Combined Heating, Cooling and Power Handbook_ Technologies & Applications

Combined Heating, Cooling & Power Handbook: Technologies & Applications

AN INTEGRATED APPROACH TO ENERGY RESOURCE OPTIMIZATIONBy Neil Petchers

THE FAIRMONT PRESS, INC. Lilburn, GA

MARCEL DEKKER, INC. New York and Basel

Copyright 2003 by The Fairmont Press.

Library of Congress Cataloging-in-Publication Data Petchers, Neil. Combined heating, cooling & power handbook : technologies and applications : an integrated approach to energy conservation/resource optimization / by Neil Petchers. p. cm. Includes index. ISBN 0-88173-433-0 (Electronic) 1. Cogeneration of electric power and heat. I. Title: Combined heating, cooling and power handbook. II. Title. TK1041 .P48 2002 621.199--dc21 2001059204 Combined heating, cooling & power handbook: technologies and applications by Neil Petchers 2003 by The Fairmont Press. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher.Published by The Fairmont Press, Inc. 700 Indian Trail, Lilburn, GA 30047 tel: 770-925-9388; fax: 770-381-9865 http://www.fairmontpress.com Distributed by Marcel Dekker, Inc. 270 Madison Avenue, New York, NY 10016 tel: 212-696-9000; fax: 212-685-4540 http://www.dekker.com Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

0-88173-433-0 (The Fairmont Press, Inc.) 0-8247-4233-8 (Marcel Dekker, Inc.)While every effort is made to provide dependable information, the publisher, authors, and editors cannot be held responsible for any errors or omissions.


Table of ContentsPart/Section/Chapter Dedication Acknowledgements Foreword Introduction PART 1 THEORY AND TECHNOLOGY SECTION I Optimizing Heat and Power Resources 1 Heat and Power Resources Overview 2 Expressing Power Cycle Performance 3 Localized vs. Central Station Power Generation 4 Selection of Power Generation Systems SECTION II Thermal Technologies 5 Heating Value and Combustion of Fuel 6 Properties and Value of Steam 7 Boilers 8 Heat Recovery SECTION III Prime Mover Technologies 9 Reciprocating Engines 10 Combustion Gas Turbines 11 Steam Turbines 12 Combined and Steam Injection Cycles 13 Controlling Prime Movers 14 Renewable and Alternative Power Technologies PART 2 OPERATING ENVIRONMENT SECTION IV Environmental Considerations 15 Air Pollution Regulatory Programs 16 Air Permitting Process 17 Emissions Control Measures 18 Refrigerants and the CFC Issue SECTION V Utility Industry and Energy Rates 19 Natural Gas Industry Overview 20 Electric Industry Overview 21 Utility Rate Structures


22

Utility Bill Analysis

PART 3 APPLICATIONS SECTION VI Localized Electric Generation 23 Localized Electric Generation Applications Overview 24 Electricity 25 Electric Generators 26 Generator Driver Applications and Selection 27 Electric Generator Switchgear and Controls 28 Interconnecting Electric Generators SECTION VII Mechanical Drive Services 29 Mechanical Drive Applications Overview 30 Air Compressors 31 Pumps 32 Fans SECTION VIII Refrigeration and Air Conditioning 33 Refrigeration Cycles and Performance Ratings 34 Psychrometrics 35 Heat Extraction Evaporators, Chilled Water, Economizers and Thermal Storage 36 Heat Rejection Condensers, Cooling Towers, Heat Pumps and Heat Recovery 37 Vapor Compression-Cycle Systems 38 Absorption Cooling Systems 39 Desiccant Dehumidification Technologies PART 4 ANALYSIS AND IMPLEMENTATION SECTION IX Integrated Approach to Energy Resource Optimization Projects 40 Integrated Approach to Energy Resource Optimization Projects 41 Technical Analysis 42 Evaluating Project Financial Potential 43 Project Contracting and Financing Options 44 Program Implementation and Operation Appendix: Conversion Tables


CHAPTER ONEHeat and Power Resources Over view

V

irtually every facility requires energy conversion for both power and heat. Power may be purchased from an electric utility or private provider, or it may be produced on site. Power is used as electricity for lights and computers and to drive equipment via electric motors. It is also used as mechanical energy in the form of a rotating shaft that directly drives equipment. Heat or thermal energy is usually produced on site from purchased fuel through various types of energy conversion devices. Heat is used to raise steam, hot water, or hot air for space heating or process use, or to produce a cooling effect through certain heat-driven cycles. Power is generally produced by application of prime movers, either on site or at centralized electric generation plants. Prime movers are devices that convert fuel or heat energy into mechanical energy, which in turn can be used to drive virtually any type of shaft-powered equipment, including electric generators and motor vehicles. Due to the laws of thermodynamics, heat is produced as a necessary by-product of power production. Much of the technology discussed in this book involves three major types of prime movers: reciprocating engines, combustion gas turbine engines, and steam turbine engines. Most of the applications in this book involve strategic deployment of prime mover and certain heat-cycle technologies in commercial, industrial, and institutional facilities. This chapter introduces a number of terms used to describe and compare the application of these technologies. Facilities rarely have a consistent requirement for power and heat. Generally, these requirements vary based on the time of use or outside ambient conditions. The portion of a facilitys power or heat requirements that is constant is referred to as baseload. The portion that varies is referred to as intermittent load. Maximum intermittent requirements are referred to as the peak load. If thermal requirements are not considered, baseload power requirements are usually met most economically through purchased power from central utility power plants rather than localized on-site production. Advantages associated with centralized power production include: economy of scale, preferential fuel purchase opportunity, lower staffing levels per unit output, diversity, and reserve capacity. These advantages are usually sufficient

to overcome inherent disadvantages of centralized power production, such as system efficiency losses associated with power transmission and distribution, as well as an assortment of regulatory obligations. Intermittent and peak load requirements, on the other hand, are usually served by centralized utility systems with lower economic efficiency. In some cases, these requirements can be served more economically by strategic application of on-site power production technologies. Examples are on-site peak shaving electric generation, which is the on-site production of electricity during peak usage and/or cost periods, and various types of mechanical drive services. If thermal energy requirements are taken into consideration, on-site power production has a significant thermodynamic efficiency advantage over centralized power production, because heat energy rejected from the power production process can be used. Centralized power plants usually have no use for this heat energy and must liberate it to the environment at an economic loss. When a facility can recover and use this heat energy, the thermodynamic efficiency advantage translates into an economic advantage that may exceed the economic advantages of centralized power production. Comparison of life-cycle costs determines the degree to which it is economical to produce shaft power on site, rather than purchase power from an electric utility or private power producer. Such decisions involve analysis of an entire facilitys energy usage characteristics, including concurrent requirements for both power and heat, since on-site prime movers can provide both. The life-cycle cost elements of an on-site prime mover are primarily capital, fuel, and operations and maintenance costs, which are also the primary constituents of electric utility and other centralized power producer costs. Electric utility rates assign different portions of these capital and operational costs to different time periods based on the utilitys cost to serve. Rate designs, which often include demand charges and seasonal and time-of-use rates, send price signals that influence consumer behavior. The relationship between these price signals and on-site energy load characteristics will largely determine which portion of electricity requirements can be provided more economically by on-site prime movers than by electricity purchased from a utility or other centralized source.



Additionally, the marginal, or incremental, cost of utility power production will largely determine whether it is economical to produce more power than is required on site and export the excess to other sellers or users. Investment in an on-site prime mover shifts many additional cost factors onto the individual facility. These costs are capital, fuel procurement, and operation and maintenance, as well as costs associated with reserve capacity, emissions control, space considerations, and insurance. The potential payoff for absorbing these added cost factors is lower operating costs and increased economic performance.

PRIME MOVER CYCLE TERMINOLOGYThere are numerous terms used to describe application of prime mover cycles. These include topping, bottoming, simple, combined, and cogeneration cycles. Definitions for these terms are flexible. Commonly, topping and bottoming are applied to cycles to indicate the stage at which an energy stream is used to produce power. Alternatively, the term base unit may be used to indicate the primary system to which a topping or bottoming cycle is applied. The topping cycle may be characterized as one that uses a high temperature working fluid to generate power followed by use of recovered heat. In contrast, in a bottoming cycle, the working fluid is used as a high-temperature heat source before being used for power generation.

overall thermal efficiency. The base system may be a preexisting system or a new one, and it normally is the major power producer. For example, if a gas turbine is fitted with a heat recovery boiler that supplies a steam turbine, then the steam turbine plant becomes a bottoming unit on the base gas turbine. Figure 1-1 illustrates a simple-cycle gas turbine base unit. Figure 1-2 illustrates the addition of a heat recovery system to this base unit. Simple cycle refers to the conventional application of a single prime mover cycle. As shown in Figure 1-3, a combined cycle, as the name implies, is the sequential linking of any topping and bottoming cycle, or two simple cycles. The classic combined cycle is a gas turbine in conjunction with a steam turbine. The gas turbine generates shaft power at the upper range of the energy stream. Its exhaust heat is converted to steam in a heat recovery steam generator, and then passed through a steam turbine to generate additional power. The entire plant would be referred to as a combined-cycle plant. From the power plant perspective, a reciprocating engine applied in mechanical drive or power generation service may be considered as a base unit since it would be the major power producer. A combustion gas turbine might be the base unit or the topping cycle, and the steam turbine might be the base unit or the bottoming cycle. Where the base demand is for process steam, conventional steam cycles with extraction steam turbines might be considered as a topping cycle. Figure 1-4 shows several variations on application concepts for repowering a power plant with 150 megawatt (MW) gas turbines. In these examples, approximately 75 MW can be recovered from its exhaust energy. The basic combined-cycle unit provides a total output of 225 MW and offers the highest thermal fuel efficiency of the various options. The fully fired boiler concept is some-

Fig. 1-1 Application of Simple-Cycle Gas Turbine Base Unit. Source: ABB

Under these definitions, topping cycles have the capacity to independently deliver mechanical or electrical energy from the conversion of fuel or heat energy. Bottoming cycles cannot operate without a preceding energy conversion cycle or process. Bottoming cycles tend to be physically large and relatively expensive due to the low quality of the energy input. In the context of large power generation plants, the terms topping and bottoming cycle may be applied to the systems that are added to base systems in order to enhance6Copyright 2003 by The Fairmont Press.

