Just one glance at the rooftop above can make a solar installer sweat. Can you spot what's wrong? If the answer is no, continue reading. In this article, we’ll explain just how important it is to make sure your solar modules work at their maximum power point level (MPP).
Solar PV modules generate DC power through the conversion of sunlight to electricity. The output power at the cell level is minimal, but when you add up the effect of multiple cells, e a higher output is achieved, enough a to back up your household supply.
The concept behind the panels is complex, involving a lot of physics and electrical terms and definitions that we do not aim to explain here. However, we do care about how this power output changes and how we can optimize its value for your home.
The first thing you need to know is that the power output changes over the day with the variations of temperature and irradiance.
As some of you may know, power in DC is equal to
P = V * I
P = Power in DC
V = Voltage
I = Current
Here you can see that two parameters modify the value of power: voltage and current.
Voltage and current also change with other factors related to the environment. For instance, the current changes with the levels of irradiance (the intensity of solar radiation per square meter). The higher the irradiance value, the higher the current.
On the other side, the voltage changes with temperature. If the temperature is high, the voltage is lower - not higher, as some people may believe.
Irradiance is a factor that changes a great deal across the day, meaning that current values tend to change a lot as well.
Temperature does not change a lot during the day but changes drastically across the year from summer to winter.
Taking into account these two parameters along with their changes due to meteorological conditions, it is possible to obtain the characteristic curve of the PV modules, known as the I/V curve.
For example, take a look at this I/V curve of a PV module:
This curve has five important points:
(1) Isc stands for short-circuit current, representing the highest current that the module can produce.
(2) Voc stands for open-circuit voltage, representing the highest voltage that the module can produce.
(3) Imp stands for maximum power current.
(4) Vmp stands for maximum power voltage.
(5) Pmax is the maximum power that the module can produce.
The fifth point is the so-called MPP or Maximum Power Point and denotes the optimum point at which the module should operate to achieve the highest power output.
In order to operate the system at the MPP, charge controllers and inverters are equipped with a maximum power point tracker or MPPT. This consists of an algorithm that tracks the MPP based on the variations of power that act as a reference to adjust the voltage level of the array to the Vmp value (4). This results in the Imp (3) and therefore, sets the power of the array at its maximum (MPP) - (5).
How Can I Identify The MPP Value Of My Module?
You must go to the datasheet and take a look at the specs. Find the Vmp (4) and the Imp (3) and multiply the two values, which will result in the authentic MPP (5) point of the module, similar to the peak power provided.
One thing that you must understand is that this MPP value is not a fixed value. As explained above, voltage and current change and in the same way, the optimum power output changes across the day and due to the time of the year as well.
It is the work of the inverter or charge controller to find this point every time.
How Can I Make Sure That My PV Array Works At Its MPP?
There are certain factors that modify the I/V curve of the module and force the system to work at a power value that is lower than the MPP.
If you want to obtain the highest energy yield from your array, then you should take into account these considerations:
1. Reduce any element that could cause shade on the PV array.
Shade reduces the power output of the modules and drastically changes the I/V curve of the panel, even leading to the creation of two MPP points (global and local) which are not the ideal MPP point of the system.
2. Avoid placing panels in multiple orientations towards the same MPPT input.
This is an undesirable design, as it leads to mismatch losses and does not allow the charge controller or inverter to accurately perform the MPPT function.
However, due to space constraints, it is sometimes necessary to place strings or panels in different orientations. If that’s your case, then make sure your inverter/charge controller has at least two MPPT inputs that will allow it to perform the tracking function of two strings in different orientations independently.
3. Never place different modules in the same array.
This is a really bad idea, as you will definitely limit the power output of one or most of the modules which will incur in higher mismatch losses and a modification of the I/V curve.
The MPP represents a crucial factor in the design of your PV system.
Even when the duty of the solar installer is to set the arrangement of your design appropriately to maximize its power output, it is also good for you to understand the importance of this operation point within the system and do everything possible to make sure that your system will work at its MPP for most of the time.
Doing so will increase your energy output and therefore, reduce your demand on electricity from the grid.
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Photo credit: Scott Moore