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Energy 60 (2013) 388e400

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Energy

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Working fluid selection for a subcritical bottoming cycle applied to ahigh exhaust gas recirculation engine

Angad S. Panesar*, Robert E. Morgan, Nicolas D.D. Miché, Morgan R. HeikalSchool of Computing, Engineering and Mathematics, University of Brighton, BN2 4GJ, United Kingdom

a r t i c l e i n f o

Article history:Received 3 April 2013Received in revised form11 July 2013Accepted 6 August 2013Available online 4 September 2013

Keywords:Heavy duty diesel engineExhaust gas recirculationWaste heat recoveryBottoming cycleWorking fluid selection

* Corresponding author. Sir Harry Ricardo LaboratEngineering, School of Computing, Engineering andBrighton, Cockcroft Building, Lewes Road, BrightonTel.: þ44 (0)1273 642313; fax: þ44 (0)1273 642330.

E-mail address: A.Panesar@brighton.ac.uk (A.S. Pa

0360-5442/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.energy.2013.08.015

a b s t r a c t

The selection of a suitable working fluid for a BC (Bottoming Cycle) system is one of the most importantsteps in maximising system performance and minimising the system size and cost. The presented workdetails a systematic approach in the selection of working fluids applied to a subcritical cycle withminimum superheat. Over 60 different synthetic, organic, and inorganic fluids were studied. The fluidselection study was decomposed into numerous fluid screening and fluid ranking criteria with commonboundary conditions and assumptions. After the cycle was optimised for maximum overall conversionefficiency, the fluid ranking criteria allowed the objective assessment of the working fluids. Acetone,dichloromethane and trans-1,2-dichloroethylene were found as the best candidates for optimal perfor-mance and system related trade-offs, contrary to commonly used R245fa, ethanol and water. A BC in-tegrated into an EGR (Exhaust Gas Recirculation) only engine platform to meet Euro 6 oxides of nitrogenemission is examined for improved fuel economy and reduced load on the engine cooling module. The BCsimulation results for EGR and partial high temperature after-cooler heat recovery using the proposednew fluids show a specific fuel saving potential between 9.8 and 13.7% for a typical cruise and high loadconditions.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The implementation of the Euro 6 NOx (oxides of nitrogen)limits of 0.4 g/kWh in 2014 will increase Diesel fuel and/or ureaconsumption relative to Euro 5 as a consequence of the additionaluse of emissions control devices. The use of such regulatory tech-niques to limit greenhouse emissions is also anticipated in thefuture, with California finalising its heavy duty tractor-trailergreenhouse regulations [1]. An approach for improving the over-all BSFC (Brake Specific Fuel Consumption) could come fromcapturing the untapped thermal energy in the exhaust of HDDE(Heavy Duty Diesel Engines).

Due to advancements in critical components, the use of fluiddriven BC (Bottoming Cycles) in the range under 200 kW to generatepower in the stationary WHR (waste heat recovery) sector hasbecome an established technology [2]. Pressure to reduce BSFC hasrevived the interest in BCs formobile HDDE platforms with high EGR(Exhaust Gas Recirculation) levels [3] and light duty applications [4].

ories, Centre for AutomotiveMathematics, University ofBN2 4GJ, United Kingdom.

nesar).

All rights reserved.

The proposed BC working fluids for automotive application areusually: fluorinated refrigerants (e.g. R245fa [5] and R245ca [6]),alcohols (e.g. ethanol [7] and methanol [8]) and water [9].

Adoption of BCs for mobileWHR is by nomeans the only way forheat to power conversion. Thermo-electric generators [10],mechanical-turbo compounding [11], electrical-turbo compound-ing [11] have also been proposed. However, the above studies showthat BCs provides the largest BSFC improvement, up-to 7.4% for atypical HDDE drive cycle [3]. BCs are also shown to be less sensitiveto vehicle load and when considered for cost comparisons ($/kW),BCs [11] can be 1/3 of the cost compared to thermo-electric gen-erators [10].

2. Bottoming cycle applied to an exhaust gas recirculationonly engine

Application of a BC has the potential to reduce the BSFC penaltyof a Euro 6 EGR only engine. However, despite the demonstration ofsubstantial fuel savings by modelled BC systems for transportapplication, the experimental setups are not reaching the fuelsavings for production maturity within the expected costs [12,13].Furthermore, HDDE and original equipment manufacturers tend toinvest in fuel saving technologies whereby the maximum paybackperiod is in the 18e24 months range [14], shorter than what BCs

Nomenclature

ATM atmospheric lifetime (years)BTE brake thermal efficiency (%)BSFC brake specific fuel consumption (g/kWh)h enthalpy (kJ/kg)_Q heat transfer (kW)_m mass flow (kg/s)M molecular mass (g/mol)_W power (kW)P pressure (bar)cp specific heat (kJ/kg K)T temperature (�C)V volume flow rate (m3/s)Avg averageBTDC before top dead centreBC bottoming cycleCAC charge air coolerCA crank angleR30 dichloromethaneD dry fluidEGR exhaust gas recirculationF NFPA flammability ratingGWP global warming potential (direct effect, relative to CO2

for an integration time horizon of 100 years)H NFPA health ratingHDDE Heavy duty diesel engineHFC hydrofluorocarbonsHFO hydrofluoroolefinsI isentropic fluidL LitreMAC mobile air conditioningNFPA National Fire Protection AssociationNOx oxides of nitrogenODP ozone depletion potential (relative to CFC-11)PSI Performance and System Index

R1130 trans-1,2-dichloroethyleneVSLS very short lived substancesVFR volume flow ratiosWHR waste heat recoveryW wet fluidWF working fluidPR pressure ratio (expansion)

SubscriptsI 1st lawII 2nd lawair radiator airb boilingc criticalcond condenserconv conversionevp evaporatorexp expansionf freezingign auto ignitionmax maximummin minimump pinch pointquad quadlateralrec heat recoveryvap heat transfer in vapour form

Symbolsr density (kg/m3)D differenceh efficiency (%)x exergy of power production (%)_I irreversibility (kW)_n specific volume (m3/kg)l thermal conductivity (W/m K)m viscosity (cP)

Table 1NOx strategy to meet Euro 6 emissions at B50 and C100, and the correspondingwaste heat quantities and qualities in different engine streams for the 10 L enginemodel.

NOx strategy Air tofuel ratio

Inlet(bar)

Inlet(�C)

EGR(%)

Start ofinjection(BTDC)

Injectionduration(�CA)

Fuel railpressure(bar)

B50(1500/970 Nm)

22.1 2.7 66 42 �3 8 2500

C100(1800/1570 Nm)

24.6 4.8 70 35 �5 30 2300

Waste heat Inter-coolerkW [�Ce�C]

After-coolerkW [�Ce�C]

EGRkW [�Ce�C]

ExhaustkW [�Ce�C]

B50(1500/970 Nm)

0 17.6[152e60]

47.9[388e80]

39.3[290e110]

C100(1800/1570 Nm)

10.5[145e121]

59.7[200e62]

100[465e95]

100[300e110]

A.S. Panesar et al. / Energy 60 (2013) 388e400 389

can presently deliver [11]. A key consideration in the research anddevelopment effort for BC systems is therefore to investigate andidentify paths that may improve the practicality and minimise thecosts of such a concept. A simulation study was undertaken toidentify adequate working fluids for the BC using a systematicmethodology. The study takes multiple design and operationalfeatures into account to reduce the difference between modelledand experimental results. The investigation focused primarily onhigh temperature heat considering over 60 working fluids to ach-ieve high overall conversion efficiency.

