Exhaust waste heat recovery Rankine system sizing -...

Exhaust Waste Heat Recovery Systems Application to Rankine system

HERGOTT Julien

17ème Cycle de conférences Cnam/SIA 15 Mars 2016

Waste heat recovery technologies

Waste heat recovery potential

Heat to heat systems

Heat to power systems

Focus on Rankine system

Cycle principles

System architectures

Components technologies

System sizing for commercial vehicle application

Design steps

Design point / operating range

Design and constraints variables

Conference agenda

17e cycle conference CNAM/SIA - PARIS - 15/03/2016 2






Cycle principles




Design steps



Conference agenda


Total thermal losses represent 60 to 80 % of fuel energy ! About 30% used to move the vehicle

About 30% used to cool engine cylinder ( maintain material temperature)

About 30% remaining in the exhaust gases (thermal cycle efficiency)

Thermal losses are mandatory and can’t be avoided

Waste Heat Recovery Potential Why do we want to recover heat ?

FECT measurements

1.6L 4Cyl SI Turbo at 100 km/h

The only way to value these losses is to use waste heat recovery systems


3,3 kW

8%

11,5kW

28%

2,7kW

6% 3,5kW

8%

12,2kW

30%

11,5kW

28%

mechanical

cooling

unbrurnt gases

radiated

exhaust

exhaust condensed

http://energy.gov/eere/vehicles/fact-880-july-6-2015-conventional-vehicle-energy-use-where-does-energy-go

Waste Heat Recovery Potential Exergy analysis

Exhaust exergetic content much higher than coolant

FUEL 100%

To Wheels : 33%

To Exhaust 600C: 15% potential work FUEL 100%

T0 = 20°C

Energy balance Exergy balance

Exhaust energetic content equivalent to coolant

Exhaust gases have the highest potential to produce work !

To Coolant 90C : 5% potential work

Destruction: 47%


Exergy is amount of energy that can be converted into useful work

Mechanical / electrical work is pure exergy

Thermal energy converted into work has an upper limit: Carnot efficiency

Exergy of heat source depends on its temperature

The higher the temperature, the higher the exergy

Waste Heat Recovery Potential Passenger Cars

Passenger car engine is largely oversized for exceptional but mandatory driving conditions

Engine is mainly used in low efficiency area

Hybrid Electric Vehicles (HEV) partially addresses that issue

It can handles urban usage where there is the lowest efficiency


4cyl 1.6L turbocharged gasoline

WLTC

Source: internal

Engine speed

En

gin

e T

orq

ue

7

Waste Heat Recovery Potential Passenger Car energy balance

Chemical

6L/100km

18.5 kW thermal

WLTC

Sources : internal

A car is mainly used in transients, especially in urban conditions

Heat to heat can support engine warming and cabin heating.

Regenerative BRAKING • Electric

• Flywheel

• Others (hydraulics)

Heat to Heat • EHRS

• EHRM

1.5L/100km

4.6 kW

1.5 L/100km

4.6 kW

3 L/100km

9 kW

Kinetic energy

0,7 L/100km

Frictions 0,8l/100

High grade Heat

4.6 kW

1.5 L/100km

20% efficient

Where is my money? What could be recovered ? How could it be recovered ? What I paid for

Low grade Heat

9 kW

3 l/100

EXHAUST

EGR

MECHANICAL

COOLING

AND OTHERS

25%

25%

50%

Heat to work • Rankine

• TEG

• Compound

• Air cycles

5-10% thermal efficient = 5-10% fuel eco

< 4 % Efficient = < 8 % FE

17e cycle conference CNAM/SIA - PARIS - 15/03/2016

Waste Heat Recovery Potential Trucks

A truck is used mainly in steady operating conditions

Quasi steady engine speed (90 km/h motorway)

Variable engine load depending on trip elevation

80% of time engine used at best efficiencies (40-45%)

