How Solar PV Works
Q. How Does Solar Power Work?
For the non-scientific minded –
A photovoltaic array converts the Sun’s energy into electricity.
An inverter converts this Direct Current (DC) electricity into Alternating Current (AC) electricity and feeds this electricity into the grid.
For the Scientific minded –
A photovoltaic cell is made from P-N silicon material (silicon doped with Boron and Phosphorous) which has an inbuilt depletion layer (electrically charged layer) that serves to sweep free electrons to electrodes at either side of the P-N material.
When enough solar radiation is absorbed by the P-N material to exceed its band gap energy, electron-hole pairs are created and the now free electrons are swept to the electrodes creating an electric charge.
Providing the depletion layer stays intact, recombination of electron-hole pairs is avoided and current can flow when the cell is connected in a circuit.
When the voltage across the cell is increased (toward its open circuit voltage) the effect of the depletion layer is reduced and recombination of the electron-hole pairs occurs which reduces the free electrons thus reducing the amount of current that can be derived from the cell.
This is why PV cells have an inverse voltage current relationship.
All PV modules are rated for their nominal DC power output at Standard Test Conditions (STC – irradiance of 1kW/m2 with an air mass density of 1.5 and cell temperature of 25 degrees C).
When a crystalline PV cell heats up to its normal operating temperature of around 50 degrees C, 2 things happen.
- The increased temperature increases kinetic energy within the cell which allows slightly more current to flow, however
- the increased temperature also causes the depletion layer to reduce, as a result re-combination of electron hole pairs occurs resulting in a loss of efficiency – typical crystalline PV cells lose power at a rate of around 0.5% / degrees C above standard test conditions (25 deg C).
Whilst the PV cell loses power when operating above STC, conversely it will also increase power output (and open circuit voltage) when operating below STC.
When designing a PV system attention must be paid to ensure that the open circuit voltage of the Array at the lowest expected temperature does not exceed the maximum voltage rating of the inverter (or charge controller).
The PV modules that we use are all selected on the basis of having a power temperature coefficient lower than their competitors, which gives them their superior output characteristics in our hot Australian operating conditions.
Click this link to view a Temperature Power Comparison Crystalline Modules between Hyundai, Kyocera, Silex, Trina, Suntech and CEEG panels.
The inverter does exactly what its name suggests, inverting the applied DC voltage below its zero reference point creating an AC voltage.
The AC waveform is achieved by switching the polarity of the applied DC voltage every half cycle and rapid pulse width modulation of this electronic switching synthesises a sine wave with relatively low harmonic distortion.
Most inverters for grid connect PV systems have inbuilt Maximum Power Point Tracking (MPPT) which varies the input impedance of the inverter to ensure that the PV modules operate at their maximum power point.
The output of the inverter synchronises with the electricity grid’s phase angle before varying its output impedance to control power flow into the grid.
In order to comply with AS4777, all grid connect inverters must have anti-islanding protection which shuts down the output of the inverter when the grid power fails.
This is a safety requirement to ensure that power is not fed into an unsafe situation (e.g. power lines down).
Typically this protection function is achieved by monitoring the impedance of the grid, any sudden change in impedance turns off the output of the inverter, which then will wait to synchronise again before feeding power to the grid.