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Entropy equation in Chapter 4: control mass approach 4... · 2018. 9. 13. · Entropy equation in...
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Energy Equation
Entropy equation in Chapter 4: control mass approach
The second law of thermodynamics
Availability (exergy)– The exergy of a system is the maximum useful work possible during a process that brings the system
into equilibrium with a heat reservoir.
– The availability function is a characteristic of the state of the system and the environment which acts as a reservoir at constant pressure (p0 ) and constant temperature (T0 ).
(4.27)
Energy Equation
Entropy equation in Chapter 4: control mass approach
Availability/exergy
Carnot cycle efficiency
,
, ,
Energy Equation
Entropy equation in Chapter 4: control mass approach
The second law of thermodynamics
From state 1 to 2
Maximum useful work
Maximum useful work obtainable from the specified change
Energy Equation
Energy and entropy equation in Chapter 4: control mass approach
The second law of thermodynamics
(4.27)
Energy Equation
Energy and entropy equation in Chapter 4: control mass approach
The second law of thermodynamics
(4.27)
the second T‐ds relation
Energy Equation
Energy and entropy equation in Chapter 4: control mass approach
The second law of thermodynamics
(4.27)
Energy Equation
Energy and entropy equation in Chapter 4: control mass approach
Entropy change of ideal gas
From the first T ds relation From the second T ds relation
Energy Equation
Energy equation in Chapter 4: control volume approach
The first law of thermodynamics
In the control volume approach, there are flow in and out.
The first law
Energy Equation
Energy equation in Chapter 4: control volume approach
Heat addition term
Work term
Work done by the mass in the control volume + work associated with mass flow
Body force
PvmmPVPW
VPxAPW
normal
normal
I
ii
I
ii gzm
dtmgzd
DtmgzDm
dtmd
DtmD
11
Energy Equation
Energy equation in Chapter 4: control volume approach
Control mass
Control volume
For stationary control volume
(4.38)
dtdVP
dtdW
normal
Energy Equation
Energy equation in Chapter 4: control volume approach
Control volume
Energy Equation
Entropy equation in Chapter 4: control volume approach
Entropy equation
For a stationary control volume
Energy Equation
Entropy equation in Chapter 4: control volume approach
Actual work
Useful work (useful part of the actual work)
Maximum useful work
Maximum useful workActual workUseful work
Energy Equation
Entropy equation in Chapter 4: control volume approach
Energy equation
Useful work (useful part of the actual work)
Useful work (or useful actual work)
Partial derivative? Stationary!
Energy Equation
Entropy equation in Chapter 4: control volume approach
Multiplying , then subtracting from (4.44)
Express the actual useful work with entropy
Energy Equation
Entropy equation in Chapter 4: control volume approach
Maximum useful work
Irreversibility
=0
Energy Equation
Entropy equation in Chapter 4: control volume approach
Irreversibility
Energy Equation
Entropy equation in Chapter 4: control volume approach
Example
Energy Equation
Entropy equation in Chapter 4: control volume approach
Example
(4.27)
Maximum useful work
Energy Equation
Entropy equation in Chapter 4: control volume approach
Example
Actual work
Useful work
Useful work
Energy Equation
Entropy equation in Chapter 4: control volume approach
Example
Irreversibility
Maximum useful work
Useful work
Energy Equation
Entropy equation in Chapter 4: control volume approach
Example
Irreversibility
Contents
1. Introduction
2. Nonflow Process
3. Thermodynamic Analysis of Nuclear Power Plants
4. Thermodynamic Analysis of A Simplified PWR System
5. More Complex Rankine Cycles: Superheat, Reheat, Regeneration, and Moisture Separation
6. Simple Brayton Cycle
7. More Complex Brayton Cycles
8. Supercritical Carbon Dioxide Brayton Cycles
Introduction
Summary of the working forms of the first and second laws
First law for control volume
Second law
Working forms
Contents
1. Introduction
2. Nonflow Process
3. Thermodynamic Analysis of Nuclear Power Plants
4. Thermodynamic Analysis of A Simplified PWR System
5. More Complex Rankine Cycles: Superheat, Reheat, Regeneration, and Moisture Separation
6. Simple Brayton Cycle
7. More Complex Brayton Cycles
8. Supercritical Carbon Dioxide Brayton Cycles
Thermodynamic Analysis of Nuclear Power Plants
Analysis of NPPs
Provides the relation between the mixed mean outlet coolant temperatures (or enthalpy forBWR) of the core through the primary and secondary systems to the generation of electricityat the turbine.
Cycles used for the various reactor types and the methods of thermodynamic analysis of these cycles are described. Rankine cycle for steam‐driven electric turbines(PWR, BWR) Brayton cycle can be considered for gas cooled reactor systems Both cycles are constant‐pressure heat addition and rejection cycles for steady‐flow operation
Thermodynamic Analysis of Nuclear Power Plants
Analysis of NPPs
Simple Brayton cycle
Maximum temperature of the Brayton cycle (T3) is set by turbine blade and gas cooledreactor core material limits far higher than those for the Rankine cycle, whichis set by liquid-cooled reactor core materials limits .
Thermodynamic Analysis of Nuclear Power Plants
Efficiency to assess the thermodynamic performance of components and cycles
Thermodynamic efficiency (or effectiveness): The ratio between the actual useful work and the maximum useful work
Isentropic efficiency: Special case of thermodynamics efficiency Adiabatic case Important for devices such as pump, turbine, compressor
Thermal efficiency: The fraction of the heat input that is converted to net work output The cycle efficiency that we are interested in.