No challenging technologies as for passenger cars

Hybridization is not considered yet

8

80% of time

40-45% brake efficiency

6 cyl 11L diesel engine

Typical real road conditions


Source: internal

Engine speed

En

gin

e T

orq

ue

9

Waste Heat Recovery Potential Trucks energy balance

13 L/100km

102 kW

7L/100km

55 kW

8 .3 L/100km

66 kW

Aerodynamics

and frictions

11 l/100km

Lost in air

High grade

55 kW

7 L/100km

EXHAUST

EGR

Chemical

30 l/100km

240 kW

thermal

Typical long

haul trip

Sources : internal

CVE usage is mainly steady state. In this mode, hybrid is not able to produce fuel saving.

In this context, Exhaust energy recovery is a promising technology.

Where is it money? What could be recovered ? How could it be recovered ? What my company

paid for ?

Low grade

66 kW

8.3 L/100km

Heat to work • Rankine

• TEG

• Compound

• Air cycles

5-13% thermal efficient = 3 - 8 % Fuel Eco

< 5 % Efficient = < 3 % FE

Kinetic energy 2 L/100km

MECHANICAL

COOLING

AND OTHERS

42.5%

23%

27.5%







Cycle principles




Design steps



Conference agenda


Main function is to transfer heat from exhaust to an other fluid (oil or engine coolant)



Heat to Heat systems

Exhaust gases

Engine coolant

Oil

Indirect fuel economy

• Thermal management (hybrid vehicles)

• Reduce friction (oil heating)

Comfort

• Increase engine coolant heating

𝑄 𝑖𝑛

𝑄 𝑜𝑢𝑡

Heat to heat technologies EHRS


How it works

• Gas exchange heat with fluid (oil or engine coolant) with heat

exchanger

• Exchanger is bypassed when engine coolant is warm or to

reduce exhaust back pressure at high exhaust mass flow rate

Application

• Hybrid vehicles : Reduces engine utilization to maintain coolant

temperature (thermal management)

• Comfort for winter conditions (faster coolant warm-up)

Exhaust Heat Recovery System

Technology constraints

• Manage exhaust pressure drop

• Avoid fluid boiling

Status : In production

Hyundai IONIQ

Application

• Gasoline turbo engine : Avoid air/fuel mixture enrichment at high engine load to maintain exhaust gases below 950 degC (turbine protection)

• Faster engine Warm-up

• Manifold durability

Heat to heat technologies EHRM


How it works

• Exhaust manifold cooled down by engine coolant flowing through

manifold walls

Exhaust Heat Recovery Manifold

Technology constraints • Efficiency limited to keep high temperature for turbine afertreatment

• In competition with integrated manifold


Ford Fusion 2013

Adsorption phase : cabin cooling Desorption phase : heat recovery

Heat to heat technologies Adsorption cycles


Application • Air conditioning system

• Refrigerated truck

How it works ?

• Cooling cycle without mechanical compressor

• It uses sorbent salt / NH3 saturation properties

• Salt acts as a “thermal” compressor

• Gas pumping when salt is cooled

• Gas discharge when salt is heated

Technology constraints • System duty cycle / Thermal inertia of reactor

• Liquid ammonia safety ?

• Component maturity

Status : Exploration

Cabin

Air

Air exhaust

Air

Cabin






Cycle principles




Design steps



Conference agenda


Heat to power technologies


Main function is to convert heat into useful power (electrical or mechanical)

Heat to Power

systems

Exhaust gases

Engine coolant

Ambient air

𝑄 𝑖𝑛

𝑄 𝑜𝑢𝑡

𝑊

Direct fuel economy

• Mechanical link to engine crankshaft

Energy storage

• Battery charging (hybrid vehicles)

Comfort • Increase temporarily maximum engine power

Heat to Power technologies TEG


How it works ?

• Heat exchanger with thermoelectric generators layers between

exhaust gases and engine coolant

• Temperature difference between plates produce electron

circulation = electric current

• Electric production with only one component and no moving

parts !

