Chapter 2 the electric power industry+distributed generation feb 2011

• The numerical difference between primary and end-use energy is made up of losses during the conversion of fuel to electricity, losses in the transmission and distribution system T&D, a

Green Energy Renewable Energy Systems

Course-Biên sọan: Nguyễn Hữu Phúc Khoa Điện- Điện Tử- Đại Học Bách Khoa TPHCM

CHAPTER 2: The Electric Power Industry

•Little more than a century ago there were no lightbulbs, refrigerators, air conditioners, or any of the other electrical marvels that we think of

as being so essential today

•Indeed, nearly 2 billion people around the globe still live without

the benefits of such basic energy services

•The electric power industry has since grown to be one of the largest enterprises on the planet, with annual sales of over $300 billion in the United States alone

•It is also one of the most polluting of all industries, responsible for

three-fourths of U.S sulfur oxides (SOX) emissions, one-third of our

carbon dioxide (CO2) and nitrogen oxides (NOX) emissions, and fourth of particulate matter and toxic heavy metals emissions.

one-Major Electricity Milestones

THE ELECTRIC UTILITY INDUSTRY TODAY

Conventional power generation, transmission, and distribution system.

Electric utilities, monopoly franchises, large central power stations, and longtransmission lines have been the principal components of the prevailing electric power paradigm since the days of Insull

Utilities and Nonutilities

Entities that provide electric power can be categorized as utilities or nonutilitiesdepending on now their business is organized and regulated

Nonutility generators (NUGs) are privately owned entities that generate

power for their own use and/or for sale to utilities and others

Nonutility generators have become a significant portion of total electricity generated in the United States From EIA Annual Energy Review 2001 (EIA, 2003).

primary energy : The energy going into power plants

end-use energy , which is the energy content of electricity that is actually delivered

to customers

• The numerical difference between primary and end-use energy is

made up of losses during the conversion of fuel to electricity, losses in the

transmission and distribution system (T&D), and energy used at the power plant itself for its own needs

• Less than one-third of primary energy actually ends up in the form of

electricity delivered to customers

•For rough approximations, it is reasonable to estimate that for every 3 units of fuel into power plants, 2 units are wasted and 1 unit is delivered to end-users

Electricity flows as a percentage of primary energy Based on EIA Annual

Energy Review 2001 (EIA, 2003).

Distribution of retail sales of electricity by end use Residential and commercial

buildings account for over two-thirds of sales Total amounts in billions of kWh (TWh) are 2001 data From EIA (2003).

The load profile for the a peak summer day in California (1999) shows

maximum demand occurs between 2 P.M and 4 P.M

Lighting and air conditioning accounts for over 40% of the peak End uses are ordered the same in the graph and legend.

From Brown and Koomey (2002).

Average retail prices of electricity, by sector (constant $1996) From EIA Annual Energy Review 2001 (EIA,

2003).

CARNOT EFFICIENCY FOR HEAT ENGINES

Over 90% of world electricity is generated in power plants that convert heat into mechanical work The heat may be the result of nuclear reactions, fossil-fuel combustion, or even concentrated sunlight focused onto a boiler Almost all of this 90% is based on a heat source boiling water to make steam that spins a

turbine and generator, but there is a rapidly growing fraction that is generated using gasturbines

The best new fossil-fuel power plants use a combination of both steam

turbines and gas turbines to generate electricity with very high efficiency

Steam engines, gas turbines, and internal-combustion engines are examples

of machines that convert heat into useful work

What we are interested in here is, How efficiently can they do so? This same question will be asked when we describe fuel cells, photovoltaics, and wind

turbines, and in each case we will encounter quite interesting, fundamental limits

to their maximum possible energy-conversion efficicencies

Heat Engines

a heat engine extracts heat QH from a

high-temperature source, such as a

boiler, converts part of that heat into

work W, usually in the form of

a rotating shaft, and rejects the

remaining heat QC into a

low-temperature sink such as the

atmosphere or a local body of water.

A heat engine converts some of the heat extracted from a high-

temperature reservoir into work, rejecting the rest into a low-

temperature sink.

