SOLAR RADIATION RESOURCE ASSESSMENT PROJECT 

Program Overview of Fiscal Year 1993 
The Solar Radiation Resource Assessment Project ........................... 1 

The Solar Resource ......................................................................... 3 

FY 1993 Accomplishments .............................................................. 7 

A View Toward the Future ....... ....................................................... 14 

NOTICE: This report was prepared as an account of work sponsored by an agency of the United States 

government. Neither the United States government nor any agency thereof, nor any of their employees, makes 

any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, 

or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would 

not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by 

trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and opinions 

of authors expressed herein do not necessarily state or reflect those of the United States government or any 

agency thereof. 
The Solar Radiation Resource 

Assessment Project provides planners 

and users of solar technologies with 

information on the best times and 

locations to use the sun's energy 

resource. The project team accomplishes this by producing and disseminating relevant and reliable 

information about solar radiation. 

The U.S. Department of Energy 

(DOE) established this assessment 

project to facilitate the deployment 

of solar technologies. Managed 

in the Analytic Studies Division of 

the National Renewable Energy 

Laboratory (NREL), the project is 

the major activity of DOE's Resource 

Assessment Program, which is funded 

and monitored by the Photovoltaics 

Division of the Office of Solar Energy 

Conversion. 

David Parsons. NREL 

To produce top-quality information 

about solar radiation, the project 

supports many activities. It provides 

technical and financial support for 

solar radiation measurement stations; 

it calibrates, tests, and evaluates 

measurement instruments; it develops 

and validates solar radiation models; 

and it creates data bases and associated 

products, such as manuals and maps. 

One major supporting activity of the 

project is the National Solar Radiation 

Data Base, a new data base that 

contains improved and up-to-date 

solar radiation and meteorological 

data. Using this updated information 

enables planners and designers to 

better gauge the performance of 

solar technologies at specific locations. 

Another supporting activity is the 

Historically Black Colleges and 

Universities Solar Monitoring 

Network. Consisting of six monitoring 

stations in the Southeast and East, 

this network has collected valuable 

solar radiation data since 1985. 

Precision instruments, known as pyranometers, 

are among the instruments used to measure 

solar radiation. 
NREL's Solar Radiation Research 

Laboratory also contributes to the 

project's output of superior data. 

This laboratory, located in Golden, 

Colorado, supplies the outdoor 

research and calibration capabilities 

for the project. At this laboratory, 

project team members calibrate and 

test solar measurement systems 

and obtain solar radiation values 

for the area. 

Warren Gretz, NREL 

In addition to these activities, the 

project keeps in touch with the needs 

of the solar industry and others by 

responding to requests for solar 

radiation resource data and assessment methods. This outreach activity 

helps the team determine what 

information is relevant and how it 

is best presented and delivered. 

Through these and other activities, 

the project supports the information 

needs of the solar industry, including 

developers, designers, suppliers, 

and installers of solar technologies; 

government agencies; educational 

organizations; researchers; and the 

general public. 

Project team member Tom Stoffel 

calibrates pyranometers at the 

Solar Radiation Resean;h Laboratory. 
The amount of solar radiation reaching the Earth's surface varies greatly 

because of changing atmospheric conditions and the changing position of 

the sun in the sky, both during the day 

and throughout the year. Cloudiness 

is the predominant atmospheric condition that determines the amount of 

solar radiation that reaches the Earth. 

Consequently, cloudy regions receive 

less solar radiation than the cloud-free 

desert climates of the southwestern 

United States. For any given location, 

Understanding the sun's interaction with 

clouds is an important part of evaluating 

the variability of the solar resource. 

the solar radiation reaching the 

Earth's surface decreases as the 

cloud cover increases. 

Local geographic features, such as 

mountains, oceans, and large lakes, 

influence the formation of clouds; 

therefore, the amount of solar 

radiation received by these areas 

may be different from that received 

by adjacent land areas. For example, 

the mountains may receive less solar 

radiation than the foothills and plains 

a short distance away because winds 

blowing against the mountains force 

air to rise; clouds form as the moisture 

in the air cools. Coastlines can also 

receive a different amount of solar 

radiation than areas farther inland. 

