Electric Power System - University of Colorado Boulderecee.colorado.edu/.../materials/lecture_notes/ElectricPowerSystem.pdf · Electric Power System ECEN 2060 Spring 2008. ... Electric

Electric Power System

ECEN 2060

Spring 2008

2ECEN2060

US Electricity Generation

3ECEN2060

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%

4ECEN2060

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)

5ECEN2060

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

6ECEN2060

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

7ECEN2060

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)

8ECEN2060

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

9ECEN2060

US Power Grids

• 986 GW total capacity (2006)

• 275,000 miles of transmission lines

10ECEN2060

A more detailed view

High-Voltage

DC (HVDC) line

example

(Pacific intertie),

846 miles, 500kV, 2 GW

11ECEN2060

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)

12ECEN2060

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

13ECEN2060

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