There different procedures used for the testing of solar PV panels. They vary in terms of their test and measurement criteria
In this article, we will present the different testing schemes for PV panels.
Before we go in depth lets discuss the most important topic the article, Solar Cell I-V Characteristic and the Solar Cell I-V Curve, it will discuss terms like Isc(Short Circuit Current) ,MPPT and Voc (Open Circuit Voltage).
The graph below is Solar Cell I-V Characteristic Curves showing the refers to a specific photovoltaic (PV) cell, module or array of current and voltage (I-V) characteristics, giving a detailed overview of its capacity to transform solar energy and performance.lets discuss in detail
The above graph represents the current-voltage (I-V) characteristics of a usual photovoltaics cell working under standard conditions. The energy produced by a solar cell is a combination of the current and voltage ( I x V ). The IV curve above is obtained at the moment for a given energy level if the multiplication is completed, point by point, for all voltages from short-circuit to open-circuit conditions.
If no load is attached, the current will be at its lowest (zero) and the voltage around the cell will be at its limit, known as the open circuit voltage of solar cells, or Voc. At the other end, the voltage across the cell is at its lowest (zero) while the solar cell is shorted, i.e. the positive and negative ends connected together, but the current flowing out of the cell hits its limit, defined as the short circuit current of the solar cells, or Isc.
The typical curve of the photovoltaic array I-V differs from short circuit current (Isc) at zero output volts to zero current at maximum open circuit voltage (Voc). In an open circuit, the maximum voltage achievable from a cell is the maximum current obtainable from a closed circuit. Neither of these two situations produces much energy, but there is a point in between where full power is produced by the photovoltaic cell.
There is, however, one particular combination of current and voltage for which, at Imp and Vmp, the power reaches its highest value. In other words, the point at which maximum energy is produced by the cell , the small black circle of the green rectangle is seen in the top right field. This is the “power point maximum” or MPP. Therefore, the optimum photovoltaic cell (or panel) output is specified to be at the maximum power point.
In the I-V curve, the maximum power point (MPP) of a photovoltaic cell is located close to the bend. From the open-circuit voltage and short circuit current, the respective values of Vmp and Imp can be estimated: Vmp x (0.8–0.90) Voc and Imp x (0.85–0.95) Isc.. As the output power of the solar cell both depend on temperature, the real output power varies with variations in atmospheric temperature.
For a single PV cell or panel, we have so far presented the Solar Cell I-V Characteristic Curve. But several photovoltaic arrays are made up of attached smaller PV panels. Then, as shown, the I-V curve of a PV array is essentially a scaled up variation of the characteristic curve of the single solar cell I-V.
Before you start
- Check the voltage (V) and current (A) ratings of your panel, you can usually find these written on the back of the panel
- Suitable conditions for producing readings on your system. To obtain the rated output of your panel you will need full, bright sunlight falling directly onto the panel.
- You need to have full understanding of how to use tool especially DMM (Digital Multi Meter)
- Disconnect panels from system completely, and when disconnecting the panels first disconnect panels from the regulator, then disconnect the regulator from the battery.
- Keep the manufacture’s data sheet for the panel we’d like to check.
CAUTION
- When exposed to light, panels produce electricity, so it is recommended that you cover the front of solar panel, and this is very important in case of panels with higher voltage.
- Never short circuit either the battery or the panel
Standard Test Conditions (STC)
STC is the set of criteria that a solar panel is tested at. Since voltage and current change based on temperature and intensity of light, among other criteria, all solar panels are tested to the same standard test conditions. This includes the cells’ temperature of 25℃ (77℉), light intensity of 1000 watts per square meter, which is basically the sun at noon, and the atmospheric density of 1.5, or the sun’s angle directly perpendicular to the solar panel at 500 feet above sea level.
Normal Operating Cell Temperature (NOCT)
NOCT takes a more experimenter view of actual real world conditions, and gives you power ratings that you will likely actually see from your solar system. Instead of 1000 watts per square meter, it uses 800- 850 watts per square meter, which is closer to a mostly sunny day with dispersed clouds.
Solar panel open circuit voltage test
The purpose of this test is to determine whether the module is working, not if it’s performing to factory specifications.
