K. Emery

Spectral effects on PV-device rating

Abstract A recently developed spectral model “SEDES2” is applied to study the effect of variations in solar spectral irradiance on the efficiency of seven particular solar cells. As a new feature, SEDES2 calculates hourly solar spectral irradiance for clear and cloudy skies from readily available site-specific meteorological data. Based on these hourly spectra, monthly and yearly efficiencies for the solar cells are derived. As a key result the efficiencies of amorphous silicon cells differ by 10% between winter and summer months because of spectral effects only. A second intention of this study is to analyse the sensitivity of power and energy rating methods to spectral irradiance but also to total irradiance and cell temperature. As an outcome, a multi-value energy rating scheme applying the concept of “critical operation periods” is proposed.

Spectral corrections based on optical air mass

The measurement of the photovoltaic (PV) performance with respect to reference conditions requires measuring the performance with respect to a reference spectrum. Procedures were developed in the mid 1980s to correct measurements for errors relating to the spectral irradiance of the light source being different from the standard and the responsivity of the irradiance detector being different from the device under test. In principle, these procedures are exact, but require the measurement of the spectral irradiance of the light source and responsivity of the test device. This is problematic for most facilities that measure module performance. It has been suggested that a polynomial fit of the short-circuit current (I/sub sc/) measured under natural sunlight divided by the total broadband irradiance as a function of air mass provides an accurate spectral correction factor. The polynomial correction factor is normalized to unity at an absolute air mass of 1.5. The polynomial correction factor is compared with the spectral correction factor for a variety of devices at two locations.

Solar simulation-problems and solutions

The authors quantify the effects of bulb aging, reflections off the simulator optics and fixturing, spectral irradiance, and variations in temporal and spatial characteristics of solar simulators on PV device performance. As the distance between the integrating optics or bulb decreases, the spatial nonuniformity parallel and perpendicular to the test plane increases. A short focal length increases the possibility for an artificial increase in the measured current. The effect on the spectral irradiance and spectral mismatch error was studied for different bulb types, different bulbs of the same type, bulb age, bulb voltage, and distance from the bulb to the test plane.<<ETX>>

Improved test method to verify the power rating of a photovoltaic (PV) project

This paper reviews the PVUSA power rating method [1–6] and presents two additional methods that seek to improve this method in terms of model precision and increased seasonal applicability. It presents the results of an evaluation of each method based upon regression analysis of over 12 MW of operating photovoltaic (PV) systems located in a wide variety of climates. These systems include a variety of PV technologies, mounting configurations, and array sizes to ensure the conclusions are applicable to a wide range of PV designs and technologies. The work presented in this paper will be submitted to ASTM for use in the development of a standard test method for certifying the power rating of PV projects.

Photovoltaic module energy rating methodology development

A consensus-based methodology to calculate the energy output of a PV module is described in this paper. The methodology develops a simple measure of PV module performance that provides for a realistic estimate of how a module will perform in specific applications. The approach makes use of the weather data profiles that describe conditions throughout the United States and emphasizes performance differences between various module types. An industry-representative Technical Review Committee has been assembled to provide feedback and guidance on the strawman and final approach used in developing the methodology.

Outdoor rating conditions for photovoltaic modules and systems

Historically, #at-plate photovoltaic modules have been given a ‘peak-watta rating indicating the power generated under 1000 W/m2 global irradiance at a standard temperature. However, questions have arisen regarding the direct-normal irradiance, ambient or cell temperature, and wind speed (when it is specied) that should be used for evaluating the performance of #at-plate and concentrator modules. By studying the conditions that are observed when the global irradiance on a 2-axis-tracked surface is 1000 W/m2, our analysis provides an objective, quantitative basis for the choice of the ‘peak-watta rating conditions for both types of collectors. These observed conditions are consistent with commonly used values of 850 W/m2 for direct-normal irradiance and 203C for ambient temperature. Evidence is given that wind speed should be increased from the commonly used 1 m/s to a more frequently observed 4 m/s. ( 2000 Elsevier Science B.V. All rights reserved.

Criteria for the design of GaInP/GaAs/Ge triple-junction cells to optimize their performance outdoors

This paper investigates which reference spectrum should be used to design GalnP/GaAs/Ge triple-junction cells (at 300 K) in order to optimize their performance outdoors (at elevated temperatures). The outdoor performance is simulated using direct spectra from the recently proposed Module Energy Rating Procedure. We find that triple-junction cells designed for AM1.5D, low-AOD and AM1.5G standard spectra at 300 K all work well for maximizing daily energy production at elevated temperatures. AM1.5G cells are the best choice for midday power production, whereas AM1.5D cells are the best choice for power production during the morning and evening. Performance of cells optimized for a newly proposed Low-AOD spectrum is intermediate between these two extremes.

Terrestrial solar spectral modeling tools and applications for photovoltaic devices

Variations in terrestrial spectral irradiance on photovoltaic devices can be an important consideration in photovoltaic device design and performance. This paper describes three available atmospheric transmission models, MODTRAN, SMARTS2, and SPCTRAL2. We describe the basics of their operation and performance, and applications in the photovoltaic community. Examples of model input and output data and comparisons between the model results for each under similar conditions are presented. The SMARTS2 model is shown to be much easier to use, as accurate as the complex MODTRAN model, and more accurate than the historical NREL SPCTRAL2 model.

Outdoor meteorological broadband and spectral conditions for evaluating photovoltaic modules

The choice of conditions that should be used for rating photovoltaic modules is revisited to assess how well they represent the real world. One part of this study looks at the meteorological conditions and broadband characteristics of the irradiance when the global irradiance is 1000 W/m/sup 2/. Global normal irradiance on a 2-axis-tracked surface of 1000 W/m/sup 2/ was found to be correlated with a direct normal irradiance of 836 W/m/sup 2/, an ambient temperature of 23.7/spl deg/C, a wind speed of 4.5 m/s, a total water vapor of 1.4 cm, an aerosol optical depth of 0.08, and an air mass of 1.43. The second part of the study investigates the direct normal spectra that are commonly observed and concludes that of the two reference spectra that are commonly used, the global 37/spl deg/ tilt reference spectrum (ASTM E892) better represents observed data. Spectral modeling identifies a set of conditions for which the direct normal spectrum is very similar to ASTM E892.

Artificial enhancements and reductions in the PV efficiency

The researcher is under great pressure to improve the conversion efficiency of photovoltaic (PV) devices. This paper addresses methods of artificially enhancing or reducing the efficiency with respect to a given set of standard reference conditions. Artificial efficiency improvements can be caused by measurement procedures, the data acquisition system, device processing, and the definition of what the standard reference conditions are for a given device. Procedural methods of enhancing the efficiency include applying a static charge prior to measurement and improper contacting. The data acquisition system can change the efficiency by sweeping the device voltage too fast; allowing the calibration to drift; neglecting the spectral mismatch error; or having spatial nonuniformities in the illumination. PV devices can be processed to give a high initial efficiency at the expense of stability. The so-called active area is the most common definition-related method of artificially improving the efficiency. Efficiency differences for thermophotovoltaic devices are dominated by differences in the reference conditions caused by a lack of standardization.