Where the changes in geography 

are less pronounced, such as in the 

Great Plains, the variability of total 

solar radiation is less. 
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Global radiation 

The elevation of the sun above the horizon, or conversely, the angle of the 

sun from the ver tical (straight up or zenith), determines air mass. Air mass 

values are higher when the sun is lower in the sky. For example, air mass is 

1 when the sun is directly overhead and the angle of the sun from the zenith 

direction is 0°; air mass is 2 when the angle is 60°. The air mass value at 

any par ticular time depends on the location (latitude), the time of day, and 

the day of the year. 

When the sun is closer to the horizon, direct beam radiation must pass 

through a longer distance in the Earth's atmosphere than when the sun is 

overhead. This longer path results in more scattering and absorption of the 

solar radiation. The atmosphere through which the solar radiation passes 

is also quite variable. Significant variables are atmospheric turbidity 

(haziness caused by aerosols such as dust), water vapor, and clouds. 

So how does the atmosphere affect solar radiation? It basically acts as 

a dynamic filter absorbing and scattering solar radiation. It creates spatial 

(geographic), temporal (hourly, daily), and spectral (wavelength) variations in 

solar radiation that researchers must characterize or describe with respect 

to these variations' effects on operating solar energy conversion systems. 

Three elements of solar radiation 

reaching the Earth's surface can 

be measured: total radiation (also 

called global radiation) and its two 

components, direct beam and diffuse 

radiation. As the name implies, direct 

beam radiation travels in a direct line 

from the sun. Diffuse radiation, on 

the other hand, has been scattered 

out of the direct beam by molecules, 

aerosols, and clouds. Because diffuse 

radiation comes from all regions of 

the sky, it is also referred to as sky 

solar radiation. On clear days, the 

portion of global solar radiation 

that is diffuse is about 10% to 20%; 

on cloudy days, that proportion can 

be as much as 100%. 
For good-quality solar resource 

assessment, at least two of the three 

elements need to be measured. 

In practice, however, it is wise to 

measure all three elements. By doing 

so, researchers can verify the quality 

of the data by using the following 

equation: 

global = diffuse + direct x cose 

where cose is the cosine of the 

solar zenith angle. 

This check can alert researchers to 

equipment and operational problems. 

Spectral Distribution 

of Solar Radiation 

The optimum use of certain technologies may require knowledge of 

the spectral, or wavelength, distribution of solar radiation because 

different types of solar technologies 

use different wavelengths of the 

spectrum. For example, photovoltaic 

devices respond primarily to wavelengths in the visible and near-infrared 

region of the spectrum, whereas solar 

detoxification units use energy from 

the ultraviolet region. Location, 

climate, and atmospheric conditions 

influence the spectral distribution 

of solar radiation. 

Researchers can measure the spectral 

distribution of solar radiation with a 

spectroradiometer. This instrument 

measures the solar radiation intensity 

at discrete wavelengths. Because spectroradiometers are complex and relatively expensive, and their operation 

and maintenance require a significant 

effort, they are not routinely used for 

5 

long-term data collection. Rather, 

researchers use spectroradiometers 

to establish data bases that provide 

values of representative spectra for a 

wide range of climates and locations. 

Data about spectral solar radiation 

can be used to study the sensitivity of 

spectrally selective solar technologies, 

such as photovoltaics, "smart windows," 

and biomass. These technologies can 

. 

be sensitive to geographical, seasonal, 

and diurnal variations in the 

solar spectrum. 
These technologies conver t sunlight into usable forms of energy. Guided 

by solar resource assessment data, designers and planners can optimally 

design and locate these solar energy technologies. 

Solar resource assessment provides 

information about the availability of 

solar radiation at different locations. 

It also involves determining how 

much solar radiation is available to 

specific energy-producing or energysaving technologies, such as photovoltaic panels or smart windows. 

Making this determination requires 

measuring and characterizing the 

nature of solar radiation and the parameters that affect it. It also involves 

measuring solar radiation at various 

locations to quantity the resource, 

developing models to estimate solar 

radiation based on available meteorological data, and performing studies 

using measured or modeled data to 

show the amount of solar radiation 

available and how it varies geographically and by season and year. 