Entropy and the Carnot Heat Engine

The definition of Entropy (extremely important quantity ) is not very intuitive

It can be described as a measure of molecular disorder, or molecular randomness

At one end of the entropy scale is a pure crystalline substance at absolute zero temperature Since every atom is locked into a predictable place, in perfect order, its entropy is defined to be zero

In general, substances in the solid phase have more ordered molecules and hence lower entropy than liquid or gaseous substances When we burn some coal, there

is more entropy in the gaseous end products than in the solid lumps we burned

That is, unlike energy, entropy is not conserved in a process In fact, for every real process that occurs, disorder increases and the total entropy of the universe

increases

if an amount of heat Q is removed from a “large” thermal reservoir at temperature

T (large enough that the temperature of the reservoir doesn’t change as a result

of this heat loss), the loss of entropy S from the reservoir is defined as

(*)

where T is an absolute temperature measured using either the Kelvin or Rankine scale.

Equation * suggests that entropy goes down as temperature goes up

•The maximum possible efficiency of a heat engine is given by

•The following constraint on the efficiency of a heat engine

STEAM-CYCLE POWER PLANTS

Conventional thermal power plants can be categorized by the thermodynamiccycles they utilize when converting heat into work

Utility-scale thermal power plants are based on either

(a) the Rankine cycle, in which a working fluid is alternately vaporized and

condensed, or

(b) the Brayton cycle, in which the working fluid remains a gas throughout the

cycle

(c) Most baseload thermal power plants, which operate more or less

continuously, are Rankine cycle plants in which steam is the working fluid

(d) Most peaking plants, which are brought on line as needed to cover the daily

rise and fall of demand, are gas turbines based on the Brayton cycle

(e) The newest generation of thermal power plants use both cycles and are called combined-cycle plants

Basic Steam Power Plants

A fuel-fired, steam-electric power plant.

Let us use the Carnot limit to estimate the maximum efficiency that a power

plant such as that shown in Fig can possibly have A reasonable estimate

of T H , the source temperature, might be the temperature of the steam from the

boiler, which is typically around 600◦C For TC we might use a typical condenseroperating temperature of about 30◦C

the average efficiency of power plants is only about half this amount

Coal-Fired Steam Power Plants

Typical modern coal-fired power plant using an electrostatic precipitator for particulate

control and a limestone-based SO2 scrubber A cooling tower is shown for thermal pollution control From Masters (1998).

the average new steam plant is about 34% efficient and has a heat rate of

approximately 10,000 Btu/kWh

The best steam plants have efficiencies near 40%

Mass flows to generate 1 kWh of electricity

in a 33.3% efficient, coal-fired power plant burning bituminous coal.

COMBUSTION GAS TURBINES

Basic Gas Turbine

A basic gas turbine driving a generator is shown in Fig In it, fresh air is

drawn into a compressor where spinning rotor blades compress the air, elevatingits temperature and pressure This hot, compressed air is mixed with fuel, usuallynatural gas, though LPG, kerosene, landfill gas, or oil are sometimes used,

and subsequently burned in the combustion chamber The hot exhaust gasesare expanded in a turbine and released to the atmosphere The compressor andturbine share a connecting shaft, so that a portion, typically more than half,

of the rotational energy created by the spinning turbine is used to power the

compressor

Basic gas turbine and generator Temperatures and efficiencies are typical.

Basic Gas Turbine

Compressor

Fuel 100%

Fresh air

Combustion chamber

Turbine

Exhaust gases 67%

Generator

AC Power 33%

1150 o C

550 o C

Brayton Cycle: Working fluid is always a gas

Most common fuel is natural gas

Gas Turbine

Source: Masters

Steam-Injected Gas Turbines (STIG)

One way to increase the efficiency of gas turbines is to add a heat exchanger, called a heat recovery steam generator (HRSG), to capture some of the waste heat from the turbine

As shown in Fig , water pumped through the HRSG turns to steam, which is injected back into the airstream coming from the compressor.

The injected steam displaces a portion of the fuel heat that would otherwise be needed in the combustion chamber These units, called steam injected gas turbines (STIG), can have

Steam-injected gas turbine (STIG) for

increased efficiency and reduced NOx

emissions Efficiencies may approach

45%.

COMBINED-CYCLE POWER PLANTS

Combined-cycle power system with representative energy flows providing a total

efficiency of 49%.

Efficiencies of up to 60% can be achieved, with even higher values when the steam is used for heating

Simple-cycle gas turbine with a steam generator for cogeneration showing typical conversion efficiencies.

GAS TURBINES AND COMBINED-CYCLE COGENERATION

Representative energy flows for a combined-cycle, cogeneration plant with back-pressure steam turbine, delivering thermal energy to a district heating system.

