What Is The Process Of Photovoltaic?

Photovoltaics (PV) is the direct conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect. PV refers to a method of generating electrical power by converting solar radiation into direct current electricity. The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy, allowing them to act as charge carriers for an electric current.

The term photovoltaic denotes the unbiased operating mode of a photodiode in which current through the device is entirely due to the transduced light energy. The photovoltaic effect was first observed by French physicist Edmond Becquerel in 1839. The first solar cell was built in 1883 by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The development of photovoltaics gained momentum in 1954 when scientists at Bell Laboratories accidently discovered that silicon doped with certain impurities was very sensitive to light. This resulted in the production of the first practical photovoltaic cells with 6% efficiency.

Today, photovoltaic cells provide a clean and virtually inexhaustible source of energy from the sun. PV has emerged as a powerful technology with applications in rooftop power generation, building facades, electronic devices, remote locations, and most recently, in utility-scale power plants. With concerns over energy supplies and environmental issues related to fossil fuel use, photovoltaics is poised to make a meaningful contribution to the world’s growing energy needs.

How Photovoltaic Cells Work

Photovoltaic cells, also called solar cells, convert sunlight directly into electricity. They are made of semiconducting materials that exhibit the photovoltaic effect. When sunlight hits the cell, photons are absorbed by the semiconducting material which causes electrons to be excited into a higher state of energy. This creates electron-hole pairs as electrons move into the conduction band leaving holes in the valence band.

The semiconducting material is specially treated to form a p-n junction with a p-type side (positively charged) and an n-type side (negatively charged). The electric field at the p-n junction causes the excited electrons in the conduction band to flow towards the n-side, while the holes flow to the p-side. Metal contacts are placed on both sides of the solar cell to collect the electrons on the n-side and holes on the p-side, generating an electrical current that can be used as electricity.

Manufacturing Photovoltaic Cells

The manufacturing process for photovoltaic cells involves several key steps, including purification of silicon, doping, photolithography, and anti-reflective coating. Silicon is the primary material used in most solar cells, and it needs to be highly purified to increase efficiency. Raw silicon is refined through chemical processes like gasification and crystal growth to remove impurities. Once purified, the silicon is treated with specific chemical agents in a process called doping, which adjusts the electrical properties of the material to make it positively or negatively charged.

Photolithography is then used to define the electrical circuits on the surface of the silicon. This process uses photographic techniques to etch microscopic grooves onto the silicon wafer. These grooves guide the flow of electrons within the cell. An anti-reflective coating is applied to increase light absorption and reduce reflection off the surface. This coating maximizes the amount of sunlight that penetrates the cell and gets converted into electricity. Through careful control of materials, processes and precision manufacturing, high quality photovoltaic cells can be mass produced to meet the growing worldwide demand for solar technology.

Connecting Cells into Modules

After the photovoltaic cells are manufactured, they need to be interconnected and encapsulated into modules (also known as solar panels). This protects the cells and allows the PV system to generate the desired electrical output.

Photovoltaic cells are very thin, usually around 0.2-0.3 mm, and produce a relatively small amount of current and voltage. To increase power output, individual cells are connected together in series strings. When cells are wired in series, the voltage adds up but the current remains the same.

The series strings of cells are then laid out, interconnected, and laminated between sheets of ethylene vinyl acetate (EVA) plastic. EVA has high transparency and electrical resistivity, protecting the cells while allowing sunlight to pass through. A transparent glass cover is added to the front of the module, providing structural support and weather protection.

The back of the module is covered with a polymer and junction box. The junction box contains the terminal connections for wiring the modules together and outputting power from the PV system. Modules are designed to provide easy interconnection into arrays with customized voltage and power output.

Photovoltaic System Components

After the photovoltaic cells are manufactured and connected into modules, there are several other key components required in a complete photovoltaic system:

Modules

The photovoltaic modules, which contain the interconnected cells, are mounted and wired together to form solar arrays. The modules are designed to be weatherproof and durable to withstand outdoor exposure to sunlight, rain, snow, and wind.

Charge Controllers

Charge controllers regulate the voltage and current coming from the solar arrays to prevent overcharging the batteries. They extend battery life by avoiding overcharging.

Inverters

Inverters convert the DC output of the modules and batteries into standard AC power for usage in homes, buildings, and on the grid. They allow solar energy systems to provide usable power for most electrical loads.

Batteries

Batteries store energy for use when the solar arrays are not generating enough power, such as at night. Deep cycle lead-acid batteries are commonly used for solar storage.

Wiring

Proper wiring connects all of the photovoltaic system components together electrically. The wiring must be properly sized and rated for the voltages and currents involved.

Installing Photovoltaic Systems

Properly installing a photovoltaic system requires careful planning, preparation, and execution. The key steps include:

Site Survey

A site survey evaluates the location’s solar resource, orientation, shading, and structural capacity for supporting PV arrays. Installers examine the roof or ground area to determine the optimal layout for capturing sunlight.

Mounting Structure

Panels are mounted on racks, poles, or directly integrated into roofing materials. The mounting structure must withstand weather, winds, and seismic activity over decades. Proper orientation, tilt angle, and spacing maximize solar capture.

