Energy Degradation and Power Generation Technologies

1. Energy Degradation and Power Generation Technologies IB Physics Topic 8

2. Background Knowledge • Sankey diagrams • Energy degradation and thermodynamics • Efficiency • Electric energy production

3. Sankey Diagrams • Show energy or material flows through a process • Relative width of arrows proportional to flow • A convenient way to show energy losses (or degradations) and efficiencies Degraded Energy Energy In Useful Energy Out

4. Example Sankey Diagrams

5. An Exercise • Prepare a simplified Sankey diagram showing the energy flows for a fossil-fuel fired steam electric power plant: where is energy added and taken out?

6. Simplified Sankey Diagram for Fossil Fuel Power Plant

7. What Are the Energy Loss Terms? A: Flue gases from furnace to stack B: Radiative and Convective heat losses in boiler C: Friction losses in generator

8. Heat Engines and Thermodynamics • Heat Engine: converts thermal energy into mechanical energy • Examples: internal combustion engine, steam engine • Thermodynamics dictates that heat engine cannot be 100% efficient: • Some energy must be rejected/wasted • The wasted energy is less available to do useful work: it is degraded • Thermal energy is most degraded form of energy; once energy has been converted to thermal energy we can’t convert it all to M.E.

9. A Typical Steam Cycle • Saturated steam from steam generator expanded in high pressure turbine to provide shaft work to turn generator • Moist steam dried and superheated in moisture separator reheater • Superheated steam expanded in low pressure turbine to provide shaft work to turn generator • Exhaust steam condensed using cooling water: thermal energy rejected to surroundings • Feed water compressed and preheated • Heat added to working fluid in steam generator by burning fuel, etc. http://www.youtube.com/watch?v=e_CcrgKLyzc

10. Heat Engine Sankey Diagram • Here’s a Sankey diagram of a heat engine operating between hot and cold temperature reservoirs: can you spot the error? Engine efficiency = __useful work__ total input energy eff = Qin – Qout Qin

11. We Depend on Electric Energy • Most of the energy utilized in homes, offices, businesses is in the form of electricity • Most electric energy generation based on Faraday’s Law: • A changing magnetic flux produces an emf • Generators operate by turning coil in magnetic field (mechanical energy  electric energy) • Photovoltaic cells generate electricity directly from sunlight using principle similar to photoelectric effect (more later)

12. How a Generator Works http://www.walter-fendt.de/ph14e/generator_e.htm The key: something to “turn the crank” on the generator

13. Producing Electric Energy • Fossil fuels: burn in boilers, generate steam, steam turns turbine/generator • Nuclear reactors: heat energy from fission, generate steam, steam turns turbine/generator • Wind energy: K.E. of wind transferred to turbine/generator • Hydroelectric: P.E. of water converted to K.E. in turbine/generator • Wave power: K.E. of waves converted to K.E. in turbine/generator

14. Energy Sources • Non-renewable: • Fossil fuels (coal, petroleum, natural gas) • Nuclear fuel (enriched uranium) • Renewable: • Solar • Wind • Waves • Tides • Geothermal • Biomass (e.g., corn for ethanol, wood, bagasse & other crop wastes)

15. World Energy Production Note the heavy dependence on non-renewable fossil fuels Source: Tsokos, World-wide averages, Total energy production

16. U.S. Energy Sources and Sectors: 2009 http://en.wikipedia.org/wiki/Energy_in_the_United_States

17. Energy Density • Energy obtained per kg of fuel/matter • Expressed as J (or some multiple)/kg • All other things being equal, fuel with higher energy density is more desirable • Typical values: • Petroleum distillates: 45-47 MJ/kg • Natural gas: 55 MJ/kg • Coal: ~30 MJ/kg • U fuel: 2100 GJ/kg • Wood: 16 MJ/kg • Water (hydro, 100 m drop): 1000 J/kg • Table of some fuel heating values: http://en.wikipedia.org/wiki/Heat_of_combustion

18. Fossil Fuel-fired Power Plant Energy Transformations

19. Fossil Fuels • Produced by decomposition of buried animal and plant matter under action of pressure, temperature, and bacteria • Coal, oil, natural gas traditionally used to produce electricity • 30% (coal) to 42% (natural gas) efficient • Gasoline and other distillate fuels used in cars and other internal combustion engines • 30-40% efficient • Major sources of air pollutants (SOx, NOx, CO2)

20. Advantages of Fossil Fuels • Relatively inexpensive • High energy density • Ease of use in current engines and other devices • Extensive infrastructure (e.g., distribution) in place

21. Fossil Fuel Disadvantages • Non-renewable • Pollution (extraction, transportation and use) • Contribute to greenhouse effect • High transportation costs (high volume) • Extensive storage facilities required • Political ramifications

22. Alternatives to Fossil Fuel Electric Power Generation • Nuclear fission • Solar: active solar devices and photovoltaics • Hydroelectric • Wind turbines • Wave power: Oscillating water column (OWC)

23. Nuclear (Fission) • Basis: Chain reactions of U with neutrons produce energy: • Nuclear reactor components: • Fuel rods: tubes containing enriched uranium (U-235) • Moderator: slows down neutrons to achieve collisions (water, others) • Control rods: absorb excess neutrons • Coolant: extracts reaction heat

24. Nuclear Power Station http://www.animatedsoftware.com/environm/nukequiz/nukequiz_one/nuke_parts/reactor_parts.swf

25. Nuclear Fuels • U-235: Makes up about 4% of the fuel mass • Fissionable using slow neutrons • Occurs in nature as a much lower percentage of the total uranium ore • To raise to the 4% used in most U.S. reactors requires enriching the fuel • U-238: Makes up about 96% of the fuel mass • Not fissionable using slow neutrons • Can form Pu isotopes using fast neutrons, some of which are fissionable using slow neutrons

