Regardless of the test method, you must know the plane of array irradiance and cell temperature to evaluate PV circuit performance. Pay attention to environmental conditions to ensure that you can interpret your I-V curves with accuracy, as rapid changes in the irradiance or cell temperature can introduce errors to your I-V curve tests. Proper sensor types and test methods like the Fluke Solmetric PVA 15i-V curve tracer should be used for reliable results.
Environmental conditions for testing
Optimal performance tests are conducted under stable weather conditions with irradiance above 700 W/m2, crucial when establishing a performance baseline at the commissioning or recommissioning process during troubleshooting. The standard test condition irradiance is 1000 W/m2, and the closer the field test conditions are to standard test conditions, the more accurate the interpretation of I-V curves will be. Good test conditions will most likely occur during the 4-hour window around solar noon.
Irradiance measurements and their impact
Irradiance measurement errors can significantly affect photovoltaic performance testing. For instance, a small error margin in irradiance can overshadow the accuracy of even high-quality I-V curve tracers like the Fluke Solmetric PVA -1500. Fast-moving clouds near the sun and high-elevation cirrus clouds are particularly problematic. One of the benefits of using I-V curve tracers for performance test measurements is that you can save critical environmental data along with the I-V data. This eliminates manual data entry errors that can cause trouble later, and minimises the opportunity for errors associated with rapid changes in test conditions.
Choice of sensors
True pyranometers are not a good choice for I-V curve testing as they have a wide, flat spectral response that differs from that of crystalline and thin-film module technologies. Hand-held irradiance sensors are not a good fit as they can be challenging to orient reliably in the plane of the array. Hand-held irradiance sensors may also have an angular response that differs substantially from fielded PV modules. Angular response is more significant early or later in the dat, or on days when the clouds scatter a significant amount of sunlight. Under these test conditions, the array and sensor must have an equally wide sky view.
Reflective light influence
Irradiance sensors should not be influenced by strong optical reflections, as this can lead to inaccurate readings. If the irradiance sensor picks up significantly more reflected light than the PV modules under test, the model will overpredict ISC, and the module will appear to be underperforming. Under certain circumstances, sunlight reflected from metal surfaces can greatly exaggerate the irradiance reading. You can usually remedy this by changing the sensor mounting location.
Temperature measurements in photovoltaic systems
While PV module performance is less sensitive to temperature variations than irradiance, it’s still a significant factor. Light-gauge thermocouples are preferred for measuring cell temperature under varying conditions. Positioning the thermocouple correctly is vital for accurate readings. Since array and module edges tend to run cool, position the thermocouple between the corner and the centre of a module located away from the cooler array perimeter. This practice aims to select a sensor attachment point that approximates the average backside temperature. The tip of the thermocouple must make good contact with the back of the PV module as air gaps will interrupt heat transfer, resulting in low temperature readings. When moving the thermocouple between identical array sections, place it at the same relative location each time to avoid introducing artificial temperature shifts.
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