, composed of 6×10 to 60×100 individual cells. Modules can then be combined into PV arrays mounted on rooftops, parking lots, or open land. The direct current (DC) electricity generated is fed into inverters to convert it into alternating current (AC) used by appliances, or fed into batteries to store.

Today PV systems are commonly used for decentralized electricity generation applications such as residential rooftop solar, commercial buildings, remote locations off the electric grid, satellites and spacecraft, roadside emergency telephones, parking meters, and more. PV offers a clean, renewable energy source with little maintenance and no emissions or waste when generating electricity.

History of Photovoltaic Cells

The photovoltaic (PV) effect was first observed in 1839 by French physicist Edmond Becquerel. However, it was not until 1954 that modern silicon PV cells were first developed. The inventors were Daryl Chapin, Calvin Fuller and Gerald Pearson at Bell Labs in the United States. Their 6% efficient solar cell was mainly intended for energy production for telephone systems.

In 1958, PV cells began to be used in the American Vanguard I satellite. Through the 1960s and 70s, PV technology continued to advance with increased funding from space programs. Manufacturing costs declined and efficiencies improved up to 14%. In the 1980s and 90s, PV adoption grew for off-grid terrestrial applications such as roadside emergency telephones, remote buildings and railroad crossings.

The early 2000s saw PV cells become cost-effective for grid-connected applications in some markets due to feed-in tariffs. Since then, global PV capacity has expanded exponentially, with key developments in manufacturing, efficiency improvements and large-scale solar farms.

How PV Cells Work

Photovoltaic (PV) cells, also known as solar cells, are made of semiconductor materials that can convert sunlight directly into electricity. When sunlight hits the PV cell, photons from the sunlight knock electrons loose from the atoms of the PV material, creating electron-hole pairs. An electric field inside the PV cell forces the electrons to flow in one direction, producing an electric current. This process is known as the photovoltaic effect.

The basic building block of a PV cell is a p-n junction. The p-type semiconductor has extra positively charged holes, while the n-type semiconductor has extra free electrons. When p-type and n-type semiconductors are placed next to each other, electrons diffuse across the junction from the n-type side to the p-type side. This leaves positively charged ions on the n-type side and negatively charged ions on the p-type side, creating an electric field across the junction.

When sunlight strikes the PV cell, photons transfer their energy to the charge carriers. Electrons on the n-type side absorb this energy and are ejected from their positions associated with parent atoms into the conduction band. These free electrons then flow through an external circuit and do electrical work. The electron-hole pairs are generated as long as light continues to shine on the solar cell.

PV Cell Materials

There are two main categories of solar cell materials used in photovoltaics: wafer-based crystalline silicon and thin films.

Silicon is by far the most common material for solar cells and consists of single crystalline silicon (Mono-Si), polycrystalline silicon (multi-Si), and amorphous silicon (A-Si). Crystalline silicon cells are more efficient but more expensive than thin film materials. Mono-Si PV cells are made from single crystalline silicon and have module efficiencies around 15-20%. Multi-Si PV cells are made from blocks of molten silicon cast to form polycrystalline structures and have module efficiencies of 13-16%.

Thin film solar cells use cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous silicon (A-Si), micromorphous silicon, and organic PV and dye-sensitized solar cells (DSSC). Thin film cells are cheaper to manufacture but less efficient, with module efficiencies of 7-13%. However, thin film technology is improving rapidly.

Emerging materials like perovskites, gallium arsenide (GaAs), quantum dots, and organic PV show promise for even higher efficiencies and lower costs, but still have challenges to overcome before wide-scale commercialization.

PV Cell Efficiency

One of the key factors for effective solar energy generation is the efficiency of the solar cells used in the photovoltaic modules. PV cell efficiency refers to how much of the sun’s energy striking the cell is converted into usable electricity. There are two main measures of efficiency to consider:

Record Lab Efficiencies

Research labs are constantly working to improve solar cell designs and materials to increase efficiency. The highest lab efficiency recorded so far is 47.1% for multi-junction concentrated PV cells. However, these cutting-edge lab cells are complex and expensive to manufacture.

Commercial Module Efficiencies

The cells used in commercial PV modules tend to have more modest efficiency ratings, typically between 15-22%. This is because they use less expensive materials that can be mass-produced. The most common technology is monocrystalline silicon with about 19% efficiency. Other commercial technologies like polycrystalline silicon (16-17%) and thin film (7-13%) offer lower performance but also lower costs.

Factors Affecting Efficiency

The operating temperature of the cells is a key factor for efficiency. Performance drops as temperature increases. Other factors like shading, dust buildup, degradation over time, and sunlight intensity also impact efficiency. Maximizing efficiency is essential to generate the highest solar energy output from a given system size.

PV System Components

Photovoltaic systems are made up of various components that work together to convert sunlight into electricity. The main components are:

Modules

The PV modules, also called solar panels, contain the solar cells that convert sunlight into DC electricity. Modules are made up of many individual solar cells wired together. The modules come in different technologies, sizes, shapes and power outputs.

