Selecting the right solar cell output power test system

The growth of the solar industry has increased the demand for test and measurement solutions for solar cells (and solar modules), and as solar cells grow in size and become more efficient, cell testing requires the use of higher currents and higher power levels, which requires more flexible test equipment.

There are several key parameters of solar cells that need to be measured. These parameters are:

● VOC——Open circuit voltage. The battery voltage when the current is equal to 0.

● ISC——Short-circuit current. The current flowing out of the battery when the load resistance is equal to 0.

● Pmax – The maximum power output of the battery. The voltage and current values ​​when the battery outputs maximum power. The Pmax point on the IV curve (Figure 1) is usually called the maximum power point (MPP).

Figure 1 This solar cell IV curve shows Pmax and its relationship to Imax and Vmax

● Vmax——Battery voltage value at Pmax point.

● Imax——The current value of the battery at the Pmax point.

● η – The conversion efficiency of the device. When the solar cell is connected to a circuit, this value is equal to the percentage of energy converted (from absorbed sunlight to electrical energy) to the energy collected. This value can be calculated by dividing Pmax by the input light irradiance (E, in W/m2, measured under standard test conditions) and multiplying it by the surface area of ​​the solar cell (AC, in square meters).

● Fill Factor (FF)—Pmax divided by VOC multiplied by ISC.

● Battery diode properties.

● Battery series resistance.

● Battery bypass resistor (or parallel resistor).

Common solutions

Today, solar cell testing solutions come in two main forms: complete turnkey systems and general-purpose test instruments.

If it is necessary to test a solar cell at its maximum output power, many research labs have low-power four-quadrant power supplies (sometimes called SMUs) that offer the following features:

● Provide precise positive and negative voltages (“provide” can also be called “apply”).

● Provides precise forward and reverse current (providing reverse current is also known as flowing current into the power supply).

● Accurately measure the voltage and current of the device under test (DUT) (measurement is also called detection).

Most high-precision four-quadrant power supplies can only provide 3A of current or 20W of continuous power.

These maximum currents and powers are acceptable when testing smaller individual cells, but as cell technology advances toward higher efficiencies, greater current densities, and larger cell sizes, the power output of the cells will quickly exceed the maximum ratings of these four-quadrant power supplies. Solar module outputs typically exceed 50W and can climb to 300W or more, which means that many tests on modules cannot be completed using a four-quadrant power supply.

In these cases, engineers should rely on readily available electronic loads, DC power supplies, DMMs, and data acquisition equipment, including temperature measurement, scanning, conversion, and data logging equipment, to provide the flexibility to perform unique tests over a wide operating range and to achieve the desired test accuracy. For example, a data acquisition system can be used to scan the temperature of the environment and the device under test, the voltage of a calibrated reference battery, and various other test parameters that need to be captured during testing.

Outdoor testing

Some engineers use turnkey solar cell test equipment for testing, which uses a solar simulator, a standardized light source that can be used to control the amount of light energy entering the solar cell. However, if the solar cell or module is very large, the solar simulator will not be able to produce enough light.

For example, the solar module being tested may be part of a larger outdoor solar energy harvesting system. In this case, the sun itself would be the only practical light source available for testing. Since it is not practical to transport a complete turnkey test system without a solar simulator outdoors, this type of testing 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 testing. Not only does the battery performance depend on temperature, but the performance of the test equipment also depends on temperature.

Many instrument vendors do not specify the performance of their test equipment outside of a very narrow range around room temperature (e.g., 25°C ±5°C). Other vendors provide a temperature coefficient specification that adjusts the accuracy specification of the test equipment to correct for operation outside of its specified operating temperature range.

Loads for higher power testing

For high power applications, standard electronic loads can be used to test solar cells. Many engineers would not think of using an electronic load to test solar cells because they are accustomed to using turnkey systems or four-quadrant power supplies.

Considering that a solar cell produces energy, when it is tested with a four-quadrant power supply, the actual operating mode of the power supply is: the solar cell applies a positive voltage to 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 a reverse current (with respect to its terminals). Under these conditions, the four-quadrant power supply can also be called a "power sink".

Electrically speaking, an instrument with a positive voltage across its terminals and current flowing in it (that is, reverse current) is called an electronic load. Therefore, for most solar cell tests where light is shining on the solar cell and the solar cell is producing energy, the four-quadrant power supply actually acts as an electronic load.

The advantage of using an electronic load is that this load is available at a wide range of current and power levels. The 3A, 20W limitation of a four-quadrant power supply can be easily overcome by using an electronic load rated at 50W or up to several thousand watts and hundreds of amps.

Electronic loads can be operated in constant voltage mode, also known as CV mode. In CV mode, the load can adjust the voltage across it by regulating the current flowing through it to maintain a constant voltage value. Therefore, CV mode can be used to create a voltage sweep, using the load to control the voltage at the output of the solar cell and then measuring the resulting current (as shown in Figure 2).

Figure 2 The IV curve of a solar cell can be measured using the CV mode of an electronic load.

Some loads, such as the Agilent N3300 series, can quickly perform a series of CV set points to sweep the output voltage in CV mode, quickly drawing an IV curve. At the same time, the load can digitize the current waveform flowing from the solar cell into the load, similar to capturing an oscilloscope trace.

By plotting the swept CV voltage versus the digitized actual current, an IV curve is created. And because it is swept so quickly, the entire test can be completed in about 1 second before the cell heats up and experiences temperature changes due to the intense light source.

However, many electronic loads have low voltage operating limits and require a minimum operating voltage to be applied between the positive and negative input terminals of the load. The minimum input voltage of common electronic loads is 2 to 3V. To overcome this limitation, a DC power supply can be connected in series with the load (as shown in Figure 3). This DC power supply is called a bias power supply because it provides a bias voltage for the load.

Figure 3. An electronic load can be configured using a DC bias supply for solar cell testing.

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, which is a voltage floating device; the DC source simply increases the voltage of the solar cell by 3V. For more information, refer to the application note "Solar Cell and Module Testing" provided by Agilent Technologies, publication number 5990-3262EN.