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