Thermo Electric Generator


• Low efficiency with actual materials (<8% thermal efficiency)

• Heat exchanger sizing challenging

• Efficiency requires temperature difference = wall thermal

resistance

• Power requires heat transfer = wall thermal conductance

• Material costs

Exhaust gases

Engine coolant

Status : Innovation ( 2025+)

Heat to heat technologies Turbocompound


How it works ?

• Additional turbine mounted downstream turbocharger turbine

• Power output can be either electrical or mechanical

• It is more used as power boost than as a fuel saving component


• Mostly valuable at high engine load

• Impact of additional pressure drop at low engine load ?

http://www.voith.com/


Scania / Volvo / Daimler

Heat to power technologies Air cycles


How it works ?

• Compressor pressurize ambient air and feed an heat exchanger

• Heat exchanger recovers heat from exhaust to increase air

temperature

• Expander expand high temperature/pressure air to recover heat

• Components architecture depends on thermodynamic cycle

• Joules-Brayton

• Stirling

• Ericsson …


• Work only at high exhaust temperature (>500 degC)

• Compression work

• Air/air heat exchanger = low overall heat transfer

coefficient

• Technology maturity

Joules-Brayton 2 isobaric

2 isentropic

Stirling

2 isochoric

2 isentropic

Heat

exchanger

expander compressor

𝑄 𝑒𝑥𝑎𝑢𝑠𝑡

𝑊 𝑜𝑢𝑡

Ambient air

𝑄 𝑜𝑢𝑡

Status : Exploration

Heat to heat technologies EHPG


Exhaust Heat Power Generator

How it works ?

• Thermal cycle based on Rankine process

• Pump feed evaporator with high pressure liquid

• Evaporator generates high pressure vapor with exhaust heat

• Expander expand vapor to produce mechanical work

• Condenser turns low pressure vapor back into liquid


• Maximum pressure / temperature

• Packaging

• System control

• Cost vs return of investment (ROI) and system mass

evaporator

pump

expander

condenser

1

2

3 4

Status : Innovation ( 2020+)






Cycle principles




Design steps



Conference agenda


Heat Source

10kW

Expander Work

1kW

Heat Sink

9.01 kW

Pump Work

0,01 kW

Electric or

mechanic Electric or

mechanic

Rankine cycle principle Basic layout

𝑄 𝑖𝑛

𝑄 𝑜𝑢𝑡

𝜂𝑡ℎ =𝑊 𝑒𝑥𝑝 −𝑊 𝑝𝑢𝑚𝑝

𝑄 𝑖𝑛


* Example values

Rankine cycle principle Temperature/Entropy diagram


Critical point

2 phase region

Supercritical region

Liquid region Superheated Vapor

region

Te

mp

era

ture

[d

eg

C]

• Saturated liquid: liquid ready to boil (any heating lead to vapor bubbles apparition)

• Saturated vapor: vapor ready to condensate (any cooling makes liquid droplets apparition)

Entropy = energy dispersion

Homogeneity = maximum entropy !

T1 T2 T3 T3 time

Low entropy High entropy

𝑻𝟏 > 𝑻𝟑 > 𝑻𝟐

Entropy [kJ/kg/K]

Rankine cycle principle Temperature/Entropy diagram


This diagram is very useful to asses graphically on cycle performances

Power output calculated with evaporator heat rate

Isobaric

Condensation

Isobaric

evaporation

compression

𝒘𝒊𝒔𝒆𝒏𝒕𝒓𝒐𝒑𝒊𝒄

Expansion

perfect = Isentropic

real

Te

mp

era

ture

[d

eg

C]

Entropy [kJ/kg/K]

Isobaric: constant pressure process

Isothermal: constant temperature process

Isentropic: constant entropy process (ideal)

Rankine cycle principle ideal vs real


coolant coolant






Cycle principles




Design steps



Conference agenda


System architectures Potential Architectures

exhaust exhaust + EGR

Heat Source

Piston Scroll Turbine

Expander

Screw

Power Output

mechanical electrical

Options

Pressure Vessel Regenerator

Heat Sink

Low temperature

Direct air cooling

High Temperature

Water cooling

Ethanol Organic

Fluid

There is a lot of potential configurations ! 17e cycle conference CNAM/SIA - PARIS - 15/03/2016 28

