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Rankine cycle engines for utilization of LH2 car fuel as a low-temperature source
0360-3199/93 $6.00+ 0.00 PergamonPress Ltd. © 1993 InternationalAssociationfor HydrogenEnergy.
Int. J. HydrogenEnergy, Vol. 18, No. 2, pp. 149-155, 1993. Printed in Great Britain.
RANKINE CYCLE ENGINES FOR UTILIZATION OF LH2 CAR FUEL AS A LOW-TEMPERATURE SOURCE S. FURUHAMA, T. NAKAJIMAand T. HONDA Musashi Institute of Technology, 1-28-1 Tamazutsumi, Setagaya-ku, Tokyo 158, Japan
(Receivedfor publication 9 September 1992) Abstract--The hydrogen fueled engine developed at Musashi Institute of Technology has a system consisting of a LH2 tank, a LH2 pump and equipment for high-pressure fuel supply by hydrogen injection into the engine. It seems feasible to obtain supplementary power by means of a Rankine cycle engine with the LH2 utilized as a heat sink and with the atmosphere or exhaust gas as the heat source if an appropriate working fluid is selected. Thermodynamic calculations on this Rankine cycle engine were carried out to determine the optimum working fluid and to choose the thermodynamic state of the cycle for its maximum output power. As a result, it was found that the output generated by this engine could be several percent of the main engine power. The aforementioned Rankine cycle engine w a s prepared for experimental study. Experimental results with the engine show the feasibility of a system with the Rankine cycle engine.
1. INTRODUCTION A hydrogen engine for automobile use has been developed at the Musashi Institute of Technology Engine Laboratory and has the following fuel supply system. Hydrogen is stored in its liquid state, compressed to high pressure with a LH2 pump, evaporated by atmospheric heat, then injected directly into an engine cylinder and ignited either by a hot surface or by electric spark. In order to utilize the low temperature of liquid hydrogen, a supplementary power generation system was experimentally studied, where 1 0 - 2 0 % of the high-pressure liquid hydrogen was heated by the atmosphere to room temperature, the heated gaseous hydrogen was expanded through a gas expander and the expanded low-pressure hydrogen was supplied to the main engine. The study demonstrated that mechanical work extracted from the gas expander was able to drive the LH2 pump [ 1 ]. During the present study, a Rankine cycle with an independent working fluid has been investigated. In this system, exhaust gas can be used as the heat source and liquid hydrogen as the heat sink. In this paper, the theoretical calculation and preliminary experiment will be presented. 2. HYDROGEN SUPPLY SYSTEM WITH RANKINE CYCLE POWER GENERATION
2.1. System description When high-pressure liquid hydrogen is heated to room temperature, its enthalpy will be increased. If exhaust gas is used as the energy source instead of atmospheric air, the
enthalpy increase in the hydrogen becomes larger and the work extracted through expansion becomes increased. However, since the hydrogen pressure should be kept high for injection into the main engine, it is desirable to utilize the liquid hydrogen merely as a heat sink. From this viewpoint, to obtain extra power output, a cryogenic fluid such as N2, Ar or CH4, whose saturation temperature can be higher than the liquid hydrogen was used as the Rankine cycle working fluid. Figure 1 i~lustrates the Rankine cycle system together with a liquid hydrogen supply line and an exhaust gas line. A working fluid pump (A) compresses the Rankine cycle working fluid, which receives heat from the exhaust gas at a heat exchanger (B) and becomes a high-pressure supercritical gas. The gas is expanded at an expander (C), where work is extracted, and becomes a low-pressure and lowtemperature gas. The gas is cooled and condensed by liquid hydrogen at a heat exchanger (D), and is then returned to a fluid tank. The extracted work at the expander (C) is used to drive both the pump (A) and LH2 pump (E). 2.2. Cycle diagram Figure 2 illustrates the T - S diagram of the Rankine cycle working fluid. The fluid in a storage tank is saturated liquid at state (1) and is entropically compressed by pump from state (1) to (2). Heat q~ is added during the process from (2) to (3), which is under constant pressure and along a supercritical path. The heat q~ is given both by the atmosphere and by engine exhaust gas. Mechanical work is done by the fluid during entropic expansion in an expander from state (3) to (4) and the process from (4) to (1) is heat rejection q2 under constant pressure.
