Shining On: primer on solar radiation and solar radiation data

8/8/2019 Shining On: primer on solar radiation and solar radiation data

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nut are solar radiation data?Solar radiation data provide informationon how much of the sun 's energy strikes asurface at a location on earth during a particular time period. The data give values ofenergy per unit of area. By showing naturally occurring changes in the amount of so larradiation over the course of days, months,and years, these data determine theamount of solar radiation for a location.The units of measurement are expressedas kilowatt-hours per square meter(kWh / m2), megajoules per square meter(Mj/m'), langleys (L), or British thermalunits per square foot (Btu/ ft') .

Today, the primary source of solar radiation data for the United States comes frommeasurements made by the NationalWeather Service at 26 SOLMET (SOlarMETeorological) stations from 1952 to1975. In addition, mathematica l modelses timated data for 222 ERSATZ (synthetidstations where no solar radiation measurements were made. Because the equipmentdid not always accurately measure thesolar radiation and the models used. werelimited in their application, the data do notalways correlate well withmore recent field measurements.To provide better data,we developed a Na tionalSolar Radiation Data Base.This data base covers 30 years(1961-1990) and comes frominformation recorded bymore accura te instrumentsand from better models. In1992, this new data base willbe available for 250 sites.

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" SOlMET ERSATZ

From 1952 to 1975 so lar radiation was measuredat 26 SOLMET stations ( . . ) and modeled for 222ERSATZ stations ( ). Most of these stations willbe included in the new National Solar RadiationData Base.

Guantanamo Bay, CubaKoror Island. PacilicKwajalein ISland. PacificSan Juan. Puerto RicoWake Island. Pacilic

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do we need solar radiation data?The earth receives a vast amount of energyfrom the sun in the fonn of solar radiation.If we converted to usable energy just 0.2%of the so lar radiation that falls on our na tion, we would meet the energy demand ofthe entire United States. A variety of solarenergy technologies arebeing developed toharness the sun's energy including: so lar electric (photovoltaid for convert

ing sunlight directly into electricity; so lar heat (thennaD for heating water for

industrial and household uses; so lar therma l electric for producingsteam to run turbines that generate

electricity; solar fuellechnologies for convertingbiomass (plants, crops, and trees) intofuels and by-products; passive solar for lighting and heating

buildings; and solar detoxification for destroying haz

ardous waste with concentrated sunlight.

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"The more accurately rue knolU thesolar resource, the better we calloptimize the system. Therefore,accurate soffir radiatio1J data are011 important factor i l l solarsys tem des ign."

o.wld F.u.nkuc:dNationIII..abonIIOrte.

The economics of these technologiesdepend on the equipment and operatingcosts, the percentage of the soL.1r rad iationthat can be converted into the desiredenergy product, and the amount of solarradiat ion available. Users of these technologies need h igh-quality solar radiationdata. If the actual solar radiation for a location is less than indicated by available datathe perfonnance and the economic goalsfor the system will not be met. On Iheothehand, if the actual solar energy at a locationis greater than indicated by the data, theperfonnance and economic projectionsmay be 100 conservative and prevent aviable technology from being used .

To minimize energy consumption, heating and air conditioning engineers alsouse solar rad iation data 10 select bu ildingconfigurations, orientations, and air conditioning syslems. Because energy costsare a significant expense in building ownership, an energy-efficient design can s ignificantly reduce the life-cycle cost of abui ld ing.

The amount of solar radiation receivedchanges throughout the day and yeardu e to weather patterns and the changingposition of the sun. and solar radiationdata reflects this variabi li ty. By knowingthe va riability, we can size storage systemsso they can provide energy at night andduring cloudy periods. For technologies


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Because of absorption and scattering by the atmosphere, the spectral distribution of so lar radiationoutside th e ahnosphere differs significant1y fromthai on earth. Also, the spectral dis lribut.ion onearth changes throughout the da y and year andis influenced by location, climate, and atmosphericconditions. Consequently, the percentage of energythat is comp osed of UV, visible, or neal'-infraredradialion, or portions thereof, 3.150 varies bylocation, time of day, and year.

wi th no energy storage, we can eva ll1 ateload matching by comparing the profile ofthe availab le so lar radiation throughout theday with the p r o f i l ~ of the energy requiredby the load. Solar radiation data also helpdetermine the best geographic loca tions forsolar energy technologies. Other factorsbeing equal, a site receiving more so larradiation w ill be more economical.

