ECE 7800: Renewable Energy SystemsTopic 3: Present Structure of Electric

Utility Industry

Spring 2010

© Pritpal Singh, 2010

Structure of Present Electrical Power SystemThe present power system structure

comprises:

• Large Centralized Generating Plants

• Primarily Fossil-Fueled (mostly coal and natural gas but also some oil) (70%) and some Nuclear and Hydropower (approx. 30%)

• A few large-scale utility companies

• Long AC transmission lines with some HVDC lines (more recently)

• Three large interconnected grids

Structure of Present Electrical Power System (cont’d)

Electric Utility CompaniesThere are several different types of

utility companies:• Investor-owned utilities – privately owned

with publicly traded stock. 5% of companies but generate 2/3 of electricity.

• Federally-owned utilities – e.g. TVA• Other publicly owned utilities (State and

local agencies). 67% of companies but generate 9% of electricity.

• Rural Electric Cooperatives – owned by groups of rural residents to supply their own power needs.

Non-Utility Electric CompaniesNon-utility electric companies do not

have power generation as their main

business. They do, however, generate

power for their own use and may sell

excess generated power back to the

local utility (e.g. Duquesne University

has a cogeneration plant to provide

much of their campus’ power).

These companies now account for

20% of the electricity generated.

Fuel Sources by State

US Energy Flow -1999 Net Primary Resource Consumption 102 Exajoules

Time of Day Electrical Usage Profile

Electricity Rates by State

Electrical Generators With the exception of a small

amount of electricity that is generated by solar, gasoline generators, diesel generators or fuel cells, most of the electricity is generated by using an energy source to force a working fluid (e.g. steam, gas, water or air) to the blades of a turbine causing a shaft to rotate. This rotational shaft energy is converted to electricity by a generator.

Simple Generators The basic principle of operation of

generators is Faraday induction. Moving a conductor through a magnetic field induces an emf in the conductor. In a simple generator a set of armature conductors (windings) are embedded in the stator of the generator. The magnetic field rotates with the rotating shaft as shown in the figure in the next slide. This results in a sinusoidal magnetic flux variation and corresponding sinusoidal emf variation.

Simple Generators (cont’d)

Single-Phase Synchronous Generators

To generate a sinusoidal signal of 60Hz, the shaft must rotate at 3,600 rpm. Such a fixed speed machine is termed a synchronous generator (since it is synchronized with the utility grid). Most conventional electricity is generated with synchronous generators (but not wind power).

Single-Phase Synchronous Generators (cont’d) Using a permanent magnet rotor

would limit the power output of a generator. Therefore, dc current is sent via slip rings and brushes through windings on the rotor to generate the magnetic field. Two types – round and salient pole rotors.

Single-Phase Synchronous Generators (cont’d) Adding more poles to a machine

results in slower rotation speed being required to achieve the same frequency of ac voltage: f= NS.p/120

Three Phase Synchronous Generators To generate 3-phase power, we need 3-phase generators. The below figure shows examples of 2-pole and 4-pole synchronous 3-phase generators.

Heat Engines A heat engine extracts heat from a

source, uses some of it to do useful work, and rejects some as waste heat.

QH

W

QC

High temperature Source, TH

Low temperature Sink, TL

Heat engine

Thermal Efficiency of Heat EnginesThe thermal efficiency of a heat engine is given by the following equation:

Thermal efficiency = Net work output = W

Total heat input QH

Since QH = W + QC, it can be shown that:

H

C

Q

QiciencyThermalEff 1

EntropyEntropy is considered to be the state of disorder of a system. This dictates the direction of flow of heat – from warm to cold.

For a heat engine, if an amount of heat Q is removed from a “large” thermal reservoir at temperature T (in K), the loss of entropy is defined as:

ΔS = Q T

Laws of ThermodynamicsThere are three basic laws ofthermodynamics:

1. Energy can neither be created nor destroyed but can only be converted from one form to another.

2. All of the heat input into a heat engine cannot be converted to work – there will inevitably be some heat rejected.

3. Absolute zero can never be reached.

Laws of Thermodynamics SimplifiedThe British scientist and author C.P. Snow had anexcellent way of remembering the three laws:

1. You cannot win (that is, you cannot get something for nothing, because matter and energy are conserved).

2. You cannot break even (you cannot return to the same energy state, because there is always an increase in disorder; entropy always increases).

3. You cannot get out of the game (because absolute zero is unattainable).

Laws of Thermodynamics (in terms of entropy)

1. Energy can neither be created nor destroyed but can only be converted from one form to another.

2. The entropy of a system increases.

3. The entropy at absolute zero is zero.

Entropy Change in a Heat EngineThe entropy of a heat engine must increase during its operation. Since there is no entropy change associated with the work done, and the heat engine’s entropy must increase, entropy added to the low temperature sink ≥ the entropy added to the high temperature reservoir.

H

H

C

C

T

Q

T

Q

H

C

H

C

T

T

Q

Q

Carnot Efficiency of a Heat EngineCombining the equations of thermal

efficiency and the relationship between QC,QH, TC and TH, we can see that the thermal efficiency can be expressed as:

Thus the maximum efficiency of a heat engine is given by:

This is known as the Carnot efficiency of a heat engine.