Solar resource assessment information 

facilitates the optimal use of solar 

technologies. Among the solar energy 

technologies that can benefit from 

information about the solar resource 

are the following: 

Solar electric (photovoltaic) for 

converting solar radiation directly 

into electricity 

Solar heat (thermal) for heating water 

for industrial and household uses 

Solar thermal electric for producing 

steam to run turbines that generate 

electricity 

Solar fuel technologies for converting 

biomass (plants, crops, and trees) 

into fuels and by-products 

Passive solar designs for lighting 

and heating buildings 

Solar detoxification for destroying 

hazardous waste using concentrated 

sunlight. 

The economics of these technologies 

not only depend on the costs of equipment and operation, but also on the 

amount of solar radiation available 

and the percentage of solar radiation 

that can be converted into the desired 

energy product. 
The project team continued its 

leadership in conducting measurements at the Historically Black 

Colleges and Universities (HBCU) 

monitoring network in the southeastern United States and at NREL's 

own Solar Radiation Research 

Laboratory. During FY 1993, the 

team accomplished improvements 

in both programs. 

HBeD Monitoring Network 

For almost a decade, NREL has funded and overseen the HBCU network. 

Consisting of six monitoring stations 

in the southeastern and eastern 

United States, the HBCU network 

continues to be a good source of 

measured solar radiation data and 

provides an opportunity for engineering students to gain experience in the 

solar energy field. 

During FY 1993, the project team 

gained remote access to the data at 

each HBCU station by installing 

dial-up modems on the data loggers. 

Now project team members can 

review daily the data collected at each 

site, which reduces the time required 

to identify instrument problems and 

expedites the data archival process. 

The HBCU network's monitoring 

stations are located at BethuneCookman College, Daytona Beach, 

Florida; Bluefield State College, 

Bluefield, West Virginia; Elizabeth 

City State University, Elizabeth City, 

North Carolina; Mississippi Valley 

State University, Itta Bena, Mississippi; 

South Carolina State University, 

Orangeburg, South Carolina; 

and Savannah State College, 

Savannah, Georgia. 

Tom "Whitney, principal investigator; 

and James Malloy, engineer; insp�

ct 

solar radiation measurement equzpment 

at South Carolina State University. 

South Carolina State University is 

one of six monitoring stations in the 

HBCU network. 
NREL's Solar Radiation Research Laboratory 

in Golden, Colorado 

Warren Gretz, NREL 

Solar Radiation Research Laboratory 

NREL's Solar Radiation Research 

Laboratory (SRRL) provides the 

outdoor research and calibration 

capabilities needed by the project. 

Located on top of South Table 

Mountain, near NREL's offices, 

the laboratory also supports the 

solar radiation and meteorological 

measurement needs of other 

programs at NREL. 

In September 1993, SRRL was the 

site of a 3-week intercomparison 

of 14 absolute cavity radiometers. 

These special instruments are used 

to calibrate solar-measuring devices 

by providing radiometer traceability 

to the World Radiometric Reference. 

The project team collaborated with 

the National Oceanic and Atmospheric 

Administration (NOAA) on its revised 

solar radiation monitoring network, 

ISIS, The revised network will have 

about six research-quality measuring 

stations, called SURFRAD (SURFace 

RADiation). In addition, approximately 

10 to 15 of the original SOLRAD 

(SOLar RADiation) stations operated 

by NOAA's National Weather Service 

since the late 1940s will be upgraded. 

Participants in the intercomparison 

included NREL, the National Oceanic 

and Atmospheric Administration, 

and the Eppley Laboratory. 

Information about solar radiation 

values obtained at SRRL was 

published in FY 1993. Summary 

Information and Data Sets for 

NREL's Solar Radiation Research 

Laboratory 1981-1991 summarizes 

information on available solar 

radiation and describes hourly 

data sets for solar radiation and 

meteorological elements measured 

from 1981 to 1991. 
When researchers lack solar radiation measurements for a specific site, 

they can use models built on meteorological data to estimate the radiation 

values. Although less accurate, this is often the only option available. Many 

models use cloudiness data based on observations made by a trained 

meteorologist who looks at the sky and estimates the amount of cloud cover 

in tenths. A clear sky rates a cloud cover value of 0 tenths, and an overcast 

sky rates a cloud cover value of 10 tenths. Researchers can derive cloud 

cover data from weather satellites as well. 

