Energy Degradation and Power Generation Technologies IB Physics Topic 8

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

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

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

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?

Simplified Sankey Diagram for Fossil Fuel Power Plant

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

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.

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

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!

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)

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

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

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)

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

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

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

Fossil Fuel-fired Power Plant Energy Transformations

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)

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

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

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

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

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

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

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

Nuclear Power Plant Energy Transformations

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

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

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

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/

Solar Power Energy Transformations Active Solar Device Photovoltaic Cell

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)

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%

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?

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

Hydroelectric Energy Transformations

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

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

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

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 http://en.wikipedia.org/wiki/Tidal_barrage

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

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/

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

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 http://www.reuk.co.uk/Calculation-of-Wind-Power.htm

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

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

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

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

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

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

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

(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

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

Power Equations Summary Hydroelectric: P = ρQgh Wind: P = Cp A(½ ρv3) Wave front: P = ½ ρLvgA2 NOVA Program: Energy Surge http://www.pbs.org/wgbh/nova/tech/power-surge.html