1、35.1CHAPTER 35SOLAR ENERGY USEQuality and Quantity of Solar Energy . 35.1Solar Energy Collection. 35.6Components . 35.11Water Heating 35.13Solar Heating and Cooling Systems 35.15Cooling by Nocturnal Radiation and Evaporation 35.16Cooling by Solar Energy 35.18Sizing Solar Heating and Cooling Systems:
2、 Energy Requirements 35.19Installation Guidelines. 35.24Design, Installation, and Operation Checklist. 35.25Photovoltaic Applications 35.27Symbols 35.28HE sun radiates considerable energy onto the earth. PuttingTthat relatively low-intensity (rarely over 300 Btu/hft2) energyto work has lead to the c
3、reation of many types of devices to convertthat energy into useful forms, mainly heat and electricity. How thatenergy is valued economically drives the ebb and flow of the globalsolar industry. This chapter discusses several different types of solarequipment and system designs for various HVAC appli
4、cations, aswell as methods to determine the solar resource.Worldwide, solar energy use varies in application and degree. InChina and, to a lesser extent, Australasia, solar energy is widelyused, particularly for water heating. In the Middle East, solar poweris used for desalination and absorption ai
5、r conditioning. Solar energyuse in the United States is relatively modest, driven by tax policy andutility programs that generally react to energy shortages or the priceof oil. In Europe, government incentives have fostered use of photo-voltaic and thermal systems for both domestic hot-water and spa
6、ceheating (solar combi systems), which have a well-established mar-ket in several countries, and solar cooling is an emerging marketwith a significant growth potential. Combined solar space heating/cooling and domestic hot-water production (solar combi-plus sys-tems) may lead to both high solar frac
7、tions and economical systemsbecause of the continuous (annual) exploitation of the solar collectorfield and other system components.Recent interest in sustainability and green buildings has led to anincreased focus on solar energy devices for their nonpolluting andrenewable qualities; replacing foss
8、il fuel with domestic, renewableenergy sources can also enhance national security by reducingdependence on imported energy.For more information on the use of solar and other energysources, see the Energy Information Administration (EIA) of theU.S. Department of Energy (www.eia.gov) and the Internati
9、onalEnergy Agency (www.iea.org).1. QUALITY AND QUANTITY OF SOLAR ENERGYSolar ConstantSolar energy approaches the earth as electromagnetic radiation,with wavelengths ranging from 0.1 m (x-rays) to 100 m (radiowaves). The earth maintains a thermal equilibrium between theannual input of shortwave radia
10、tion (0.3 to 2.0 m) from the sun andthe outward flux of longwave radiation (3.0 to 30 m). Only a lim-ited band need be considered in terrestrial applications, because 99%of the suns radiant energy has wavelengths between 0.28 and 4.96m. The current value of the solar constant (which is defined as th
11、eintensity of solar radiation on a surface normal to the suns rays, justbeyond the earths atmosphere at the average earth-sun distance) is433.4 Btu/hft2(ASTM Standard E490). Chapter 14 of the 2013ASHRAE HandbookFundamentals has further information on theavailable extraterrestrial solar radiation.Sol
12、ar AnglesThe axis about which the earth rotates is tilted at an angle of23.45 to the plane of the earths orbital plane and the suns equator.The earths tilted axis results in a day-by-day variation of the anglebetween the earth-sun line and the earths equatorial plane, called thesolar declination . T
13、his angle varies with the date, as shown in Fig-ure 1, and may be estimated by the following equation: = 23.45 sin (1)where N = day of year, with January 1 = 1.The relationship between and the date from year to year variesto an insignificant degree. The daily change in the declination is theprimary
14、reason for the changing seasons, with their variation in thedistribution of solar radiation over the earths surface and the varyingnumber of hours of daylight and darkness. Note that the followingsections are based in the northern hemisphere; sites in the southernhemisphere will be 180from the examp
15、les (e.g., a solar panelshould face north).The earths rotation causes the suns apparent motion (Figure 2).The position of the sun can be defined in terms of its altitude abovethe horizon (angle HOQ) and its azimuth , measured as angle HOSin the horizontal plane.At solar noon, the sun is exactly on t
16、he meridian, which containsthe south-north line. Consequently, the solar azimuth is 0. Thenoon altitude Nis given by the following equation asThe preparation of this chapter is assigned to TC 6.7, Solar Energy Utili-zation.Fig. 1 Variation of Declination (degrees) and Equation of Time ET as Function
17、 of Day of Year360284 N+365-35.2 2015 ASHRAE HandbookHVAC ApplicationsN= 90 LAT + (2)where LAT = latitude.Because the earths daily rotation and its annual orbit aroundthe sun are regular and predictable, the solar altitude and azimuthmay be readily calculated for any desired time of day when the lat
18、-itude, longitude, and date (declination) are specified. Apparentsolar time (AST) must be used, expressed in terms of the hourangle H, where(3)Solar TimeApparent solar time (AST) generally differs from local standardtime (LST) or daylight saving time (DST), and the difference can besignificant, part
