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F. Vignola

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Director of the University of Oregon Solar Radiation Monitoring Laboratory. Been in the solar resource assessment field for over 45 years. Semi-retired but still busy working on projects. My goal has been to provide the basic information for the planning, deployment, and operating solar electric system.

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University of Zagreb

Uppsala University

University of East London

University of Leicester

Gwen Robbins Schug

University of North Carolina at Greensboro

Gabriel Gutierrez-Alonso

University of Salamanca

Macquarie University

Universidade Federal do Rio Grande do Sul

Swansea University

Jesper Hoffmeyer

University of Copenhagen

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Papers by F. Vignola

Direct Normal Irradiance

Performance of PV Inverters

Proc. of the 37th ASES …, 2008

Testing a Model of IR Radiative Losses Preprint Testing a model of IR radiative losses

Thermopile pyranometers exhibit IR radiative losses that affect global and diffuse shortwave meas... more Thermopile pyranometers exhibit IR radiative losses that affect global and diffuse shortwave measurements made with first class thermopile based instruments. Pyrgeometers can be used to measure the sky temperature and are used to calculate the pyranometer's IR radiative losses. Few solar monitoring sites are equipped with pyrgeometers necessary to account for the IR radiative losses associated with the pyranometers. High quality data from the Solar Radiation Research Laboratory (SRRL) at the National Renewable Energy Laboratory are used to test and further develop a model for the IR radiative losses without the use of pyrgeometer data. The various methods for obtaining IR radiative loss values are compared and contrasted using the SRRL data. A simple scaling method is proposed and tested to adjust the non-pyrgeometer based correlation models to sites with different sky temperature characteristics.

Alternative method for characterizing photodiode pyranometers for Rotating Shadowband Irradiometers

Campbell Scientific control system for an Absolute Cavity Radiometer

The SRML is a regional solar radiation data center. The goal of the lab is to provide high qualit... more The SRML is a regional solar radiation data center. The goal of the lab is to provide high quality solar resource data for planning, design, deployment, and operation of solar electric facilities in the Pacific Northwest United States. The SRML began operation in 1975 with a simple need to understand the solar resource in the region. Since that time, it has developed into one of the premier monitoring networks in the country.

Low irradiance losses of photovoltaic modules

Solar Energy, 2017

The efficiency of a photovoltaic cell/module changes, as the intensity of incident irradiance dec... more The efficiency of a photovoltaic cell/module changes, as the intensity of incident irradiance decreases, in a non linear way and these changes are referred to as low irradiance losses. In this study data from field experiments, developed and organized by the National Renewable Energy Laboratory, are used to evaluate the low irradiance losses for a variety of module technologies. The results demonstrate that the ratio of the normalized power divided by the normalized short circuit current provide a good measure of the module's low light efficiency losses after both the maximum power and the short circuit current are adjusted for temperature effects. The normalized efficiencies determined through the field data, spanning for several months, are in good agreement with those determined under controlled conditions in a solar simulator. An analytical relation for the normalized efficiency is proposed based on existing formulation for the fill factor. Despite the approximate nature of the fill factor relation, this approach produces reliable results. It will be shown that a normalized efficiency curve can be used to extract information on the series and shunt resistances of the PV module and that the shunt resistance as a function of solar irradiance can be studied. Alternately, this formulation can be used to study the low irradiance losses of a module when the internal resistances are known.

Atmospheric turbidity determinations from irradiance ratios

Towards Production of an Updated National Solar Radiation Data Base

ABSTRACT The National Renewable Energy Laboratory and collaborating agencies are working towards ... more

Testing a Model of IR Radiative Losses: Preprint

Thermopile pyranometers exhibit IR radiative losses that affect global and diffuse shortwave meas... more Thermopile pyranometers exhibit IR radiative losses that affect global and diffuse shortwave measurements made with first class thermopile based instruments. Pyrgeometers can be used to measure the sky temperature and are used to calculate the pyranometer's IR radiative losses. Few solar monitoring sites are equipped with pyrgeometers necessary to account for the IR radiative losses associated with the pyranometers. High quality data from the Solar Radiation Research Laboratory (SRRL) at the National Renewable Energy Laboratory are used to test and further develop a model for the IR radiative losses without the use of pyrgeometer data. The various methods for obtaining IR radiative loss values are compared and contrasted using the SRRL data. A simple scaling method is proposed and tested to adjust the non-pyrgeometer based correlation models to sites with different sky temperature characteristics.

