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HomeMy WebLinkAboutSolar Radiation Assessment for Anchorage, Alaska, January 1983SOLAR RADIATION ASSESSMENT FOR ANCHORAGE, ALASKA By Richard Becker, Jr. And Lynn D. Leslie January 1983 This analysis was performed under Alaska Council on Science and Technology Grant Number W81-01 ACKNOWLEDGEMENTS The authors wish to thank their associates at AEIDC for the opportunity to produce this report from the data collected at this site. Specifically we wish to acknowledge James L. Wise for his support and expertise regarding solar energy. Thanks also to Suzanne K. Whitehurst for her secretarial support and Judith Brogan for her editorial assistance. We appreciate the efforts of Laura Larson, Car- tographer, Aubrey Nixon, Typesetter, and the entire Production Department for their diligent work on this report. ABSTRACT This report summarizes data from in- struments (pyranometers) measuring: incoming solar radiation reaching the roof of the Arctic Environmental Information and Data Center building in downtown Anchorage. Receptors mounted on a south-facing 61-degree inclined plane and a 180-degree horizontal plane have been taking continuous readings for three and one-half years. A third instrument with a 90-degree vertical south-facing plane has been operational for two and one-half years. Mount angles were chosen to demonstrate the relation- ship between collector angle and energy recep- tivity. Effects of local obstructions and climatic variables are discussed as well as aspects of physical meteorology that pertain to solar radiation and energy in Anchorage. On an annual basis the 61-degree inclined plane absorbs the most radiation. Seasonally the horizontal receptor gathers the most radi- tion in summer and the vertical gathers the greatest amount in winter. Cloudy days are warmer than sunny days in fall, winter, and spring. Both the inclined and vertical planes are used as practical solar collectors. The vertical collec- tor is a simple cost-effective solution if the prime purpose is energy efficiency during the coldest months of the year. The inclined collec- tor has additional year-round benefits but has some design disadvantages. Funds for the Solar Radiation Assessment were provided by the Alaska Council on Science and Technology under the auspices of the Geophysical Institute, University of Alaska, Fairbanks, Grant No. W81-01. TABLE OF CONTENTS Page Acknowledgements ........ OMIA sl 52 2.250 2 n9556 List of Symbols ........... List of Tailes 2. .....55.... List of Figures ............ ENUOGRCHION: «ooo. s seine MisCUMION, 26.6 555585. Physical Meteorology BERSCER Corina asec AMEE OOEN 9555251535500 101 ye foo lola sa ws cxeiarerarereioieioww leaieieotowions 4 Black Body Radiation 5 PRUMTORDINOTIC EAIOCtS ooo Sos ears oa oo Se ee eee 6 The Albedo .......... 6 aes SClONES: is. on cece ce 6 Climate and Local Variables 8 TORE OMRON | 555.555.5588 aie wis 5:5 0510 G1 Ws 51a, FT 018) ore a:e1ei vre'eie o: Viv ote 519 HE 8 Descriptiofiof the Seasons .....2......... 0. cece cece ces ceceseccewes 8 ee an ne er rer rrr rr 9 ESEERIMNCRRLEMOEN 555051 Sha cas fw p6 fe pays ft ie SG 1w so BAe See re 1g Fe os od ors 1) os0r0i0 oss he 10 PYFANOMECLETS 2.0... ccc ccc eect cece cee ese renee cesseeseseeseees 10 RB ois cars ara sig! ay ays 12 vroruiiel sis sve oy ere Bk elo « wi ayeaiasTare ms CSN vieRle say 12 Monthly Results “12, Seasonal and Annual Results cece Mle alad RetperMtUhe: © ois: o.5 = viens os fale d+ wicie 34/8 0icie) Marcio ws sis"oik 6 std lores os 19 Conclusion Appendix I Appendix II Appendix III References I* oF °K m,cm, um LIST OF SYMBOLS planetary albedo speed of light in cm s-! degrees Celcius photon energy in Joules irradiance in Watts m ~2 black body irradiance in Watts m -2 frequency in cycles per second degrees Fahrenheit Plank’s constant in Joules - sec. energy in Joules degrees Kelvin wavelength in micrometers length in meters, centimeters, micrometers radius of the Earth in meters the solar constant in Watts m -2 time in seconds Stephan-Boltzman constant in Watts m ~2 K ~4 absolute temperature in degrees K. power in Watts; iv LIST OF FIGURES FIGURE Page Da li OL AIAG 5s as sass rae ok me gc goo eds se ie ew Some we 2. Panoramic view from measuring site ............ 000 eee eee ee eee 3. A portion of the electromagnetic spectrum 4 Bharti: G Facets ACG noon n oo ace oie oid wales tus Bib Sle aHe DAG NS 5. Pyranometer: illustration .......... 0.00. cece eee eee eee eee ee 6. Pyranometer: illustration of mechanism ................00.eeeeee 7. Mean daily insolation comparison ........... 00. ee eee e cence 8. Anchorage airport cloudiness ........... 0. eee e eee eee cette eee 9. Insolation vs. temperature: winter-sunny 10. Insolation vs. temperature: winter-cloudy 11. Insolation vs. temperature: spring-sunny 12. Insolation vs. temperature: spring-cloudy 13. Insolation vs. temperature: summer-sunny .. 14. Insolation vs. temperature: summer-cloudy ................0.000 26 15. Insolation vs. temperature: fall-sunny .............0..eee eee eee 27 16. Insolation vs. temperature: fall-cloudy ................00-eeeeeee 28 LIST OF TABLES TABLE Page 1. Albedo of common surfaces ......... 0000s cece eee eee ee eens 6 2. Horizontal receptor: monthly values ............. 2.2.00 cece e eee 13 3. Inclined receptor: monthly values ............. 20 ee eee eee ee eens 14 4. Vertical receptor: monthly values ............ 000 e cece eee eens 15 5. Monthly insolation means, maximums and minimums .............. 