A Euro 6 EGR only NOx reduction strategy was derived from theresults of a 2 Litre (L) single cylinder research engine at typicalcruise (B50) and high load (C100) conditions. The selected engineoperating points offer high weight factor for European StationaryCycle test. The resulting NOx strategy from the 2 L engine wasapplied to a 10 L engine simulation model. The 10 L engine modelconstructed in Ricardo WAVE 8.1 [15] is representative of a long-haul HDDE. Table 1 shows the applied NOx strategy and the wasteheat available in different heat exchangers with hot and coldtemperatures.

The 10 L model features a dual-stage turbo-charger, as currentlyemployed by major truck manufacturers [14]. The cooling moduleof the 10 L model at the C100 operating condition is depicted inFig. 1. An optional exhaust heat exchanger is also shown to repre-sent the large tail pipe waste heat. For an EGR rate of 35% at C100,the EGR cooler requires a maximum cooling capacity of around

100 kW. The energy balance for the 10 L engine model at C100 isalso summarised in Fig. 1, which shows near equal fuel chemicalenergy available in the EGR cooler and exhaust heat exchanger; CAC(inter-cooler þ after-cooler) and engine cooling.

Fig. 1 shows an example of a high speed turbine-generator BCunit for EGR WHR. Also shown is an optional loop for recoveringpartial high temperature after-cooler heat. The BC hardware willadd an estimated 50 kg to the EGR only platform due to the addition

Fig. 1. Schematic of the BC integrated into a 10 L EGR only engine with energy balance derived from WAVE simulation at C100 operating condition.

A.S. Panesar et al. / Energy 60 (2013) 388e400390

of turbine, generator, pump and pipes [11]. With the fuel savings ona tractor-trailer amounting to an estimated 1% per 450 kg of massreduction, this additional weight should have a negligible effect onBSFC [11]. Depending on the selected working fluid, a pressurisedcylinder or a liquid storage tank with 20% free capacity to allow forexpansionwill have to be fitted after the condenser. A filter-drier toremove any moisture, acid and particles is needed prior to thepump inlet. A bypass valve will provide a parallel loop to enable thefluid to be circulated with the expansion stage bypassed. This canbe used for setting up the required pressure, temperature and flowconditions, bypass when waste heat level is too low and as a safetyfeature in the event of expansion machine failure.

3. Fluid selection methodology

The working fluid of the BC directly impacts the safety, size,performance and cost-effectiveness of the system. However, theselection and evaluation of the relevant fluids suitable for BCapplied to EGRWHR is limited within the literature. To address this,a detailed fluid selection method summarised by Fig. 2 wasdeveloped. Multiple fluid screening criteria were implemented tonarrow down the vast list of possible fluids. Then, the 26 fluids thatmet the screening requirements were simulated using the sameboundary conditions and assumptions about the equipment per-formance using Aspen HYSYS V7.3 software package [16]. Onceoptimised for maximum power recovery with minimum superheat,the cycles were then assessed according to thermodynamic,thermo-physical, environmental, safety and economic ranking pa-rameters. A PSI (Performance and System Index) was developedusing multiple fluid ranking criteria for the comparison of the in-fluence of different working fluids on the EGRWHR system. The PSIbenchmarks the fluid performance against optimal properties ofR245fa, ethanol and water, the preferred options for automotiveapplication in present literature. The equation takes the form asshown in equation (1). This helps in the rapid identification of theworking fluids of interest and highlights their favourable processproperties.

PSIWF ¼ 1n

Xn

i¼1

264

Parameter1ðWFÞMax:Parameter1ðR245fa;Ethanol;WaterÞ

þ :::

þMin:Parameter2ðR245fa;Ethanol;WaterÞParameter2ðWFÞ

þ :::

375 (1)

3.1. Screening criteria, boundary conditions and assumptions

The environmental criteria discount the Chlorofluorocarbonsand Hydrochlorofluorocarbons as they have high ODP (OzoneDepletion Potential) and are being phased out [17]. Per-fluorocarbons have no ODP, but are chemically very stable, result-ing in very long (>1000 years) ATM (Atmospheric Lifetime) due tothe high numbers of carbon-fluorine bonds [18]. Hence, they werealso eliminated. The US NFPA (National Fire Protection Association)704 standard [19] was used to assess the fluids toxicity, flamma-bility and stability [20]. Using the maximum NFPA health hazardlimit of 2 (moderate), fluids like carbon-disulfide and allyl-chloridewere rejected. To avoid highly reactive fluids with serious NFPAinstability rating (�3), alkynes which have electron dense areas andfluids like 1,2-butadiene were also omitted. A major disadvantagewith the use of hydrocarbons like n-heptane and n-pentane forhigh temperatures is not only their decomposition temperaturesbut also their low ignition temperatures. The screening criteriadisregard fluids with auto-ignition below 300 �C. Some stationaryBCs use highly thermal stable fluids (e.g. Dowtherm A), Siloxanes(D4, D5 and D6) and Xylene (m, o and p- Xylene) [2]. These fluidscan operate up-to 400 �C but are excluded, as they require highlypressurised containers.

From the initial 60 fluids, the 26 fluids (and their classes) whichmet the screening criteria and were used for modelling the BC areshown in Fig. 3. Considering a sub-critical cycle, the maximum sys-tem pressure was restricted to 40 bar or 98% of critical pressure (Pc),which ever was lower. If the fluid decomposition temperature wasnot available, a maximum temperature of no more than 25 �C abovecritical temperature (Tc) was taken. A minimum pinch temperatureof 30 �C was assumed for all heat exchangers. The BC minimum

Fig. 2. Method overview of the fluid selection study.

A.S. Panesar et al. / Energy 60 (2013) 388e400 391

pressure that was determined according to the fixed condensingtemperature of 65 �C varied with different fluids, leading to sub-atmospheric and super-atmospheric condensation pressures.

3.2. Optimised subcritical cycle for power recovery

The simplest configuration of the BC as shown in Fig. 1 wasanalysed. The simplicity of the cycle arrangement is expected toresult in low heat exchanger area per kW output, which is animportant factor in the final sizing of BCs for automotive applica-tion. This is due to the higher thermal conductivity associated withliquid and two phase in heat exchangers, and hence, lower areacompared to a high vapour heat transfer cycles like recuperated orcycles with large levels of superheat/de-superheat. The cycleanalysis is based on the expansion of fluids starting from drysaturated vapour. In order to limit turbine damage, wet fluids weresuperheated to ensure a turbine exit vapour fraction of 0.99.Optimising the cycle operating conditions involved determiningthe subcritical turbine inlet pressures and temperatures corre-sponding to optimum power recovery, given only the initial EGRand the cooling air conditions.

Fig. 4A shows the variation in the thermal efficiency withincreasing expansion inlet pressures for EGR WHR. Results of threedistinct fluids types are shown. Working fluids include, wet/largelatent heat inorganicfluid e.g. water, dry/lowcritical pressure organicfluid e.g. R245fa and wet/high critical pressure organic fluid e.g.ethanol. Fig. 4A demonstrates that the thermal efficiency increases

with the increment in the expansion inlet pressure. The trend isconsistent for all thefluids, withfluidswith a high boiling point beingmore efficient. From Fig. 4A it can be seen that water provides thehighest first law efficiency of 22.5%. Within a fixed evaporator andcondenser temperature limit, working fluids with high latent heatwill give higher net power output per unit heat absorbed.

The potential advantage of a working fluid for EGR WHR doesnot only depend on the thermal efficiency but also on the overallconversion efficiency. Fig. 4B shows the net power produced usingthe three fluids. The maximum power is not produced using water.Increasing the evaporator pressure for water from 22 to 40 barincreases the thermal efficiency (from 19.9 to 22.5%) but reducesthe net power (from 14.35 to 13.9 kW). With increasing evaporatorpressure, less quantity but high quality heat is being exchanged inthe EGR cooler. Hence, increasing the evaporator pressure beyond22 bar has a marginal but negative effect on the net power pro-duced. For other working fluids greater net power is achievedabove 22 bar, nevertheless with a decreased cycle efficiencycompared to water. This is due to the additional power beingderived from a stream of steadily decreasing EGR temperature.Within the constraints of the study, a 40 bar evaporator pressurewas considered optimal for all fluids, the exceptions were water(22 bar) and R245fa (98% of critical pressure). The thermodynamicproperties of the various fluids have been calculated using thePeng-Robinson property package [16]. Table 2 contains 12 fluids ofinterest out of the 26 fluids that passed the initial screening. Alsoincluded are R245fa, ethanol and water.

