2. Syst Flashcards
What is a systems? (2)
a definable part of the world that is interesting to us.
It is seperated from its environment by a system boundary.
How can systems be categorized and distinguished? (3)
Can be categorized in:
Isolated systems
Closed systems
Open systems
What types of energy transfer exist? (3)
mechanical
thermal
through material flow
Types of energy transfer
mechanical: Energy in the form of ?(1)? crosses the system boundary
thermal: Energy in the form of ?(2)? is transferred across the system boundaries
through material flow: Different types of energy (e.g. kinetic energy) cross the system boundary with the ?(3)?
(1) work (W)
(2) heat (Q)
(3) material flow
Beispiele für welchen system typ jeweils?
1) Themos flask
2) Cooled turbo compressor
3) Closed cylinder of a combustion engine
1) Isolated system
2) Open system
3) Closed cylinder of a combustion engine
Characteristics of an isolated system? (2)
Matter and energy impermeable
No energy transit possible
Characteristics of an closed system? (2)
Matter impermeable
Energy transfer in the form of:
Work
Heat
Characteristics of an open system? (2)
Matter permeable
Energy transfer in the form of:
Work
Heat
Material-bound energy transport
What is the difference between a system and a model?
A system is the real-world entity being studied, while a model is a simplified abstraction of the system tailored to solve a specific problem.
(System: We analyze the world by diving it into smaller subsystems and sub-subsystems, which are then studied using methmatical methods.
Model: Constructing models simplifies complex systems by creating a simplified description, often using mathematical equations. Models help reduce complexity but are not exact replicas of reality.
From system to model: A model is closely related to the purpose of the problem being investigated. The problem determines the system, system boundaries and the relevant system parameters
Setting up a conceptual model using a box representation
-> p.6
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What is a system analysis? (Definition)
Is a system-technical way of describing a system through:
Elements
A cause-effect-relationship
In a ??, the easiest black-box structure is found in order to describe the relationship between the state of the system and the state of the surroundings
system analysis
What is energy system analysis? (Definition)
Investigation of the structural elements of a system, i.e. the representation of how the system works.
Goals of systems research and systems analysis?
Support decisions in energy policy and energy research with regard to technologies and infrastructures for energy supply and energy conversion on a knowledge-based and systematic basis.
What is thermodynamics? (4)
Could be defined as a “general energy theory”
teaches to distinguish between different forms of energy
describes their conversions into each other by using energy balances of the 1st law of thermodynamics
clarifies the conditions and limits for the conversion during natural and technical processes by applying the statements of the 2nd law of thermodynamics
Energy balancing:
Change = ??
= Transport + Generation - Losses
What are the elements of thermodynamics? (3)
- System and system boundaries
- State/status and variables describing the status
- Process and process variables
What are state variables (Zustandsgrößen)?
variables like mass, temperatur, volume etc.
(The system is in a certain state if it can be described at any moment by a unique set of variables (state variables))
Name some process variables? (4)
for example: Work (W), Power (P), Heat (Q), Heat flow (QPunkt)
(If a system is in energetic interaction with its environment, the state of the system changes, it goes through a process. The energy in the form of e.g. heat or work (process variables) is supplied or removed over the system boundaries)
Energy units, Definition and Conversion Factors
-> slide15!!!!
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Name some common energy units!
Kilojoule (kJ)
Oil equivalent (OE)
Coal equivalent (CE)
Kilocalorie (Kcal)
Kilowatt hour (kWh)
British Thermal Unit (BTU)
Oil (barrels (bbl)):
What is 1 bbl in liter and in tonnes?
1 bbl = 159l ~= 0.136 t
Gas
What is 1 cubic feet/day in m^3/years?
10.34 m^3 / yearc
What is the 1st Law of Themodynamics?
Law of conservation of energy: There can be no generation or destruction of energy
The different forms of energy can be transformed into each other, but they cannot be generated or destroyed.
Law of Conservation of Energy (1st Law of Thermodynamics)
The sum of all forms of energy always remains the same, but:
Energy = ??
Exergy + Anergy
What is the part of the energy that can be converted into any other form of energy under given thermodynamic conditions of the environment?
Exergy
What is the part of the energy that cannot be converted into other forms of energy, e.g. thermal energy at the temperature level of the environment.
Anergy
Energy consumption in the thermodynamic sense does not exist, as the sum of exergy and anergy is constant (1st law)
true/false?
true
Energy consumption in the thermodynamic sense does not exist, as the sum of exergy and anergy is constant (1st law)
However, energy can be degraded from exergy to anergy and it is never possible to convert anergy into exergy. (true/false?)
true (2nd law of thermodynamics)
-> this principle could be interpreted as energy consumption
The second law of thermodynamics describes the principle of?
energy degradation
All natural processes are irreversible
true/false?
true (all natural processes can only run by themselves in one direction)
Entropy is a measure for what?
the irreversibility of a process
The entropy of a system changes by: ?? (3)
Heat transport across the system boundaries
Mass transport across the system boundaries
Irreversible processes inside the system (Entropy generation, Sirr)
The entropy generated by irreversibilites can only assume positive values.