17 6. Seasonal and annual insolation means ..............00- eee e eee eee 18 INTRODUCTION The Solar Energy Meteorological Research and Training Site Alaska Project began in fall 1977. The objectives of this program included measuring solar, atmospheric, and outgoing long-wave fluxes at various locations in Alaska. Anchorage area homes and offices are not designed to take advantage of solar energy. This study with solar receptors mounted at various angles collecting energy over a signifi- cant period of record, can help determine the cost effectivness and efficiency of solar energy in Alaska. This report gives the results of three and one-half years (beginning in September 1978) of solar radiation measurements at the downtown Anchorage site (61 °13’22’’N, 149 °53’11’’W) at an elevation of 107 feet above sea level. Only a few locations in Alaska have insolation measurement records of 20 or more years (Figure 1). Note that the rooftop of the two-story Arctic Environmental Information and Data Center building, where the instrumen- tation is mounted, experiences effects of sur- rounding obstructions (Figure 2). The three main pyranometers are mounted horizontally, south-facing vertically and south-facing in- clined 61 degrees, which is roughly our latitude. Insolation data collected at this site will vary with receptor angle and season. Meteorological and climatic factors presented herein illustrate why less heating energy is needed on cloudy days during the coldest months of the year. Different types of solar energy collectors are available which have varying feasibility for high-latitude buildings. Contractors, architects, and private builders acquainted with these facts and figures will see that despite objections of high latitude and fre- quently overcast skies, the facts favor solar energy use in Alaska. ee FIGURE 1. MAP OF ALASKA SJ LOCATION WITH A HISTORY OF / | 20 YEARS OR MORE OF / | / | Borrow S INSOLATION DATA AVAILABLE Pole Elevation 5° $11.5°E Figure 2. Panoramic View around Measuring Site. North Edge Federal Building Captain Cook Elevation 14° Elevation 3° N 84,59 W N 77.59 Ww M 096 N «SF00 UOMeAaTy ASTUT OW “41, PHYSICAL METEOROLOGY Electromagnetic Radiation All matter emits energy in the form of elec- tromagnetic radiation. Intensity is a function of temperature. Electromagnetic radiation could be characterized as a continuum of waves which travel through a vacuum at the speed of light (c = 3x 108 ms-!). These waves exhibit a con- tinuous range of wavelengths called the elec- tromagnetic spectrum. A portion of the elec- tromagnetic spectrum emitted by the sun is significant to the atmospheric energy budget (Figure 3). The narrow range of wavelengths, called visible light, can be further divided into subranges called color. The sun emits electromagnetic energy at all wavelengths but most strongly in the visible and near IR bands. Radiation emitted by the Earth and its atmosphere is largely confined to the IR (Wallace and Hobbs 1977). This difference in wavelength of maximum emission can be ex- plained by the difference in the temperatures of the two bodies. The sun radiates at a temperature of 5780° K, whereas the Earth radiates at 255° K (measured from outer space). SHORTWAVE Sra oep Laa ULTRAVIOLET NEAR INFRARED The energy transmitted by electromagnetic radiation exists in discreet units called photons. The amount of energy E associated with a photon of radiation can be expressed as qd) where f is the frequency of the radiation or the number of waves passing a given point per se- cond and h is Plank’s constant (h = 6.626 x 10 -34 J . s). Since the speed of light is constant, a straightforward ratio between wavelength and frequency can be shown by the following. (2) f =c/r where c = speed of light and in wavelength. Thus, it follows that the amount of energy con- tained in a photon of radiation is inversely pro- portional to wavelength and directly propor- tional to frequency and temperature. Equation (3) combines 1 and 2 to illustrate this relation- ship. E=hf @) E = he/) LONGWAVE INFRARED ceca MICROWAVE WAVELENGTH A 14m =1x10%m FIGURE 3. A PORTION OF THE ELECTROMAGNETIC SPECTRUM Black Body Radiation To better illustrate the property of the dependence of wavelength upon temperature, a hypothetical object called a black body is used. A black body absorbs all incident radiation at all wavelengths striking its surface, hence the term black. In all wavelengths and in all direc- tions maximum emission is realized (Wallace and Hobbs 1977). When the total energy in equals the total energy out, the body is said to be in radiative equilibrium and will stabilize at a constant temperature. The following equation expresses black body irradiance over all wavelengths. (4) It = oT4 where T is absolute temperature and o isa constant called the Stephan-Boltzman constant ( o = 5.67 x 10°8Wm-2deg-4). Neither the sun nor the Earth is a perfect black body, but both approach its properties in their wavebands of maximum emission. The Earth is in radiative equilibrium with the sun, and thus its temperature from above the at- mosphere does not change over time. For exam- ple, a decrease in solar energy striking the Earth would result in a lowered outgoing IR radiation corresponding to the new reduced equilibrium temperature. A drop of the globally averaged temperature of only 2 °-4° K (or C) is thought to be enough to trigger an ice age. Fortunately for us the sun maintains a nearly constant temperature and radiates a very stable rate of energy called the solar constant (S = 1.38 x 103 W m2), which can be shown as the Earth/sun energy balance (Figure 4). The radiative equilibrium relationship is given in equation 5. (5) (1- A) Sm R2 =14 9 R2 In the preceding equation A is the Earth’s albedo, which will be explained shortly; S is the solar constant, R is the radius of the Earth; and I is the irradiance of the Earth in its wavelength of maximum emission. NORTHERN HEMISPHERIC SUMMER SUN FIGURE 4 EARTH'S RADIATION BALANCE ENERGY (IN) = ENERGY (OUT) —————+ Incoming Paralle! Beam Solar Radiation ~_/7 ~~ Outgoing Longwave Planetary Radiation Atmospheric Effects Not all of the solar radiation striking the top of the Earth’s atmosphere reaches the Earth’s surface. Attenuation processes called absorb- tion, reflection, and scattering take place when the light photons strike the gas molecules in our atmosphere. Scattered light does not strike ob- jects in parallel