Some engineers use a turnkey solar cell test device that uses a solar simulator, a standardized light source that controls the light energy entering the solar cell. However, if the solar cell or module is very large, the solar simulator will not produce enough light.
For example, the solar module being tested may be part of a large outdoor solar energy harvesting system. In this case, the sun itself will be the only practical source available in the test. Since it is virtually impossible to transport a complete turnkey test system without a solar simulator outdoors, this test needs to be performed using some other test solution modified from standard test equipment.
Another factor to consider for outdoor testing is temperature. Because the performance of the battery is affected by temperature, it is necessary to monitor the temperature during the test. Not only is battery performance dependent on temperature, but the performance of the test equipment is also dependent on temperature.
Many instrument vendors do not specify the performance of their test equipment when the temperature is outside a very narrow range around room temperature (eg, 25 ° C ± 5 ° C). Other suppliers offer a temperature coefficient specification that adjusts the accuracy specifications of the test equipment to correct for operation outside its specified operating temperature range.
Load for higher power testing
For high power applications, standard electronic loads can be used to test solar cells. Many engineers don't think of using electronic loads to test solar cells because they are used to using turnkey systems or four-quadrant power supplies.
Considering that solar cells generate energy, when using a four-quadrant power supply to test it, the actual mode of operation of the power supply is: the solar cell applies a positive voltage across the terminals of the power supply. At the same time, current flows from the solar cell into the terminals of the four-quadrant power supply, which means that the four-quadrant power supply sees reverse current (as far as its terminals are concerned). Under these conditions, it can also be said that the four-quadrant power supply is "power sinking".
Electrically, an instrument with a positive voltage applied across it and a current flow (ie, reverse current) is called an electronic load. Therefore, for most solar cell tests that have light illumination and solar cells generate energy, the four-quadrant power supply actually acts as an electronic load.
The advantage of using an electronic load is that this load can be used at various current and power levels. With an electronic load rated at 50W or up to several kilowatts and hundreds of amps, the 3A, 20W limit of four-quadrant power supplies can be easily overcome.
The electronic load can operate in constant voltage mode, also known as CV mode. In CV mode, the load can be adjusted to maintain a constant voltage by adjusting the current flowing through it to adjust the voltage across it. Therefore, the CV mode can be used to create a voltage sweep, use the load to control the voltage at the output of the solar cell, and then measure the resulting current.
Some loads, such as the Agilent N3300 Series, can quickly execute a series of CV positioning points to scan the output voltage in CV mode to quickly plot the IV curve. At the same time, the load can digitize the current waveform that flows out of the solar cell into the load, similar to capturing an oscilloscope curve.
An IV curve can be created by plotting the CV voltage controlled by the scan versus the digitized actual current. And because this is done by a quick scan, the entire test can be completed in about 1 second, when the battery does not heat up due to a strong light source and temperature changes occur.
However, many electronic loads have low voltage operating limits that require a minimum operating voltage to be applied between the positive and negative inputs of the load. The minimum input voltage for common electronic loads is 2 to 3V. To overcome this limitation, a DC power supply can be connected in series for the load. This DC power supply is called a bias supply because it provides a bias voltage to the load.
Typically, the bias supply is set to 3V to ensure that the minimum voltage requirement of the load is always met. The voltage of the DC source has no effect on the solar cell, the latter is a voltage floating device; the DC source only increases the voltage of the solar cell by 3V.
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