How it Works
The solar panels convert solar energy to Direct Current (DC) and the inverters(Power conditioning unit) convert Direct Current(DC) to Alternating Current (AC) to power up the building with or without the grid current.
Power Output of the Solar Rooftop
Power outputs for Solar rooftops are small compared to ground mounted Photovoltaic systems. Output for Residential purposes can be in the range of 10-20 Kilowatts and the power output for commercial purposes can reach 100 KW and more.
Different types of Solar PV systems
1) Direct or direct-coupled systems
These are the systems where the DC output of the solar pv is connected to the DC load. These are the simplest type of Solar PV systems – Only components are PV and a load. This can be used for fans, water pumping systems and other equipments that use Direct current.
2) Grid-tied or Utility-Interactive PV systems
These systems are directly connected to the grid and work in tandem with the grid. This systems work in 2 ways :
- In case of surplus power generated by the Solar PV system the excess power is supplied to the grid and
- In case of a higher demand for power/electricity surplus current is drawn from the grid.
They are easy to install and maintain since there are no batteries used in this case and the grid functions as a virtual battery. These systems use Grid-tied inverters for the 2 way electric flow.
These systems disconnect from the grid whenever there is no grid power to ensure safety (Also known as Anti-islanding)
3) Off-grid systems
These are the systems that work independently of the grid. This kind of a system is usually used for remote homes to power up an entire building for the required load. There are batteries used to store the electricity generated by the solar pv system. In case of extra power needed other backup devices like diesel generators are used.
4) Grid Interactive PV systems
These type of systems is a combination of both Grid-tied and Off-grid systems. In this case, the inverters will produce as much power as possible when operating in utility interactive mode. When the utility is interrupted, the inverters disconnect from the grid and switch to standalone mode and make as much power as the load needs.
There are 2 basic types of stand alone PV systems:
DC coupled systems
These are most common and simple off grid systems. The main components used are:
- Solar modules
- Charge controller
In DC-coupled systems, the Solar output is connected to the DC side of PV system. The harvested solar energy first flows to a battery bank via a charge controller and then to AC loads via a battery based inverter.
Generally, they are not able to store the grid power.
The flow is like this: PV Array -> Charge Controller -> Battery Bank <-> Battery-Based Inverter <-> AC Loads <-> Utility
Note that the battery-based inverter in this application is responsible for disconnecting the local electrical system from the utility.
AC Coupled systems
In AC-coupled systems, the Solar output is connected to the AC side of PV system. The current generated by the solar panels, first passes through the batteryless inverter which converts DC to AC. This is used by the AC loads through the AC load panel. Any extra current generated is stored into the battery bank using the battery inverters or supplied to grid in case of grid tied systems.
AC Coupled systems are complicated and are less common. They use 2 types of inverters used.
- Battery Inverters
- Grid Tied inverters
Generally a grid-tied inverter would not work in case there is no grid power, but with the battery inverter it synchronizes with it. Battery inverters create voltage so that grid tied inverters can work when there is no utility.
Main components used are
- Solar modules
- Grid tied inverters or batteryless inverter
- Off grid inverter or battery inverter
The existing grid tied system can be converted to this with a battery backup without much changes to the existing systems wiring. This system is a lot more efficient compared to the battery based system since most of the conversion happens through the grid tied inverter.