The development of the photovoltaic solar industry has raised the demand for inspection and precise measurement solutions for solar panels (and photovoltaic solar panels), and along with the increase in solar panel form factor and efficiency, cell inspection has to apply larger instantaneous currents and higher power levels, which requires the use of more flexible inspection equipment.
General To accurately measure the important parameters of solar panels. Some of these parameters are.
●ISC - short-circuit current. When the load resistance is equal to zero, the instantaneous current flowing from the cell.
●VOC - Voltage Open Circuit. The voltage of the battery when the instantaneous current is equal to 0.
●Vmax - At the point of Pmax, the voltage value of the battery.
●Pmax - The maximum power output of the battery. The Pmax point on the I-V curve (Figure 1) is generally referred to as the maximum power point (MPP).
●η - The conversion efficiency of the element. When a solar panel is connected to a circuit, this value is calculated as a percentage of the energy converted (from absorbed sunlight to electrical energy) to the energy captured. This value can be calculated by dividing Pmax by the input light irradiance (E, in W/m2, measured precisely under standard test conditions) and multiplying it by the surface area of the solar panel (AC, in square meters).
●Imax - At the point of Pmax, the current value of the cell.
●Fill factor (FF) - Pmax divided by VOC and multiplied by ISC.
●Battery bypass resistance (or shunt resistance).
●Battery diode property.
● Battery series resistance.
Common Solutions
Nowadays, there are two main forms of solar panel testing solutions: complete turnkey systems and universal test instruments.
To test solar panels at their maximum output power, many research labs have low-power quadrant power supplies (sometimes referred to as SMUs) with both.
● Accurately and precisely measure the voltage and current of the component under test (DUT) (accurate measurement is also known as detection).
● Provide accurate forward and reverse currents (providing reverse current is also referred to as current flow into the power supply).
● Provide accurate positive and negative voltages ("provide" can also be called "apply").
Most high-precision four-quadrant power supplies can only provide 3A of current or 20W of continuous power.
In such cases, technical engineers should rely on existing DC electronic loads, DC regulated power supplies, DMMs and data collection equipment, including temperature detection, scanning, conversion and data logging equipment, to facilitate the flexibility to carry out specific tests over a wide range of operations and to achieve the predicted detection accuracy. For example, the data collection system can also be used to scan the temperature of the environment and the device to be tested, the voltage of a calibrated reference cell, and various other inspection parameters that need to be captured during the inspection.
These maximum currents and powers are acceptable when testing smaller individual cells, but as cell technology advances to higher efficiencies, greater current densities, and larger cell form factors, the power output of cells will soon exceed the maximum ratings of these four-quadrant power supplies. The output of PV solar panels will typically exceed 50W and may climb to 300W or more, which means that many panel-specific tests cannot be done with a four-quadrant power supply.
Outdoor Testing
Some technical engineers use turnkey solar panel inspection equipment to perform the inspection, which uses a type of solar simulator, which is a standardized light source that controls the light energy entering the solar panel. However, if the solar panel or module is particularly large, the solar simulator will not be able to form an adequate amount of light.
For example, the solar module under test may be part of a medium to large outdoor solar collection system. In such a situation, the sun itself would be the only actual light source available for the test. Since it is unlikely that a complete turnkey inspection system without a solar simulator can actually be delivered outdoors, this type of inspection requires the use of some other inspection solution modified from standard test equipment to be implemented.
Many instrumentation suppliers do not specify the performance of their testing equipment when the temperature is outside of a very narrow range near room temperature (e.g., 25°C ± 5°C). Other suppliers provide a temperature coefficient specification that adjusts the accuracy specification of the inspection equipment to calibrate for operation outside of its specific operating temperature range.
Another factor to consider for outdoor testing is temperature. Since battery performance is affected by temperature, temperature needs to be monitored during testing. Not only is battery performance dependent on temperature, but so is the performance of the testing equipment.
Loads for higher power detection
For high power applications, standard DC electronic loads can be used to inspect solar panels. Many technical engineers would not think of using DC electronic loads to inspect solar panels because they are used to using turnkey systems or four-quadrant power supplies.
Considering that solar panels form energy, when a four-quadrant power supply is used to carry out inspection on it, the actual mode of operation of the power supply is that the solar panel applies a positive voltage to the terminals of the power supply. At the same time, current flows from the solar panel to the terminals of the quadrant power supply, which means that the quadrant power supply sees a reverse current (with respect to its terminals). Under these conditions, the quadrant power supply can also be called a "power sink".
Electrically speaking, the instrumentation with positive voltage and current flowing at both ends (i.e., reverse current) is called a DC electronic load. Therefore, for most solar panel testing where light is shining and solar panels are also forming energy, the four-quadrant power supply actually functions as a DC electronic load.
DC electronic loads can operate in constant voltage mode, also known as CV mode. In CV mode, the load is able to maintain a stable operating voltage value by regulating the current flowing through itself and thus the operating voltage at its terminals. Thus, CV mode can be used to create an operating voltage sweep, using the load to control the operating voltage at the output of the solar panel and subsequently measure the current generated.
The advantage of using DC electronic loads is that these loads are available in a variety of current and power levels. Using DC electronic loads rated at 50W or up to thousands of watts and hundreds of amps can also easily overcome the 3A, 20W limit imposed by a four-quadrant power supply.
Ability to use the CV mode of a DC electronic load to measure the I-V curve of a solar panel
Some loads are able to quickly perform a series of CV locating points to facilitate scanning of the output operating voltage in CV mode, which in turn quickly depicts the I-V graph. In addition, the load is able to inform the current waveform flowing from the solar panel into the load, similar to capturing a digital oscilloscope graph.
However, many DC electronic loads have low voltage operating limits, and a minimum operating voltage must be applied between the positive and negative inputs of the load. The minimum input operating voltage for a common DC electronic load is 2 to 3 V. To get rid of these limits, a DC power supply can be connected in series with the load. These DC power supplies are called bias supplies because it provides a bias voltage for the load.
With the help of drawing a graph of the CV operating voltage controlled by the scan against the actual current of the information, it is possible to create an I-V graph. And since this is also obtained with the help of a rapid scan, the full detection can be completed in about 1 second, when the battery does not heat up and produce temperature changes due to the intense light source.
Ability to configure DC electronic loads using a DC bias power supply
Generally, the bias power supply is set to 3V to ensure that the minimum operating voltage required for the load is always met. The operating voltage of the DC source has no effect on the solar panel, which is a floating operating voltage device; the DC source simply raises the operating voltage of the solar panel by 3V.
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