As of 2018, solar energy capacity has reached an overall capacity of approximately 60 GW in the United States alone (SEIA). As solar installations increase across the country, their signature photovoltaic modules have become easily recognizable. However, there are other key components of a solar system that are necessary to convert the sun’s light into usable electricity. One of these parts is an inverter. Let’s get a basic overview of what an inverter is and how it functions.
In basic terms, electricity is a flow of electrons. The process of generating solar electricity starts when photons of sunlight hit the photovoltaic (PV) cell, freeing the electrons. The typical commercial solar cell contains two sides: N- and P-type silicon. N-type silicon is negatively charged, while P-type silicon is positively charged. Connecting the two sides is a P-N junction, which controls the flow of electrons in a single direction and produces direct current (DC). Direct current flows at a relatively steady voltage. Each solar module is made of a series of cells (usually 60 or 72) and has a negative and a positive connector. Modules get connected in series (usually between 16 and 30) in what is called a string of modules.
An inverter plays a critical role in a photovoltaic (PV) system and solar energy generation, converting the DC output of a string of PV modules panel into AC power.
There are several reasons why AC power is preferred over DC power. An important advantage of AC is that it can be stepped up in voltage via transformer more easily than DC and is more cost-effective to send over long distances. Most electricity grids operate on AC, and many pieces of industrial equipment with high power requirements have been designed to use AC power. Since solar panels produce DC, it is often necessary to convert to AC.
There are a few different options available when it comes to selecting inverters for a PV system: string inverters, central inverters and microinverters. Battery systems use a different kind of inverter.
Before diving into the specifics of each inverter, it is important to note the concept of shading. Shading occurs when a panel or part of a panel receives less sunlight than the surrounding panels in the array. It is critical to take shading into account during the design stage, because the shading on one module in a string will affect the performance of the other modules in the same string. The reason is that if one panel’s output is less than the others, the output of the others in the string will be reduced to that of the lowest-producing panel. In the following sections, we will explain how the various types of inverters can play an important role in helping mitigate the effect of shading on the system’s energy output.
Central inverters have been around for a long time. They are typically mounted on a pad at the ground level. They usually come with several optimization modules (called MPPTs, Maximum Power Point Tracking systems) that ensure that groups of strings run at the maximum power based on irradiance. Since they optimize production for a large number of modules at a time, they are best used in larger installations where there is consistent production across the array. These inverters are popular for utility-scale systems, however, the current models do not always comply with the latest safety requirements for rooftop installations (rapid shutdown for emergency responders, arc fault detection). They usually require the use of combiner boxes near the modules.
One of the disadvantages of central inverters is that they are a central point of failure – if a component inside the inverter needs to be replaced, the whole system will be down until that happens. They are also less standardized and can have longer lead times to replace.
Commercial string inverters became common about ten years ago when safety requirements for rooftop installations became more stringent, and inverter manufacturers decided to adapt residential inverters to be used on commercial three-phase installations.
They are ideal for locations with little shading issues and where large groups of panels are all facing the same direction. For example, a warehouse with two roof orientations would have separate string inverters for each roof orientation.
String inverters have several major advantages: cost, ease of installation and maintenance, code compliance, etc. This inverter type is generally less expensive than microinverters, which are installed at each panel. They also generally comply with the latest safety requirements in the electrical code (up to the 2014 version). They do not require any space at the ground level as they can be mounted on racking or on walls and they can easily be serviced or swapped without taking the whole system offline.
Power optimizers are installed on each solar panel (sometimes each pair of solar panels), creating what is referred to as a “smart module.” They help mitigate the negative effects of shading and variable output and work well when combined with string or central inverters. Power optimizers enable module-level maximum power point tracking (MPPT), which increases efficiency by controlling the voltage output per panel.
In states following the 2017 version of the National Electrical Code, power optimizers are a cost-effective way to comply with the rapid shutdown requirements provided they have that capability.
Installed on each solar panel, microinverters convert DC power to AC power at the panel, circumventing the need for a string or central inverter altogether. Microinverters also help mitigate the negative impact of shading and prevent having a single point of failure within the system. These inverters are significantly more expensive in terms of hardware and labor and create more potential points of failure within the system. As such, they are rarely used in commercial systems.
Batteries output DC power, similar to solar panels, so they also require inverters. The difference with solar inverters is that battery inverters are usually connected to a site controller that decides when to charge or discharge the batteries. These inverters can operate in both directions, allowing AC power to convert to DC power to charge batteries.
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