true/false?
true
(Sirr > 0 for irreversible processes, Sirr = 0 for reversible processes (ideal borderline case)
?(1)? = fully usable, unlimitedly convertible part of energy (e.g. work); entropy-free
?(2)? = non-usable part of the energy in the considered environment; afflicated with entropy
(1) Exergy
(2) Anergy
The ?? share is decisive for the usability of energy.
exergy
(since energy with a high exergy share is more valuable than energy with a high anergy share)
Energy that is used in many different processes and converted into different energies is likely to have a high ?(1)? share.
exergy
“It is never possible to turn anergy into exergy.”
true/false
true
?(1)? occur during the conversion of exergy into anergy
If the conversion is ideal, the borderline case occurs and no ?(2)? is generated.
The measurement of irreversibilities should be a method to determine the proportions of exergy and anergy of a system.
(1) Irreversibilities
(2) anergy
Definitions of energy? (3)
Physical capacity to work (Exergy, Anergy)
Stored energy (Mechanical energy, thermal energy, chemically bound energy, nuclear energy, gravitational energy)
Process energy (heat, radiation, mechanical work, electrical work)
E = Epot + Ekin + U = const
-> Principle of energy conservation: Energy of a constant mass can neither be “produced” nor “destroyed”, but is conserved, 1st law of thermod.)
…
Examples for “U” (Internal energy):
Thermal kinetic energy of the molecules in a body, a liquid, or within a gas (measured as temperature)
Force effect of the moving molecules hitting the system boundaries (measured as pressure)
Binding energy between the molecules of a substance or the atoms of a molecule (chemical energy)
Binding energy between the components of an atomic nucleus (nuclear energy)
…
Equations for a Closed System
-> see p.23
…
Equations for an Open System:
-> Stationary Flow Process (p. 24)
-> Transient Flow Process (p. 25)
…
How to build a mass, energy, power or entropy balance for different thermodynamic systems?
see page 23 - 25
What is a circular process?
Is a stationary process where the initial state is identical to the final state
1) The power balance for a cicular process is simplified to?
2) Therefore the energy balance results as?
1) Summe(QPunkt) + Summe(P) = 0
2) P = Qpunktin - Qpunktout
Circular Process
-> siehe Formeln slide 27
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1) What is the Carnot Cycle?
2) Consists of? (4)
a theoretical thermodynamic cycle that provides the maximum efficiency for a heat engine operating between two temperature reservoirs.
It consists of four reversible processes:
1-> 2: Isothermal Expansion
2 -> 3: Isentropic expansion
3 -> 4: Isothermal compression
4 -> 1: Isentropic compression
(abhängig davon wo man anfängt…)
Name The Carnot Principles! (2)
- The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between same two thermal reservoirs. (eta irr < eta rev)
- The efficiencies of all reversible heat engines operating between the same two thermal reservoirs are the same. (etaA = etaB = …)
What is the Carnot Efficiency?
Is the maximum efficiency that a heat engine may have operating between the two temperatures.
The realization of circular processes in reality: The ??
Joule Process (Gas Turbine, Aircraft Engine)
What is the formula for the Carnot efficiency? (Carnot-Wirkungsgrad)
etasubc = 1 - (Tm, out / Tm,in)
The Gas Turbine (Joule Process)
-> see slide 28!
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The Aircraft Engine (Joule Process)
-> see slide 29!
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Describe the energy conversion chain of a gas turbine!
- Chemical (Kerosene)
- Thermal (Combustion)
- Mechanical (Pressure, rotation) & Kinetic (Recoil)
Describe the energy conversion chain of an aircraft engine!
- Chemical (Kerosene)
- Thermal (Combustion)
- Mechanical (Pressure, rotation) & Kinetic (Recoil)
1) Draw a simplified schematic of a gas power plant.
2) Show which working fluid is used.
3) name all components (3)
4) Name all the process steps.
5) Draw the circular process in a T,s-diagram (Joule Process)
1) Drawing slide 31
2) Drawing slide 31
3)
Compressor (Compression of the air to the high pressure in the combustion chamber)
Combustion chamber (Combustion under supply of fuel)
Turbine (Expansion of the combustion gas to approximately the ambient pressure)
4)
Isentropic compression in the compressor (1->2)
Isobaric heat supply in combustion chamber (2->3)
Isentropic expansion in the turbine (3->4)
Isobaric heat dissipation (open process) (4->1)
5) See slide 31+32!
Joule Process (of a Gas Turbine Plant)
Draw pV- and Ts-diagram !
slide 32!
Joule Process (of a Gas Turbine)
-W = ??
-W = Win - Wout = Qin - Qout
Joule Process (of a Gas Turbine)
-W = Win - Wout = Qin - Qout
How can the usable energy be maximized?
Idea: Increase heat input without exceeding T3
Anwer: Conversion of the process into a rectangle in the T,s-diagram with T2 = T3 and T1 = T4
Theoretical Optimization of the Joule Process: The Carnot Process
Draw the Carnot Process in pV- and Ts-diagram!
slide 33
