1. EE 394J-10 Distributed Generation Technologies Fall 2012

2. 2 © Alexis Kwasinski, 2012 Photovoltaic (PV) modules are made by connecting several PV cells. PV arrays are made by connecting several PV modules. Although the sun will eventually die as a white dwarf star in about 4.5 Billion years, solar power can be considered a renewable source of energy because we can expect that for the next couple of billion years the sun will still radiate power without making the Earth inhabitable. Solar power is radiated through space. Solar power is generated by nuclear fusion. Light propagation can be represented through waves or through particles (dual representation). To represent electricity production in PV cells, the particle (photon) representation is used Photovoltaic modules

3. 3 © Alexis Kwasinski, 2012 Photons are created at the center or the Sun. It takes an average of 10 million years for the photons to emerge (they collide many times in the Sun interior). Then it takes 8 minutes for a photon to reach the Earth. Fusion reactions: Step 1: ( represents an atom of deuterium = an hydrogen isotope formed by a proton and a neutron, a positron ( p + ) or antielectron is an electron with a positive charge, a neutrino n 0 are very low mass- no charge elementary particles). This reaction requires extreme temperatures and pressures to bring two protons so close (< 10 -15 m) that the repulsion force between them disappears. Step 2: where γ represent a photon. Step 3.1: Step 3.2: where is tritium an hydrogen isotope formed by 2 neutrons and a proton Photons’ Journey into Electricity

4. 4 © Alexis Kwasinski, 2012 Fusion reactions (continue): Step 4.1: Step 4.2: The overall reaction is: This reaction releases 26 MeV All photons are created equal. So why photons leaving the sun have different energy (as indicated by their different frequency in the dual wave model)? The emitted photons have high energy. This energy is mostly lost in collisions with atoms as the photons leave the sun. This reaction can only occur due to the high pressure generated by the mass contraction at the Sun’ s center. The Sun is mostly composed of hydrogen (73 %) and Helium (25 %). These proportions are changing. Eventually the sun will start the fusion process of heavier elements. Photons’ Journey into Electricity

5. 5 © Alexis Kwasinski, 2012 Ideal radiation of energy is described by the black body radiation. Black bodies radiate energy at different wavelengths as indicated by The Sun closely behaves like a black body at a temperature T=5800 K (the Sun’s surface temperature) http://en.wikipedia.org/wiki/Image:EffectiveTemperature_300dpi_e.png Total blackbody radiation rate (area under the curve): E=AσT 4 For the Sun it equals 1.37 kW/m 2 Wavelength for the maximum: For the Sun it approximately equals 0.5 μm Photons’ Journey into Electricity

6. 6 © Alexis Kwasinski, 2012 Finally, the photons reach the Earth. US Solar Insolation Map: NREL Photons’ Journey into Electricity

7. 7 © Alexis Kwasinski, 2012 The incident power has 3 components depending on the final photons path. Reflected radiation Direct-beam radiation Diffuse radiation Photons’ Journey into Electricity

8. 8 © Alexis Kwasinski, 2012 Direct-beam radiation: The extraterrestrial solar insolation is given by This is the solar insolation before entering the Earth’s atmosphere. In the equation, SC is the solar constant an equals 1.37 kW/m 2 and n is the day number (January 1 is day #1). The day number takes into consideration that the Earth-Sun distance changes through the year. The solar insolation is attenuated as it passes through the atmosphere. The portion that reaches the earth’s surface. where A and k are constants and m is the air mass ratio that takes into account that the sun’s beam path length through the atmosphere changes with the sun relative position with respect to the earth surface at the location where the analysis is made. Photons’ Journey into Electricity

9. 9 © Alexis Kwasinski, 2012 Sun’s location terms Photons’ Journey into Electricity

10. 10 © Alexis Kwasinski, 2012 Magnetic vs. celestial poles: Magnetic poles: Created by Earth’s magnetic field Can be located with a compass They move along Earth’s surface! Celestial poles: Created by Earth’s rotation. They are two imaginary stationary points in the sky. Important for PV system applications. Geological Survey of Canada Photons’ Journey into Electricity

11. 11 © Alexis Kwasinski, 2012 NOON 1 PM 3 PM Jun Dec Sep Sun’s position in the sky throughout the day and during an entire year. Photons’ Journey into Electricity

12. 12 © Alexis Kwasinski, 2012 Photons’ Journey into Electricity The direct-beam insolation I BC depends on the PV module orientation with respect to the sun. If the PV module is fixed, this insolation will change in a deterministic way throughout the day and the year: if the incident angle θ is given by Then, the direct-beam insolation is

