Measuring the power output of solar cells using test equipment

Preface

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

Typical Measurements

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 done using a four-quadrant power supply.

In these cases, engineers should turn to 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 with the desired test accuracy. For example, a data acquisition system can be used to scan the ambient and DUT temperatures, the voltage of a calibrated reference battery, and a variety of other test parameters that need to be captured during testing.

Solar cell measurements typically include the following key parameters.

Open circuit voltage (Voc) – the voltage of a battery when the current is zero.

Short Circuit Current (Isc) – The current drawn by a battery when the load resistance is zero.

Maximum power output of a battery (Pmax) – The voltage and current point at which the battery produces the most power. The Pmax point on the IV curve is often referred to as the maximum power point (MPP).

Pmax voltage (Vmax) - The battery voltage at Pmax.

Pmax Current (Imax) – The battery current at Pmax.

Today, solar cell testing solutions come in two main forms: complete turnkey systems and general-purpose test instruments. If a solar cell needs to be tested at its maximum output power, many research labs have low-power four-quadrant power supplies.

The conversion efficiency (η) of a device is the percentage of power converted (from light to electrical energy) and collected when the solar cell is connected to a circuit. η is calculated by dividing the maximum power point Pmax by the input light irradiance (E, in W/m2) under standard test conditions (STC) and the solar cell surface area (Ac, in square meters).

Duty factor (FF) – Maximum power point Pmax divided by open circuit voltage (Voc) and short circuit current (Isc)

Battery diode performance

Battery series resistance

Battery parallel resistance

Figure 1: Current-voltage curve of a solar cell

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Common solutions

Currently, solar cell testing solutions fall into two categories: turnkey systems and general-purpose test instruments. Turnkey systems are suitable for validation and manufacturing test phases. These systems ensure test repeatability because they are programmed to perform a series of cell tests on solar cells.

Researchers usually use general-purpose test instruments found in semiconductor design labs. They use semiconductor device parameter analyzers to measure diode device characteristics and LCR meters (inductance capacitance resistance meters) to measure the inductance, capacitance, and resistance of materials/devices.

When testing the full solar cell output power, many research labs use a low power 4-quadrant power supply (sometimes referred to as an SMU) that can:

Accurately supply positive and negative voltages (supply is also called apply);

Accurately supply positive and negative current (supplying negative current is the process of drawing current into a power source);

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

4-quadrant power supplies are very versatile, but the maximum current and power they can provide to the device under test is relatively small. Most precision 4-quadrant power supplies can only supply 3 A or 20 W of continuous power. This maximum current and power is suitable for small stand-alone battery testing, but as battery technology advances, battery efficiency, current density and size have increased significantly, battery power output may soon exceed .

To this end, engineers must use existing standard electronic loads, DC power supplies, digital multimeters, and data acquisition equipment to form a flexible test system to test these solar cell modules over a wide range of operating ranges while ensuring measurement accuracy. For example, you can use a data acquisition system to scan the ambient temperature, the temperature of the device under test, the voltage of the calibrated reference cell, and other test parameters that need to be captured during the test.

Outdoor testing

Some engineers use turnkey solar cell test equipment to perform testing. This equipment uses a solar simulator, which is a standardized light source that can be used to control the amount of light 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 large outdoor solar harvesting system. In this case, the sun itself will 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 requires the use of some other test solution modified from standard test equipment to perform outdoor testing. Another factor to consider is temperature, because the performance of the battery is affected by temperature, so the temperature needs to be monitored 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 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 loads operating outside of its specified operating temperature range.

For high power applications, you can use a standard electronic load for solar cell testing. Many engineers don't think of electronic loads when testing solar cells because they are used to using a packaged system or a 4-quadrant power supply. Since solar cells can generate energy, when testing them with a 4-quadrant power supply, the actual operation of the power supply is as follows: the solar cell applies a positive voltage to the terminals of the power supply. At the same time, current flows from the solar cell to the terminals of the 4-quadrant power supply, which means that the 4-quadrant power supply sees a negative current (relative to its terminals). At this point, the 4-quadrant power supply can also be said to be absorbing current. Electrically, a power supply that has a positive voltage applied to its terminals and current flowing toward it (i.e., absorbing current) is called an electronic load. Therefore, for most solar cell testing, if light is shining on the solar cell and the cell is generating power, the 4-quadrant power supply is used as an electronic load. The advantage of using an electronic load is that it can accommodate all currents and powers: using an electronic load of 50W or more (up to thousands of W and hundreds of A), we can break out of the 3A, 20 W power supply limitation of a 4-quadrant power supply.

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

Electronic loads can operate in constant voltage mode, also known as CV mode. In CV mode, the load adjusts the voltage across it to maintain a constant voltage value by regulating the current flowing through it. 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.

Some loads (such as the M9700 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.

Electronic loads can operate in constant voltage (or CV) mode. In constant voltage mode, the load adjusts the current flowing through it to regulate the voltage at its terminals to keep it at a constant value. Therefore, constant voltage mode can be used to create a voltage sweep: use the load to control the voltage output by the solar cell, and then measure the resulting current (as shown in Figure 2). Some loads (such as the Agilent N3300 series) can quickly execute a CV setpoint list to sweep the output voltage in constant voltage mode, thereby quickly drawing an IV curve. At the same time, the load can convert the current waveform flowing from the solar cell to the load into a digital waveform (similar to capturing an oscilloscope trace). By plotting the swept controlled CV voltage and the actual current image converted to digital, you can create an IV curve. Because this can all be done in a short period of time as a fast sweep, the entire test can be achieved in about one second, that is, before the cell heats up and the temperature changes due to the intense light source.

Figure 2: IV curve measured using an electronic load in CV mode

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Determine maximum power using the product of V and I

Many electronic loads have a lower operating voltage limit because most electronic loads are designed based on FETs. To conduct current correctly, FETs require a minimum voltage across the FET, which means there is a minimum operating voltage between the + and – input terminals of the load. Typically, the minimum input voltage of an electronic load is 2 to 3 W. Connecting a DC power supply in series with the electronic load can eliminate this limitation. Referring to Figure 3, the DC power supply used to provide compensation voltage to the electronic load is called a compensation power supply. Typically, the compensation power supply is set to 3 V to ensure that the minimum voltage requirement of the electronic load is met. The voltage of the DC power supply will not affect the solar cell. The DC power supply is a floating device and will bias the solar cell by a maximum of 3 V.

Figure 3: Electronic load and compensation power supply configured for solar cell testing

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Conclusion and more information

The urgent global demand for clean, renewable energy is driving the rapid development of solar cell technology. As solar cells increase in size and efficiency, battery testing may encounter greater current and power, so the market needs more flexible test equipment. At this time, the complete solution may not meet the needs, and engineers can use existing electronic loads to test solar cells. If configured and applied properly, electronic loads can be used to make all power-related measurements of solar cell or solar cell module output. Electronic loads currently on the market offer a wide range of voltage, current, power and measurement accuracy. The combination of loads, digital multimeters and data acquisition equipment can meet your measurement needs when the complete system is not flexible enough.