19、icularly when DST is in effect. Because the sunappears to move at the rate of 360 in 24 h, its apparent rate ofmotion is 4 min per degree of longitude. The AST can be deter-mined from the following equation:AST = LST + ET+ (4 min)(LST meridian Local longitude) (4)All standard meridians are multiples
20、 of 15 east or west of theprime meridian, which is at the Royal Observatory in Greenwich,U.K. The longitude correction is a positive value for the westernhemisphere and negative for the eastern hemisphere. The longitudesof the seven standard time meridians that affect North America areAtlantic ST, 6
21、0W; Eastern ST, 75W; Central ST, 90W; MountainST, 105W; Pacific ST, 120W; Alaska ST, 135W; and Hawaii-Aleutian ST, 150W. Starting with the prime meridian throughGreenwich, many European countries define their standard meridi-ans based on legal, political, and economic as well as purely physi-cal or
22、geographical criteria. The longitudes of the three standardtime meridians that affect Europe are western European ST (U.K.,Ireland, and Portugal), 0; central European ST, 15E e.g., Spain(except for Canary Islands) to the south, Serbia to the east, and Swe-den to the north; and eastern European ST, 3
23、0E (e.g., Greece andCyprus to the south, Turkey to the east, Finland to the north).The equation of time (ET) is the measure, in minutes, of theextent by which solar time, as determined by a sundial, runs fasteror slower than local standard time (LST), as determined by a clockthat runs at a uniform r
24、ate. The equation of time may be estimatedby the following equation:ET = 9.87 sin 2B 7.53 cos B 1.5 sin B (5)where B = 0.989(N 81).Example 1. Find AST at noon DST on July 21 for Washington, D.C., lon-gitude = 77W; for Chicago, longitude = 87.6W; and for Athens,Greece, longitude = 23.75E.Solution: No
25、on DST is 11:00 AM LST. Washington is in the easterntime zone, and the LST meridian is 75W. From Equation (5), the equa-tion of time for July 21 (N = 202) is 6.07 min. Thus, from Equation(4), noon DST for Washington isAST = 11:00 6.07 + 4(75 77) = 10:45.93 AST = 10.77 hChicago is in the central time
26、 zone, and the LST meridian is 90W.Thus, from Equation (4), noon central DST isAST = 11:00 6.07 + 4(90 87.6) = 11:03.53 AST = 11.06 hAthens is in the eastern European time zone, and the LST meridianis 30E. Thus, from Equation (4), noon DST isAST = 11:00 6.07 4(30 23.75) = 10:28.93 AST = 10.48 hThe h
27、our angles H for these three examples (see Figure 3) arefor Washington, H = (12.00 10.77)15 = 18.6 eastfor Chicago, H = (12.00 11.06)15 = 14.1 eastfor Athens, H = (12.00 10.48) 15 = 22.8 eastTo find the solar altitude and the azimuth when the hour angleH, latitude LAT, and declination are known, the
28、 following equa-tions may be used:sin = cos(LAT) cos cos H + sin(LAT) sin (6)sin = cos sin H/cos (7)or cos = (cos H cos sin LAT sin cos LAT)/cos (8)Tables 15 to 21 in Chapter 29 of the 1997 ASHRAE HandbookFundamentals give values for latitudes from 16 to 64 north. For anyother date or latitude, inte
29、rpolation between the tabulated valueswill give sufficiently accurate results.Incident AngleThe angle between the line normal to the irradiated surface (OPin Figure 3) and the earth-sun line OQ is called the incident angle .It is important in solar technology because it affects the intensity ofthe d
30、irect component of solar radiation striking the surface and thesurfaces ability to absorb, transmit, or reflect the suns rays.To determine , the surface azimuth and the surface-solar azi-muth must be known. The surface azimuth (angle POS in Figure3) is the angle between the south-north line SO and t
31、he normal POto the intersection of the irradiated surface with the horizontalplane, shown as line OM. The surface-solar azimuth, angle HOP, isdesignated by and is the angular difference between the solar azi-muth and the surface azimuth . For surfaces facing east of south, = in the morning and = + i
32、n the afternoon. For surfacesfacing west of south, = + in the morning and = in theafternoon. For south-facing surfaces, = 0, so = for all condi-tions. The angles , , and are always positive.For a surface with a tilt angle (measured from the horizontal),the angle of incidence between the direct solar
33、 beam and the nor-mal to the surface (angle QOP in Figure 3) is given bycos = cos cos sin + sin cos (9)For vertical surfaces, = 90, cos = 0, and sin = 1.0, so Equa-tion (9) becomesFig. 2 Apparent Daily Path of the Sun Showing Solar Altitude and Solar Azimuth H Number of hours from solar noon15=Numbe
34、r of minutes from solar noon4-=Solar Energy Use 35.3cos = cos cos (10)For horizontal surfaces, = 0, sin = 0, and cos = 1.0, soEquation (9) leads toH= 90 (11)Example 2. Find for a south-facing surface tilted upward 30 from thehorizontal at 40 north latitude at 4:00 PM, AST, on August 21.Solution: Fro
35、m Equation (3), at 4:00 PM on August 21,H = 4 15 = 60From Equation (1) for August 21 (N = 233), = 11.8From Equation (6),sin = cos 40 cos 11.8 cos 60 + sin 40 sin 11.8 = 30.4From Equation (7),sin = cos 11.8 sin 60/cos 30.4 = 79.4The surface faces south, so = . From Equation (9),cos = cos 30.4 cos 79.