Depends on OEM engine architecture. EGR trends not clear yet

Not enough exergy on exhaust gases

(system specific power)

For now EGR is not spread on gasoline engine

System Architecture Heat Source selection

Gasoline LV

Exhaust EGR EGR + Exhaust Exhaust EGR EGR + Exhaust

Diesel CVE

Exhaust EGR EGR + Exhaust

Diesel LV

Exhaust gases always used as primary heat source

If available, EGR interest should be valuated with a complete economical study


High capacity flow Low capacity flow

Condensing pressure is higher with low capacity flow

System Architecture Heat Sink selection

Heat sink characteristics

Capacity flow

𝐶 = 𝑚 × 𝑐𝑝

Temperature

A good heat sink is a sink at low temperature and high capacity flow !

• ability to absorb heat with limited temperature increase

• Low heat capacities can be compensated by mass flow rates

• Cooling potential

• the lower the source temperature, the higher the cooling potential


𝑻𝒔𝒂𝒕 ↑

Engine coolant Ambient Air

Main circuit Dedicated circuit

Temperature

High

80 to 100 degC

Medium 60 degC

Low − 41 degC in Mouthe (Winter)

40 degC in Toulouse (Summer)

Capacity flow

High

3 – 10 kW / K

Medium To be designed !

Low 0.3 – 1 kW/K for LV

1 – 3 kW / K for CVE

Rankine Architecture Heat Sink selection

Engine coolant circuit is the most stable source in temperature and capacity flow

Coolant loop is designed to absorb heat with limited temperature increase

Care has to be taken to avoid overheating !

Dedicated coolant loop is a good alternative, but others technologies should bring the need

Air as heat sink allows lower temperatures but higher temperature drops !

It also increase inlet temperature of following heat exchanger (AC & Front end radiator)


System Architecture Energy output


mechanical

electrical

thermal

Mechanical

• Expander is mechanically plugged to engine

Benefits

• Mechanic to mechanic : high efficiency

Drawbacks

• Live usage : not possible to store energy

• Expander speed linked to engine speed

Electrical

• Expander is plugged to a generator to feed battery

Benefits

• Versatile energy management

• More suitable for high speed expander (turbine )

• Expander speed management

Drawbacks

• Mechanic to electric conversion efficiency

• Battery charge/discharge efficiency

Mechanical vs electrical ?

Rendement mécaniques supérieur mais meilleure gestion de l’énergie

Aucun intérêt electrique si pas d’hybridation

ENGINE

CLUTCH

EHPG ENGINE EHPG

GENERATOR BATTERY






Cycle principles




Design steps



Conference agenda


Components technologies Working fluids


What is a good working fluid ?

Thermodynamic

properties

SAFETY / ENVIRONEMENT

• Low Flammability

• Low Global Warming potential

COST

• As low as possible

• Material compatibility

THERMODYNAMIC PROPERTIES

• High evaporating pressure

• Low condensing pressure

• Low Pumping work

• Dry expansion

• High Degradation temperature

• Low freezing point

Thermodynamic properties are not the only criterion to select fluid

Dry / wet / isentropic fluids

Positive displacement 𝑾𝒐𝒓𝒌 = 𝚫𝑽𝒐𝒍𝒖𝒎𝒆

Dynamic 𝒘𝒐𝒓𝒌 = 𝚫𝒌𝒊𝒏𝒆𝒕𝒊𝒌 𝒆𝒏𝒆𝒓𝒈𝒚

• Impose mass flow rate (incompressible liquid)

• High pressure ratio / low mass flow rate (oil pump, dosing pump …)

