UE 02 + 03: Systems + Energy

Question 1

A “…” is a definable part of the world that is interesting to us. It is separated from its “…” by a “…”.

Answer 1

“system”

“environment”

“system boundary”

Question 2

What types of energy transfer do you know?

Answer 2

• 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!

Question 3

What types of systems do you know?

Answer 3

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

Question 4

The energy change of a closed system during “…” is equal to the net work and net heat transfer between the system and its environment.

Answer 4

“a process”

Question 5

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.

Answer 5

“State variables”

“Process variables”

Question 6

What thermal state variables do you know?

Answer 6

Temperature: T

Pressure: p

Volume: V

Question 7

What caloric state variables do you know?

Answer 7

Internal energy: U

Enthalpy: H

Entropy: S

Question 8

What process variables do you know?

Answer 8

Work: W

Power: P = W° = dW/dt

Heat: Q

Heat flow: Q° = dQ/dt

Question 9

What is an adiabatic process?

Answer 9

An adiabatic process is a thermodynamic process in which no heat (Q) is exchanged between the system and its surroundings.

Question 10

1) What are extensive variables? Which do you know?

2) What are intensive variables? Which do you know?

Answer 10

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

Question 11

Value of Energy

1) What is the law of conservation of energy?

2) What is exergy?

3) What is anergy?

Answer 11

1) 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

Question 12

1) What does the 1st law of thermodynamics say?

2) What does the 2nd law of thermodynamics say?

Answer 12

1) 1st law of thermodynamics

• Basis: law of energy conservation
  –> Energy =exergy + anergy = const.
  –> 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

Question 13

In a closed system the transfer of heat Q° and work W° will increase “…”.

Answer 13

“the internal energy U”

Question 14

True or false?

A higher initial transfer temperature T_0 adds more entropy S_q to the system.

Answer 14

False!

A higher initial transfer temperature T_0 adds less entropy S_q to the system.

S_q = Q° / T_0

Question 15

True or false?

For a given system the level of entropy can never decrease.

Answer 15

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!

Question 16

True or false?

For a given system the level of entropy can never decrease through reversibilities of irreversibilities.

Answer 16

True!

S_irr > 0 –> irreversible process

S_irr = 0 –> reversible process

S_irr < 0 –> impossible process

Question 17

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

Answer 17

E_pot

• Example for ideal conversion: leverage
• Max. exergy share of the converted energy: 100%

E_kin

• 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 (&laquo_space;100 %)

n_c: Carnot efficiency

Question 18

What is the degree of utilization?

Answer 18

Degree of utilization
- Average efficiency determined over a longer period of time

Question 19

True of false?

The energetic efficiency is always greater or equal to the exergetic efficiency.

Give example(s).

Answer 19

False!

Example: Industrial furnace

• n_ex = (n_c * Q°_out) / P_in
• Energetic efficiency (n_en) >= exergetic efficiency (n_ex)

Example: Steam turbine

• n_ex = P_out / (n_c * Q°_in)
• Energetic efficiency (n_en) <= exergetic efficiency (n_ex)

Question 20

2nd law of thermodynamics

What is entropy?

Answer 20

Entropy

• All natural processes are irreversible.
• Entropy is a measure for the irreversibility of a process
• 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/T0
  –> S > 0 (for irreversible processes)
  –> S = 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.

Question 21

True or false

The entropy of a fixes mass does not change during a process that is internally reversible and adiabatic –> isentropic process.

Answer 21

True

Question 22

What is the reversible power and the irreversibility rate of a heat engine?

Answer 22

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

Question 23

In a T-S diagram for internally reversible processes, the area under the process curve represents the “…”.

Answer 23

“heat transfer”

integral_1_2 T dS = delta_Q

Question 24

The “…” appears as a vertical line segment on a T-s diagram.

Answer 24

“isentropic process”

isentropic process = internally reversible (Sirr = 0) and adiabatic (delta_Q = 0)

Question 25

True oder false?

The change of the entropy of an ideal circular process is zero.

Answer 25

True

Question 26

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

Answer 26

1) Ideal Vapor Power Plants

2)

1. Boiler (q_in)
2. Turbine (w_turb,out)
3. Condenser (q_out)
4. 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

Question 27

True or false?

Electricity can be completely converted into other forms of energy. Therefore the exergetic efficiency is 100 %.

Answer 27

True

Question 28

What types of energy do you know?

Answer 28

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

Question 29

According to “…”, energy is always converted and not generated or destroyed

Answer 29

“the 1st law of thermodynamics”

Question 30

What is enthalpy?

Answer 30

Enthalpy (h) is the internal energy of mass due its heat (u_e), pressure (p_e) and volume (v_e).

h = u_e + p_e * v_e

Question 31

True or false?

A higher transport temperature leads to a higher entropy increase.

Answer 31

False!

S_Q = Q / T_transfer

A higher transport temperature leads to a LOWER entropy increase.

Question 32

How do you determine the exergy of a heat flow?

Ex(Q°_out) = ?

Answer 32

Ex(Q°_out) = n_c * Q°_out

n_c: Carnot efficincy

Question 33

True or false?

The following is always valid:

n_en <= n_ex <= n_c

Answer 33

False!

Each of the following cases is possible:

n_en = n_ex

n_en <= n_c <= n_ex

n_ex <= n_c <= n_en

Question 34

Provide the p-V- and T-s-diagram of water.

Answer 34

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

Question 35

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.

Answer 35

True

Question 36

Which ideal process enables the most efficient transformation of heat to exergy in a thermodynamic cycle?

Answer 36

Carnot process

Question 37

Provide the steps of an ideal Rankine process and the corresponding power plant components.

Draw the p-V- and T-s-diagram.

Answer 37

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

Question 38

Provide the steps of an ideal Carnot process and the corresponding power plant components.

Draw the p-V- and T-s-diagram.

Answer 38

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

Question 39

Provide the steps of an ideal Joule process and the corresponding power plant components.

Draw the p-V- and T-s-diagram.

Answer 39

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.

Question 40

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) “…”

Answer 40

“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”

Question 41

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

Answer 41

“Ground source heat pumps”

“Water source heat pump”

“Air source heat pump”

Question 42

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 heat pump only a refrigerant circulates.

Answer 42

True!