Measuring Global Irradiance

Progress on an Updated National Solar Radiation Data Base for the United States

includes the production of a two-year test data set, which allowed us to evaluate models, investi... more includes the production of a two-year test data set, which allowed us to evaluate models, investigate the availability and suitability of model input data, survey the availability of surface meteorological data, expand the scope of a new satellite-based solar radiation model, and propose a plan for a 1991-2000 NSRDB update. This paper describes recent work and evaluation of the test data set. Preliminary recommendations will be reviewed by an expert committee involved since the inception of the update project, and from that review we will develop a full data base update plan.

Optical Modeling and Measurements for Solar Energy Systems (Proceedings Volume)

Nanoantennas, coupled to rectifying nanodiodes ("rectennas"), could be used for convert... more

Building a bankable solar radiation dataset

Solar Energy, 2012

A robust solar radiation dataset is essential for securing competitive financing for solar power ... more A robust solar radiation dataset is essential for securing competitive financing for solar power projects. The majority of solar radiation datasets are derived from publically available data, though there are an increasing number of proprietary datasets being developed and marketed. Most of these new datasets represent models based on satellite images and validated with ground measured data. This paper focuses on the strengths and weaknesses of the existing publically available solar radiation databases, though the commentary is equally relevant to the newer commercial datasets. While the financing community generally views the solar resource as stable, it also views the material miscalculation of the solar resource as one of the biggest risks in a solar project. Therefore, lenders and rating agencies alike require verification of the solar resource dataset to be utilized at each project location as this translates directly into electric energy production forecasts and revenues. The variability of the solar resource as exhibited by the historical solar data and the accuracy of the dataset play significant roles in estimating the probability of future performance and influences the financial contract that the project is likely to receive.

Accuracy of ground surface broadband shortwave radiation monitoring

Journal of Geophysical Research: Atmospheres, 2014

The uncertainty of broadband shortwave radiation monitoring is determined for direct, diffuse, an... more The uncertainty of broadband shortwave radiation monitoring is determined for direct, diffuse, and global irradiance for measurements obtained at the Payerne (Switzerland) station of the Baseline Surface Radiation Network (BSRN). The uncertainty estimates include sources that reflect realistic long-term operation conditions. The uncertainties are derived using the methodology specified by the "Guide to the expression of uncertainty in measurement." The differences between redundant determinations of direct, diffuse, and global irradiance are analyzed and are shown to be compatible with the uncertainties. In addition, the signatures of some uncertainty sources are sought within the error statistics to find out if corrections can be applied and what their magnitude is. The global and diffuse irradiance uncertainties range from 1.8% to 2.4% without correction and are less than 1.8% with corrections. These uncertainties are close to or satisfy the BSRN targets for large signals (global: 1000 W m À2 , diffuse: 500 W m À2). For small signals (50 W m À2), the targets are not achieved, mainly as a result of uncertainties associated with the data acquisition electronics (DAQ). The direct irradiance uncertainty is~1.5%, 3 times larger than the BSRN uncertainty target. An accuracy gain can also be achieved at the DAQ level, but even without considering the DAQ uncertainty, the target is exceeded by a factor of about 2. The direct irradiance uncertainty remains~1% even using an absolute cavity radiometer as transfer standard for correcting the pyrheliometer sensitivity. Thus, the direct irradiance accuracy target of 0.5% is probably not achievable with the best commercially available technology.

