Gestão de Sistemas Energéticos 2015/2016 Exergy Analysis Prof. Tânia Sousa

Exergetic Efficiency Energy and exergy balances at steady-state? Moran et al., 2014 T0T0 Heat lost

Exergetic Efficiency Energy and exergy balances at steady-state Moran et al., 2014 T0T0 Heat lost

Exergetic Efficiency Energy efficiency? Moran et al., 2014 T0T0 Heat lost

Exergetic Efficiency Energy efficiency Moran et al., 2014 T0T0 Heat lost

Exergetic Efficiency Exergy efficiency? Moran et al., 2014 T0T0 Heat lost

Exergetic Efficiency Exergy efficiency Moran et al., 2014 T0T0 Heat lost

Exergetic Efficiency Exergy efficiency Exergy analysis: (mis)match between energy used and end-use Moran et al., 2014 T0T0 Heat lost

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

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

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? W 1 2 P 1 =5 bar T 1 =320 ºC s 1 = kJ.kg.K -1 h 1 = kJ.kg P 2 =1 bar s 2 = kJ.kg.K -1 h 2 = kJ.kg T 0 =298 K

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? – What is the heat released if specific work equals net specific exergy? W 1 2 P 1 =5 bar T 1 =320 ºC s 1 = kJ.kg.K -1 h 1 = kJ.kg P 2 =1 bar s 2 = kJ.kg.K -1 h 2 = kJ.kg T 0 =298 K

Exergetic Efficiency Energy and exergy efficiency of an adiabatic counterflow heat-exchanger?

Exergetic Efficiency Compute the energy and exergy efficiencies of the heat-exchanger: State h (kJ/kg) e f (kJ/kg) m c /m h = 1/3

Exergetic Efficiency Discuss differences in energy and exergy efficiencies State h (kJ/kg) e f (kJ/kg) m c /m h = 1/3

Exergetic Efficiency What is the “optimal” exergy efficiency value? State h (kJ/kg) e f (kJ/kg) m c /m h = 1/3

Exergy analysis in design What are the sources of exergy destruction?

Exergy analysis in design What are the sources of exergy destruction? What can we do to minimize it? Cost  T ave

Exergy analysis in design What are the sources of exergy destruction? What can we do to minimize it?

Exergy analysis for allocating costs Cogeneration system

Exergy analysis for allocating costs

Cost accounting: C F is the cost rate of the fuel, C 1 is the cost rate of the high pressure steam, and Z b accounts for the cost rate associated with owning and operating the boiler

Exergy analysis for allocating costs Cost accounting: C F is the cost rate of the fuel, C 1 is the cost rate of the high pressure steam, and Z b accounts for the cost rate associated with owning and operating the boiler Relationship between the unit costs c 1 and c F ?

Exergy analysis for allocating costs What is the unit cost of electricity?

Exergy analysis for allocating costs The unit cost of electricity What are the three possibilities?

Exergy analysis for allocating costs c 2 =0 c 2 =c 1 c 2 =c e

Exergy analysis for allocating costs The unit cost of electricity If on an exergy basis what will happen on an energy basis? c 2 =c 1

Exergy analysis for allocating costs Power and exergy rate that exits the turbine (steady- state with negligible heat transfer)?

Exergy analysis for allocating costs Unit costs of steam exiting the boiler, steam exiting the turbine and power?

Exergy analysis for allocating costs Cost rates of steam exiting the turbine and power?

Cogeneration as a share of national power production in EU

Cogeneration Simultaneous production of usable heat (steam or hot water), electricity and in certain cases cooling (chilled water) Traditional Power Plants vs. CHP

Cogeneration How cogeneration saves energy? Benefits and problems?

Cogeneration Cogeneration system design options

Cogeneration Systems Heat demands satisfied through cogeneration: –Residential and commercial needs that require large amounts of heat at low temperatures –Industrial processes that require heat at a wide range of temperatures Large cogeneration systems: –A steam turbine in Switzerland that generates 465 MW of thermal power and 135 MW of electrical power Micro and small scale CHP systems (individual buildings): –In the range between 15 kWe to 1MWe

Cogeneration Typical commercial cogeneration system

Absorption refrigeration system Ammonia is the refrigerant and water the aborbent Heat drives ammonia vapour out of the solution

Simple-cycle cogeneration Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)

Cogeneration: Topping Cycle Supplied fuel first produces power followed by thermal energy Thermal energy is a by product used for process heat or other Most popular method of cogeneration

Cogeneration: Bottoming Cycle Primary fuel produces high temperature thermal energy Rejected heat is used to generate power Suitable for manufacturing processes

Efficiencies of separate processes

Benefits of Cogeneration Substitution of fuel driven cogeneration with fuel driven electrical generation and fuel heating Comparison using first law efficiencies? What are the exergy efficiencies assuming that the energy and exergy of the fuel are identical? T 0 = 288K 423K

Benefits of Cogeneration Substitution of fuel driven cogeneration with fuel driven electrical generation and electrical heating Comparison using first law efficiencies? What are the exergy efficiencies assuming that the energy and exergy of the fuel are identical? T 0 = 288K 423K

Benefits of Cogeneration Cogeneration increases the energy and exergy efficiencies compared to separate processes The exergy efficiency is much lower than the energy efficiency for cogeneration and non-cogeneration. Why? Separate SystemCogeneration Caso a)  =66.2%  =30.9%  =92%  =43% Caso c)  =35.5%  =16.6%  =92%  =43%