Chapter 6 Flashcards
Heat Engines
The devices that convert heat to work
- They receive heat from a high-temperature source
- They convert part of this heat to work (usually in the form of a rotating shaft)
- They reject the remaining waste to a low-temperature sink
- They operate on a cycle
High-temperature source
Solar energy, oil furnace, nuclear reactor, etc.
Low-temperature sink
The atmosphere, rivers, etc.
Heat engines and other cycle devices usually involve a fluid to and from which heat is transferred while undergoing a cycle. This fluid is called the _____
Working fluid
Work can always be converted to ___ directly and completely
Heat
The reverse is not true
Qin
Amount of heat supplied to steam in boiler from a high-temperature source (furnace)
Qout
Amount of heat reject from steam in condenser to a low-temperature sink
Wout
Amount of work delivered by steam as it expands in turbine
Win
Amount of work required to compress water to boiler pressure
Steam power plant
A portion of the work output of a heat engine is consumed internally to maintain continuous operation
Wnet,out = Wout-Win (kJ)
Wnet,out = Qin-Qout (kJ)
Thermal efficiency
Net work output/total heat input
The most efficient heat engines
Reject almost one-half of the energy they receive as waste heat
Can we save Qout?
In a steam power plant, the condenser is the device where large quantities of waste heat is rejected to rivers, lakes, or the atmosphere
Can we not just take the condenser out of the plant and save all that waste energy?
No - without a heat rejection process in a condenser, the cycle cannot be completed
A heat-engine cycle cannot be completed without…
Rejecting some heat to a low-temperature sink
The Second Law of Thermodynamics: Kelvin Planck Statement
It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work
No heat engine can have a thermal efficiency of 100%, or as for a power plant to operate, the working fluid must exchange heat with the environment as well as the furnace
The impossibility of having a 100% efficient heat engine
Limitation that applies to both the idealized and the actual heat engines
Not due to friction or other dissipating effects
The transfer of heat from a low-temperature medium to a high-temperature one requires special devices called _____
Refrigerators
Refrigerators
Cyclic devices
The working fluid used in the refrigeration cycle is called a _____
Refrigerant
The most frequently used refrigeration cycle is the
Vapor-compression refrigeration cycle
In a household refrigerator, the freezer compartment where heat is absorbed by the refrigerant serves as the _____
Evaporator
In a household refrigerator, the coils usually behind the refrigerator where heat is dissipated to the kitchen air serve as the ______
Condenser
Basic components of a refrigeration system
Expansion valve, condenser, compressor, evaporator
The efficiency of a refrigerator is expressed in terms of the
Coefficient of performance (COP)
The objective of a refrigerator is to…
Remove heat (Ql) from the refrigerated space (cooled space)
Can the value of COPr be greater than unity (1)?
Yes
Can the value of COPhp be lower than unity?
No
What does COPhp = 1 represent?
Violates 2nd law (Ql = 0)
The objective of a heat pump
Supply heat Qh into the warmer space
Most heat pumps in operation today have a seasonally averaged COP of ____
2 to 3
Most existing heat pumps use ____ as the heat source in winter
The cold outside air
Heat pump efficiency in cold climates
Drops considerably when temperatures are below the freezing point
Geothermal (ground-source) HP that use the ground as the heat source can be used
Geothermal HP
More expensive to install but more efficient
_____ are basically refrigerators whose refrigerated space is a room or a building instead of the food compartment
Air conditioner
The COP of a refrigerator decreases with…
Decreasing refrigeration temperature
Not economical to refrigerate to a lower temperature than needed
The amount of heat removed from the cooled space in Btu’s for 1 Wh of electricity consumed
Energy Efficiency Rating (EER)
EER = 3.412 COPr
The work supplied to a heat pump is used…
To extract energy from the cold outdoors and carry it into the warm indoors
The Second Law of Thermodynamics: Clausius Statement
It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer or heat from a lower-temperature body to a higher-temperature body
States that a refrigerator cannot operate unless its compressor is driven by an external power source, such as an electric motor
Kelvin-Planck and the Clausius statements
Are equivalent in their consequences, and either statement can be used as the expression of the second law of thermodynamics
Any device that violates the Kelvin-Planck statement also violates the Clausius statement, and vice versa
A cult of hot coffee
Does not get hotter in a cooler room
Transferring heat to a wire
Will not generate electricity
Transferring heat to a paddle wheel