Fig. 1-2 Application of Heat Recovery Unit to Gas Turbine Base Unit. Source: ABB

Heat and Power Resources Overview

times referred to as hot windbox refiring. From the power plant perspective, it is considered a topping cycle in that the gas turbine provides only 150 MW of a total plant output of 600 MW. The gas turbine can also be applied in a topping cycle as shown in the feedwater heat exchanger repowering, which can be well utilized in a wide plant capacity range. Note that plant efficiency increases as the repowered plant size becomes smaller relative to the capacity of the gas turbine. Figure 1-5 shows the potential performance achieved with each of these options as a function of repowered plant output. Cogeneration is the sequential use of fuel energy to produce more than one finished energy product, such as electric power, steam, refrigeration, thermal drying, air heating, or a host of others. While two finished products can be made by splitting the output of a single-boiler steam supply between a steam turbine power cycle and a heating application, this is not cogeneration. What distinguishes cogeneration and the thermodynamic efficiency benefits it produces is the operative concept of sequential use and production of both power and usable thermal energy. Cogeneration may be applied to both simple and combined cycles. Heat recovery turns a relatively inefficient simplecycle power generation process into a more efficient cogeneration or combined-cycle process. Heat recovery is the effective capture and use of heat rejected from the power cycles. Rejected heat is the energy associated with streams of air, exhaust gasses, and liquids that exit the system and enter the environment as waste products.

As shown in Figure 1-2, heat recovery is applied to the simple-cycle gas turbine for the purpose of sequentially providing thermal energy to a process, thereby transforming that system into a cogeneration cycle. As shown in Figure 1-3, heat recovery is applied to a simple-cycle gas turbine for the purpose of powering a steam turbine, thereby transforming the system into a combined-cycle system. Figure 1-6 shows the application of heat recovery twice. Heat is first recovered from the simple-cycle gas turbine, transforming the system into a combined cycle. Heat is then recovered again from the steam-turbine cycle and used for a process application, transforming the entire system into a cogeneration combined cycle.

REGULATORY TERMINOLOGY APPLIED PRIME MOVER CYCLES

TO

To establish federal regulations and Qualifying Facility (QF) cogeneration system efficiency standards, the Federal Energy Regulatory Commission (FERC) defined cogeneration as the combined production of electric power and useful thermal energy by sequential use of energy from one source of fuel. As defined by FERC, a topping cycle first uses thermal energy to produce electricity, and then uses the remaining energy for thermal process. In a bottoming cycle, the process is reversed. Figures 1-7 through 1-11 are diagrammatic examples of cogeneration topping, bottoming, and combined cycles, consistent with commonly used regulatory definitions.

Fig 1-3 Combined-Cycle System Featuring Gas Turbine, Heat Recovery Unit and Steam Turbine. Source: ABB

7Copyright 2003 by The Fairmont Press.


Gas Turbine 150 MW Exhaust Energy for Use in Steam Cycle Approximately 75 MW

Repowering with an HRSG 75 MW Steam Plant ~54%**Approximate Repowered Net Plant EfficienciesGas Turbine Reheat Steam Turbine

Repowering with a Fully Fired 450 MW Steam Plant ~49%*

Repowering with a Feedwater Heater 750 MW ~46%*

Repowering with a Feedwater Heater 200 MW ~50%*

Gas Turbine Reheat Steam Turbine Fully Fired Steam Generator Heat Exchanger

Gas Turbine Reheat Steam Turbine

Heat Recovery Steam Generator

Fig. 1-4 Various Repowering Options Featuring Addition of 150 MW Gas Turbine. Source: Siemens Power Corp.

SUMMARYFacilities have numerous options for meeting their power and heat resource requirements. Electricity will most commonly be purchased from electric utilities or other centralized power producers, which will employ simple- or combined-cycle systems to generate the power. Facilities may also employ simple or combined power cycles to generate their own power in the form of electricity or direct shaft power output, thereby reducing or eliminating their purchase of electricity. Facilities may also employ cogeneration cycles, which sequentially serve both power and heat requirements. To determine if it is economical to apply prime mover technology on site, a facility should perform a life-cycle cost-benefit analysis. Terms such as thermal efficiency, fuel rate, fuel credit, net fuel rate, and fuel and cost chargeableto-power are useful in this pursuit. Still, the repeated statement that mechanical or electrical energy is more valuable than heat energy must be considered within the context of available market alternatives. The relative values of heat and power are not fixed, but are ever changing along with the energy market and the technologies available.

% 55

Combined Cycle Plants with Heat Recovery Steam Generators

Net Plant Efficiency

50

Combined Cycle Plants with Feedwater Heat Exchanges Combined Cycle Plants with Fully Fired Steam Generators (Hot Windbox)

45

Steam Turbine Power Plant1350 750 450 300 150 75MW

Steam Turbine Output1500 900 600 450 300 225 MW

Repowered Plant Output

Fig. 1-5 Efficiency Range of Repowered Plant with 150 MW Gas Turbine as a Function of Repowered Plant Output. Source: Siemens Power Corp.


Heat and Power Resources Overview

Figure 1-6 Cogeneration Combined Cycle with Heat Recovered from Gas Turbine and Steam Turbine Cycles. Source: ABB

Mechanical Inefficiency

Generator Inefficiency Generator Outputs Electricity Optional Outputs Process Heat

Air and Fuel Gas Turbine

High-Temperature Exhaust

Low-Temperature Exhaust

Preheated Combustion Air Process Heat Heat Exchanger

Process Steam

Water

Waste-heat Recovery Boiler

Fig. 1-7 Gas-Turbine Topping Cycle. Source: U.S. DoE

Diesel Engine

Generator Inefficiency Generator

Air and Fuel

Electricity

High-Temperature Exhaust Low-Temperature Exhaust Jacket Cooling Water Process Steam

Heat-recovery Boiler Fig. 1-8 Reciprocating Engine Topping Cycle. Source: U.S. DoE



Exhaust


Generator Inefficiency Electricity

Fuel Water Steam Generator (boiler) Fig. 1-9 Steam-Turbine Topping Cycle. Source: U.S. DoE Exhaust

Generator Back-pressure Low-pressure Turbine Process Steam

Mechanical Inefficiency Source (waste heat)


Turbine Vapor Generator

Generator

Coolant In

Coolant Out

Condenser

Feed pump Regenerator Fig. 1-10 Steam Turbine Rankine Bottoming Cycle. Source: U.S. DoE Mechanical Inefficiency Air and Fuel Generator Inefficiency Electricity Generator Gas Turbine Low-temperature Exhaust

High-temperature Exhaust



Fuel (optional) Steam Turbine Water Heat-recovery boiler Process Steam

Generator

Fig. 1-11 Combined-Cycle. Source: U.S. DoE


LE PHASES MPRESSION ANSION NSION MPRESSION c b ABS PRESSURE a b c d b c d a c

(B) OTTO CYCLE PHAS ISENTROPIC COMPRE CONSTANT VOLUME b ISENTROPIC EXPANSIO CONSTANT VOLUME c p = ABS PRESSURE

CHAPTER TWO

b

b a

E x pdr e s s i n g P o w e r Cay c l e Pde r f o r m a n c eda

T

he thermodynamic and economic performance of heat and power systems can be evaluated and expressed in many ways. This chapter presents some of the concepts and equations that may be used to determine the various performance characteristics of systems featuring simple cycles, cogeneration cycles, and combined cycles. General definitions are first provided, followed by equations, presented in generic format. These are followed by examples featuring commonly used power and thermal energy units.

ENERGYThere are two general types of energy: kinetic and potential. Kinetic energy (KE) is the energy of a given mass of material due to its motion relative to another body. Potential energy (PE) is the energy of a given mass of material as a result of the materials position in a force field. Generally, potential forms can be easily converted into kinetic forms and vice versa. Energy can be transformed from one form to another. For example, a rock resting on the top of a hill has the potential to roll down, or fuel has the potential to be combusted and liberate heat energy. The more common classifications of energy include: mechanical, electrical, electromagnetic, chemical, nuclear, gravitational, and thermal. Thermal energy is associated with atomic and molecular vibration. It is considered a basic energy form because all other energy forms can be completely converted into thermal energy. But the second law of thermodynamics limits conversion of thermal energy into other forms. Enthalpy (H or h) is the thermodynamic property defined as: H = U + PV (2-1)