beam radiation but is diffused. This reaction is predominantly caused by the collision of photons with water vapor, which both diminishes energy and randomly alters the direction of the beam. All of the gaseous consti- tuents in the atmosphere absorb, reflect, and transmit electromagnetic radiation selectively as a function of wavelength. These are complex photochemical reactions and beyond the scope of this paper. Nonetheless, it is important for Alaskans to note this attenuation by the atmosphere. At high latitudes the sun strikes the Earth at an oblique angle and thus passes through a deeper layer of air than in lower latitudes. This path through the air is called optical depth, and the greater its thickness, the less radiation strikes the Earth’s surface. Another effect of the atmosphere on the global radiation balance appears to be partially a man-made phenomenon. Carbon dioxide (CO) is a weak absorber in the visible and a strong absorber in the IR. Consequently, in- coming solar radiation can penetrate to the ground while much of the longwave outgoing radiation is trapped in the atmosphere and reemitted back to the ground. This warming of the surface temperature has erroneously been called ‘‘the greenhouse effect.’ Actually, greenhouses attain higher temperatures than the outside because the glass cover restricts up- ward motion of warm air and eliminates wind chill. The Albedo The monochromatic (single-wave band) radiation striking any opaque surface is either absorbed or reflected. At any wavelength strong reflectors are weak absorbers (for exam- ple, snow at visible wavelengths). The fraction of insolation reflected back to space from various surfaces on the globe is called the albedo. The combined surface reflection of the entire globe is the planetary albedo and is ap- proximately 30 to 35 percent The albedo of some common surfaces are given in Table 1. TABLE 1 Percent Ground Cover Reflected Bare soil 10-25 % Sand, desert 25-40% Grass 15-25% Forest 10-20% Snow (clean, dry) 75-95 % Snow (dirty, wet) 25-75% Sea surface (sun 25° above horizon) 10% Sea surface (low sun angle) 10-70% Thin clouds 20-50% Thick clouds 50-80% Total planetary albedo 30-35 % Adapted from Kondratyev, 1969 Clouds The effect of cloud cover on incident radia- tion at the Earth’s surface is twofold. Clouds are very nearly black bodies in the IR and highly reflective in the visible. Therefore, much of the solar radiation reaching clouds is screened by absorbtion or reflected back to space, preventing it from reaching the ground. The second effect of clouds is to absorb a very large fraction of the outgoing IR from the sur- face and to emit its own long-wave radiation in all directions. Paradoxically, cloudy days are usually warmer than clear sunny days from late fall through early spring due to the insulating properties of clouds. The impact of clouds on the energy budget is a function of both cloud thickness and height. The thicker the cloud deck, the more effectively it screens incoming long-wave radiation and the greater its albedo. The height of the cloud base and cloud top determines the temperature gra- dient through the cloud depth. The atmosphere cools with height, partially determining the lapse rate within the cloud layer. Recall that clouds radiate IR as indicated by the black body formulation, and have a strong dependence on temperature (see equation 4). Thus it follows that clouds with a low ceiling and warm base temperature will radiate a greater long-wave radiation downward than a high cold cloud. A very high cloud top altitude means that very lit- tle long-wave radiation is emitted to the upper atmosphere and to outer space. The radiative properties of clouds are interdependent on cloud height and thickness. CLIMATE AND LOCAL VARIABLES Local Geography Anchorage, Alaska is located in the southcentral region of the state at 61° 13’ N latitude and 149° 53’ W longitude. The city is surrounded by water on three sides. Cook Inlet and its Knik and Turnagain arms are to the west, north, and south, respectively. The ter- rain rises gradually to the east until it reaches the Chugach Mountains approximately 10 miles from the city center and the port facility. The mountain range is oriented north-northeast to south-southwest and has an average height of 4,000 to 5,000 feet, with the tallest peaks ap- proximately 8,000 to 10,000 feet. The moun- tains act as a barrier for Anchorage, holding back the influx of warm, moist air from the Gulf of Alaska. Consequently Anchorage has a low annual precipitation value of 14.7 inches compared to those nearby stations along the Gulf of Alaska that are five to 10 times greater. In the winter extreme cold weather associated with interior Alaska is blocked by the Alaska Range that runs in an arc from southwest through northwest to northeast, approximately 100 miles from Anchorage. A high-pressure system that forms over the Interior that brings temperatures in the minus 50 to 60 degree Fahrenheit range in the Interior will cause clear skies in Anchorage but the temperature will be only minus 15 to 30 degrees in Anchorage, due to the mountain barrier blocking the cold air and the modifying effect of the moist air of Cook Inlet. Description of the Seasons Anchorage has four distinct seasons, but their length and characteristics may differ somewhat from what is considered the normal standards in middle latitudes. Winter, by far the longest season, runs from mid-October to mid-April. A possible sunshine of five hours and 28 minutes marks the winter solstice, the shortest day of the year on or about December 21. The pattern of weather fluctuates between clear and cold and cloudy and mild throughout the winter. During clear, cold periods, it is not unusual for a fog bank to settle in on An- chorage. The cause of this moisture is Cook In- let and its arms. Even in midwinter it is a low- level moisture source due to extreme tides, which maintain some open water throughout the