John H. Lienhard IV, John H. Lienhard V. A Heat Transfer Textbook (776116), страница 83
Текст из файла (страница 83)
Atmospheric575Radiative heat transfer57645% reachesthe earth’ssurface§10.645% istransmittedto the earthdirectly andby diffuseradiation33% isreflectedback tospace22% is absorbedin theatmosphereSensible heattransfer toatmosphereRadiation that reaches the outeratmosphere from the sunNetradiationfromsurfaceEvaporationThe flow of energy fromthe earth's surface back to and through - the earth'satmosphereFigure 10.24 The approximate distribution of the flow of thesun’s energy to and from the earth’s surface [10.16].gases also irradiate the surface.
This irradiation is quite important tomaintaining the temperature of objects on the surface.In Section 10.5, saw that the energy radiated by a gas depends uponthe depth of the gas, its temperature, and the molecules present in it.The emissivity of the atmosphere has been characterized in detail [10.16,10.17, 10.18]. For practical calculations, however, it is often convenientto treat the sky as a black radiator having some appropriate temperature.Solar energy§10.6577This effective sky temperature usually lies between 5 and 30 K belowthe ground level air temperature.
The sky temperature decreases as theamount of water vapor in the air goes down. For cloudless skies, the skytemperature may be estimated using the dew-point temperature, Tdp , andthe hour past midnight, t:Tsky = Tair 0.711 + 0.0056 Tdp1/42+ 7.3 × 10−5 Tdp+ 0.013 cos(2π t/24)(10.55)where Tsky and Tair are in kelvin and Tdp is in ◦ C. This equation appliesfor dew points from −20◦ C to 30◦ C [10.19].It is fortunate that sky temperatures are relatively warm.
In the absence of an atmosphere, not only would more of the sun’s radiation reachthe ground during the day, but at night heat would radiate directly intothe bitter cold of outer space. Such conditions prevail on the Moon, whereaverage daytime surface temperatures are about 110◦ C while averagenighttime temperatures plunge to about −150◦ C.Selective emitters, absorbers, and transmittersWe have noted that most of the sun’s energy lies at wavelengths nearthe visible region of the electromagnetic spectrum and that most of theradiation from objects at temperatures typical of the earth’s surface ison much longer, infrared wavelengths (see pg.
535). One result is thatmaterials may be chosen or designed to be selectively good emitters orreflectors of both solar and infrared radiation.Table 10.4 shows the infrared emittance and solar absorptance forseveral materials. Among these, we identify several particularly selectivesolar absorbers and solar reflectors. The selective absorbers have a highabsorptance for solar radiation and a low emittance for infrared radiation. Consequently, they do not strongly reradiate the solar energy thatthey absorb.
The selective solar reflectors, on the other hand, reflect solar energy strongly and also radiate heat efficiently in the infrared. Solarreflectors stay much cooler than solar absorbers in bright sunlight.Example 10.12In Section 10.2, we discussed white paint on a roof as a selectivesolar absorber. Consider now a barn roof under a sunlit sky. Thesolar radiation on the plane of the roof is 600 W/m2 , the air temperature is 35◦ C, and a light breeze produces a convective heat transfer578Radiative heat transfer§10.6Table 10.4 Solar absorptance and infrared emittance for several surfaces near 300 K [10.4, 10.15].SurfaceαsolarεIRAluminum, pureCarbon black in acrylic binderCopper, polished0.090.940.30.10.830.04Selective Solar absorbersBlack Cr on Ni plateCuO on Cu (Ebanol C)Nickel black on steelSputtered cermet on steel0.950.900.810.960.090.160.170.160.140.2–0.350.70.820.260.12–0.180.900.93Selective Solar ReflectorsMagnesium oxideSnowWhite paintAcrylicZinc Oxidecoefficient of h = 8 W/m2 K.
The sky temperature is 18◦ C. Find thetemperature of the roof if it is painted with either white acrylic paintor a non-selective black paint having ε = 0.9.Solution. Heat loss from the roof to the inside of the barn will lowerthe roof temperature. Since we don’t have enough information to evaluate that loss, we can make an upper bound on the roof temperatureby assuming that no heat is transferred to the interior. Then, an energy balance on the roof must account for radiation absorbed fromthe sun and the sky and for heat lost by convection and reradiation:44= h (Troof − Tair ) + εIR σ Troofαsolar qsolar + εIR σ TskyRearranging and substituting the given numbers,48 [Troof − (273 + 35)] + εIR (5.67 × 10−8 ) Troof− (273 + 18)4= αsolar (600)For the non-selective black paint, αsolar = εIR = 0.90.
Solving by§10.6Solar energyiteration, we findTroof = 338 K = 65◦ CFor white acrylic paint, from Table 10.4, αsolar = 0.26 and εIR = 0.90.We findTroof = 312 K = 39◦ CThe white painted roof is only a few degrees warmer than the air.Ordinary window glass is a very selective transmitter of solar radiation. Glass is nearly transparent to wavelengths below 2.7 µm or so, passing more than 90% of the incident solar energy. At longer wavelengths,in the infrared, glass is virtually opaque to radiation. A consequence ofthis fact is that solar energy passing through a window cannot pass backout as infrared reradiation.