Where: U = Internal energy of system P = System pressure V = System volume Enthalpy is a general measure of the internally stored energy per unit mass. Commonly used units for expressing enthalpy are Btu/lbm and kJ/kg. Internal energy (U) includes all forms of energy of a given system, except its

gross kinetic and potential energy. It is associated with the thermodynamic state of the system. Two key properties that are used to measure energy levels are temperature and pressure: Temperature is a property that is the measure of the average kinetic energy possessed by the molecules of a substance. The higher the temperature, the greater the kinetic energy or molecular activity of the substance. The common scales of temperature are called the Fahrenheit (F) and Celsius or Centigrade (C) temperatures. These are defined by using the ice point and the boiling point of water at atmospheric pressure at sea level. The ice point is 32F (0C) and the boiling point is 212F (100C). Fahrenheit temperature can be determined from Centigrade temperature as follows: F = (C x 9/5) + 32. Centigrade temperature can be determined from Fahrenheit temperature as follows: C = (F 32) x 5/9. The scales of absolute temperature are called the Rankine (R) and Kelvin (K) temperature scales. These temperature scales are commonly used with steam applications. Both of these temperature scales use absolute zero as a point of origin. Since absolute zero is measured as -459.59F, Rankine temperature can be determined by adding 459.59 to the Fahrenheit temperature. Similarly, since absolute zero is measured as -273.15C, Kelvin temperature can be determined by adding 273.15 to the Celsius temperature. Pressure (p) is the force per unit area exerted on or by a fluid. Pressure, expressed in pounds per square inch (psi), Pascal, or bar, is typically expressed as either absolute pressure, e.g., psi absolute (lbf/in2 abs, or psia), or gauge pressure, e.g., psi gauge (lbf/in2 g or psig). Absolute pressure (psia), which is generally used in steam tables and most fluid and thermodynamic equations, is the true force per unit of area expressed as pounds per square inch exerted by a fluid on the wall of the vessel containing it. Standard atmospheric pressure is 14.696 lbf/in2 abs, or 29.92 in. of mercury atmospheric (in. Hg atm) at sea level, or 406.8 inches of water (in. wg). In SI units, standard atmospheric pressure is 1.013 Bar, 101,325 Pascal (P), or 76.0 centimeters of mercury (cm HgA). Gauge pressure (psig) is the difference between absolute pressure of a fluid and ambient atmospheric


T = ABS TEMPERATURE

c


pressure. Since atmospheric pressure at sea level is about 14.7 psia (101,325 P), absolute or true pressure is determined, using approximate values, simply by adding 14.7 (101,325) to gauge pressure. Vacuum, or negative gauge pressure, is pressure below atmospheric pressure.AND WORK In more general terms, all energy in a thermodynamic system can be classified as either heat or work. Heat is thermal energy that is transferred across the boundary of systems with differing temperatures, always in the direction of the lower temperature. Heat transfer occurs when two adjacent bodies of mass are not in equilibrium due to a difference in temperature. Heat is commonly expressed in calories or British thermal units.

is the product of the mass of an object (m) and its acceleration (a) caused by that force. Mass, expressed in lbm slugs, or kilograms (kg), is a measure of the quantity of matter of which an object is composed. Acceleration, expressed in feet per second per second (ft/s2) or in meters per second per second (m/s2), is a measure of the time rate of change of velocity. Thus: F = ma (2-2)

HEAT

British thermal unit (Btu) is the amount of energy required to raise the temperature of 1 lbm of water by 1.0F, standardized at 60F (i.e., from 59.5F to 60.5F). Calorie (c) is the amount of heat necessary to raise the temperature of 1 gram of water from 14.5C to 15.5C. Other commonly used heat units, which are defined below under the definition of work, are the Joule (J) and the Watt-hour (Wh). In SI units, kilo (k) and mega (M) are used to indicate 1,000 and 1,000,000 units, respectively (e.g., kcal, kJ, kWh, MJ, and MWh). Correspondingly, in English units, M and MM are used to indicate 1,000 and 1,000,000 units, respectively (e.g., MBtu and MMBtu). Work (w), or mechanical energy, is done when a force acts through a distance. Work is the product of force and the displacement along the line of force. Doing work requires the expenditure of energy. Ft-lbf, Joule (J), horsepower-hour (hp-h), and kWh are the common measures of work. A Joule is the amount of energy equal to the work done by a force of 1 Newton (N) when the point at which the force is applied is displaced 1 meter in the direction of the force. This is also known as 1 Newton-meter of energy. The following terms are useful in more fully understanding the concept of work. Force (F) is the action that will cause acceleration of a mass. Change in velocity of an object is caused by force. If no force acts on an object, it moves at constant velocity. Newtons first law of motion states that a body at rest will stay at rest and a body in uniform motion will continue its uniform motion unless acted upon by a force. Force, expressed in pound-force (lbf ) or in Newton (N),

Torque is the force applied at a distance from an axis of rotation and is expressed in lbf-ft or Newton-meter (Nm). It is the product of, for example, the force applied to a lever, or crank arm, and the perpendicular distance from the line of action of the force to the axis of rotation. In rotating machines, the forces required to accelerate (or decelerate) the speed of rotation add up to a torque that is proportional to the angular acceleration times the moment of inertia of the machine. Power (P) is the rate of doing work and is given by the formula: P= dE dt (2-3)

where power (P) is equal to the rate of energy expenditure (dE) over a given time interval (dt). Thus, power refers to the rate of mechanical energy expenditure over a given time interval or the rate of doing work. In rotating machines, power is equal to torque times the speed of rotation. Therefore, if speed is held constant, torque and power are proportional. Commonly used units for expressing power are Btu per hour (Btu/h), Joule per second (J/s), ft-lbf per minute (ft-lbf/m), meter-kg per minute (m-kg/m), horsepower (hp), and Watt (W). Horsepower (hp) is an English system unit based on the power needed to raise a weight of 550 lbm through a height of 1 foot in 1 second (550 ft-lbm of energy per second) and was originally derived from the estimated power of one horse pulling a load. One hp is approximately equal to 2,545 Btu/h and 745.7 Watt. Brake power is a measure of the power generated by a prime mover. It can be measured at the crankshaft or the flywheel. The term brake refers to a mechanical arrangement used to measure the output torque, which, when multiplied by rotational speed, yields the power. Watt (W) is the unit of power equal to 1 J/s and approximately 3.413 Btu/h. 1 kilowatt (kW) is approximately equal to 1.34 hp.


Expressing Power Cycle Performance

Metric horsepower (PS) is the power that raises a mass of 75 kg through a height of 1 meter in 1 second. One PS is approximately equal to 0.9863 hp and 735.5 Watt. Given that power is the rate of doing work, applying a unit of time to a unit of power yields a measure of work. Hence, hp-h or Wh are the common work units. Given that power already has a unit of time, work expressions can be reduced to basic energy units such as Btu, calorie, or Joule. In many thermodynamic applications, Btu is considered to be a more convenient English system unit than ft-lbf. In terms of potential work energy, it is defined by the relationship 1 Btu = 778.16 ft-lbf. One hp is approximately equal to 2,545 Btu. The Joule, which may be considered the mechanical equivalent to heat, is equal to 4.1855 calories.

energy include heat and work. The first law of thermodynamics for a system undergoing a cycle is: Q = W For the non-cycle process, the law is: Q = dE + W (2-4a) (2-4)

POWER CYCLESEnergy cycles are a series of thermodynamic processes during which the working fluid undergoes changes involving energy transitions and is then returned to its original state. In this process, the working fluid undergoes changes involving pressure, temperature, and energy levels while producing a usable transfer of energy. The purpose of any practical thermodynamic cycle is to convert energy from one form to another more useful form. Thus, the practical goals are either to convert heat into work or, in the reverse, to use work to remove heat from a cold to hot region. The classic example used to demonstrate thermodynamic cycles is the heat engine. In the heat engine, only heat and work flow across the operating systems boundaries. The engine can deliver work to external devices or receive work from an external device and cause heat to flow from a low temperature level to a high temperature level. Power cycles are processes in which heat energy is converted to work energy. During these processes, the energy of a fuel is converted into heat energy, which in turn is used to produce electrical or mechanical energy in the form of shaft power. The processes in the cycle are governed by the principle of the first law of thermodynamics, also known as the law of the conservation of energy.

Where: Q = The heat transferred to the system during the process W = The work done by the system during the process E = Total energy of the system (internal, kinetic, and potential) in the given state (this energy may be associated with the motion and position of the molecules, with the structure of the atoms, with chemical reactions, with gravity, or with any of a number of other forms of interaction)

SECOND LAW

OF

THERMODYNAMICS

The second law of thermodynamics states that energy always seeks a lower level, or, in a manner of speaking, only runs downhill. Energy is only useful when it moves through a device from a higher level to a lower level. The level of obtainable benefit is proportional to the decrease in level that is available. The decrease in level is denoted by an increase in molecular disorder, or entropy (s) Entropy is the thermodynamic property of the system held constant in a reversible adiabatic process. The entropy is a useful parameter in accounting of energy conversions that result in production of work as well as energy unavailable to do work (i.e., heat loss from the system and loss due to internal irreversibilities). It measures the relative molecular disorder of a given system. Entropy can be expressed as: ds =

( )Q T

(2-5)

FIRST LAW

OF

THERMODYNAMICS

The first law of thermodynamics, or conservation of energy, states that energy can be neither created nor destroyed, but only converted from one form to another. The motion of the molecules causes thermal or internal energy embodied within a system. Transient forms of

Where: s = Entropy Q = Heat T = Temperature at the boundary Commonly used units for expressing entropy are Btu/lbm per F and kJ/kg per K. This increase in entropy is unavoidable because energy never develops enough work to restore itself to the



original level. The conversion of energy is, therefore, never completely reversible, hence, there are no perpetual motion machines. The second law of thermodynamics can be stated in a number of equivalent ways. One statement is: no heat engine, either actual or ideal, when operating in a cycle, can convert all the heat energy supplied to it into work. Some of the heat energy must be transferred to a heat sink at a temperature lower than the temperature at which the heat energy is supplied. A consequence of the second law is that for any actual process, dssystem 0 Where: s = Entropy The principle of the increase of entropy, based on the second law of thermodynamics, is that the only processes that can occur are those in which the net change in the entropy of system plus the surrounding region increases. Therefore, (2-7) dsystem + dsurrounding 0 The important concepts operating in a thermodynamic system are then energy and entropy. A unit of energy represents a certain potential to do work. The entropy change of the system is a measure of the irreversibility of that process and the degree to which energy has not been made available to perform work. (2-6)

converted to heat, the converse is not true. When these concepts are applied to a heat engine, there is a range of possibilities. At one end of the spectrum, a large portion of the input energy can be converted to prime power (useful shaft work) and most of the rest can be converted to useful thermal energy. At the other end of the spectrum, a small fraction of the input may be converted to prime power and excess energy may be a costly nuisance requiring rejection through the use of additional energy in such forms as pumps, fans, and cooling equipment. From an economic perspective, the same quantity of energy resulting from the same process can have a positive value on a cold day as a heat source and a negative value on a hot day as an excess heat load. Power producing equipment (i.e., prime movers) operate on several different cycles. Generally, reciprocating engines operate on either the Otto cycle or the Diesel cycle, combustion turbines operate on the Brayton cycle, and steam turbines operate on the Rankine cycle. In each of these power cycles, the initial and final states of the system are identical. At the end of the cycle, all of the properties have the same value they had at the beginning of the cycle, except that heat has been added, heat has been rejected, and work has been done. An analysis of the power cycle involves an accounting of all of the energy exchanges occurring at each of the processes, so that the sum of all energy inputs equals the sum of all outputs.

