The Basics of a Photovoltaic System

Stand-alone systems supply DC and sometimes AC electrical loads independently of the electric utility grid. Excess solar electricity is stored in batteries for use at night.

PV systems can also back feed electricity into the grid at times when load demand exceeds PV production. This can help reduce the amount of voltage drops and reverse power flows that occur on distribution feeders.

Materials

The basic building block of a PV system is a solar cell, which converts sunlight into electricity. It consists of a semiconductor material – usually silicon (monocrystalline, polycrystalline or amorphous), gallium arsenide or metal chalcogenides – and a glass cover.

When light hits the cell, it raises the energy levels of some of the electrons in the semiconductor material – knocking them Photovoltaic System from their lower valence band into the higher conduction band. The free electrons flow through conductive metal contacts on the back of the cell to create an electric current that can be used to power your home or the grid.

When several cells are connected together, they become a solar module. Solar modules come in a variety of sizes and shapes, from flexible thin-film types to large monocrystalline or polycrystalline silicon panels.

Cells

Solar cells are semiconductor wafers doped with different impurities to create a junction that can absorb sunlight in order to generate electricity. A photovoltaic cell converts solar energy directly into direct current electricity without generating harmful pollution.

A photovoltaic cell contains two layers of semiconductor material, one with a positive charge and the other negative. When the sun shines on a photovoltaic cell, it energizes electrons in the negative layer, freeing them to flow through an external circuit and back into the positively charged layer where they produce electric current. The power produced by a single PV cell is typically low, so multiple cells are connected in series to produce greater voltage and current. They are then packaged tightly into a weatherproof container known as a solar module.

The performance of a PV cell depends on the wavelengths and intensity of the sunlight it can absorb. It is also influenced by the cell’s efficiency, open-circuit voltage (VOC), VOC ratio and fill factor values. Solar cells are sensitive to shading, which reduces the cell’s electrical output. When a single cell or a string of cells in a solar panel is shaded, the entire string’s output drops dramatically due to internal short-circuiting (electrons reversing their direction of flow through the shaded cell).

Modules

The photovoltaic modules convert the Sun’s radiation into electricity. They are typically mounted on ground-mounted frames, or on a tracking system that moves to continually face the Sun throughout the day. The modules are connected to an inverter, which converts the DC power from the solar cells into AC energy. The inverter output is either used to supply on-site DC electrical loads, or fed into the electric grid as part of a grid-connected PV system.

Depending on their size and the type of PV cell, a solar module can produce different amounts of current. Larger cells or those exposed to more intense sunlight can generate more current. Multiple cells are wired in parallel to increase current, and in series to increase voltage.

Whether used to power homes or businesses, or to supplement an existing electricity system, a PV system reduces dependence on fossil fuels, helping mitigate climate change and reducing harmful air emissions such as carbon dioxide, nitrogen oxides, and sulfur oxides. It also helps avoid the costs associated with transmission lines, as well as “capacity” revenues paid to power plants when they are needed to supply electricity during peak demand periods.

Inverters

The direct current that each PV module produces is not enough to power appliances, so modules are connected together in series and then parallel to produce higher voltages and larger currents. This is why a PV system needs an inverter.

Inverters convert DC electricity into AC electricity. Different types of inverters are used for different applications. Selecting the incorrect inverter can decrease a PV system’s performance.

Solar PV inverters are often equipped with maximum power point tracking (MPPT) solar battery backup for home circuitry that optimizes energy production by adjusting the cells to their most efficient operating points. This feature is important in regions with varying temperatures and solar irradiance conditions.

In grid connected systems, the inverter must also provide regulated AC electricity that is synchronized with the grid frequency and limit feed in voltage to no higher than the utility grid supply. This is called anti-islanding functionality. Grid-connected PV inverters are also capable of providing DC power to critical loads during a blackout, as well as supplying the power it generates to the grid. This is known as intentional islanding.

Installation

The cost of PV systems has been dropping as the industry scales up manufacturing and incrementally improves materials. However, they are still more expensive than conventional fuel systems.

The power that a single solar cell produces is not enough to power most equipment, so a number of cells are grouped together to form an array. The array is then connected to an inverter that converts the DC current from the cells into alternating current that can power appliances and electric equipment.

The array is then connected to either the electricity grid or a battery system, depending on the energy needs of the site. Grid-connected systems can supply on-site electrical loads and back feed the electricity grid at night or during periods when the PV system output is greater than the local load demand. Battery systems store the energy for use later, at lower cost than a utility system. PV systems can be constructed in a variety of sizes based on energy requirements and can be installed on flat or pitched rooftops or in ground-mounted arrays, including solar trees that mimic the appearance of a tree canopy at night.