Influence of Diffuse and Ground-Reflected Irradiance on the Spectral Modeling of Solar Reference Cells

Proceedings of the SOLAR 2021 Conference, 2021

Application of the Clear Sky Spectral Error for Radiometer Classification in ISO 9060

Proceedings of the ISES Solar World Congress 2019, 2019

A recent update of the ISO 9060 standard for the classification of solar radiometers introduces t... more A recent update of the ISO 9060 standard for the classification of solar radiometers introduces the use of the clear sky spectral error. The spectral error is the change in responsivity that may occur when the spectral distribution of the incident solar radiation differs from the spectral distribution present at the time of calibration. Spectral errors may occur if the radiometer does not have a completely uniform spectral responsivity. This use of the clear sky spectral error is a significant change compared to the previous version of the 9060 standard as now not only thermopile radiometers, but also photodiode radiometers are covered by the standard. We explain the method used to derive the spectral error and present spectral errors for several radiometers of different technologies. The results demonstrate that the new method is helpful to classify radiometers, distinguishing between different radiometer types and excluding inappropriate instruments from the classification as pyra...

University of Oregon: GPS-based Precipitable Water Vapor (PWV)

A partnership with the University of Oregon and U.S. Department of Energy's National Renewabl... more

Comparison of Pyranometers and Reference Cells on Fixed and One-axis Tracking Surfaces

Proceedings of the SOLAR 2017 Conference, 2017

Photovoltaic (PV) system perfomance is monitored by a wide variety of sensors. These instruments ... more Photovoltaic (PV) system perfomance is monitored by a wide variety of sensors. These instruments range from secondary standard pyranometers to photodiode-based pyranometers to reference cells. Although instruments are mounted in the plane of array of the modules a wide range of results have been obtained. Some of these difference have been assumed to come from systematic uncertainties associated with the irradiance sensors. This study is an attempt to quantify these differences by comparing the output of selected thermopile-based pyranometers to photodiode-based pyranometers and reference cells on a horizontal surface, a fixed-tilt surface, and a one-axis tracking surface. This analysis focuses on clear-sky results from two sites with different climatic conditions. Several important features were observed. Photodiode-based pyranometers and reference cells produce widely different results under clear skies, especially at larger angles-of-incidence even though both instruments are based on measuring the short circuit current of solar cells. The difference is caused by the scattering of light as it passes through the glazing of the reference cell or the diffuser lens of the photodiodedbase pyranometer. Both instruments are shown to have similar response to the spectral distribution of the irradiance when compared to the thermopile-based pyranometer that has a response nearly independent of the wavelength of light used by PV modules.

Modeling Photovoltaic Power

Engineering, Technology & Applied Science Research, 2016

A robust and reliable model describing the power produced by a photovoltaic system is needed in o... more A robust and reliable model describing the power produced by a photovoltaic system is needed in order to be able to detect module failures, inverter malfunction, shadowing effects and other factors that may result to energy losses. In addition, a reliable model enables an investor to perform accurate estimates of the system energy production, payback times etc. The model utilizes the global irradiance reaching the plane of the photovoltaic modules since in almost all Photovoltaic (PV) facilities the beam and the diffuse solar irradiances are not recorded. The airmass, the angle of incidence and the efficiency drop due to low values of solar irradiance are taken into account. Currently, the model is validated through the use of high quality data available from the National Renewable Energy Laboratory (USA). The data were acquired with IV tracers while the meteorological conditions were also recorded. Several modules of different technologies were deployed but here we present results ...

Verification of deterministic solar forecasts

Solar Energy, 2020

The field of energy forecasting has attracted many researchers from different fields (e.g., meteo... more The field of energy forecasting has attracted many researchers from different fields (e.g., meteorology, data sciences, mechanical or electrical engineering) over the last decade. Solar forecasting is a fast-growing subdomain of energy forecasting. Despite several previous attempts, the methods and measures used for verification of deterministic (also known as single-valued or point) solar forecasts are still far from being standardized, making forecast analysis and comparison difficult. To analyze and compare solar forecasts, the well-established Murphy-Winkler framework for distribution-oriented forecast verification is recommended as a standard practice. This framework examines aspects of forecast quality, such as reliability, resolution, association, or discrimination, and analyzes the joint distribution of forecasts and observations, which contains all time-independent information relevant to verification. To verify forecasts, one can use any graphical display or mathematical/statistical measure to provide insights and summarize the aspects of forecast quality. The majority of graphical methods and accuracy measures known to solar forecasters are specific methods under this general framework. Additionally, measuring the overall skillfulness of forecasters is also of general interest. The use of the root mean square error (RMSE) skill score based on the optimal convex combination of climatology and persistence methods is highly recommended. By standardizing the accuracy measure and reference forecasting method, the RMSE skill score allows-with appropriate caveats-comparison of forecasts made using different models, across different locations and time periods.

