Exergy presentation
description
Transcript of Exergy presentation
-
Exergy in Processes
Flows and Destruction of Exergy
-
Exergy of Different Forms of Energy
Chemical Energy
Heat Energy
Pressurised Gas
Electricity
Kinetic Energy
-
Oxidation of Methane
H = -890.1 kJ/mol
S = -242.8 J/(mol.K)
Exergy available = - H + T0 *S
If T0 = 298K, then:
Exergy = 817.9 kJ/mol
Energy quality = 92%
-
Heat
If T0 = 10C (283K)
Heat at 2000C (2273K), energy quality = 87.5%
Heat at 100C (373K), energy quality = 24.1%
Heat sink at -100C?
-
Heat
Water at 100C , reference T 10C
As heat is taken from it, its temperature gradually decreases.
So, the exergy of the first heat removed is that of heat at 100C (energy quality 24.1%)
The exergy of the last heat removed is that of heat at just above 10C (energy quality zero)
The average energy quality of all the heat can be calculated either by doing a mathematical integration or by looking up thermodynamic data and calculating the changes in H and in S.
The result is 13%
-
Heat
Steam at 100C
Step 1 condense steam becomes water at 100C, about 2260 kJ/kg of enthalpy, all at 100C.
Exergy = 544.7 kJ/kg
Energy quality = 24.1%
Step 2 as for water at 100C
Total Exergy = 594.3 kJ/kg, energy quality = 22.6%
-
Compressed Air
1 L volume of air at 2 atmospheres pressure, expanded into 1 L of vacuum
Enthalpy of decompression .. zero!
Entropy change 0.47 J/K
If T0 = 298K, then
Exergy = 139 J
Energy quality. 139/0 ????
-
Electricity
No entropy
Nothing random about it.
If DC, the voltage is always the same.
If AC, the voltage is completely predictable.
-
Kinetic Energy
Movement of a body
(Laminar) flow of fluid
both predictable no entropy
Thermal motion
random entropy depends on temperature
-
Destruction of Exergy
Irreversible events during the process
leak
pressure drop in flowing fluid
heat transfer
friction
electric circuit losses
combustion
-
Effect of Irreversibility
Starting Entropy
S
Endpoint Entropy
Reversible
Reversible
Endpoint Entropy
Starting Entropy
Irreversible
Reversible
Reversible only With irreversible event
S
-
Exergy DestructionReversible Process Only
Enthalpy change H
Entropy change S
Exergy = - H + T0 * S
With Irreversible Event
Enthalpy change H
Reversible entropy change S Sirr
Exergy =- H + T0 * (S Sirr )
Exergy destroyed = T0 * Sirr
-
Exergy Loss
Irreversible event find S
How?
Use literature information on entropy of before and after states
Look at heat flow from high T to lower
Look at reversible route for the same change and evaluate the integral of dq/T
-
Exergy Loss
Example combustion
Definitely irreversible, and generally no work or heat transfer take place during the event
Gases react, forming combustion products
Use H to calculate temperature achieved
Get entropy numbers for products
Compare total entropy of products with entropy of the starting materials at the starting temperature
Result is the entropy change its all irreversible if there is no heat transfer
Exergy loss is T0 S
-
Exergy Loss
Example heat transfer
Heat q moves from reservoir at T1 to reservoir at T2
Entropy of first reservoir decreases by q/T1
Entropy of second reservoir increases by q/T2
Increase is q(1/T2 1/T1 )
Exergy loss is T0 * q(1/T2 1/T1 )
-
Exergy Loss
Ideal gas expands to double its volume (leak)
What is an equivalent reversible process?
Isothermal expansion, doing work (heat in, work out)
If n moles of gas are at pressure P, temperature T, then work out is: nRT ln(2)
heat in is also nRT ln(2)
So: S = nR ln(2)
Exergy loss = T0 nR ln(2)
-
Basic Heat Power Cycle
Heat in
Heat out
Power out
Power inPump Motor
Pressure high
Pressure low
-
Power Plant the Exergy View
Boiler
Turbine
Pump
Condenser
Power
Cooling Water
Gas
Air
Exhaust Water
Steam
-
1 - Combustion
Burn methane in just sufficient air to provide the oxygen required. (Start at 25C, 298K)
Temperature reaches 1950C, 2223K.
