GRID INTERACTIVE BATTERY INVERTERS

When it comes to Solar Engineering, it is either gird connected or outside the gird that primarily comes to our mind but there are certain cases of hybrid inverters that work on battery as well as the grid and direct solar, probability being the power to remain for the maximum hours. Well the thinkers do not stop there to bring the concept of Grid interactive battery inverters now which is still not much popular but would eventually become the most effective. 
A large portion of the energy generated by a typical PV system will be exported to the grid and there is increasing demand for solutions to enable better use of this energy through the use of energy storage.

• Adding battery storage enables this excess generation to be stored for use later when consumption is higher than generation. 

• Some systems can also provide backup power in the event of grid failure.

• These inverters use a battery bank for energy storage and will not operate without batteries. They are used in addition to grid connect solar inverters.

Grid-interactive battery inverters, can export power to the utility grid, can charge a battery using surplus energy for use in times of low generation and some can also supply backup power to protected loads during a grid outage.Depending upon inverter type and which loads need to be powered an automatic transfer switch that enables them to safely operate off-grid during a blackout may also be required.For back-up applications the inverter is connected to the battery bank (usually 24 or 48 VDC), an AC distribution board for loads needing back-up, and the building supply, using an automatic transfer switch if required.
The battery bank is charged by the PV array connected through a charge controller or through the battery inverter via AC coupling and an energy meter is used to measure import/export.
If more power is being generated than being used then instead of it being exported to the grid it is instead sent to charge the battery. As loads increase above generation and power starts to be imported it is instead taken from that stored in the battery.
During a power cut, the switch box will automatically disconnect from the grid and supply AC power to the protected loads by drawing energy from the battery bank and solar array. When the grid is restored, the inverter will automatically switch back to grid-connect operation and recharge the batteries.

Some energy is used to maintain the battery bank. Because of this systems with battery backup produce less energy (kWh) per kW of PV array than equivalent grid-tie systems that don’t include batteries.

Adding Storage to Grid Connected PV Systems

A large portion of the energy generated by a typical PV system will be exported to the grid and there is increasing demand for solutions to enable better use of this energy through the use of energy storage.

Adding battery storage enables this excess generation to be stored for use later when consumption is higher than generation. As electricity prices rise this sort of battery storage system will become more and more cost effective. Some systems can also provide backup power in the event of grid failure.

Types of Energy Storage Systems

These can be broadly described as one of three types with the following features:

a) DC Coupled Energy Storage

• Here the battery is connected on the PV side of the solar inverter
• A high voltage battery (to match PV voltage) is used to store the energy when it’s generated for when it’s needed
• There is some (slight) reduction in the PV feed in tariff due to energy used in battery charge/discharge process
• No requirement to inform the DNO about changes to the system connected to the grid
• Not possible to charge the battery from cheap rate electricity
• Usually no operation in the event of a power cut

b) AC Coupled Energy Storage

• Here the battery is connected on the AC side of the solar inverter
• An additional battery inverter/charger is required to convert battery power to 230 Vac and vice versa
• There is no effect on the PV feed in tariff
• DNO approval needs to be obtained
• Possible to charge the battery from cheap rate electricity
• Back-up may be possible with additional switchgear

c) AC Coupled Energy Storage with Back-up

• Allows operation in event of a power cut
• Extra protection relays are required to enable AC loads to be run either from grid or from the battery and to provide a new neutral earth connection
• These can be included in inverter/charger
• Requires DNO approval

Not all installers are familiar with the issues and benefits of energy storage. We run regular training courses for installers on this topic and can provide recommendations:

 Find an installer

Why implement grid backup?

• Get security for the future - from power cuts and rising energy bills
• Have the ability to use most or all of your generation - your energy costs less than importing grid energy
• Bask in the knowledge that you are your own power station

The system consists of:

• A solar inverter - to connect the solar photovoltaic (PV) panels.
• An inverter/charger - to convert battery power to 230V AC (grid power) and vice versa.
• Batteries - to store the energy when it’s generated, for use when it’s needed. Lead acid or lithium ion.
• Controls for monitoring and control of the system - to make a more efficient system and give you information on the distribution of energy.
• Cabling and switchgear - for efficient and safe operation

Typically a battery store of approximately 5kWh (~200 Ah 48VDC) usable storage might be required. Sealed lead acid batteries are a good choice for low maintenance and safe installation in a house. Lithium Ion batteries offer optimum performance and space savings.

Source: http://www.windandsun.co.uk/information/types-of-system/grid-connect-system-with-battery-storage.aspx