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How much current does a solar cell provide?

PTB calibrates reference solar cells for real conditions by means of a novel measuring setup

PTB-News 3.2016
01.09.2016
Especially interesting for

manufacturers of solar cells

operators of solar power facilities

Conventional measurement procedures for determining the performance of solar cells make it difficult to obtain yield forecasts under real operating conditions. By means of a novel measuring setup which uses a laser-based spectral measurement procedure, PTB researchers have successfully characterized solar cells comprehensively enough that their yield can be calculated for any given climatic condition. This measuring setup also allows reference solar cells to be calibrated under the established standard test conditions (STCs) with a measurement uncertainty of less than 0.4 % – the only setup in the world to achieve an uncertainty this low.

In this PTB laboratory, solar cells are irradiated with white light as well as withmonochromatic light (via the opening in the middle).

Because of the boom in the renewable energy market, it will become increasingly important in the future to compare the performance of solar cells, as they are different in terms of their efficiency. A cell made by one manufacturer may achieve greater electrical power at a certain level of solar radiation (irradiance) than a comparable cell made by another manufacturer. Here, although electrical power can be measured with relative ease, it is considerably more difficult to determine irradiance. To this end, PTB calibrates reference solar cells whose short-circuit current is a measure of irradiance. Shortcircuit current is the largest possible current which a module or a cell can produce. Calibration takes place under standard test conditions (STCs), i.e., under irradiation with a standardized solar spectrum, at 1000 watts per square meter and a solar cell temperature of 25 °C. However, in reality, the spectrum of sunlight varies depending on the time of day and on the season, as well as on the atmospheric composition; furthermore, the temperature, the angle of incidence and the irradiance are different depending on where the module/cell is installed.

To be able to generate yield forecasts for these cells, which are used throughout the world, PTB has expanded its solar cell measuring setup. For comparability measurements, the so-called differential spectral responsivity (DSR) method is used; this method was recently further developed to become the laser DSR method. This method allows the yield for any given climatic condition to be calculated, such as for solar cell temperatures between 15 °C and 75 °C, and for irradiances from 0 W/m2 to over 1100 W/m2. In addition, the wavelength and the angle of incidence of the light can be varied. Ultimately, all of these measurements allow the performance of different solar cells to be compared.

For conventional (lamp-based) DSR procedures, white light is separated into individual wavelengths and temporally modulated by means of a monochromator and a chopper wheel, and then directed onto the solar cell via an optical element. In this way, the system can be set to all colors from ultraviolet to infrared light. At the same time, the cell is irradiated with white light, as this is the only way to achieve the 1000 W/m2 required for the measurement. Yet the current generated by means of white light is up to a billion times greater than that generated by means of monochromatic light, often leading to a poor signal-to-noise ratio. By means of the laser-based DSR procedure, the noise problem – depending on the wavelength – has successfully been reduced by a factor of 100 to 10 000. In this way, the overall measurement uncertainty has been improved to the record value of less than 0.4 %. One additional advantage should be mentioned: Until now, only reference solar cells of a size of 20 mm × 20 mm could be calibrated. With the new facility, frequenit has become possible to calibrate solar cells with sizes of up to 15 cm × 15 cm. The expanded portfolio of calibration services with the reduced measurement uncertainties mentioned above is passed on to testing and calibration laboratories, where these services make more precise traceability possible.

Contact

Ingo Kröger
Department 4.1
Photometry and Applied Radiometry
Phone: +49 (0)531 592-4147
ingo.kroeger(at)ptb.de

Scientific publication

S. Winter, T. Fey, I. Kröger, D. Friedrich, K. Ladner, B. Ortel, S. Pendsa, F. Witt: Design, realization and uncertainty analysis of a laser-based primary calibration facility for solar cells at PTB. Measurement 51, 457–463 (2014)