Direct Normal Irradiance

Performance of PV Inverters

Proc. of the 37th ASES …, 2008

Testing a Model of IR Radiative Losses Preprint Testing a model of IR radiative losses

Thermopile pyranometers exhibit IR radiative losses that affect global and diffuse shortwave meas... more Thermopile pyranometers exhibit IR radiative losses that affect global and diffuse shortwave measurements made with first class thermopile based instruments. Pyrgeometers can be used to measure the sky temperature and are used to calculate the pyranometer's IR radiative losses. Few solar monitoring sites are equipped with pyrgeometers necessary to account for the IR radiative losses associated with the pyranometers. High quality data from the Solar Radiation Research Laboratory (SRRL) at the National Renewable Energy Laboratory are used to test and further develop a model for the IR radiative losses without the use of pyrgeometer data. The various methods for obtaining IR radiative loss values are compared and contrasted using the SRRL data. A simple scaling method is proposed and tested to adjust the non-pyrgeometer based correlation models to sites with different sky temperature characteristics.

Alternative method for characterizing photodiode pyranometers for Rotating Shadowband Irradiometers

Campbell Scientific control system for an Absolute Cavity Radiometer

The SRML is a regional solar radiation data center. The goal of the lab is to provide high qualit... more The SRML is a regional solar radiation data center. The goal of the lab is to provide high quality solar resource data for planning, design, deployment, and operation of solar electric facilities in the Pacific Northwest United States. The SRML began operation in 1975 with a simple need to understand the solar resource in the region. Since that time, it has developed into one of the premier monitoring networks in the country.

Low irradiance losses of photovoltaic modules

Solar Energy, 2017

The efficiency of a photovoltaic cell/module changes, as the intensity of incident irradiance dec... more The efficiency of a photovoltaic cell/module changes, as the intensity of incident irradiance decreases, in a non linear way and these changes are referred to as low irradiance losses. In this study data from field experiments, developed and organized by the National Renewable Energy Laboratory, are used to evaluate the low irradiance losses for a variety of module technologies. The results demonstrate that the ratio of the normalized power divided by the normalized short circuit current provide a good measure of the module's low light efficiency losses after both the maximum power and the short circuit current are adjusted for temperature effects. The normalized efficiencies determined through the field data, spanning for several months, are in good agreement with those determined under controlled conditions in a solar simulator. An analytical relation for the normalized efficiency is proposed based on existing formulation for the fill factor. Despite the approximate nature of the fill factor relation, this approach produces reliable results. It will be shown that a normalized efficiency curve can be used to extract information on the series and shunt resistances of the PV module and that the shunt resistance as a function of solar irradiance can be studied. Alternately, this formulation can be used to study the low irradiance losses of a module when the internal resistances are known.

Atmospheric turbidity determinations from irradiance ratios

Towards Production of an Updated National Solar Radiation Data Base

ABSTRACT The National Renewable Energy Laboratory and collaborating agencies are working towards ... more

Testing a Model of IR Radiative Losses: Preprint

Thermopile pyranometers exhibit IR radiative losses that affect global and diffuse shortwave meas... more Thermopile pyranometers exhibit IR radiative losses that affect global and diffuse shortwave measurements made with first class thermopile based instruments. Pyrgeometers can be used to measure the sky temperature and are used to calculate the pyranometer's IR radiative losses. Few solar monitoring sites are equipped with pyrgeometers necessary to account for the IR radiative losses associated with the pyranometers. High quality data from the Solar Radiation Research Laboratory (SRRL) at the National Renewable Energy Laboratory are used to test and further develop a model for the IR radiative losses without the use of pyrgeometer data. The various methods for obtaining IR radiative loss values are compared and contrasted using the SRRL data. A simple scaling method is proposed and tested to adjust the non-pyrgeometer based correlation models to sites with different sky temperature characteristics.