13. 13 © Alexis Kwasinski, 2012 June 21 December 21 March 21 September 21 Equator Tropic of Cancer Latitude 23.45 o Tropic of Capricorn Latitude -23.45 o Austin’s Latitude: 30 o 23.45 o 30 o Edge of PV module (for incidence angle calculation) Earth’s surface (for air mass ratio calculation) Photons’ Journey into Electricity Impact of the sun’s position for the calculation of the direct-beam radiation with respect to the incidence angle and the air mass ratio

14. 14 © Alexis Kwasinski, 2012 Photons’ Journey into Electricity Assuming that the diffuse radiation does not depends on the sun’s position in a clear sky, then it is modeled using the following equation:\ where C is the sky diffuse factor which can be obtained from ASHRAE. This is another deterministic value. The reflected radiation can be calculated by considering the reflectance ρ of the surface in front of the PV module: This is another deterministic value. The total radiation rate on a PV module is, therefore, given by

15. 15 © Alexis Kwasinski, 2012 After a long journey, photons are converted into electricity in semiconductors: Whenever a photon with enough energy hits an atom, an electron may jump the energy gap into the conduction band. Once in the conduction band the electron is free to move in an electric circuit. If the circuit is open or if the load requires less current (charge per time) than the one being produced, the free electrons will eventually decay again. Since it is assumed a continuous slow varying incident solar energy, electrons are freed at a constant rate. Hence, a constant voltage is produced. Photons’ Journey into Electricity

16. 16 © Alexis Kwasinski, 2012 Photons’ Journey into Electricity Atom’s energy model: Photons energy is quantized. The energy of a photon with a wavelength of λ ( or a frequency of υ ) is where h is Planck’s constant Gap EgEg Conduction band (partially filled) Forbidden band Filled band Electron Energy Gap EgEg Conduction band (Empty at T = 0K) Forbidden band Filled band Electron Energy Metalssemiconductors

17. 17 © Alexis Kwasinski, 2012 Photons’ Journey into Electricity if the last equation is plotted we obtain that Hence, there is a theoretical limit to a PV cell power output which depends on the semiconductor material being used. For different semiconductors we have that: From the course’s recommended book Lost in heat From the course’s recommended book

18. 18 © Alexis Kwasinski, 2012 Photons’ Journey into Electricity Efficiency limit can be understood by comparing the following two figures: So for an air mass ratio of 1.5 the efficiencies are (see next slide) From the course’s recommended book http://en.wikipedia.org/wiki/Image:EffectiveTemperature_300dpi_e.png Insufficient energy Excess energy

19. 19 © Alexis Kwasinski, 2012 For silicon and an air mass of 1.5 the maximum efficiency is about 50% As the band gap energy decreases the efficiency improves somewhat. However, the cost increases significantly. Next class: PV cells electrical characteristics and technologies. Photons’ Journey into Electricity

20. 20 © Alexis Kwasinski, 2012 PV Cells Technologies Characterization criterion: Thickness: Conventional – thick cells (200 - 500 μm) Thin film (1 – 10 μm). Tend to be less costly than conventional (think) cells but they also tend to be less reliable and efficient. Crystalline configuration: Single crystal Multicrystalline: cell formed by 1mm to 10cm single crystal areas. Polycrystalline: cell formed by 1μm to 1mm single crystal areas. Microcrystalline: cell formed by areas of less than 1μm across. Amorphous: No single crystal areas. p and n region materials: Same material: homojunction (Si) Different material: heterojunction (CdS and CuInSe 2 )

21. 21 © Alexis Kwasinski, 2012 BP SX170B PolycrystallineBP SX170B Monocrystalline Mitsubishi PV-TD 190MF5 Polycrystalline Uni-Solar Laminate PVL-136 Amorphous Uni-Solar solar shingle PV Modules at ENS PV Cells Technologies

22. 22 © Alexis Kwasinski, 2012 PV Cells Technologies Thick film fabrication techniques: Czochraski’s (CZ): for single-crystal silicon. Costly. Float zone process (FZ): also for single-crystal silicon. Costly Ribbon silicon Cast silicon: for multicrystalline cells. Less costly. Thin film Can be used embedded in semitransparent windows. Techniques: Amorphous Silicon: can achieve higher efficiencies (in the order of 42% thanks to the multijunction (different multiple layers) in which each layer absorb photons with different energy. Gallium Arsenide (GaAs): relatively high theoretical efficiency (29 %) which is not significantly affected by temperature. Less sensitive to radiation. Gallium makes this solution relatively expensive. Gallium Indium Phosphide (GaInP): similar to GaAs. Cadmium Telluride (CdTe): Issue: Cd is a health hazard (it is very toxic). Copper Indium Diselenide (CIS or CuInSe2): relatively good efficiency) Silicon Nitrade (N 4 Si 3 )