POWER CYCLE THERMAL EFFICIENCYEfficiency is a general concept used to describe the effectiveness of energy conversion from one state or form to another. Thermodynamic efficiency and coefficient of performance (COP) are terms used for expressing efficiency for devices that operate in cycles or for individual system components that operate in processes. The term COP is usually reserved for refrigeration systems. The thermal efficiency of a cycle is the ratio of output energy to input energy. In a heat engine, the desired form of energy is the work; the form of energy that costs money is heat from the high-temperature source (directly or indirectly, the cost of the fuel).

SUMMARY

OF

FIRST

AND

SECOND LAWS

From the second law, it follows that with each process, interaction, reaction, or exchange of a stream of energy, some of its potential to do work is lost due to process irreversibilities, characterized by an increase in molecular disorder (increased entropy). In practice, in addition to these losses, processes may experience energetic losses, which are essentially heat losses (i.e., radiation, conduction, and convection). When a fuel is combusted, it automatically loses energy. From the moment heat is collected on a solar plate, or rushing water is used to generate power, the stream of energy continues to decline in its total useful value to do work. The first law shows that on a global level, no energy is lost. It is only lost from within a given system to another. The second law states that even on a global level, availability to do work is lost, since entropy increases throughout the universe with no compensation. Whereas work and all other forms of energy can be wholly

CARNOT CYCLEThe Carnot cycle is of particular importance because its thermal efficiency represents the maximum value obtainable for any heat engine. The Carnot cycle represents a theoretical efficiency that serves as a standard for cycle efficiencies. It is based on the concept that the thermal efficiency of a reversible engine is a function



solely of the upper and lower temperatures of the cycle and not a function of the working substances. The Carnot cycle for both vapor and gas cycles typifies ideal performance for power producing cycles. In the Carnot cycle, heat is taken from an infinite reservoir, at temperature TH, isothermally (without temperature change) and reversibly. The energy received produces work by expanding a working fluid reversibly and adiabatically in an ideal frictionless engine. During expansion, the engine produces a net work output. The working fluid, at temperature TL, rejects heat at constant temperature reversibly and isothermally to an infinite sink and is then adiabatically and reversibly compressed to its initial state. Figure 2-1 shows a temperature-entropy (T-s) diagram of the Carnot cycle for a heat engine. The four basic steps or processes are indicated below. Step A-B Adiabatic isentropic compression (no change in entropy) Step B-C Isothermal addition of heat enters cycle at constant temperature TH with an increase in entropy Step C-D Adiabatic isentropic expansion with no change in entropy (ds = 0); with expansion work and an equivalent decrease in enthalpy Step D-A Isothermal rejection of heat; heat is rejected at constant temperature TL with a reduction of entropy In Step B-C, the heat added to the cycle is indicated by Qin. In Step D-A, the heat rejected is indicated by Qout.

In Steps B-C-D-A, the net work of the cycle is indicated by Wnet. From these values, cycle efficiency (th) can be expressed as follows:th =

(TH TL ) (Qin Qout ) = TH Qin

(2-8)

Where: th = Cycle efficiency TH = Initial temperature at which heat is added to cycle = Final temperature at which heat is rejected TL from cycle Qin = Heat energy input Qout = Heat energy rejected The following example shows ideal cycle efficiency based on Equation 2-8, given an initial temperature, TH = 540F, and a final temperature, TL = 40F. Temperatures are expressed in degrees R (F + 460 = R).th =

(TH TL ) TL

x 100% =

1,000 500 x 100% = 50% 1,000

Figure 2-2 shows the Carnot cycle with steam as the working fluid based on the temperatures shown in the above example. The figure includes both a pressurevolume (PV) diagram and a temperature-entropy (Ts) diagram, indicating the relationship between pressure and temperature as measures of energy. As shown, the four basic steps are similar to that of the Carnot heat engine.

Fig. 2-2 Carnot Cycle with Steam as Working Fluid. Source: Babcock &Wilcox

Step A-B Step B-C

Fig. 2-1 Temperature-Entropy Diagram for Carnot Heat Engine.

Isentropic compression Constant pressure (hence, constant temperature heat addition) Step C-D Isentropic expansion Step D-A Constant pressure (hence, constant temperature heat rejection) Figure 2-3 shows the pressure-volume and temperatureentropy diagrams for Carnot, Otto, Diesel, and Brayton gas



b

T = ABS TEMPERATURE

T = ABS TEMPERATURE

c p = ABS PRESSURE

b

c p = ABS PRESSURE

c

c d

b

a d

b a

a

d

d a VL = VOLUME

DISPLACEMENT VL = VOLUME a b c d b c b c d a

s = ENTROPY

s = ENTROPY

(A) CARNOT CYCLE PHASES ISENTROPIC COMPRESSION ISOTHERMAL EXPANSION ISENTROPIC EXPANSION ISOTHERMAL COMPRESSION c T = ABS TEMPERATURE b p = ABS PRESSURE d b a a b c d b c d a c

(B) OTTO CYCLE PHASES ISENTROPIC COMPRESSION CONSTANT VOLUME HEAT ADDITION ISENTROPIC EXPANSION CONSTANT VOLUME HEAT REJECTION c T = ABS TEMPERATURE d b a

p = ABS PRESSURE

d a VL = VOLUME a b c d b c d a

a VL = VOLUME

d

s = ENTROPY

s = ENTROPY

(C) DIESEL CYCLE PHASES ISENTROPIC COMPRESSION CONSTANT PRESSURE HEAT ADDITION ISENTROPIC EXPANSION CONSTANT VOLUME HEAT REJECTION a b c d b c d a

(D) BRAYTON CYCLE PHASES ISENTROPIC COMPRESSION CONSTANT PRESSURE HEAT ADDITION ISENTROPIC EXPANSION CONSTANT PRESSURE HEAT REJECTION

Fig. 2-3 P-V and T-s Diagrams for Carnot, Otto, Diesel, and Brayton Gas Cycles. Source: Babcock & Wilcox

cycles. Work for each cycle is again represented by area A-B-C-D. Note that mean effective pressure (MEP) is the work of the cycle divided by the displacement.

PRACTICE VERSUS THEORYGiven the first law of thermodynamics, all of the energy that goes into an operating system is work or heat. The first law treats heat and work as being interchangeable, though some qualifications must apply. Work and all other forms of energy can be wholly converted to heat, but the converse is not generally true. The second law of thermodynamics shows that given a source of heat, only a portion of the heat can be converted to work in a heat-work cycle. The rest must be rejected to a heat sink. Thus, it is impossible to have a heat engine that is 100% efficient and even the most perfect cycle must be less than perfectly efficient. Further, the ideal performance of a given power cycle is always less than the Carnot efficiency. During an ideal (reversible) process, it is theoretically possible to reach the maximum potential efficiency for the specific process. Cycle design and operational improvements are initiated in an effort to approach the theoretical limits of the Carnot cycle. There

are, however, factors that render the process irreversible. The practical limit of power cycle efficiencies is set by metallurgical limits or strength of available materials (ability to operate under high temperatures and pressures) and by the ambient temperature of the heat sink. In addition, all practical applications of cycles and any other thermal process will be subject to energetic or heat losses resulting from friction, sustained expansion, convection, and conduction. Thus, reversible thermodynamic processes exist in theory only, defining only the limiting case for heat flow and work processes. From the comparison of reversible and irreversible processes and cycles, it becomes clear that a critical concern in evaluating various cycles is theoretical and practical cycle efficiency. Theoretical efficiency shows the maximum efficiency that could, in theory, be attained from any given cycle. Practical efficiency shows what can be expected from a particular system operating on a given cycle. In the ensuing chapters covering prime mover technologies, thermal efficiency and other expressions of performance are presented first in theory and then in a practical context that can be applied to actual applications. The following series of expressions are used through-



out the various chapters that present combined heat and power system technologies and applications. They are used to present practical thermodynamic and economic performance measurements, based primarily on fuel and heat input and work and heat output.

