Technical Concepts and Overview Flashcards
What does the first law of thermodynamics state?
“Energy is neither created nor destroyed but converted from one form to another”
What does the second law of thermodynamics state?
“A device operating continuously cannot export only work from an isolated source of heat”
What implications for the design of energy systems does the second law of thermodynamics have?
The design must account for the inevitable loss of heat energy as they operate
Even though energy is conserved . . . the flow of energy is []
irreversible
To happens to the “grade” of energy as it flows through a process?
It deteriorates
What is “exergy” as a measure of energy grade?
The theoretical maximum extractable work from a flow of energy
The most energy that can be EXERTED from a flow at any one time
What happens to the exergy of a heat source as it’s temperature approaches infinity?
Exergy nears pure energy, it is said to be very high grade
Explain first law efficiency
accounts only energy arriving at the process, irrespective of grade
n = required energy / total derived energy input
Explain primary energy efficiency
replaces energy delivered from a primary source such as gas, sunlight etc. by the primary energy required to produce it.
Essentially processed energy over the source energy
n = required energy / “primary equivalent of delivered energy” input
Explain exergy efficiency
compares actual consumption against that required by a thermodynamically ideal process
n = required exergy / total exergy input
Energy systems accept [] energy flows and transforms them to produce one or more flows of a different []
accept one or more energy flows
flows of a different grade
Why is exergy efficiency useful?
Helps to understand where exergy is being destroyed in an energy process
In a conventional power station, where is the exergy loss typically located?
between combustion flame and the steam entering a turbine
What does CCGT stand for?
combined cycle gas turbine
Why is a CCGT better than a conventional power station?
a CCGT uses gas turbines to take exergy from the gas flame and pass cooled down gases to drive a secondary steam turbine. (Uses much more of the available energy)
What does a heat pump do?
uses power to take energy from a cold source and pass heat to a hot source. The inverse of a heat engine
What is a Carnot engine?
An ideal heat engine
What is the equation of a Carnot engine?
Q^. = W^. +q^. all without loss of exergy in the process
A Carnot engine takes energy at a rate of Q^. from [] and discharge waste at rate q^. to []
heat source Th (T hot)
heat sink Tc (T cold)
The higher the [] and the lower the [] the more [] is converted to work
higher the source temp
lower the sink temp
think of the waterfall with a waterwheel in it analogy
In a heat pump, energy is taken from [] and power is supplied to raise its [] and discharge heat at a higher []
taken from a low grade heat source
raise its grade
discharge heat at a higher temp
An example of a heat pump is
A refrigerator
air conditioning unit
air to air heat pump
How does an air conditioning unit work?
removes heat from a room, discharges heat at a higher temp to exterior
How does an air to air heat pump work?
take zero-grade heat from ambient
raises its grade by adding the exergy of its shaft power to the flow of energy into the building