26. Nuclear Fuels (cont.) • Pu isotopes • Formed in ordinary reactors from reactions of fast neutrons with U-238: • Some of these can serve as fuel in ordinary slow-neutron reactors • Pu-239 can be used as fuel in fast neutron reactors and in weapons production

27. Nuclear Power PlantEnergy Transformations

28. Nuclear Power Advantages • High energy density/power output per unit of fuel • Large reserves of nuclear fuels available • No air pollutants, including greenhouse gases

29. Nuclear Power Disadvantages • Mining safety issues • High-level radioactive waste disposal issues • Potential for production of nuclear weapons materials • Major public health hazard in case of accident

30. Solar Power: Active Solar Devices • Sunlight used directly to heat water or air for household purposes • Advantages: cheap, simple • Disadvantages: collectors are bulky, depend on sunlight availability

31. Another Solar Technology: Photovoltaic Cells • Promising technology for electricity production from sunlight • Used extensively in space program • Based on solid-state physics of semiconductors: similar to photoelectric effect • - Electrons absorb energy from photons, transition from valence band energy levels to conduction band levels http://www.jc-solarhomes.com/photovolt.htm http://www.teachersdomain.org/asset/psu06-e21_vid_pv4/

32. Solar Power Energy Transformations Active Solar Device Photovoltaic Cell

33. Photovoltaic Cell Advantages/Disadvantages • Advantages: • Use “free”, inexhaustible solar energy • Clean, non-polluting • Disadvantages: • Availability issues: daytime, sunny days only; not easily stored (batteries?) • Low power output • Require large areas • High initial costs (equipment, distribution)

34. Physics of Solar Power • Sun’s total power output (i.e., luminosity) P = 3.9 x 1026 W • Intensity: power per unit area at distance r from sun: I = P/(4πr2) • at earth’s mean distance from sun = 1400 W/m2 at top of atmosphere: SOLAR CONSTANT • at surface depends on latitude, angle of incidence (i.e., season) • variations due to solar power output ±1.5%

35. Daily Insolation • Daily insolation is the energy per m2 of earth surface per day (units: watt/hr∙m2) • Here’s a map of average U.S. values: Where is this resource located?

36. Hydroelectric Power • Principle: P.E. of water falling from a height h converted to K.E. http://www.tutorvista.com/content/physics/physics-ii/fission-and-fusion/hydroelectric-power-plants.php

37. Hydroelectric Energy Transformations

38. Hydroelectric Power Computations • P = mgh/Δt = (ρΔV)gh/Δt = ρ(ΔV/Δt)gh • ΔV/Δt = Q (volumetric flow rate) • So P = ρQgh • To generate large amounts of power a hydroelectric station requires • Large flows of water, Q • Large heights, h

39. Hydro Advantages/Disadvantages • Advantages: • “Free” energy source • Inexhaustible • Clean, non-polluting • Disadvantages: • Very dependent on location • Creates large changes to environment • Very high initial costs

40. Other Hydroelectric Technologies • So far we only considered the technology based on water storage in a reservoir (lake) behind the dam • Other technologies vary primarily in the mode of water resource storage • They include: • Tidal water storage • Pumped hydro

41. Tidal Water Hydroelectric Facility The dam (barrage) is opened during high tide and then closed. The trapped water is then used to produce electric power http://en.wikipedia.org/wiki/Tidal_barrage

42. Pumped Hydroelectric Facility Such a facility is, because of efficiency considerations, always a net consumer of electric power. The benefit is that the utility can sell more energy during times of peak demand (and therefore peak cost) When demand is low water is pumped back up to a reservoir. This then serves as the source during times of high demand. http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity

43. Wind Power • Principle: extract part of the K.E. of the wind and convert to electricity using generator Horizontal Axis Turbine Vertical Axis Turbine http://www1.eere.energy.gov/wind/

44. Wind Power Energy Transformations

45. Wind Power Computation • If the area swept by the wind turbine blades is A, wind velocity v, air density ρ, then • Mass flow rate of air past blades in time Δt = (density)(volumetric flow rate)(Δt): m = ρ(Av)Δt • K.E. of air = ½ mv2= ½ (ρAvΔt)v2 = ½ρAΔtv3 • Power = K.E./Δt = ½ ρAv3 http://www.reuk.co.uk/Calculation-of-Wind-Power.htm

46. Wind Power Computations (cont.) • Extractable power is less than shown • Actual power based on a “power coefficient” Cp • It’s an efficiency factor: you don’t really convert all of the wind’s kinetic energy to useful power and there are mechanical losses • Cp varies between 0.35 and 0.45 P = Cp A(½ ρv3) Doubling turbine area doubles power extracted Doubling wind speed increases power by 8X

47. Wind Power Advantages/Disadvantages • Advantages: • “Free” energy source • Inexhaustible • Clean, no air emissions • Ideal for remote locations (e.g., islands) • Disadvantages: • Not 100% dependable (wind varies) • Low power output • Aesthetic objections (large numbers, noisy) • High transmission costs (remote locations) • High maintenance, capital costs

48. Wind Resources Serious wind power generation requires wind speeds of 6-14 m/s

49. Wave Power • Based on harnessing energy of deep-water long-wavelength ocean waves • Involves converting wave’s kinetic and potential energy to electric energy • A number of possible technologies have been proposed: one is oscillating water column (OWC) concept OWC is actually a land-based wave power technology

50. OWC Schematic and Animation http://www.archipelago.co.uk/our-work/wave-power-animation