Lecture 7: Exergy

Question 1

Define First Law of thermodynamics

Answer 1

· Conservation of energy - energy cant be created or destroyed, only changed from one form to another
dQ=dE+dW

Question 2

Define entropy

Answer 2

• Measure of amount of molecular disorder within system
  
  • High entropy - high degree of molecular disorder
  • Low entropy - low degree of molecular disorder
  • System can only generate (NOT DESTROY) entropy
  • System entropy increased/decreased by energy transport across system boundary
  • Heat transfer to system increases its disordered energy
  Heat transfer from system reduces its disordered energy

Question 3

Define Second Law of thermodynamics

Answer 3

• No process possible where only result is heat transfer from low temp body to body of high temp
  
  • No process possible where sole result is heat absorption from reservoir and complete conversion into work
  • In energy transfer within closed system, system entropy increases

dS≥dQ/T

Question 4

Define quality of energy

Answer 4

• High quality - organised or concentrated to perform useful work
• Low quality - dispersed or disorganised and has little ability to do work
Entropy is measure of disorder of energy, more disorder less useful it is

Question 5

Define heat engine

Answer 5

• Heat moves from hot reservoir to cold reservoir => some thermal energy extracted for mechanical work W
  
  • Qh= heat removed from hot reservoir
  • Qc = heat deposited in cold reservoir
  • W = mechanical work done

Q_ℎ=Q_C+W

Question 6

Define heat pump

Answer 6

opposite of heat engine

Question 7

Define closed system

Answer 7

• Environment - temp and pressure
• System can exchange heat and work with environment
System does not exchange matter with environment
**availability **

Question 8

Define Flow system

Answer 8

• Environment - temp and pressure
• System can exchange energy and volume with environment
Streams of material enter/exit the system
exergy

Question 9

Define exergy and its characteristics

Answer 9

• Maximum shaft work that can be done by composite of the system and a specified reference environment => infinite reference environment, in equilibrium, and to enclose all other systems

B=∆H −T_0 ∆S

· Characteristics 
- System in complete equilibrium doesn’t have exergy => no temp, pressure or conc difference therefore no driving force
- System exergy increase the more it deviates from environment => higher exergy amount of hot water in winter than summer or block of ice has little exergy in winter but high in summer
- Exergy destroyed when energy loses its quality 
- Exergy is useful part of energy => has economic value 
- Depends on environment state => as well as state of system or flow  Exergy efficiency measure of approach to ideality