UE 02 + 03: Systems + Energy Flashcards
A “…” is a definable part of the world that is interesting to us. It is separated from its “…” by a “…”.
“system”
“environment”
“system boundary”
What types of energy transfer do you know?
- Mechanical
–> Energy in the form of work (W) crosses the system boundary - Thermal
–> Energy in the form of heat (Q) is transferred across the system boundaries - Material flow
–> Different types of energy (e. g. kinetic energy) cross the system boundary with the material flow
–> Only in open systems relevant!
What types of systems do you know?
Isolated system
- No transport of matter
- No transport of energy
- Example: Thermos flask
Closed system
- No transport of matter
- Transport of energy possible: work (W) and heat (Q)
- Example: Closed cylinder of a combustion engine
Open system
- Transport of matter possible
–> Material-bound energy transport (e. g. kinetic energy) possible - Transport of energy possible: work (W) and heat (Q)
- Example: Cooled turbo compressor
The energy change of a closed system during “…” is equal to the “…” between the system and its environment.
“a process”
“net work and net heat transfer”
What is it about?
“…”
- A system can be assigned physical quantities or variables that describe its thermodynamic properties.
- The system is in a certain state if it can be described at any moment by a unique set of variables.
“…”
- If a system is in energetic interaction with its environment, the state of the system changes, it goes through a process.
“State variables”
“Process variables”
What thermal state variables do you know?
Temperature: T
Pressure: p
Volume: V
What caloric state variables do you know?
Internal energy: U
Enthalpy: H
Entropy: S
What process variables do you know?
Work: W
Power: P = W° = dW/dt
Heat: Q
Heat flow: Q° = dQ/dt
What is an adiabatic process?
An adiabatic process is a thermodynamic process in which no heat (Q) is exchanged between the system and its surroundings.
1) What are extensive variables? Which do you know?
2) What are intensive variables? Which do you know?
1) Extensive variables
- Describe properties which respective values (Z) is the sum of the corresponding state variable (Z_A, Z_B, …) of all parts of the system (part A, part B …)
–> Z = Z_A + Z_B + … - Volume V, internal energy U, enthalpy H, entropy S, work W, power P, heat Q, heat flow Q°
2) Intensive variables
- Describe properties which are not additive
- Temperature: T, pressure: p, specific internal energy: u, specific enthalpy: h, specific entropy: s
Value of Energy
1) What is the 1st law of thermodynamics?
2) What is exergy?
3) What is anergy?
1) 1st law of thermodynamics = law of conservation of energy
–> The sum of all forms of energy always remains the same
–> Energy = Exergy + Anergy = const. (1st law)
2) Exergy (Availability)
–> The part of the energy that can be converted into any other form of energy under given thermodynamic conditions of the environment
–> Fully usable, unlimitedly convertible part of energy (e.g. work); entropy- free
–> The exergy share decreases in all conversion processes.
3) Anergy
–> The part of the energy that cannot be converted into other forms of energy under given thermodynamic conditions of the environment
–> Non-usable part of the energy in the considered environment; afflicted with entropy
–> E.g. thermal energy at the temperature level of the environment
1) What does the 1st law of thermodynamics say?
2) What does the 2nd law of thermodynamics say?
1) 1st law of thermodynamics
- Basis: law of energy conservation
–> Energy =exergy + anergy = const. - Energy balances
–> Energy = internal energy + external energy = U + E_pot + E_kin = const. - Energy consumption or generation in the thermodynamic sense does not exist
2) 2nd law of thermodynamics
- Basis: law of energy degradation
–> If exergy becomes anergy this process is irreversible
–> This principle could be interpreted as energy consumption - The irreversibility of a process is quantified through entropy
–> All natural processes are irreversible
In a closed system the transfer of heat Q° and work W° will increase “…”.
“the internal energy U”
True or false?
A higher initial transfer temperature T_0 adds more entropy S_q to the system.
False!
A higher initial transfer temperature T_0 adds less entropy S_q to the system.
S_q = Q° / T_0
True or false?
For a given system the level of entropy can never decrease.
False!
For a given system the level of entropy can only decrease if heat is emitted (Q° < 0 –> S_q < 0). In this case the entropy of the environment increases.
S_irr < 0 is impossible!
True or false?
For a given system the level of entropy can never decrease through reversibilities or irreversibilities.
True!
S_irr > 0 –> irreversible process
S_irr = 0 –> reversible process
S_irr < 0 –> impossible process
Anergy and exergy forms
For each energy form provide an example for the ideal conversion as well as the maximum exergy share of the converted energy.
E_pot, E_kin, U_Electricity, U_Chemical binding energy, U_Thermal energy
E_pot (part of E_mech)
- Example for ideal conversion: leverage
- Max. exergy share of the converted energy: 100%
E_kin (part of E_mech)
- Example for ideal conversion: Impact
- Max. exergy share of the converted energy: 100%
U_Electricity
- Example for ideal conversion: Movement of load carriers
- Max. exergy share of the converted energy: 100%
U_Chemical binding
- Example for ideal conversion: Fuel cell
- Max. exergy share of the converted energy: < 100 %
U_Thermal energy
- Example for ideal conversion: Thermal engine
- Max. exergy share of the converted energy: = Q° * n_c («_space;100 %)
n_c: Carnot efficiency
What is the Carnot efficiency?
