spectral solar radiation data base, models, and instrumentation

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Solar Cells, 27 (1989) 279 - 287 279 SPECTRAL SOLAR RADIATION DATA BASE, MODELS, AND INSTRUMENTATION R. HULSTROM, C. RIORDAN and T. CANNON Solar Energy Research Institute (SERI), Golden, CO 80401 (U.S.A.) Summary This paper summarizes recent results of the SERI Resource Assessment and Instrumentation Branch on a spectral solar radiation data base, models, and instrumentation. The objective of such research has been to support the photovoltaics community by helping to develop a thorough under- standing of the relationships between the natural spectral solar radiation environment and the design, performance, and performance testing of photovoltaic devices. A measured spectral data base consisting of about 3000 spectra has been produced. A new instrument, called the Atmospheric Optical Calibration Systems has been developed and patented. This instru- ment will provide real-time data comparing outdoor atmospheric optical conditions, during PV device testing, with reference conditions. A plan has been developed for possibly improving SERFs current simple spectral solar radiation model (SPCTRAL2) by comparing it with the measured spectral data base and with a recently upgraded rigorous model (LOW- TRAN) produced by the Air Force Geophysics Laboratory. A spectro- radiometer has been under development to provide better measurements of the complete solar spectrum from 0.3 to 3.0 pm. 1. Introduction The primary objective of our spectral solar radiation research is to help develop a fundamental scientific understanding of the relationships between the natural solar radiation environment and the design, perfor- mance, and performance measurements of spectrally sensitive solar energy conversion devices (e.g. photovoltaics (PV)). This understanding is needed to guide the development and optimization of PV devices to meet the goals of the Department of Energy's (DOE's) National Photovoltaics Pro- gram. To accomplish this primary objective, we need to be able to charac- terize thoroughly the natural spectral solar radiation environment. With this aim, three major technical thrusts have been established: the develop- ment of mathematical models to simulate spectral solar irradiance for various atmospheric conditions; the development of instrumentation to 0379-6787/89/$3.50 © Elsevier Sequoia/Printed in The Netherlands

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Page 1: Spectral solar radiation data base, models, and instrumentation

Solar Cells, 27 ( 1 9 8 9 ) 279 - 287 279

SPECTRAL SOLAR RADIATION DATA BASE, MODELS, AND INSTRUMENTATION

R. H U L S T R O M , C. R I O R D A N and T. C A N N O N

Solar Energy Research Institute (SERI), Golden, CO 80401 (U.S.A.)

Summary

This paper summarizes recent results of the SERI Resource Assessment and Instrumentation Branch on a spectral solar radiation data base, models, and instrumentation. The objective of such research has been to support the photovoltaics communi ty by helping to develop a thorough under- standing of the relationships between the natural spectral solar radiation environment and the design, performance, and performance testing of photovoltaic devices. A measured spectral data base consisting of about 3000 spectra has been produced. A new instrument, called the Atmospheric Optical Calibration Systems has been developed and patented. This instru- ment will provide real-time data comparing outdoor atmospheric optical conditions, during PV device testing, with reference conditions. A plan has been developed for possibly improving SERFs current simple spectral solar radiation model (SPCTRAL2) by comparing it with the measured spectral data base and with a recently upgraded rigorous model (LOW- TRAN) produced by the Air Force Geophysics Laboratory. A spectro- radiometer has been under development to provide better measurements of the complete solar spectrum from 0.3 to 3.0 pm.

1. Introduction

The primary objective of our spectral solar radiation research is to help develop a fundamental scientific understanding of the relationships between the natural solar radiation environment and the design, perfor- mance, and performance measurements of spectrally sensitive solar energy conversion devices (e.g. photovoltaics (PV)). This understanding is needed to guide the development and optimization of PV devices to meet the goals of the Department of Energy's (DOE's) National Photovoltaics Pro- gram.

To accomplish this primary objective, we need to be able to charac- terize thoroughly the natural spectral solar radiation environment. With this aim, three major technical thrusts have been established: the develop- ment of mathematical models to simulate spectral solar irradiance for various atmospheric conditions; the development of instrumentation to

0 3 7 9 - 6 7 8 7 / 8 9 / $ 3 . 5 0 © Elsevier Sequo i a /P r in t ed in The Ne the r l ands

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measure both spectral solar irradiance and pertinent atmospheric optical transmittance properties; and the development of a measured spectral solar irradiance data base representing a range of spectra from the natural environment. Each technical thrust results in "p roduc t s" that the PV com- muni ty can then use to produce the understanding necessary to develop, optimize, and test (e.g. by performance measurements) PV devices. For example, a model that simulates spectral solar irradiance characteristics can be combined with a PV device model to study multijunction bandgap selection and design strategies.

