EFS Chapter 9 Centralized Electric Power Systems
Chapter 9: Centralized Electric Power Systems
Introduction
Industrialized world takes electricity for granted
1-2 billion people globally still without basic energy services
Electricity infrastructure in North America:
275,000 miles of high-voltage transmission lines
1000 gigawatts of generating capacity
Serves a customer base of about 300 million people
Cost of electricity infrastructure: over $1.4 trillion
Reliable electricity requires real-time control and coordination of power plants
National Academy of Engineering describes the grid as "The Greatest Engineering Achievement of the 20th Century"
Summary of total 2015 electricity generation in the United States:
Carbon-free nuclear power and renewables provide about one-third of electricity
Fossil fuels generate the other two-thirds, with equal percentages from coal and natural gas
Electromagnetism: The Technology behind Electric Power
Scientists in the early nineteenth century explored electromagnetism
Voltage can be created in an electrical conductor by moving it through a magnetic field
Direct current dynamos and alternating current generators were developed based on this phenomenon
Current flowing through a wire in a magnetic field creates a force that wants to move the wire
Electric motors convert electric current into mechanical power
Electric motor in hybrid electric vehicles acts as a generator when the brakes are engaged
First electromagnet created by William Sturgeon in 1825
Development of generators and motors followed
First practical direct current (dc) motor/generator, called a dynamo, developed by Zénobe Gramme
Gramme's dynamo demonstrated the potential to generate power at one location and transmit it to a distant location
Creating the Modern Electric Utility: Edison, Westinghouse, and Insull
First major electric power market developed around the need for illumination
Thomas Alva Edison created the first workable incandescent lamp in 1879
Edison Electric Light Company provided electricity and lightbulbs
Edison's system was based on direct current
Edison's Pearl Street Station in Manhattan became the first investor-owned utility in the nation
Centralized Electric Power Systems
Edison's customers had to be located within a mile or two of a generating station
Power stations were located every few blocks around the city
Difficulty in moving power from one place to another without high losses in power lines
The Important Role of Transformers
Power delivered by power lines is equal to voltage times current (P = VI)
Higher voltage leads to lower line losses
Modern transmission lines operate at high voltages to minimize line losses
The Battle between Edison and Westinghouse
Edison bet on DC power, while Westinghouse recognized the advantages of AC power
Westinghouse launched a competing company based on AC power
Edison stuck with DC and launched a campaign to discredit AC
Edison demonstrated the lethality of AC by electrocuting animals in front of the press
High-voltage transmission had overwhelming advantages
Edison's insistence on DC led to the disintegration of his electric utility enterprise
Insull Develops the Business Side of Utilities
Samuel Insull developed the business side of utilities
Spread high fixed costs over as many customers as possible
Aggressively marketed the advantages of electric power, especially for daytime use
Centralized Electric Power Systems (Page 5)
Insull's idea was to integrate loads to use the same generation and transmission equipment more efficiently.
This resulted in lower prices and increased demand.
Insull promoted rural electrification to expand the customer base.
Building bigger power stations took advantage of economies of scale, decreasing electricity prices and increasing profits.
Insull introduced the idea of selling utility common stock to raise large sums of capital.
Insull helped establish regulated monopolies with franchise territories and controlled prices.
Electric Power Infrastructure (Page 5)
The electric power industry in the US is worth over a trillion dollars with annual sales exceeding $300 billion.
About 40% of total US primary energy is used to generate electricity, with two-thirds coming from fossil fuels.
Combustion of fossil fuels in power generation is responsible for a significant portion of emissions.
Power plants generate electricity, and transmission and distribution systems carry it to customers.
The system consists of generating stations, high-voltage transmission lines, distribution substations, and local power lines.
The North American Power Grid (Page 6)
The power grid consists of interconnected transmission and distribution lines.
Electricity flows at nearly the speed of light, seeking the path of least resistance.
The North American power grid is divided into three separate interconnected grids: Eastern Interconnect, Western Interconnect, and Texas.
Interconnections between the grids are made using high-voltage dc (HVDC) links.
Within each interconnection zone, circuits operate at the same frequency.
There are seven major independent system operators (ISOs) and regional transmission organizations (RTOs) responsible for operating the transmission systems.
ISOs and RTOs allocate transmission rights through auctions and spot-market transactions.
Balancing the Grid (Page 6)
Routine management of the grid involves balancing power supply with customer demand.
