Photovoltaic Systems

When combining an inverter with mechanical and electrical hardware and a solar panel, we use the sun’s energy to create electricity. This is called a photovoltaic system (PV).

These systems range from off-grid to portable or rooftop units to massive industrial-scale units.

Here we will introduce the utility or grid-tied versions.

How do they work?

Photons comprise of particles of solar energy. When they collide with a solar panel, they create a current; this process is called the PV effect.

Individually, a panel will produce some energy, but when multiple panels are linked, they produce enough to form a solar array.

This solar array is (DC) direct current. You’ll find that many devices use DC, but they also require (AC) alternating current, which is present in standard utility grid electricity.

We need to convert the DC to AC by passing it through an inverter.

Once converted, the AC power can charge and power your household devices and be stored or forwarded to the national grid.

Important Components

Here we will look at the BOS, the balance of the system.

The PV system has multiple components such as combiners, circuit breakers, inverters, electric meters, disconnects, wiring and racking. You’ll find that these account for in excess of 50% of the cost and the majority of the future maintenance.

Solar Panel

Within a solar panel, there are numerous cells with semiconductor properties encased in a protective coating. This is where the light is captured, specifically the sun’s protons, which are converted into electricity via the PV effect.

The semiconductor is surrounded by conducting material which gathers the produced electricity.

On either side of the semiconductor is a layer of conducting material which “collects” the electricity produced.  

The illuminated side of the panel also contains an anti-reflection coating to minimise the losses due to reflection. The majority of solar panels produced worldwide are made from crystalline silicon, which has a theoretical efficiency limit of 33% for converting the sun’s energy into electricity. Many other semiconductor materials and solar cell technologies that operate at higher efficiencies have been developed, but these come with a higher cost to manufacture.

Inverters

An inverter is an electrical device that accepts electrical current in direct current (DC) and converts it to alternating current (AC). For solar energy systems, the DC current from the solar array is fed through an inverter which converts it to AC. This conversion is necessary to operate most electric devices or interface with the electrical grid. Inverters are essential for almost all solar energy systems and are typically the most expensive component after the solar panels themselves.

Most inverters have conversion efficiencies of 90% or higher and contain important safety features, including ground fault circuit interruption and anti-islanding. These shut down the PV system when there is a loss of grid power.

Racking

Racking refers to the mounting apparatus which fixes the solar array to the ground or rooftop. Typically constructed from steel or aluminium, these apparatuses mechanically fix the solar panels in place with a high level of precision. Racking systems should be designed to withstand extreme weather events such as hurricane or tornado-level wind speeds and/or high accumulations of snow. Another critical feature of racking systems is electrically bonding and grounding the solar array to prevent electrocution. Rooftop racking systems typically come in two variations, including flat roof systems and pitched roof systems. For flat rooftops, it is typical for the racking system to include weighted ballast to hold the array to the roof using gravity. The racking system must be mechanically anchored to the roof structure on pitched rooftops. Ground-mounted PV systems can also use either ballast or mechanical anchors to fix the array to the ground. Some ground-mounted racking systems also incorporate tracking systems which use motors and sensors to track the sun through the sky, increasing the amount of energy generated at a higher equipment and maintenance cost.

Other Components

The remaining components of a typical solar PV system include combiners, disconnects, breakers, meters and wiring. As the name suggests, a solar combiner combines two or more electrical cables into one larger one. Combiners typically include fuses for protection and are used on all medium to large and utility-scale solar arrays. Disconnects are electrical gates or switches that allow for manual electrical wire disconnection. Typically used on either side of an inverter, namely the “DC disconnect” and “AC disconnect”, these devices provide electrical isolation when an inverter needs to be installed or replaced. Circuit breakers or breakers protect electrical systems from currents or surges. Designed to trigger automatically when the current reaches a predetermined amount, breakers can be operated manually, acting as an additional disconnect. An electric meter measures the amount of energy that passes through it and is commonly used by electric utility companies to measure and charge customers. For solar PV systems, a special bi-directional electric meter is used to measure both the incoming energy from the utility and the outgoing energy from the solar PV system. Finally, the wiring or cables transport the electrical energy from and between each component and must be adequately sized to carry the current. Wiring exposed to sunlight must have protection against UV exposure, and wires carrying DC sometimes require metal sheathing for added protection.