Combined Heat and Power (CHP)

Turbine

Exhaust gases

Generator

AC Power 33%

Heat recovery steam generator (HRSG)

Water pump Feedwater

Exhaust 14%

Steam 53%

Process heat Absorption cooling Space & water heating

Overall Thermal Efficiency = 33% (Electricity) + 53% (Heat) = 86%

BASELOAD, INTERMEDIATE AND

PEAKING POWER PLANTS

Example of weekly load fluctuations and

roughly how power plants can be

categorized as baseload, intermediate,

or peaking plants.

The fluctuations in demand suggest that during the peak demand, most of a utility’s power plants will be operating, while in the valleys, many are likely to be idling or shut off entirely

In other words, many power plants don’t operate with

a schedule anything like full output all of the time It has also been mentioned that some power plants, especially large coal-fired plants as well as

hydroelectric plants, are expensive to build but relatively cheap to operate, so they should be run

more or less continuously as baseload plants; others,

such as simple-cycle gas turbines, are relatively

inexpensive to build but expensive to operate.

They are most appropriately used as peaking power

plants, turned on only during periods of highest

demand Other plants have characteristics that are

somewhere in between; these intermediate load

plants are often run for most of the daytime and then

cycled as necessary to follow the evening load

Figure suggests these designations of baseload, intermediate, and peaking power plants applied to a weeklong demand curve.

Screening Curves

screening curves that show annual revenues required to pay fixed

and variable costs as a function of hours per year that the plant is operated

capacity factor as the ratio

of average power to rated power

The average cost of electricity is the slope of the line drawn

from the origin to point on the revenue curve that corresponds

to the capacity factor

Screening curves for coal-steam, combustion turbine, and combined-cycle

plants based on data in Table 3.3 For plants operated less than 1675 h/yr,

combustion turbines are least expensive; for plants operated more than 6565 h/yr,

a coal-steam plant is cheapest; otherwise, a combined-cycle plant is least

expensive

Load–Duration Curves

A load–duration curve is simply the hour-by-hour load curve rearranged from chronological order into an order based on magnitude The area under the curve is the total kWh/yr

Interpreting a load–duration curve

Plotting the crossover points from screening curves onto the load–duration curve to

determine an optimum mix of power plants.

The fraction of each horizontal rectangle that is

shaded is the capacity factor for that portion of

the generation facilities.

TRANSMISSION AND DISTRIBUTION

Transmission and distribution (T&D) construction expenditures

at U.S investor-owned utilities compared with generation

Except for the anomalous spurt

in power plant construction during the 1970s and early 1980s, T&D costs have generally exceeded generation From

Lovins et al (2002), using Edison Electric Institute data.

A simple distribution station

For simplication, this is drawn

as a one-line diagram, which

means that a single conductor

on the diagram corresponds to

the three lines in a three-phase

system.

A one-line diagram of a dc link between ac systems The inverter and rectifier can switch roles to allow bidirectional power flow.

Transmission Lines

Examples of transmission towers: (a) 500 kV; (b) 230-kV steel pole; (c) 69-kV wood tower; (d) 46-kV wood tower.

Aluminum conductor with steel reinforcing (ACSR).

Technology Motivating Restructuring

The era of bigger-is-better that dominated power plant construction for most of the twentieth century shifted rather abruptly around 1990 From Bayliss (1994).

Restructuring of the electric power industry has been motivated by the emergence

of new, small power plants, especially gas turbines and combined-cycle plants, that offer both reduced first cost and operating costs compared with almost all of the

generation facilities already on line

The economies of scale that motivated ever-larger power plants in the past seem to have played out, as illustrated in Fig By 2000 the largest plants being built were

only a few hundred megawatts, whereas in the previous decade they were closer to

1400 MW

Distributed Generation

ELECTRICITY GENERATION IN TRANSITION

•The traditional, vertically integrated utility incorporating generation, transmission,

•distribution, and customer energy services is in the beginning stages of

•what could prove to be quite revolutionary changes

•The era of ever-larger central power stations seems to have ended

•The opening of the transmission and distribution grid to independent power

producers who offer cheaper, more efficient, smaller-scale plants is well underway

•Attempts to restructure the regulatory side of utilities to help create competition

among generators and allow customers to choose their source of power have been

initiated in a number of states, but with mixed success.

On the customer side of the meter, the power business is beginning to look more like

it did in the early part of the twentieth century when more than half of electricity was self-generated with small, isolated systems for direct use by industrial firms

Those old steam-powered, engine generators used for heat and power have modern

equivalents in the form of microturbines, fuel cells, internal-combustion engines, and small gas turbines

Using these technologies, customers are rediscovering the economic advantages of on-site

cogeneration of heat and power , or trigeneration for heating, electric power, and cooling.