If demand exceeds supply, turbine generators slow down slightly to convert kinetic energy into extra electrical power.
This results in a slight drop in the grid's expected frequency.
Automatic governors increase torque to bring the generator back up to speed.
If demand decreases, turbines speed up before being brought back under control.
Balancing the grid is compared to managing a bathtub with faucets representing electricity supply and drains representing electricity demand.
Centralized Electric Power Systems
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Grid frequency is represented by the depth of tub water
Goal is to maintain grid frequency between 60.02 Hz and 59.98 Hz
Tools for balancing the grid include mechanical inertia, governors, and frequency regulation units
Frequency regulation units can respond within a few seconds
They can reduce their output when frequency rises and increase their output when frequency drops
They are paid a monthly fee per megawatt of regulation services available
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ISOs and RTOs balance supply and demand using day-ahead, hourly demand auctions
Almost real-time, 5-minute load forecasts are used to dispatch the lowest-cost plants to meet demand
Bathtub analogy is used to explain grid balancing
Actions to balance the grid include frequency regulation, battery storage systems, demand response, and reserves
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Battery storage systems are beginning to provide frequency regulation services
Demand response involves end-use customers varying their loads to help balance the grid
Reserves such as spinning reserves, operating reserves, and capacity reserves can be called upon to balance loads
Large deviations in frequency can cause damage to equipment and automatic shutdowns of portions of the grid
Blackouts can occur when the grid is running at or near capacity, often during hot days with high demand for air conditioning
Insufficient management of tree growth within transmission line rights-of-way can trigger blackouts
Load duration curves show the variation in power demands throughout the day
Baseload plants operate continuously, while load-following plants and peakers are used to meet varying demand
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Load curve and load duration curve
Load curve is a chronological hour-by-hour version of a load curve
Load duration curve is a rearrangement of the vertical slices of the load curve from highest to lowest demand
Baseload, load-following, and peaker power plants
Baseload plants run all the time, load-following plants run about half the time, and peaker plants are needed for less than 800 hours per year
Cost implications of peaker plants
Peaker plants represent $14 billion worth of plant that has to recover its capital cost through high-priced electricity sales during peak periods
This translates into higher costs during peak demand periods
Future solutions for peak demand periods
Load shifting and battery storage opportunities can help deal with peak demand periods
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Electric power generation
Most electricity is generated in large, central stations with power capacities measured in hundreds or thousands of megawatts
Power plants are often clustered together into power stations
Fuel sources for electricity generation
Coal and natural gas provide two-thirds of electricity
Nuclear power delivers less than 20% of electricity
Renewables make up about 13% of electricity generation and are growing quickly
Introduction to small-scale, distributed generation technologies in the next chapter
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Conventional coal-fired steam power plants
Pulverized coal is burned in a boiler to produce high-temperature, high-pressure steam
Steam expands in the turbine blades, causing the turbine shaft to rotate and generate electric power
The expanded steam is condensed back to liquid state and reused in the boiler
Reasons for condensing the steam
Creating a large pressure difference across the turbine
Avoiding waste of water and damage to the turbine blades
Improving the efficiency of the turbine
Energy balance and environmental impact of coal-fired power plants
Only about one-third of the fuel energy is converted into electricity
About 85% of the remaining energy leaves the plant as waste heat in cooling water
Cooling water demands are enormous, with approximately 1 billion gallons of water per day being withdrawn and returned to the source
Use of cooling towers to transfer heat into the atmosphere and cool the water
Page 13: Centralized Electric Power Systems
Two-thirds of the energy put into steam power plants ends up in cooling water
Cooling water is at a low temperature and not useful
Principal disadvantage of centralized power generation
Principal advantage of small-scale, decentralized systems
Combined-heat-and-power (CHP) systems can generate electricity at the site of the end user
Waste heat can be put to work in CHP systems
Page 13: Flue Gas Emission Controls
Power plants emit toxic pollutants, including SOx, NOx, particulate matter, and CO2
Emission control devices can help remove pollutants from flue gases
Electrostatic precipitator (ESP) adds a charge to particulates in the gas stream for collection
Flue gas desulfurization (FGD) system, or scrubber, precipitates sulfur to form calcium sulfite sludge
Nitrogen oxides (NOx) have two sources: thermal NOx and fuel NOx
Selective catalytic reduction (SCR) technology can reduce NOx emissions
SCR in a coal station is similar to catalytic converters in cars
Page 14: Flue Gas Emission Controls
Flue gas emission controls are expensive and account for a significant portion of the capital cost of a new coal plant
Emission controls also reduce the overall efficiency of the plant by about 5%
Page 14: Combustion Turbines
Natural gas as a fuel for power plants has environmental advantages over coal-fired power plants
Natural gas burns cleaner and is less carbon intensive
Natural gas power plants do not require boiling water
Page 14: Using Fly Ash to Reduce Carbon Emissions from Cement Production
Cement production contributes to global carbon emissions
Fly ash from power plants can replace a portion of cement production and reduce CO2 emissions
Concrete made with fly ash has been shown to be stronger and more durable
Only a small percentage of fly ash generated annually is currently being recycled
Page 15: Centralized Electric Power Systems
Gas plants use a turbine similar to a jet engine to make steam.