One of the program's more notable modeling achievements is the develop-­

ment of the National Solar Radiation Data Base, completed in the summer 

of 1992. Because of a limited amount of measured data, NREL researchers, 

using meteorological data from National Weather Service stations, developed 

a model to estimate more than 90% of the solar radiation data for this 

data base. The model, METSTAT (METeorological-STATisticai), 

uses meteorological data and statistical algorithms 

to produce a data base of long-term data similar 

to one developed from measured data. 

Researchers will give METSTAT extensive 

documentation and validation over the 

next several years and will study its 

use for enhancing international 

data bases. 

Cloud cover information and 

other data obtained from 

satellites also can be used 

to estimate solar radiation at 

any location on Earth. 

The project team has plans 

to use satellite data for 

enhancing the National 

Solar Radiation Data Base 

and for developing international solar resource 

information. 
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Annual average 

direct nO.l'mal 

solar. radiation 

Annual average 

global horizontal 

solar radiation 
In FY 1993, the project team presented the National Solar Radiation 

Data Base and developed a variety 

of products from the new data base. 

National Solar Radiation Data Base 

In the summer of 1992, the project 

team completed a 3-year update 

and upgrade of our nation's primary 

solar radiation data base, the 

SOLMETjERSATZ data base. Using 

information from the new data base, 

the National Solar Radiation Data 

Base, researchers can produce 

reports, manuals, charts, and maps in 

user-friendly formats. One such product, soon to be published, is a solar 

radiation data manual for flat-plate 

,md concentrating collectors. 

Seminar about Solar Radiation 

Resource Assessment 

To introduce the new solar radiation 

data base, project team members 

hosted the seminar Recent Advancements in Solar Radiation Resource 

Assessment. This 3-day seminar took 

place in Denver, Colorado, 

during November 1992. About 

70 people attended, representing 

electric utilities, universities, national 

laboratories, private industry, 

and DOE. 

The National Solar Radiation Data Base contains hourly values of measured 

or modeled solar radiation data and meteorological data for 239 National 

Weather Service stations from 1961 to 1990. NREL completed Version 1.0 

in 1992. Version 1.1 will be completed in early 1994. 

Data from two types of stations-primary and secondary-make up this data 

base. Primary stations, of which there are 56, measured solar radiation for 

at least part of the 30 years. The remaining 183 secondary stations made 

no solar radiation measurements at all. For these stations, researchers 

used the METSTAT model, which is derived from meteorological data such as 

cloud cover. 

The National Solar Radiation Data Base is distributed on CD-ROM by the 

National Climatic Data Center, Asheville, North Carolina, as a three-volume 

set (eastern, central, and western United States). Each volume is $100. 

Team members explained the contents of the new data base and the 

scientific principles and methods 

used for its development. In addition, 

invited speakers presented information on other recent advancements 

pertaining to resource assessment. 

On the last day, a panel discussion on 

resource assessment needs took place 

to help prioritize future resource 

assessment activities. 

CD-ROM Version of the 

New Data Base 

Project team members provided 

technical support and review to 

the National Climatic Data Center 

(NCDC) When it put the National 

Solar Radiation Data Base on a threevolume CD-ROM set. This set can be 

ordered from NCDC by contacting 

User Support at (704) 259-0682. 
Comparison of the New Data Base 

with the SOLMET /ERSATZ Data Base 

Team members compared the 

National Solar Radiation Data Base 

with the SOLMET/ERSATZ data 

base and published the results in 

A Comparison oj Data from 

SOLAfET/ERSATZ with the 

National Solar Radiation Data Base 

(NREL/TP-463-5118). This stationby-station comparison of monthly 

and annual values of global hOlizontal 

and direct beam radiation revealed 

significant differences in the data 

bases. Consequently, using the new 

data for analysis or design of solar 

energy systems will give different 

results, which, based on the factors 

mentioned in the accompanying sidebar, are deemed to be more realistic. 

Solar Radiation Data Manual for FlatPlate and Concentrating Collectors 

The project team is developing a 

new solar radiation data manual 

based on the new data base. The 

manual will contain, for each of the 

239 stations, 30-year monthly and 

annual averages of solar radiation 

values for 14 configurations of flatplate and concentrating collectors. 

