Daryl R. Myers

Projected performance of three- and four-junction devices using GaAs and GaInP

This paper explores the efficiencies expected for three- and four-junction devices for both space and terrestrial applications. For space applications, the effects of temperature and low concentration are investigated. For terrestrial applications, a concentration of 500 suns is assumed and the theoretical efficiencies are calculated as a function of spectral variations including the effects of air mass, turbidity, and water-vapor content.

Recent Progress in Reducing the Uncertainty in and Improving Pyranometer Calibrations

The Measurements and Instrumentation Team within the Distributed Energy Resources Center at the National Renewable Energy Laboratory, NREL, calibrates pyranometers for outdoor testing solar energy conversion systems. The team also supports climate change research programs. These activities led NREL to improve pyranometer calibrations. Low thermal-offset radiometers measuring the sky diffuse component of the reference solar irradiance removes bias errors on the order of 20 Watts per square meter (W/m 2 ) in the calibration reference irradiance. Zenith angle dependent corrections to responsivities of pyranometers removes 15 to 30 W/m 2 bias errors from field measurements. Detailed uncertainty analysis of our outdoor calibration process shows a 20% reduction in the uncertainty in the responsivity of pyranometers. These improvements affect photovoltaic module and array performance characterization, assessment of solar resources for design, sizing, and deployment of solar renewable energy systems, and ground-based validation of satellite-derived solar radiation fluxes.

Spectral solar radiation data base at SERI

Abstract This article describes the results to date of a cooperative project among the Solar Energy Research Institute (SERI), the Electric Power Research Institute, the Florida Solar Energy Center (FSEC), and the Pacific Gas and Electric Company (PG&E) to build a spectral solar radiation data base. The objective is to build a data base representing a range of air masses and atmospheric conditions (or climates) that is applicable to different photovoltaic (PV) collector types and is accessible by the PV and general scientific community. Approximately 1300 spectra and concurrent broadband solar radiation and meteorological data have been collected at FSEC and PG&E and archived at SERI in the data base as of October 1987.

A comparison of photovoltaic module performance evaluation methodologies for energy ratings

The rating of photovoltaic (PV) modules has always been a controversial topic in the PV community. There is no industry standard methodology to evaluate PV modules for energy production. This issue must be discussed and resolved for the benefit of system planners, utilities, and other consumers. Several methodologies are available to rate a modules peak power, but do any accurately predict energy output for flat-plate modules? This paper analyzes the energy performance of PV modules using six different energy calculation techniques and compares the results to the measured amount of energy produced. The results indicate which methods are the most effective for predicting energy output in Golden, Colorado, under prevailing meteorological conditions.

What is the appropriate reference spectrum for characterizing concentrator cells

Consensus standards for determining the efficiency of a concentrator cell or module have not been developed. NREL, Sandia National Laboratory, the Fraunhofer Institute for Solar Energy in Germany, and the Progress in Photovoltaics Efficiency Table authors have informally agreed upon concentrator-cell reference conditions. These conditions are 25/spl deg/C cell temperature, 1-sun = 1000 W/m/sup 2/ total irradiance, and the ASTM E891-87 direct-normal reference spectrum. Deficiencies in the direct reference spectrum are discussed, and a more representative reference spectrum for evaluating concentrator cells is proposed. The spectrum was generated by the SMARTS model, and the atmospheric parameters are as close as possible to the existing direct spectrum, with the exception that the aerosol optical depth at 500 nm is reduced from 0.27 to 0.085.

Revising and Validating Spectral Irradiance Reference Standards for Photovoltaic Performance Evaluation

In 1982, the American Society for Testing and Materials (ASTM) adopted consensus standard direct-normal and global-tilted solar terrestrial spectra (ASTM E891/E892). These standard spectra were intended to evaluate photovoltaic (PV) device performance and other solar-related applications. The International Standards Organization (ISO) and International Electrotechnical Commission (IEC) adopted these spectra as spectral standards ISO 9845-1 and IEC 60904-3. Additional information and more accurately representative spectra are needed by today’s PV community. Modern terrestrial spectral radiation models, knowledge of atmospheric physics, and measured radiometric quantities are applied to develop new reference spectra for consideration by ASTM.Copyright