For certain technologies, we also need toknow the spectral, or wavelength, distribution of the solar radiation. For example,photovol taic dev ices respond primari ly towavelengths in the visible and near-infrared region of the spectrum, while solardetoxification uses energy from theultraviolet (UV) region. Location, climate,and atmospheric conditions influence thespectral di stribution of solar radia tion.

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Th is remote wate ...leve l-monitoring station uses photovoliaics for chargingstorage batteries that supply electric power. Solar radiation data prov ideinformat ion for determining the size of the photovoltaic and battery systemneeded to su ppl y remote stations like this with reliable elecb'ic se rvice.

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n u ~ influences the amount of solar radiation?The amount of solar radiation reaching theea rth's surface varies greatly because ofchanging atmospheric conditionsand thechanging position of the sun, both duringthe day and throughout the year. Cloudsare the predominant atmospheric conditionthat detennines the amount of solar radia-tion that reaches the earth . Consequently,regions of the nation with cloudy climatesreceive less solar radiation than the cloud-free desert climates of the southwesternUnited States. For any given location, thesolar radiation reaching the earth's surfacedecreases with increasing cloud cover.

Local geographical features, such asmountains, oceans, and large lakes, in-fluence the formation of douds; therefore,the amount of solar radiation received forthese areas may be different from thatreceived by adjacent land areas. For ex-ample, m01.Ultains may receive less solarradiation than adjacent foothills and plainslocated a short distance away. Winds blow-ing against mountains force some of the ai rto rise, and douds fonn from the moisturein the air as it cools. Coastlines ma y alsoreceive a different amount of solar radia-tion than areas further inland. Where thechanges in geography are less pronounced,such as in the Great Plains, the amount ofsolar radiation varies less.

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Many atmospheric scientistbelieve that the eruption ofMOllnt Pinatubo in June 199will have worldwide effectsduring the next few years.This was on e of the largestvolcanic eruptions of the 20century. Volcanic ash andsulfur dioxide spewed highabove the Philippines andinto the stratosphere; theresulting dust cloud spreadaround the earth's equatorand toward higher latitudesThe increased dust diminishthe solar radiation receivedat the earth's surface. Peakeffects will occur in 1992,but colder winters andcooler summers may ensueuntil near the middle ofthis decade. Long-termmeasurement of solarradiation at numerous sitespermits naturally occurringevents such as this to beevaluated with respect totheir impact on the solarresource and the climate.


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Clouds are the predominant atmospheric co ndition that detennines the amountof solar radiation reaching the earth.

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The amount of solar radiation also vardepending on the time of day and theseason. In general, more solar radiation ispresent during midday than during eithethe early morning or late afternoon. At mday, the sun is positioned high in the skyand the path of the sun 's rays through thearth's atmosphere is shortened. Consequently, less solar radiation is scattered oabsorbed, and more solar radiation reachthe earth's surface. In the northern hemisphere, south-facing collectors also receivmore so lar radiation during m idday because the sun's rays are nearly perpendicular to the collector su rface. Trackingcollectors can increase the amount of so laradiation received by tracking the sun ankeeping its rays perpendicular to the colltor throughout the day. In the northernhemisphere, we also expect more so larradiation during the summer than duringthe winter because there are more daylighours. This is more pronounced at higherlatitudes.

