Aron Habte

Evaluation of Radiometers Deployed at the National Renewable Energy Laboratory's Solar Radiation Research Laboratory

This study analyzes the performance of various commercially available radiometers used for measuring global horizontal irradiances and direct normal irradiances. These include pyranometers, pyrheliometers, rotating shadowband radiometers, and a pyranometer with fixed internal shading and are all deployed at the National Renewable Energy Laboratorys Solar Radiation Research Laboratory. Data from 32 global horizontal irradiance and 19 direct normal irradiance radiometers are presented. The radiometers in this study were deployed for one year (from April 1, 2011, through March 31, 2012) and compared to measurements from radiometers with the lowest values of estimated measurement uncertainties for producing reference global horizontal irradiances and direct normal irradiances.

Uncertainty of rotating shadowband irradiometers and Si-pyranometers including the spectral irradiance error

Concentrating solar power projects require accurate direct normal irradiance (DNI) data including uncertainty specifications for plant layout and cost calculations. Ground measured data are necessary to obtain the required level of accuracy and are often obtained with Rotating Shadowband Irradiometers (RSI) that use photodiode pyranometers and correction functions to account for systematic effects. The uncertainty of Si-pyranometers has been investigated, but so far basically empirical studies were published or decisive uncertainty influences had to be estimated based on experience in analytical studies. One of the most crucial estimated influences is the spectral irradiance error because Si-photodiode-pyranometers only detect visible and color infrared radiation and have a spectral response that varies strongly within this wavelength interval. Furthermore, analytic studies did not discuss the role of correction functions and the uncertainty introduced by imperfect shading. In order to further improve the...

Optimization of multijunction solar cells through indoor energy yield measurements

The variability of the solar spectra in the field may reduce the annual energy yield of multijunction solar cells. It would, therefore, be desirable to implement a cell design procedure based on the maximization of the annual energy yield. In this study, we present a measurement technique to generate maps of the real performance of the solar cell for a range of light spectrum contents using a solar simulator with a computer-controllable spectral content. These performance maps are demonstrated to be a powerful tool for analyzing the characteristics of any given set of annual spectra representative of a site and their influence on the energy yield of any solar cell. The effect of luminescence coupling on buffering against variations of the spectrum and improving the annual energy yield is demonstrated using this method.

Best Practices Handbook for the Collection and Use of Solar Resource Data for Solar Energy Applications: Second Edition

This report presents detailed information about solar resource data and the resulting data products needed for each stage of a solar energy project, from initial site selection to systems operations. It also contains a summary of solar forecasting and its development throughout the last few years. The U.S. Department of Energys Solar Energy Technologies Office, project developers, engineering procurement construction firms, utility companies, system operators, energy suppliers, financial investors, and others involved in solar energy systems planning and development will find this handbook to be a valuable resource for the collection and interpretation of solar resource data. This handbook is expected to be used as a reference during each project stage.

Energy yield determination of concentrator solar cells using laboratory measurements

The annual energy conversion efficiency is calculated for a four junction inverted metamorphic solar cell that has been completely characterized in the laboratory at room temperature using measurements fit to a comprehensive optoelectronic model of the multijunction solar cells. A simple model of the temperature dependence is used predict the performance of the solar cell under varying temperature and spectra characteristic of Golden, CO for an entire year. The annual energy conversion efficiency is calculated by integrating the predicted cell performance over the entire year. The effects of geometric concentration, CPV system thermal characteristics, and luminescent coupling are highlighted.

Quantifying the impact of incidence-angle dependence on solar radiometric calibration

Evaluating photovoltaic cells, modules, arrays, and system performance relies on accurate measurements of the solar radiation resources available for power conversion. Measuring solar resources accurately can lead to a reduction in the investment risks associated with installing and operating solar energy systems. The National Renewable Energy Laboratorys Solar Radiation Research Laboratory collects and disseminates solar irradiance data and provides calibrations of broadband radiometers that are traceable to the international standards. It is essential that radiometric data are traceable to the international system of units, e.g., through the World Radiometer Reference and World Infrared Standard Group. This paper demonstrates the importance and application of an existing approach that ultimately reduces the uncertainty of radiometric measurements. Almost all commercially available broadband radiometers use a single responsivity value that is generated at a 45° solar zenith angle (incident angle) based on outdoor calibrations or transfers between radiometers inside integrating spheres or that responsivity is generated using normal incident radiation based on indoor calibrations using lamps and comparisons to reference radiometers to compute measured irradiance data. However, based on our experience and that of other experts in the radiometric science community, this method introduces increased uncertainty to the data. If a single responsivity value is used, the radiometer will overestimate or underestimate the irradiance data compared to the reference irradiance. This was demonstrated in Myers [1], Reda [2], and Reda et al. [3]. Further, by using responsivity as a function of solar zenith angle, the uncertainty for some instruments in the responsivity value can be reduced by as much as 50% compared to using a single responsivity calculated at 45° [2, 3].