Primary reference cell calibrations, at SERI: history and methods

The tabular calibration method used at the Solar Energy Research Institute (SERI) for primary reference solar cells is derived and described in detail. An uncertainty analysis shows that the tabular method should have a total uncertainty of +or-1.0%; data from five years of calibrations are then shown to support this analysis. Results from SERIs tabular method are compared with terrestrial calibrations performed by the NASA Lewis Research Center in the late 1970s.<<ETX>>

An Extensive Comparison of Commercial Pyrheliometers under a Wide Range of Routine Observing Conditions

In the most comprehensive pyrheliometer comparison known to date, 33 instruments were deployed to measure direct normal solar radiation over a 10-month period in Golden, Colorado. The goal was to determine their performance relative to four electrical-substitution cavity radiometers that were calibrated against the World Radiometric Reference (WRR) that is maintained at the World Radiation Center in Davos, Switzerland. Because of intermittentcabling problems with one of the cavity radiometers, the average of three windowed, electrical-substitution cavity radiometers served as the reference irradiance for 29 test instruments during the 10-month study. To keep the size of this work manageable, comparisons are limited to stable sunny conditions, passing clouds, calm and windy conditions, and hot and cold temperatures. Other variables could have been analyzed, or the conditions analyzed could have employed higher resolution. A more complete study should be possible now that the instruments are identified; note that this analysis was performed without any knowledge on the part of the analyst of the instruments’ manufacturers or models. Apart from the windowed cavities that provided the best measurements, two categories of performance emerged during the comparison. All instruments exceeded expectations in that they measured with lower uncertainties than the manufacturers’ own specifications. Operational 95% uncertainties for the three classes of instruments, which include the uncertainties of the open cavities used for calibration, were about 0.5%, 0.8%,and 1.4%.The open cavitiesthat wereused for calibrationof allpyrheliometers havean estimated 95% uncertainty of 0.4%‐0.45%, which includes the conservative estimate of 0.3% uncertainty for the WRR.

Using a Blackbody to Calculate Net Longwave Responsivity of Shortwave Solar Pyranometers to Correct for Their Thermal Offset Error during Outdoor Calibration Using the Component Sum Method

Abstract Thermopile pyranometers’ thermal offset has been recognized since the pyranometer’s inception. This offset is often overlooked or ignored because its magnitude is small compared to the overall solar signal at higher irradiance. With the demand of smaller uncertainty in measuring solar radiation, recent publications have described a renewed interest in this offset, its magnitude, and its effect on solar measurement networks for atmospheric science and solar energy applications. Recently, it was suggested that the magnitude of the pyranometer thermal offset is the same if the pyranometer is shaded or unshaded. Therefore, calibrating a pyranometer using a method known as the shade/unshade method would result in accurate responsivity calculations because the thermal offset error is canceled. When using the component sum method for the pyranometer calibration, the thermal offset error, which is typically negative when the sky is cloudless, does not cancel, resulting in an underestimated shortwave resp...

Pyrgeometer calibration at the National Renewable Energy Laboratory (NREL)

Abstract Pyrgeometers are used to measure longwave terrestrial radiation. Regular pyrgeometer calibration against an internationally recognized standard is required in order to measure the longwave radiation consistently at different sites around the globe. At present, there is no internationally recognized standard to calibrate pyrgeometers. A well-characterized blackbody is, however, an accepted approach. This paper describes a method of establishing a precise blackbody reference and using it to calibrate a group of four transfer reference pyrgeometers. The group is then deployed outdoors to evaluate the precision of the blackbody calibration. The results from the outdoor data shows that the percentage mean-square-error of each transfer reference pyrgeometer is 0.12%, 0.07%, 0.46%, and 0.10% with a resultant percentage root-mean-square of 0.43%. The errors are calculated with respect to the average of the irradiance readings of the transfer reference pyrgeometers. To minimize the number of transfer reference pyrgeometers and to allow more space for calibrating test pyrgeometers, a sub-set of the transfer reference pyrgeometers is then used to calibrate a test pyrgeometer outdoors. The calibration of the test pyrgeometer resulted in reducing its error from +4.00% to ±0.32% with respect to the irradiance measured by the sub-set of the transfer reference pyrgeometers. The outdoor calibration method can minimize the calibration cost resulting from using the lengthy and costly blackbody calibration because many pyrgeometers can be calibrated at the same time. Appendix A shows a diagram that describes the papers concept.