Both man-made and naturally occurrinevents can limit the amount of solar radiation at the earth's surface. Urban air pollution, smoke from forest fires, and airbornash resulting from volcanic activity reducthe solar resource by increasing the scatteing and absorption of so lar radiation.Thihas a larger impact on radiation coming ia direct line from the sun (direct beam)than on the total (global) solar rad iation.Some of the direct beam radiation is sca ttered toward earth and is called diffuse (skradia tion (global = direct + diffuse):Consquently, concentrators that use on ly direcbeam solar radiation are more adverselyaffected than collectors that use global soradiation. On a day with severely polluteair (smog alert), the direct beam solar radtion can be reduced by 40% whereas theglobal solar radiation is reduced by 15% t


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25%. A large volcanic eruption maydecrease, over a large portion of the earth,the direct beam solar radiation by 20% andthe global solar radia tion by nearly 10% for6 months to 2 years. As the volcanic ashfalls out of the atmosphere, the effect isdiminished, but complete removal of theash may take several years.

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parts of solar radiation are measured?The total or global solar radiation striking acollector has two components: (1) directbeam radiation, and (2) diffuse radiation.Additionally, radiation reflected by thesurface in front of a collector contributes tothe solar radiation received. But unless thecollector is tilted at a steep angle from thehorizontal and the ground is highly reflective (e.g., snow), this contribution is small.

As the name implies, direct beam radiation comes in a direct line from the sun.For sunny days with clear skies, most of

the so lar radiation is direct beam radiatioOn overcast days, the sun is o b ~ bythe clouds and the direct beam radiationis zero.

Diffuse radiation is scattered out of thedirect beam by molecules, aerosols, anddouds. Because it comes from all regionsof the sky, it is also referred to as sky radiation. The portion of total so lar radiationthat is diffuse is about 10% to 20% for cleaskies and up to 100% for cloudy skies.

Some of th e solar radiation entering the earth's abnosphere is absorbed and sca ttered. Direct beamradiation comes in a direct line from the sun. Diffuse radiation is scattered ou t of the direct beam bymolecules, aerosols, and clouds. The sum of the direct beam, diffuse, and ground-reflected radiationarriving at the surface is caUed total or global solar radiation.

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The type of data needed and the fundsavailable help determine the number andkinds of instruments used at a site tomeasure solar radiation. A complete solarradiation monitoring station has instrumentation for measuring three quantities:(1) to tal or global radiation on a horizontalsurface, (2) diffuse radiation on a horizontalsurface, and (3) direct beam radiation.Measuring all three quantities providessufficient information for understandingthe solar resource and for rigorous qualityassessment of the data . Any two of themeasured quantities can be used to calculate a range of acceptable values for thethird . Many monitoring stations also haveequipment for measuring solar radiation ontilted and tracking surfaces and for measuring meteorological parameters such as am-bient temperature, relative humidity, andwind speed and direction.

A station with a lower level of fundingmay only measure two quantities; the thirdis calculated. For example, the direct beamcomponent can be derived by subtractingthe diffuse radiation from the globalradiation and applying trigonometTicrelationships to account for the positionof the sun. The trade-off for this approach isthat the calculated direct beam data are lessaccurate than if the direct beam data weremeasured .

HistOrically, many stations havemeasured only the global radiation on ahorizontal surface. This necessitates calculating both the diffuse and direct beamso lar radiation, which results in lessaccurate values for these two quantitiesthan if they were measured.

In the absence of any solar radiationmeasurements, we employ models usingmeteorological data such as cloudinessand minutes of sunshine to estimate solarradiation. Although much less accurate,this is often the only option we have forlocations where solar radiation is notmeasured. Cloudiness data are based onobservations by a trained meteorologistwho looks at the sky and estimates theamount of cloud cover in tenths. A dearsky rates a cloud cover value of 0 tenths,and an overcast sky rates a cloud covervalue of 10 tenths. Minutes of sunshine arerecorded by an instrument that measuresthe time during the day when the sun isnot obscured by clouds.