H

C

H

C

T

T

Q

QiciencyThermalEff 11

H

C

T

T1max

Carnot Efficiency ExampleApprox. Carnot efficiency of a solar cell:

TH = 6000K (approx. black body radiation temp. of sun)

TC = 300K (ambient room temperature)

Note: This calculation is not quite right because a solar cell is not a classical heat engine.

K

K

T

T

H

CCarnot 6000

30011

Steam-Cycle Power Plants Conventional thermal power plants are of

two types:

1. Rankine Cycle – a working fluid is alternately vaporized and condensed; and

2. Brayton Cycle – working fluid remains a gas throughout the cycle.

Baseload thermal power plants are usually Rankine cycle plants with steam as the working fluid. Peaking power plants are usually gas turbine Brayton cycle plants. The newest generation of power plants use both cycles (combined cycle power plants).

Steam Cycle Power Plants A basic fossil-fueled power plant is

shown below:

http://www.youtube.com/watch?v=SeXG8K5_UvU&feature=related

Coal-Fired Steam Power Plants Coal-fired power plants are the most

common type of power plant providing over half the electricity in the US. Although they are responsible for over 75% of SO2 emissions, recent emission control requirements have made coal-fired plants much cleaner. A schematic diagram of a coal-fired plant is shown on the next slide.

Coal-Fired Steam Power Plants (cont’d)

Coal-Fired Steam Power Plants (cont’d)

Coal is pulverized and conveyed to the furnace and burned. The electrostatic precipitator adds a charge to particulates in the waste stream which allows them to be attracted to electrodes from where they are removed. A wet spray of limestone slurry is sprayed on the flue gas in the scrubber to remove sulfur from the waste gas stream.

Example 3.2

Combustion Gas Turbines Gas turbines are typically used for

providing peaking power whereas coal-fired steam power plants are used to provide baseload power.

The schematic of a basic gas turbine is shown below.

Combustion Gas Turbines (cont’d)• Siemens SGT-100 Turbine for Power

Generation (ISO) 4.35/4.70/5.05/5.25MW(e)

• The SGT-100 single-shaft industrial gas turbine is a proven unit for all electrical power generation and cogeneration applications. It offers high efficiency and reliability and operates on a wide range of gaseous and liquid fuels.

Combustion Gas Turbines (cont’d)

A steam injected gas turbine includes a heat exchanger to capture some of the waste heat and thereby increase the efficiency of the generator.

Combined-Cycle Power Plants In a combined-cycle power plant the waste

heat from a gas turbine plant is used to boil water to steam which drives a steam turbine and, in turn, a generator. This results in increased efficiency (>50%). A schematic of the generator is shown below:

Co-Generation Plants In co-generation plants the waste heat

from the power generator is used to provide heat either for industrial processes or for space heating. A simple gas turbine cogeneration plant is shown below:

Co-Generation Plants (cont’d) A combined-cycle power plant used

in a cogeneration system is shown in the below diagram:

Baseload, Intermediate and Peaking Power PlantsThree regimes of loads:

1. Baseload – covers constant power demand

2. Peaking – covers high power peak requirements

3. Intermediate – between baseload and peak

Screening Curves Screening curves are plots of

revenues required to pay fixed and variable costs for a power plant as a function of hours per year that the plant is operated. They are used to optimize the mix of power plants to meet the load needs for a particular location.

Screening Curves (cont’d)

Fixed and Variable Costs for a Power Plant

The capital costs of a power plant can be annualized by multiplying them by a fixed charge rate (usually 11-18%). The annualized fixed costs (on a per-kW basis) can be written as:

Annualized fixed capital costs ($/yr.-kW) = capital cost ($/kW) x Fixed Charge rate (yr-1)

Variable costs are given by:

Variable($/yr.-kW) = [Fuel cost ($/Btu x heat rate (Btu/kWh) + O&M ($/kWh)] x h/yr.

Fixed and Variable Costs for a Power Plant (cont’d)

Example 3.3

Load-Duration Curves A load duration curve is simply an hour-

by-hour load curve reordered in terms of decreasing load (see figure below):

Load-Duration Curve Interpretation

Optimizing Power Plant Mix

Optimized Power Plant Mix (cont’d)

Transmission and Distribution Power plants may be located at some

distance from the place where the power will be used. As a result the power must be transmitted from point of generation to point of use.

The transmission lines carry AC voltages at levels ranging between 161 kV to 765 kV. Lower voltage sub-transmission lines carry to distribution centers near the loads. At substations, the voltage is lowered to 4.16 - 24.94 kV for distribution over feeders to the customer.

US National Electric Grid

Ref: MSN Encarta

Transmission and Distribution (cont’d)

A simple distribution station including the protection and isolation circuit components is shown below:

Transmission and Distribution (cont’d)

The US has over 275,000 miles of transmission lines forming a National Transmission Grid. Fig. 3.35 shows the regional breakdown of this power transmission network. Most of this grid is 3-phase AC power. However, high voltage DC (HVDC) is used for particular applications. Over large distances, using HVDC transmission avoids having to exactly match frequency and phase of the AC voltage.

Transmission and Distribution (cont’d)

HVDC links require converters at both ends as shown in the diagram below:

Transmission Line Losses Nominal standard transmission and

distribution system voltages are given in Table 3.5.

As the current propagates down the transmission line, losses occur due to the I2R losses in transmission.

Let us consider an example of calculating power losses in a transmission line (Example 3.4).