Thomas Stoffel

Measurement of Broadband Diffuse Solar Irradiance Using Current Commercial Instrumentation with a Correction for Thermal Offset Errors

Abstract Diffuse-sky solar irradiance is an important quantity for radiation budget research, particularly as it relates to climate. Diffuse irradiance is one component of the total downwelling solar irradiance and contains information on the amount of downward-scattered, as opposed to directly transmitted, solar radiation. Additionally, the diffuse component is often required when calibrating total irradiance radiometers. A variety of pyranometers are commonly used to measure solar diffuse irradiance. An examination of some instruments for measuring diffuse irradiance using solar tracking shade disks is presented, along with an evaluation of the achieved accuracy. A data correction procedure that is intended to account for the offset caused by thermal IR exchange between the detector and filter domes in certain common diffuse pyranometers is developed and validated. The correction factor is derived from outputs of a collocated pyrgeometer that measures atmospheric infrared irradiance.

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.

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.

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.

Joint EPRI/SERI project to evaluate solar radiation measurement systems for electric utility solar radiation resource assessment

A description is presented of a cooperative project of the Electric Power Research Institute (EPRI) and the Solar Energy Research Institute (SERI) to compare several solar radiation measurement systems as options for performing solar radiation resource assessments in a utilitys service territory. Preliminary results are given for a six-month field comparison of SERIs measurement systems with two different implementations of the rotating shadowband radiometer concept.<<ETX>>

Accuracy of silicon versus thermopile radiometers for daily and monthly integrated total hemispherical solar radiation

Measurement stations for solar radiation resource assessment data are expensive and labor intensive. For this reason, long-term solar radiation measurements are not widely available. Growing interest in solar renewable energy systems has generated a great number of questions about the quality of data obtained from inexpensive silicon photodiode radiometers versus costly thermopile radiometers. We analyze a year of daily total and monthly mean global horizontal irradiance measurements derived from 1-minute averages of 3-second samples of pyranometer signals. The data were collected simultaneously from both types of radiometers at the Solar Radiation Research Laboratory (SRRL) operated by the National Renewable Energy Laboratory in Golden, Colorado. All broadband radiometers in service at SRRL are calibrated annually using an outdoor method with reference radiometers traceable to the World Radiometric Reference. We summarized the data by daily total and monthly mean daily total amounts of solar radiation. Our results show that systematic and random errors (identified in our outdoor calibration process) in each type of radiometer cancel out over periods of one day or more. Daily total and mean monthly daily total solar energy measured by the two pyranometer types compare within 1% to 2%. The individual daily variations among different models of thermopile radiometers may be up to twice as large, up to ±5%, being highest in the winter (higher average solar zenith angle conditions) and lowest in summer, consistent with the lower solar zenith angle conditions.

Performance testing using silicon devices - Analysis of accuracy

Accurately determining PV module performance in the field requires accurate measurements of solar irradiance reaching the PV panel (i.e., Plane-of-Array - POA Irradiance) with known measurement uncertainty. Pyranometers are commonly based on thermopile or silicon photodiode detectors. Silicon detectors, including PV reference cells, are an attractive choice for reasons that include faster time response (10 microseconds (μs)) than thermopile detectors (1 s to 5 s) and lower cost and maintenance. The main drawback of silicon detectors is their limited spectral response. Therefore, to determine broadband POA solar irradiance, a pyranometer calibration factor that converts the narrowband response to broadband is required. Normally, this calibration factor is a single number determined under clear-sky conditions with respect to a broadband reference radiometer. The pyranometer is then used for various scenarios including varying airmass, panel orientation, and atmospheric conditions. This would not be an issue if all irradiance wavelengths that form the broadband spectrum responded uniformly to atmospheric constituents. Unfortunately, the scattering and absorption signature varies widely with wavelength and the calibration factor for the silicon photodiode pyranometer is not appropriate for other conditions. This paper reviews the issues that will arise from the use of silicon detectors for PV performance measurement in the field based on measurements from a group of pyranometers mounted on a 1-axis solar tracker. We also present a comparison of simultaneous spectral and broadband measurements from silicon and thermopile detectors and estimated measurement errors when using silicon devices for both array performance and resource assessment.

Southern Nevada Renewable Resource Assessment

University of Nevada, Las Vegas Renewable Energy Center (UNLV-REC) currently monitors three meteorological stations in southern Nevada under the direction of the National Renewable Energy Laboratory (NREL) and is funded by the Nevada Southwest Energy Partnership (NSWEP). The three station locations are Eldorado Valley, UNLV-REC Solar Site, and Nevada Power Company Clark Station. The installation dates for each of the locations were October of 2004 for Eldorado Valley station, August of 2003 for the UNLV-REC Solar Site, and March of 2006 for the Nevada Power Clark Station. Publicly available data from each site have been archived since installation completion. This paper discusses the installation of the equipment for each site and images of the setup. The data that is being collected between the sites is also compared. Data comparisons between the sites include net monthly solar energy; monthly peak direct normal irradiance (DNI), average daily wind speed, monthly wind roses, and average monthly dry bulb temperatures. The recently measured data is also compared to resource maps developed by NREL and to TMY data. With these meteorological resources, microclimatic variations can be studied for the area and used as a renewable energy resource for renewable installations in southern Nevada.Copyright