For operation on natural gas, Equation 2-12 becomes: Fuel (gas) rate = Btuinput /hp-h Btu/cf In SI units, the corresponding expression would be in m3/kWhm. = cf/hp-h (2-12a)

POWER CYCLE PERFORMANCE EXPRESSIONSThermodynamic (or thermal) efficiency is obtained using the first law of thermodynamics and is calculated as the net work produced divided by the heat energy consumed. It is generically expressed in percent as:th =

FUEL-SPECIFIC EFFICIENCY

AND

HEAT RATE

Net work output Heat energy input

x 100%

(2-9)

In the English system, work output is commonly expressed as either hp (shaft output) or kWe (electric generator output). Heat energy input is typically given as Btu/h. Alternative English and SI expressions for thermal efficiency are listed in Table 2-1. To express efficiency in terms of fuel energy input (e.g., to a steam boiler serving turbine-driven equipment), boiler efficiency must be included in the expression. Thus, the fuel utilization efficiency, TF, would be expressed as:TF

=

th

x

Boiler

(2-10)

Heat rate is the inverse of thermal efficiency or the amount of energy input required (heat added) to generate shaft work. It is expressed as: Heat rate = Heat energy input Net work output In English units, heat rate is commonly expressed as Btu per horsepower-hour (Btu/hp-h) or Btu per kilowatthour (Btu/kWh). Alternative English unit and SI expressions are listed in Table 2-1. Fuel rate is the heat rate expressed in terms of units of fuel added (or input) per unit of work unit output. Fuel units are commonly expressed on a mass-unit basis as pounds (lbm), tons, or kilograms (kg) or on a volumetricunit basis as gallons (gal), liters (L), barrels, cubic feet (ft3 or cf), or cubic meters (m3 or cm). Fuel rate is expressed as: Fuel rate = Fuel input Net work output (2-12) (2-11)

Conversion from heat rate to fuel rate requires a knowledge of the energy density, or heating value, of the particular fuel. Liquid fuels generally are expressed on an energy per unit mass or energy per unit volume basis (Btu/lbm or Btu/gal). For gaseous fuels, values are expressed on a volume basis, and the reference conditions for the fuel volume measurements (pressure, temperature, and degree saturation with water vapor), as well as test conditions, have to be stated explicitly. Care must be taken when converting heat rates to fuel rates to identify whether the fuel energy density refers to the higher heating value (HHV) or lower heating value (LHV) of fuel. During the combustion of hydrocarbon fuels, some of the oxygen is combined with hydrogen, forming water vapor that may leave the combustion device either in vapor or condensed to liquid state. When the latent heat of vaporization is extracted from the flue products, causing the water to become liquid, the fuels energy density is identified as HHV. When the equipment used allows the water to remain in the vapor state, the energy density is identified as LHV. Since different fuels have varying amounts of hydrogen, the numerical relationship between LHV and HHV varies. Prime mover (reciprocating engine and combustion gas turbine) performance is usually based on LHV, while fuel energy is often expressed in HHV. Thus, in order to convert heat rates specified in LHV to purchasable fuel units, one must know the energy density of the fuel and the ratio of LHV to HHV. Refer to chapter 5 for additional details. To express true fuel-specific thermal efficiency, when heat added is expressed on an LHV basis, Equation 2-9 becomes:th (HHV)

=

Net work x (LHV/HHV) x 100% Heat added (LHV) (2-13)

To express true fuel-specific heat rate, when heat added is expressed on an LHV basis, Equation 2-11 becomes: Heat rate (HHV) = Heat added (LHV) x (HHV/LHV) Work units of output (2-14)



CONSIDERATION

OF

RECOVERED HEAT

Overall thermal efficiency is the ratio of work output plus heat recovered to the heat input. Equation 2-9 thus becomes:th =

(2-15) Since mechanical or electrical power is usually more valuable than heat energy, combined product values are not highly informative and can result in misleading conclusions about the relative thermodynamic and economic value of a cogeneration cycle. Though not fully definitive, the terms net thermal efficiency and net heat rate are often more revealing. Net thermal efficiency reflects incremental energy usage for power generation by subtracting recovered heat from the total energy input, assuming recovered heat replaces other energy usage. Equation 2-9 thus becomes:th

(

Net work + Recovered heat Heat added

)

x 100%

This displaced energy is commonly referred to as an energy credit or fuel credit. The amount of energy displaced by recovered heat is a function of the efficiency of the displaced boiler (or other energy conversion equipment). Displaced energy resulting from heat recovery can be expressed as: Energy credit = Heat recovered Efficiency of displaced boiler (2-18)

Examples If prime mover heat recovery provides 100,000 Btu of usable thermal energy, reducing boiler operation at 84% conversion efficiency, Equation 2-18 becomes: Energy credit = 100,000 Btu 0.84 = 119,000 Btu

=

(2-16) Net heat rate is the energy input required to generate shaft work minus the amount of heat recovered: Net heat rate = Heat added Heat recovered Net work output (2-17)

(

Net work output Heat added Heat recovered

)

x 100%

, ENERGY-, FUEL- AND COST-CHARGEABLE-TO-POWERAn important distinction needs to be made in comparing prime mover applications, which use rejected heat (or pass on thermal energy to useful processes), with typical simple-cycle processes. Whereas simple cycle thermal efficiencies refer to a single energy product, thermal efficiency measurements of cogeneration-type applications refer to combined production of prime energy and thermal energy. When a facility is considering an investment in some type of prime mover system, the investment analysis is based on economic performance. The following indices are useful in expressing the performance of cogeneration systems.

(2-18a) It is important to note that the displaced energy is not necessarily fuel energy. If, for example, recovered heat were used to displace an electric heat source with an efficiency of 98%, the displaced energy would be expressed as displaced electricity and Equation 2-18 would become: 100,000 Btu Displaced = = 29.9 kWh electricity 3,413 Btu/kWh x 0.98 (2-18b) Alternatively, if recovered heat were used to generate chilled water via a double-effect absorption chiller (at a rate of 10,000 Btu/ton-h), the displaced energy could be expressed as a chilled water or cooling credit: Displaced = 100,000 Btu = 10.0 ton-h cooling 10,000 Btu/ton-h

(2-18c) If operation of the absorption chiller resulted in the displacement of an electric chiller, there would be an electricity credit calculated by considering the efficiency of the original chiller (kW/ton) and related auxiliaries.

Energy-Chargeable-to-PowerThe difference between the total energy input to the prime mover and the energy credit represents the actual amount of fuel energy consumed for the purpose of generating shaft power. Energy-chargeable-to-power (ECP) provides a useful means of expressing the net heat rate of prime mover applications that include heat recovery: Total heat added Displaced fuel energy Net power output

Energy CreditThe value of heat recovery can be measured by the cost avoided in using recovered thermal energy (or heat) for a specific purpose, as opposed to using another source of energy. Most commonly, recovered heat replaces thermal energy output from some type of fuel-burning equipment, usually a boiler or furnace. In these cases, the value of recovered thermal energy is equivalent to the cost of fuel energy that would have otherwise been consumed.

ECP =

(2-19)



where the displaced fuel energy is computed from Equation 2-18. Calculation of ECP using English and SI units is illustrated in Table 2-1. For a back-pressure steam turbine with exhaust heat recovery, ECP can be expressed using Equation 2-19 based on boiler input, heat recovery fuel savings, and turbine power output. Alternatively, ECP can be calculated based on steam flow and turbine inlet and exhaust conditions. When steam flow is expressed in lbm/h and steam energy content (enthalpy) in Btu/lbm, Equation 2-19 becomes: ECP = lbm/h (Btu/lbmInput Btu/lbmDischarge ) hp xBoiler

Operating Cost-Chargeable-to-Power

The operating cost-chargeable-to-power (CCP) provides a means of expressing economic performance, or the net cost of prime mover (cogeneration) applications that effectively use rejected heat. The CCP expression follows the same format as the ECP and FCP expressions. In addition to energy operating costs, the CCP expression may be extended to include all operating costs associated with the system, such as operations and maintenance (O&M) costs, water costs, and environmental compliance costs. Equation 2-21 becomes: Fuel cost Fuel credit value + Other operating costs Net power outputOF

= Btu/hp-h (2-19a)

CCP =

(2-22)

In the case of combined-cycle operation, the net power output of the system includes the output of the topping cycle (typically a gas turbine or reciprocating engine) and the bottoming cycle (typically a steam turbine). When recoverable heat is used both to generate additional power and to displace other fuel usage, Equation 2-19 becomes: ECP = Total energy input Displaced energy TCOutput + BCOutput Auxiliary power input (2-20)

Applying ECP to Combined Cycles

RELATIONSHIP

ECP, FCP,

AND

CCP

If the type and per-unit cost of fuel is the same for both the prime mover and the fuel use displaced by heat recovery, FCP can be derived from ECP, and CCP can be expressed as a function of ECP or FCP: FCP = ECP Energy units per unit of fuel (2-23)

Where: TC = Topping cycle BC = Bottoming cycle

CCP = ECP x Cost per energy unit

(2-24)

Fuel-Chargeable-to-Power

CCP = FCP x Cost per fuel unit

(2-25)

Fuel-chargeable-to-power (FCP) provides an alternative means of expressing the net fuel rate of prime mover (cogeneration) applications that effectively use rejected heat. It expresses the total fuel rate of power generation minus a fuel credit (if rejected heat is used), divided by power production minus any auxiliary power requirements. When energy input and displaced fuel energy are replaced with fuel input and fuel displaced (or fuel savings), Equation 2-19 becomes: FCP = Total fuel input Fuel displaced Net power input (2-21)

Commonly, however, the fuel used by the prime mover may be different than the fuel used by the heat energy source that is displaced. Alternatively, the same fuel type may be used but at different prices, due to contract specifics, such as the ability to operate on an alternative fuel. In either of these cases, CCP cannot be expressed as a function of ECP or FCP. Instead, a separate fuel credit term must be used. The displaced fuel energy resulting from heat recovery is expressed by Equation 2-18. By assigning a separate financial value or avoided cost to this, Equation 2-18 becomes: Fuel credit value = Fuel credit x Avoided cost per fuel unit (2-26) The CCP expression can be manipulated to express performance for a variety of practical applications. The displaced energy use, for example, may in fact be electricity as opposed to fuel. Still, the basic form of Equation 2-22

When work rate is expressed in English units as hp and fuel rate as cf of natural gas, Equation 2-21 becomes: FCP =

(cfInput cfDisplaced )hp-hNet output

= cf/hp-h

(2-21a)



applies the value of the energy credit is subtracted from the cost of the total energy input, plus any additional operating costs, and this net cost is divided by the net power output to yield the CCP.

assigned to each year, based on the assumed discount rate. The present value factor serves to reduce CCP in each future year to an equivalent cost in the present based on a given discount rate.