winter. Snow comes during the periods of cloudy and mild. The first measurable occur- rence of snow on average takes place on Oc- tober 15, and the latest measurable snow of the season takes place on average on April 15. Snows have occurred as early as September 20 and as late as May 9. Most snowstorms in mid- winter amount to less than 4 inches of ac- cumulation per storm and visibility of 1 to 3 miles during the storm. Generally ground cover does not exceed 15 inches of accumulated snow pack in the Anchorage bowl. The winds during the winter are usually light and from the north. But once or twice during the winter it is not unusual to have strong, gusty blows from the north caused by extreme low pressure south of Anchorage in the nearby Gulf of Alaska and a large well-defined high pressure system north over the Interior. Spring follows ‘‘breakup”’ and is character- ized by warm days and cool nights. Daily temperature values are on a constant rise and precipitation amounts are minimal. Summer is the season of long days and short nights. A total possible sunshine of 19 hours, 30 minutes falls on or about June 21, the summer solstice. The summer season is from June through mid-September. Summer can be split into two seasons—the first dry and the second wet. The second season starts in mid July with a marked increase in cloudiness. This second season also accounts for approximately 40 per- cent of Anchorage’s annual precipitation. Autumn, the shortest season, usually spans from mid-September to mid-October. The trees will turn to fall colors and lose their leaves in approximately a two-week time span. The cloudiness and precipitation of late summer will not decline until early October. Measureable snowfall can occur as early as September, but this is rare. October, on the other hand, has been known to have snowfalls of 10 to 12 inches. October 1982 set a record of 27.1 inches of snow. Autumn winds are generally from the south. They are post frontal in nature, forming from the southern Bering Sea or Bristol Bay and tracking northeastward across Alaska’s In- terior. Occasional winds from a southeasterly direction bring gusts greater than 100 miles per hour down the northwestern slopes of the Chugach Mountains that can cause con- siderable damage to roofs, power lines, and trailers in the community. Local Obstructions The major obstruction to the pyranometers used in this report starting in the east to southeast, is the Chugach Mountains (Figure 2). With an average elevation of three to five degrees, the mountains effectively delay sunrise by 30 to 45 minutes. This has a greater effect on the inclined and vertical receptor than on the horizontal. In midsummer when the sun comes up in the northeast, the Sheraton Hotel Building blocks the horizontal (global, 360°) in- strument for approximately 15 minutes. Mov- ing around to the south there are no major in- terferences, just a few telephone poles that af- fect the instrumentation for less than five minutes in winter. The only large building to the south is far enough away that it does not in- terfere with the insolation values at any time. Moving to the west, the Federal Building blocks direct gain in spring and fall when the sun drops behind the building late in the day. In summer the total increases due to reflection from the glass on the building which is coated with a solar shade that reflects the sun off the nor- theast corner late in the afternoon and early evening. In midsummer, some insolation very late in the day may be lost due to the skyline of downtown Anchorage. INSTRUMENTATION Pyranometers The pyranometer is a type of actinometer. The Van Nostrand’s Scientific Encyclopedia defines actinometer as ‘‘the general name for any instrument used to measure the intensity of radiant energy. In earlier usage, the term was often restricted to the measurement of photochemically active radiation, but is now used more generally.’’ There are three major types of actinometers used in meteorological and astronomical applications. The first is the pyrheliometer, which measures direct solar radiation intensity. The second instrument, the pyrgeometer, measures the effective terrestrial radiation. The third, the pyranometer, measures the combined intensity of direct solar radiation and diffuse sky radiation (Figure 5). It consists of a recorder and a radiation-sensing element mounted so that it views the entire sky. The four pyranometers used in this study have their sensing elements at different angles to evaluate radiation intensity as a function of angle. Two of the pyranometers are facing south. The vertical (90°) instrument evaluates how much of the total sky radiation strikes a vertical surface. The other south-facing instru- ment is inclined at 61 degrees to measure how much radiation will strike a pitched wall or roof in the Anchorage area. The other two pyranometers are on a horizontal plane (global 180°). The first, which has been in operation since the start of this project in late September 10 1978, is measuring direct solar and diffused sky radiation. The second horizontal instrument was installed in August 1982 and has a shadow band apparatus attached around it. The shadow band shades out all the incoming direct radiation and only allows the diffused sky radiation to be measured (Figure 6). The actual internal workings of a pyranometer are beyond the scope of this paper. Simply put, it is made up of an absorbing black surface thermocouple and a reflective white thermocouple that send signals to a potentiometer. The potentiometer reads the difference in the thermal elec- tromotive force and passes that on to the recorder. Direct solar radiation is that portion of the radiant energy received at the instrument direct- ly from the sun. Diffused sky radiation reaches the Earth’s surface after being scattered from the direct beam by molecules or suspended par- ticles in the atmosphere. Of the total light removed from the direct beam by scattering in