Pure working fluids

Synthetic Natural

g

Naturanthetic

HFER7000

HFC*R245caR245fa

y

HFE HFC* InorganicWater

Ammonia

Organicrganic InorganI

Hydrocarbons

Saturated (alkanes)

2-Methyl-Pentane

CycloalkanesCyclopentane

Aromatic Hydrocarbons

Benzene

00 R245caR245fa

g

rocarbonsy

turated

Functional group

g

al ona

n

CycloalkanesCyclopentane

ro tic rocarbons

Benzene

Cycloalk

ne

omat

*Phase down (High GWP)

FluoroalkanesFluorobenzene

ChloroalkanesTrichloroethylene

R1501-Chlorobutane

R1130R30

IodoalkanesIsopropyl-Iodide

Ethyl-Iodide

Contain C-halogen Contain C-O

Alcohols Ethanol

MethanolIsopropyl-Alcohol

KetonesMethyl-Ethyl-Ketone

Acetone

Contain nitrogen

NitrileAcetonitrile

Contain sulphur

ThiolPropylmercaptan

EsterIsopropyl-Acetate

Ethyl-AcetateMethyl-Acetate

g p

Contaig

Nitril

p

Thiol

cohols KetoneM

Ester

kanesethylene50butane300

Iodoalka

g

I

lk

roalkanes

ChlorTrichlor

R1-Chlo

R1R

roe15ro

113R30

rooal

Fig. 3. Shortlisted fluids used for modelling BC and their fluid classification.

A.S. Panesar et al. / Energy 60 (2013) 388e400392

4. Maximum theoretical efficiency

All practical WHR systems involve a variable temperature pro-cess like the one shown in Fig. 5A. To benchmark a BC performanceagainst the theoretical maximum, more appropriate limits thanthose derived from Carnot efficiency are needed. It has been shownthat an ideal system for power recovery under finite source/sinkconditions is equivalent to a succession of infinitesimal Carnot cy-cles. They are described as ideal trilateral or ideal quadlateral,depending on whether the source stream is cooled to the ambientor an intermediate temperature [21]. The ideal quadlateral cycleapplied to the EGR stream in Fig. 5A assumes isentropic expansion(100%) and perfect heat transfer (0 �C pinch point). However, a realquadlateral cycle includes heat transfer losses and a non-reversibleexpansion process with increased entropy as shown in Fig. 5B.

0

5

10

15

20

10 20 30 40

Net

pow

er (

kW)

Max. cycle pressure (bar)

WaterEthanolR245fa

B

0

5

10

15

20

25

10 20 30 40

The

rmal

eff

icie

ncy

(%)

Max. cycle pressure (bar)

A

Fig. 4. Effect of expansion inlet pressure on (A) Thermal efficiency, (B) Net power.

Assuming air temperature increase from 20 to 35 �C and EGRtemperature drop from 465 to 95 �C, a real quadlateral cycle(Fig. 5D) with 30 �C overall pinch point and 75% efficient expansionwill be 25.4% efficient (ideal quadlateral 43.4%, Fig. 5C).

Ideal quadlateral and real quadlateral efficiencies can be usedfor any source/sink temperatures, with assumed pinch points inheat exchangers and expansion efficiency. Considering values forexhaust heat, after cooler, intercooler, and engine cooling, the sinkparameters remain same and the EGR temperature limits arereplaced with respective source inlet and exit temperatures. Usingtemperatures from Table 1, Table 3 shows the calculated Carnot,ideal quadlateral, and real quadlateral efficiencies (30 �C overallpinch point, 75% efficient expansion) for different streams at C100.The real quadlateral cycle efficiency calculated for exhaust heat,after-cooler, inter-cooler and engine cooling of 18.8%, 9.7%, 9% and3%, respectively, highlights the challenges for using low tempera-ture waste heat. Although heat rejection to the engine coolant isaround 70% heat from the EGR cooler, its small conversion potentialto power makes it of the least interest. Generally, the higher theoverall conversion efficiency, the greater the amount of powerproduced for a given heat transfer. The concept of ideal quadlateralcycle for EGR cooling, as shown in Fig. 5A, applies to both BCs andthermo-electric generators. Yet, they have not been cited as anupper thermodynamic limit for performance comparison by otherauthors investigating WHR applied to automotive application. Forperformance evaluation, apart from the first law efficiency thispaper evaluates fluids also on: heat recovery efficiency, overallconversion efficiency, second law efficiency [22] (defined as relativeCarnot efficiency), exergy of power production [23] (defined as theratio of net power to the incoming exergy flow) and irreversibilitiesin the BC components [22]. The quadlateral, energy and exergycalculation procedures for a BC applied to EGRWHR are detailed inTable 4.

5. Results and discussion

Table 5 details the performance and property values of the BCwith the 15 fluids mentioned in Table 2 for the same heat sourceand installation component parameters. The next stage, the fluidranking criteria as shown in Fig. 2, involves setting up of maximumand minimumworking fluid performance and property guidelines.The molecular makeup of the working fluids fundamentally pre-cludes the possibility of an ideal fluid. An extensive trade-off amongthe selected fluids was therefore undertaken. The trade-off be-tween the pure fluids showed acetone, dichloromethane (R30) andtrans-1,2-dichloroethylene (R1130) as suitable alternatives toR245fa, ethanol and water. The following section discusses the re-sults of the analysis and the trade-offs that influence fluid selectionfor WHR applied to HDDE.

5.1. Acetone and R245fa e comparing heat transfer irreversibilitiesand GWP

The fluorine content in R245fa results in low toxicity andflammability; it is classified by NFPA as moderate hazard (2) andminimal hazard (0). However, the fluorine is also responsible forthe increase in ATM, resulting in a high GWP of 1030 [18]. This hasresulted in consideration of R245fa as a potential global warminggas and its phase down is being considered. Directive 2006/40/ECbans fluids with a GWP higher than 150. This directive is currentlyonly applied to MAC (Mobile Air Conditioning) systems, but withthe increasing global thermal problem such directives may beextended for fluids used on WHR systems.

An alternative fluid identified in the present study is acetone, aketone (hydrocarbon derivative) as shown in Fig. 3. Hydrocarbons

Table

2Th

ermod

ynam

ic,thermo-phy

sical,en

vironmen

tal[17],a

ndsafety

prope

rties[20]

oftheselected

workingfluids.

Critical

temperature

T c� C

Critical

pressure

P cba

r

Atm

osph

eric

boilingpoint

T b� C

Molecular

mass

Mg/mol

Atm

ospheric

liquid

den

sity

rkg

/m3

Free

zing

point

T f� C

Vap

ourslop

e(w

et/dry/

isen

trop

ic)

W/D

/I

Atm

ospheric

lifetim

eATM

yr.

