Introduction
Reference engine and design point choice
Engine-ORC design point | Symbol | Unit | Value |
---|---|---|---|
Engine brake power |
P
eng
| (kW) | 302 |
Engine speed |
N
| (rpm) | 2000 |
Engine torque |
τ
| (Nm) | 1443 |
Exhaust gas mass flow rate |
\(\dot{m}_{exh}\)
| (kg s−1) | 0.36 |
Exhaust gas temperature (after low pressure turbine) |
T
exh
| (°C) | 509 |
Exhaust gas thermal power (cooling limit to 90 °C) |
\(\dot{Q}_{exh}\)
| (kW) | 186 |
EGR gas mass flow rate |
\(\dot{m}_{EGR}\)
| (kg s−1) | 0.12 |
EGR gas temperature (EGR cooler inlet) |
T
EGR,IN
| (°C) | 699 |
EGR gas temperature (EGR cooler outlet) |
T
EGR,OUT
| (°C) | 145 |
EGR cooler thermal power |
\(\dot{Q}_{EGR}\)
| (kW) | 81 |
Modelling and methodology
Organic Rankine cycle (ORC)
Heat sink data | Symbol | Unit | IC1 | IC2 |
---|---|---|---|---|
Coolant mass flow rate |
\(\dot{m}_{cf}\)
| (kg s−1) | 3.2 | var. |
Coolant temperature at the ORC condenser inlet |
T
cf,IN
| (°C) | 93.6 | 50 |
-
pressure drops and heat losses have not been considered in the components and in the pipes;
-
pump isentropic efficiency, η is, P , has been set to 70%;
-
expander isentropic efficiency, η is,E , has been set to 80%, considering the possibility of using a radial expander, due to the stable operating profile;
-
expander mechanical efficiency, η mech,E , has been set to 85%, considering possible mechanical coupling with the engine crankshaft using a belt. Electrical coupling could also be assumed, in first approximation, to have a similar efficiency value when considering the electric generator and a driving belt;
-
heat exchangers are counter-flow, divided in single-phase and two-phase zones and modelled with fixed boundaries technique;
-
a sub-cooling degree, \(\Delta T_{sub - cool}\), of 2 °C has been imposed at the outlet of the condenser in order to obtain working fluid always in a liquid state at the pump inlet and avoid cavitation problems. A fluid reservoir is not modelled in this preliminary study;
-
in first approximation, exhaust gas and EGR gas are assumed to have the same properties of dry air;
-
the circulator pumps of the cooling circuits are not considered in the overall power balance;
-
ORC net power output (considering mechanical efficiency also in the balance):
-
ORC efficiency:$$\eta_{ORC} = \frac{{P_{ORC,net} }}{{\dot{Q}_{ORC,IN} }}$$(11)
-
brake-specific fuel consumption (BSFC) of the combined engine-ORC system (%):$$BSFC_{eng + ORC} = \frac{{\dot{m}_{f} }}{{P_{eng + ORC} }} \cdot 100$$(12)
-
BSFC improvement (compared to baseline engine) (%):$$BSFC_{impr} = \frac{{\left( {BSFC_{eng} - BSFC_{eng + ORC} } \right)}}{{BSFC_{eng} }} \cdot 100$$(13)
-
ORC system rejected heat (kW) at the condenser:$$\dot{Q}_{ORC,out} = \dot{Q}_{de - superh} + \dot{Q}_{cond} + \dot{Q}_{sub - cool}$$(14)
Parameter | Reference component | Description |
---|---|---|
P
ORC,net
| ORC System | ORC net power output |
η
ORC
| ORC System | ORC efficiency |
BSFC
impr
| Combined system | BSFC improvement compared to baseline engine |
\(\dot{Q}_{ORC,OUT}\)
| ORC system | ORC rejected heat in the condenser |
\(\mathop \sum \limits_{i} U_{i} A_{HX,i} = \frac{1}{{\dot{m}_{exh/EGR} }}\mathop \sum \limits_{i} \left( {\frac{{\dot{Q}_{HX,i} }}{{\Delta T_{LMTD,i} }}} \right)\)
| Heat exchangers | Sum of the conductance of the HXs (global surface index) per unit recovered mass flow |
Heat sink and radiator