• Oil pump , fuel pump

• Impose pressure head

• Low pressure ratio / high mass flow rate

• engine coolant pump , fuel pump

Components technologies Pump

Gear Vane Lobe Centrifugal Piston

Specification

• Fluid compatibility

• High efficiency

• Voltage for electric motor

• 24V truck

• 12 V cars

Main Function

Pump the fluid

Low pressure

liquid High pressure

liquid

Electricity /

mechanic


Rankine components Evaporator

Preheating zone

Gas / Liquid

Boiling zone

Gas / 2phase

Superheating zone

Gas/ vapor

Fluid out

Gas in

Fluid in

Gas out

Counter flow temperature profile

T

x

wall Gas side

Fluid side

Gas in

Fluid in

Gas out

Fluid out

Counter flow configuration

𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =𝑄

𝑄 𝑚𝑎𝑥

≈𝑇𝑔𝑜𝑢𝑡

− 𝑇𝑔𝑖𝑛𝑇𝑔𝑖𝑛

− 𝑇𝑓𝑖𝑛

• Heat recovery vision: gas as reference for efficiency

• 100% efficiency: gas cooled to fluid inlet temperature


Pinch Point

Tube / shell • fluid flow in the tube

• Gas flow around , in the shell

Plate

• fluid and gas at each side of the plate

Rankine components Evaporator

Main Function

Recover heat

High pressure

subcooled liquid High pressure

superheated vapor

Exhaust gases

Specification

• Compact

• Low gas thermal resistance and low internal fluid volume

• Low exhaust and fluid pressure drop

• Resist to high pressure

• Durability to pressure / temperature variation

Finned round tube corrugated tube

Helicoidal coil tube chevron Plate/fin

Heat exchange area designed to fit a heat recovery target

Exhaust pressure drop must be as low as possible


Spiral

Positive displacement Dynamic

• Work = Volume variation

• Expander absorb volume flow rate in function of expander speed

• Work = kinetic energy variation

• Sonic flow : speed of sound at throat , volume flow independent

of expander speed

Rankine components Expander

Gear Vane Lobe Piston

Specification


• Compact

• Wet fluid : Accept liquid droplet

• Match pressure and temperature

Main Function

Today, there is no preferred technology for automotive application

Produce

mechanical work

High pressure

superheated vapor Low pressure superheated

vapor or biphasic


Rankine components Condenser

Main Function

Evacuate heat

Low pressure

Superheated vapor or biphasic Low pressure

subcooled liquid

Heat sink

Specification


• Compact

• Low pressure drop

• Efficient with low average temperature difference

Same technologies as for evaporator, but not same constraints

Small temperature difference

Low pressure, but has to remains above 1 bar

Technology selection depends on heat sink

Direct air cooling

Low global heat transfer coefficient

Engine coolant High global heat transfer coefficient

Plates Tube/Fins


Focus on Rankine system Part 2 conclusions

There is a lot of system architectures combinations

Fluids / Expander technologies / Cold source ….

There is no preferred system layout yet

Many compromises to deal with

One or two component technology should be frozen to highlight full architecture


System design need to be optimized for each OEM architecture and constraints !






Cycle principles




Design steps



Conference agenda


System sizing for commercial vehicle application Design steps


While no regulation, return of investment is the only criterion to sell the system.

1. Analyze Exhaust and cooling flux • Mission profile analysis on a reference trip

• Define operating conditions

2. Evaluate realistic performances • Define proper design variable and constraints

• Study different architecture ( heat sink / fluid / heat source)