This is precisely why we make greenhousesout of glass. A greenhouse is a structure in which we use glass trap solarenergy in a lower temperature space.The atmospheric greenhouse effect and global warmingThe atmosphere creates a greenhouse effect on the earth’s surface thatis very similar to that caused by a pane of glass. Solar energy passesthrough the atmosphere, arriving mainly on wavelengths between about0.3 and 3 µm. The earth’s surface, having a mean temperature of 15◦ Cor so, radiates mainly on infrared wavelengths longer than 5 µm.
Certainatmospheric gases have strong absorption bands at these longer wavelengths. Those gases absorb energy radiated from the surface, and thenreemit it toward both the surface and outer space. The result is that thesurface remains some 30 K warmer than the atmosphere. In effect, theatmosphere functions as a radiation shield against infrared heat loss tospace.The gases mainly responsible for the the atmospheric greenhouse effect are CO2 , H2 O, CH4 , N2 O, O3 , and some chlorofluorcarbons [10.20]. Ifthe concentration of these gases rises or falls, the strength of the greenhouse effect will change and the surface temperature will also rise or fall.With the exception of the chlorofluorocarbons, each of these gases is created, in part, by natural processes: H2 O by evaporation, CO2 by animalrespiration, CH4 through plant decay and digestion by livestock, and soon.
Human activities, however, have significantly increased the concentrations of all of the gases. Fossil fuel combustion increased the CO2579Radiative heat transfer580§10.60.8Temperature Anomaly, ˚C0.60.40.20.0-0.2Annual mean5-year mean-0.4-0.61880190019201940196019802000YearFigure 10.25 Global surface temperature change relative tothe mean temperature from 1950–1980 (Courtesy of the NASAGoddard Institute for Space Studies [10.21]).concentration by more than 30% during the twentieth century. Methaneconcentrations have risen through the transportation and leakage of hydrocarbon fuels. Ground level ozone concentrations have risen as a resultof photochemical interactions of other pollutants.
Chlorofluorocarbonsare human-made chemicals.In parallel to the rising concentrations of these gases, the surfacetemperature of the earth has risen significantly. Over the course of thetwentieth century, a rise of 0.6–0.7 K occurred, with 0.4–0.5 K of thatrise coming after 1950 (see Fig. 10.25). The data showing this rise areextensive, are derived from multiple sources, and have been the subjectof detailed scrutiny: there is relatively little doubt that surface temperatures have increased [10.21, 10.22]. The question of how much of therise should be attributed to anthropogenic greenhouse gases, however,was a subject of intense debate throughout the 1990’s.Many factors must be considered in examining the causes of globalwarming. Carbon dioxide, for example, is present in such high concentrations that adding more of it increases absorption less rapidly than mightbe expected.
Other gases that are present in smaller concentrations, suchas methane, have far stronger effects per additional kilogram. The con-§10.6Solar energycentration of water vapor in the atmosphere rises with increasing surfacetemperature, amplifying any warming trend. Increased cloud cover hasboth warming and cooling effects. The melting of polar ice caps as temperatures rise reduces the planet’s reflectivity, or albedo, allowing moresolar energy to be absorbed.
Small temperature rises that have beenobserved in the oceans store enormous amounts of energy that mustaccounted. Atmospheric aerosols (two-thirds of which are produced bysulfate and carbon pollution from fossil fuels) also tend to reduce thegreenhouse effect. All of these factors must be built into an accurateclimate model (see, for example, [10.23]).The current consensus among mainstream researchers is that theglobal warming seen during the last half of the twentieth century ismainly attributable to human activity, principally through the combustion of fossil fuels [10.22].
Numerical models have been used to projecta continuing temperature rise in the twenty-first century, subject to various assumptions about the use of fossil fuels and government policiesfor reducing greenhouse gas emissions. Regrettably, the outlook is notvery positive, with predictions of twenty-first century warming rangingfrom 1.4–5.8 K.The potential for solar powerOne alternative to the continuing use of fossil fuels is solar energy. Withso much solar energy falling upon all parts of the world, and with theapparent safety, reliability, and cleanliness of most schemes for utilizing solar energy, one might ask why we do not generally use solar poweralready.
The reason is that solar power involves many serious heat transfer and thermodynamics design problems and may pose environmentalthreats of its own. We shall discuss the problems qualitatively and referthe reader to [10.15], [10.24], or [10.25] for detailed discussions of thedesign of solar energy systems.Solar energy reaches the earth with very low intensity. We began thisdiscussion in Chapter 1 by noting that human beings can interface withonly a few hundred watts of energy. We could not live on earth if the sunwere not relatively gentle. It follows that any large solar power sourcemust concentrate the energy that falls on a huge area.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