This paper summarizes recent progress in each of the three major technical thrusts: spectral solar radiation data base, spectral solar radiation models, and instrumentation.

2. SERI's spectral solar radiation data base

A description and history of the spectral solar radiation data base has been published in Solar Energy [1]. Briefly, SERI cooperated with the Florida Solar Energy Center (FSEC), Cape Canaveral, and the Pacific Gas and Electric Co. (PG&E), San Ramon, CA, to build a data base for a range of atmospheric conditions, air masses, and measurement modes (direct-normal and global). Spectral solar radiation measurements from 0.3 to 1.1 pm and supporting meteorological and insolation (i.e. total solar irradiance from 0.3 to 3.0 pm) data were collected at FSEC and PG&E and sent to SERI for quality control, documentat ion, and adding to the data base, together with data collected by SERI for special research projects.

Data were collected at FSEC, PG&E, and SERI through March 1988. FSEC collected data almost daily, and PG&E collected data periodically as resources permitted. SERI collected data during the winter 1987-1988 for urban (Denver, CO) air pollution studies. About 3000 spectral data sets (i.e. spectra) from these three measurement sites are now held in the data base. These data cover a range of atmospheric conditions (clear and cloudy, high and low precipitable water vapor) and measurement modes (direct-normal, global on a tilted surface, global on a horizontal surface, and global on a surface tracking the Sun).

Our major activities on the data base during the past year were to guide data collection at the measurement sites, perform preliminary quality control checks on the data as they arrived, add the data to the data base and document them, and perform periodic recalibrations of the spectro- radiometers. The preliminary quality control processing was used to alert SERI to any field problems. Results from this processing, and field notes, are included in the data base documentat ion.

SERI recalibrated and intercompared the spectroradiometers every 6 months, and performed a final post-measurement calibration in May 1988 (Fig. 1). The results of the calibrations were used to determine total

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Fig. 1. Outdoor intercomparison of spectroradiometers used to collect data for the SERI spectral solar radiation data base.

spectral measurement uncertainty for each measurement period and mea- surement configuration. The results of the calibration and measurement uncertainty analysis are shown in Fig. 2.

The total measurement uncertainty is plot ted as a function of wave- length for the various measurement modes of global-normal (GN), d i rec t - normal (DN), global-horizontal (GH), and global-t i l t (GT). Global-normal measurements were taken with either a Teflon-dome diffuser or an inte- grating-sphere as the receiver optics of the spectroradiometer(s). Total measurement uncertainty is defined as the 95% confidence interval of the data. The integrating-sphere GH and GT data have a significantly higher measurement uncertainty because of nonuniformities (when a strong direct- beam is present) in the angular response of the integrating-sphere optics

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"-~00 400 500 600 700 800 Wavelength (nm)

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900 1000 1100

Fig. 2. Total measurement uncertainty of SERI's spectral data base: (a) integrating- sphere global-horizontal and tilt measurements are the ex treme upper and lower curves, and the integrating-sphere global-normal measurements are the extreme upper and middle curves; (b) Teflon-dome, global-normal, measurements; (c) direct-normal mea- surements.

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Standard Air 1 2 Time mass I 1 12:31pm 10,

1.2 2 11:31 a.m. 1.07 \,' 3 10:31 a.m. 1.16

1.0 4 4 9:31 a.m. 1.37 5 8:31 a.m. 1.79

08 ]- ,",~/ ^/ '~vV " f ' ~ 6 7:31 a.m. 2.72 A

300 400 500 600 700 800 900 1000 1100 Wavelength (nm)

F ig . 3. A n e x a m p l e o f s o l a r r a d i a t i o n s p e c t r a c o n t a i n e d i n t h e S E R I s p e c t r a l d a t a base.

Shown are DN s c a n s t a k e n at different times of the day, representing different air masses at the Florida Solar Energy Center, Cape Canaveral, FL.

used to measure global (i.e. direct-beam + diffuse solar irradiance) irradiance. In the DN and GN modes the direct-beam irradiance is normal to the aper- ture of the integrating~sphere, which is the same orientation as during the instruments laboratory calibration; therefore, the angular response uncer- tainty does no t have to be accounted for.

Each measurement site maintained and calibrated its own insolation and meteorological instruments. Information on instrument types and calibrations was supplied by the operators and included in instrument configuration files in the data base.

The first phase of the spectral data base will be completed by June 1989. This includes quality control processing, documentat ion, and a data base file that will be disseminated on computer disk/tape. SERI is currently working with the National Climatic Data Center (NCDC) to provide public dissemination of our spectral solar radiation data base. An example of a set of spectra is shown in Fig. 3.

