OptiTrac Global – MPPT is a mechanism in the inverter that ensures that the solar panel is not optimally loaded by the inverter and therefore obtains the maximum power from the solar panel. Before we explain how that works, we first make a small trip to the internal combustion engine.
We do this to show that it is quite normal for a device to not perform at its maximum if you load it heavily. A combustion engine in the car does not give the most power and torque at the highest possible speed of the engine. At low speed, power and torque are also not maximum. Somewhere between these two extremes, a combustion engine delivers the maximum power and torque. If you want to get the best out of your car as a preparer, you will have to drive around this optimal speed point. That is then the point of maximum power (or torque).
OptiTrac Global – MPPT
It is no different with in a pv system. When the inverter loads the solar panel heavily or lightly, it does not take the maximum power out of the panel. The inverter connected to the solar panel has to charge the solar panel in such a way that it gets the maximum power out of the panel.
The electronics in the inverter are capable of determining for themselves how much current flows from the solar panel to the inverter. The inverter thus influences the output voltage of the solar panel (see solid lines in the graph). The inverter therefore determines how heavily it loads the solar panel, comparable to the variation of the speed at the combustion engine.
When the inverter supplies the solar panel with the maximum achievable current (at the far left of the graph), the output voltage of the solar panel has dropped to 0 Volt. This is the point at the far left in the graph at the upper solid line. The current is then 8.3 Amps and the voltage is 0 Volts. The power is then 0 Volt x 8.3 Ampere = 0 Watt. You have nothing to do with that.
The other extreme is if the inverter does not supply the solar panel with power. We are then on the right side of the graph. The current is then 0 Ampere but the voltage is then maximum, here about 37 Volts. Even then, the power delivered by the panel is 37 Volt x 0 Ampere = 0 Watt. That is not smart either. Somewhere between these two extremes is a point where the solar panel will transfer the most power to the inverter.
This point of maximum power delivery (Maximum Power Point) is not a fixed point. This point varies with the irradiation, the temperature and any partial shadow on the solar panel. That is why the best point must always be sought. Hence the word “tracker” in the term Maximum Power Point Tracker.
In the graph the interrupted three lines show a power curve at three different irradiation levels (on the right y-axis the corresponding power is in watts). At the bottom dashed line (low irradiation level) the maximum is achieved at a voltage of approximately 27 Volts and 4.6 Ampere (first find the top at the dashed line and then go down perpendicular and find the corresponding point on the solid line where you can derive the voltage and current). The power is then maximum and is about 126 watts. At the top dashed line (full sun) this point is at about 29.5 Volt and 7.6 Ampere. The solar panel then gives an output of approximately 225 watts.
The conclusion you can draw here is that when the sun is not shining so strongly, the inverter will get the most out of the panel with a solar voltage of 27 volts. But at full sun that voltage is 29.5 volts. The MPPT will always, at different irradiation levels, adjust the current, including the voltage, so that this maximum power point is looked up in the graph.
The graph shows these differences at a solar cell temperature of 25 degrees Celsius. At lower and higher temperatures the curves of this graph change. Then the point of maximum power delivery will also shift. The MPP tracker will constantly try to find the best point for all these variables and thus get the most power from the solar panels.
OptiTrac Global – MPPT
When a solar panel is already in the shade a little, even though only one of the 60 or 72 cells is partially shaded, the power graph as shown above will change shape. The current that the partly shaded cell can supply collapses. Because the solar cells are in series in one panel, it affects the performance of the entire panel. In fact, the matter is even more complex because the modern solar panels use by-pass diodes and technically the solar panel consists of (for example) three small solar panels where the power curve of the shaded part is different than the non-shaded parts. In partial shade, think of the shadow of a chimney, dormer, bird droppings, fallen leaves, or even worse shadow of branches of a tree, the MPPT mechanism will try to get the best out of your solar panel.
MPPT scanning – OptiTrac Global Peak
With most solar panel installations the solar panels are in series, also known as a string. If you suffer from partial shading on one or more solar panels in such an installation, the MPPT mechanism may not get the maximum power from the solar panel. In such a situation, an inverter that can make an MPPT scan is a wise choice. At regular intervals, for example every 15 minutes, the inverter will vary the current from maximum to zero and then measure what power this produces. In fact, the inverter will follow the power curve in the graph from the beginning to the end and determine where the maximum power operating points are and then choose the best power point.
This scanning takes a few seconds and at that moment the power delivery will not be optimal. This reduces the yield of your panels for a short time. That temporary decrease in revenue is more than offset by the fact that with this MPPT scanning technique you always get the maximum power from your solar panels.
However, if you have solar panels that never lie in the sun for most of the day, the power loss during scanning will not work to your advantage. In that case it is wise to turn off or leave this MPPT scanning technique.
Realize that not all string inverters have an MPPT scan. If they have it you have to see if you can turn this MPPT scan on and off. With SMA inverters, this MPPT scanning technique is called “OptiTrac Global Peak” and can be turned on and off. Other manufacturers have invented other beautiful names and sometimes this is standard (which is not always wise).
If the description of the inverter does not mention anything about this MPPT scanning technique then it is almost certain that the inverter does not have this scanning technique and, in the case of solar panel installations that suffer from (a little) shadow, will generate much less energy losses.