Ozo

ne

dep

letion

poten

tial

ODP

Globa

lwarming

poten

tial

GW

P

NFP

Ahea

lth

hazardH

NFP

Aflam

mab

ility

hazardF

Auto

ignition

temperature

T ign

� C

Water

373.9

220.6

100.0

18.0

998.1

0W

<0.01

90

00

0n/a

Trichloroethylen

e29

7.9

49.1

87.0

131.4

1457

.6�8

4.8

I0.03

60.00

037

n/a

21

420

1,2-dichloroethan

e(R15

0)28

8.5

53.7

83.4

99.0

1246

.5�3

5.7

I0.30

4<0.00

1n/a

23

413

Ethyl-Iod

ide

280.9

47.1

72.5

156.0

1922

.9�1

11.1

In/a

0n/a

22

n/a

Acetonitrile

272.4

48.3

81.6

41.1

776.7

�43.8

Wn/a

0n/a

23

524

n-propylmercaptan

263.5

46.3

67.7

76.2

835.4

�113

.2D

n/a

0n/a

23

n/a

2-ch

lorobu

tane

247.5

39.0

68.1

92.6

868.1

�131

.3D

n/a

0n/a

23

n/a

Tran

s-1,2-dichloroethylen

e(R11

30)

243.4

55.1

47.7

96.9

1248

.7�4

9.8

I0.03

480.00

024

252

346

0

Ethan

ol24

0.9

61.4

78.3

46.1

791.9

�114

.1W

n/a

0n/a

23

363

Methan

ol23

9.4

80.8

64.7

32.0

794.4

�97.7

Wn/a

0n/a

13

464

Dichloromethan

e(R30

)23

6.9

60.8

39.8

84.9

1318

.2�9

5.1

W0.39

4<0.00

110

21

556

Isop

ropyl-alcoh

ol23

5.2

47.7

82.2

60.1

787.4

�87.9

In/a

0n/a

13

399

Acetone

235.1

47.0

56.3

58.1

784.9

�94.7

In/a

0n/a

13

465

Methyl-acetate

233.4

47.5

56.9

74.1

928.0

�98.0

In/a

0n/a

13

502

R24

5fa

154.1

36.4

15.3

134.0

1364

.4n/a

D7.6

010

302

041

2

TT Tegr,in

Tegr,out

Tair,in Tair,out

S

t

TT

S

Tegr,in

Tegr,out

Tair,in Tair,out

A B

Exp

ansi

on

loss

T

0

100

200

300

400

500

0 25 50 75 100T

empe

ratu

re

C

Heat (power) kWC

0

100

200

300

400

500

0 25 50 75 100

Tem

pera

ture

C

Heat (power) kWD

Fig. 5. (A) Ideal quadlateral cycle TeS (B) Real quadlateral cycle TeS (C) Ideal quad-lateral cycle TeQ (D) Real quadlateral cycle TeQ for EGR heat recovery.

A.S. Panesar et al. / Energy 60 (2013) 388e400 393

exhibit cardiac sensitization potentials and anaesthetic effects;however, they generally decompose quickly enough, thus reducingtheir overall toxicity risks. When compared to R245fa, acetone has areduced toxicity classification by NFPA, slight hazard (1), due to itshydrogen content [18]. Hydrocarbons also have short ATM, andtheir GWP is usually around 20 [17]. The disadvantage of thehydrogen content is the increased flammability. NFPA classifiesacetone’s flammability as serious hazard (3). Nevertheless, this isstill lower than the classification of n-pentane, which is a preferredhydrocarbon in stationary WHR systems. Also, when subjected tosolar irradiation hydrocarbons act as a factor in smog formation,reducing urban air quality [18]. Acetone is also used as an industrialsolvent. It is compatible with common engineering metals and al-loys. The preferred O-ring materials for acetone are ChemRaz,Kalrez, EPDM and Kel-F.

Fig. 6A shows results of dry saturated vapour cycle analysis forR245fa and acetone compared to the real quadlateral cycle. For realfluids, the combination of sensible heat and latent heat of vapor-isation results in a pinch point. This is located at a single point,either in the preheater or at the preheater exit, rather than theconstant overall pinch point as assumed for the fluid in the realquadlateral cycle. The efficiency of a BC depends on the losses from

Table 3Calculated Carnot, ideal quadlateral and real quadlateral efficiencies at C100.

EGR(C100)

Exhaust(C100)

After-cooler(C100)

Inter-cooler(C100)

Enginecoolant(C100)

Carnot h % 60.3 48.8 38.1 29.9 21.4Ideal quadlateral h % 43.4 36.3 24.9 25.9 18.3Real quadlateral h % 25.4 18.8 9.7 9 3

Table 4Energy and exergy equations used for performance evaluations.

Ideal quadlateral efficiency

hquad;ideal ¼ 1� Tair;out�Tair;inTEGR;in�TEGR;out

$ln

TEGR;inTEGR;out

lnTair;outTair;in

Real quadlateral efficiency

hquad;real ¼ hexp

26641� ðTair;out�TpÞ�ðTair;in�TpÞ

ðTEGR;in�TpÞ�ðTEGR;out�TpÞ$ln

TEGR;in�TpTEGR;out�Tp

lnTair;out�TpTair;in�Tp

3775

First law efficiency

hI ¼ _W turbine� _Wpump_Q in

Recovery efficiency

hrec ¼ _Q in_Qmax

Conversion efficiencyhconv ¼ hI$hrecIrreversibility pump

I:

pump ¼ Tair;in$ _mWFðspump;out � spump;inÞIrreversibility turbine

I:

turbine ¼ Tair;in$ _mWFðsturbine;out � sturbine;inÞIrreversibility EGR cooler

I:

EGR ¼ TEGR;in$ _mWF

h�sEGR;out � sEGR;inÞ �

�hEGR;out�hEGR;in

TEGR;in

�i

Irreversibility condenser

I:

cond ¼ Tair;in$ _mWF

h�scond;out � scond;inÞ �

�hcond;out�hcond;in

Tair;in

�i

Second law efficiency

hII ¼ _Wnet

_Q in

�1� Tair;in

TEGR;in

�

Exergy of power production

xp ¼ _Wnet

_mEGR :cp;EGR

h�TEGR;in�Tair;in�Tair;in$ln

TEGR;inTair;in

i

A.S. Panesar et al. / Energy 60 (2013) 388e400394

irreversible heat transfer, represented by the magnitude differencein matching of theworking fluid TeQ curves to the source/sink. As aresult, even the higher overall conversion efficiencies seen inTable 5 for different working fluids will be only around 75% of thereal quadlateral cycle as shown in Table 3. This highlights the dif-ficulty for a real system in even approximating to such limits.

Comparing values from Tables 2 and 5 and the TeS diagram(Fig. 6B) for R245fa and acetone. It is seen that R245fa has a rela-tively high critical pressure (36.4 vs. 47 bar) at a much lower criticaltemperature (154.1 vs. 235.1 �C). Hence, the BC using R245faoperates at higher super-atmospheric condensing pressures (5.27vs. 1.32 bar). This implies denser vapours with relatively lowercondensing specific volume (0.034 vs. 0.33 m3/kg). Due to this, thelower pressure ratios (6.6:1 vs. 27.1:1) and expansion volume flowratios (13.4:1 vs. 37.9:1) calculated also allow the possibility ofusing a positive displacement expander instead of a turbine.

Due to the much higher molecular mass of R245fa (134 vs.58.1 g/mol), the specific enthalpy drop in expansion is relativelysmall (21.3 vs. 112 kJ/kg). Hence, by default, the feed pump workneeds to be relatively large (2.03 vs. 1.25 kW). For a maximum cyclepressure of 36 bar, R245fa gives a cycle efficiency of 9.21%with a netpower of 9.04 kW, a reduction of 18% from the total turbine/expander power output. For the EGR temperature levels consid-ered, fluorinated hydrocarbon R245fa delivers poor overall con-version efficiency (9.04 vs. 16.08%). This is primarily due to thelower average heat addition temperature leading to large irre-versibility (32.13 vs. 24.19 kW) in the EGR cooler as a result of itslower critical point conditions as seen in Fig. 6B. R245fa is bettersuited for heat recovery between 150 and 250 �C, where bettertemperature matching between the heat stream and the workingfluid is possible.