Thermodynamic Analysis of Nuclear Power Plants
Thermodynamic efficiency (or effectiveness):
Maximum useful work No entropy generation
Fixed, non‐deformable control volume with zero shear work Negligible kinetic and potential energy differences
Thermodynamic Analysis of Nuclear Power Plants
Isentropic efficiency:
Maximum useful work
For steady‐state condition
Thermodynamic Analysis of Nuclear Power Plants
Isentropic efficiency:
Useful actual work for an adiabatic control volume with zero shear and negligible kinetic andpotential energy differences
For steady state
Thermodynamic Analysis of Nuclear Power Plants
Thermal efficiency:
Cycle efficiency
For adiabatic systems, it is not useful.
Thermodynamic Analysis of Nuclear Power Plants
Example problem (a)
Thermal efficiency:
Thermodynamic efficiency (or effectiveness): 100
70
30
30100 30%
, 1 1300600 · 100 50
(a) (b)
, 1 13001000 · 100 70
3050 60%
3070 42.9%
Thermodynamic Analysis of Nuclear Power Plants
Example problem (b)
Maximum useful work no entropy generationEntropy in = entropy out
Entropy decrease in hot reservoirs = entropy increase in cold reservoir
Thermodynamic Analysis of Nuclear Power Plants
Example problem (b)
1275 16%
, 35.2
, 39.8
1239.8 30.2%
Contents
1. Introduction
2. Nonflow Process
3. Thermodynamic Analysis of Nuclear Power Plants
4. Thermodynamic Analysis of A Simplified PWR System
5. More Complex Rankine Cycles: Superheat, Reheat, Regeneration, and Moisture Separation
6. Simple Brayton Cycle
7. More Complex Brayton Cycles
8. Supercritical Carbon Dioxide Brayton Cycles
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
One component with multiple inlet and outlet flow streams operating at steady state and surround it with a non‐deformable, stationary control volume
Energy equation
Steady Negligible Non‐deformable No shear
Negligible kinetic energy
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
One component with multiple inlet and outlet flow streams operating at steady state and surround it with a non‐deformable, stationary control volume
Energy equation
Mass conservation equation
• Turbine 0, 0 Left side > 0
• Condenser 0, 0 Left side > 0
• Pump 0, 0 Left side < 0
• Steam generator 0, 0 Left side < 0
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Turbine
Shaft work, no heat addition adiabatic turbine
Actual turbine work is related to the ideal work
by the component isentropic efficiency.
Isentropic efficiency
Pump
Shaft work, no heat addition adiabatic pump
,
Recirculating steam generator
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Steam generator
Heat transfer between the primary and the secondary
Pinch point temperature
– Minimum temperature difference between the primary and the secondary
– Inversely related to the heat transfer area
Small temperature difference: large heat transfer area
Large temperature difference: small heat transfer area
– Directly related to the irreversibility of the SG
Large temperature difference: large irreversibility
– Important design choice based on tradeoff between
cost and irreversibility
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Steam generator
Reactor plant
Actual work
Positive or negative?
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Reactor plant
Maximum useful work
– In section 6.4.2.3
Maximum useful work expressed
with the secondary side properties
Thermodynamic efficiency or effectiveness:
Single turbine case
Review
First law analysis in a simplified PWR system Stationary, non‐deformable control volume operating at steady state
Turbine and pump
Steam generator and condenser
=0
Review
First law analysis in a simplified PWR system Stationary, non‐deformable control volume operating at steady state
Thermodynamic efficiency and cycle thermal efficiency
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Assume that the turbine and pump have isentropic efficiencies of 85%.
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Example 6.4: Thermodynamic Analysis of a Simplified PWR Plant
Draw the temperature‐entropy (T‐s) diagram for this cycle.
7.75 MPa
6.89 kPa
15.5 MPa
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Example 6.4: Thermodynamic Analysis of a Simplified PWR Plant
Compute the ratio of the primary to secondary flow rates.
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Example 6.4: Thermodynamic Analysis of a Simplified PWR Plant
Compute the nuclear plant thermodynamic efficiency.
– Calculate the enthalpies of each state!
State 1
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Example 6.4: Thermodynamic Analysis of a Simplified PWR Plant
State 2
– Isentropic efficiencies of 85%.
– Isentropic process
– ,
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Example 6.4: Thermodynamic Analysis of a Simplified PWR Plant
State 3
–
State 4
– Isentropic efficiency
– Isentropic process
1 · ·
1 · ·
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Example 6.4: Thermodynamic Analysis of a Simplified PWR Plant
Compute the nuclear plant thermodynamic efficiency.
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Example 6.4: Thermodynamic Analysis of a Simplified PWR Plant
Compute the cycle thermal efficiency.
–
– ,
Thermal efficiency (cycle thermal efficiency)
= plant thermodynamic efficiency
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Improvements in thermal efficiency for this simple saturated cycle
Pressure
– Increase the pump outlet pressure/decrease turbine outlet pressure
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Improvements in thermal efficiency for this simple saturated cycle
Pressure
– Increase the pump outlet pressure/decrease turbine outlet pressure
Added heat
Rejected heat
Net work
Efficiency
Thermodynamic Analysis of A Simplified PWR System
First Law Analysis of a Simplified PWR System
Improvements in thermal efficiency for this simple saturated cycle
Pressure
– Increase the pump outlet pressure/decrease turbine outlet pressure
Added heat
Rejected heat
Net work
Efficiency