Gestão de Sistemas Energéticos 2016/2017 Exergy Analysis Prof. Tânia Sousa taniasousa@tecnico.ulisboa.pt

5 kW

5 kW

Exergy Rate Balance for Closed Systems The exergy rate balance Steady-state

Energy Balance in Open Systems Nos três acetatos seguintes coloquei bonecos para poderes explicar melhor as leis da ternodinâmica

Energy Balance in Open Systems Mass Change =  Mass Flows Energy Change = Heat + Work + Energy in Mass Flow Nos três acetatos seguintes coloquei bonecos para poderes explicar melhor as leis da ternodinâmica Enthalpy of component j Flows at the boundaries

Entropy Balance in Open Systems Nos três acetatos seguintes coloquei bonecos para poderes explicar melhor as leis da ternodinâmica

Exergy Rate Balance for Control Volume Systems The exergy rate balance for closed systems The exergy rate balance for open systems

Exergy Rate Balance for Control Volume Systems The exergy rate balance for closed systems The exergy rate balance for open systems Steady-state?

Exergy Rate Balance for Control Volume Systems The exergy rate balance for closed systems The exergy rate balance for open systems Steady-state?

The specific flow exergy What is the specific flow exergy?

The specific flow exergy Exergy is the maximum Wc obtainable from a mass flow plus the environment as it comes into equilibrium (goes to the dead state T0 & P0). Obtain an expression for the specific flow exergy

The specific flow exergy Flow exergy is the maximum Wc obtainable from a mass flow plus the environment as it comes into equilibrium (goes to the dead state T0 & P0). The specific flow exergy is: The change in specific flow exergy is:

The specific flow exergy Flow exergy is the maximum Wc obtainable from a mass flow plus the environment as it comes into equilibrium (goes to the dead state T0 & P0). The specific flow exergy is:

Exercise sº1=2.21952 kJ.kg.K-1 h1 =503.02 kJ.kg sº2=1.76690 kJ.kg.K-1

Exercise Figure shows a device to develop power using a heat transfer from a high-temperature industrial process together with a steam input. The figure provides data for steady-state operation. All surfaces are well insulated, except for the one at 500°C, across which heat transfer occurs at a rate of 4.25 kW. The device develops power at a rate of 6.1 kW. Determine, in kW,(a) the rate exergy enters accompanying heat transfer. (b) the net rate exergy is carried in by the steam, (Ef1 - Ef2). (c) the rate of exergy destruction within the device. Ignore the effects of motion and gravity and let T0 = 293 K, p0 = 1 bar. Moran et al., 2014

Specific flow exergy vs. specific exergy What is the difference between specific flow exergy and specific exergy?

Specific flow exergy vs. specific exergy What is the difference between specific flow exergy and specific exergy? The exergy of flow work

Exergetic Efficiency Energy and exergy balances at steady-state? T0 Heat lost T0 Moran et al., 2014

Exergetic Efficiency Energy and exergy balances at steady-state T0 Heat lost T0 Moran et al., 2014

Exergetic Efficiency Energy efficiency? T0 Moran et al., 2014 Heat lost T0 Moran et al., 2014

Exergetic Efficiency Energy efficiency Heat lost T0 Moran et al., 2014

Exergetic Efficiency Exergy efficiency? T0 Moran et al., 2014 Heat lost T0 Moran et al., 2014

Exergetic Efficiency Exergy efficiency Heat lost T0 Moran et al., 2014

Exergetic Efficiency Exergy efficiency Exergy analysis: (mis)match between energy used and end-use Heat lost T0 Moran et al., 2014

Exergetic Efficiency How does exergy efficiency varies assuming ? Exergy analysis: (mis)match between energy used and end-use Ts=2200K =100%  Tu Moran et al., 2014

Exergetic Efficiency Exergy efficiency Exergy analysis: (mis)match between energy used and end-use Ts=2200K =100% Moran et al., 2014

Exergetic Efficiency Exergy efficiency of a turbine in steady-state with no heat transfer? W 1 2

Exergetic Efficiency Exergy efficiency of a turbine in steady-state with no heat transfer W 1 2

Exergetic Efficiency Steam turbine in steady-state with no heat transfer. What is the specific work? What is the net specific flow exergy? What is the meaning of the net specific flow exergy? T0=298 K P1=5 bar T1=320 ºC s1=7.5308 kJ.kg.K-1 h1=3105.6 kJ.kg W 1 2 P2=1 bar s2=7.7134 kJ.kg.K-1 h2=2850.96 kJ.kg

Exergetic Efficiency Steam turbine in steady-state with no heat transfer. What is the specific work? What is the net specific flow exergy? What is the meaning of the net specific flow exergy? T0=298 K P1=5 bar T1=320 ºC s1=7.5308 kJ.kg.K-1 h1=3105.6 kJ.kg W 1 2 P2=1 bar s2=7.7134 kJ.kg.K-1 h2=2850.96 kJ.kg