How the Grid-Tied Photovoltaic System Works with Hybrid Inverter & Energy Storage.

Tempo di lettura ca.: 2 minuti, 56 secondi

In this article we will explain in a very simple way and a few steps how a photovoltaic system can be integrated to your home when your home is connected to the national grid.

The system is widely applicable to all grid-connected properties, warhouses or commercial buildings. Our turnkey solution is designed for grid-connected installation (single or three phase) with the ultimate goal to reduce the dependence on the supply of the grid and to maximise the self-consumption for the energy produced by the solar panels.

It is obligatory and all the DNOs require a certified product with export limitations, for the UK market, the inverter should G98/99 compliant.

The primary goal of a self-consumption system is to optimise the use of solar and/or wind power. The major obstacle in such a system is that power generation times do not match
the actual times of power use. This results in a system being forced to import energy from the grid and export it when there is a surplus. In an optimised self-consumption system, surplus energy is stored locally for local on-demand use. Such energy storage is becoming an increasingly attractive proposition, especially with feed-in tariffs decreasing and grid supplies becoming less stable and more expensive.

It is important to mention that the system is always connected to the grid but the grid supplies in parallel with the inverter/solar panels the energy demand of the household.

Characteristic of hybrid inverters for self-consumption

The inverter will be the main source of electricity for the household
The grid will supply any surplus energy if the consumption exceeds the power rating of the inverter
On cloudy days the grid will top up the batteries and supply energy to the house
The batteries are charged from solar first
A charge timer will be used to charge the batteries with the economy rate (low-cost electricity)
Inverter and grid run in parallel feeding power to the loads.
Power flow is bi-directional
Export to the grid can be controlled from 0Watt to maximum power.

Parallel operation with the grid

It is important to explain that a hybrid inverter will power the AC-loads but if the energy demand exceeds the capacity of the inverter or the batteries are not fully charged, the surplus energy will be withdrawn from the grid. In simple terms if the load is 5kW but the inverter can only supply 4kW then 1kW will be supplied by the grid.

This is a major difference between off-grid inverters and hybrid grid inverters, the off-grid system will go into bypass mode if the power demand exceeds the rating of the inverter and all the energy will come from the grid (read more about off-grid set up here)

How to connect the inverter to the consumer unit of the house

We collected some pictures from real installations, the energy from the photovoltaic system is converted to 230VAC single or three phases, and the output is connected directly to the switchgear of the property using a standard MCB 32A or an RCBO (with integrated RCD).

The picture with current sensor (clamps meter, C.T) and digital energy meter feeding data back to Solis 5kW 5G hybrid inverter

Similar setup with single phase consumer unit connected to the hybrid inverter.

Voltacon Hybrid 5.5kW inverter AC input is connected to the consumer unit.

Energy Cycle: Charging and Discharging Batteries with Solar and Grid Support

Let’s see now using our simple but very useful diagram of how the system behaves during in 24 hours. The graphs are easy to understand, we used the most informative and meaningful quantities to plot the results. One diagram is adequate to proof without any doubts that solar is the way to become power independent. The key points of our graph:

Late at night in 4 hours, the electric vehicle (in this case a 24kWh Nissan Leaf) tops up the batteries for 2 hours using a 13A domestic charger. The home batteries continue to be charged until 4:30. The chargins process started with 6kw from the grid and dropped to 2.4kW.
From 4:30 am the batteries start to supply the house with low cost electricity
The sun managed to contribute with 7kWh this particular day in April.
During the day the batteries have been recharged back to 100%

Below, there is another scenario with the night battery charger switched off. There was energy reserves into the batteries around 70% and next days was expected energy generation from the sun. 11.4kWh of free energy was produced from solar panels. Below is the Solis monitoring platform view of the power generation and consumption. The blue line represents the PV generation and the doted red light represents the homeowners power consumption. This system has the 24-hour feature enabled.

It works with existing Grid-Tie PV Systems with Feed In Tarriff

When upgrading the grid-tied system to an energy storage system the only part that changes is the AC Coupled battery inverter add-on. The existing solar PV system doesn’t need to change at all. The AC coupled battery inverter is installed alongside batteries which is then connected directly to your panel or mains. If the customer wants critical load backup, then those loads will be moved to the backup port (ac output for off-grid mode) . This will give customers the opportunity to select loads that they define as “critical”. Critical loads will remain online during a grid failure until the solar PV generation is null and/or the battery state of charge is null. Examples of critical loads would be refrigerators, hot water tanks, emergency lighting, etc.

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