36、4 sin 30 + sin 30.4 cos 30 = 58.8ASHRAE Standard 93-2010 provides tabulated values of forhorizontal and vertical surfaces and for south-facing surfaces tiltedupward at angles equal to the latitude minus 10, the latitude, the lat-itude plus 10, and the latitude plus 20. These tables cover the lat-itu
37、des from 24 to 64 north, in 8 intervals.Solar SpectrumBeyond the earths atmosphere, the effective blackbody temper-ature of the sun is 10,370R. The maximum spectral intensity occursat 0.48 m in the green portion of the visible spectrum. Thekaekara(1973) presents tables and charts of the suns extrate
38、rrestrial spec-tral irradiance from 0.120 to 100 m, the range in which most of thesuns radiant energy is contained. The ultraviolet portion of thespectrum below 0.40 m contains 8.73% of the total, another38.15% is contained in the visible region between 0.40 and 0.70 m,and the infrared region contai
39、ns the remaining 53.12%.Solar Radiation at the Earths SurfaceIn passing through the earths atmosphere, some of the sunsdirect radiation is scattered by nitrogen, oxygen, and other mole-cules, which are small compared to the wavelengths of the radiation;and by aerosols, water droplets, dust, and othe
40、r particles with diam-eters comparable to the wavelengths (Gates 1966). This scatteredradiation causes the sky to appear blue on clear days, and some of itreaches the earth as diffuse radiation.Attenuation of the solar rays is also caused by absorption, first bythe ozone in the outer atmosphere, whi
41、ch causes a sharp cutoff at0.29 m of the ultraviolet radiation reaching the earths surface(Figure 4). In the longer wavelengths, there are series of absorptionbands caused by water vapor, carbon dioxide, and ozone. The totalamount of attenuation at any given location is determined by (1) thelength o
42、f the atmospheric path through which the rays travel and(2) the composition of the atmosphere. The path length is expressedin terms of the air mass m, which is the ratio of the mass of atmo-sphere in the actual earth-sun path to the mass that would exist if thesun were directly overhead at sea level
43、 (m = 1.0). For all practicalpurposes, at sea level, m = 1.0/sin . Beyond the earths atmosphere,m = 0.Before 1967, solar radiation data were based on an assumedsolar constant of 419.7 Btu/hft2and on a standard sea-level atmo-sphere containing the equivalent depth of 2.8 mm of ozone, 20 mmof precipit
44、able moisture, and 300 dust particles per cm3. Threlkeldand Jordan (1958) considered the wide variation of water vapor inthe atmosphere above the United States at any given time, and par-ticularly the seasonal variation, which finds three times as muchmoisture in the atmosphere in midsummer as in De
45、cember, Janu-ary, and February. The basic atmosphere was assumed to be at sea-level barometric pressure, with 2.5 mm of ozone, 200 dust particlesper cm3, and an actual precipitable moisture content that variedthroughout the year from 8 mm in midwinter to 28 mm in mid-July.Figure 5 shows the variatio
46、n of the direct normal irradiation IDNwith solar altitude, as estimated for clear atmospheres and for an at-mosphere with variable moisture content.The ASHRAE clear-sky model described in previous editions ofthis chapter suffered from several well-known limitations. For ex-ample, the clearness numbe
47、r was not universally available; its valuevaried between 0.85 and 1.20 according to location and season inthe United States, and was often taken as unity for lack of better datain other countries. The model also lacked universal applicability andthe values of its coefficients had to be altered accor
48、ding to location.In addition, the model was derived from a very limited number ofmeasurements, and its applicability outside the United States hadnever been demonstrated. Finally, clear-sky diffuse irradiance wasproportional to beam irradiance, a fact that runs contrary to intuition(i.e., hazier skies should lead to less direct but more diffuse solar ir-radiation). To overcome these limitations, ASHRAE research proj-ects RP-1363 and RP-1453 developed a significant update of themodel (Thevena