Minimum and maximum monthly 

and annual values also will be included, 

as well as pertinent climatic data. 

On an annual basis, the comparison of the old data base with the National 

Solar Radiation Data Base showed that global horizontal radiation data for 

40% of the monitoring stations differed by more than 5%; data from some 

stations differed by as much as 18%. For direct beam radiation, data for 

60% of the stations differed by more than 5%; data for some stations showed 

differences of as much as 33% .. Differences between the two data bases 

were even greater when compared on a monthly basis. 

Data from the new data base accounts for any recent climate changes and 

provides more accurate values of solar radiation. These values are believed 

to be more accurate for several reasons: 

The new data base contains more measured data. 

Researchers used a better model to produce estimated values {more 

than 90% of the solar radiation data in both data bases are modeled}. 

Researchers used improved calibration methods. 

Researchers used a more effective means to assess the quality 

of the data. 
The solar industry cooperated in 

the development of this manual 

by providing suggestions and ideas. 

The manual will be completed and 

available for distribution in early 1994. 

New Typical Meteorological Years 

Improving the typical meteorological 

year (TMY) for each monitoring 

station is possible now that the project 

team has completed the National 

Solar Radiation Data Base. TMYs 

are serially complete data sets of 

hourly values of solar radiation and 

meteorological elements for 1 year. 

They represent conditions judged to 

be typical over a long period, such as 

30 years. TMYs are used for computer 

simulations of solar energy conversion 

systems and buildings. Because they 

are condensed into a I-year time 

frame, they are easier and faster 

to use than 30-year data sets. 

In FY 1993, the project team reviewed 

applicable methods for deriving TMYs, 

developed a plan, and distributed it 

for internal and external peer review. 

Comments and ideas of the reviewers 

are included in the final plan being 

implemented. The new TMYs will be 

completed and available for distribution in 1994. 

Solar Data Display Using 

a Geographic Information System 

The project team incorporated data 

from the new data base into NREL's 

Geographic Information System 

(GIS), a powerful tool for displaying 

geographically based data sets in ways 

that can facilitate the deployment 

of renewable energy technologies. 

From that data, the GIS produced 

preliminary color contour maps 

depicting solar resources in the 

United States. 

GIS's capabilities permit layering data 

on one map to provide color-coded 

answers to complex problems. For 

example, three types of data, such as 

solar radiation values, land use restrictions, and electric transmission line 

locations, can be layered on one map. 

These layers of data show where 

opportunities exist for using solar 

technologies based on where the solar 

resource is good, where no land use 

restrictions exist, and where access to 

electric transmission lines is available. 

Users Manual for Quality Assessment 

Produced by project team members, 

the Users Manual for the Quality 

Assessment of Solar Radiation Data 

provides monitoring station and 

network operators with technical 

information and computer software 

instructions for assessing the quality 

of solar radiation data. 

Reprinting of Shining On: A Primer 

on Solar Radiation Data 

Requests for copies of Shining On 

for use in workshops sponsored by 

the Electric Power Research Institute 

prompted the reprinting of this 

publication. It uses a friendly 

question-and-answer format to 

explain the fundamentals about 

solar radiation data and their uses. 
An 

DOE, through NREL, plans to meet 

the renewable energy industry's 

growing needs for high-quality 

resource information by expanding 

the Resource Assessment Program. 

This expansion will be supported 

with planned increased funding 

beginning in FY 1994. 

The expanded program will focus 

on the development of a Renewable 

Energy Resource Data Center at 

NREL. Project team members plan 

to incorporate data from the Solar 

Radiation Resource Assessment 

Project, as well as data about other 

renewable energy technologies 

(wind, biomass, hydro, and geothermal), into a Comprehensive 

Renewable Energy Data Base. 

Through the development of this 

data base, users will have ready 

access, for the first time, to renewable resource information presented 

in common formats for all technologies. This access will greatly improve 

the ability to plan and implement 

renewable energy systems, including 

hybrid systems. 

