Chapter 7, Problem 7.12E

Starting with energy and entropy balances, derive the general exergy balance relation for a closed system.

Define the exergy destruction, which is the wasted work potential during a process as a result of irreversibilities.

Define the Mechanisms of exergy transfer.

Define exergy, which is the maximum useful work that could be obtained from the system at a given state in a specified environment.

As in mechanical systems, friction in the workplace is bound to generate entropy and reduce performance.

1. A source of heat at 1000 K transfers 1000 kW of power to a power generation device, while producing 300 kW of useful work. Determine: a. The rate of exergy destruction in this process if the environment is at 300K. b. The second law efficiency of the system.

If heating from saturated liquid to saturated vapor would occur at 100°C (373.15 K), evaluate the exergy transfers accompanying heat transfer and work, each in kJ/kg. Ans. 484, 0.

Exergy balance for a closed system when the direction of heat transfer is taken to be to the system and the direction of work from the system.

A system undergoes a refrigeration cycle while receiving Qc by heat transfer at temperature Tc and discharging energy Qu by heat transfer at a higher temperature TH. There are no other heat transfers. (a) Using energy and exergy balances, show that the net work input to the cycle cannot be zero. (b) Show that the coefficient of performance of the cycle can be expressed as: Tc TH – TeA'¯ T(Qn – Q). B = where E, is the exergy destruction and To is the temperature of the exergy reference environment. (c) Using the result of part (b), obtain an expression for the maximum theoretical value for the coefficient of performance.