To investigate the spectral distributionof solar radiation, an instrument called aspectroradiometer measures the solarradiation intensity at discrete wavelengths.Spectroradiometers are complex andrelatively expensive instruments, andtheir operation and maintenance requireSignificant effort. Consequently, spectroradiometers are not routinely used forlong-tenn data collection. Rather, theyhelp establish data bases that have sufficient information to validate models thatpredict the spectral distribution basedon meteorological data and the positionof the sun .

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ow do we use solar radiation data?Solar energy technologies rely on solarradiation to provide energy for producingelec tricity, heating water, destroying toxicwastes, and lighting and heating buildings.Common to these technologies is that theend-use product is, for the most part, adirect function of the amount of solar radiation received and the conversion efficiency.

Windows can significantly affect the heating and cooling loads of buildings.Engineers and architects can use solar radiation data 10 evaluate the effectsthat windows will have on the energy consumption of a building and hencedelennine the size of heating and air conditioning equipment needed.

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That is, if the amount of solar radiationis increased, then the end-use productincreases also. Th is is also true for solarfuel production, in which crops are grownand then converted into fuels and byproducts. Although dependent on theso il type and rainfa ll, crops also depend onthe amount of so lar radiation received.

To determine the performance an d economics of solar conversion technologies,designers and eng ineers use so lar radiatiodata to estimate how much solar energy isavailable for a site. Depending on the pa r-ticular technology, the solar collector mighbe a photovoltaic array, a concentratingpa rabolic trough, a domestic hot watercollector, a w indow, a skylight, or a canopyof fo liage. Desig ners and engineers usehand calculations or computer simulationsto estimate the solar radiation striking acollector.

Hand calculations are approp riate w henusing solar radiation data that representan average for an extended period. Forexample, designers of remote photovoitaicpowered systems for charging batteries useaverage daily solar radiation for the monthto d etermine the size of the photovoltaicarray. The criteria for this application is notthe amount of solar radiation for a g ivenhour or day but whether or not the averagdaily solar radiation for the month is sufficient to prevent the batteries from becoming d ischarged over several days.

The month used in the design processdepends on the relative amount of solarradia tion available compared to the energyrequired by the load. For a system in whichthe load is constant throughout the year,solar rad iation data for December orJanuary are usua lly used for the northernhemisphere.


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Computer simulations are an effectivetool when an hour-by-hour performanceanalysis is needed. Utility engineers maywant to know if the output of a solar .electric power plant could reliably andeconomically help meet their daytimeelectric demand. (One of the potentialbenefits of a solar electric power plant isthat its output may coincide with the utilitypeak electric demand for summertime airconditioning loads.) By using the hourlysolar radiation data for its location, theutility can run computer programs thatshow how much energy could beproduced on an hour-by-hour basisthroughout the year by the solar electricpower plant.Some solar energy conversion technologies require a threshold value of solarradiation before certain operations canbegin or be sustained. As an example, acentral receiver solar thermal electricpower plant may require direct normalsolar radiation values above 450 W1m2 toproduce stearn for the turbine generator.Consequently, to evaluate a site's potentialfor solar thermal electric production, adesigner examines the solar radiation datato determine the times of day when thesolar radiation exceeds the threshold value.Heating and air conditioning engineersuse solar tadiation'data to optimize building designs for energy efficiency. Forexample, window orientation and size canaffect the heating and cooling of the building. South-facing windows transmit solarenergy in the winter that is beneficial inreducing heating requirements. But in theswnmer, solar energy transmitted throughwindows (primarily those that face east orwes t), must be offset by increased operationof the air conditioning system. By havingaccess to solar radiation data for their location, engineers and architects can evaluatethe effects of window orientation and size

o 4 8 12 16 20 24lime of day (hour)

Computer simulation using solar radiation data shows how the output of twophotovoltaic power systems could be added to the utility's generation to helpmeet peak electric demand in the summer. The fixed-tilt array faces south andis tilted from the horizontal at an angle equal 10 the site's lati tude. The trackingarray uses motors and gear drives to point the array at the sun throughout theday. Depending on location, the photovoltaic system with the 2-axis trackingarray receives annually 25% to 4 0 ' 7 more global solar radiation than the fixedtilt photovoltaic system and provides more power for longer periods. Thismust be weighed against the higher initial cost and maintenance requiredfor the tracker.

