The inverter can be thought of as the “brain” of a solar PV system. This is because the inverter is the one that manages how it operates along with many other functions and protection features. In terms of a desktop computer, you may think of the inverter as the CPU or the central processing unit of the solar PV system. In this article, we will go through the basic functions of an inverter, and the different types of inverter used for solar PV applications. We will also go in detail about each of the inverter specifications and functions and compare how each type of inverter differs from each other based on these.
An inverter is a power electronic device that is not exclusively used for solar PV applications. Its most basic function is to convert DC (direct current) to AC (alternating current). The difference between the two and their specific applications are detailed below:
This is described by a flow of electric current in one direction only. Sources of DC electricity can be distinguished through the presence of positive and negative polarities. The electric current from such sources flows from the positive terminal to the negative terminal. PV modules or solar panels and batteries produce this kind of electric current.
In this type, the direction of the electric current reverses its direction periodically in a sine wave manner. Conventional sources of electricity, like coal, natural gas and nuclear energy all produce AC through the use of generators. This is why this is the type of electricity that we use in our homes.
We use AC electricity in our homes because it has features that are useful in terms of electric transmission and distribution. One of these is the ability to increase or decrease its voltage to the required level. This is especially important in the transmission of electricity through huge distances because using a higher voltage level will result to lower energy losses. This fact, along with all conventional energy sources producing AC, is why our whole electric grid uses AC instead of DC.
However, with the advent of renewable energy technologies such as solar PV that produce DC, a device that can convert DC to AC is required to be able to integrate these energy sources into our electric grid. This is why these technologies require having inverters to be useful to us.
There are 3 types of inverters today that are used today: central, string and micro inverters. All of these perform basically the same functions, the only difference being the scope of their applications.
This type of inverter is the largest in terms of capacity and is the one that is most commonly used for utility-scale systems such as solar farms. Their sizes can range from 100kW to a few megawatts. These inverters are usually designed to connect directly to the electric grid, which is why they usually come in a package that includes a power station. They also have the advantage of being cheaper in terms of per kilowatt cost and easier installation and management.
This type of inverter is the one that is usually used for residential and commercial systems, with sizes that range from 1kW to 100kW. They also come in 3 different types: on-grid, off-grid and hybrid, the use of each depending on the project's specific requirements.
This type of inverters is the youngest among the 3, but is now slowly gaining popularity and market share. As their name suggests, micro inverters are the smallest out of all the 3 types, which limits their application to residential solar PV systems. However, as micro inverter technology improves over time, it is also slowly being used for commercial and even utility-scale applications. Their sizes usually range from 250W to 1kW, which accommodates 1-4 PV modules
Many people nowadays confuse micro inverters with DC-DC converters and use the two terms interchangeably. However, DC-DC converters are not inverters and only converts the DC voltage and current that is produced by PV modules to levels that allow for maximum energy production. Because of this, DC-DC converters still require the use of inverters for proper functioning.