Inverters

Inverters convert the DC electricity from the solar modules into AC electricity that can power lights, appliances, and electronics in buildings. Inverters range in size from microinverters to central inverters depending on the system design.

Batteries

Batteries store excess solar energy for use when the sun isn’t shining. They allow solar PV systems to provide backup power and maximize self-consumption. Batteries are more common in off-grid systems but can also be added to grid-tied systems.

Wiring and Mounts

Wiring connects all of the PV system components together into a working system. The modules are mounted onto racks or frames using mounting hardware specifically designed for solar panels. Proper wiring and mounting are critical for safety and performance.

Applications of Solar Photovoltaic Systems

Solar photovoltaic (PV) systems have three main applications: residential, commercial, and utility-scale.

Residential Solar PV Systems

Residential solar PV systems are installed on homes to offset electricity usage. Typical system sizes range from 3-10 kilowatts. Residential systems may be roof-mounted or ground-mounted. Any excess electricity produced can be fed back into the grid through net metering programs. Residential PV systems allow homeowners to reduce their electricity bills and their home’s carbon footprint.

Commercial Solar PV Systems

Commercial solar PV systems are installed on business properties like retail stores, factories, and office buildings. System sizes range from 10 kilowatts to megawatts. Commercial systems may be installed as rooftop arrays or ground-mounted solar farms. Businesses can use solar PV to reduce overhead costs and meet sustainability goals. Many companies install solar to power their operations during daytime hours.

Utility-Scale Solar PV Systems

Utility-scale solar refers to ground-mounted PV systems that feed into the electric grid. These large-scale solar farms have system capacities in the megawatt to gigawatt range and are owned by power companies, Independent Power Producers, or other large entities. Utility-scale solar provides renewable electricity to the grid and allows utilities to meet renewable portfolio standards and emission reduction goals.

Costs

The costs of solar PV systems have declined significantly in the past decade, making solar more affordable and accessible. The main driver of this cost reduction is the falling price of PV modules. Since 2010, module prices have declined by around 90%, from $1.60/W to $0.20/W in 2020. This drastic price drop is due to economies of scale in manufacturing as well as technology improvements yielding higher efficiency modules.

In addition to module costs, there are balance of system (BOS) costs which include inverters, racking, wiring, labor, permitting, etc. BOS costs now make up over half of the total installed cost of residential systems. BOS costs have also declined but not as rapidly as module prices.

The levelized cost of energy (LCOE) is a metric used to compare different electricity generation technologies over their lifetime. Utility-scale solar PV LCOE has dropped from around $0.20/kWh in 2010 to $0.03-0.05/kWh currently, making it cost competitive with fossil fuels. Residential solar LCOE is higher at around $0.14-0.18/kWh but can still provide savings versus retail electricity prices in many markets.

Environmental Impact

Solar PV has significant environmental benefits compared to fossil fuel electricity generation. PV systems do not produce air pollution, greenhouse gases, or radioactive waste during operation. The environmental impacts of solar PV mainly come from manufacturing the panels.

Producing solar panels does generate some carbon emissions and toxic chemicals. However, the carbon emissions over the lifetime of a solar PV system are estimated to be just 5-30 grams per kilowatt-hour. This compares very favorably to around 1,000 gCO2/kWh for coal power plants.

Solar PV needs a significant amount of land area for large-scale installations. However, the panels can be installed on land not usable for other purposes like rooftops. And the area underneath panels can still be used for grazing or growing crops.

There are some concerns around properly recycling solar panels at the end of their roughly 30 year lifespan. Most panels contain toxic elements like lead and cadmium. But new recycling processes are being developed and regulations put in place to ensure responsible recycling.

Future Outlook

The future looks bright for solar PV, with analysts projecting strong continued growth in the coming decades. This is driven by several key factors:

Growth Projections: According to the International Energy Agency (IEA), solar PV capacity could grow from about 600 GW in 2019 to over 4,500 GW by 2040 in their optimistic Sustainable Development Scenario. This represents an average annual growth rate of 15%. Other analysts predict over 8,000 GW of solar PV globally by 2050. This growth will be fueled by declining costs, supportive government policies, and increased adoption by utilities, businesses, and homeowners.

New Technologies: Advances in solar cell technology will enable increased efficiency and lower costs. Emerging technologies like perovskite solar cells, organic PV, and tandem/multi-junction cells could achieve efficiencies above 30% compared to around 20% for standard silicon cells today. Novel manufacturing techniques like inkjet and 3D printing have the potential to reduce processing costs. New solar module designs and trackers will increase energy yield.

Challenges: While the future is overall positive, some challenges remain. The intermittency of solar power creates grid integration issues once it reaches high levels of penetration. Storage solutions like batteries can help smooth out supply. The solar industry must continue to drive down costs and improve efficiency to stay competitive with conventional power sources. Environmental impacts from manufacturing should be mitigated. Policy and regulatory uncertainty remains a challenge in some markets. But the solar industry has proven adept at overcoming challenges in the past.