Measuring Global Irradiance

Progress on an Updated National Solar Radiation Data Base for the United States

includes the production of a two-year test data set, which allowed us to evaluate models, investi... more includes the production of a two-year test data set, which allowed us to evaluate models, investigate the availability and suitability of model input data, survey the availability of surface meteorological data, expand the scope of a new satellite-based solar radiation model, and propose a plan for a 1991-2000 NSRDB update. This paper describes recent work and evaluation of the test data set. Preliminary recommendations will be reviewed by an expert committee involved since the inception of the update project, and from that review we will develop a full data base update plan.

Optical Modeling and Measurements for Solar Energy Systems (Proceedings Volume)

Nanoantennas, coupled to rectifying nanodiodes ("rectennas"), could be used for convert... more

Building a bankable solar radiation dataset

Solar Energy, 2012

A robust solar radiation dataset is essential for securing competitive financing for solar power ... more A robust solar radiation dataset is essential for securing competitive financing for solar power projects. The majority of solar radiation datasets are derived from publically available data, though there are an increasing number of proprietary datasets being developed and marketed. Most of these new datasets represent models based on satellite images and validated with ground measured data. This paper focuses on the strengths and weaknesses of the existing publically available solar radiation databases, though the commentary is equally relevant to the newer commercial datasets. While the financing community generally views the solar resource as stable, it also views the material miscalculation of the solar resource as one of the biggest risks in a solar project. Therefore, lenders and rating agencies alike require verification of the solar resource dataset to be utilized at each project location as this translates directly into electric energy production forecasts and revenues. The variability of the solar resource as exhibited by the historical solar data and the accuracy of the dataset play significant roles in estimating the probability of future performance and influences the financial contract that the project is likely to receive.

Accuracy of ground surface broadband shortwave radiation monitoring

Journal of Geophysical Research: Atmospheres, 2014

The uncertainty of broadband shortwave radiation monitoring is determined for direct, diffuse, an... more The uncertainty of broadband shortwave radiation monitoring is determined for direct, diffuse, and global irradiance for measurements obtained at the Payerne (Switzerland) station of the Baseline Surface Radiation Network (BSRN). The uncertainty estimates include sources that reflect realistic long-term operation conditions. The uncertainties are derived using the methodology specified by the "Guide to the expression of uncertainty in measurement." The differences between redundant determinations of direct, diffuse, and global irradiance are analyzed and are shown to be compatible with the uncertainties. In addition, the signatures of some uncertainty sources are sought within the error statistics to find out if corrections can be applied and what their magnitude is. The global and diffuse irradiance uncertainties range from 1.8% to 2.4% without correction and are less than 1.8% with corrections. These uncertainties are close to or satisfy the BSRN targets for large signals (global: 1000 W m À2 , diffuse: 500 W m À2). For small signals (50 W m À2), the targets are not achieved, mainly as a result of uncertainties associated with the data acquisition electronics (DAQ). The direct irradiance uncertainty is~1.5%, 3 times larger than the BSRN uncertainty target. An accuracy gain can also be achieved at the DAQ level, but even without considering the DAQ uncertainty, the target is exceeded by a factor of about 2. The direct irradiance uncertainty remains~1% even using an absolute cavity radiometer as transfer standard for correcting the pyrheliometer sensitivity. Thus, the direct irradiance accuracy target of 0.5% is probably not achievable with the best commercially available technology.

Influence of Diffuse and Ground-Reflected Irradiance on the Spectral Modeling of Solar Reference Cells

Proceedings of the SOLAR 2021 Conference, 2021

Application of the Clear Sky Spectral Error for Radiometer Classification in ISO 9060

Proceedings of the ISES Solar World Congress 2019, 2019

A recent update of the ISO 9060 standard for the classification of solar radiometers introduces t... more A recent update of the ISO 9060 standard for the classification of solar radiometers introduces the use of the clear sky spectral error. The spectral error is the change in responsivity that may occur when the spectral distribution of the incident solar radiation differs from the spectral distribution present at the time of calibration. Spectral errors may occur if the radiometer does not have a completely uniform spectral responsivity. This use of the clear sky spectral error is a significant change compared to the previous version of the 9060 standard as now not only thermopile radiometers, but also photodiode radiometers are covered by the standard. We explain the method used to derive the spectral error and present spectral errors for several radiometers of different technologies. The results demonstrate that the new method is helpful to classify radiometers, distinguishing between different radiometer types and excluding inappropriate instruments from the classification as pyra...