Open circuit voltage is how many volts the solar panel outputs with no load on it. If you just measure with a voltmeter across the plus and minus leads, you will read Voc. Since the solar panel isn’t connected to anything, there is no load on it, and it is producing no current.
This is a very important number, as it is the maximum voltage that the solar panel can produce under standard test conditions, so this is the number to use when determining how many solar panels you can wire in series going into your inverter or charge controller.
PV rated as 12 volts – why Voc much higher?
The Voc is always higher for any source. A typical single crystalline cell generates about 0.5-0.6 volts, so around 36cells give about 20v open circuit voltage (Voc)
For reference the chart below to find typical VOC values for different types of crystalline PV modules.
Nominal Voltage | Voc | No. of Cells |
12 | 21 | 36 |
18 | 30 | 48 |
20 | 36 | 60 |
24 | 42 | 72 |
What happens to panel voltage as it delivers amps?
Once the load is connected the current starts flowing, the internal resistance of the source causes a drop proportional to the current. In case of solar panel alone, the resistance is dependent on how exposed it is to the sun. The 20v open circuit voltage drops to 12–14v depending on the load.
Now, depending upon the requirement panels are connected in series, parallel and sometime series-parallel configuration
Parallel Connections of Panel
Parallel connections are usually utilized in smaller, more basic systems, and mostly with PWM (Pulse Width Modulation) Controllers. Connecting your panels in parallel will increase the amps and keep the voltage the same. This is often used in 12V systems with multiple panels as wiring 12V panels in parallel allows you to keep your charging capabilities 12V.
The disadvantage of this configuration is that high amperage is difficult to travel long distances without using very thick wires. Systems as high as 1000 Watts might end up outputting over 50 amps which is very difficult to transfer, especially in the systems were your panels are more than 10 feet from your controller, in which case you would have to go to 4 AWG or thicker which can be expensive in long run. Also, paralleling systems require extra equipment such as branch connectors or combiner box.
TO SET UP YOUR SYSTEM IN PARALLEL
A Parallel connection is done by joining the positives of two panels together, as well as the negatives of each panel together. The branch connector has a Y shape, and one has two inputs for positive, which changes to one, along with two inputs for negative, which changes for one.
Series Connection of Panel
Series connections are mostly utilized in smaller systems with a MPPT (Maximum Power Point Track) Controller. This configuration is done when we need to keep the amperage same and high voltage. The reason why series connections are utilized with MPPT controllers is that MPPT Controllers actually are able to accept a higher voltage input, and still be able to charge your 12V or more batteries. MPPT Controllers can accept 100 Volts input. The benefit of series is that it is easy to transfer over long distances.
The disadvantage series systems is shading problems. As they all wired in series if one panel is shaded it will affect the whole string.
TO SET UP YOUR SYSTEM IN SERIES
By joining the positive of one panel to the negative of the other panel together a series connection is established with this you do not need any additional equipment except for the panel leads provided.
Solar panel short circuit current test
This test is performed to check the degradation of the solar module or to check the string current.
Caution: when performing the Isc test confirm the maximum current being measured is less than maximum meter rating. An arc is very common when breaking the connection, therefore, it should be done quickly.
The Isc Test should be done on the single module or string level, as the currents should be kept to 10A or less.
The test should be done on a sunny day, and the measured value should be linear with the sunlight conditions available. A solar radiation meter can be very useful in determining the sunlight conditions.
To perform the test using an inline amp meter, place the positive lead on the positive module terminal and the negative lead on the module negative terminal.
The measured value should be within 20% of the module rating adjusted for sunlight conditions. For Example, on a partly sunny day if the sunlight conditions are about half, then the Isc current will be about half.
Can a panel be short-circuited with no damage?
Yes, you will not damage a solar panel by shorting it. Solar panels are designed to be continuously operated at very close to their short circuit current.
A good easy test of a solar panel is to run it short circuited into an ammeter. While it is plausible that a solar panel may be damaged while running under short circuit, if it is then it is faulty and would also have been damaged by operating it at its design full power point. The optimum operating point of a solar panel is typically about 90%+ of its short circuit current and about 70% to 85% of its open circuit voltage.
