Bài giảng Green energy course syllabus - Chapter 2: The electric power industry

Green Energy Course-

Renewable Energy Systems

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 one-

fourth of particulate matter and toxic heavy metals emissions.

Major Electricity Milestones

THE ELECTRIC UTILITY INDUSTRY TODAY

Conventional power generation, transmission, and distribution system.

Electric utilities, monopoly franchises, large central power stations, and long

transmission 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 nonutilities

depending 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 following constraint on the efficiency of a heat engine

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

STEAM-CYCLE POWER PLANTS

Conventional thermal power plants can be categorized by the thermodynamic

cycles 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 TH , 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 condenser

operating temperature of about 30◦C.

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

Coal-Fired Steam Power Plants

Mass flows to generate 1 kWh of electricity

in a 33.3% efficient, coal-fired power plant

burning bituminous coal.

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%

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, elevating

its temperature and pressure. This hot, compressed air is mixed with fuel, usually

natural gas, though LPG, kerosene, landfill gas, or oil are sometimes used,

and subsequently burned in the combustion chamber. The hot exhaust gases

are expanded in a turbine and released to the atmosphere. The compressor and

turbine 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

Fuel

100%

Combustion

AC

Power

33%

1150 ^oC

chamber

Generator

Compressor

Turbine

550 ^oC

Fresh

air

Exhaust

gases 67%

Brayton Cycle: Working fluid is always a gas

Maximum Efficiency

550  273

1150  273

1

 42%

Most common fuel is natural gas

Typical efficiency is around 30 to 35%

Gas Turbine

Source: Masters

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.

Steam-Injected Gas Turbines (STIG)

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

efficiencies approaching 45%.

Moreover, the injected steam reduces

combustion temperatures, which helps control

NOx emissions.

Steam-injected gas turbine (STIG) for

increased efficiency and reduced NOx

emissions. Efficiencies may approach

45%.

They are considerably more expensive than

simple gas turbines due to the extra cost of the

HRSG, and the care that must be taken to purify

incoming feedwater.

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

GAS TURBINES AND COMBINED-CYCLE COGENERATION

Simple-cycle gas turbine with a steam

generator for cogeneration showing

typical conversion efficiencies.

Representative energy flows for a

combined-cycle, cogeneration plant

with back-pressure steam turbine,

delivering thermal energy to a district

heating system.

Bài giảng Green energy course syllabus - Chapter 2: The electric power industry - Nguyễn Hữu Phúc