Section 5: Heating, cooling and solar
Photovoltaic power systems
7. Grid connected (GC) systems
The principle of energy transfer is the same, but although the PV array is generally the same (or similar), the inverter is different. It must provide power into the electricity grid in the same format as the grid itself (both voltage and frequency and phase in a three phase system). This must be at the most efficient operating point of the PV array.
A PV module has one location on its operating characteristic (which changes with light intensity) where the combination of voltage and current output is at a maximum, this is known as the maximum power point (MPPt), and is the ideal operating point.
Electronic devices known as MPPT’s (maximum power point trackers) are in regular use with PV (and inbuilt into GC inverters) to ensure the most efficient transfer of power occurs, regardless of the operating light levels.
These devices (MPPTs) should not be confused with mechanical tracking devices which track the sun’s path in order to gain more daily output energy from the system. Of course a GC system with mechanical tracking and MPPT operation will have a higher daily output than say a fixed array with no MPPT included.
Generally there are no storage batteries in GC systems, as the task is to provide as much power as possible into the grid as long as sunlight is available
One variation of this is known as a uninterruptible power supply(UPS) system, where PV energy is also stored in batteries as well as supplying power to the grid) such that if the mains power fails, the system transfers to normal SPS operation and provides power to the load, domestic or otherwise.
Grid connected systems can have other sources of primary renewable energy, and wind farms and hydro systems are excellent examples of this.