Components of a simple-cycle combustion turbine (CT): compressor, combustion chamber, power turbine.
Air is drawn in, compressed, and accelerated in the compressor.
Fuel is injected and ignited in the combustion chamber, creating high-pressure, high-temperature gas.
The expanding hot gases spin the turbine and are then exhausted.
Industrial gas turbines tend to be large, heavy machines with low efficiencies (20% to 30%).
Aeroderivative gas turbines are lightweight and compact, with fast starts and quick acceleration.
They adjust easily to rapid load changes and startup/shutdown events.
They have higher efficiencies (30% to 40%) than industrial counterparts.
9.5.4 Combined-Cycle Power Plants
The exhaust gases from a simple-cycle gas turbine can be used to generate steam.
Heat recovery steam generator (HRSG) is used to capture the heat and produce steam.
The steam can be used for industrial process heat or water and space heating.
Combined-cycle power plants use a second-stage steam turbine to generate more electricity.
Combined-cycle plants have achieved fuel-to-electricity efficiencies exceeding 60%.
Gas turbines in combined-cycle plants can provide easily controllable, variable output.
9.5.5 Clean Coal: Integrated Gasification Combined-Cycle (IGCC) Power Plants
Coal cannot be used in a gas turbine due to erosion and corrosion of turbine blades.
Integrated gasification combined-cycle (IGCC) power plants convert coal into a synthetic gas.
The synthetic gas is cleaned up and burned in a gas turbine.
IGCC plants are more expensive and have trouble competing with natural gas-fired plants.
Carbon emissions from coal-fired power plants can be controlled through carbon sequestration.
Carbon sequestration involves capturing CO2 and storing it permanently.
Promising methods of carbon storage include injecting CO2 into oil fields and storing it in deep brine aquifers.
Centralized Electric Power Systems (Page 17)
Nuclear power has had a rocky history, from being thought of as "too cheap to meter" to "too expensive to matter."
Nuclear power is a carbon-free source of electric power and is experiencing a resurgence of interest.
Public misgivings after the Fukushima disaster, radioactive waste disposal, siting concerns, and nuclear proliferation are challenges for nuclear power.
The economic competitiveness of nuclear power compared to natural gas plants and renewable energy systems is a key factor.
Nuclear reactor technology is similar to fossil fuel power plants, but the heat is created by nuclear reactions instead of combustion.
Light Water Reactors (LWRs) use ordinary water as a moderator to slow down neutrons.
Boiling Water Reactors (BWRs) make steam by boiling water within the reactor core.
Pressurized Water Reactors (PWRs) use a separate heat exchanger called a steam generator.
Small Modular Reactors (SMRs) are being developed as a new generation of smaller reactors.
Heavy Water Reactors (CANDU) use heavy water, which is more effective in slowing down neutrons than ordinary hydrogen.
The nuclear fuel "cycle" includes mining and processing uranium ores, enrichment, fuel fabrication, and shipment to reactors.
Spent fuel is stored on-site in short-term storage facilities while awaiting a longer-term storage solution.
Reactor decommissioning and disposal of radioactive components are necessary after around 40 years.
Nuclear Fuel and Waste (Page 18)
Plutonium, with a half-life of 24,390 years, is a major concern in nuclear waste.
Nuclear wastes remain dangerously radioactive for tens of thousands of years.
Removing plutonium from nuclear wastes has been proposed to shorten the decay period, but it introduces the risk of nuclear weapons proliferation.
