Solar Panels and Optimisers
A typical solar panel electrical specification might look like this:
The parameters are:
- Rated Maximum Power (Pmax): the power output from the panel under standard test conditions
- Open Circuit Voltage (Voc): the voltage presented by the panel when no current is being drawn
- Max Power Voltage (Vmp): the voltage presented when the panel is delivering maximum power
- Short Circuit Current (Isc): the current flow if the panel terminals are shorted together
- Max Power Current (Imp): the current flow when the panel is delivering maximum power. Vmp * Imp = Pmax
- Temperature Coefficients: the variation in the parameters with changes in temperature compared to STC (see below)
The characteristics show how the parameters vary with the level of light and temperature:
Parameters are usually provided for 2 conditions: Standard Test Conditions (STC, normally 1000w/m2 and 25c) and nominal operating cell temperature (NOCT, typically 800w/m2 and 20c).
Where the temperature coefficient is +ve, the parameter value increases with temperature, where it is -ve, the parameter value decreases. For this panel, there is a relatively small increase in current with temperature but a bigger fall in voltage so the maximum power from the panel falls as temperature increases.
The converse is also true: the maximum power increases with falling temperature, so you might get more power generated on a clear and cold winters day than on a clear summers day.
When working out the overall voltage presented by a string of panels connected in series, both Vmp and Voc need to be considered. Vmp x number of panels will be the voltage presented when maximum power is being drawn. However, Voc x number of panels will be voltage presented if the inverter does not draw power from the panels if, for example, the power output from the inverter is limited or there is a grid fault.
Optimisers help to reduce the effect of shadows crossing across panels. When any one panel in a string is shaded, it reduces the current that can be drawn from the string and this limits all of the panels in that string.
An optimiser, such as Tigo TS4-A-O, sits in-line with the string and solar panel and detects when the panel is being shaded and bypasses the panels. This allows other panels in the string to generate more power.
This depends on the situation of your solar panels and whether they are shaded at certain times of the day.
One way to detect if optimisers will help is to look at the power and voltage output from your strings on a relatively bright and clear day. This chart shows the output from an East (PV1) and West (PV2) facing array and exhibits some key indicators that shading might be an issue:
Firstly, PV1 Power does not have an dome or bell shape but is flattened at the start of the day and then rises swiftly as the shadow moves off the string. If you project the bell shape backwards, you can see the power loss. In addition to this, you can see that the PV1 Voltage rises higher than normal (the difference between Vmp and Voc), indicating the panels are generating power that cannot be drawn off by the inverter. This is the effect of a tree that shades half of the string until the sun rises higher and fully illuminates it.
Secondly, PV2 Power exhibits a saw tooth effect at the end of the day, also with corresponding peaks in PV2 Voltage. This is the effect of the sun tracking West, casting the shadow of the edge of a building across the string. As the shadow moves left to right, it progressively covers more of the end of the string. As the solar cells are arranged in columns, the drops demonstrate the way the panel uses internal bypass diodes to try to reduce the effects of partial shading. However, there is no bypass diode between the panels so the output drops once the first panel is fully shaded.