Conversely, acetone with a higher critical pressure and tem-perature is better suited to EGR temperature levels. For a slightlyhigher cycle pressure of 40 bar, it will produce 16.08 kW of net

power, with a 16.36% efficient cycle. Under such conditions thepump work will be relatively low, only 7% of the total turbine po-wer. Due to the nearly equivalent heat recovery by R245fa andacetone and the higher net power produced by acetone, the averageheat exchanger’s surface area per kW output (UA/kW) of acetone islower than that of R245fa. Furthermore, acetone has nearly twicethe values of average thermal conductivity when compared toR245fa (0.086 vs. 0.048 W/mK). For acetone, turbines will be bettersuited as expansion devices. Hence, by offering higher overallconversion efficiency, higher thermal conductivity and lower GWP,acetone can be considered as a suitable alternative for EGR WHR.

5.2. Dichloromethane and ethanol e comparing flammability,volume flow ratios and condensing pressure

Table 2 includes four chloroalkanes, trichloroethylene, 1,2-dichloroethane (R150), trans-1,2-dichloroethylene (R1130) anddichloromethane (R30). They contain chlorine molecules, theaddition of which leads to ODP [18]. However, the selected fourfluids are categorised as VSLS (Very Short Lived Substances). Nearzero ODP (<0.001) and ATM <0.4 years have been calculated fortrichloroethylene [24], R150 [25], R1130 [26] and R30 [25]. Thesestudies show that chlorine from the selected VSLSs are unlikely toaffect ozone at quantities likely to be emitted to the atmosphere.Since over 90% emissions of the ozone loss due to these VSLSs takeplace in the troposphere, most emissions will decompose beforereaching the stratosphere. As their atmospheric persistence will becomparatively short, they will have low GWP values (<25). Also,the US Environmental Protection Agency has already authorisednon-ozone-depleting chlorinated solvents, trichloroethylene,R1130 and R30 as substitutes [27]. The MAC industry have recentlystarted using unsaturated, double bonded compounds. However,R1130 and R30, are unsaturated, double bonded historic re-frigerants, with use dating back to the 1920’s. Along with acetone,the two more proposed alternative working fluids for EGR WHR inthis study are R1130 and R30.

Fig. 7A, shows nearly equal percentage of heat distribution inpreheater, evaporator and superheater, and performance whencomparing ethanol to R30. Comparing the TeS diagram in Fig. 7Bthe entropy of the saturated vapour decreases with increasingtemperatures for both the fluids, hence, they require superheatingto avoid condensation at turbine exit. With an equal NFPA toxicityclassification of moderate hazard (2), R30 is considered as only aslight flammability hazard (1) compared to ethanol’s seriousflammability hazard (3). Furthermore, due to the highest autoignition temperature (556 �C) of R30 within the considered organicfluids, the risk of leak, leading tomixing of EGR stream and R30 andcausing explosion is greatly reduced. The EGR cooler surface tem-peratures stay below the ignition temperature of R30 by amargin of90 �C over any of the engine conditions considered. This addedsafety is absent when using ethanol whose auto ignition temper-ature is only 363 �C.

R30 is a recommended alternative chlorinated solvent; it is notlisted in the Montreal Protocol, and is expected to have no signifi-cant impact on stratospheric ozone depletion. R30’s contribution toglobal warming, acid rain and smog formation is negligible. In fact,R30 has a low photochemical ozone creation potential in thetroposphere (0.9), when compared with ethanol (27) [28]. Thechlorine content of R30 also gives it lubricity and improvedmiscibility [18]. R30 is presently absent in the WHR market, how-ever, it is available on the general market and affordable [29].

Ethanol and R30, have similar critical temperatures (240.9,236.9 �C) and pressures (61.4, 60.8 bar). For a maximum cyclepressure of 40 bar the net power for both the cycles is around16.5 kW, with a cycle efficiency of 16.8%. For approximately equal

Table 5BC process and fluid ranking parameters of the selected working fluids for EGR WHR.

Water Trichloroethylene R150 Ethyl-iodide Acetonitrile n-Propylmercaptan 2-Chlorobutane R1130 Ethanol Methanol R30 Isopropyl-alcohol Acetone Methyl-acetate R245fa

Tevp �C 205 281 266 267 259 252 245 220 216 199 206 225 224 223 153TWF,max

�C 398 281 266 267 290 252 246 220 230 265 227 225 224 223 153TEGR,out �C 195 99 98 100 110 98 98 98 97 102 98 97 97 97 98_mWF kg/s 0.024 0.291 0.210 0.405 0.091 0.170 0.202 0.283 0.094 0.069 0.262 0.127 0.155 0.188 0.521_Wpump kW 0.08 1.28 1.08 1.68 0.76 1.29 1.41 1.42 0.76 0.55 1.23 1.04 1.25 1.28 2.03_Wturbine kW 14.38 20.23 19.62 20.03 19.75 18.20 16.93 17.69 17.28 17.38 17.71 16.74 17.33 16.98 11.07_Wnet kW 14.30 18.95 18.54 18.35 18.99 16.90 15.52 16.27 16.52 16.82 16.48 15.70 16.08 15.70 9.04Qsuperheat kW 11.2 0.0 0.0 0.0 11.1 0.0 0.0 0.0 3.8 13.4 7.7 0.0 0.0 0.0 0.0Qde�superheat kW 0.0 7.9 6.4 4.7 0.0 14.6 16.5 3.3 0.0 0.0 0.0 5.0 3.3 6.6 5.3Avg. UA/kW 87.7 107.1 107.8 112.1 114.1 96.5 99.9 124.1 126.7 127.4 130.3 115.8 117.0 110.5 219.2Pmin bar 0.25 0.52 0.49 0.84 0.55 0.93 0.90 1.69 0.62 1.05 2.28 0.68 1.33 1.32 5.27Pmax bar 22 40 40 40 40 40 38 40 40 40 40 40 40 40 36PR (expansion) 55.0 60.0 62.2 40.4 56.8 37.2 36.1 21.7 51.9 33.3 16.5 48.1 27.1 27.3 6.6Dhturbine kJ/kg 609.6 69.5 93.3 49.5 216.8 107.2 83.9 62.6 183.6 253.5 67.5 131.8 112.0 90.3 21.3Vexit m3/s 338 371 365 276 317 243 242 156 266 177 123 267 184 183 64VFR (expansion) 29.2 80.5 78.3 54.8 51.2 59.0 72.8 26.2 51.7 24.8 15.8 69.3 37.9 40.6 13.4_vcond m3=kg 3.983 0.354 0.482 0.189 0.965 0.398 0.333 0.153 0.785 0.719 0.130 0.584 0.330 0.270 0.034lavg W/mK 0.354 0.060 0.074 0.047 0.108 0.078 0.067 0.061 0.098 0.116 0.071 0.083 0.086 0.083 0.048mavg cP 0.230 0.208 0.246 0.203 0.127 0.142 0.140 0.146 0.271 0.179 0.156 0.351 0.116 0.130 0.135hI % 20.45 19.39 18.93 18.82 20.04 17.21 15.80 16.59 16.77 17.34 16.80 15.96 16.36 15.97 9.21hrec % 69.95 97.74 97.95 97.47 94.77 98.19 98.20 98.08 98.50 96.99 98.10 98.41 98.27 98.26 98.14hconv % 14.30 18.95 18.54 18.35 18.99 16.90 15.52 16.27 16.52 16.82 16.48 15.70 16.08 15.70 9.04_Ipump kW 0.02 0.41 0.34 0.62 0.02 0.41 0.46 0.48 0.20 0.12 0.36 0.29 0.30 0.36 0.63_Iturbine kW 4.01 5.46 5.36 5.62 5.59 4.58 4.25 5.06 4.92 4.97 5.08 4.70 4.92 4.68 3.18_IEGR kW 13.46 19.80 20.52 20.45 19.21 22.51 23.89 23.77 23.69 23.16 23.82 24.43 24.19 24.56 32.13_Icond kW 8.92 12.27 12.25 11.70 11.39 12.75 13.04 11.51 12.01 11.38 11.35 12.15 11.70 11.89 12.14hII % 35.13 33.31 32.52 32.34 34.43 29.58 27.15 28.51 28.81 29.80 28.87 27.41 28.11 27.45 15.83xP % 30.43 40.33 39.45 39.04 40.41 35.97 33.02 34.63 35.14 35.79 35.07 33.41 34.22 33.40 19.24PSI values 0.676 0.668 0.669 0.733 0.663 0.623 0.579 0.776 0.669 0.653 0.767 0.621 0.724 0.647 0.651

A.S.Panesar

etal./

Energy60

(2013)388

e400

395

Fig. 6. (A) TeQ diagram comparing heat transfer irreversibilities between real quad-lateral cycle, R245fa and acetone, (B) TeS diagram of R245fa and acetone.