Wiring Modules

Modules are wired together into arrays, then connected to inverters and electrical panels. Proper wiring, overcurrent protection, and grounding ensure safe, reliable operation. DC optimizers or microinverters can be integrated to maximize energy harvest.

Grid Connection

For grid-connected systems, the inverter synchronizes PV output with the electricity grid. Permits, inspections, and utility approvals ensure local codes and safety standards are met prior to operation.

Efficiency and Performance

The efficiency of a photovoltaic system refers to how much of the sun’s energy striking the system is converted into electricity. There are several key metrics used to characterize the efficiency and performance of PV systems.

Efficiency Factors

The conversion efficiency of a PV cell is determined by how much of the solar energy spectrum it can absorb and convert to electricity. Typical crystalline silicon cells have an efficiency of 15-20%. More advanced multi-junction cells can achieve over 40% efficiency by using multiple stacked cells to absorb more of the solar spectrum.

Capacity Factor

The capacity factor compares a PV system’s actual production over a period of time to its potential output if it operated at full rated capacity. Capacity factors for PV systems typically range from 15-30%, depending on the location and PV technology used.

Performance Ratio

The performance ratio looks at the overall effect of losses on a system, from temperature effects to inverter inefficiency. A higher performance ratio indicates a better performing system overall. Typical values range from 0.5 to 0.9 for well-designed systems.

Degradation

Over time, the performance and efficiency of a PV system will degrade slightly each year. Typical degradation rates are around 0.5-1% annually. This is due to aging and weathering effects on the PV modules and system components.

Financial Analysis of Photovoltaic Systems

The financial viability of photovoltaic systems depends on several factors, including system costs, available incentives, electricity rates, solar resource, and financing options. Here is an overview of the financial considerations for residential and commercial photovoltaic systems:

System Costs

The upfront cost of a photovoltaic system includes the solar panels, inverter, racking, wiring, permits, and installation labor. For a typical residential system, total costs range from $15,000 to $25,000 before incentives. Commercial systems are typically larger and more expensive on a per watt basis, with costs ranging from $2 to $4 per watt of installed capacity.

Incentives

Many federal, state, and local incentives are available to reduce the upfront cost of a photovoltaic system. These include tax credits, cash rebates, solar renewable energy credits, and net metering programs. The federal investment tax credit (ITC) offers a 26% tax credit for systems installed in 2020-2022, which can reduce overall system costs significantly.

Payback Period

The payback period is the amount of time it takes for a photovoltaic system to pay for itself through electricity bill savings. Most residential systems have a payback period of 6-12 years. This depends on system costs, electricity rates, solar resource, and incentives. Faster payback periods can be achieved in areas with high electricity costs and strong financial incentives.

Levelized Cost

The levelized cost of electricity (LCOE) is a measure of the overall cost competitiveness of a photovoltaic system over its lifespan. The LCOE represents the per kilowatt-hour cost (in today’s money) of building and operating a system. The LCOE from solar photovoltaics has declined dramatically in the last decade and is now cost competitive with retail electricity prices in many regions.

Environmental Impact of Photovoltaics

The environmental impact of photovoltaic systems is overwhelmingly positive compared to fossil fuel energy sources. By generating electricity from sunlight without any emissions, photovoltaics can significantly reduce the carbon footprint of energy production.

The carbon footprint of photovoltaics comes mainly from the energy required to manufacture the panels and system components. But within just 1 to 4 years of operation, a PV system generates enough clean electricity to offset the carbon emissions from its production. Over the 25-30 year lifetime of a system, it will produce 50-80 times more energy than was used in its manufacturing.

In addition to eliminating carbon emissions from electricity generation, photovoltaics avoid air pollutants like sulfur oxides, nitrogen oxides, and particulates. Widespread adoption of solar power can greatly improve air quality and public health.

There are some concerns around the toxic materials like lead and cadmium used in manufacturing some solar panels. Proper end-of-life recycling and disposal is important. But strict regulations ensure the environmental risks during production and after decommissioning are minimized.

Overall, photovoltaics represent a sustainable energy source with enormous potential to reduce greenhouse gas emissions and address climate change when implemented at scale. The environmental benefits outweigh the limited impacts.

Future Outlook

The future looks bright for photovoltaics as researchers continue to find ways to improve efficiency and reduce costs. Some key areas where we expect to see innovations include:

Improving Efficiency

There are many ways to potentially increase the efficiency of photovoltaic cells. Scientists are exploring the use of new materials, nanotechnology, light trapping techniques, and tandem/multi-junction cells that can capture more of the light spectrum. Efficiencies in the lab have already exceeded 45%, and there is optimism that commercial cells could reach 30% in the coming decades.

New Materials

Silicon has long been the dominant material for photovoltaics, but researchers are now looking at other semiconductors like perovskites that could potentially be cheaper and easier to manufacture at scale. Flexible thin-film cells using cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) are also gaining traction. The diversity of materials provides more options for optimizing efficiency and cost.

Growth Projections

Photovoltaic technology has been on an exponential growth curve as costs have come down dramatically. Solar power is now price competitive with fossil fuels in many markets and adoption is accelerating globally. Projections estimate solar PV installments could grow from about 600 gigawatts today to over 25,000 gigawatts by 2050. Government incentives, improving economics, and concern about climate change are driving rapid expansion of photovoltaics worldwide.