Field spectra binning for energy production calculations and multijunction solar cell design

Annual spectra sets must be used for accurate energy production prediction and multijunction solar cell design for maximum energy production at a specific site. These spectra sets contain a large quantity of data that is cumbersome to manage during solar cell design calculations and impractical to reproduce in solar simulators for indoor energy production measurements. However, it should be possible to bin together spectra with similar spectral contents, and then use this reduced set with little loss of accuracy. We present two binning algorithms which judiciously bin together similar spectra to create a much smaller “proxy” set, for which the total measurement time, energy production calculation and solar cell optimization decreases to a matter of seconds. These algorithms are assessed against their accuracy in representing the whole spectra sets for solar cell design and energy production prediction. We find that a set of just five spectra fulfills this requirement. In addition, the sets of proxy spectra act as “fingerprints” of specific sites, and provide an efficient and effective way to understand how cell design and performance vary from site to site. Furthermore, the process of reducing a full data set to a few proxy spectra can help assess the quality of the dataset for multijunction applications, and contribute to improvements to the datasets and data collection methods.

Improved Instrumentation for Solar Resource Measurements and Forecasting: Cooperative Research and Development Final Report, CRADA Number CRD-09-349

of CRADA Work: Under this Agreement, NREL will work with Participant to improve the instrumentation and measurement systems available for measuring and monitoring solar radiation elements needed by the electric utilities and solar power system integrators to adequately characterize the spatial and temporal variations of the renewable energy resources. This work includes, but is not limited to, research and development for making cost effective radiometers, solar trackers, and related instrumentation for solar resource measurements and forecasting. Cooperative R&D is proposed in three areas: Improved radiometric systems for cost effective irradiance measurements (direct normal, global, diffuse and plane-of-array); evaluation of innovative disruptive technology approaches for deriving the constituent solar energy components utilizing new fast-response thermopile measurement technologies, coupled with innovative shading geometry and tracker motion control; and low-cost solutions for sky imaging needed for resource forecasting. This work will be conducted at NREL and Participant facilities.

Shortwave radiometer calibration methods comparison and resulting solar irradiance measurement differences: A user perspective

Banks financing solar energy projects require assurance that these systems will produce the energy predicted. Furthermore, utility planners and grid system operators need to understand the impact of the variable solar resource on solar energy conversion system performance. Accurate solar radiation data sets reduce the expense associated with mitigating performance risk and assist in understanding the impacts of solar resource variability. The accuracy of solar radiation measured by radiometers depends on the instrument performance specification, installation method, calibration procedure, measurement conditions, maintenance practices, location, and environmental conditions. This study addresses the effect of different calibration methods provided by radiometric calibration service providers, such as NREL and manufacturers of radiometers, on the resulting calibration responsivity. Some of these radiometers are calibrated indoors and some outdoors. To establish or understand the differences in calibration methodology, we processed and analyzed field-measured data from these radiometers. This study investigates calibration responsivities provided by NRELs broadband outdoor radiometer calibration (BORCAL) and a few prominent manufacturers. The BORCAL method provides the outdoor calibration responsivity of pyranometers and pyrheliometers at 45° solar zenith angle, and as a function of solar zenith angle determined by clear-sky comparisons with reference irradiance. The BORCAL method also employs a thermal offset correction to the calibration responsivity of single-black thermopile detectors used in pyranometers. Indoor calibrations of radiometers by their manufacturers are performed using a stable artificial light source in a side-by-side comparison between the test radiometer under calibration and a reference radiometer of the same type. In both methods, the reference radiometer calibrations are traceable to the World Radiometric Reference (WRR). These different methods of calibration demonstrated +1% to +2% differences in solar irradiance measurement. Analyzing these differences will ultimately help determine the uncertainty of the field radiometer data and guide the development of a consensus standard for calibration. Further advancing procedures for precisely calibrating radiometers to world reference standards that reduce measurement uncertainty will allow more accurate prediction of solar output and improve the bankability of solar projects.

Comparing Measured and Satellite-Derived Surface Irradiance

The purpose of this study is two-fold: 1) To examine the performance of the Global Solar Insolation Project (GSIP) physics-based model in characterizing global horizontal solar radiation across the United States by comparing to the ground measured data, and 2) to examine improvements of the GSIP data to address temporal and spatial variations. The study enumerates and examines the spatial and temporal limitations of the GSIP model. Most comparisons demonstrate relatively good statistical agreement. However, the methodology used in the satellite model to distinguish microclimate conditions presents significant challenges, and the model requires refinement in addressing aerosol estimates, water vapor estimates, and clear sky optical properties. Satellite derived datasets are only available at half-hour intervals. Surface measurement can easily be made at temporal resolution in the order of seconds. Therefore intra-hour variability, an important quantity for understanding how power production in power plants will vary, cannot be directly derived from satellites. This paper illustrates how intra-hour variability in ground measurements cannot be captured by the satellite based datasets. We also discuss the potential for improved next-generation geostationary satellite data to improve the accuracy of surface radiation estimates.Copyright