organic rankine cycle for light duty passenger vehicles · 2014-03-04 · organic rankine cycle for...
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Organic Rankine Cycle for Light Duty Passenger Vehicles
Quazi Hussain and David Brigham Research & Advanced Engineering
Ford Motor Company
Directions in Engine-Efficiency and Emissions Research (DEER) 2011 Conference
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System Configuration
EVAPORATOR
CONDENSER PUMP
EXPANDER EXHAUST IN EXHAUST OUT
0
5
10
15
20
25
30
35
40
200 250 300 350 400 450 500 550
h (kJ/kg)
P (b
ar)
1
2 3
4
1
2 3
4
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Key Questions • Can we generate power from waste heat using ORC in a light duty
passenger vehicle? • Is the amount of power generated sufficient to bring about a fuel
economy benefit? • What are the power output characteristics for a conventional and a
hybrid vehicle? • What is the backpressure effect?
Content Overview • Approach • Design Sensitivities and Trade-offs • System Behavior • Power Output • Backpressure Effect • Summary
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Approach: Overview
Exhaust enthalpy vs. time (downstream of cat converters)
- Mech. power vs. time - Elec. power vs. time - System backpressure
Model
Features: • Fully transient • Includes effect of fluid and component warm-up • Includes dependencies on key design parameters for each component
• Allows Design of Experiment based investigations and optimization
- ORC Operating conditions - Component size/geometry
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Approach: Working Fluid Selection
• Advantageous properties of organic fluids: – Very low (-100 C) freezing point – “Dry fluid” - after expansion it stays in superheated region no need to
superheat – Low condensation temperature Coolant heat can be used to preheat the fluid
which can increase power output • Issue with organic fluids:
– Cooling with ambient air requires large condenser surface area due to small temperature difference.
• R245fa selected for this study: – 1,1,1,3,3,-pentafluoropropane; CF3CH2CHF2 – No Ozone impact – Low Global Warming Impact – Non-flammable – Thermodynamic state properties evaluated from NIST database REFPROP V7
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Approach: Model Features
– The evaporator is shell & tube type with exhaust gas passing through the tubes.
– Heat transfer on gas side use familiar Re-Pr correlations – The evaporator uses Rohsenow nucleate pool boiling correlation. – The high side and low side pressures are held at a fixed value. – The mass flow of working fluid varies with the available heat. – The efficiency of the pump and turbine are assumed to be
constant . – No heat is lost to the surroundings. – Condenser is modeled as a heat sink.
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Design Sensitivity Studies
• Evaporator Design: – Length: 200mm → 400 mm – Number of tubes: 100 → 200 – tube diameter: 5mm → 7 mm – volume: 1.125L → 8.0L
• High Side Pressure: 25 bar → 30 bar • Low Side Pressure: 3.54 bar • Component Efficiencies
– Expander: 50% – Pump: 50% – Heat exchanger: 50% – Generator: 80%
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DOE Results
2009 Escape Highway Drive Cycle
LoadAccessoryVehicleGeneratedPowerFactorPower =
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Critical Trade-Offs
• Increasing evaporator size: → Increased power output → Large thermal mass poor drive cycle transient performance
• Small tube diameter/tube number: → Smaller package → High backpressure
• Increasing peak working pressure: → Increased power output → Increased expander cost
• Designs selected for detailed study: – L = 200mm, D = 5mm, N = 100 – 25 bar peak pressure
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System Behaviors (HEV)
City drive cycle
Highway drive cycle
PF = 0.323
PF = 0.98
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System Behaviors (Conventional)
Conventional City drive cycle
Conventional Highway drive cycle
PF = 0.513
PF = 0.9
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Muffler 1 Muffler 2
Muffler 1 Muffler 2 Heat Exchanger
PA
PB
Hot Gas
Cooler Gas
• For the above case, engine back pressure actually decreases due to lower temperature entering the muffler system
• The back pressure effect depends on the heat exchanger design, e.g. if the heat exchanger is too restrictive to flow then backpressure will increase
PB < PA
Hot Gas
ambient
ambient
Engine
Engine
Large Temp drop at muffler inlet
Lower backpressure
PB
PA
Hot Gas
Backpressure Effects
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2.55
1.13
0.41
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Bac
kpre
ssur
e (k
Pa)
Muffler Assembly Heat Exchanger
Baseline system without exhaust heat recovery
System with exhaust heat recovery
Whether backpressure would go up , go down, or stay the same depends on the heat exchanger design
Cycle Avg. Backpressure
PA
PB
1.54
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Summary
• Organic Rankine Cycle using the exhaust waste heat in a light duty passenger vehicle can generate enough electricity to partially offset the accessory load.
• The amount generated varies depending on the type of vehicle, drive cycle, and system size.
• Under EPA highway drive cycle, the system comes close to meeting the electric accessory load demand of the vehicle.
• The output during EPA city cycle is less than the accessory load demand. • HEV output in city driving is much lower compared to a conventional vehicle
due to lower exhaust energy caused by frequent engine shut down events. • HEV highway output is slightly higher than a conventional vehicle due to
higher engine work (higher exhaust energy) needed to propel a heavier vehicle.
• Due to excessive cooling of the exhaust gas in the evaporator, the net backpressure can be actually lower with a well designed ORC system.