"Because the solar load is the largestcompO/len t for building exteriorsurfaces, and because windows arethe most sensitive to the solar load,solar radiation data are essential forthe accurate and energtJ efficientdesigtl ofbuildings and their aircondition ing sys tems."

Jack F. Roberts, PE.American Society of Heating, Refrigeratingand Air-Conditioning Engineers

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Concentrator collectors (top) use direct beam solar radiation; flat-plate collectors(bottom) use direct beam radiation, diffuse (sky) radiation, and ground-reflectedradiation.

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on the energy constunption of the buildingand determine the size of the heating andair conditioning equipment needed. Theycan use this information, combined withdesired levels of natural lighting and thebuilding aesthetics, to fonnulate the finalbuilding design.

Except for concentrator systems, solarradiation data cannot be used without firstaccounting for the orientation of the solarcollector. Concentrators track the sun andfocus only direct beam radiation, but flatplate collectors receive a combination ofdirect beam radiation, diffuse (sky) radiation, and radiation reflected from the groundin front of the collector. Depending on thedirection the collector is facing and its tiltfrom the horizontal, flat-plate collectorsreceive different amounts of direct beamradiation, diffuse radiation, and groundreflected radiation. Designers employequations to calculate the total or globalradiation on a flat-plate collector. Theequations use values of the direct beamradiation, the diffuse radiation on ahorizontal surface, and the orientationof the collector.

To maximize the amount of solar radiation received during the year, flat-platecollectors in the northern hemisphere facesouth and tilt from the horizontal at anangle approximately equal to the site'slatitude. The annual energy productionis not very sensitive to the tilt angle as longas it is within plus or minus 15 of thelatitude. As a general rule, to optimize theperfonnance in the winter, the collector canbe tilted 15 greater than the latitude. Tooptimize performance in the summer, thecollector can be tilted 15 less than thelatitude. Solar radiation data combinedwith computer simulations can definethese relationships more precisely.

In the initial design stage, d esigners ofcells used in photovoltaic mod ules canuse spectral solar radiation da ta ba ses and


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models to optimize the ceUs for maximumenergy production. Because the spectralcontent of solar radiation changes throughout the day and season, photovoltaic ceUsare tailored for a specific range of solarradiation wavelengths that will producethe most energy. Different photovoltaicmaterials have different peak responses;performance models using spectral solarradiation data bases can compare two ormore photovoltaic materials operatingunder a range of seasons and climates.This results in optimizing the design earlyand eliminates the expense and time thatwould otherwise be needed for preliminary field testing.

"For sizing stand-a lonePV systems, we calcli late theIlwnber of PV modliles required to keep the batteriescharged by lIsing the average daily solar radiation incidenton the collector for the month of the year with the smallestratio of solar radiation to electric load demand."

RIchard N. ChapmanSandIa NaUonal Laboratories

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ne1'e can you obtain solar radiation data?The National Weather Service of theNational Oceanic and AtmosphericAdministration (NOAA) opera tes monitor-ing stations in the United States to collectand disseminate information about solarrad iation. This information is available oncompu ter readable magnetic tape fromNOAA's National Climatic Data Center(NCOC), Federal Building, Asheville, NC28801 (704) 259-0682 .

Most of NOAA's solar radiation datasets are from 26 SOLMET stations and222 ERSATZ stations and consist of hourlyvalues of solar radiation and meteorologi-cal data from 1952 to 1975. For theSOLMETstations, instruments measured the globalhorizontal so lar radiation and researchersmodeled the direct beam solar radia tiondata. For the ERSATZ stations, although nosolar radiation measurements were made,researchers modeled global horizontalradiation based on observed meteorologi-cal data such as cloudiness and minutes ofsunshine; the ERSAlZ data do not includedirect beam radiation. Because all theERSATZ data are modeled, these data areless accurate than the SOLMET data.

