Energy and the Environment

Energy and the Environment Fall 2012 Instructor: Xiaodong Chu Email：chuxd@sdu.edu.cn Office Tel.: 81696127

Flashbacks of Last Lecture • The time rate of exchange of work and heat quantities determine the mechanical or thermalpower that can be produced • In most cases of steady heat transfer from a hot to a cold environment, the time rate of heat transfer where u is the heat transfer coefficient,and uA is called the thermal conductance

Flashbacks of Last Lecture • The most common fossil fuels are hydrocarbons (碳氢化合物), i.e., mixture of molecules composed of carbon and hydrogen • When a mixture of fuel and air is burned, heat may be transferred from the hot combustion products to a colder fluid and the amount of heat available for this purpose is called the fuel heating value (燃料热值) and is usually expressed in energy units per unit mass of fuel

Flashbacks of Last Lecture • Generating mechanical power from fossil fuel must utilize the combustion process to change the temperature and/or pressure of a fluid and then find a way to use the fluid to make mechanical work by moving a piston or turning a turbine • Analyze ideal devices in which a fluid is heated and cooled, and produces or absorbs work, as the fluid moves through a cycle and such a device can be called a heat engine • The thermodynamic efficiency(热力学效率)ηthisthe amount of work produced (w) in proportion to the amount of heat that is added (q), i.e., ηth ≡w/q

Flashbacks of Last Lecture • The Carnot cycle • The Carnot cycle is illustrative of the second law limits on the simplest of heat engine cycles • It is sustained by two heat reservoirs(热库), a hot one of temperature Th and a cold one of Tc • Consider the heat engine to be a cylinder equipped with a movable piston and enclosing a fluid of unit mass • The cycle consists of four parts: two expansions(膨胀) and two compressions (压缩)

Thermodynamic Principles: Ideal Heat Engine Cycles • The Rankine cycle • The Rankine cycle is a steam cycle by “burning” fuel to generate mechanical power; for instance, in a steam power plant, fuel mixed with air is burned to heat water in a boiler to convert to steam, which then powers a turbine • In an efficient steam plant, nearly all the fuel’s heating value is transferred to the boiler fluid, but of course only part of that amount is converted to turbine work

Thermodynamic Principles: Ideal Heat Engine Cycles • In an idealized Rankine cycle, since the process of 5->6 is adiabatic (绝热) and isentropic (等熵), steady flow work per unit mass of steam produced by the turbine is equal to the enthalpy change across the turbine (by equation 3.20) and can be expressed (by equation 3.18) as • There is a similar expression for the work required to operate the pump since the process of 1->2isalso adiabatic and isentropic • The net work produced in the cycle can be expressed as

Thermodynamic Principles: Ideal Heat Engine Cycles • Because the heating and cooling processes of the ideal Rankine cycle (2->5, 6->1) occur at constant pressure while the work processes are isentropic (5->6, 1->2), the thermodynamic efficiency may be expressed as

Thermodynamic Principles: Ideal Heat Engine Cycles • Thermodynamic efficiency of the Rankine cycle depends explicitly upon the properties of the working fluid, e.g., its pressure or temperature • The cycle efficiency is increase if the boiler pressure (and steam temperature) is increased • Critical point (临界点)of water • Superheating(过热) • The basic cycle is capable of improvements in efficiency by use of internal heat exchange at intermediate pressure levels • Reheating(再热) • Regenerative feed water heating(给水回热)

Thermodynamic Principles: Ideal Heat Engine Cycles • Thermodynamic efficiency of the ideal Rankine cycle is in the range of 30-45%, but actual steam plants have lower-than-ideal efficiencies (even the best ones seldom exceed 40% thermal efficiency) • The steam turbine and feed water pumps are not 100% efficient, resulting in less net work • Mechanical power is required to operate the boiler fans and condenser cooling water pumps, reducing the net power output • The boiler does not transfer all of the fuel higher heating value to the working fluid because the flue gases exit from the boiler and excess air is used (above that required for stoichiometric combustion)

Thermodynamic Principles: Ideal Heat Engine Cycles • The steam turbine for an electric power plant experiences a large change in pressure between entrance and exit, during which the steam density decreases greatly, requiring ever longer turbine to extract power from the steam flow

Thermodynamic Principles: Ideal Heat Engine Cycles • The Brayton cycle • The Brayton cycle is associated with gas turbine that can be used for aircraft propulsive engine, naval vessel propulsion, high-speed locomotives, and electric power production • Thesimplest gas turbine plant consists of a compressor and turbine in tandem attached to the shaft that delivers mechanical power, and a combustion chamber is situated between the compressor and turbine

Thermodynamic Principles: Ideal Heat Engine Cycles • Since the power produced by the turbine and that absorbed by the compressor is each greater than the net power output, the total power is considerably greater than the net power, which means the aerodynamic efficiencies of the compressor and turbine need be high so that as much net power is produced as possible • The thermodynamic efficiency of the cycle can be improved by increasing the turbine inlet temperature, but the latter is limited by the high-temperature strength of the turbine blades • For the simple Brayton cycle, the best thermodynamic efficiencies are about 33% • By use of heat exchange between the hot exhaust gas and the compressed gas entering the combustion chamber, this efficiency may be increased by about four percentage points

Thermodynamic Principles: Ideal Heat Engine Cycles • In the Brayton cycle, the steady flow work produced by the turbine and that absorbed by the compressor are each equal to the change in enthalpy of the fluid flowing through them • The net work per unit mass of fluid is the difference between the turbine work and the compressor work • The heat q added to the fluid leaving the compressor is just the increase in enthalpy in the constant pressure process 2->3

Thermodynamic Principles: Ideal Heat Engine Cycles • The thermodynamic efficiency of the Brayton cycle is • The thermodynamic efficiency of the ideal Brayton cycle depends upon the pressure ratio p2/p1 = p3/p4 (increases with increasing pressure ratio) and the thermodynamic properties of air and combustion products, expressed as

Thermodynamic Principles: Ideal Heat Engine Cycles • The compressor and turbine of a gas turbine power plant are usually built into a single rotor as shown with the combustion chamber sandwiched between the compressor and turbine • The rotor shaft delivers the net power difference between the turbine and compressor powers to the electric generator

Thermodynamic Principles: Ideal Heat Engine Cycles • Combined Brayton and Rankine cycle • The combustion products gas stream leaving the gas turbine carries with it portion of the fuel heating value that was not converted to work, which may be used to generate steam in a boiler and produce additional work without requiring the burning of more fuel • The use of a gas turbine and steam plant to produce more work from a given amount of fuel than either alone could produce is called a combined cycle

Thermodynamic Principles: Ideal Heat Engine Cycles • The thermodynamic efficiency of a combined cycle is • In the combined cycle gas plus steam power plant, the thermal efficiency of the steam cycle is considerably lower than that for the most efficient steam-only power plant, because the gas turbine exhaust gas is not as hot as the combustion gas in a normal boiler and because the gas turbine requires much more excess air than does a steam boiler, but nevertheless the combined cycle plant provides an overall fuel efficiency that is higher than that for any single cycle plant

Thermodynamic Principles: Ideal Heat Engine Cycles • You should master processes of the Rankine cycle and the Brayton cycle on page 48 and page 53

Energy and the Environment