Beginning in FY 1994, the project 

team will begin collaborative 

discussions with researchers involved 

in the evaluation of resources in other 

renewable technology areas to determine the format and availability of 

resource data that can be incorporated into the Renewable Energy 

Resource Data Center. In addition, 

these collaborations may lead to a 

pilot study with the wind program to 

develop a hybrid wind/solar data 

base. This data base would be one of 

the first major products of the center. 

The expanded program will also 

focus on the establishment of a 

National Cooperative Solar Network 

(NCSN). Managed through regional 

centers selected by NREL from a 

competitive solicitation, this program 

will archive solar irradiance measurements collected by many existing 

solar and meteorological monitoring 

networks across the United States. 

In exchange for the data, NREL 

will provide technical assistance and 

offer calibration support to the 

participating networks through 

the regional centers. 
The 1'<'"" ..

 ", •• "" 

Resoluce Data 

In FY 1993, the project team began 

to design and build the Renewable 

Energy Resource Data Center. Plans 

are for this activity to be substantially 

increased in FY 1994 as work progresses toward developing a center 

modeled after the Distributed Active 

Archive Centers. These centers have 

been established around the United 

States to archive and disseminate 

large, diverse data sets. 

The Renewable Energy Resource 

Data Center will make available the 

information in the Comprehensive 

Renewable Energy Data Base. 

Furthermore, because of the 

importance of displaying the 

geographic variability of various 

renewable energy resources, along 

with other important energy-related 

data bases (such as the location 

of transmission lines and energygenerating stations) and demographic 

data bases, a geographic information 

system will be one of the primary tools 

of the new center. Center users will be 

able to access data via electronic mail. 

On-line user support, including 

menu-driven options and assistance, 

will be a primary feature of the center. 

Transmission 

Lines 

Capacity (kV) kWhjm2 per day --115 IlIlI 3.5 to 4.0 

------ 138 � 4.0 to 4.5 f-+-

 161 0 4.5 to 5.0 

u. 230 0 5.0 to 5.5 

If-fH 345 0 5.5 to 6.0 

�---.500 o 6.0 to 6.5 

!i2l 6.5 to 7.0 

The Renewable Energy Resource Data Center will include GIS capabilities 

that will allow users to overlay resource information with other important 

data. This map of Texas, showing the distribution of direct normal solar 

radiation and the location of transmission lines, is an example of such 

a GIS product. This type of analysis helps users investigate the opportunity 

for deploying solar technologies near major transmission lines. 
The Solar Radiation Resource 

Assessment Project continues to 

broaden its activities to meet the 

increasing needs of the renewable 

energy industry. For FY 1994 and 

beyond, the activities of the project 

will include the following: 

Publishing the validation of the 

National Solar Radiation Data Base, 

the data manual for flat-plate and 

concentrating coilectors, and the 

new TMY data sets 

Using GIS capabilities and the new 

national data base to generate 

seasonal and annual solar radiation 

maps for the United States and its 

territories tailored for various 

types of solar collectors 

Initiating a National Cooperative 

Solar Measurement Network Program 

to increase the number of stations 

measuring high-quality solar 

radiation data 

Expanding access to international 

solar radiation data by assisting 

the World. Radiation Data Center 

in St. Petersburg, Russia 

Using satellite imagery to estimate 

soiar radiation 

Providing users with electronic 

access to the Renewable Energ), 

Resource Data Center at NREL 

The project team will use satellite data from the NASA/Langley Research 

Center in Hampton, Virginia, to provide gridded, monthly average estimates 

of global horizontal solar radiation for any iocation on Earth. This gridded 

map of �vlexico for July 1985 is an example of how the data will be displayed. 

Steve Wilcox. NREL 

The Main ,f'f),n"''''lf,71. Observatory in 

St. VPTpn,'YI

"'rr Russia, houses the World 

Radiation Data Ceniel: Project team members 

will continue to assist with the 

and communications 

center. This will 

NEEL and others to access the center's 

data archives r:/e,crnml"caLJ!'I!. 
The vision of NREL 's Resource Assessment Program 

is to be recognized throughout the world as the primary center 

for objective, accurate, timely, and relevant information on 

renewable energy resources and to enable industry and others 

to use this center of excellence to facilitate the deployment 

of renewable energy technologies. 
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