the atmosphere, about two-thirds ultimately reaches the Earth as diffuse sky radiation. The presence of clouds also increases the ratio of diffuse to direct radiation. Randomly scattered incoming solar radiation has a 50 percent chance of being deflected downwards. Conse- quently, receptivity to the horizontal plane is least reduced. That radiation which is scattered anywhere above the Earth’s surface has little probability of striking either a vertical or inclin- ed receptor mounted at the ground. FIGURE 5. PYRANOMETER Source: Instrumentation for the Measurement of the Components of Solar and Terrestrial Radiation, Eppley Laboratory, Inc. FIGURE 6. THE PRINCIPLE OF A PYRANOMETER Source: Physical Meteorology by Johnson, John C. DIRECT DIFFUSED SOLAR RADIATION SKY RADIATION hermocouples to potentiometer. The sun shade blocks out Thermal EMF difference read. the direct rays of the sun. 11 RESULTS Over several years, monthly and seasonal values of. solar energy (in Watt-hours per square meter) do not seem to vary greatly. Oc- casional anomalous maximums and minimums can occur for any given month or season, but their effects on the data are smoothed over time. Consequently, values averaged over long periods of record will be most meaningful. Monthly Results The period of record for the horizontal and 61 degree inclined receptors begins in October 1978 and extends through July 1982. The period of record for the vertical receptor begins in April 1979 and ends in July 1982 but is not inclusive due to an instrument failure (Table 4). Tables 2, 3, and 4 break down the resultant means, maximums, and minimums for each month and each receptor over the entire period of record. This presentation shows the relative consistency from year to year during any given month and the most obvious anomalies. For in- stance, February 1979 stands out as an atypical- ly sunny winter month and July 1981 appears to have been much cloudier than normal. These three tables will also be helpful for those who 12 wish to compare the latest year’s data with that presented previously (Becker 1981). A more succinct interpretation of these data is presented in Figure 7 and Table 5. These il- lustrate the long-term means found over the en- tire period of record on a monthly basis. The averaged values and greatest extremes found will be of interest to anyone involved in design or construction of solar energy collectors. Examination of all the charts and tables discussed up to this point shows a pattern of monthly insolation values as ranked according to receptor angle. In January and February the most efficient receptor is the inclined, followed by the vertical and finally the horizontal. In March and April the inclined receptor still leads, the vertical is the next, and the horizontal receives the least insolation. May through August receive their greatest insolation upon the horizontal receptor, the inclined receives the second greatest values, and the vertical the least. September’s ranking is horizontal, inclin- ed, then vertical. October, November, and December absorb the most insolation upon the vertical receptor, and the horizontal trails the inclined during this period. €I HORIZONTAL RECEPTOR: 2 Monthly Values Wh/m 78-79 79-80 80-81 81-82 Mean Max Min Mean Max Min Mean Max Min Dail Dail Dail Daily Daily Daily | Daily Daily Daily Daily Daily Daily a “ 2246.7 105.2 999.5 2848.9 162.5 2132.0 38.2 382.6 1128.1 114.7 731.5 66.9 316.0 698.0 105.2 879.5 105.1 238.3 578.4 38.4 2992.3 95.6 714.5 2007.6 95.5 4030.4 640.5 2639.4 4741.5 945.3 6338.4 841.2 5069.7 6672.9 1510.5 7842.0 1348.1 5065.8 7954.0 1061.2 8164.1 1499.7 6170.2 9110.8 = 1775.1 7944.3 1271.5 3807.2 7074.2 1089.8 6950.1 1348.0 2923.1 6615.6 898.7 5315.3 554.4 2734.9 4837.4 889.2 Note: The units chosen for the table are Watt hours per square meter. These values can be converted to BTU’s per square foot and Langleys by using the conversion factors shown in Appendix II. TABLE 2. HORIZONTAL RECEPTOR: MONTHLY VALUES bl INCLINED RECEPTOR: Monthly Values Whim? 78-79 79-80 80-81 81-82 Mean Max Min Mean Max Min Mean Max Min Mean Max Min Daily Daily Daily | Daily Daily Daily Daily Daily Daily Daily Daily Daily 1791.6 5900.0 1226.8 1037.8 = -- | 847.0 3690.2 907.5 = -- | 824.5 2250.2 875.0 34989 133.2 | 1006.5 3493.0 5049.6 6620.7 276.3 | 2189.6 6334.7 2841.3 7844.2 227.9 | 3939.7. 7725.9 4604.9 8431.4 1041.0 | 4613.2 8367.0 4881.1 8352.3 1914.2 3840.7 8239.0 4435.1 8387.0 1095.2 | 4287.4 8179.8 4452.3 8102.3 1434.8 | 4407.3 7794.9 4073.1 7687.3 979.5 | 3689.6 7548.6 3804.8 7845.2 661.1 2970.8 6854.5 Note: The units chosen for the table are Watt hours per square meter. These values can be converted to BTU’s per square foot and Langleys by using the conversion factors shown in Appendix II. TABLE 3. INCLINED RECEPTOR: MONTHLY VALUES SI VERTICAL RECEPTOR: Monthly Values Whim? 78-79 79-80 80-81 81-82 Mean Max Min Mean Max Min Mean Max Min Mean Max Daily Daily Daily Min Daily 81.2 Month Daily Daily Daily Daily Daily Daily Daily Daily 1613.0 6011.7 81.2 559.3 809.4 3639.6 75.5 1193.4 3296.6 §2.2 DEC 1126.2 2244.1 69.8 JAN 455.9 2046.5 40.6 FEB 1301.4 5197.6 69.6 MAR 3450.7. 6152.9 500.0 APR 4764.3 6127.8 883.7 MAY 3275.2 5348.8 453.6 3601.7. 5290.7 976.8 2182.4 4406.9 442.0 2009.9 5569.7 476.7 3211.0 6199.6 441.9 Note: The units chosen for the tabie are Watt hours per square meter. These values can be converted to BTU’s per square foot and Langleys by using the conversion factors shown in Appendix II. TABLE 4. VERTICAL RECEPTOR: MONTHLY VALUES 91 6000 HORIZONTAL INSOLATION | 5000 INCLINED INSOLATION 4000 VLLLLLL VERTICAL INSOLATION 3000 RRR 2000 1000 FIGURE 7. MEAN DAILY INSOLATION COMPARISON HORIZONTAL vs INCLINED vs VERTICAL INSOLATION IN Wh/Sq m 7 i oe WOW Lax xnona Ss M SR ERR ER in, Es SESS SE ESI > R f] ) tt yn iW |p Ry | mi id i id oo WY Ue IY a lif y HH 6=—s UK M i ) h A OR TR Od ite TR mA I AN IY RR ee RO Vy mS IS a a Ss AB ee oe A MU ol APR MAY JUN JUL AUG SEP OCT NOV DEC MONTH ul MONTHLY INSOLATION: Means, Maximums and Minimums Period of Record: Oct. 1978—June 1982 Wh/m? HORIZONTAL SURFACE INCLINED SURFACE VERTICAL SURFACE Mean Maximum Minimum} Mean Maximum Minimum} Mean Maximum Minimum Daily Daily Daily Daily Daily Daily Daily Daily Daily 369.1 1156.8 860.2 3498.9 455.9 2046.5 1225.7 2992.3 : 2946.6 6620.7 5 1301.4 5197.6 2327.6 4741.5 3412.1 7844.2 3079.7 6570.7 4267.7 6672.9 4817.7 8431.4 3869.7 6127.8 4920.4 8250.4 4383.6 8352.3 3034.6 5424.0 5194.4 9110.8 4237.3 8387.0 2930.5 5290.7 4453.5 8303.8 3787.7 8102.3 2531.8 5395.4 3546.6 7096.3 3460.1 7687.3 2453.8 5918.7 2625.3 5898.4 3463.7 7845.2 2931.9 6255.7 1001.7 3374.7 1551.2 5900.0 . 