Electric Power System - University of Colorado...
Transcript of Electric Power System - University of Colorado...
Electric Power System
ECEN 2060
Spring 2008
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US Electricity Generation
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US Electricity Flow (2006)
Energy Information Administration, http://www.eia.doe.gov/(excellent source of energy-related data)
1 quadrillion Btu = 1015 Btu = 1.055x1018 J = 2.93x1011 kWh = 293 TWh
Loss: 64.7%
35.3%
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Energy conversions in electricity generation Chemical
(e.g. coal)
Nuclearheat mechanical electrical
Carnot efficiency limit of heat engines
η < 1−TC/TH < 70%
Water flow
Radiation
steam
flow
Betz efficiency limit:
η < 60%
mechanical electrical
Air flow mechanical electrical
electrical
Material bandgap efficiency limit:
η < 50%
Hydro and wind
Photovoltaic
(kinetic)
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Coal-fired power plant
Energy conversion and emissions: Example 3.2
Plant heat rate (thermal energy required to produce 1kWh or electricity): 10,800 kJ/kWh
Heat released per kg of coal: 27,300 kJ/kgThermal efficiency (3600 kJ/kWh)/(10,800 kJ/kWh) = 33.3 %Coal rate = (10,800 kJ/kWh)/(27,300 kJ/kg) = 0.4 kg/kWhCarbon emissions (75% of coal is carbon): 1.09 kg/kWh
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Coal-fired steam power plants
• More than 50% of all electricity generated in the US
• Approx 30% thermal efficiency (from generated heat to generated electricity)
• Emissions� Carbon dioxide
� Sulfur oxide (75%)
� Mercury
� Nitrogen oxides
� Particulate matter (and radiation exposure!)*
*Radiation doses from airborne effluents of model coal-fired and nuclear power plants (1000 megawatts electric) are
compared. Assuming a 1 percent ash release to the atmosphere (Environmental Protection Agency regulation) and
1 part per million of uranium and 2 parts per million of thorium in the coal (approximately the U.S. average),
population doses from the coal plant are typically higher than those from pressurized-water or boiling-water reactors that meet government regulations. Higher radionuclide contents and ash releases are common and would result in
increased doses from the coal plant. The study does not assess the impact of non-radiological pollutants or the total
radiological impacts of a coal versus a nuclear economy.
McBride JP, Moore RE, Witherspoon JP, Blanco RE, Radiological Impact of Airborne Effluents of Coal and Nuclear
Plants, Science. 1978 Dec 8;202(4372):1045-1050
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Electricity Generation in Colorado
• 11.2 GW total capacity
• Total electricity generated (2006): 50.7 TWh, 70% coal-fired
• Average retail price: 7.61 cents/kWh
• Boulder Valmont power plant: coal-fired (186 MW), gas combustion (43 MW)
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Electric Power System
• Generation, transmission and distribution of constant
frequency (60±0.1 Hz in the US) 3-phase AC
� Constant-frequency AC:
• Simple voltage step-up or step-down using transformers
• Efficient transmission and distribution
� 3-phase:
• Constant instantaneous power
• Efficient, reliable generators
• Efficient transmission and distribution
• Complex interconnected system (grid*) with many
generators adding power to meet time-varying demands for electricity
*There are 3 separate power grids in the US
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US Power Grids
• 986 GW total capacity (2006)
• 275,000 miles of transmission lines
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A more detailed view
High-Voltage
DC (HVDC) line
example
(Pacific intertie),
846 miles, 500kV, 2 GW
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Demand fluctuations
Types of power plants
• Baseload plants (e.g. coal-fired or
nuclear)
• Intermediate (e.g. hydro or
combined-cycle)
• Peaking (e.g. combustion gas
turbines)
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Generation, transmission and distribution
Typical residential wiring
• One of the distribution phases (4 kV in the example shown) dropped to two 120V circuits using transformer with a center-
tapped secondary
• Available AC voltages: 120Vrms and 240Vrms (single-phase)
Voltage levels in transmission and distribution are based on efficiency/cost trade-offs
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Grid-Connected Renewable Sources
• Must interface to the existing 60 Hz utility grid and comply to utility regulations
• Challenges and opportunities
� Geographical availability of grid transmission/distribution
lines
� Intermittency of renewable sources such as wind or solar
� Increased needs for energy storage
� Distributed generation
• Distributed synchronization and system controls
� Economics