I-V measurement at 25 degrees C and EL image shooting can be done at same time
I-V characteristic measurement under STC as well as EL (electro luminance) image shooting are done in the movable PV laboratory.
EL images are used to check for micro cracks in solar cells (power generation elements) and to check connection states between cells and inter connectors.
To use the limited space on the box-shape bed, the laboratory was designed so that two operations, the measurement of I-V characteristics and shooting of EL images, are performed continuously by one piece of measurement equipment (Fig. 3). The workload for panel setting and removal and operation of equipment was reduced, compared with the method where different instruments are used for the two operations.
Panels are fixed on a horizontal stage for I-V characteristic measurement, and, then, EL images are shot. The light irradiation equipment needed for I-V characteristic measurement, EL image shooting equipment and others are mounted below the bed. The height of the stage is changed because the distance from the panels for light irradiation to measure I-V characteristic is different from the distance for image shooting to obtain EL images. After I-V characteristic measurement, the stage is lifted up for EL image shooting.
The number of panels that can be measured in one day is 60 to 80 at maximum. According to the company, the bottleneck in increasing measurement speed is panel temperature control.
The box-type bed where the measurement is done is tightly closed during measurement, and the temperature is maintained at 25°C by using air conditioners and heaters. The interior walls are painted black, similar to the environment for indoor testing, to prevent the influences of light irradiation and shooting.
Panels are carried in from the rear of the bed, 20 sheets at a time, and are set against a rack. The temperature is controlled at 25°C.
The panels carried into the laboratory are often as hot as about 50°C, because, for example, they are subjected to sunlight. It takes about one hour to stabilize the panel temperature at 25°C.
When 20 panels are placed in the laboratory, two hours are required for the series of operations including temperature stabilization, measurement, shooting and panel replacement. Therefore, the maximum number of panels that can be measured and shot per day is 60 to 80.
In a sampling inspection performed at a mega solar power plant of about 3MW output, 60 solar panels were inspected, and this number was close to the maximum number of panels that can be measured in one day.