Overview.
Solar power system configuration refers to the arrangement and connection of various components that make up a solar power system. The configuration determines how the system operates, its efficiency, and its overall performance.
Many off-grid PV system configurations are possible. The following will introduce to you some typical designs:
- Small off-grid PV system
- Very small off-grid PV system (e.g. Pico solar home system (Pico SHS) – very small system producing only DC power
o Off-grid PV system for a single home – DC or AC power
- Permanently installed system
- Plug-and-play SHS
- Off-grid PV hybrid system
- DC-coupled hybrid system
- AC-coupled mini-grid system (including hybrid)
- ‘Fuel saver’ system (PV-diesel hybrid system that only runs when the diesel generator is running)
Small off-grid PV system configurations
Describing different small off-grid PV system configurations
Around 90% of all off-grid PV systems are very small, powering individual applications (e.g. solar street lighting) or buildings (e.g. solar home systems). The different configurations include:
- Very small off-grid PV systems can be as small as 10 W and usually provide 5 V DC power for mobile phone charging via a Universal Serial Bus (USB) port.
- An off-grid PV system for a single home is often called a solar home system (approximately 20– 1,000 W). Solar home systems usually provide 12 V DC power using a vendor-specific outlet port. Larger systems also incorporate inverters to provide AC power.
- An off-grid PV hybrid system has an additional power source—usually a diesel generator (which can be run on biodiesel/biogas), but sometimes also comprising small wind turbines or Pico/micro hydro plants. Many off-grid PV systems of over 1000 W have an additional power source. PV-diesel hybrid systems are often diesel generator systems to which PV modules are added, rather than systems designed from the outset as PV-diesel hybrid systems.
Left: a DC-only off-grid PV system – the most common type of off-grid PV system. Right: an off-grid PV system producing AC power through an inverter (second-most common type of off-grid system). (Source: Solar Magazine Uganda)
Off- grid PV systems producing AC power through the inverters(Source: Green Power International LTD.)
Mini-grid and hybrid configurations
In the end, you should be able to.
- explain the differences between DC-coupled and AC-coupled mini-grids.
Mini-grid systems (sometimes referred to as micro-grids) generally serve several buildings or a large building complex.
- In a DC-coupled system, the PV array is connected to the batteries (DC bus/line), as in a traditional off-grid PV system. An inverter is used to power AC loads. In this configuration, the PV array needs to be close to the batteries. Most systems over 1000 W also have a diesel generator or any other UMEME source to power heavy, infrequently used loads and contribute to battery charging during periods of low solar radiation. This has the significant advantage of allowing the battery bank capacity to be reduced. While many larger systems of this type exist with PV array sizes above 4000 W, beyond a certain size, AC-coupled systems are often the more attractive option.
- In an AC-coupled mini-grid, a special grid-forming battery inverter is used, which creates its own small AC mini-grid. The PV array is then connected directly to the AC mini-grid side of the installation via grid-connected inverters. System design and installation therefore require knowledge of grid-connected PV technology. One significant advantage of these increasingly common systems is that the PV array does not need to be close to the batteries; rather, it can be anywhere along the AC line. The system size is usually over 2000 W. Three-phase systems are also available.
Simplified schematics of DC-coupled system (left) and AC-coupled mini-grid (right). (Source: Solar Magazine Uganda).
Special case: ‘Fuel Saver’ concept
Let’s discus the fuel saver concept, its benefits and drawbacks.
While most mini-grids (incorporating PV) require some sort of battery storage, not all do. One such system is SMA Solar Technology’s Fuel Saver concept. Grid-connected inverters are used to feed PV electricity into a small local grid powered by diesel generators , thus reducing the amount of electricity (and, hence, fuel consumption) that the diesel generators need to provide. PV electricity is fed into the grid when the diesel generators are running, which means it is more suitable for industrial and agroindustry applications where power requirements are high during the day, when solar power is available. The system can respond flexibly to sudden changes in supply or demand, e.g. if heavy loads are switched on or off, or it suddenly becomes cloudy, thereby reducing PV output. However, the system will not work when the diesel generators are turned off. Nevertheless, fuel costs can be cut by up to 60%, depending on factors such as insolation, the load profile, and generator type. Some of these systems also incorporate relatively small battery banks to cover lighter loads in off-peak periods.
SMA Solar Technology Fuel Saver concept schematic (Source: SMA)
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