Fig. 7. (A) TeQ diagram for ethanol and R30 showing similar heating profile andexpansion work, (B) TeS diagram of ethanol and R30.

A.S. Panesar et al. / Energy 60 (2013) 388e400396

average heat addition temperature, the pump power of ethanol isslightly lower (0.76 kW) compared to R30 (1.23 kW), due to itslower density (791.9 vs. 1318.2 kg/m3) and high latent heat ofvaporisation (385.2 vs. 154.2 kJ/kg).

The maximum and minimum cycle pressures must be main-tained at low levels, yet super-atmospheric, in order to prevent airor moisture ingress into the system which reduces system effi-ciency. The higher boiling point of ethanol (78.3 �C) compared toR30 (39.8 �C), will lead to lower condensing pressures. For theselected 65 �C condensing temperature, ethanol shows vacuumpressures (0.62 bar) involving the use of expensive equipment,compared to a super-atmospheric pressure by R30 (2.28 bar).Increasing the condensing pressure to 1 bar, to avoid a vacuumwillreduce ethanol’s operational performance by over 10% to 14.8 kW.

When comparing heat exchangers, the heat transfer coefficientsdepend on the thermo-physical properties and hydrodynamicregime of the fluids, predominantly thermal conductivity and vis-cosity. The average thermal conductivity slightly favours ethanol(0.098 vs. 0.071 W/mK), whereas the average viscosity favours R30(0.156 vs. 0.271 cP). For an overall conversion efficiency of 16.5%,their required heat exchanger area per unit output is nearly equal.

The use of highly efficient and compact positive displacementexpanders and turbines depend on small volumetric expansion ratiosand low volumetric flow rates. R30, compared to ethanol, has ahigher molecular mass (84.9 vs. 46.1 g/mol). It also gives much lowerexpansion pressure ratios (16.5:1 vs. 51.9:1), volume flow ratios(15.7:1 vs. 51.7:1) and expansion exit flow rates (123 vs. 266 m3/s).This allows R30 to use a piston expander without reducing opera-tional pressures and hence, the operational performance; which isnot possible with ethanol for the pressure limits considered in this

study. Hence, R30 is preferred over ethanol due to its lower flam-mability, higher auto ignition temperature, low photochemical ozonecreation potential, small volumetric expansion ratios and low volu-metric flow rates. The super-atmospheric condensation pressurereduces condenser volume, eliminates infiltration gases, and im-proves cold weather performance by reducing condensing temper-atures while remaining at super-atmospheric pressures.

5.3. Trans-1,2-dichloroethylene and water e comparing overallconversion efficiency

Boiler grade water is non-toxic, non-flammable, environmen-tally friendly and an inexpensive working fluid. Water has thehighest average thermal conductivity (0.354 W/mK) of all theworking fluids considered in this study. High working fluid tem-peratures are possible with water as it has no thermal degradationproblems at any practical temperatures. In comparison, organic andsynthetic fluids decompose when temperatures exceed certainlimiting values. This is usually encountered above the criticaltemperature [30]. Detrimental changes in the chemical makeup oforganic and synthetic fluids are not limited to the decompositiontemperatures. In the case of water, moisture will cause corrosionafter a period of time. However, moisture mixed with the fluorineor chlorine content in the working fluid will slowly hydrolyse intohydrofluoric and hydrochloric acids. These acids greatly acceleratemetal corrosion; the filter drier has to be changed frequently toprovide a moisture-free system.

As shown in Table 5, water gives a higher thermal efficiency(20.45%) when compared to R1130 (16.6%). However, the first law

Fig. 8. (A) TeQ diagram for water and R1130 showing the implication of sensible heatand latent heat on heat recovery efficiency, (B) TeS diagram of water and R1130.

A.S. Panesar et al. / Energy 60 (2013) 388e400 397

does not distinguish between the qualities of the energy that isreceived by the two BCs. The overall conversion efficiency and exergyof power production efficiency are shown in Table 4 (which take theEGR temperature drop into account) are more appropriate indicatorsof how effectively the available heat is being converted formaximumpower recovery. The system should be selected and optimised toprovide maximum EGR cooling and high overall conversion effi-ciency (whilst minimising component size and hence cost).

Table 6Rational for the chosen pairings of working fluids.

Current pairing

- Acetone vs. R245fa

Can highlight acetone’s drawback of higher flammability due to hydrogen content(3 vs. 1).Can highlight acetone’s drawback of higher volume flow ratio across expanderunit power output (2.35 vs. 1.48).

- R30 vs. ethanol

Can highlight R30’s drawback of lower thermal conductivity (0.071 vs. 0.098 W/mK).Chighlight R30’s drawback of higher feed pump power (7 vs. 4.4% of expansion power)

- R1130 vs. water.

Water can be paired with any of the three fluids to highlight the large latent heat drawbHowever, paring with a fluid with chlorine content also allows in pointing towards thdrawback of chlorine (and fluorine) content when mixed with moisture.

Trouton’s rule shows that lower molecular weight fluids willhave higher latent heat of vaporisation [31]. Since, R1130 and waterdiffer significantly, it is important to consider the heat exchangerimplications associated with heat capacity and latent heat ofvaporisation as they influence sensible heating and evaporation.Fig. 8A, showswater cooling the EGR stream from 465 to 195 �C andrecovering 70 kWof heat with contributions of 14, 45 and 11 kW inthe preheater, evaporator and superheater. Due to the high latentheat to sensible heat ratio, the volume of vapour produced per unitmass at the turbine inlet is very large compared to that of the liquidentering the evaporator. Hence, the expansion work (14.38 kW) ismuch greater than the pumping work (0.08 kW). Therefore, even ifthe pump has a reduced efficiency, its effect on the net power issmall.

Due to water’s large latent heat as shown in Fig. 8A, the largestpart of the heat transfer takes place in the evaporator at constanttemperature. During the evaporation process, the EGR temperaturecontinuously decreases. This high degradation in EGR temperatureto evaporate the steam results in reduced overall conversion effi-ciency and exergy of power production efficiency of 14.3% and30.43%. As a result, 25% of the potential heat recovery or enthalpydrop remains unused (Fig. 8A). Increasing the current evaporationtemperature (205 �C) and corresponding pressure (22 bar) willincrease the thermal efficiency, but will also reduce the overallconversion efficiency. Additionally, if the 11 kW superheat neededto avoid turbine damage is eliminated, then the evaporation tem-perature can only be marginally raised. In contrast to this, R1130has a much lower latent heat to sensible heat ratio. It can recover98 kW of heat from the EGR cooler and hence cool the EGR tem-perature down to 98 �C. The cycle will produce a net power of16.27 kW, with contributions of 62 and 36 kW in the preheater andevaporator. Hence, irrespective of around 20% lower thermal effi-ciency, it will have a higher overall conversion efficiency and exergyof power production efficiency of 16.3% and 34.63%.

