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 “…” between the system and its environment.

Answer 4

“a process”

“net work and net heat transfer”

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 1st law of thermodynamics?

2) What is exergy?

3) What is anergy?

Answer 11

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

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 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

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 or 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 (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 (&laquo_space;100 %)

n_c: Carnot efficiency

Question 18

What is the Carnot efficiency?

Answer 18

• 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

Question 19

What is the degree of utilization?

Answer 19

Degree of utilization

• Average efficiency determined over a longer period of time
• Efficiency >= degree of utilization

Question 20

A real heat engine is given.

How do you calculate the …

1) … reversible power?
2) … irreversibility rate?

Answer 20

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

Question 21

True of false?

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

Give example(s).

Answer 21

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)

Question 22

True or false?

All of these are possible:

n_en <= n_c <= n_ex

n_ex <= n_c <= n_en

n_en = n_ex

Answer 22

True!

Question 23

2nd law of thermodynamics

What is entropy?

Answer 23

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.

Question 24

True or false

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

Answer 24

True

Question 25

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

Answer 25

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 26

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

Answer 26

"heat transfer" integral_1_2 T dS = delta_Q

Question 27

The "..." appears as a vertical line segment on a T-s diagram.

Answer 27

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

Question 28

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 28

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 29

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

Answer 29

True

Question 30

What types of energy do you know?

Answer 30

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 31

According to "...", energy is always converted and not generated or destroyed

Answer 31

"the 1st law of thermodynamics"

Question 32

What is enthalpy?

Answer 32

Enthalpy (H) is the internal energy of mass due its heat (U), pressure (p) and volume (V). H = U + p * V

Question 33

True or false? A higher transport temperature leads to a higher entropy increase.

Answer 33

False! S_Q = Q / T_transfer A higher transport temperature leads to a LOWER entropy increase.

Question 34

How do you determine the exergy of a heat flow? Ex(Q°_out) = ?

Answer 34

Ex(Q°_out) = n_c * Q°_out n_c: Carnot efficincy

Question 35

True or false? The following is always valid: n_en <= n_ex <= n_c

Answer 35

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 36

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

Answer 36

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 37

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 37

True

Question 38

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

Answer 38

Carnot process

Question 39

Provide the steps of an ideal Rankine process and the corresponding power plant components. Draw the p-V- and T-s-diagram.

Answer 39

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 40

Provide the steps of an ideal Carnot process and the corresponding power plant components. Draw the p-V- and T-s-diagram.

Answer 40

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 41

Provide the steps of an ideal Joule process and the corresponding power plant components. Draw the p-V- and T-s-diagram.

Answer 41

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 42

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 42

"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 43

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 43

"Ground source heat pumps" "Water source heat pump" "Air source heat pump"

Question 44

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: "..."

Answer 44

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**

Question 45

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.

Answer 45

True!

Question 46

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.

Answer 46

Exergy share: n_c = 1 - T_l/T_h = 6 %

Question 47

True or false? Heat engines are always closed systems with circular processes.

Answer 47

False! Heat engines are always circular processes. However, they are **not always a closed system (e.g. gas turbine process)**.

Question 48

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?

Answer 48

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