1608.7 6011.7 438.8 2132.0 . 893.3 3690.2 5 1001.4 3639.6 265.9 731.5 774.2 2250.2 1126.2 2244.1 Note: The units chosen for the table are Watt hours per square meter. These values can be converted to BTU’s per square foot and Langleys by using the conversion factors shown in Appendix II. TABLE 5. MONTHLY INSOLATION: MEANS, MAXIMUMS AND MINIMUMS Seasonal and Annual Results Solar seasons center around the solstices and equinoxes and are not necessarily those months with similar temperature and weather regimes. Table 6 lists the seasonal and annual averaged insolation values over the entire period of record at this latitude (61 ° N). The ideal collec- tor angle changes dramatically with season. The inclined plane is the dominant receptor on an annual basis, receiving an average of 2882 Wh/m2 daily. The horizontal and_ vertical receive 2553 Wh/m2 and 2194 Wh/m2, respec- tively. During solar summer (May, June, and July) the horizontal receptor dominates. The mean daily radiation during this season is as follows: horizontal, 4856.1 Wh/m2; inclined, 4136.2 Wh/m2; and vertical, 2832.3 Wh/m2. In winter (November, December, and January) the vertical receptor becomes dominant. The vertical daily averaged insolation value for winter is 861.1 Wh/m2, The inclined receptor values are close behind at 842.6 Wh/m2, and the horizontal collector receives only 358.0 Wh/m2 on a daily average. Spring and fall daylight hours are in rapid transition from the short days of winter and long days of summer, and the inclined receptor dominates both in dai- ly mean insolation values. In spring mean daily values are inclined plane, 3725.5 Wh/m2; ver- tical plane, 2750.3 Wh/m2; and horizontal plane, 2607.0 Wh/m2. In fall these values are inclined plane, 2825.0 Wh/m2; horizontal plane, 2391.2 Wh/m?; and vertical plane, 2331.5 Wh/m2, TABLE 6. SEASONAL AND ANNUAL INSOLATION: MEANS Oct. 1978—Jun. 1982 Period of Record: Whim? HORIZONTAL INCLINED VERTICAL SURFACE SURFACE SURFACE Solar Seasons Mean Mean Mean slat WINTER 357.9 842.6 861.1 SPRING 2607.0 3725.5. 2750.3 SUMMER 4856.1 4136.2 2832.3 AUTUMN 2391.2 2825.0 2331.5 : + [ANNUAL MEANS| 2553 Wh/m?2/Day | 2882 Wh/m?/Day 2194 Wh/m?/Day Note: The units chosen for the table are Watt hours per square meter. These values can be converted to BTU’s per square foot and Langleys by using the conversion factors shown in Appendix Il. 18 Clouds and Temperature The amount of cloud cover largely deter- mines the amount of incoming solar radiation an area receives. In Anchorage an average of only 64 days each year are clear (0-30 percent sky cover), 63 days are partly cloudy (40-70 per- cent sky cover), and 238 days are cloudy (80-100 percent sky cover). On an annual basis sky cover sunrise to sunset is 74 percent. Therefore, Anchorage receives the total available solar radiation only 26 percent of the time. Figure 8 illustrates Anchorage cloudiness comparing the 28-year mean to the 5-year (period of record) mean to the one year (new data) mean. Predictably, a strong coorelation between cloudiness and insolation values emerges from data on the pyranometer strip chart. The relationship between insolation and temperature is a function of water vapor and clouds in the atmosphere. Figures 9 through 16 show one cloudy day and one sunny day in each solar season. Higher temperatures occur on cloudy days when the earth’s surface is covered by an insulating layer. Also, the air masses are more likely to be maritime air from the Earth on cloudy days and continental polar air from the north on clear days. This is particularly evi- 19 dent in spring, winter, and fall. In winter the cloudy day’s temperature ranges in the 30’s (°F), whereas just a few days earlier, on a clear sunny day, the temperature was only in the single digits or teens (°F). The insolation values on the cloudy winter day are approximately an order of magnitude less than those on the sunny winter day. Analogous, though less dramatic correlations occur in spring and fall. Insolation values are still less by nearly an order of magnitude on cloudy days as compared to sun- ny days, but the temperature increase on these same days is only around 10°F, whereas in the winter it was seen to be nearly twice that varia- tion. The warming properties of clouds are least evident in summer. Inspection of Figures 15 and 16 once again illustrate reduced insolation under cloudy skies as compared to sunny skies, but on both days the temperatures hover in the 50’s (°F). Because home and office heating is less necessary in summer, the lack of insulation by clouds is of less concern. Typically, a well- constructed building in Anchorage needs very little heating or cooling since the ambient air temperature lies well within the comfortable range. FIGURE 8. ANCHORAGE INT’L AIRPORT CLOUDINESS 28YR. MEAN vs 5YR. MEAN vs 1YR. MEAN PERCENT OF CLOUD COVER 100) -— = 52 ee ee eee DEC SSS ee ee] > Be > Ss a Ra ee ki ease thee sage hee Rad oO = x x << x = SS SS ee] SS Se Soe Se eS ee | OO [icicles i csceina ill danieastehal i honin cited cntanartindttAaaiicaiental iain aaceeaeelodniade © SSS ee OO Re eS eS a So ee eee = LLY Se ee ae en i nelcnilinnamaaa O° SSS ee ee OO Se SS CE NR NS NGM) Ce ee ee ee <f ll te ta el Rl ll Re el eel nl tel ai SSS Sc ee a ee |) fo ee ee epee ae ee eee Ns: ee] Oe Se Se 28 YEAR MEAN 90 {- i sn aeioincietes as cans neem aenerendeillllaeeteacadeDidaaatadlt deecteegandell eee tiasanpeeetadatibitebee mene “— ———L————T_ x XT eX Kh SN ee MAR APR = Disease tce al Sa eo i el CD GP. ED WRI: a | | ee FEB Yj y ] y Uh ‘ i y i} i i hi i i i I p } Gade ee eg a 80 hs So oO So So Oo o o oO N = 70 50 4 5 YEAR MEAN 1 YEAR MEAN MONTHS 17 1200 HORIZONTAL 4499 INCLINED 900 800 700 600 renames 500 TEMPERATURE 400 300 200 100 0 WATT HRS PER SQ METER | FIGURE 9. INSOLATION vs TEMPERATURE WINTER—SUNNY: DEC. 10, 1981 DEGREES FAHRENHEIT BR ha MEM Mecca ees een a 27 SrA 25e Oa 8 9 10 Al 12.19514 45 16° 17 16. 