Outline MAE 493R/593V- Renewable Energy Devices Solar Energy Electromagnetic wave Solar spectrum Solar global radiation Solar thermal energy Solar thermal collectors Solar thermal power plants Photovoltaics (Solar cells) P-n junction solar cells Dye-sensitized solar cells Organic solar cells CO 2 capture and photoelectrochemical cells http://www.flickr.com/photos/royal65/3167556443/ How much is the solar radiation on the earth surface? What is light?-- Dual nature of light: Photons as particle: Photons have energy but no mass Photons as wave: Electric and magnetic fields oscillating in space and time Power density (W/cm 2 )? Radiation is absorbed & emitted in photons. The defining characteristic of a photon is that its energy cannot be split into smaller pieces. Light - Electromagnetic wave spectrum Each photon s energy is defined by its frequency (ν) or wave length (λ) or wave number ( ) E photon = hν = hc/λ = hc h = Planck s constant, 6.63 x 10-34 J s c = speed of light, 3.00 x 10 8 m s -1 (or 3.00 x 10 10 cm s -1 ) Wavenumber, = 1/λ Energy unit Wavelength (λ): meter or micron Frequency (ν): Hz (cycl/s) Wavenumber ( ): cm -1 1 ev = 8065.54 cm -1 = 96.4853 kj/mol = 23.0605 kcal/mol 1
Visible light Nuclear fusion in the Sun Two protons combine to form a deuterium (hydrogen atom with one neutron) Energy White light is dispersed by a prism into the colors of the optical spectrum. http://en.wikipedia.org/wiki/visible_spectrum A blackbody absorbs and emits radiation at 100% efficiency (experimentally, they use graphite, or carbon nanotubes Wein s Law: determine the peak wavelength of the emission of a balckbody at a certain body temperature energy in = energy out Across all wavelengths λ max is the peak wavelength of emitted radiation (in µm) a = 2898, constant T emitter temperature (in K) Recall that T (K) = T ( C) + 273.15 Sun s temperature is 5800K What s its wavelength? =0.5 um The Multi-Wavelength Sun X-Ray ultraviolet Visible http://en.wikipedia.org/wiki/sunlight Infrared Composite Radio http://en.wikipedia.org/wiki/sunlight 2
What s the peak wavelength of your emission light? (a = 2898) Your body is 37 C (310 K) λ max =? 9.4 µm (far infrared) Stefan-Boltzmann Law: Energy emitted by a black body is greatly dependent on its temperature: I = (1-a) σ T 4 Where: I - power density of the radiation from a black body σ - Constant= 5.67x10-8 Wm -2 K -4 T - temperature (K) a - albedo ( reflectivity ) Example: Sun surface is 5800K, so I = 6.4 x 10 7 W/m 2 Calculate total solar emission output: Calculate Sun temperature assuming it behaves as a blackbody (knowing that λ sun = 0.5 µm). The effect of distance on radiation: the 1 / r 2 rule From S-B law: I sun = 6.4 x 10 7 W/m 2 We need surface area of sun: Area = 4πr 2 = 4π (6.96x10 8 m) = 6.2 x 10 18 m 2 Total Sun emission: 3.86 x 10 26 W Solar Emission Power: Sun emission decreases in proportion to 1 / r 2 of the Sun-Planet distance Mars is 1.52 AU Calculate solar energy reaching the Earth: (1 AU = earth-sun distance = 1.5 x 10 11 m) Using 1/ r 2 rule 1 / (1.5*1.5) = 0.44 Simple Geometry. (recall the inverse square law..) Earth-Sun distance (D): 1.5 x 10 11 m Area of sphere = 4π r 2 Mars receives ~44% of the Earth s solar radiation. So, 3.86 x 10 26 W / (4π (1.5 x 10 11 m) 2 ) Earth s incoming solar radiation: 1365 W/m 2 3
1365 W/m 2 Solar constant or air-mass zero (AM0): the total intensity above earth s atmosphere, approximately constant at 1365 kw m -2 On sunny day, light intensity on earth s surface is about 51 % of the intensity above the atmosphere. Absorption and scattering effects increase with the sun beam s path through the atmosphere. Air mass m (AMm): the ratio of the actual radiation path h to the shortest path h 0, m = h/h 0. Air mass one (AM1): The shortest path through the atmosphere is when the sun is directly above that location and the received spectrum is called. Since h = h 0 secθ, AMm is AMsecθ. h 0 AM0 Atmosphere h θ α α http://www.askmeaboutgreen.com/ Source: University of South Alabama Earth AM1 AM(secθ) Tilted PV device The atmospheric path for any zenith angle is simply described relative to the overhead air mass (See Figure). The actual path length can correspond to air mass of less than 1 (high altitude sites) to very high air mass values just before sunset. AM1.5: The spectral distribution AM1.5 has several sharp absorption peaks at certain wavelengths due to those wavelength being absorbed by various molecule in the atmosphere, such as ozone, air, and water vapor. Dust particles scatter the sun light reduces the intensity and gives rise to the sun s rays arriving at random angle. 2.5 The path length in units of Air Mass, changes with the zenith angle Source: New Port Spectral Intensity dw cm -2 (μm) -1 or kw m -2 (μm) -1 Source: University of South Alabama 2.0 1.5 1.0 0.5 0 0 Black body radiation at 6000 K AM0 AM1.5 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Wavelength (μm) Earth Radiation Earth energy in = energy out You have I earth, solve for T earth Stefan - Boltzmann law: I earth = (1-a) σ T earth 4 Incoming solar radiation: 1365 W/m 2 About 30% is reflected away by ice, clouds, etc.: reduced to 955 W/m 2 Incoming on dayside only (DISK), but outgoing everywhere (SPHERE), so outgoing is 1/4 of incoming, or 239 W/m 2 Earth Radiation Solve for T effective = 255K Earth Effective temp: 255 K, or -18 C Earth Actual temp: 288K, or +15 C the difference of +33 C is due to the natural greenhouse effect. that is: (0.7)*(0.25)*1365 = solar energy that reaches Earth surface Energy in = 239 W/m 2 = σ T 4 4
Earth Radiation Emission Spectra: Sun and Earth What Earth s radiation wavelength? Where: λ max is wavelength of emitted radiation (in µm) a = 2898, constant T emitter temperature (in K) 0.5 µm 11.4 µm If Earth effective temperature is 255K, What s the wavelength? =11.4 um Potential for Photovoltaic annual radiation * >800 kwh/m 2 yr excellent Very good good Annual Radiation Map G(SD) 2 Rhino Toolbar Potential for Passive Thermal Heating annual radiation during the heating season * >216 kwh/m 2 yr Potential for Solar Hot Water annual radiation * >400 kwh/m 2 yr Simulation: Diego Ibarra *) Compagnon 2004, Energy and Buildings (number derived for Switzerland) 5