The solar power production varies during the day time (even with a clear sky); it has the maximum output at noon time when the sun intensity is at its peak.
Also the daily pattern varies round the year. In summer, the days are long and the sun rises high, while in the winter the days are short and the sun is low in the sky, with spring and autumn in between, and the solar power output follows the same periodic seasons. The power fluctuates quite a lot with passing clouds too.
Definition of Hybrid Systems
Hybrid Systems are the simultaneous integration or combination of different power sources such as the photovoltaic system, the Grid (Electricité Du Liban), the batteries and the back-up generators.
The logic operation mode of a Hybrid PV system proposed by DCE is as follows:
(1) Sunny period & grid ON: The consumption priority is from the power generated by the solar panels. In case of production surplus the panels will charge the battery pack, and when the batteries are fully charged (100%) the production surplus will be injected into the public power grid. (NB. an electrical net?metering counter should be available for
(2) Sunny period & grid OFF: The consumption priority is from the power generated by the solar panels. In case of extra demand the battery pack will satisfy the needed energy as back-up. (NB. During grid failure, the surplus solar energy production cannot be injected into the public power grid).
(3) No sunny period & grid ON: The consumption is from the public power grid simultaneously charging the battery packs if needed.
(4) No sunny period & grid OFF: The consumption is fulfilled by the battery pack. In case of additional demand is needed and the battery packs is at its lower level then the diesel generator will be turned ON.
Consumption priority: Knowing that the solar management system is capable of combining the energy sources, the order of priority of the energy sources is always as follows:
1- Photovoltaic panels
2- Utility company (public grid)
3- Battery pack
4- Diesel generator
AC System vs. DC System
According to each project, installers can choose between two types of stand-alone or off-grid solar power systems.
As the photons radiated from the sun hit the Silicon atoms of the solar cells they transform their energy to loose electrons which produce Direct Current (DC). This DC current is fed directly into a string inverter. The string inverter function is to convert the Direct Current produced by the solar panels to Alternative Current (AC) with a constant voltage and frequency that is fed to an inverter/charger (also called off-grid inverter or grid tied inverter).
The inverter/charger is coupled to an AC-bus which supplies the AC loads, and at the same time it is coupled to the Automatic Transfer Switch (Grid and/or Generator) and to the battery bank.
The inverter/charger duty is to manage and control the energy input and energy output of the whole system and to convert from AC to DC and vice versa in order to charge and discharge the batteries. It is responsible also to supply the AC loads with a stable AC current by using the energy supplied from the grid and/or the string inverter or by using the battery bank when there is no energy input in the system. The inverter charger is a multi-task device it can combine two or more different energy sources to supply the AC loads and charge the batteries at the same time. Moreover, the inverter/charger is capable to feed electrical energy into the utility grid via a Net Metering electrical meter in case of excess of production and to request the generator in case the batteries reach a low state of charge (SOC).
AC coupled off-grid solar power system
In a DC coupling systems, solar panels are connected to a charge controller that charges directly the batteries. The charger is responsible of charging the batteries properly by regulating the current and the voltage coming from the solar panels.
An inverter/charger is coupled to the AC-bus which supplies the AC loads, and it is coupled also to the battery bank and ATS (Grid and/or Generator) and the batteries. The inverter /charger duty is to manage the power supplied by the ATS and the batteries according to the load demand. The inverter supply the electrical loads using the energy stored in the battery bank. During covered periods or during the night, the inverter/charger is also capable to charge the batteries from the ATS. Similar to the AC coupling, the inverter/charger is capable to feed electrical energy into the grid (Net Metering) in case of excess of production and to request the generator in case the batteries reach a low state of charge (SOC).
As a conclusion, it should be noted that AC coupling is addressed for large installations while the DC coupling is addressed for smaller ones. AC coupling system preserves the life of the battery bank by reducing its cycling because the energy produced by the panels is directly converted to AC and can be fed directly to the loads in contrast with the DC coupling which stores the energy produced by the panels in the batteries in order to use it as backup power.
It is the simplest configuration where one inverter/charger forms a single-phase current.
Single Cluster and Multi-Cluster Systems
A single cluster is the connection of three inverters/chargers in parallel or in series with one master and two slaves. The three inverters communicate with each other to supply power efficiently in a mono or three phase system.
Single Cluster - Single Phase
When the inverters/chargers are connected in series, this configuration is used in a monophasic installation where the powers of the inverters are added up together.
Single Cluster - Multiphase
On the other hand, when the inverters/chargers are connected in parallel, this configuration is used in a tri-phasic installation where the three inverters/chargers are synchronized in a manner to supply a three-phase current where the master is leading.
Multicluster systems consist of several three-phase clusters. The individual clusters must be connected to a Multicluster Box. The Multicluster Box is the AC main distribution board in a multicluster system.
DC coupled off-grid solar power system
Solar Panels Daily Power Variation