Total and Life-Cycle CCP

Following are four examples of how the terms and formulae provided above are applied to specific representative prime mover systems. All values are given in English units and the assumed fuel is pipeline grade natural gas with an HHV of 1,020 Btu/cf and LHV of 918 Btu/cf. Fuel costs are given for each example on the basis of 1,000 cf (1 Mcf). For simplicity, the performance CCPTotal = Fuel cost Energy credit + Operating costs + Annualized capital cost expressions are applied under idealized full load operating conditions. In actual Net power output (2-27) applications, prime mover performance This expression can be extended further to reflect will be directly related to continually varying operating conlife-cycle CCP. Based on an assumed financial discount ditions. For example, operation under varying load condirate, the life cycle CCP would be the sum of the present tions or at varying inlet air temperatures usually results in a values of the annualized CCP over the life of the systems. change in heat rate from rated full load operation. Numerous To perform this calculation, a present value factor is other factors will also influence actual performance.

The CCP may also be extended to include capital costs as well as operating costs by including an annualized capital cost component based on an assumed time-valued cost of capital. The change () in operating costs represents any increases or decreases in related costs, such as O&M, chemical treatment, water use, or sewer use. The total annual CCP can then be expressed as:

SUMMARY EXAMPLES

TermThermal efficiency (Eq. 2-9)

English Unitshp x 2,545 Btu/hp-h x 100% Btu/h Btu/h = Btu/hp-h hp

SI UnitskWm x 3,600 kJ/kWh x 100% kJ/h or kWm x 100% kWhh /h kJ/h = kJ/kWhm kWm kWhh /h or = kWhh /kWhm kWm

Net work output x 100% Heat energy inputHeat rate (Eq. 2-11)

Heat energy input Net work inputNet thermal efficiency (Eq. 2-16)

(

hp x 2,545 Btu/hp-h kWm x 3,600 kJ/kWhm Net work output x 100% BtuHeat added /h BtuHeat recovered /h x 100% kJ x100% Heat added/h kJHeat recovered /h Heat added Heat recovered

)

(

)

(

)

Net heat rate (Eq. 2-17)

Heat added Heat recovered Net work outputEnergy-chargeable-to-power (Eq. 2-19)

BtuEnergy added /h BtuHeat recovered /h hp BtuHeat added BtuHeat recovered x

kJEnergy added /h kJHeat recovered /h kWm

Boiler

kJHeat added kJHeat recovered x

Boiler

Total heat added Displaced fuel energy Net power output

hp-hOutput hp-hAuxilliary input

kWhmOutput kWhm Auxiliary input

Table 2-1 English and SI Units Commonly Used in Performance Terms.



Example 1 Reciprocating Engine Applied to Mechanical Drive ServiceFull-load Performance ValuesTotal fuel input (LHV) per hp-h output Ratio of fuel HHV to LHV Total energy recovered per hp-h output Efficiency of displaced boiler O&M cost for total system per hp-h output Fuel cost (natural gas) Horsepower to Btu conversion factor = = = = = = = 6,758 Btu (LHV) 1.11 3,003 Btu (40%) 82% $0.007/hp-h $3.75/Mcf 2,545 Btu/hp-h

Example 2 Back-Pressure Steam Turbine Applied to Mechanical Drive ServiceFull-load Performance ValuesSteam turbine inlet enthalpy (@ 250 psig, saturated) Steam turbine exhaust enthalpy Condensate return enthalpy Total lbm of steam input per hp-h output Boiler efficiency Actual enthalpy drop in turbine per lbm of steam Incremental O&M cost for turbine per hp-h output Fuel cost (natural gas) = = = = = = = = 1,201.7 Btu/lbm 1,129.0 Btu/lbm 170.3 Btu/lbm 35.0 lbm/hp-h 83% 72.7 Btu/lbm $0.003/hp-h $3.75/Mcf

Fuel input is calculated on an HHV basis, using Equation 2-14 as follows: 6,758 Btu/hp x 1.11 = 7,507 Btu/hp-h The thermal efficiency (HHV basis) of the system for simple-cycle operation, based on Equation 2-9, is: 2,545 BtuShaft /hp-h 7,507 BtuInput /hp-h x 100% = 33.9%

The thermal efficiency of the system for simple-cycle operation (full steam use without use of back-pressure steam), based on Equation 2-9, is: 2,545 BtuShaft /hp-h 35.0 lbm/hp-h x (1,201.7 170.3) Btu/lbm x 100% = 7.05%

The fuel energy efficiency for simple-cycle operation, based on Equation 2-10, is: 7.05% x 0.83 = 5.86% The fuel rate for simple-cycle operation on natural gas, based on Equation 2-12, is:

The fuel rate for simple-cycle operation when operating on natural gas, using Equation 2-12, is: 7,507 Btu/hp-h 1,020 Btu/cf = 7.360 cf/hp-h

(

35.0 lbm/hp-h x (1,201.7 170.3) Btu/lbm = 42.640 cf/hp-h 1,020 Btu/cf x 0.83 The overall thermal efficiency (HHV basis), based on Equation 2-15, is: The ECP (assuming full use of back-pressure steam), based on Equation 2-19, is: 2,545 BtuShaft /hp-h + 3,003 BtuHeat /hp-h x 100% = 73.9% 7,507 BtuEnergy input /hp-h

)

The ECP, based on Equation 2-19, is: 7,507 Btu /hp-h 3,003 Btu /hp-h = 3,845 Btu/hp-h 0.082

35.0 lbm/hp-h x

(1,201.7 1,129.0) Btu/lbm = 3,066 Btu/hp-h 1,020 Btu/cf x 0.83

The FCP, based on Equation 2-21, is: 3,066 Btu/hp-h = 3.006 cf /hp-h 1,020 Btu/cf The CCP, based on Equation 2-22, is: 3.006 cf /hp-h x $3.75/Mcf x 1 Mcf /1,000 cf + $0.003/hp-h = $0.011/hp-h + $0.003/hp-h = $0.014/hp-h

The FCP, based on Equation 2-21, is: 3,845 Btu/hp-h 1,020 Btu/cf = 3.770 cf /hp-h

The CCP, based on Equation 2-22 is: 3.770 cf/hp-h x $3.75/Mcf x 1 Mcf /1,000 cf + $0.007/hp-h = $0.014/hp-h + $0.007/hp-h = $0.021/hp-h



Example 3 Combustion Gas Turbine Applied to Electric Power CogenerationFull-load Performance ValuesTotal fuel input (LHV) per kWh output Total energy recovered per kWh output Efficiency of displaced boiler O&M cost for total system per kWh output Fuel cost (natural gas) for the gas turbine Fuel cost (natural gas) for the displaced boiler = = = = = = 11,045 Btu (LHV) 6,380 Btu (52%) 82% $0.008/kWh $3.50/Mcf $2.70/Mcf

Example 4 Combined-Cycle Cogeneration System Applied to Electric Power Generation (Gas Turbine System with Back-Pressure Steam Turbine)Full-load Performance ValuesTotal fuel input (LHV) per kWh gas turbine output Steam turbine kW output per kW gas turbine output Energy recovered per kWh combined cycle output Efficiency of displaced boiler O&M cost for total system per kWh output Fuel cost (natural gas) = 11,300 Btu (LHV) = 0.175 kW (597 Btu/kWh) = 4,540 Btu = 83% = $0.006/kWh = $3.35/Mcf

Fuel input (HHV Basis) = 11,045 Btu/hp-h x 1.11 = 12,279 Btu/hp-h The thermal efficiency (HHV Basis) for simple-cycle operation, based on Equation 2-9, is:

(

3,413 Btu/kWh 12,270 Btu/kWh

)

x 100% = 27.8%

Fuel input (HHV basis) = 11,300 Btu/kWh x 1.11 = 12,553 Btu/kWh The heat rate of the combined-cycle system, based on Equation 2-11, is: 12,553 Btu = 10,683 Btu/kWh (HHV Basis) 1.175 kWh The fuel rate, on an HHV basis, based on Equation 2-12, is: 10,683 Btu/kWh = 10.474 cf/kWh 1,020 Btu/cf The energy credit for recovered heat, on an HHV basis, based on Equation 2-18, is: 4,540 Btu/kWhTotal 0.83 and the fuel credit is: 5,470 Btu/kWh 1,020 Btu/cf = 5,470 Btu/kWhTotal

