ThermoSolutions-CHAPTER08

ThermoSolutions-CHAPTER08 - 8-1 Chapter 8 EXERGY A MEASURE...

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8-1 Chapter 8 EXERGY – A MEASURE OF WORK POTENTIAL Exergy, Irreversibility, Reversible Work, and Second-Law Efficiency 8-1C Reversible work differs from the useful work by irreversibilities. For reversible processes both are identical. W u = W rev -I. 8-2C Reversible work and irreversibility are identical for processes that involve no actual useful work. 8-3C The dead state. 8-4C Yes; exergy is a function of the state of the surroundings as well as the state of the system. 8-5C Useful work differs from the actual work by the surroundings work. They are identical for systems that involve no surroundings work such as steady-flow systems. 8-6C Yes. 8-7C No, not necessarily. The well with the higher temperature will have a higher exergy. 8-8C The system that is at the temperature of the surroundings has zero exergy. But the system that is at a lower temperature than the surroundings has some exergy since we can run a heat engine between these two temperature levels. 8-9C They would be identical. 8-10C The second-law efficiency is a measure of the performance of a device relative to its performance under reversible conditions. It differs from the first law efficiency in that it is not a conversion efficiency. 8-11C No. The power plant that has a lower thermal efficiency may have a higher second-law efficiency. 8-12C No. The refrigerator that has a lower COP may have a higher second-law efficiency. 8-13C A processes with W rev = 0 is reversible if it involves no actual useful work. Otherwise it is irreversible. 8-14C Yes.
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8-2 8-15 Windmills are to be installed at a location with steady winds to generate power. The minimum number of windmills that need to be installed is to be determined. Assumptions Air is at standard conditions of 1 atm and 25 q C Properties The gas constant of air is 0.287 kPa.m 3 /kg.K (Table A-1). Analysis The exergy or work potential of the blowing air is the kinetic energy it possesses, Exergy = kJ/kg 032 . 0 s / m 1000 kJ/kg 1 2 m/s) 8 ( 2 2 2 2 2 ¸ ¹ · ¨ © § ke V At standard atmospheric conditions (25 q C, 101 kPa), the density and the mass flow rate of air are U ¡¡ P RT 101 kPa (0.287 kPa m kg K)(298 K) mk g 3 3 / ./ 118 and Thus, kW 23.74 = kJ/kg) 2 kg/s)(0.03 742 ( ke Power Available kg/s 742 = m/s) 8 ( m) 10 )( 4 / )( kg/m 18 . 1 ( 4 2 3 1 2 1 m D AV m & & S V The minimum number of windmills that needs to be installed is windmills 26 # 3 . 25 kW 23.74 kW 600 total W W N & & 8-16 Water is to be pumped to a high elevation lake at times of low electric demand for use in a hydroelectric turbine at times of high demand. For a specified energy storage capacity, the minimum amount of water that needs to be stored in the lake is to be determined. Assumptions The evaporation of water from the lake is negligible. 75 m Analysis The exergy or work potential of the water is the potential energy it possesses, Thus, kg 10 2.45 10 u ¸ ¸ ¹ · ¨ ¨ © § ¡ ¸ ¹ · ¨ © § u s/kg kW 1 s / m 1000 h 1 s 3600 m) 75 )( m/s 8 . 9 ( kWh 10 5 mgh = PE = Exergy 2 2 2 6 gh PE m
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8-3 8-17 A heat reservoir at a specified temperature can supply heat at a specified rate. The exergy of this heat supplied is to be determined.
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This note was uploaded on 09/21/2009 for the course ME 311 taught by Professor Ferrenberg during the Fall '09 term at Nevada.

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ThermoSolutions-CHAPTER08 - 8-1 Chapter 8 EXERGY A MEASURE...

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