3. Spectral solar radiation models

SERFs spectral solar radiation model, SPCTRAL2 [2], was developed for cloudless-sky conditions. The plan is to improve the model, as necessary, to accommodate other atmospheric conditions (such as cloudy and polluted skies) based on validation and verification of SERI's model (and other models) with measured data.

The intent is to develop a hybrid model based on SPCTRAL2 and models developed by other laboratories, such as the AFGL's LOWTRAN model [3]. The authors attended an AFGL conference on atmospheric transmittance models in May-June 1988 and agreed to cooperate with

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AFGL to compare their new LOWTRAN-7 code with our measured data and SPCTRAL2.

To make model improvements, it is essential to know spectral mea- surement uncertainty to determine whether proposed improvements are outside the measurement uncertainty limits. The spectroradiometer calibra- tion and intercomparison data acquired in 1988, as part of the spectral data base activities, allow estimation of this uncertainty (Fig. 2).

Investigations of cloudy sky modifiers for SPCTRAL2 were postponed until we could

(1) perform the thorough uncertainty analysis on the measured data, and

(2) extend our spectral measurement capability beyond the current 1.1-~m limit so that we could better define cloud transmittance in the near- IR (1.0 - 3.0 pro).

A new spectroradiometer with a wavelength range of 0.3 - 3.0 ~m was purchased in 1988 (see Section 4.2). This instrument was originally designed for geophysical applications. In 1989 we are

(1) developing the software to acquire irradiance data; (2) developing an improved integrating sphere for global irradiance

measurements; and (3) developing a view-limiting tube for DN measurements. SERI has been disseminating SPCTRAL2 upon request. For exam-

ple, requestors in 1988 included the University of Nebraska, Villanova University, 3M Company, University of California at Davis, and others. SPCTRAL2 is now used extensively by both the national and international research communities.

4. Instrumentation development

4.1. Atmospheric optical calibration system The design objectives and applications of the AOCS to outdoor PV

testing have been described previously [4]. Briefly, it is designed to yield real-time data and analyses on atmospheric properties, such as haziness and water vapor, and solar radiation characteristics that affect PV device performance. More recently, progress was made in the following areas:

(1) A patent, No. 4 779 980, entitled "Atmospheric Optical Calibra- tion System" was issued by the U.S. Patent and Trademark Office to the Midwest Research Institute (MRI). An international application was pub- fished, providing defensive or provisional patent protection, or both, in countries designated by the Chapter II Patent Cooperation Treaty (PCT). A PCT Chapter II demand was filed in all of the designated countries, and a separate national application was filed in Canada. MRI Ventures issued an AOCS Business (i.e. commercialization) Opportunity Announce ment to many U.S. and foreign companies.

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Fig. 4. The direct-beam module of the SERI Atmospheric Optical Calibration System- Working Unit (AOCSWU).

(2) An AOCS evaluation experiment, described in detail in the FY 1987 PV AR&D Annual report [5], was completed at our Solar Radiation Re- search Laboratory. The results experimentally confirmed the validity of the AOCS concept.

(3) A prototype AOCS Working Unit (AOCSWU) was designed and constructed, and new software was developed based on results of the evalua- tion experiment.

The AOCSWU direct-beam module is shown in Fig. 4. The larger cylinder contains four narrowband optical channels used to determine atmospheric turbidi ty (0.368 and 0.862 /~m) and water vapor (0.862 and 0.942 /~m), as well as a broadband channel for measuring direct-beam total irradiance. The smaller cylinder contains a sensor to measure photon flux over the 0.4 - 0.7 pm region.

Each channel consists of a sapphire window, an optical filter (except for the total-irradiance channel), a silicon detector, an operational am- plifier, apertures to define the field-of-view and slope angles, and baffles to limit the stray light. The design field-of-view angles for the narrowband and broadband channels are 2.3 ° and 5.7 ° , respectively. Hybrid detector- amplifiers are used for the narrowband channels.

Removable desiccant cartridges are installed to monitor and control internal humidity. The direct-beam module also contains the electronics for signal processing and analog-to-digital conversion. A push but ton is used to initiate the data acquisition and processing sequence, and col- ored lamps indicate the status of the data acquisition and calculation se- quence.

In addition to the direct-beam module, there are four broadband detectors (one each total irradiance and photon flux in both horizontal and plane-of-array) and a computer for controlling the data acquisition, making all calculations (including modeled terrestrial solar spectral irra- diance), and storing the data. These components are shown in Fig. 5.

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Fig. 5. The complete AOCSWU, showing, from right to left, the computer system, the four broadband detectors, and the direct-beam module.