149
150
S. FURUHAMA et al. Room
"~
(c) D)
•
to E n g i n e
Temperature
W
(B TT=T1
m k
O
T6
Entropy Workinc
Fluid
s
Fig. 3. Hydrogen T - S diagram.
Tank LH2 -'rank
{A) (BI (C) (D)
L i q u i d Working F l u i d lleat Exchanger Expander lleat Exchanger
Pump 3. SELECTION OF WORKING FLUID AND CYCLE OPTIMIZATION 3.1. Cycle analysis
(E) Lll2-Pum p (F) llydrogen Engine Fig. 1. Rankine cycle system, together with a liquid hydrogen supply line and an exhaust gas line.
Thermodynamic calculation was carried out to determine the working fluid and thermodynamic state of the cycle which maximized its power output. Figure 4 indicates how the net amount of work varies with heat sink temperature T~ and peak temperature T3. In this figure, qt and q2 denote the amount of heat which a unit mass of 1 kg of working fluid can receive and reject, respectively, and they are expressed in terms of enthalpy difference as:
j®
q, = h3 - h2,
(1)
q2
(2)
= h4 -
hi.
The area enclosed by the cycle (1) - (2) - (3) - (4) - (5) - (1) in the T - S diagram is equal to the net amount of work w which can be extracted from the cycle with 1 kg of working
O 14
m k~ Q) E Q)
T3+AT 3 T
/® P2 T3.h3
T,
.Aw3 Entropy
s
Fig. 2. Rankine cycle working fluid T - S diagram.
P2,T2.
Figure 3 illustrates the hydrogen T - S diagram. The saturated liquid hydrogen at state (5) is compressed by the LH2 pump to state (6). During a constant-pressure process from (6) to (7), the hydrogen receives heat q2 from the Rankine cycle working fluid. Thus, hydrogen is then heated by the atmosphere from T7 to room temperature under constant pressure.
~--",,\
T4 h 4
T T I -A T
q21 A%~ I s t
,
-
S5 S
Fig. 4. T - S cycle diagram with Ti and /'3 being varied.
RANKINE CYCLE ENGINES UTILIZING LH2 CAR FUEL
oor
'~ ~°°I . . . . .
~
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0.3 0.40.5
K
l;s
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8'5
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,3--
~ 6 o o K
oor
9"f
'r 1 (K) •
3°°r
151
(K) I
o.1
I
|
0.2 o.~ o.t Pl
(MPa)
(HPa)
(b) kr
(a) N 2 J O00r __
T3=
,-, 6ool- /
_~.~.~...~5ooK
~
200~
~---~------
. . . .
'Jlo
.,,~.-4.00K
. _ ~___--~__./300 K
I;.~ 1~o d5
d0
T I ,(K)
oll
0.2
o'.3 o'.~
Pl (MPa) (c) p2= ....
CH4 8MPa
P2=20MPa
Fig. 5. Effect of TL(p]) and 7"3 on w for three different working fluids; N2, Ar and CH4.