18) 20° .21 22 23 24 HOURS 60° 50° 40° 30° 20° 10° 0° FIGURE 10. INSOLATION vs TEMPERATURE WINTER—CLOUDY: DEC. 18, 1981 WATT HRS PER SQ METER DEGREES FAHRENHEIT 1200 . pate ene HORIZONTAL +400 —-—-— 1000 C INCLINED ~“ 800 | 40° 799 8 VERTICAL ‘os age ---—-- — 500 TEMPERATURE 400 20° 300 200 10° 100 0 Peddie dA ole VT. 2234.5 6-7 8 9 10°11 12/13: 14-15 16°17 18 19 20 21° 22 23 24 HOURS £7 HORIZONTAL INCLINED TEMPERATURE 1200 1100 1000 900 800 700 600 500 400 300 200 100 FIGURE 11. INSOLATION vs TEMPERATURE SPRING—SUNNY: MAR.13, 1982 WATT HRS PER SQ METER DEGREES FAHRENHEIT - A 2 13 74-5 6 7 8 9-10 17 12:13; 14 15: 16° 17 18> 19 20° 21.22 23 24 HOURS 60° 50° 40° { 30° 20° 10° 0° FIGURE 12. INSOLATION vs TEMPERATURE SPRING—CLOUDY: MAR.7, 1982 WATT HRS PER SQ METER DEGREES FAHRENHEIT 1200 — —— 60° HORIZONTAL 4400 tamed: ara 1] — 50° INCLINED °° 800 + 40° 99 VERTICAL gy a —--—--- 500 TEMPERATURE 400 — 20° 300 200 ig * 400 0 0° 132.946 67 8 9 10-11 12/13 -14°45 16-17 16°19 20 21 22 23 24 HOURS st HORIZONTAL INCLINED TEMPERATURE 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 FIGURE 13. INSOLATION vs TEMPERATURE SUMMER—SUNNY: JUN.7, 1981 WATT HRS PER SQ METER DEGREES FAHRENHEIT | 60° 50° 4 40° — 30° 20° 10° Ge + 2-34 56.7 8 9 10: 11--12:13.14°15 16. 17.18 18:20 21 22 23 24 HOURS 9% 1200 HORIZONTAL 4499 —-——-—— 1000 INCLINED = °° 800 700 600 -—--—- 500 TEMPERATURE 400 300 200 100 0 WATT HRS PER SQ METER FIGURE 14. INSOLATION vs TEMPERATURE SUMMER—CLOUDY: JUN.29, 1981 DEGREES FAHRENHEIT — te TT TT TT T 1 Ol es \ ee 2:-3°4 5. 6 7 & 9 40 11 12 13:14°15 16°17 18.19 20 21 22 23 24 HOURS 60° 50° 40° 30° 20° 10° LZ HORIZONTAL TEMPERATURE 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 FIGURE 15. INSOLATION vs TEMPERATURE FALL—SUNNY: SEP. 22, 1981 WATT HRS PER SQ METER DEGREES FAHRENHEIT PER SQ METER “Jee eas ;FAHRENHETT [ L aN sa \ j Lo i ew ee oe / \\ oe I 40° L / \\ vo L / | \ z 30° L jf u-~\ A P- Ze | | 20° 1 10° a 12°93. 426 6-7. 8 -9 10°11 12:/18 14°45 16 17. 18° 19° 20 21: 22° 29.94 HOURS FIGURE 16. INSOLATION vs TEMPERATURE FALL—CLOUDY: SEP. 18, 1981 1200 WATT HRS PER SQ METER DEGREES FAHRENHEIT 60° HORIZONTAL 41100 t sms 1000 FN Ca ee rat ea | le 50° 900 INCLINED 800 f- _| 40° 700 8 VERTICAL — goo = aps --—--— 500 | a | TEMPERATURE 400 4 20° 300 + 200 10° 100 0 0° Ws et OARS 67% BO WOT Aa 18. 14 1S AG" 17 182 19: 20 -21°22° 23 24 HOURS CONCLUSION The angle of greatest insolation value throughout our study for Anchorage, Alaska is the 61 degree south-facing inclined plane. Com- parison of insolation values by season shows that during the season of highest energy need (winter) the vertical 90 degree south-facing angle is the prime receptor. In summer and under overcast conditions at any time of the year the horizontal 180 degree plane received the greatest amount of insolation. In overcast sky conditions the horizontal receptor is domi- nant due to the scattering of incoming solar radiation. As an alternative energy source in Anchor- age, Alaska, it would not be cost effective (due to initial cost of equipment for so little insola- tion) to put in an active system, which uses a flat plate solar collector. A passive system, on the other hand, is very viable. (A passive system captures the heat without supplementary energy to move the heat.) Conduction, convection, or radiation are the prime sources used in moving heat in passive solar technologies. A passive solar home uses the whole building as a collec- tor and storage facility by employing south- facing glazing to capture the solar radiation. The initial cost of constructing a building that utilizes passive solar can be somewhat higher 29 than the average building of equivalent size and amenities, but the pay-back comes in so short a time that it is usually well worth the extra cost. Our data reveal that both inclined and ver- tical receptors have their advantages and disad- vantages. The inclined collector on a year- round basis would outperform the vertical but is more costly to install. Also, technologically there are some flaws in moisture buildup, icing, and drainage on tilted glazing. The vertical col- lector, on the other hand, does not have as great an installation cost but except for the solar winter months and early spring is not as effective as the inclined. Remember, though, that in summer Anchorage buildings require lit- tle heating or cooling because the mean air temperature generally falls within the accepted comfort zone. The acquisition of new equipment at this site will introduce a record of new data variables. The gaps in the present data record caused by equipment failure need to be filled by increas- ing the record length of all existing measurements. The funding for these solar radiation assessments should continue so that a future, more statistically significant data analysis can be conducted. APPENDIX I — INSTRUMENTATION RADIATION SENSORS 1 Lambda Instruments Corporation pyramometer - vertical - serial number PY 1690-7903 1 Kipp and Zonen pyramometer - horizontal - serial number CMS5-711054 1 Kipp and Zonen pyramometer - inclined plane - serial number CMS5-711217, replaced 8/6/80 by CMS-785447, replaced 9/8/82 by 818085 1 Kipp and Zonen pyramometer - horizontal diffused - serial number installed 8/3/82 1 Eppley Laboratory, Inc. Shadow Band, serial number 7640 Leads and Northrup Speedmax recorder - serial