NOAA also has available more recentdata for the periods 1977 to 1980 and 1988to the present. The data include hourlyvalues of measured global horizontal solarradiation for 38 s tations, measured directbeam solar radiation for 32 stations, andmeasured diffuse horizontal radiation fo rnine stations.

Two of NOAA's data sets are of partic-ular interest to designers and engineers: thetypical meteorological year (TMY) data setand the weather year for energy calculations(WYEC) da ta set. For these, researchersextracted infonnation from SOLMETERSATZ data to make data sets of hourlyva lues spanning one year. For the ERSATZ

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TMY data, researchersincluded values of directbeam radiation withmodeled values of globalhorizontal radiation. These data sets repre-sent typical values occurring from 1952 to1975, and not the minimum or maximumvalues. For example, a cloudy year in thispe riod may ha ve had an annual solar radia-tion value 10% below theTMYvalue, and avery cloudy month in this period may havehad a solar radiation value 40% percentbelow its TMY value. A difference betweenTMY and WYEC data is that the TMY dataare weighted toward solar radiation valuesand their hourly distribution, whereas theWYEC data are weighted toward averagemonthly values of temperatures and solarradiation. Researchers recently revisedthe WYEC data to include estimates ofdirect beam and diffuse solar radiation


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and es timates of illuminance for ligh tingapp lications. llluminance refers to solarrad iation in the visible region of the solarspectrum to which the human eyeresponds.

Solar radiation data derived from theSOLMET/ERSATZ data sets are also published in tabular fonn by the NationalTechnicallnfonnation Service (NT IS), U.s.Department of Commerce, 5285 Port RoyalRoad, Springfield, VA 22161. Two of thesetabular data sets are listed below.

Illsolatioll Data Manunl and Direct Nonna iSowr Rndiatioll Data Malmal, SERl/ TP-22O-3880, Gold en, CO: Solar Energy ResearchInstitute, July 1990.

This map shows the globa lsolar radiation for a southfacing collector tilled at anangle equal 10 the si telatitude as an annual dailyaverage for differentlocations in the UnitedStates. one numbers onthe map represent MJ /m2;mu ltiply by 0.2778 toobtain kWhlm1.)

This manual contains monthly averagesof global horizontal and direct beam solarradiation, ambient temperature, the ratioof global horizontal solar radia tion on earthto that outside the earth 's ahnosphere (Kt),and heating and cooling degree-days.This infonnation is presented for all theSOLMET/ERSATZ sta tions.

Stand-AlonePllOtovo/taic Systems: AHandOOok ofRecommended Design Prod ices,SAND87-7023, Albuquerque, NM: SandiaNa tional Laboratories, March 1990.

The appendix of this handbook containsmonthly estimates of solar radiation striking collectors. These estimates are ca lculated for different tilts and sun-tracking

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NOAA's National O imatic Data Center has solar radiation data ava ilable oncomputer readable magnetic tape. The data sets are fo r 26 SOLMET stationsand 222 ERSATZ stations and consist of hourly values of solar radialion andmeteorological data from 1952 to 1975.

schemes. The estimates are for a selected setof 38 SOLMET(ERSATZ stations and arebased on theS OLMET(ERSATZ data.

Maps are available that dep ict long-termaverage solar radiation data for each month.This is a convenient way to show variationsin the amount of solar radiation and for interpola ting data between sta tions. For theUnited States, these maps were made usingso lar radiation data from the SOLMET/ERSATZ data base. The Solar RndiafiollEnergy Resource Atlas of tlte United States ItNlSpublished by the Superintendent of Docu-ments, but is out of print. This a tlas is available a t some university and city libraries.The University of Lowell compiled anintemational so lar radiation da ta base forlocations ou ts ide the United States. Thisdata base presents average d a ily valuesby mon th and year for global horizontalso lar radiation. It is ava ilable from theUniversity of Lowell Photovoltaic Program,1 University Avenue, Lowell, MA 01854(508) 934-3377.