fluid. It is clearly illustrated in this figure that decrease of T~ by ATL and increase of T3 by AT3 both result in an increase of w by Awl and Aw3, respectively. The net amount of work w was then calculated as a function of T~ (P0 and T3 for three different working fluids N2, Ar and CH4, and their results were compared in Fig. 5. It should be noted that temperature 7"1 is automatically determined by expander exit pressure p~, because state (1) is on the saturation line. The figure shows that a larger amount of work w can be obtained by decreasing temperature T~ and pressure pt. Since each fluid has its own saturation line, temperature T~ will be different for each different working fluid even if pressure p~ is kept the same. Thus, the amount of work w among N2, Ar and CH4 under the same expander exit pressure p~ and the same amount of heat input q~ is compared in Fig. 6, where the amount of work for N2 is set at unity and those for the other two fluids are expressed as a fraction. It is clearly shown in Fig. 6 that the amount of work w linearly decreases as temperature 7", increases. The relationship between w and T3 (peak temperature) is also shown in Fig. 5, where higher /'3 results in larger w for all working fluids under the condition of constant 7"1. Figure 7 illustrates the effect of peak pressure p2 on the net amount of work w. As in the figure, ifp2 is increased, w is increased by Aw2, hut is decreased by Aw4 at the
same time. Thus, the net amount of work w for N2 was calculated as peak pressure was varied, and was plotted in Fig. 8. In each case with a different peak temperature T3, a maximum value of w exists. This is because higher peak pressure not only increases the expansion work w2 in the expander, but also increases the compression work w~ that should be given by the pump. For high p2, the increase of w~ surpasses that of w2, as shown in Fig. 8, which results
1.0
ql = c o n s t P2=8MPa Pl =0. IMPa
e N2 "". ~ Ar
T 3 =300K
~ 0.9 "~ CR 4
0.8 I 7
I 87
I 112
T1
(K)
Fig. 6. w Comparison among N2, Ar and CH4 in view of the difference in TI.
152
S. FURUHAMA et al. Constant-Pressure Line of P2+AP i~
T1
500
CII4 40C
CH4
300 I
N2 20~ Ar
"---.--- ~. w 4
I0~
"-®
~.~
21o
1.0
~
• P2= 8MPa • p2 =20MPa TI= 80K T3=300K Pl =0. I MPa
Ar 1
-
G=Ikg .const
&@ N2
310
4.°
210
cp (kJ/kg.K)
I I s1
Fig. 9. Effect on cp of T~, pz, T3 and p2 being varied.
• s4 s
input, the working fluid with the largest c? allows the largest w, as indicated in this figure.
Fig. 7. T - S cycle diagram with p2 being varied.
w
500
It has been demonstrated so far that the net amount of work is expected to increase if a fluid with larger Cp and lower boiling temperature 7", is chosen, and if cycle peak temperature T3 is made higher. However, LH2 fuel whose flow rate is to be determined by the main engine operation is used as the heat sink in this system. Therefore, the amount of heat rejection from the cycle is proportional to the temperature difference between T~ and the LH2 temperature in its tank, which indicates that when Tj is decreased, the flow rate of the working fluid should also be decreased to completely condense the fluid. The power output w of the system was calculated by the following equation in which the working fluid flow rate was taken into consideration:
500K
-- 400
300
3.2. C o m p a r i s o n o f p o w e r output
G=I kg const, T3 =600K
. ~ -
200
/,,
/
/
/
/
/
/
400K
/
300K
I00
T3 = 600 K
w2
]'-!
5001'[
/
400K 300K
N = (ql - q2) X G = (ql - q2) x
'2.00
Wl b,,
L'-T'~
l
20
30
= (ql Iq2 -
410
)
1) X
c? dT x C~,
(4)
/6
Fig. 8. Effect of P2 on w for N2.
