Solar Energy overview

1. Solar Energy overview

2. Solar Principle: Lots of sunlight incident on Earth’s surface: 1.3x1017 kWh/yr insolation; total human energy use (estimated for *all* history) 2.7x1012 kWh Largest potential source • Diffuse • Needs lots of land • Could use “free surface” (as roofs of built areas) • Variable (like wind, but less so) • Sun only shines half of day… • Weather/year cycle? Harness through: • Thermal conversion (including passive solar heating/cooling) • Photovoltaics (direct conversion to electricity)

3. Solar energy uses “hot water” solar thermal not discussed here • Low grade heat can be used as industrial process heat Neither is heating/cooling/daylighting • But daylighting is cheapest way to displace electrical use

4. Which Solar Technology? Break even between PV and Thermal at ca. 1300 kWh.m-2.yr-1

6. Concentrated Solar Thermal Power not so new…

7. Solar Radiation

8. Solar Collector • Flat Plate, T max ~70˚C • Hot water, space heating • 30-50% heat loss

9. Solar Collector

10. Concentrating Collectors • Motivation • Increase intensity at collector • Less heat loss over a smaller area • Higher maximum temperatures • Smaller area = less material = lower cost • Types • Trough • Dish • Heliostat/Central Receiver

11. Concentrating Collectors Aa, Ar =Area of aperture, receiver [m2] Ar Aa

13. Concentrating Collectors Larger CR means a more efficient collector ηo= optical efficiency (includes absorbed fraction at collector and reflectivity of concentrating optics)

14. Concentrating Collectors r R Ar Aa (Tiwari, 2004)

15. Radiation from a body • Bodies at above 0K emit radiation • Emissivity: ratio of emissive power of a surface to that of a black body (ε=1.0). • For a blackbody: Q=AσT4 • For generic (“gray”) body: Q=AεσT4 • Higher temperatures lead to more energy lost by emitted radiation

16. Temperature has mixed effect

17. Troughs ~ 300˚C • CR~10-50

18. Parabolic Trough Schematic • Focuses parallel rays to a line • A black pipe is placed with its center at the focus • Pipe can be in a vacuum or could have a glass cover tube to reduce convection • Cylindrical reflector can be on one half of the vacuum tube and approximates the parabolic shape 040208

19. Dish ~ 700˚C • CR~200-500

20. Heliostat/Central Receiver ~ 800-1000˚C • CR~500-3000

21. Central Receiver: Solar Two

22. Central Receiver: Sandia CRTF 5 MW power Flux to 280 W/cm2 Each heliostat is separately driven to focus its beam on the receiver

24. Central Receiver and Energy Storage: Sandia CRTF • The large tank stores energy to use during cloud passage or at dusk • The output power is extracted at a constant rate 090211

25. Solar Thermal Energy Storage • Latent Heat/PCM • Wax • Salts • Eutectics

26. What are concentrators made of? • Have to withstand extreme conditions (heat, wind, temperature variation) • Silvered Glass, with low Fe content • Thick Glass • Thin Glass • Polished Alumina • Silvered Polymer

27. Economics • Current: Glass ~$65 /m2 • Emerging: Polymer rolls + Al substrate ~$30/m2 • NREL Targets: >90% reflectance 10-30 yr lifespan $10/m2

28. Combined Cycle

29. Working Fluid Choice • Temperature Stability • Safe, non-toxic • Cheap • Wetting vs. Drying Fluid

30. Organic Rankine • Lower quality (temperature) heat • Drying fluid (fluid still superheated after turbine expansion) • CFCs: R-1XX • Hydrocarbons • Isobutane • Methanol • Pentane • Many others

32. What are Solar Cells? Load Solar cells are diodes Light (photons) generate free carriers (electrons and holes) which are collected by the electric field of the diode junction The output current is a fraction of this photocurrent The output voltage is a fraction of the diode built-in voltage - + p-type n-type Open-circuit voltage Voltage Maximum Power Point Current Short-circuit current

33. Energy-band Diagrams Electrons in solids fill states until you run out • Conduction band – top band, electrons are the charge carriers (support current flow) • Valence band – bottom band, electrons normally live here unless excited to conduction band (by heat or light) • An electron must acquire the band gap energy to jump across to the conduction band, measured in electron-volts eV • Silicon band gap energy is 1.12 eV • Also remember energy and wavelenght are related

34. Energy-band Diagrams The probability of finding an electron in a state is the Fermi distribution Fermi level is the energy at which the probability of finding an electron is 0.5 http://upload.wikimedia.org/wikipedia/commons/c/c7/Isolator-metal.svg

35. Charge carriers: Electrons and Holes Electrons (which are, um…, electrons) • Electrons move in the conduction band • Force is “electric field” Holes (the “absence of an electron” in that state) • Holes move in valence band Electrons create holes when they jump to the conduction band • Photons with enough energy move electron to CB • Create hole-electron pairs in a semiconductor For a specific material, the charge carrier density is a constant

36. Solar Cell Max Efficiency Photons need to have at least bandgap energy (Egap) Photons with a shorter wavelength but more energy than Egap dissipate the extra energy as heat This limits effectively the maximum efficiency of a single junction cell to 30% • Multiple junction cells limit is 68% for infinite number of layers Concept known as the Shockley-Queisser Limit

38. The “p-n junction” “n-type” has excess electrons (can donate electrons) “p-type” has electron deficit (can accept electrons) Connecting an n-type semiconductor (doped to have extra electrons) to a p-type material (extra holes) creates “p-n junction” • n-type carriers diffuse into p-type material (fill available energy states) • Result is excess positive charge at surface of n-type, excess negative charge at surface of p-type • Creates a “built in electric field” at p-n junction • Region where carriers have diffused is “depletion width”

39. p-n junction diagram http://en.wikipedia.org/wiki/File:Pn-junction-equilibrium.png

40. p-n junctions in the dark Electrons diffuse from high to low concentration region Electric field at junction pushes electrons away from junction (same for holes) Under no applied external potential, these are in equilibrium No current http://en.wikipedia.org/wiki/File:Pn-junction-equilibrium.png

41. The p-n Junction Diode

42. p-n junctions in PV devices Photons generate hole electron pairs in p-n junction E-field at junction pulls electrons to n-type (similar for holes) Flow of holes and electrons creates a current

43. Photogenerated Charge Carrier Generation Efficiency

44. Sizes important to PV: Absorption coefficient Thicker is better. You need at least 2 absorption lengths even with a back surface reflector.

45. Cell current under illumination (photocurrent)

46. PV cell model

47. Maximizing voltage produced in cell -Vmax has log dependence on light intensity -You would like to use materials with large Lp, Ln (light doping) Lp, Ln: Minority carrier diffusion lengths tp, tn: Minority carrier lifetimes pn, np: Minority carrier concentrations

48. Sizes important to PV: carrier diffusion Thinner is better (Need to be able to diffuse to the contacts!) Optimal performance: 10 nm for organics 1-2 microns for CdTe, CIS, a-Si:H 2-10 microns for GaAs 20-100 microns for Si, Ge

49. Recombination and losses within cell Generated hole/electron pairs can recombine at defects • Impurities • Grain boundaries Looks like further current loss within the cell • Use very pure single crystal material…

50. Single crystal vs. Polycrystalline Si