Baseline engine radiator data | Symbol | Unit | Value |
---|---|---|---|
Coolant volume flow |
\(\dot{V}_{cf}\)
| (m3 s−1) | 4.8 |
Coolant radiator inlet temperature |
T
cf,air,IN
| (°C) | 95.7 |
Cooling air volume flow |
\(\dot{V}_{air}\)
| (m3/s) | 11.8 |
Cooling air radiator inlet temperature (after CAC and AC HX) |
T
air,rad,IN
| (°C) | 50 |
Radiator heat rejection |
\(\dot{Q}_{rad}\)
| (kW) | 202.5 |
Radiator height |
H
| (m) | 1.13 |
Radiator width |
W
| (m) | 0.89 |
Radiator depth |
D
| (m) | 0.08 |
Radiator core frontal area |
A
f,rad
| (m2) | 1 |
Radiator core volume |
V
rad
| (m3) | 0.08 |
Fan power consumption |
P
fan
| (kW) | 21 |
Case | Configuration ORC-heat sink | Heat sink position |
---|---|---|
1 | Simple cycle (SC)–IC1 | a |
2 | Parallel cycle (PC)–IC1 | a |
3 | Simple cycle (SC)–IC2 | b |
4 | Parallel cycle (PC)–IC2 | b |
Working fluid selection
Working fluid |
T
c
(°C) |
p
c
(bar) |
T
boil
(°C) |
T
freeze
(°C) | Health hazard (H) | Flammability hazard (F) | GWP (100) |
---|---|---|---|---|---|---|---|
Ethanol | 241.6 | 62.7 | 78.5 | −114.2 | 0 | 3 | n/a |
Methanol | 239.5 | 81 | 64.5 | −97.6 | 1 | 3 | 2.8 |
Toluene | 318.6 | 41.3 | 110.6 | −95.2 | 2 | 3 | 2.7 |
Cyclopentane | 238.6 | 45.7 | 49.3 | −93.5 | 1 | 3 | n/a |
MDM | 290.9 | 14.2 | 152.5 | −86 | 0 | 2 | n/a |
Acetone | 235 | 47 | 56.1 | −94.7 | 1 | 3 | 0.5 |
R-141b | 204.4 | 42.1 | 32.1 | −103.5 | 2 | 1 | 725 |
R-123 | 183.7 | 36.6 | 27.8 | −107.2 | 2 | 0 | 77 |
R-245fa | 154 | 36.5 | 15.1 | −102.1 | 2 | 1 | 1030 |
Water–steam | 374 | 220.6 | 100 | 0 | 0 | 0 | <1 |
Optimization procedure
Independent variable | Unit | Simple cycle (SC) | Parallel cycle (PC) |
---|---|---|---|
Working fluid mass flow | (kg s−1) |
\(\dot{m}_{wf}\)
|
\(\dot{m}_{wf}\)
|
Condensing pressure | (bar) |
p
cond
|
p
cond
|
Pressure ratio | (–) |
PR
|
PR
|
Superheating degree in the ORC exhaust circuit | (°C) |
\(\Delta T_{sh,exh}\)
|
\(\Delta T_{sh,exh}\)
|
Superheating degree in the ORC EGR circuit | (°C) | – |
\(\Delta T_{sh,EGR}\)
|
Cooling fluid mass flow | (kg/s) |
\(\dot{m}_{cf}\)
|
\(\dot{m}_{cf}\)
|
Working fluid rate in the ORC EGR circuit | (%) | – |
a
EGR
|
Variable | Unit | Simple cycle (SC) | Parallel cycle (PC) |
---|---|---|---|
Pinch point temperature difference in the evaporators and condensers | (°C) |
\(\Delta T_{PP,evap/cond} \ge 10\)
|
\(\Delta T_{PP,evap/cond} \ge 10\)
|
Superheating level in the ORC exhaust and EGR circuits | (°C) |
\(\Delta T_{sh,exh/EGR} \le 100\)
|
\(\Delta T_{sh,exh/EGR} \le 100\)
|
Evaporation pressure | (bar) |
p
evap
≤ 30 (or \(0.9 \cdot p_{c}\)) |
p
evap
≤ 30 (or \(0.9 \cdot p_{c}\)) |
Condensing pressure | (bar) |
p
cond
≥ 1.2 |
p
cond
≥ 1.2 |
Evaporation temperature | (°C) |
T
evap
≥ 50 |
T
evap
≥ 50 |
Condensing temperature | (°C) |
T
cond
≥ 50 |
T
cond
≥ 50 |
Exhaust gas temperature at evaporator outlet | (°C) |
T
exh,OUT
≥ 90 |
T
exh,OUT
≥ 90 |
EGR gas temperature at EGR cooler outlet | (°C) | – |
T
EGR,OUT
= 145 |
Vapour quality at expansion outlet | (–) |
x
E,OUT
≥ 0.9 |
x
E,OUT
≥ 0.9 |
Cooling fluid temperature at condenser outlet | (°C) |
T
cf,cond,OUT
≤ 125 |
T
cf,cond,OUT
≤ 125 |
Maximum working fluid temperature (expander inlet) | (°C) |
T
wf,exp,IN
≤ T
c
|
T
wf,exp,IN
≤ T
c
|
Cooling fluid mass flow | (kg s−1) |
\(\dot{m}_{cf} \le 5\)
|
\(\dot{m}_{cf} \le 5\)
|
-