in a decrease of the net amount of work w. However, the effect of peak pressure p2 is relatively small, especially for P2 higher than 10 MPa where it is nearly constant. The relationship between w and specific heat at constant pressure Cp can be expressed by the following equation: W = ql -- q2
where C_m represents the consumption rate of hydrogen. Operating conditions of the main hydrogen engine listed in Table 1 were used for this calculation and the results shown in Fig. 10 are power output N and flow rate G obtained for N2 with 7', being varied and p2 being kept constant. Four different temperature levels, at 300, 400, 500 and 600 K, were chosen as the constant T3 and in each case there was
Table 1. Main hydrogen engine and its operating conditions
cpdT1"2
cp d T x Cmlq2
7"6
__
I
10
P2 (MPa)
=
(l
cpdT+
Tt(s5
-
sj)
(3)
T5
The effect of cp was calculated with Ti, pl, T3 and p2 being fixed and results for At, N2 and CH4 are shown in Fig. 9. Since a larger Cp results in a larger amount of heat
Displacement Rotation frequency Excess air ratio Power output NE H2 consumption rate GH
3839 cm 3 2000 rpm 1.57 60 kW 2.0 )< 10 -3 kg s -1
153
RANKINE CYCLE ENGINES UTILIZING LH2 CAR FUEL P2= 8MPa ------ p2=2OMPa
2.5
~'~'~'---~
2.0
4.0
[02= 8MPa
,.,
~
t"
/
\ 600K
/
3.5
/
3.(
J.5
~
"
"
~
300K
v.
¢3--" × 10"* 300Kl10
i
i
65
~0
75
2.5
/
soo,,:/
..... i
z
.o,,.], ; i
i
i
80
8.5
90
/
/ w / / / / / ~ //~ / // .//
400 K
CH4
/ Ar
/ /
\ ',~>+
/
/
/
/
/CH
4
~r
~
N2
2.0
,
95
T I (K)
0/02 0?05 0:t
o.'2 9:3 o'.4
1.5
Pl (MPa) 1.0
Fig. 10. Effect on G and N of 7"1 being varied for N2.
I
300
1
I
4.00
500
I
600
T 3 (K)
Fig. 11. Effect on N of 7"3 being varied at optimum value of TI. a maximum value of N. This was because two opposite effects appear when T~ is decreased. The first effect is an increase in the net amount of work for a unit mass of work- If, on the other hand, the exhaust gas of the main engine ing fluid and the second is a decrease in working fluid flow is used as the heat source, i.e. T3=600K, the ratio rate. For high TI, the first effect surpasses the second and becomes 6.2%. power output N increases until its maximum as TI is lowered. However, for low Tt, the second effect becomes dominant and further decrease of Ti results in smaller N. 4. EXPERIMENTAL STUDY The dotted line in Fig. 10 was drawn by connecting these Since the theoretical investigation presented so far sugmaximum values of N. A similar calculation was also carried out for Ar and gested that the Rankine cycle system could be feasible, an CH4. Figure 11 compares the theoretical power output N expander and a heat exchanger were fabricated for its which can be obtained for the optimum value of 7"1. It is experimental study. demonstrated in this figure that CI-L, allows the largest 4.1. Test expander value of N among the three working fluids. Figure 12 shows the dependence of N on p2. The effect Figure 13 shows a drawing of the assembled expander. of P2 can be recognized to be more significant in this The inner diameter, stroke, volume and expansion ratio of figure than in Fig. 8, where the relationship between w and the expander are 50 ram, 70 mm, 137 cm 3 and 10, respecp2 is shown. There is no maximum value of N in Fig. 12. tively. A commercial engine crank-case was utilized and its It can be pointed out that the reason for this is that higher piston was used as a cross-head, which was connected to p2 decreases entropy s4 (see Fig. 7) and rejected heat q2 for the expander piston by means of a piston-rod. The pistoncondensation, and that as a result, the working fluid flow rod was long enough to minimize heat inflow through it, rate G can be increased. and a thermal insulation layer was installed on the cylinderWhen the hydrogen main engine is operated under the liner. The P - V diagram shown in Fig. 14 was obtained for conditions listed in Table 1, the theoretically required p2=8 MPa and T3=300K. A supply valve was opened at power for driving the LH2 pump, Np, is 0.22 kW. Thus, top dead center (TDC) and was kept open from (a) through the net power output which can be used as supplement to (b) to (c) for about 14 ° crank-angle (CA), during which the main engine power NE Can be evaluated by subtracting high-pressure working fluid was taken into the expander. Np from N. If the atmosphere is used as the heat source, After the supply valve was closed, the fluid expanded and i.e. T3=300 K, the ratio of the supplementary power is pressed down the piston. During this process, work was calculated to be 2.8% from the following equation: done and extracted through a crankshaft. An exhaust valve was opened at bottom dead center (BDC) and as the piston (N - Np)INE x 100 = 2.8%. went up to TDC, low-pressure and low-temperature work-
154
S. FURUHAMA et al.
T3=
3.5
600K
/ 3.0
A
500K
6
2-
400K 2.