number D71-30809-1-1, replaced 8/3/82 by a Datel Systems, Inc. Data Logger II, Model DL-2, serial number 03720582H TEMPERATURE/HUMIDITY Wihl Lambrecht K.G. Cottingon Hydrothermograph - serial number 373613 Humidity records for a hair hygrometer and temperature from a bi-metallic strip. Chart is changed every Monday. Stevenson weather shelter and stand WIND EQUIPMENT September 1978 to July 1979 - Taylor Anemometer and Wind Vane System on a 2-m tower. Sensor deemed unreliable; it malfunctioned much of the time. Esterline/Angus recorders for direction and speed serial numbers 75405 and 88565 July 1979 to Present - Weathertronic Wind Recorder, model no. 2350, operated successfully for two months and then had to be returned to distributor for repair. It has worked satisfactorily since November 1979. Model 2130 wind speed and direction sensor. Recorder has separate strip charts for speed and direction. 30 APPENDIX II Conversion Factors To Obtain: Multiply Wh/m? BTU/ft? 3172 Langleys -086045 Joules/m? 2.778 x 10 * Temperature Scale Conversions °C = 5/9 (°F-32) Celsius or Centigrade °F = 9/5 °C + 32 Fahrenheit ‘K = C + 273 Kelvin °K = [5/9 (°F — 32)] + 273 31 APPENDIX III - GLOSSARY Albedo - the ratio of the radiation reflected from an object to the total amount inci- dent upon it. Black body - an ideal (but nonexistent) body which would absorb all and reflect none of the radiation striking its surface; hence it would be black. Also, it will emit the maximum possible radiation in all wavelengths as uniquely determined by its temperature. Diffusion or scattering - the displacement of photons caused by their collision with atmospheric particles of varying sizes ranging from microscopic specks down to electrons. This scattering is in all directions and all wavelengths. Direct or Parallel beam radiation - the sun’s rays are emitted from such a great astronomical distance that any divergence of beams is negligible to an observer on Earth. Direct radiation is brighter and more intense than diffuse radiation. Electromagnetic radiation - radiation associated with periodically varying electric and magnetic fields traveling at the speed of light along a continuum of frequencies and wavelengths. Electromagnetic spectrum - the continuous ensemble of electromagnetic waves rang- ing from cosmic and gamma rays through microwave and radio waves, including visible light. Equinox - the liné where the Earth’s equator passes through the plane of the ecliptic of our revolution around the sun. On these days hours of light will equal hours of darkness at the equator. In the Northern Hemisphere the vernal (spring) equinox oc- curs on March 21st and the sun moves north of the equator; the autumnal equinox falls on September 22nd and the sun ‘retreats’ into the Southern Hemisphere. Frequency - the number of times a wave passes a given point in space in a given unit of time; usually measured in cycles per second. Insolation - incident solar radiation. Kelvin (°K) - a temperature scale based upon absolute molecular motion. 0°K is ab- solute zero, the complete absence of all molecular kinetic energy (O°K = -273°C = -459.4°F). Optical depth (in the atmosphere) - the product of the path length through which the sun’s rays travel and ratio by which that beam is attenuated by scattering. Photon - a quantity of electromagnetic energy proportional to its frequency and containing mass. Pyranometer (also called actinometer) - an instrument which measures the combined intensity of direct and diffuse solar radiation. It consists of a recorder and a radia- tion sensing element which views the whole sky. 32 Radiative equilibrium - a steady state condition where energy in equals energy out of a body and a constant temperature is realized. Solstice - either of the two points where the Earth’s equator’s apparent displacement due to the tilt of its axis is farthest north or south. The summer solstice falls on June 21 when the Earth is farthest north and is the longest day in the Northern Hemisphere. The winter solstice, December 21st, is the shortest day in the Northern Hemisphere and is the southernmost point on the earth’s journey around the sun. Wavelength - the space period from one wave cycle to the next. A length scale which measures the perpendicular distance between any two points whose phase is separated by 27r. Thermocouple - a device which measures the temperature difference and direction of heat flow between two objects. Potentiometer - an instrument used for the measurement of differences in potential electric field or electromotive force. Photovoltaic - same as photoelectric; changes in electric characteristics of substances due to radiation, generally in the form of light. 33) REFERENCES Becker, Jr., Richard, 1981. Comparison of solar radiation measurements on a horizontal, inclined, and vertical surface in Anchorage, Alaska. Arctic En- vironmental Information and Data Center, University of Alaska. Report for U.S. Department of Energy. 26 pp. Considine, Douglas M., 1976. Van Nostrand’s Scientific Encyclopedia. New York. Van Nostrand Reinhold Company. Sth ed. 2370 pp. Critchfield, Howard J., 1974. General Climatology. New Jersey. 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Bulletin of the In- stitute of Water Resources. Volume 1. University of Alaska, Fairbanks. 163 pp. U.S. National Climatic Center. 1980. Local climatological data - Anchorage, Alaska. U.S. National Oceanic and Atmospheric Administration. 4 pp. Wallace, John M., and Hobbs, Peter V., 1977. Atmospheric science: An introduc- tory survey. New York. Academic Press. 467 pp. 34 Wise, James L. 1979. Alaska solar radiation analysis. Arctic Environmental Infor- mation and Data Center, University of Alaska. Report for U.S. Department of Energy. 27 pp. ____, 1980. Analysis of solar radiation measurements on an inclined surface in Anchorage, Alaska. Arctic Environmental Information and Data Center, University of Alaska. Report for U.S. Department of Energy. 20 pp. 35