The fuel rate, on an HHV basis, for simple-cycle operation, based on Equation 2-12, is: 12,270 Btu/kWh = 12.029 cf/kWh 1,020 Btu/cf The ECP (HHV Basis), based on Equation 2-19, is: 12,270 Btu/kWh 6,380 Btu/kWh = 4,486 Btu/kWh 0.82 In this case, the CCP will not be computed as a function of ECP or FCP because the cost of fuel input to the gas turbine and the cost of fuel that is being displaced by use of recovered heat are different. Therefore, the fuel credit must be calculated separately and then applied to the CCP expression. The fuel credit, based on Equation 2-18, is: 0.52 12.029 cf/kWh x = 7.628 cf/kWh 0.82 To apply the CCP expression, the cost of the total fuel input and the value of the fuel credit must be calculated separately as follows: Total fuel input cost = 12.029 cf/kWh x $3.50/Mcf x 1 Mcf/1,000 cf = $0.042/kWh Fuel credit value = 7.628 cf/kWh x $2.70/Mcf x 1 Mcf/1,000 cf = $0.021/kWh The CCP, based on Equation 2-22, is: $fuel cost $fuel credit + $O &M cost = $Operating cost $0.042/kWh $0.021/kWh + $0.008/kWh = $0.029/kWh

= 5.363 cf/kWh

The ECP, on an HHV basis, based on Equation 2-20, is: 10,683 Btu/kWh 4,540 Btu/kWh 0.83 = 5,213 Btu/kWh

The FCP, based on Equation 2-23, is: 5,213 Btu/kWh 1,020 Btu/cf = 5.111 cf/kWh

The CCP, based on Equations 2-22 and 2-24, is: 5.111 cf/kWh x $3.35/Mcf x 1 Mcf/1,000 cf + $0.006/kWh = $0.017/kWh + $0.006/kWh = $0.023/kWh



AND CCP Absent from the above discussion of performance indicators are several essential factors that affect actual project economic performance for any prime mover application. When considering an investment in a prime mover system, the analysis is based not on the potential output and efficiency of the system, but the match of that potential output with the facilitys internal requirements or ability to sell the outputs. While the CCP of a given system provides useful and compelling information, it provides only a limited perspective from which to make an investment decision. In most cases, higher cogeneration heat recovery indicates a higher overall heat and fuel rate, even if the ECP or FCP remains the same. This is because as more input energy is converted to thermal energy, less is converted into power, and more fuel must be used per unit of power output. Depending on the available load or rejected energy sink of displaceable thermal energy, it is often better to consider a more thermally efficient power producing system with less heat recovery. With an unlimited sink of thermal energy requirement, power generating thermal efficiency becomes less of a consideration. A relatively inefficient power system may

BEYOND ECP, FCP,

be considered because decreased capital costs and increased thermal energy generation efficiency compensate for decreased power output. However, if there is a limited amount of thermal energy that can be used, the emphasis shifts to more thermally efficient prime movers that produce a higher percentage of shaft power and a lower percentage of recoverable thermal energy. Each prime mover system considered will have a different heat rate and different combination of power and thermal outputs. Available temperatures and pressures of thermal outputs will also vary. There will be different emissions rates for different pollutants, different space requirement, and different expected service lives. Moreover, these performance characteristics will vary as operating conditions vary. Additionally, each of the prime mover systems will require different levels of maintenance and operating attention, and they will have different capital costs. All of these factors will vary, even for the same equipment, depending on site conditions. Translating performance into cost and values requires a detailed analysis of the site conditions, the equipment performance under these conditions, and the interaction of all factors for each alternative.


CHAPTER THREELocalized vs Central Station Power Generation

Most large utility plants rely on the conventional Rankine steam cycle, in which high-pressure steam is expanded through a set of turbine wheels to generate mechanical energy, which then drives a generator to make electric power. The high-pressure steam is produced in a steam generator, or boiler, which may be fueled by coal, oil, natural gas, wood, refuse, or nuclear fission. Figure 3-1 is a simplified schematic representation of a large utility supercritical steam power plant, featuring a high-pressure and a low-pressure turbine, with regeneration CENTRAL UTILITY PLANTS and reheat. The boiler section may be divided into three The electric utility industry, which still produces the functional components: an economizer, which heats majority of the nations power, has traditionally relied on two incoming feedwater nearly to its boiling point; an evaporabasic thermodynamic cycles to generate electric power: the tor, which adds energy to convert the feedwater into steam; conventional steam turbine Rankine cycle for baseload and a superheater, which raises the temperature of the and intermediate load applications, and the gas turbine steam prior to its use in the steam turbine. Large utility boilBrayton cycle for peaking applications. While these cycles ers may employ several stages of economizer, evaporator, and have proven reliable for the utility industry, the use of nat- superheater, as well as other components, in order to maxiural gas or oil fuels and heat recovery technology opens up mize the thermodynamic efficiency of the system. These feaa number of other cycles that offer significantly better tures are generally uneconomical in smaller capacity plants. In the basic cycle, superheated steam exits the boiler and enters the first stage of the steam turbine. Most of the steam passes completely through the steam turbine and is exhausted, drawn to a partial vacuum at a condenser served by external cooling water. The lower the pressure at the steam exhaust, which is set by the condenser operating temperature, the greater the amount of mechanical energy that can be obtained from the expanding steam, and the higher the power cycle efficiency. Typical condenser operating temperature and pressure are 90F (32C) and 1.5 in (3.8 cm) of Hg (about 0.75 psi), respectively. The condenser returns the exhaust Fig. 3-1 Simplified Diagram of Large Utility Steam Generation Reheat Cycle. Source: steam to liquid state by removing its latentCogen Designs, Inc.

or a given application, an on-site prime mover possesses several potential cost advantages over an electrically driven end-use device. These advantages are due largely to a higher utilization of input energy when compared with a central electric generation plant. Potential advantages include recovery of high-grade heat (as opposed to rejection of low-grade heat by central electric generation plants) and reduction of electrical transmission and end-use device inefficiencies. Economics do not always come out in favor of a more efficient end-use. Large centralized power plants also possess several advantages over the smaller user, including economy of scale, preferential fuel cost, relatively low staffing levels, and reserve capacity.

F

thermodynamic performance for large central power plants, as well as smaller facilities.

CONVENTIONAL UTILITY RANKINE STEAM CYCLE PLANTS



heat, which is rejected to the outside environment in the condensing part of the cycle. Liquid condensate is returned to the boiler through the deaerator to restart the process. Although boiler feedwater is chemically treated to remove impurities that contribute to scaling and fouling of boiler surfaces, concentrations of dissolved materials, which tend to build up over time, must be eliminated through a blowdown process that removes a small percentage of boiler feedwater from the system. To replace this blowdown loss, fresh make-up water is added before the feedwater enters the deaerator. In the deaerator, extraction steam is used to strip dissolved air from the feedwater. Larger utility plants may also use a reheat cycle to optimize cycle efficiency while avoiding excessive moisture in the low-pressure stages of the turbine. In the reheat cycle, the steam is withdrawn after partial expansion, re-superheated at constant pressure in the boiler, and introduced into a low-pressure turbine. A regenerative cycle is an additional efficiency enhancement that reduces condenser losses by using a

portion of available latent heat for deaeration and feedwater heating. Steam is extracted between high-pressure and low-pressure turbines and at intermediate points in the lowpressure turbine for use by the deaerator and feedwater heating heat exchangers. Figure 3-2 is a simplified schematic representation of a large utility subcritical steam power plant. It features a high-pressure and a low-pressure turbine, with regeneration and reheat. Table 3-1 lists historical average heat rates for electric utility steam-electric power plants. Recent fossil-fueled heat rates of 10,300 Btu/kWh (10,860 kJ/kWh) correspond to a thermal efficiency of 33%, with heat rates for nuclear steam-electric plants being slightly higher. Cycle efficiency is proportional to the difference between the temperature at which energy is added (at the boiler) and rejected (at the condenser). However, increases in system pressure and temperature are limited by the tolerance of component materials and decreases in condenser pressure and temperature are limited by ambient environmental conditions. As such, efficiency levels of conventional

Fig. 3-2 Subcritical Pressure, 2,400 psig (166 bar) Steam Turbine Cycle Heat Balance Diagram. Source: Babcock & Wilcox


Localized Versus Central Station Power Generation

Period1965-69 1970-74 1975-79 1980-84 1985-89 1990-93

Fossil-Fueled (Btu/kWh)10,429 10,436 10,386 10,451 10,391 10,323

Nuclear (Btu/kWh)11,463 10,934 10,930 10,952 10,771 10,694

Table 3-1 Average Heat Rates for Utility Steam-Electric Plants, 1965-1993. Source: U.S. DoE/EIA

plants have been relatively constant over the past two decades. Individual steam plant thermal efficiencies can range from under 30% to more than 40%. At the higher end, the more efficient plants use supercritical steam conditions and several stages of reheat. Due to cost and technical considerations, these types of plants have not been widely used in the United States, where, currently, the best steam cycles offer a thermal efficiency of about 34%. Based on these typical thermal efficiencies, a conventional steam cycle releases almost two-thirds of the energy in the fuel into the environment. Figure 3-3 illustrates how this occurs.

these energy losses, which, in this example, amount to 12% of the fuel energy content, are rejected to the outside environment through the boiler stack. Expansion of steam through the steam turbine is represented by the diagonal arrow running from the boiler to the stacked bar on the right entitled condenser In this . example, expansion releases about 452 Btu of useful mechanical work per pound (1,046 kJ per kg) of steam, which is converted by the generator into electric power. Efficiency losses in the generator itself, and other parasitic and auxiliary electric demands of the generating station, slightly reduce the electric energy available for export from the plant. By far, the largest source of inefficiency in the conventional steam cycle is fundamental to the thermodynamic principles underlying the cycle. The latent heat released through the condenser to the environment from the condensing steam after expansion in the steam turbine serves no useful purpose in the power cycle. In this example, roughly 830 Btu/lbm (1,930 kJ/kg) of steam is lost through the condenser. An additional 150 Btu/lbm (349 kJ/kg) of steam is described as water heating on the condenser bar below the condenser losses. This energy represents the steam extracted from the steam turbine and recovered for deaeration and feedwater heating. In summary, this representative steam cycle example indicates a fuel energy input requirement of 1,458 Btu (1,538 kJ) to produce 452 Btu (479 kJ) of useful work, for an overall power cycle efficiency of 31%. On a heat rate basis, this cycle requires about 11,010 Btu/kWh (11,613 kJ/kWh) of electric energy produced.