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Solar radiation data recorded forl-minute intervals are available for fourlocations: Albany, Ne w York; Atlanta,Georgia; Davis, California; and San AntonTexas. The data were recorded over perioof 1 year or more by university meteorolcal research and training stations. Becausof the time scale used, these da ta areprimarily of interest to researchers studying transient responses in solar energy tenology sys tems. These data are availablefrom the National Renewable EnergyLabora tory (NREL), 1617 Cole BoulevardGo lden, CO 80401.

A spectral so lar radiation data base repsenting a range of atmospheric and climaconditions is also available from NREL.This da ta base includes mo re than 3()(x)spectra measured over a wavelengthrange from 300 to 1100 nanometers at2-nanometer increments (1 nanome ter isone-billionth of a meter) and is the resul tof a cooperative effort between NREL,the Electric Power Research Institute, theRorida Solar Energy Center, and the PacGas a nd Electric Company. Spectral solarradiation was measured at three sites: CaCanaveral, Rorida; San Ramon, Californiand Denver, Colorado. This data base ca nhelp determine whether spectraUy selecttechnologies (such as photovol taics andbiomass) are optimized for a particularlocation and climate.

Additionally, other sources of so lar radtion data are state and local governmentsutilities, and uni versities. Examples incluthe Padfic Gas and Electric Solar InsolatiMonitoring Program, the University ofOregon/Pacific Northwes t Solar RadiatioData Network, and the Historically BlackCo lleges and Universities Solar RadiationMonitoring Network.


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awwillwemeet our solar radiation data needs?One of the goals of the Solar RadiationResource Assessment Project at NREL is toprovide accurate information about solarradiation to minimize the economic riskof implementing solar energy conversiontechnologies. The data must accuratelyrepresent the spatial (geographid, temporal (hourly, daily, and seasonal), andspectral (wavelength distribution) variability of the solar radiation resource atdifferent locations.

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The new National Solar Radiation DataBase (1961-1990) for the United States willimprove data quality over the existingSOLMET/ERSATZ (1952-1975) data base.For this new data base, NOAA used betteequipment for measuring solar radiationat more sites and NREL used better modeing techniques for synthetic stations.Scheduled for completion in 1992, this newdata base will include data for 250 sites.After completing the data base we willproduce special purpose products such astypical meteorological year (TMy) datasets, maps, and data summaries.

By continuing the long-tenn measurement of solar radiation at numerous sites,we can assess changes in climate and addnew data to existing data bases. We canimprove the quality of the solar radiationdata base for the United States by workinwith existing regional solar radiationnetworks and establishing educationalinitiatives so that data are being collectedat several hundred sites in the UnitedStates. This large number of measuremensites will improve the quality of the solarradiation data base, better represent thegeographic distribution of solar radiationin the United States, and provide researchdata to develop techniques to estimatesolar radiation where there are no measurment stations.

This type of research involves developing spatial interpolation techniques, suchas mapping s ~ a r radiation using cloudcover infonnation from satellites, toestimate solar radiation between measurement stations. This cloud-cover mappingtechnique promises high spatial resolutionfor the optimum siting of solar energy conversion technologies and enables estimating solar radiation for countries whereno solar radiation data base exists.


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NREL is improving the equipment andtechniques used to measure solar radiationand the models and methods used to deter-mine the performance of solar conversiontechnologies.Our recent activities include: angular response characterization anduncertainty analysis of solar radiometers, development of improved quality assess-ment procedures for solar radiation data, calibration of radiometers for industry

and members of the scientific community, development of both broadband andspectral solar irradiance models, and conhibutions to the development of

solar trackers and spectroradiometers.For information about solar radiationdata, models, and assessments contactthe NREL Technical Inquiry Service at

303/231-7303.Ooud-cover infonnation. analyzed from photographstaken by satelli tes, has the potential for estimating solarradiation at any location on earth.

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Documents

Shining On: primer on solar radiation and solar radiation data