Testing de panel 5.5v 2w 180x80mm Arduino
Understanding the manufacturer data they provide us
5.5v -> effective or nominal voltage (not max or min)
2w -> a little tricky one. this is not a real output power. its the Vmax(open circuit)* Imax(short circuit)
* Later it will be deduced (empirically and theoretically) that Vmax is around 7v and Imax around 285 mA (7v x 285 mA = 2 watt)
Theoretical performance:
Assuming 2 watt output power
V.I = 2 w
5.5v * I =
2w => I = 2/5.5 = 0.3636 A = 363 mA
Practical performance:
We will proceed to test this panel in full sun-power with a variable-load
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I[mA]
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V[v]
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P[w]
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24
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5,8
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0,14
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110
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5
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0,55
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|
142
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4
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0,568
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150
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4,7
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0,705
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156
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4,6
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0,71
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195
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2,61
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0,51
|
|
197
|
3,43
|
0,67
|
Influence of the protection diode in the calculation:
Before carrying out an analysis of the empirically obtained data, it should be noted that next to the solar panel, a diode is placed (in this 1N4148) in order to avoid a reverse polarization that causes the "load" to become a "generator" and vice versa, being able to damage the solar panel.
Since the calculations are slightly affected by the diode, it is important to know in which regions of the "polarization curve" the data was taken to understand the drops we will have in it.
In the following curve corresponding to the 1N4148 diode datasheet, the polarization areas in which the diode worked in this test are marked in blue.
For most of the cases, in the diode it falls between 0.9 and 1v, which makes sense, since the panel is presented as 5.5v (effective / working voltage) and we in the best of cases managed to get 4.7v (at maximum energy transfer).
Conclusions: The panel presents a zone of maximum power hovering around 156 mA of maximum current. Less than half of the manufacturer's proposal *.
When we exceed this value, the voltage begins to drastically decrease, also causing the power delivered to the load to drop. This is a technological feature of solar panels.
* It is assumed that the manufacturer takes the AC voltage (maximum voltage) and multiplies it by the current in DC (maximum current), resulting in a power that can only be taken as a reference parameter.
Solar panel modeling
In order to predict the behavior of a solar panel in the face of load variations, the panel is modeled.
Structures of a solar panel
Physical model:
Basic CELL:
Each cell contributes 0.6v to the system, and can be modeled as follows:
IL: This current generator represents the solar radiation received with the panel. It means that the current generator value will depend of the sun it receives. In the absence of value, it supplies 0 current.
Id: this "diode" is part of the circuit model and is "polarized" by the IL current gen. It will maintain a voltage of 0.6v (unless we short the output terminals or we do not have the sun-power to polarize it)
Rs: represents the series resistance of the quadrupole
Rsh: represents the parallel resistance of the quadrupole.
A module is made up of a quantity "n" of cells in parallel. For example, a 22v output cell will have 22 / 0.6 = 36 cells in series. as the picture shows
A panel can be made up of one or more modules, it can also be called an “array” as illustrated in the following figure:
The LTSpice simulator is used for this.
The model give us the possibility of change all the parameters of the basic cell. The cell quantity is represented by "n" (depending on the voltage of the panel that we have, each cell add 0.6v to the entire system).
For the case of the arduino 5.5v panels, we will use n = 14 as shown in the following graph:
Ics1 represents the solar power source, the current will go from 1 mA to 360 mA (It is not the output current of the panel!)
Vpv will represent the supplied load, it will allow us to do a voltage sweep on the load . Then we will obtain different values of the panel output current. Thus being able to locate the maximum power in the load for each value of Ics1 (solar energy)
Because of the fluctuation of the curves with a high sun-power dependence and load dependence, the normal solution is to make a power tracker. The problem with this low-power project is that i dont have enough input power to make a entire power-tracker-system.
One of the proposed solutions would be to "regulate" the output voltage to the value where we obtain the maximum power (for the first 5 levels of solar intensity plotted in the simulator), which would be more less at the 4.3v output, however, we have a really small input voltage to make a correct polarization (necesary to make a voltage-regulation) .
Here is when i had a idea. our friend "TP4056", a smart li-ion charger that let us make some kind of "fixed load" that could help us to solve this problem,.. maybe ! ( in the next chapter ! )