CONVENTIONAL UTILITY GAS TURBINE CYCLE PEAKING PLANTSFig. 3-3 Conventional Power Cycle Energy Utilization. Source: Cogen Designs, Inc.

The left stacked bar, entitled Boiler, represents the energy added to a pound of feedwater as it passes into and through the three sections of the steam generator. Since not all of the energy content in the fuel is available to increase the steam energy content, and since the boiler itself has energy losses, in total, 1,458 Btu (1,538 kJ) of fuel energy must be added per pound of steam to increase the energy content of the steam from approximately 200 Btu/lbm (465 kJ/kg) to 1,500 Btu/lbm (3,488 kJ/kg), a 1,300 Btu/lbm (3,023 kJ/kg) increase. The majority of

The other basic thermodynamic cycle used by electric utility plants to generate electric power is the open-cycle gas turbine, which operates on the Brayton cycle. Whereas steam cycle plants are used for base and intermediate load applications, gas turbine cycle plants are used by many utilities to serve peaking power requirements. Whereas the steam cycle operates essentially on a closed loop, the gas turbine uses air in a once-through open cycle. Since air is essentially a non-condensable fluid at normal operating temperatures, the gas turbine cycle does not use a boiler and condenser. Instead, a combustor burns fuels such as natural gas or oil in direct contact with compressed air, directing the mixture into a power turbine, which then expands the gases and exhausts them directly into the



atmosphere. While the steam cycle uses boiler feed pumps to elevate feedwater to the required operating pressures, the gas turbine uses a multiple-stage compressor to elevate the pressure of the incoming air stream to operating pressure. The power turbine, through a common shaft, supplies the energy required for this air compression. There are numerous other differences between the conventional gas turbine cycle and the conventional steam turbine cycle described above. Whereas the steam turbine operates at high pressure of up to 2,000 psig (139 bar) and a relatively moderate temperature of about 1,000F (538C), the gas turbine operates at a relatively moderate pressure of up to 460 psi (33 bar) and a high temperature of about 2,000F (1,093C). Whereas the steam turbine exhausts to a deep vacuum of about 0.75 psia (5 kPa) at a low temperature, the gas turbine exhausts to atmospheric pressure (about 14.7 psi or 101.4 kPa), but at a temperature of about 1,000F (538C). Despite these differences, the thermodynamic efficiency of the two cycles are relatively close, though historically, the basic gas turbine cycle has been slightly less thermally efficient than the steam cycle. Figure 3-4 is a basic heat balance diagram for a simplecycle gas turbine system. Currently, some of the newest gas turbine plants offer thermodynamic efficiencies superior to the conventional steam cycle plants with simple-cycle heat rates below 10,000 Btu/kWh (10,548 kJ/kWh) and thermal efficiencies greater than 34%. Still, gas turbine plants often do not match the operating cost-efficiency of conventional steam plants because they operate on higher-cost natural gas and distillate oil fuels, as opposed to lower-cost boiler fuels such as coal. However, due to their relative simplicity, gas turbine-cycle plants have much lower capital costs than steam-cycle plants with full environmental control systems. As a result, the gas turbine plants offer distinct

Fig. 3-4 Basic Heat balance Diagram for a Simple-Cycle Gas Turbine System.

advantages over conventional steam-cycle plants for low load factor peaking service, where the impact of higher operating cost is less significant than capital cost.

COMBINED-CYCLESThe fact that a gas turbine exhausts large quantities of air/gases at temperatures around those of a fairly efficient steam cycle (1,000F or 538C) allows for significant enhancements in overall cycle efficiency. As shown in Figure 3-5, if the exhaust from a gas turbine is fed to a heat recovery steam generator (HRSG), the steam that is raised can drive a steam-powered cycle. The steam cycle converts about 15 to 20% of the rejected heat energy into additional electric power, resulting in a combined-cycle thermal efficiency of about 43 to 50%. On a heat rate basis, this plant will require only about 7,000 to 8,000 Btu/kWh (7,380 to 8,440 kJ/kWh), compared with 10,000 to 11,000 Btu/kWh (10,550 to 11,600 kJ/kWh) for conventional-cycle plants. The combined-cycle plant capital cost per kW falls between that of a simple gas turbine cycle and a conven-

Fig. 3-5 Combined Cycle System Featuring Gas Turbine, Heat Recovery Steam Generator and Condensing Steam Turbine.


Localized Versus Central Station Power Generation

tional steam cycle. Its exceptional thermal efficiency ratings allow it to compete favorably for intermediate and baseload service.

OTHER ADVANCED CYCLESThere are a number of other advanced cycles that can be applied to gas turbine or combined cycle systems: The steam injection cycle is similar in many respects to a combined-cycle plant. These plants generate steam from recovered heat from the basic gas turbine cycle. However, instead of using the steam to drive a second power cycle, steam injection-cycle plants inject the steam directly into the gas turbine. The increased total mass flow and energy input to the gas turbine result in significantly enhanced system capacity and overall cycle efficiency. Since steam is used in an open cycle as opposed to a closed condensing steam turbine cycle, the overall efficiency will be slightly lower than that achieved with a combinedcycle. Water usage is also considerably greater. A regenerator cycle uses a heat exchanger, or recuperator, which transfers heat from turbine exhaust to compressor discharge air prior to combustion of fuel. Recovered heat displaces a portion of fuel that would otherwise be required, thereby enhancing overall cycle efficiency. The reheat cycle uses an additional combustor or reheat element in which additional fuel is combusted using the oxygen present in the exhaust gas. The reheat cycle increases the thermal efficiency of the turbine cycle by increasing the average temperature of the gases doing expansion work in the turbine section. The intercooling cycle is used to decrease the work of compression required by the gas turbine cycle by cooling the air in the middle of its compression cycle. For this purpose, two or more compressor sections are used. The intercooler is a heat exchanger through which air exiting the low-pressure compressor passes prior to entering the high-pressure compressor. Intercooling results in lower high-pressure compressor exit temperature, which allows for higher pressure ratios and, therefore, a significant increase in turbine capacity. The humid air turbine (HAT) cycle is currently under development and is expected to be in production within a few years. In the HAT cycle, exhaust heat from the gas turbine is used to heat and humidify the combustion air. The HAT cycle will operate with intercooling and high-pressure ratios and is expected to offer a thermal efficiency of 45% (LHV) or greater. The HAT cycle is also being designed to operate with fuel from a coal gasifier.

There have also been advancements in coal-fired steam cycle plants. Development has been driven by the demand to minimize air emissions and waste production as much as for improved cycle efficiency. Fluidized-bed combustion (FBC) is a technology that has widespread appeal because of its low emission characteristics. In an FBC unit, solid, liquid, or gaseous fuels, together with inert materials such as sand, silica, or alumina, and/or sorbents such as limestone are kept suspended through the action of primary air distributed below the combustor floor. Fluidization promotes turbulence, which makes the mass of solids behave more like a liquid. The results of FBC are lower and more uniform distribution of temperature. Fluidized-bed configurations include bubbling-bed and circulating-fluidized-bed designs, with atmospheric- or elevated-pressure operations. Research and development for enhancing cycle efficiency of both conventional steam and gas turbine cycle plants is largely focused on improved materials. Given that specific capacity and cycle efficiency are closely tied to increased firing temperatures, materials technology has become a limiting factor. Component development technology has centered on ceramics and advanced alloys for high temperature. Increasing pressure and temperature tolerance due to improved component material strength and turbine blade cooling will allow for improved cycle efficiency.

POWER PLANT COSTSTable 3-2 shows comparative characteristics of current and developing fossil-fueled generating cycle technologies by one nationally recognized source. Included are comparative heat rates, capital (overnight), and operations and maintenance (O&M) costs in 1987 $/MWh. O&M costs are differentiated between fixed and variable components. Overnight costs do not include financing costs during construction and other peripheral costs, such as site purchase and development. Actual turnkey costs can, therefore, be significantly higher. Additionally, the labor and material costs associated with centralized power plant construction, as well as localized on-site systems, vary from region to region. Regional multipliers for new construction costs that may be applied to the various new fossil-fueled generating technologies can be found in standard construction estimating references.

On-Site Application of Conventional and Advanced Cycle Plants

Use of the conventional and advanced steam turbine



Technology Pulverized coal Advanced coal Oil/gas steam Combined cycle Advanced combined-cycle Combustion turbine Advanced combustion turbine

Year Available 1990 2000 1990 1990 2005 1990 1990

Overnight Costs 1987 $/kW 1,213 1,345 785 486 476 352 566

Heat Rate Btu/kWh 9,649 8,809 9,477 7,707 7,369 11,456 9,149

Fixed O&M 1987 $/kW 17.6 33.3 5.3 3.7 6.7 0.6 10.0

Variable O&M 1987 $/MWh 4.3 2.5 5.2 3.7 2.6 6.2 5.0

Table 3-2 Characteristics of New Fossil Fuel Electric Power Generation Technologies. Source: U.S. DOE/EIA (Argonne National laboratory)

Rankine cycle and gas turbine Brayton cycle are not limited to large utility applications. A

Combined Heating, Cooling and Power Handbook_ Technologies & Applications

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