As a working fluid, water attains very low condensing pressuresat 65 �C.Water’s sub-atmospheric condensing pressure (0.25 bar) ismuch below R1130’s super-atmospheric pressure (1.69 bar).Moreover, water has the disadvantage of very higher condensingspecific volume (3.983 m3/kg), in contrast to a much lower volume(0.153 m3/kg) of R1130, resulting in larger condensers. The highvolumes imply that the volume flows (338 vs. 156 m3/s) and vol-ume ratios (29.2:1 vs. 26.2:1) in the expansion are high resulting inlarger and expensive machines. The major disadvantage of usingwater as aworking fluid in the kWoutput range is its lowmolecular

Alternative paring

per

- Acetone vs. ethanol

Cannot highlight acetone’s drawback of higher flammability due tohydrogen content (3 vs. 3).Cannot highlight acetone’s drawback of higher volume flow ratio acrossexpander per unit power output (2.35 vs. 3.13).

an

- R30 vs. R245fa

Cannot highlight R30’s drawback of lower thermal conductivity (0.071vs. 0.048 W/mK)Cannot highlight R30’s drawback of higher feed pump power (7 vs. 18%of expansion power)

ack.e

Table 7Influence of PSI parameters.

hII;WFhII;Water

Parameter 1: Indicates 2nd law efficiency performance.

hconv;WFhconv;Ethanol

Parameter 2: Indicates overall heat to net power conversionperformance.

_IEGR;WF_IEGR;Water

Parameter 3: Indicates the heat source to working fluidirreversibilities. As the heat transfer irreversibilities inthe EGR cooler has the highest percentage (e.g. ethanol:pump 0.5%, EGR cooler 58%, expansion 12% andcondensing 29.5%) it is considered as a suitable indicatorof the overall cycle irreversibility.

ðPmax=PminÞPRWFðPmax=PminÞPRR245fa

Parameter 4: Indicates the use of fluids with higher criticalpressures and super atmospheric condensation. ConsiderR30 and R245fa, both of which have super atmosphericcondensation. But due to the much lower critical pressureof R245fa, the optimal evaporator pressure within theconstraints of the subcritical cycle considered is 98%(35.6 bar) of the critical pressure. Whereas, R30 due tothe much higher critical pressure operates at an evaporatorpressure which is 66% (40 bar) of the critical pressure.The lower critical pressure working fluid with superatmospheric condensation has three disadvantages. Firstly,to control evaporator pressure precisely between 95 and100% of critical pressure is a challenging task. Secondly,with evaporator pressures nearing the critical pressurethe feed pump power consumption becomes a greaterpercentage of the expansion power. Thirdly, apart fromdecomposition due to high temperatures at boundarylayer of tube walls, the expansion inlet temperatures nearto that of the fluid critical temperature may compoundthe degradation effect.

UA= _Wnet;Water

UA= _Wnet;WFParameter 5: Indicates the overall heat exchanger footprint of the BC per net power produced.

VFR= _Wnet;R245fa

VFR= _Wnet;WFParameter 6: Indicates the expansion machine size pernet power produced.The largest percentage of the BC cost will be due to theexpansion machine and the heat exchangers. Hence,having a compact turbine/expander with efficienttransmission and low heat exchanger areas is vital.Parameter 5 and 6 indirectly make an economiccomparison between different BCs giving relativeindication of the overall heat exchanger foot printand the expansion machine size per net power produced.

_vR245fa_vWF

Parameter 7: Indicative of the volume occupiedby the working fluid.

_Q vap=Wnet;Ethanol_Qvap=Wnet;WF

Parameter 8: Indicative of the vapour heat exchangerfoot print._Qvap is the total heat transferred in vapour form by thecycle i.e. summation of the total superheating andde-superheating load. As the volume will be high andthermal conductivity will be low for a working fluid invapour state compared to liquid phase, this parameter willpoint towards the use of fluids with near vertical dT/dS.

A.S. Panesar et al. / Energy 60 (2013) 388e400398

weight (18 g/mol) compared to R1130 (96.9 g/mol). As sonic ve-locity of the vapour is roughly inversely proportional to the squareroot of its molecular weight, water will have higher turbine/alter-nator speed (under the same temperature limits). The largeexpansion pressure ratio (55:1 vs. 21.7:1) may require multiplestaging to realise its efficiency potential.

For EGR WHR the advantages of R1130 include: compactequipment (turbine and air condenser), lower pressure ratio, dryexpansion and super-atmospheric condensation. Using R1130, theEGR heat source characteristics can be closely matched resulting inhigher overall conversion efficiency. The higher molecular mass ofR1130 also enables a mass flow rate 12 times higher than water,maintaining higher turbine efficiencies. Furthermore, R1130 has alower freezing temperature (�49.8 �C), thus eliminating condenserfreeze-up concerns or equipment expansion issues.

The rational for the chosen pairings of working fluids in Sections5.1e5.3 was twofold. Firstly, related to the TeS curves: acetone andR245fa do not require superheating, whereas R30 and ethanol

require superheating. Secondly, related to highlighting the unfav-ourable process properties of the proposed fluids. This can be un-derstood by considering Table 6 which shows alternative paringscompared to current pairings.

5.4. Performance and System Index

The PSI (Performance and System Index) developed usingequation (1) was used to benchmark 8 selected parameters of anyworking fluid against the optimal values obtained from water,ethanol or R245fa as shown in equation (2). The first four param-eters of equation (2) take the system performance into account andthe rest assess the compactness of the overall system. Table 7presents the 8 parameters used in calculating the PSI value andtheir influence. The 8 selected parameters help in the rapid iden-tification of working fluid of interest and highlight their favourableprocess properties. Themost suitable working fluid will then have ahigh 2nd law efficiency, high overall conversion efficiency, low EGRcooler irreversibility, high pressure ratio with super-atmosphericcondensation, low heat exchanger area, low expansion volumeflow ratio, low condensing specific volume and low heat transfer invapour form. Fluids with favourable process properties result inhigh PSI values and vice versa.

PSIWF ¼ 18

26666664

hII;WFhII;Water

þ hconv;WFhconv;Ethanol

þ _IEGR;WF_IEGR;Water

þ

ðPmax=PminÞPRWFðPmax=PminÞPRR245fa

þ UA= _Wnet;Water

UA= _Wnet;WFþ

VFR= _Wnet;R245fa

VFR= _Wnet;WFþ _vR245fa

_vWFþ _Qvap=Wnet;Ethanol

_Q vap=Wnet;WF

37777775

(2)

The overall performance and system suitability of acetone, R30and R1130 is also reflected in their higher PSI values as shown inTable 5 of 0.724, 0.767 and 0.776, compared to lower values of 0.651,0.669 and 0.676 for R245fa, ethanol and water, respectively. Inclockwise direction for decreasing PSI values starting from R1130,Fig. 9 shows the normalised values for the selected 8 parameters forall the shortlisted working fluids with respect to R245fa, ethanoland water.

As BC parameters present a multi-dimensional surface onwhich an optimum condition can be found within prescribedconstraints, it is important to point out the influence on the PSIvalue by partial selection of the 8 PSI parameters. For this,consider two options, Fig. 10A and B. Fig. 10A only takes parameter1, 2, 3 and 5 into account for the PSI value calculation. This set ofparameters will then be biased towards a high thermodynamicefficiency and lower heat exchanger foot print reasoning. Fig. 10Atherefore points towards the suitability of fluids with highercritical temperatures. High critical temperature fluids like water,trichloroethylene, R150 and acetonitrile seem to be very favour-able. Fluids like R1130, ethanol, methanol, R30, isopropyl-alcohol,acetone and methyl-acetate with similar critical temperatures(243.4e233.4 �C) show no significant difference. Fig. 10B onlytakes parameter 2, 4, 6, 7 and 8 into account for the PSI valuecalculation. This set of parameters will then be biased towards thecompact expander and near isentropic fluid reasoning. Forexample R245fa which was the worst fluid in Fig. 10A turns out tobe one of the most favourable fluids. Whereas water due to is verylow condensing pressure and the large superheating appears to bethe worst. Fig. 9, inclusion of all the 8 PSI parameters consideredin the present work attempts to be least biased towards any of thefluids usually considered. The main objective of the 8 parametersis to find the working fluid that could show the best performanceand system (size and cost) trade-off to achieve a better integrationwithin an HDDE.

