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
Upper left is Sankey diagram for an ipod. Lower left: a car. Upper right: a production cycle including recycle

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.

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
The amount of energy rejected to the cold reservoir = energy from hot reservoir: no work could be done in this case!

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
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: 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

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: 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:

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

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 Plant Energy 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

32. Solar Power Energy Transformations
Active Solar Device
Photovoltaic Cell

33. Photovoltaic Cell Advantages/Disadvantages
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.

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
“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
Barrage is pronounced BAR ij
The dam (barrage) is opened during high tide and then closed.
The trapped water is then used to produce electric power

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.

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

44. Wind Power Energy Transformations
Winds occur when different layers of the atmosphere are heated by the sun to different temperatures, causing pressure differences due to hot air rising/cold air sinking, resulting in air flows

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 = ½ ρAΔtv3
Power = K.E./Δt = ½ ρAv3

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
“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
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
Of the many possible technologies, IB has chosen to emphasize OWC in the curriculum
OWC is actually a land-based wave power technology

50. OWC Schematic and Animation

51. Basis of OWC Technology
As wave crest approaches cavity in device, column of water rises and pushes air above it upwards
Air pushed upward turns turbine and is released to atmosphere
As trough of wave approaches, water level falls
Air drawn in also turns turbine

52. Wave front Power: Simplified Analysis
Energy in wave alternates between PE as water wave rises and KE as it falls
Volume of water in a single wave = A(λ/2)(L)
Mass of water in single wave = ρV = ρ(Aλ/2)(L)
Loss of P.E. of this water = mgh = (ρAλ/2)(L)(g)(A)
Waves passing a point in unit time = f = v/λ

53. (Power proportional to A2, as expected for a wave)
Wave Power (cont.)
P.E. loss per unit time = (ρA2λ/2)(Lg)(v/λ)
Maximum power available = this P.E. loss per unit time:
P = ½ ρLvgA2
(Power proportional to A2, as expected for a wave)
Finally, maximum power available per unit length of wave front:
P/L = ½ ρvgA2

54. Wave Power Advantages/Disadvantages
“Free”, inexhaustible energy source
Reasonably high energy density
Clean, no air emissions
Disadvantages:
Location specific: requires large amplitude waves
Irregularity in wave patterns lowers efficiency
Difficulty in coupling low-frequency waves to high frequency turbine motion (50-60 Hz)
Very high installation, transmission, and maintenance costs
Need to design for very hostile environment: hurricanes, gale-force storms

55. Power Equations Summary
Hydroelectric: P = ρQgh
Wind: P = Cp A(½ ρv3)
Wave front: P = ½ ρLvgA2
NOVA Program: Energy Surge