University of Oregon: GPS-based Precipitable Water Vapor (PWV)

A partnership with the University of Oregon and U.S. Department of Energy's National Renewabl... more

Comparison of Pyranometers and Reference Cells on Fixed and One-axis Tracking Surfaces

Proceedings of the SOLAR 2017 Conference, 2017

Photovoltaic (PV) system perfomance is monitored by a wide variety of sensors. These instruments ... more Photovoltaic (PV) system perfomance is monitored by a wide variety of sensors. These instruments range from secondary standard pyranometers to photodiode-based pyranometers to reference cells. Although instruments are mounted in the plane of array of the modules a wide range of results have been obtained. Some of these difference have been assumed to come from systematic uncertainties associated with the irradiance sensors. This study is an attempt to quantify these differences by comparing the output of selected thermopile-based pyranometers to photodiode-based pyranometers and reference cells on a horizontal surface, a fixed-tilt surface, and a one-axis tracking surface. This analysis focuses on clear-sky results from two sites with different climatic conditions. Several important features were observed. Photodiode-based pyranometers and reference cells produce widely different results under clear skies, especially at larger angles-of-incidence even though both instruments are based on measuring the short circuit current of solar cells. The difference is caused by the scattering of light as it passes through the glazing of the reference cell or the diffuser lens of the photodiodedbase pyranometer. Both instruments are shown to have similar response to the spectral distribution of the irradiance when compared to the thermopile-based pyranometer that has a response nearly independent of the wavelength of light used by PV modules.

Modeling Photovoltaic Power

Engineering, Technology & Applied Science Research, 2016

A robust and reliable model describing the power produced by a photovoltaic system is needed in o... more A robust and reliable model describing the power produced by a photovoltaic system is needed in order to be able to detect module failures, inverter malfunction, shadowing effects and other factors that may result to energy losses. In addition, a reliable model enables an investor to perform accurate estimates of the system energy production, payback times etc. The model utilizes the global irradiance reaching the plane of the photovoltaic modules since in almost all Photovoltaic (PV) facilities the beam and the diffuse solar irradiances are not recorded. The airmass, the angle of incidence and the efficiency drop due to low values of solar irradiance are taken into account. Currently, the model is validated through the use of high quality data available from the National Renewable Energy Laboratory (USA). The data were acquired with IV tracers while the meteorological conditions were also recorded. Several modules of different technologies were deployed but here we present results ...

Verification of deterministic solar forecasts

Solar Energy, 2020

The field of energy forecasting has attracted many researchers from different fields (e.g., meteo... more The field of energy forecasting has attracted many researchers from different fields (e.g., meteorology, data sciences, mechanical or electrical engineering) over the last decade. Solar forecasting is a fast-growing subdomain of energy forecasting. Despite several previous attempts, the methods and measures used for verification of deterministic (also known as single-valued or point) solar forecasts are still far from being standardized, making forecast analysis and comparison difficult. To analyze and compare solar forecasts, the well-established Murphy-Winkler framework for distribution-oriented forecast verification is recommended as a standard practice. This framework examines aspects of forecast quality, such as reliability, resolution, association, or discrimination, and analyzes the joint distribution of forecasts and observations, which contains all time-independent information relevant to verification. To verify forecasts, one can use any graphical display or mathematical/statistical measure to provide insights and summarize the aspects of forecast quality. The majority of graphical methods and accuracy measures known to solar forecasters are specific methods under this general framework. Additionally, measuring the overall skillfulness of forecasters is also of general interest. The use of the root mean square error (RMSE) skill score based on the optimal convex combination of climatology and persistence methods is highly recommended. By standardizing the accuracy measure and reference forecasting method, the RMSE skill score allows-with appropriate caveats-comparison of forecasts made using different models, across different locations and time periods.