0

0.7

1.4R1130

R30

Acetone

Ethyl-Iodide

Water

R150

Acetonitrile

EthanolTrichloroethylene

R245fa

Methanol

Methyl-Acetate

n-Propylmercaptan

Isopropyl-Alcohol

2-Chlorobutane

PSI value2nd law efficiency (to water)Conversion efficiency (to ethanol)EGR irreversibility (to water)Cycle PR x expansion PR (to R245fa)

0

0.7

1.4R1130

R30

Acetone

Ethyl-Iodide

Water

R150

Acetonitrile

EthanolTrichloroethylene

R245fa

Methanol

Methyl-Acetate

n-Propylmercaptan

Isopropyl-Alcohol

2-Chlorobutane

PSI valueVFR/Wnet (to R245fa)Condensing specific volume (to R245fa)UA/Wnet (to water)Heat transfer in vapour phase (to ethanol)

Fig. 9. Working fluid normalised values for the selected 8 parameters, showing performance and system advantages of R1130, R30 and acetone over water, ethanol and R245fa.

A.S. Panesar et al. / Energy 60 (2013) 388e400 399

6. Exhaust gas recirculation and partial after-cooler wasteheat recovery

For systems approach, the recovery of EGR and partial hightemperature after-cooler heat was conducted. The potential bene-fits of using the proposed three fluids are shown in Table 8. All ofthe three fluids have high auto-ignition temperatures (>460 �C),low GWP (<25) and super-atmospheric condensation at 65 �C.With different ratios of latent heat of vaporisation to sensibleheating, the cooled EGR and cooled after-cooler temperatures andhence the heat recovery efficiency differ. For the two selected testpoints, the BSFC improvement varies between 9.8 and 13.7 g/kWh,with 2e2.8% point increase in BTE (Brake Thermal Efficiency).

For maximum BSFC improvement, the analysis favours acetone(10.2% at B50 and 13.7% at C100), and shows its ability to recover

0

0.7

1.4R1130

R30

Acetone

Ethyl-Iodide

Water

R150

AcetonitrileEthanolTrichloroethylene

R245fa

Methanol

Methyl-Acetate

n-Propylmercaptan

Isopropyl-Alcohol

2-Chlorobutane

A

0

0.7

1.4R1130

R30

Acetone

Ethyl-Iodide

Water

R150

AcetonitrileEthanolTrichloroethylene

R245fa

Methanol

Methyl-Acetate

n-Propylmercaptan

Isopropyl-Alcohol

2-Chlorobutane

B

Fig. 10. Sensitivity study of PSI values to the choice of parameters (A) Biased towards ahigh thermodynamic efficiency and lower heat exchanger foot print reasoning (B)Biased towards the compact expander and near isentropic fluid reasoning.

high and even low temperature waste heat. The maximum reduc-tion of load on the engine cooling module depicted in Fig. 1 is22.6 kW (i.e. the power produced by the turbine) using BC withacetone. However, the overall trade-off marginally favours R30irrespective of its slightly reduced performance (9.8% at B50 and12.2% at C100). This is due to the lowest flammability rating,condensing specific volume, pressure ratio, expansion volume flowratio and highest auto-ignition temperature of R30 out of the threeworking fluids.

7. Conclusion

The simulation study conducted for an EGR only engine coupledwith a BC showed an alternative route to meet the Euro 6 NOxrequirements while reducing the specific fuel consumption of thebase engine. As the process performance and system parameters ofa BC are dependent on the selected fluid, a detailed fluid selectionstudy was conducted to show trade-offs among desired propertiesand hence identify optimal solutions. With multiple maximisedandminimised indices for a subcritical cycle with minimumvapourheat transfer, the study identified acetone, R30 and R1130 as themost suitable working fluids for EGR WHR. Comparing the threeproposed fluids, acetone and R1130 are near isentropic, whereasR30 is a wetting fluid and requires slight superheating when usedwith turbines. Acetone showed the highest overall conversion

Table 8Performance improvement of the 10 L EGR only engine with BC recovering EGR andpartial after-cooler heat with the three selected optimal fluids at B50 and C100condition.

Acetone(B50)

R30(B50)

R1130(B50)

Acetone(C100)

R30(C100)

R1130(C100)

TEGR, out �C 118.8 125.7 124.9 115.0 136.6 124.5Taftercooler, out �C 119.7 130.6 121.8 108.3 138.8 125.9QEGR [kW] 42.5 41.5 41.6 88.7 83.4 86.3Qaftercooler [kW] 6.1 4.0 5.7 39.5 26.4 32.0_Wnet [kW] 7.9 7.6 7.8 20.7 18.3 19.4hrec [%] 71.5 67.0 69.6 82.7 70.8 76.3hconv [%] 11.6 11.1 11.4 13.4 11.8 12.5% increase in

power5.2 5.0 5.1 7.0 6.2 6.6

D BSFCimprovement

10.2 9.8 10.1 13.7 12.2 12.9

% point BTEimprovement

2.1 2.0 2.1 2.8 2.5 2.6

A.S. Panesar et al. / Energy 60 (2013) 388e400400

efficiency for EGR and partial after-cooler heat recovery resulting in10.2 and 13.7% BSFC improvement at B50 and C100.

For the examined EGR WHR, the proposed three fluids offerhigher overall conversion efficiency between 16.08 and 16.48%,when compared to 9.04% and 14.03% for R245fa and waterrespectively. When compared to R245fa they also offer higheraverage thermal conductivity (between 0.061 and 0.086 vs.0.048 W/mK) and lower GWP (<25 vs. 1030). Water has no healthor environmental impact and has the highest thermal conductivity.However, the large amounts of heat needed to evaporate it, andfurthermore, the heat required to superheat in order to avoid tur-bine damage results in poor overall conversion efficiencies. Systemsusing water will also require freeze protection capabilities. Theproposed three fluids maintain super-atmospheric condensationpressure at 65 �C which avoid infiltration of air, whereas ethanoland water show sub-atmospheric pressures. The low saturatedvapour specific volume of the proposed fluids (between 0.13 and0.33 m3/kg) compared to high values of ethanol (0.785 m3/kg) andwater (3.983 m3/kg) gives a relative indication of the compactsystem size. The additional problems associatedwith the expansionof ethanol and water is the huge volume flow rates of 226 and338 m3/s and low molecular mass of 46.1 and 18 g/mol.

When R30 is compared to ethanol for EGRWHR, R30 offers lowerflammability rating (slight vs. serious hazard), higher auto-ignitiontemperatures (556 vs. 367 �C), lower pressure ratios (16.5:1 vs.51.9:1) and lower volume flow ratios (15.8:1 vs. 51.7:1). The practicaland suitable operating pressures seen with R30 allow the use of apiston expander which can be directly coupled to the crank shaft.Hence, when comparing between the three proposed fluids acetone,R30 and R1130, irrespective of R30’s marginally reduced perfor-mance of 9.8 and 12.2% BSFC improvement at B50 and C100, R30 isconsidered most suitable. As a consequence of the R30 BC integratedinto an EGR only engine, the BTE will improve by an estimated 2e2.5% point when recovering EGR and partial after-cooler heat.

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