4.2. Ex tended wavelength spectroradiometer The need to make accurate spectral irradiance measurements of both

outdoor and simulated solar irradiance at wavelengths beyond the 1.1-pm silicon detector cut-off has been accentuated by research on PV materials that respond to longer wavelengths than silicon does. In the past, to measure solar spectra over the entire wavelength region from 0.3 to 3 pm, it has been necessary to use two spectroradiometers or to change gratings and detectors manually part way through the spectral scan, or measure over a restricted wavelength range and extend the range with a model.

The market was surveyed for spectroradiometers that would automat- ically scan the entire solar wavelength range. Design criteria included auto- matic scanning, filter and grating changes over the 0.35 - 3 ~m wavelength range, a spectral bandwidth of 0.006 pm from 0.35 to 1.1 pm and 0.010 pm from 1.1 to 3 pm, and a total scan time of less than 40 s. Only one instrument was commercially available that could meet these resolution and wavelength requirements. The instrument selected is the Single Channel Infrared Intelligent Spectroradiometer (SIRIS) manufactured by Geo- physical Environmental Research (GER). This instrument, with a factory retrofit detector, covers the 0.3- 3.0-pm range with specified bandwidths. The total scan time is about 3 min, but shorter scan times can be program- med at the expense of less spectral detail, a lower signal-to-noise ratio, or both. The entire system is portable (see Fig. 6) and can be easily used either outdoors or indoors (for measuring solar simulators).

4.3. Sunphotometer calibration system Sunphotometers are used to measure atmospheric turbidity and water

vapor content; both affect the outdoor performance of PV devices. Calibra- t ion of sunphotometers has always been a difficult and tedious process because it requires outdoor measurements to be made under near-ideal and constant atmospheric conditions. The authors have developed a sun-

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Fig. 6. The extended wavelength spectroradiometer, showing from right to left, the integrating-sphere, the optical/electronics unit, and the computer data acquisition unit.

Fig. 7. The SERI sunphotometer calibration system, showing from right to left, the light source, the control optics, and a commercial sunphotometer.

photometer calibration system to measure the calibration (i.e. response) change with time (i.e. stability) under laboratory conditions.

The system is shown in Fig. 7, configured to calibrate a commercial sunphotometer . A very stable, intense beam of light is provided from a 1000 W tungsten-halide DXW lamp. Current to this bulb is regulated to abou t 0.01% by a Gamma Scientific Model 5000 Lamp Monitor and Control designed specifically to regulate current to NIST (National Institute of Standards and Technology, formerly the National Bureau of Standards) radiance standards. Optical components are used to produce a narrow beam of high-intensity light that overfills each sunphotometer detector. The sunphotometer ' s ou tpu t signal is normalized to the signal from a refer- ence detector.

This system will be used to calibrate the AOCS, as well as to check periodically the calibration of other sunphotometers. Initial tests indicated that changes of less than 2% in the response of the AOCS or a sunphotom- eter will be detectable.

5. Discussion

To date, research performed by the SERI Resource Assessment and Instrumentat ion Branch has resulted in a variety of scientific tools that will significantly help the PV communi ty design, develop, and test devices. These scientific tools include a spectral solar radiation simulation model (SPCTRAL2), an Atmospheric Optical Calibration System (AOCS), a 3000- spectra spectral solar radiation data base, an advanced spectroradiometer that covers the total spectrum, and a laboratory calibration system for sunphotometers .

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With respect to the significance of spectral solar radiation charac- teristics, the subject research produced a report entitled "Summary of Studies that Examine the Effects of Spectral Solar Radiation Variations on PV Device Design and Performance" [6]. The results documented by that report underscore the importance of spectral solar radiation research in support of the design and development of PV devices and technology.

References

1 C. Riordan, D. Myers, M. Rymes, R. Hulstrom, W. Marion, C. Jennings and C. Whita- ker, SoL Energy, 42(1) (1989) 67.

2 R. E. Bird and C. Riordan, J. Clim. Appl. Meteorol., 25(1)(1986)87. 3 F. X. Kneizys, R. L. Hulstrom, C. J. Riordan and T. Cannon, Users Guide to LOW-

TRAN 7, Environmental Research Papers, 1010 Air Force Geophysics Laboratory, Hanscom AFB, MA, 1988.

4 R. Hulstrom and T. Cannon, Sol. Cells, 21 (1987) 329. 5 R. Hulstrom, T. Cannon and C. Riordan, Photovoltaic Advanced Research and

Development Project, Solar Radiation Research, Annual Report, for 1 October 1986-30 September 1987, Solar Energy Research Institute, Golden, CO, 1988.

6 C. Riordan and R. Hulstrom, Summary of Studies that Examine the Effects of Spectral Solar Radiation Variations on PV Device Design and Performance, Solar Energy Research Institute, Golden, CO, 1989.