They are not the sexiest part of the solar power system (definitely not as sexy as the panels) but happen to be the most vital and are the brain of the system. Also known as converters or PV inverters, they convert direct solar current (DC) into alternating current that can be fed to the grid or used by an off-grid electrical network.
It is also a safety mechanism that shuts off the system when the grid goes down. Beyond that, by converting the solar energy, it allows solar power to work just like any other energy source.
There are various types of solar power inverters.
1. String Inverters
This is the best alternative for small installations. It’s typical for residential and commercial applications for power less than 1MW.
String inverters are inverters used to convert energy when the panels are installed in a row each of them being on a string. Example, if you have 30 panels, arranged six rows of 5 panels and each row has one string. In this case, energy from all the panels on one string flow through the single string into the inverter then to the power grid.
They are best for use in places where they are no shading issues. Shade will lead to a decline in performance of the entire system. If one panel is shaded for only a portion of the day, reducing its performance, the performance of other panels is automatically reduced to the level of the struggling panel. For the effectiveness of this inverter, the panels should be on a single plane.
- Has lower balance of systems costs
- Its maintenance costs are low, and
- It is simpler in design and modularity. The modularity is better for systems with different array angles.
- It requires more inverter connections
- Needs more space to install the inverters and
- Newer and are less field tested
Just like power optimisers, they are module level electronics. However, they convert energy from DC to AC right at the panel, and thus string inverters are not required.
This panel level conversion of power has its perks. One being the fact that the performance of the system will not be affected if one of the panels is shaded making it perfect for areas with shading issues. These micro-inverters are also reliable where an installation has multiple planes facing different directions.
They also monitor the performance of each panel, are more efficient but expensive relative to string inverters.
3. Central Inverters
Similar to string inverters just a bit larger and with the capacity to support more strings.
The difference between Central and a string inverter is that the multiple strings from the solar panels have to be connected by running them through a combiner box. The combiner then runs the DC to the inverter where it is converted into AC and finally moved to the power grid.
- The DC watt unit cost is much lower relative to string inverters
- Has a proven field reliability
- Optimal for large systems
- Has a large inverter pad footprints
- Its installation costs are quite high
- Higher DC wiring and combiner costs
4. Battery-based inverters/chargers
These are tricky to understand. They are bidirectional in nature and are inverters that manage energy between the array and grid while keeping the batteries charged. It can also be defined as an inverter that converts DC from batteries into AC.
The battery-based inverter is becoming more important with the growth of solar and storage. It is quite beneficial since it continuously provides and ensures operation of critical condition irrespective of the grid condition e.g. in the event of an outage.
They are of two types (grid interactive and off grid) and require a battery to operate.
- Grid-interactive inverters
Also known as hybrid inverters, they perform a dual function. They export excess power into the grid while at the same time providing backup power to protected loads during outages.
They use a battery bank to store excess energy. In normal conditions, excess energy is exported, but during a blackout, it uses an automatic transfer enabling them to operate off grid safely. With this, it disconnects from the grid and supplies AC to protected load from the battery. However, it provides about 5%to 10% less energy than other systems that do not rely on batteries. This is because energy is required to maintain the batteries.
- Off-grid inverters
These inverters convert DC from batteries into AC as per the requirements of the load. Solar panels charge the batteries through a charge controller and only power required is converted.
As per eurosolar guidelines, the inverter and battery should be properly sized because the excess energy cannot be exported to the grid.
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