- Thermal efficiency of an ideal (closed and reversible (S_irr = 0)) heat engine
- Maximum thermal efficiency that a heat engine can theoretically reach permitted by the 2nd law of thermodynamics
- n_c = 1 - T_l / T_h = 1 - Q°_out / Q°_in
What is the degree of utilization?
Degree of utilization
- Average efficiency determined over a longer period of time
- Efficiency >= degree of utilization
A real heat engine is given.
How do you calculate the …
1) … reversible power?
2) … irreversibility rate?
Heat engine (= closed system + circular process)
1) Reversible power
Closed system + reversible process –> n_th = n_c
W°_rev = n_th * Q°_in = n_c * Q°_in
2) Irreversibility rate
I° = W°_rev - W°_real
True of false?
The energetic efficiency is always greater or equal to the exergetic efficiency.
Give example(s).
False!
Example: Industrial furnace
- n_en = Q°_out / W°_in
- n_ex = (n_c * Q°_out) / W°_in
- Energetic efficiency (n_en) >= exergetic efficiency (n_ex)
Example: Steam turbine
- n_en = W°_out / Q°_in
- n_ex = W°_out / (n_c * Q°_in)
- Energetic efficiency (n_en) <= exergetic efficiency (n_ex)
True or false?
All of these are possible:
n_en <= n_c <= n_ex
n_ex <= n_c <= n_en
n_en = n_ex
True!
2nd law of thermodynamics
What is entropy?
Entropy
- Entropy is a measure for the irreversibility of a process
–> All natural processes are irreversible. - Entropy of a system: ∆S = S_q + S_irr
- The entropy of a system changes by
–> Heat transport across the system boundary (Entropy transport with heat, 𝑆𝑞)
–> The entropy transported with the amount of heat, S_q = ∆𝑄/T0
–> S_irr > 0 (for irreversible processes)
–> S_irr = 0 (for reversible processes (ideal borderline case) - The entropy of a fixed mass can be changed by (1) heat transfer and (2) irreversibilities. Then it follows that the entropy of a fixes mass does not change during a process that is internally reversible and adiabatic –> isentropic process.
True or false
The entropy of a fixes mass does not change during a process that is internally reversible and adiabatic –> isentropic process.
True
What is the reversible power and the irreversibility rate of a heat engine?
Reversible power (W°_rev)
- Theoretically maximum power output of heat engine if reversibility is assumed
- In this case the process runs at n_th = n_c
–> W°_rev = n_c * Q°_in
Irreversibility rate (I°)
- The process is assumed to be irreversible or in other words to be a real process
- In this case the process runs at n_th < n_c
- In this case the irreversibility rate is the amout of power lost due to irreversibilities
- I° = W°_rev - W°_irrev
In a T-S diagram for internally reversible processes, the area under the process curve represents the “…”.
“heat transfer”
integral_1_2 T dS = delta_Q
The “…” appears as a vertical line segment on a T-s diagram.
“isentropic process”
isentropic process = internally reversible (Sirr = 0) and adiabatic (delta_Q = 0)
1) What is the Rankine Cycle?
2) Draw the scheme of a respective powerplant.
3) Draw the ideal T-s-diagram
4) Draw the real T-s-diagramm
5) Draw the ideal T-s-diagram in case of a reheater
1) Ideal Vapor Power Plants
2)
- Boiler (q_in)
- Turbine (w_turb,out)
- Condenser (q_out)
- Pump (w_pump, in)
3) Compare slide 23
1-2: Isentropic pumping
2-3: Isobaric heat addition in a boiler
3-4: Isentropic expansion in an turbine
4-1: Isobaric heat rejection in a condenser
4) Compare slide 25
1-2: pumping with irreversibilities (s increases)
2-3: boiler with pressure drop (p decreases)
3-4: turbine with irreversibilities (s increases)
4-1: condenser with pressure drop (p decreases)
5) Compare slide 27
True or false?
Electricity can be completely converted into other forms of energy. Therefore the exergetic efficiency is 100 %.
True
What types of energy do you know?
Physical capacity to work
- Exergy
- Anergy
Stored energy
- Mechanical energy (= potential and kinetic energy)
- Thermal energy
- Chemically bound energy
- Nuclear energy
- Gravitational energy
Process energy
- Heat
- Radiation
- Mechanical work
- Electrical work
According to “…”, energy is always converted and not generated or destroyed
“the 1st law of thermodynamics”
What is enthalpy?
Enthalpy (H) is the internal energy of mass due its heat (U), pressure (p) and volume (V).
H = U + p * V
True or false?
A higher transport temperature leads to a higher entropy increase.
False!
