Corso di Fisica Te T cnica Ambientale Solar Radiation

Transcription

1 Solar Radiation

2 Solar radiation i The Sun The Sun is the primary natural energy source for our planet. It has a diameter D = 1.39x10 6 km and a mass M = 1.989x10 30 kg and it is constituted by 1/3 of He and approximately 2/3 of H. Solar structure Photosphere T f 6000 K Cromosphere T cr K Corona T c K T K Power emitted by the Sun P = W 2

3 The Sun We introduce the Solar Constant( C s ) defined as the radiative flux integrated all over the electromagnetic spectrum that intercept a surface located at the superior limit of the atmosphere at a mean distance from the Sun of approximately 1 A. U. (d = km) at normal incidence: C W/m 2 Some considerations: C s = (W)/{4 [ (m)] 2 } = 1372 W/m 2 F = 1372(W/m 2 ) R T2 = W F = (TJ/anno)/G = Mtep Approx times The world energy annual consumption! s G = kj/kg oil heat power R T = 6400 km 3

4 The Sun The Sun can be assimilated il to ablack body at uniform temperature: t using the Stefan- Boltzmann law it is possible to evaluate the black body equivalent temperature: 4 P R 4 T T 2 s 5800K If we use the Wien law, the Sun equivalent temperature evaluated considering the maximum power emitted at a wavelength of λ ~ μm is: A T 6100 K With A: m K

5 The Sun The extraterrestrial solar spectrum with Planck's law radiation superimposed. 5

6 How solar radiation interacts with our planet? Part of it reaches the surface and part is reflected by the clouds, the surface and the atmospheric gases. Albedo = Reflected/Incident A avg = Over 1 m 2 of the earth surface: F ST = 1370 W/m 2 (1-A avg ) R T2 /4 R T2 = 230 W/m 2 Radiative Balance at the earth surface F 4 ST F T P T P -20 C measured: T P = 15 C!?! 6

7 Composition of the Earth atmosphere Atmospheric composition near the earth surface (dry air) (Vittori. 1992). Gas % by volume Gas % by volume N ± He (5.239 ± 0.05) 10-4 O ± Kr (1.14 ± 0.01) 10-4 Ar ± Xe (8.7 ± 0.1) 10-6 CO ± H 2 ~ Ne (1.821 ± 0.004) 10-3 Mean values of concentration of variable gases (Vittori. 1992). Gas % by volume Gas % by volume H 2 O O CH NO CO Rn SO NO tracce N 2 O

8 The Earth atmosphere extends to approximately 1000 km from the surface and it can be divided in areas that are characterized by a particular temperature trend with ihaltitude. li The Troposphere starts at the ground or sea and reaches up to km height. The troposphere contains about 80 % of the mass of the atmosphere and also contains almost all of water of the atmosphere. It's lowest layer of about 1 to 2.5 km is called the planetray boundary layer (PBL). The stratosphere t lies above the troposphere at km. It is characterized by an increase of ozone with a maximum ozone concentration at about 30 km of hihtt height. Temperature increase with height is mainly caused by the ozone that absorbs the UV radiation from the sun. 8

9 Air Mass (AM) is defined as the ratio of a sun ray path in the atmosphere over the minimum sun ray path (zenith). AM l l 1 1 sen s AM = 0 if radiation is incident on a surface outside the atmosphere AM = 1 at zenith AM = 2 for s = 30 ; AM = 1.15 for s = 60. 9

10 When solar radiation passes through the atmopsheric gases it is subjected to absorption and diffusion processes. This phenomena cause the reduction of radiation intensity along the direction of propagation according to the Bouguer (or Lambert. or Beer) law: s, o e, i where ( ) e s.i.o () = spectral transparency (transmission) coefficient = extintion i coefficient i s = optical depth (it depends on gas density and thickness). x s x dx 0 10 x

11 Il l is the radiation path at the zenith and l 1 is the generic radiation path it can be defined: AM l1 l1 l AM l For the Beer Law: l e And for the transmission i coefficient: i e l sen s 11

12 Diffusion phrenomena are responsible for the radiation intensity differences in the sky. It is a continuum function of λ and is caused by the interaction of the solar radiation with the air molecules, aerosols and water vapor. Diffusion is responsible for blu sky and red sunset. The transparency of the atmosphere due to diffusion processes can be written as: where p = pressure φ = particle concentration g = amount of water fallen AM = air mass p diff air aerosol g AM 20 water 12

13 Absorption is a discontinous function of λ and it depends on the amount and temperature of asimmetric molecules like CO 2 and H 2 O. The figure shows the absorption bands of atmospheric compounds in the IR: CO 2 has a max for λ= 2.7μmμ eλ= 4.3μmμ Water vapor shows a strong absorption in the IR In the near IR it shows a max for λ = 2.7μmμ Nitrogen and oxigen absorb in the X ray band while ozone (O 3 ) absorbs in the UV band protecting us from this dangerous radiation. ass O 3 H2O CO2 13

14 Total spectral transparency of the atmosphere diff ass The figure shows the total spectral transparency for AM = 1. It can be noted as in the near IR the maximum absorption is due to CO 2 and water vapor. 14

15 In the figure are shown the spectral irradiance at AM = 0 and AM = 2; black areas represent the absorption of the atmospheric gases. 15

16 Total transparency If we integrate the spectral transparency over the full spectrum we can obtain the Total transparency. generic irradiance: extra-atrmospheric total irradiance: D 0 d o 0 d 0 0 Both D and represents the direct component of irradiance. D 0 d 0 d 0 16

17 Radiative Balance for the atmosphere surface system 17

18 Surface interaction Solar Radiation incident on a surface at ground can be expressed as: D d a D = direct radiation = diffuse radiation d a = reflected radiation 18