300K
1
T~ -J
I
x I 0 "' 300K l0 A
_ _ _ _ _ _ ~ ~
600K
5
z I0
210
i
30
-180
(TDC) IB0 (o C A ) (a) p-@ Diagram
@ ©
.-W v L9
I
40
P2 (MPa ) Fig. 12. Effect on N ofp2 being varied.
2i
TDC ~ Exhaust Cam
w
@
9 (b) p-V Diagram
BDC
Cam Fig. 14. Pressure diagram of cylinder of expander.
Exhaust Valve Exhaust Port Adiabatic Space Piston Ro(
,g Gas Port n
QN~QH N 2 ConstantPressure Line Cll4 Boiling / Point r ..... / r IMPa
Commercia Engine
t Fig. 13. Assembly drawing of experimental expander.
Fig. 15. T - S diagram of H2 and N:.
155
RANKINE CYCLE ENGINES UTILIZING LH2 CAR FUEL
G N2 Heat Exchanger To Flow Meter . u.~, ~ ~ LN2
Table 3. Results for power output and thermal efficiency Power output w LN2 consumptionrate Carnot cycle theoretical thermal efficiency 7/c Rankine cycle theoretical thermal efficiency T/R Experimental thermal efficiency 7/
~Dynamometer
ing fluid was exhausted. The pressure inside the cylinder was kept almost the same as that in the exhaust line during this process from (d) to (a). 4.2. Experimental set-up and results For the experimental study, CH4 was used as working fluid and LN2 as the heat sink instead of LH2. Their T - S plots are shown in Fig. 15. The pressure of LN2 was set at 1 MPa and its flow rate was controlled so that the amount of transferred heat QN from CH4 to LN2 was equal to the amount Q, that would have been expected at the heat exchanger (d) in Fig. 1 if LH2 had been used. It should also be noted that the upper limit of the heat sink temperature for both LN2 and LH2 was assumed to be the CH4 boiling temperature. The experimental set-up for the present study is illustrated in Fig. 16 and the results of the temperature measurement are tabulated in Table 2. The presence of CH4 in its storage tank was set at 0.15 MPa, where the saturation temperature was I17K. Since the measured temperature at the heat exchanger exit was 115K, it was believed that the working fluid condensed almost completely at the heat exchanger. The power output of the cycle and LN2 consumption rate are shown in Table 3. The experimentally obtained thermal efficiency is also compared with the theoretical efficiencies in this table. The measured net power output in Table 2. Results of temperature measurement
N2
1. Expander inlet 2. Expander exit 3. Heat exchanger inlet 4. Heat exchanger exit 5. Heat exchanger inlet 6. Heat exchanger exit
26.0%
Rate of expander power output to enthalpy difference Wexp/Ah Power transmissionefficiencyof LCH4 pump Rate of design amount of heat transfer to experimental for heat exchanger
Fig. 16. Experimentalset-up.
CH4
41.4%
Table 4. Performanceof each component
~LCHgTank
Measuring point
0.89 kW 12.9 × 10 -3 kg s -I 63.5%
Temperature 315 K 169 K 178 K 115 K 83 K 125 K
Pressure of Cl-h in its storage tank was set at 0.15 MPa, where the saturationtemperature was 117 K.