S_Q = Q / T_transfer
A higher transport temperature leads to a LOWER entropy increase.
How do you determine the exergy of a heat flow?
Ex(Q°_out) = ?
Ex(Q°_out) = n_c * Q°_out
n_c: Carnot efficincy
True or false?
The following is always valid:
n_en <= n_ex <= n_c
False!
Each of the following cases is possible:
n_en = n_ex
n_en <= n_c <= n_ex
n_ex <= n_c <= n_en
Provide the p-V- and T-s-diagram of water.
compare with notes
p-V-diagram of water
–> Isotherms (top left to bottom right)
T-s-diagram of water
–> Isobars (bottom left to top right)
left to right: compressed liquid region, superated liquid-vapor region, superheated vapor region
left to right: saturated liquid line, saturated vapor line
True of false?
In a given p-V-diagram the area below the line connecting two states equals the conducted work W.
In a given T-s-diagram the area below the line connecting two states equals the conducted heat Q.
True
Which ideal process enables the most efficient transformation of heat to exergy in a thermodynamic cycle?
Carnot process
Provide the steps of an ideal Rankine process and the corresponding power plant components.
Draw the p-V- and T-s-diagram.
Ideal Rankine process
- Vapor process
1-2: Isentropic pumping
2-3: Isobaric heat addition in boiler
3-4: Isentropic expansion in turbine
4-1: Isobaric heat rejection in condenser
Compare notes for diagrams
Provide the steps of an ideal Carnot process and the corresponding power plant components.
Draw the p-V- and T-s-diagram.
Carnot process
1-2: Isentropic compression
2-3: Isothermic expansion + heat supply
3-4: Isentropic expansion
4-1: Isothermic compression + heat rejection
Compare notes for diagrams
Provide the steps of an ideal Joule process and the corresponding power plant components.
Draw the p-V- and T-s-diagram.
Joule process
1-2: Isentropic compression (air intake + compressor)
2-3: Isobaric heat addition (combustion chamber)
3-4: Isentropic expansion (turbine)
4-1: Isobaric heat rejection (exhaust gases into atmosphere)
Compare notes.
Combined cycle power plant
- In a combined cycle power plant for example a “…” are combined.
- The thermal energy of the still very hot “…” is transferred into the steam cycle via “…”.
- This combines the advantages of both processes: a) “…” and b) “…”
“gas turbine process (Joule process) and a water-steam cycle (Rankine process) (CCGT)”
“exhaust gases from the gas turbine”
“heat exchanger”
“the high inlet temperature of the gas turbine”
“the lower waste heat temperature in the water-steam process”
Types of heat pumps by heat source
“…”
- Using geothermal heat
- High efficiency
- Low operational cost
- High installation cost
- High space requirement
“…”
- High efficiency
- Lowest operational cost
- Very high investment cost
- Additional regulations regarding ground water use
“…”
- Low efficiency and high operational cost in winter
- Low investment cost due to less complex heat source development
- Lowest impact on environment
“Ground source heat pumps”
“Water source heat pump”
“Air source heat pump”
What is missing?
Types of heat pumps by heat source
Ground source heat pumps
- Using geothermal heat
- Efficiency: “…”
- Operational cost: “…”
- Investment cost: “…”
- High space requirement
Water source heat pump
- Efficiency: “…”
- Operational cost: “…”
- Investment cost: “…”
- Additional regulations regarding “…”
Air source heat pump
- Efficiency: “…”
- Operational cost: “…” in winter
- Investment cost: “…” due to less complex heat source development
- Impact on environment: “…”
Types of heat pumps by heat source
Ground source heat pumps
- Using geothermal heat
- Efficiency: High
- Operational cost: Low
- Investment cost: High
- High space requirement
Water source heat pump
- Efficiency: High
- Operational cost: Lowest
- Investment cost: Very high
- Additional regulations regarding ground water use
Air source heat pump
- Efficiency: Low
- Operational cost: High in winter
- Investment cost: Low due to less complex heat source development
- Impact on environment: Lowest
True or false?
In most geothermal heat pumps brine flows into the evaporator and emits heat into the refrigerant which is used in the refrigeration cycle.
Water however can be used in the condenser if for example the heat pump is connected to a building’s heating circuit. In this case the refrigerant emits heat into the water flow.
Within the refrigeration cycle of the heat pump only a refrigerant circulates.
True!
A building’s total annual heat demand is 200 MWh/a. The heating temperature is 40°C, whereas the room temperature is 20°C.
Specify the exergy share.
Exergy share: n_c = 1 - T_l/T_h = 6 %
True or false?
Heat engines are always closed systems with circular processes.
True!
80 liters of water at a temperature of 45°C (1) are mixed with 10 liters of tap water at 15°C (2). What is the temperature of the mixture?
What is the approach to solve this exercise?
General approach
- Heat transfer from higher to lower temperatures (1 –> 2)
Q_in = |Q_out|
= m_1 * c * (T_1 - T_mix) = m_2 * c * (T_mix - T_2)
- Solve for T_mix
