Solar cells are a valuable technology because they can convert free sunlight into electricity, one of the most useful forms of energy used by modern society. However, photovoltaic cells have a key limitation that must be considered: by themselves they can only deliver direct current, and most electrical equipment used in homes and businesses is designed for alternating current. In a commercial solar power system, the conversion from DC to AC power is carried out with one or more inverters, which means they are fundamental pieces of equipment.
Although all inverters have the same basic purpose, there are several possible layouts for commercial solar power systems. This article will provide an overview of the three main inverter configurations used, describing their advantages and disadvantages:
No inverter type can be considered superior to the rest under all circumstances. Like in any technical decision, there are trade-offs when comparing different technologies. In general, string inverters are the most economic option, while microinverters and power optimizers increase performance for an additional cost.
String inverters are the most common type. They get their name because individual solar panels are wired in series circuits, or strings, which are then connected to the inverter. Small residential PV systems typically have a single string of panels, while larger commercial solar systems use multiple strings. Solar panels connected in a string circuit carry the same current, while their voltages add up.
For a given generation capacity, solar power systems with string inverters tend to have the lowest cost, due to the simple connection and reduced number of electronic components. These photovoltaic arrays are also the less demanding in terms of maintenance, since each solar module only has a junction box and a series connection.
The main limitation of string inverters is that they make solar panels in the same circuit dependent on each other. In other words, if one solar panel is covered by shadows or malfunctioning, the entire string will suffer from reduced output. This is not an issue for rooftops without shading issues and where all panels have the same orientation, but the effect becomes noticeable as shading or rooftop orientation become more variable.
String inverters normally include monitoring capabilities, but these are limited to the level of string circuits. Inverters with this capability can detect the affected string when there is a fault, but the specific panel can only be identified through inspection.
As implied by their name, microinverters are smaller versions of string inverters, and they are designed for direct installation on individual solar modules. The type of connection also differs: while string inverters use a series connection, microinverters use a parallel connection like the electrical appliances in your home. In simple terms, a string circuit has the same current for all solar modules and adds their voltages, while a microinverter circuit has the same AC voltage for all modules and adds their current output.
The parallel connection of commercial solar arrays with microinverters provides two significant advantages:
If the microinverters have built-in monitoring, any fault can be found with ease, with no need to check all the solar panels in a circuit. When an individual solar panel is malfunctioning, its power output is affected and the respective microinverter detects it immediately.
Since microinverters manage the power output of each solar panel individually, the overall energy output is often increased, compared with a system of the same capacity using string inverters. However, the addition of a small-scale inverter for every panel comes with a higher price.
Commercial solar arrays with microinverters are also more demanding in terms of maintenance, since each photovoltaic module is equipped with a sensitive electronic component not found with a string inverter configuration.
Commercial solar arrays with power optimisers are hybrid between the string inverter configuration and the microinverter configuration. Photovoltaic modules are connected in series circuits with a main inverter, but each module is equipped with small power conditioning unit. These power optimisers manage the current and voltage output of each panel, so that the string circuit is not affected if one panel is malfunctioning.
Just like microinverters, power optimisers can be designed with monitoring capabilities, which simplifies troubleshooting. A malfunctioning solar panel stands out immediately, and maintenance personnel does not waste time checking an entire string of panels.
This system configuration uses main inverters, but they are specially designed to operate along with power optimisers – conventional string inverters are not compatible. Although this system configuration is more expensive than using conventional string inverters, the cost tends to be lower than that of a microinverter system.
Microinverters and power optimisers can increase the output of a commercial solar array, but a technical and financial assessment is very important to determine if these enhancements make sense for your property. If you have a uniform rooftop without shadows you can use conventional string inverters, since all solar panels are subject to roughly the same operating conditions.
Together, microinverters and power optimizers are referred to as module-level power electronics. String inverters are simpler and have been in the market longer, which gives them a cost advantage. However, there are cases where microinverters or power optimizers can win the cost-benefit analysis.
Module-level power electronics can be a viable solution when you want to maximize the output of a commercial solar array with limited area. If there is plenty of area and you want more electricity production, using a larger photovoltaic array can be a more cost-effective solution.