62 % 55% 86%
this system is 0.89 kW, which corresponds to 1.5% of the main hydrogen engine output. When the inlet and exit temperatures are 315K and lI5K, respectively, Carnot cycle efficiency is 63.5% and the ideal CH4 Rankine cycle efficiency is 41.4%. On the other hand, the experimental result shows that the efficiency of the present system is 26.0%. The performance of each component is tabulated in Table 4, which suggests that the improvement of the expander and the LH2 pump would allow the system efficiency to increase toward its theoretical value; such an improvement requires future work. 5. CONCLUSION The present studies, both theoretical and experimental, lead to the following conclusions. (1) As a working fluid, CH4 allows the largest amount of power output among the three examined fluids, N2, Ar and CH4, because the largest value of cp of the above for CH4 enables heat input to the fluid to be a maximum. (2) Under a fixed amount of heat input, the output work linearly increases with respect to temperature difference (T3- T0. (3) The amount of power output increases as peak temperature T3 increases. (4) The amount of power output also increases as peak pressure increases. (5) The theoretical study and preliminary experiment demonstrated that if a Rankine cycle with an independent working fluid was integrated with the LH2 supply system, a supplementary power output of several percent was obtainable. REFERENCES
I. S. Furuharna, T. Matusita, T. Nakajima and K. Yamaura, Hydrogen injection spark ignition engine with LH2-pump driven by high pressure hydrogenexpander, Hydrogen Energy Progress VII, Vol. 3, pp. 1975-1987 (0000).
Int. J. HydrogenEnergy, Vol. 18, No. 2, pp. 149-155, 1993. Printed in Great Britain.
RANKINE CYCLE ENGINES FOR UTILIZATION OF LH2 CAR FUEL AS A LOW-TEMPERATURE SOURCE S. FURUHAMA, T. NAKAJIMAand T. HONDA Musashi Institute of Technology, 1-28-1 Tamazutsumi, Setagaya-ku, Tokyo 158, Japan
(Receivedfor publication 9 September 1992) Abstract--The hydrogen fueled engine developed at Musashi Institute of Technology has a system consisting of a LH2 tank, a LH2 pump and equipment for high-pressure fuel supply by hydrogen injection into the engine. It seems feasible to obtain supplementary power by means of a Rankine cycle engine with the LH2 utilized as a heat sink and with the atmosphere or exhaust gas as the heat source if an appropriate working fluid is selected. Thermodynamic calculations on this Rankine cycle engine were carried out to determine the optimum working fluid and to choose the thermodynamic state of the cycle for its maximum output power. As a result, it was found that the output generated by this engine could be several percent of the main engine power. The aforementioned Rankine cycle engine w a s prepared for experimental study. Experimental results with the engine show the feasibility of a system with the Rankine cycle engine.
1. INTRODUCTION A hydrogen engine for automobile use has been developed at the Musashi Institute of Technology Engine Laboratory and has the following fuel supply system. Hydrogen is stored in its liquid state, compressed to high pressure with a LH2 pump, evaporated by atmospheric heat, then injected directly into an engine cylinder and ignited either by a hot surface or by electric spark. In order to utilize the low temperature of liquid hydrogen, a supplementary power generation system was experimentally studied, where 1 0 - 2 0 % of the high-pressure liquid hydrogen was heated by the atmosphere to room temperature, the heated gaseous hydrogen was expanded through a gas expander and the expanded low-pressure hydrogen was supplied to the main engine. The study demonstrated that mechanical work extracted from the gas expander was able to drive the LH2 pump [ 1 ]. During the present study, a Rankine cycle with an independent working fluid has been investigated. In this system, exhaust gas can be used as the heat source and liquid hydrogen as the heat sink. In this paper, the theoretical calculation and preliminary experiment will be presented. 2. HYDROGEN SUPPLY SYSTEM WITH RANKINE CYCLE POWER GENERATION
