Exergy HW 8 MC

9. The difference between the reversible work and the useful work for a process is called irreversibility. a) true b) false

True

Exergy is a composite property. T or F

True

18. Liquid water enters an adiabatic piping system at 15ºC at a rate of 3 kg/s. It is observed that the water temperature rises by 0.3ºC in the pipe due to friction. If the environment temperature is also 15ºC, the rate of exergy destruction in the pipe is a) 3800 W b) 24000 W c) 72000 W d) 98000 W

a) 3800 W

23. Air is throttled from 50ºC and 800 kPa to a pressure of 200 kPa at a rate of 0.5 kg/s in an environment at 25ºC. The change in kinetic energy is negligible, and no heat transfer occurs during the process. The power potential wasted during this process is a) 59 kW b) 0 c) 0.2 kW d) 119 kW

a) 59 kW

8. If the final state of the system is the dead state, the ____ and the ____ become identical. a) reversible work, exergy b) irreversible work, exergy c) reversible work, irreversible work d) none of the above

a) reversible work, exergy

6. The surrounding work is zero for a) system with fixed boundaries b) system with flexible boundaries c) system with open boundaries d) all of the above

a) system with fixed boundaries

13. For a process that involves no irreversibilities, will the actual useful work for that process be equal to the reversible work? a) yes b) perhaps c) no d) yes and no

a) yes

12. Can the exergy of a system be increased by altering the environment? a) yes, with difficulty b) yes, very easily c) no d) possibly

a) yes, with difficulty

Useful work is given by a). difference between the actual work and the surrounding work b). Work of the surroundings c). W-p(V2-V1) where V2 is the volume at state 2 and V1 is the volume at state 1 d). all of the above

a). difference between the actual work and the surrounding work

10. For a totally reversible process, a) reversible work is the non- useful work b) irreversibility = 0 c) both of the above d) none of the above

b) irreversibility = 0

7. The maximum amount of useful work that can be obtained from a system as it undergoes a process between any two specified states is called a) exergy of the environment b) reversible work c) energy d) options b and c

b) reversible work

15. For a process that involves no entropy generation a) the exergy is high b) the exergy destruction is zero c) the exergy associated with heat is zero d) options a and b

b) the exergy destruction is zero

11. Consider two systems that are at the same pressure as the environment. The first system is at the same temperature as the environment, whereas the second system is at a lower temperature than the environment. How would you compare the exergies of these two systems? a) the first system has zero exergy, and the second system zero exergy b) the first system has zero exergy, and the second system some exergy c) the first and second system have the same level of exergy d) the first system has zero exergy, but it is hard to tell if the second has any

b) the first system has zero exergy, and the second system some exergy

____ is conserved but ______ is not conserved. a). exergy, energy b). energy, exery c). entropy, exergy d). energy, irreversibility

b). energy, exergy

21. A furnace can supply heat steadily at 1300 K at a rate of 500 kJ/s. The maximum amount of power that can be produced by using the heat supplied by this furnace in an environment at 300 K. a) 115 kW b) 192 kW c) 385 kW d) 500 kW

c) 385 kW

20. A house is maintained at 21ºC in winter by electric resistance heaters. If the outdoor temperature is 3ºC, the second-law efficiency of the resistance heaters is a) 0% b) 4.1% c) 6.1% d) 61%

c) 6.1%

22. A heat engine receives heat from, a source at 1500 K at a rate of 600 kJ/s and rejects the waste heat to a sink at 300 K. If the power output of the engine is 400 kW, the second-law efficiency of this heat engine is a) 42% b) 53% c) 83% d) 67%

c) 83%

16. Keeping the limitations imposed by the 2nd law of thermodynamics in mind, choose the wrong statement below: a) A heat engine cannot have a thermal efficiency of 100% b) For all reversible processes, the second law efficiency is 100% c) The 2nd law efficiency of a heat engine cannot be greater than its thermal efficiency d) The 2nd law efficiency of a process is 100% if no entropy is generated during that process

c) The 2nd law efficiency of a heat engine cannot be greater than its thermal efficiency

14. Two geothermal wells whose energy contents are estimated to be the same. Will the exergies of these wells necessarily be the same? a) yes, if they do not have the same temperature b) no, if they have the same temperatures c) no, if they have different temperatures d) not at all, irrespective of their temperatures

c) no, if they have different temperatures

A dead state a). is in equilibrium with its surroundings b). has zero exergy c). both of the above d). none of the above

c). both of the above

19. A water reservoir contains 100 tons of water at an average elevation of 60 m. The maximum amount of electric power that can be generated from this water is a) 8000 kWh b) 16000 kWh c) 1630000 kWh d) 16 kWh

d) 16 kWh

17. Heat is lost through a plane wall steadily at a rate of 800 W. If the inner and outer surface temperatures of the wall are 20ºC and 9ºC, respectively, and the environment temperature is 0ºC, the rate of exergy destruction within the wall is a) 0 W b) 11 W c) 15 W d) 29 W

d) 29 W

24. A 12-kg solid whose specific heat is 2.8 kJ/kgºC is at a uniform temperature of -10ºC. For an environment temperature of 20ºC, the exergy content of this solid is a) Less than zero b) 0 kJ c) 4.6 kJ d) 55 kJ

d) 55 kJ

Which of the following statements is true? a). kinetic energy is entirely available energy b). potential energy is entirely exergy c). the exergy of thermal energy of reservoirs is equivalent to the work output of a Carnot engine operating between the reservoir at temperature T and environment To d). all of the above

d) all of the above