Entropy increase from start is 802.0 J/(mol.K). This is an irreversible process.
Exergy destruction is 239.0 kJ/mol, or 29% of the starting exergy.
-
Combustion
Flame
Air, 25C
Gases, 1950CMethane, 25C
Exergy loss 29%
Energy loss - nil
-
2 Heat Transfer
Hot gases from combustion transfer heat to water at 25C, making steam at 538C and critical pressure (217.7 atm)
Combustion gases cooled to 25C, and water condensed
Gas entropy decreases by 1060.3 J/K per mol of methane
Water entropy increases by 1661.4 J/K per mol of methane
Net entropy increase of 601.1 J/K per mol of methane
Exergy destruction 179.1 kJ/mol, or 22% of the starting exergy.
Total destroyed so far is 51%
-
Heat Transfer
HeatExchanger
Gases, 1950CGases + condensedwater, 25C
Water, 25C, 217atmSteam, 538C, 217atm
-
Turbine and Condenser
A big steam turbine can extract 80-90% of the theoretically available energy
In this example, the turbine might produce work equivalent to 30% of the exergy, and destroy 7%.
Condensers have big heat flows, but at temperatures not much above ambient, so exergy losses there are about 3%
-
Power Plant Energy Flows
Cooling Water 60
BoilerFuel 100
Stack 5
Steam 95 Shaft Power 32
Steam 60
Other Losses 3
TurbineCondenser
-
Power Plant Exergy Flows and Destruction
27Fuel 92
Stack 2
Steam 43
7Shaft Power 32
2Steam 3
Other Losses 1
Cooling Water 1
Turbine
Condenser
Combustion
2065
HeatTransfer
-
Gas Turbine
Air is compressed
Natural gas is burned in the compressed air
A turbine takes power from the hot compressed air
There is still combustion, but no heat exchanger
-
Gas Turbine
Air in
Compressor, 15x, 85% efficient
Gas in
Shaft power Shaft power out
Turbine Inlet Temperature 1000 C
Turbine, 85% efficient
-
Gas Turbine Energy Flows
Air in
Compressor, 15x, 85% efficient
Gas in 100
Shaft power 59 Shaft power out 32
Turbine Inlet Temperature 1000 C
Turbine, 85% efficient
Heat out 6859 159
-
Gas Turbine Exergy Flows and Destruction
Air in
Compressor, 15x, 85% efficient
Gas in 92
Shaft power 59 Shaft power out 32
Turbine Inlet Temperature 1000 C
Turbine, 85% efficient
Heat out 1654 115
531
8
-
Home Furnace Losses 1st Law
Fuel 100
Exhaust 5
Heat to Building 95
-
Home Furnace Exergy Flows and Destruction
27 Heat Transfer58Fuel 92
Combustion
Heat to Building 6
Exhaust 1
-
Energy Efficiency
Usually defined as the fraction of energy that goes where you want it to.
The denominator is the enthalpy available
The numerator is the electricity produced, the heat that goes to the purpose intended, a total of the two (cogeneration)
-
Apples and Oranges
Power generation 50% is very good
House furnace 70% is very poor!
Its easy to avoid energy losses
Its very difficult to avoid exergy destruction.
-
Exergy Analysis
Levels the energy playing field
Consistent method to present the value of energy that is in different forms
Choice of reference temperature depends on the purpose of the analysis
Exergy in ProcessesExergy of Different Forms of EnergyOxidation of MethaneHeatHeatHeatCompressed AirElectricityKinetic EnergyDestruction of ExergyEffect of IrreversibilityExergy DestructionExergy LossExergy LossExergy LossExergy LossBasic Heat Power CyclePower Plant the Exergy View1 - CombustionCombustion2 Heat TransferHeat TransferTurbine and CondenserPower Plant Energy FlowsPower Plant Exergy Flows and DestructionGas TurbineGas TurbineGas Turbine Energy FlowsGas Turbine Exergy Flows and DestructionHome Furnace Losses 1st LawHome Furnace Exergy Flows and DestructionEnergy EfficiencyApples and OrangesExergy Analysis