Plutonium can be used as a reactor fuel if separated from the wastes, as done in France, Japan, Russia, and the United Kingdom.
The United States has not allowed commercial reprocessing of wastes due to proliferation risks.
A 2001 MIT study recommended against pursuing reprocessing.
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Hydropower is a significant source of electricity, with close to 1 TW of capacity delivering 16.5% (3400 TWh) of the total global supply.
Hydropower supplies more than 90% of electricity in more than two dozen countries.
Most new hydro facilities are being installed in Asia (led by China) and Latin America (led by Brazil).
China has the greatest installed capacity (210 GW) and is aggressively pursuing new projects.
Hydropower generates about 6% of U.S. electricity, which is about the same as the amount delivered by wind plus solar plants in 2016.
The focus in the United States and other OECD countries has shifted from developing new sites to improving existing facilities and adding generation capabilities to existing nonpowered dams.
Hydropower is a flexible source of power that can provide baseload power, peaking power, spinning reserve, and energy storage.
Hydropower facilities often serve multiple purposes besides power generation, including urban water supply, flood control, irrigation, and recreation.
There is a debate in the United States about whether large hydroelectric facilities should be counted as renewable energy systems under state-by-state renewable portfolio standard (RPS) frameworks.
Page 20: Some Economics for Conventional Power Plants
An economic analysis is crucial for making decisions about which generation technologies to use.
Costs of construction, fuel, operations and maintenance (O&M), and financing are crucial factors.
Externalities, such as healthcare costs and other costs of pollution, are not usually included in cost calculations.
Vulnerability to natural disasters, terrorism, and war is a complicating factor.
Electric utilities can be categorized into investor-owned, federally owned, other publicly owned, and cooperatively owned.
Investor-owned utilities (IOUs) are privately owned and regulated, with stock that is publicly traded.
Federally owned utilities produce power at facilities run by entities such as the Tennessee Valley Authority (TVA) and the U.S. Army Corps of Engineers.
Publicly owned utilities are state and local government agencies that generally sell power at lower cost than IOUs.
Rural electric cooperatives are owned by groups of residents in rural areas and provide services primarily to their own members.
Independent power producers (IPPs) and merchant power plants are privately owned entities that generate power for their own use or for sale to utilities and others.
Power plant ownership has shifted dramatically in the past two decades, with IOUs and IPPs each accounting for about 40% of generation capacity in the United States.
The levelized cost of electricity (LCOE) takes into account fixed costs and variable costs to estimate the average cost of electricity generated by a power plant.
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Introduction to fixed charge rate (FCR) for annualizing fixed costs of power plants
Different types of power plant ownership: merchant plants, IOUs, and POUs
Financing of power plants with a mix of loans and equity
Weighted average cost of capital (WACC) for estimating FCR
Calculation of capital recovery factor (CRF) for annualizing capital cost
Components of FCR: insurance, property taxes, fixed O&M, and corporate taxes
Additional percentage points added by the California Energy Commission for certain factors
Summary of total fixed charge rates in Table 9.2
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Introduction to variable costs of power plants
Factors influencing variable costs: power plant efficiency, fuel price, operation-related O&M, and capacity factor
Description of power plant efficiency in terms of heat rate
Calculation of annual energy delivered by a power plant using rated power and capacity factor
Introduction of levelizing factor (LF) for accounting for varying fuel prices over the life of the project
Calculation of annualized fuel cost using energy, heat rate, fuel cost, and LF
Solution Box 9.1 for combining costs and energy production estimates to find the average cost of electricity
Illustration of fixed and variable cost components of electricity cost in Figure 9.21
Sensitivity analysis comparing coal, nuclear, and combined-cycle gas plants in Figure 9.22
Potential impact of carbon costs and other externalities on power plants
Note: The transcript contains information from pages 21 and 22 of a document. It discusses the fixed charge rate (FCR) for annualizing fixed costs of power plants, different types of power plant ownership, financing methods, weighted average cost of capital (WACC), capital recovery factor (CRF), components of FCR, variable costs of power plants, power plant efficiency, heat rate, annual energy delivered, fuel cost, levelizing factor (LF), average cost of electricity, and potential impact of carbon costs and externalities.
