Eos IAM Characterization System
Indoor IAM Characterization System for
The Research-Grade Incidence Angle Modifier Characterization
Photovoltaic (PV) cells and modules are not created equally, especially regarding their finer performance parameters. Not only should cell short-circuit current at normal incidence [ISC(θ=0)] be maximized, the dependence of short-circuit current on angle-of-incidence [ISC(θ)] should follow the cosine law as closely as possible – perhaps even exceeding it with state-of-the-art optical designs. Careful optimization of ISC(θ) is particularly important for building-integrated PV (BIPV) applications and other fixed-tilt deployments. ISC(θ) can be optimized by careful choices of optical components, their interfaces, and the manufacturing process. Since the manufactured properties of each will affect ISC(θ), high-resolution characterization of ISC(θ) of manufactured photovoltaics will reveal system-level details that are otherwise difficult to uncover.
Eos is the research-grade incidence angle modifier (IAM) characterization system for all photovoltaic devices. The turn-key Eos system consists of an adjustable and highly collimated broadband “white” light source, a light-source laser-alignment system, a stepper-motor-controlled sample stage, high-resolution measurement electronics, and a supplied PC with software that enables fully automated measurements. The Eos system is customized for each end-user’s application, particularly with respect to PV device form-factor and available working distance. An important system parameter is the working distance between the light-source and the sample stage, as light-source collimation increases with working distance. The working distance is mainly limited by space contraints of the installation location. Measurement acquisition times increase with angle-of-incidence (AOI), with larger AOI values corresponding to less measurement signal. Aquisition times are minimized at each AOI using an optimized low-noise measurement architecture and statistical signal-processing algorithms. These system features enable unmatched measurement resolutions.
Figure 1 is a comparison of Eos IAM characterization data acquired from a typical silicon PV module with a historically applied default IAM model. The simplified ASHRAE model underestimates relative response at lower AOIs and overestimates relative response at AOIs above approximately 70°.
Figure 1: Eos IAM characterization data of a typical silicon PV module compared to the historically accepted ASHRAE model
The Eos characterization data are derived from the AVG column of Table 1.
Eos measurement repeatability data taken over thirteen independent measurement trials of a typical 72-cell silicon PV module are provided in Table 1. For these measurement trials, the test module was removed from and reinstalled in the sample stage between trials. The measurements were otherwise fully automated.
Table 1: Eos IAM measurement repeatability data
The data were acquired from a single silicon PV module over thirteen trials with the averages (AVG) and the standard deviations (STDEV) of the measurement series.
In order to demonstrate the value provided by Eos, twelve different silicon PV module models made by various manufacturers were characterized. Three samples of each model were measured to determine possible sample-to-sample variations. These data are provided in Table 2. The minimum, average, and maximum measured IAM values at each AOI over the set of thirty-six PV modules are presented to highlight the ranges of IAM performances possible between competing PV module manufacturers. IAM values greater than one can indicate enhanced optical coupling (independent of the cosine law) of incident irradiance into the active material relative to normal incidence.
Table 2: Eos IAM characterization data of twelve different silicon PV modules from various manufacturers
Three samples were characterized per module model and the average values (AVG) are provided along with the standard deviations (STDEV) of the three samples.
Explanation of the Aggregate Statistics columns:
- The MIN column provides the minimum values measured across the three samples of twelve module types (over a total of thirty-six module samples).
- The AVG column provides the average values measured across the three samples of twelve module types (for a total of thirty-six module samples).
- The MAX column provides the maximum values measured across the three samples of twelve module types (for a total of thirty-six module samples).
- The STDEV column provides the average standard deviation values of twelve module models (i.e. the standard deviation values of each module model averaged over the twelve module models). This method of evaluation was chosen because differences between module models should not be reflected in measurement system statistics. The point being made is that the measurement repeatability errors in addition to possible sample-to-sample variations within a model set are less than the measured performance range over the module models.
Eos measurement errors are provided in Table 3. It’s important to note that when comparing PV modules with identical cell dimensions (e.g. standard six-inch polycrystalline PV cells), the relative measurement error (for example, when comparing the relative performances of test samples or products) is simply the repeatability error. Therefore, the accuracies of product comparisons are unmatched by other IAM characterization techniques.
Table 3: Eos raw measurement repeatability errors and expanded total measurement errors
The repeatability errors are simply the ratio of the STDEV values to the AVG values of Table 1 (in percent), while the expanded total measurement error values also include systematic (bias) errors and are estimated according to the ISO/IEC Guide 98-3:2008 – Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in measurement (GUM:1995).