• Build component specifications

3. Lower the ratio 𝒔𝒚𝒔𝒕𝒆𝒎 𝒄𝒐𝒔𝒕

𝑷𝒐𝒘𝒆𝒓 𝒐𝒖𝒕𝒑𝒖𝒕

• Select best component technologies for ideal architecture

• Optimize design paramters to lower the system cost






Cycle principles




Design steps



Conference agenda


System sizing for commercial vehicle application Mission profile analysis

Long haul truck journey selection Vehicle simulation or test

Engine operating range Cooling potential Energy 2D distribution

Mission profile analysis tool

Exhaust available heat

Engine performances

Exhaust flow , temp

Cooling performances

Exhaust and cooling flow/ temperature for operating range


Engine data


Operating range


Evaporating pressure variation

• Boiling Pressure: proportional to boiler heat

• Fixing nominal pressure will fix pressure operating range

Cooling capacity

Without fan

Rejected

by engine

Available for

condenser

Cooling capacity

Without fan

Rejected

by engine

Available for

condenser

System operating range

Cooling limit Min expander

pressure

Exhaust available power range

Maximum cooling capacity

Low vehicle speed / high

engine load 𝑸 𝒃𝒐𝒊𝒍

𝑷𝒃𝒐𝒊𝒍

𝑷𝒎𝒊𝒏

𝑷𝒎𝒂𝒙

Nominal speed and load

Max pressure limit

Design 1

Design 2

Impacted by evaporating pressure variation and cooling capacity

Min Max






Cycle principles




Design steps



Conference agenda


System sizing for commercial vehicle application Performances prediction / Component specifications

What are the design variables and constraints ?

Design variables Variable that define system performances

Design Constraints Upper / Lower limits to design a realistic system

A good knowledge of system is required to define constraints

Realistic constraints = appropriate design variables = realistic components specs

Evaporating pressure , fluid super heat…. Max pressure, min pressure, subcool ….

Thermo Analysis tool

Performances/

Component spec


Design variable and constraints Evaporating pressure


The highest pressure is often not the optimal design in term of cost

Design point pressure define system operating range !

Effect of evaporating pressure increase

• Increase net isentropic work

• Increase required boiler heat transfer area

• Can reduce max recoverable work (pinch location)

• Increase pumping work

Constraints

• Has to remains bellow max allowable pressure

• Expander matching

Design variable and constraints Fluid superheating


Effect of fluid superheat increase


• Avoid droplet formations of wet fluid during expansion

• Increase required boiler area

Constraints

• Fluid outlet temperature bellow degradation temperature

• Minimum superheat for wet fluids and turbine

Fluid superheat is costly in term of boiler area for limited power output increase due to mass flow decrease

Superheat will be kept at a minimum level (10-20 K for dry fluids / 30-50 K for wet fluids)

Design variable and constraints Evaporator performance


Effect of efficiency increase

• Increase recovered heat rate => power output

• Increase quickly required boiler area

Constraints

• Pinch location : pinch located inside evaporator limit max

heat rate

• Evaporator size

Evaporator efficiency designs how much heat will be recovered

Pinch analysis should be done to specify reasonable efficiency / boiler size

No internal pinch

𝜼𝒎𝒂𝒙 = 𝟏𝟎𝟎% internal pinch

𝜼𝒎𝒂𝒙 < 𝟏𝟎𝟎%

Temperature profile

Counter-flow evaporator

𝜂𝑏𝑜𝑖𝑙𝑒𝑟 =𝑄

𝑄 𝑚𝑎𝑥

Design variable and constraints Cooling loop : condensing pressure and subcool


Effect of condenser pressure decrease


• Increase condenser area

• Increase subcool required

Constraints • 𝑃𝑚𝑖𝑛= 1 bar to avoid vacuum (air into system)

• 𝑃𝑖𝑛𝑐ℎ > Pinchmin : depends on sink characteristics

• Subcool > subcool min : avoid pump cavitation

Condensing pressure will be chosen as low as possible while it meets all the constraints

𝑠𝑢𝑏𝑐𝑜𝑜𝑙 = 𝑇𝑠𝑎𝑡 − 𝑇

Design variable and constraints Design variable / constraints summary


With all the constraints, design area is limited.

Optimization work can begin …

General conclusion

Design of Rankine system is challenging, but has the highest short term fuel

economy potential for trucks

Faurecia as technical solutions supplier, wants to supply a full system

System design

Component selection

control

Faurecia develops engineering tools to study such system and support

development and technical assessment

Simulation

Test bench

Next steps : demonstration system !


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