Will not cause it to rotate
Major uses of the second law
- The second law may be used to identify the direction of processes
- The second law also asserts that energy has quality as well as quantity
- The second law is also used in determining the theoretical limits for the performance of commonly used engineering systems, such as heat engines and refrigerators, as well as predicting the degree of completion of chemical reactions
Second law - process
Occur in a certain direction and not in the reverse direction
Must satisfy both the 1st and 2nd laws of thermodynamics to proceed
Bodies with relatively large thermal masses can be modeled as _____
Thermal energy reservoirs
Thermal energy capacity
Mass * specific heat
A hypothetical body with a relatively large thermal energy capacity that can supply or absorb final amounts of heat without undergoing any change in temperature is call a _____
Thermal energy reservoir or just a reservoir
In practice, large bodies of water such as oceans, lakes, and rivers as well as the atmospheric air can be modeled accurately as thermal energy reservoirs because…
Of their large thermal energy storage capabilities or thermal masses
Source
Supplies energy in the form of heat
Sink
Absorbs energy
Any device that violates the first or second law
Perpetual-motion machine
A device that violates the first law (by creating energy)
PMM1
A device that violates the second law
PMM2
A process that can be reversed without leaving any trance on the surroundings
Reversible process
Deliver the most and consume the least work
A process that is not reversible
Irreversible process
All the processes occurring in nature are ____
Irreversible
Why are we interested in reversible processes?
- They are easy to analyze
2. They serve as idealized models (theoretical limits) to which actual processes can be compared
The facts that cause a process to be irreversible are called ________
Irreversibilities
Irreversibilities include:
Friction, unrestrained expansion, mixing of two fluids, heat transfer across a finite temperature difference, electric resistance, inelastic deformation of solids, and chemical reactions
Heat transfer through a temperature difference is ____
Irreversible
If no irreversibilities occur within the boundaries of the system during the process
Internally reversible process
If no irreversibilities occur outside the system boundaries
Externally reversible
It involves no irreversibilities within the system or its surroundings
Totally reversible process
A totally reversible process involves:
No heat transfer through a finite temperature difference, no nonquasi-equilibrium changes, and no friction or other dissipative effects
A reversible process involves no ____
Internal and external irreversibilities
Reversible Isothermal Expansion
Th = constant
Reversible Adiabatic Expansion
Temperature drops from TH to TL
Reversible Isothermal Compression
TL = constant
Reversible Adiabatic Compression
Temperature rises from TL to TH
The Reversed Carnot Cycle
The Carnot heat-engine cycle is a totally reversible cycle
Therefore, all the processes that comprise that it can be reversed, in which case it becomes the Carnot refrigeration cycle
The Carnot Principles
- The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs
- The efficiencies of all reversible heat engines operating between the same two reservoirs are the same
All reversible heat engines operating between the same two reservoirs have the ____
Same efficiency (2nd Carnot principle)
A temperature scale that is independent of the properties of the substances that are used to measure temperature is called a _____
Thermodynamic Temperature scale
Thermodynamic Temperature Scale is called the ____
Kelvin Scale
The temperatures on the Thermodynamic Temperature Scale are called _____
Temperatures
Efficiency > reverse efficiency (heat engine)
Impossible heat engine
Efficiency < reversible efficiency (heat engine)
Irreversible heat engine
Efficiency = reversible efficiency (heat engine)
Reversible heat engine
_____ the most efficient of all heat engines operating between the same high- and low-temperature reservoirs
Carnot heat engine
Can we use C unit for temperature?
No
How do you increase the thermal efficiency of a Carnot heat engine?
Decrease TL
Increase TH
How do you increase the thermal efficiency for actual heat engines
Decrease QL
Increase QH
How do you increase the COP of a Carnot refrigerator or heat pump?
Increase TL
Decrease TH
How do you increase the COP of actual refrigerators or heat pumps?
Increase QL
Decrease QH
COPr < COPr, rev
Irreversible refrigerator
COPr = COPr, rev
Reversible refrigerator
COPr > COPr, rev
Impossible refrigerator
The COP of a reversible refrigerator or heat pump is the…
Maximum theoretical value for the specified temperature limits
The COPs of both the refrigerators and heat pumps ____ as TL ____
Decrease, decrease
Requires more work to absorb heat from lower-temperature media