Page 23: Centralized Electric Power Systems 309 Solution Box 9.1 The Cost of Electricity for a Natural Gas, Combined-Cycle Plant
Levelized cost of energy (LCOE) for a natural gas, 700-kW combined-cycle power plant
Cost factors:
Plant size: 700 kW
Capital cost: $950/kW
Fixed charge rate: 16%/yr
Average heat rate: 6600 Btu/kWh
Current cost of natural gas: $2.50 per million Btu
Fuel levelization factor: 1.4
Capacity factor: 0.60
Calculation:
Annualized capital cost = 1 kW × $950/kW × 0.16/yr = $152/yr
Annual energy produced = 1 kW × 8760 hr/yr × 0.60 = 5256 kWh/yr
Fuel = 5256 kWh/yr × 6600 Btu/kWh × $2.50/106 Btu × 1.4 = $121.41/yr
Total annual costs = $152.00 + $121.41 = $273.41
LCOE = $273.41/yr/5256 kWh/yr = $0.052/kWh = 5.2 ¢/kWh
Final cost of electricity does not depend on plant size
Page 23: Figure 9.21 Impact of Capacity Factor on Cost of Electricity for Solution Box 9.1
Illustrates the impact of capacity factor on the cost of electricity for the natural gas combined-cycle plant
Page 23: Figure 9.22 Levelized Cost of Energy for New Natural Gas Combined-Cycle (NGCC), Coal, and Nuclear Plants
Compares the levelized cost of energy for new natural gas combined-cycle (NGCC), coal, and nuclear plants
Assumptions are given in Table 9.3
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Gas plants have increased efficiency compared to coal plants
Lower carbon intensity of natural gas
Table 9.4 summarizes important characteristics for average real plants in the United States in 2014
Estimates of added cost of electricity with a price on carbon emissions
Carbon tax of $50 per metric ton of carbon increases cost of coal-generated electricity by 4.7¢/kWh
Natural gas combined-cycle (NGCC) plant sees an increase of only 2¢/kWh
Bituminous coal releases about 75% more carbon per delivered Btu compared to natural gas
Page 24: Figure 9.23 Equivalent CO2 Emissions Including Methane Leakage
Illustrates equivalent CO2 emissions including methane leakage
Methane leakage during drilling and transporting natural gas can dramatically increase climate impacts of gas-fired power plants
Leakage rates vary from around 1% to almost 10%
Page 25: Centralized Electric Power Systems 311 leakage occurs is a hotly debated research topic
Methane leakage is a hotly debated research topic
Estimates of methane leakage rates vary from around 1% to almost 10%
Epstein et al. (2011) estimate that the life-cycle cost of coal emissions and associated waste streams exceeds $300 billion per year in the United States alone
Externalities add between 9.5¢ and 26.9¢/kWh to the cost of coal-based electricity
Coal plants are more expensive than wind, solar, and other non-fossil fuel power generation
Page 25: Solution Box 9.2 Methane Leakage
Compare the global warming impact of a natural gas-fired power plant with a coal-fired power plant
Assume 2% well-to-plant methane leakage
Calculation:
31.91 lbs of carbon in the form of methane released per million Btu
Assumed 2% leakage rate sends 0.851 lb CH4 into the atmosphere
20-year global warming potential (GWP) for methane is 86
Equivalent CO2 leakage = 0.851 lb CH4 × 86 = 73.18 lb CO2
CO2 emissions at the power plant: 114.66 lb CO2
Total equivalent CO2 emissions = 73.18 + 114.66 = 187.8 lb CO2
Normalized emissions per kWh of energy generated at the plant: 2.0 lb CO2/kWh
Similar emission rate to a conventional coal-fired power plant
Page 25: Figure 9.23 illustrates the impact of methane leakage
Figure 9.23 illustrates the impact of methane leakage on CO2 emissions
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9.7 Summary
The American grid is considered the greatest engineering achievement of the twentieth century.
The grid has provided safe, reliable, and affordable electricity.
The grid is undergoing radical changes due to concerns about global warming, competition from renewables, and new regulatory mechanisms.
Grid operators face the challenge of balancing varying electricity demand with the right combination of power plants, transmission lines, and controls.
Load duration curves highlight the importance of peaker plants that meet peak summer air conditioning loads.
Wholesale electricity prices can increase dramatically during peak times.
The analysis in this chapter estimates the potential financial impact of a future where carbon emissions are no longer free.
Methane leakage rates can eliminate the carbon advantage of natural gas-fired power plants over coal.
The next chapter explores distributed energy resources challenging the traditional