The 2nd Law of Thermodynamics
-energy moves from higher states to lower states
-energy has both a quality and a quantity
-used to determine theoretical limits of device performance and the completion rate of chemical reactions
Quality
implies the direction as well as the degree of degradation of energy in a process
A process must satisfy
both the 1st and 2nd laws of thermodynamics to proceed
Work can always...
be converted to heat directly and completely, but the reverse is not true
Heat engines
-devices that convert heat to work
-receive heat
-convert part of this heat to work
-reject remaining waste heat
-operate on a cycle
Working fluid
a fluid that heat engines involve that heat is transferred by in a cycle
thermal reservoirs
-bodies with relatively large thermal masses
-has relatively large thermal energy capacity that can supply or absorb finite amounts of heat without undergoing any change in temperature
Examples of thermal bodies
lakes, river, oceans, and atmospheric air
How much energy that is received from waste heat is rejected by presently engineered engines?
about one half
It is ___________ for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work
impossible
no ______________ can have a thermal efficiency of 100%
heat engine
What causes a heat engine to have an efficiency of less than 100%
a limitation that applies to both the ldealized and the actual heat engines
refrigerators
transfers heat from a low-temperature medium to a high-temperature medium
refrigerators are ______________, like heat engines
cyclic devices
What is the most used refrigeration cycle?
vapor-compression refrigeration cycle
What is the objective of a refrigerator?
to remove heat (QL) from the refrigerated space
What is the objective of a heat pump?
supply QH into the warmer space
What is the seasonal average of a heat pump in operation today?
2 to 3
What is used as a heat source when the efficiency drops considerably due to temperatures below freezing?
geothermal HP
Geothermal HP are_____________ to install but are _____________
more expensive; more efficient
The COP of a refrigerator ____________________ with decreasing refrigeration temperature
decreases
The Energy Efficiency Rating (EER)
the amount of heat removed from the cooled space in Btu's for 1 watthour of electricity consumed
Clausius Statement
-It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from lower- temp body to a higher-temp body
-It states that a refrigerator cannot operate unless its compressor is driven by an externa
reversible process
a process that can be reversed without leaving any trace on the surroundings
all processes in nature are _________________
irreversible
Why are we interested in reversible processes?
they are easy to analyze and serve as idealized models
Reversible processes deliver the ______________ work and consume the ______________ work
most; least
What are two familiar reversible processes?
frictionless pendulum and a quasi-equilibrium expansion and compression of a gas
Friction renders a process __________________
irreversible
irreversibilites
factors that cause a process to be irreversible
irreversibilities include:
friction, unrestrained expansion, mixing of two fluids, heat transfer across a finite temperature difference, electric resistance, inelastic deformation of solids, chemical reactions
Heat transfer through a temperature difference is
irreversible
internally reversible process
If no irreversibilities occur within the boundaries of the system during the process
externally reversible process
If no irreversibilities occur outside the system boundaries
totally reversible process
it involves no irreversibilities within the system or its surroundings
a totally irreversible process involves no...
heat transfer through a finite temperature diff, non-quasi equilibrium changes, friction
the carnot heat engine cycle is __________________
completely reversible
The Carnot Principles
1. The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs
2. The efficiencies of all reversible heat engines operating between the same two reservoirs are the same
thermodynamic temperature scale
A temperature scale that is independent of the properties of the substances that are used to measure temperature
What is the most effective of all heat engines?
the carnot heat engine
The higher the ___________ of the thermal energy, the ___________ its quality
higher, higher
no refrigerator can have a higher ______________________ than a reversible refrigerator
COP
the COP of a reversible refrigerator or heat pump
the maximum theoretical value for the specified temperature units
The COP of both refrigerators and heat pumps _______________ as TL _____________
decreases, decreases
PMM 1
violates 1st law of thermo.
spontaneously adds or removes energy without any excess energy being added
PMM 2
makes irreversible processes reversible
violates 2nd law of thermo
PPM 0
doesnt really exist
violates zeroth law of thermo
Carnot Cycle clue words
-carnot
-reversible
-maximum power output/ maximum possible affect
-minimum power input
-consume least heat/produce most work
the increase of entropy principle
for any real processes, entropy must always increase of remain constant
For a reversible process,
entropy stays constant
Processes can occur in a certain __________ only
direction
Processes most proceed in the direction that....
complies with the increase in energy principle
Entropy is a ______________________ property
non-conserved; however, it is conserved during the idealized reversible process
the performance of engineering systems is degraded by....
the presence of irreversibilites; and entropy generation is a measure of the magnitudes of the irreversibilites
the entropy change of a system can be _____________, but the entropy generation cannot be
negative
the value of entropy is __________ once the state of the system is ___________
Fixed, fixed
isentropic process
a process during which the entropy remains constant, that has no irreversibilites
for an isentropic reversible process,
s2=s1