the pinch point value of 10 °C has been considered as a trade-off between heat exchanger performance and cost-dimensions;
-
the working fluid evaporation pressure has been limited to 30 bar or 90% of the fluid critical pressure due to safety reasons and possible fluid chemical instability;
-
the working fluid condensing pressure has been imposed to be higher than 1.2 bar in order to avoid ambient air leaking into the system and expensive sealing;
-
the evaporating and condensing temperatures have been imposed higher than 50 °C in order to avoid inverse heat transfer during particularly hot ambient conditions;
-
the exhaust gas temperature at the outlet of the evaporator has been limited to 90 °C in order to avoid acid condensation and corrosion problems (low sulphur content diesel fuel assumed);
-
the EGR cooler gas outlet temperature has been fixed to 145 °C in order to fulfil combustion requirements for the engine;
-
the coolant temperature at the condenser outlet has been limited to 125 °C to avoid the cooling mixture to boil. In the IC2 heat sink layout, the coolant mass flow has been imposed lower than 5 kg/s to keep the design similar to the main engine cooling circuit;
-
the vapour quality at the expander outlet has been imposed to be higher than 0.9 in order to avoid liquid droplets formation and possible damaging problems, especially when using turbo-expanders;
Results
ORC performance optimization
Simple cycle (SC)—exhaust gas heat recovery
Simple Cycle (SC) | |||
---|---|---|---|
IC1 | IC2 | ||
Water–steam | 6.4 | Methanol | 7.7 |
Toluene | 6.3 | Acetone | 7.6 |
Ethanol | 5.1 | Ethanol | 7.1 |
Acetone | 4.9 | Cyclopentane | 7.0 |
Methanol | 4.8 | Water–steam | 6.7 |
Cyclopentane | 4.6 | R-141b | 6.3 |
R-141b | 3.7 | Toluene | 6.3 |
R-123 | 3.1 | R-123 | 5.7 |
MDM | 2.8 | R-245fa | 4.4 |
R-245fa | 1.7 | MDM | 2.8 |
Parallel cycle (PC)—exhaust gas and EGR heat recovery
Parallel Cycle (SC) | |||
---|---|---|---|
IC1 | IC2 | ||
Toluene | 9.2 | Methanol | 10.6 |
Water–steam | 9.1 | Acetone | 10.2 |
Ethanol | 6.8 | Water–steam | 10.0 |
Acetone | 6.6 | Cyclopentane | 9.9 |
Cyclopentane | 6.3 | Ethanol | 9.9 |
Methanol | 5.9 | Toluene | 9.2 |
R-141b | 4.9 | R-141b | 8.7 |
MDM | 4.2 | R-123 | 7.7 |
R-123 | 4.1 | R-245fa | 6.1 |
R-245fa | 1.9 | MDM | 4.2 |
Heat sink study
\(\dot{Q}_{ORC,OUT}\) (kW) | ||||
---|---|---|---|---|
Fluids | SC–IC1 | SC–IC2 | PC–IC1 | PC–IC2 |
Toluene | 128.2 | 128.4 | 191.5 | 191.5 |
Water–steam | 107.9 | 107.8 | 171.8 | 171.1 |
Ethanol | 135 | 138.9 | 200.7 | 202.2 |
Acetone | 135.4 | 137.5 | 201.4 | 201.9 |
Methanol | 136 | 137.2 | 203.6 | 200 |
Cyclopentane | 137 | 138.2 | 203.3 | 202.8 |
R141b | 139.9 | 142 | 207.9 | 207.8 |
R123 | 142 | 142.2 | 211 | 211.9 |
MDM | 125.4 | 125.4 | 192.1 | 192.1 |
R245fa | 147.4 | 150 | 205.1 | 218.7 |
Simple cycle—indirect condensation 1 (SC–IC1)
Parallel cycle—indirect condensation 1 (PC–IC1)
Simple cycle—indirect condensation 2 (SC–IC2)
Parallel cycle—indirect condensation 2 (PC–IC2)
Overall results
Fluid | Config. |
A
f,rad,incr
(%) |
V
rad,incr
(%) |
P
fan
(kW) |
P
ORC,net
(kW) |
\(\Delta P\) (kW) |
---|---|---|---|---|---|---|
Water–steam | PC–IC1 | 40 | 90 | 6.2 | 30.2 | 24.0 |
Toluene | PC–IC1 | 40 | 90 | 8.2 | 30.6 | 22.4 |
Ethanol | PC–IC2 | 100 | 275 | 15.1 | 33.3 | 18.2 |
Acetone | PC–IC1 | 40 | 90 | 9.3 | 21.4 | 12.1 |
Cyclopentane | PC–IC1 | 40 | 90 | 9.7 | 20.3 | 10.6 |
Methanol | PC–IC1 | 40 | 90 | 9.7 | 19.1 | 9.4 |