In addition to economic benefits, other motivations helping to drive the transition toward small-scale, decentralized energy systems include increased concern for environmental impacts of generation, most especially those related to climate change, increased concern for the vulnerability of our centralized energy systems to terrorist attacks, and increased demands for electricity reliability in the digital economy.

Distributed Resources

Examples of distributed resources (Based on Lovins et al (2002).

DISTRIBUTED GENERATION WITH FOSSIL FUELS

•Distributed generation (DG) is the term used to describe small-scale

power generation, usually in sizes up to around 50 MW, located on the

distribution system close to the point of consumption

•Such generators may be owned by a utility or, more likely, owned by a customer who may use all of the power on site or who may sell a portion,

or perhaps all of it, to the local utility.

• The process of capturing and using waste heat while generating

electricity is sometimes called cogeneration and sometimes combined

heat and power (CHP).

•When a fuel is burned, some of the energy released ends up as latent heat in the water vapor produced (about 1060 Btu per pound of vapor, or 2465 kJ/kg)

•Usually that water vapor, along with the latent heat it contains, exits the stack along with all the other combustion gases, and its heating value is, in essence, lost In

some cases, however, that is not the case

•For example, the most fuel-efficient, modern furnaces used for space-heating

homes achieve their high efficiencies (over 90%) by causing the combustion

gases to cool enough to condense the water vapor before it leaves the stack

•Whether or not the latent heat in water vapor is included leads to two different

values of what is called the heat of combustion for a fuel

•The higher heating value (HHV), also known as the gross heat of combustion ,

includes the latent heat, while the lower heating value (LHV), or net heat of

combustion , does not.

HHV and LHV

For natural gas, the difference between HHV and LHV is about 10%.

The thermal efficiency of power plants is often expressed as a heat rate ,

which is the thermal input (Btu or kJ) required to deliver 1 kWh of electrical

output (1 Btu/kWh = 1.055 kJ/kWh)

The smaller the heat rate, the higher the efficiency In the United States, heat rates are usually expressed in Btu/kWh, which results in the following relationship

between it and thermal efficiency, η:

where the LHV/HHV ratio can be found from Table

Example 4.1 Microturbine Efficiency

A microturbine has a natural gas input of 13,700 Btu (LHV) per kWh of electricity generated Find its LHV efficiency and its HHV efficiency

Solution In Section 3.5.2 the relationship between efficiency and heat rates (in

American units) is given by (3.16)

Efficiency = 3412 Btu/kWh/ Heat rate (Btu input/kWh output)

Using the LHV for fuel gives the LHV efficiency:

Efficiency (LHV) = 3412 Btu/kWh/ 13,700 Btu/kWh = 0.2491 = 24.91%

Using the LHV/HHV ratio of 0.9010 for natural gas (Table 4.2) in Eq (4.1) gives

the HHV efficiency for this turbine:Efficiency (HHV) = 24.91% × 0.901 = 22.44%

Microcombustion Turbines

More recently, a new generation of very small gas turbines has entered the

marketplace Often referred to as microturbines, these units generate anywhere

from about 500 watts to several hundred kilowatts

Figure illustrates the basic configuration including compressor, turbine, and

permanent-magnet generator, in this case all mounted on a single shaft Incoming air is compressed to three or four atmospheres of pressure and sent through a

heat exchanger called a recuperator, where its temperature is elevated by the hot

exhaust gases

By preheating the compressed incoming air, the recuperator helps boost the

efficiency of the unit

The hot, compressed air is mixed with fuel in the combustion chamber and is

burned The expansion of hot gases through the turbine spins the compressor and generator

The exhaust is released to the atmosphere after transferring much of its heat to the incoming compressed air in the recuperator

Microturbine power plant Air is compressed (1), preheated in the recuperator (2), combusted with natural gas (3), expanded through the turbine (4), cooled in the recuperator (5), and exhausted (6) From Cler and Shepard (1996).

Example specifications of several microturbines are given in Table 4.3

For example, the Capstone Turbine Corporation manufactures several

refrigeratorsize microturbines that generate up to 30 kW and 60 kW These

turbines have only one moving part—the common shaft with compressor, turbine, and generator, which spins at up to 96,000 rpm on air bearings that require no lubrication There are no gearboxes, lubricants, coolants, or pumps that could

require maintenance