2.1. System description When high-pressure liquid hydrogen is heated to room temperature, its enthalpy will be increased. If exhaust gas is used as the energy source instead of atmospheric air, the
enthalpy increase in the hydrogen becomes larger and the work extracted through expansion becomes increased. However, since the hydrogen pressure should be kept high for injection into the main engine, it is desirable to utilize the liquid hydrogen merely as a heat sink. From this viewpoint, to obtain extra power output, a cryogenic fluid such as N2, Ar or CH4, whose saturation temperature can be higher than the liquid hydrogen was used as the Rankine cycle working fluid. Figure 1 i~lustrates the Rankine cycle system together with a liquid hydrogen supply line and an exhaust gas line. A working fluid pump (A) compresses the Rankine cycle working fluid, which receives heat from the exhaust gas at a heat exchanger (B) and becomes a high-pressure supercritical gas. The gas is expanded at an expander (C), where work is extracted, and becomes a low-pressure and lowtemperature gas. The gas is cooled and condensed by liquid hydrogen at a heat exchanger (D), and is then returned to a fluid tank. The extracted work at the expander (C) is used to drive both the pump (A) and LH2 pump (E). 2.2. Cycle diagram Figure 2 illustrates the T - S diagram of the Rankine cycle working fluid. The fluid in a storage tank is saturated liquid at state (1) and is entropically compressed by pump from state (1) to (2). Heat q~ is added during the process from (2) to (3), which is under constant pressure and along a supercritical path. The heat q~ is given both by the atmosphere and by engine exhaust gas. Mechanical work is done by the fluid during entropic expansion in an expander from state (3) to (4) and the process from (4) to (1) is heat rejection q2 under constant pressure.
149
150
S. FURUHAMA et al. Room
"~
(c) D)
•
to E n g i n e
Temperature
W
(B TT=T1
m k
O
T6
Entropy Workinc
Fluid
s
Fig. 3. Hydrogen T - S diagram.
Tank LH2 -'rank
{A) (BI (C) (D)
L i q u i d Working F l u i d lleat Exchanger Expander lleat Exchanger
Pump 3. SELECTION OF WORKING FLUID AND CYCLE OPTIMIZATION 3.1. Cycle analysis
(E) Lll2-Pum p (F) llydrogen Engine Fig. 1. Rankine cycle system, together with a liquid hydrogen supply line and an exhaust gas line.
Thermodynamic calculation was carried out to determine the working fluid and thermodynamic state of the cycle which maximized its power output. Figure 4 indicates how the net amount of work varies with heat sink temperature T~ and peak temperature T3. In this figure, qt and q2 denote the amount of heat which a unit mass of 1 kg of working fluid can receive and reject, respectively, and they are expressed in terms of enthalpy difference as:
j®
q, = h3 - h2,
(1)
q2
(2)
= h4 -
hi.
The area enclosed by the cycle (1) - (2) - (3) - (4) - (5) - (1) in the T - S diagram is equal to the net amount of work w which can be extracted from the cycle with 1 kg of working
O 14
m k~ Q) E Q)
T3+AT 3 T
/® P2 T3.h3
T,
.Aw3 Entropy
s
Fig. 2. Rankine cycle working fluid T - S diagram.
P2,T2.
Figure 3 illustrates the hydrogen T - S diagram. The saturated liquid hydrogen at state (5) is compressed by the LH2 pump to state (6). During a constant-pressure process from (6) to (7), the hydrogen receives heat q2 from the Rankine cycle working fluid. Thus, hydrogen is then heated by the atmosphere from T7 to room temperature under constant pressure.
~--",,\
T4 h 4
T T I -A T
q21 A%~ I s t
,
-
S5 S
Fig. 4. T - S cycle diagram with Ti and /'3 being varied.
RANKINE CYCLE ENGINES UTILIZING LH2 CAR FUEL
oor
'~ ~°°I . . . . .
~
r
-
?'5
s'o i5
io
o'.i o'.2 Pl
/
| ~
.
o
.
I
0.3 0.40.5
K
l;s
g'o Cs ,~o
8'5
T~ •
,3--
~ 6 o o K
oor
9"f
'r 1 (K) •
3°°r
151
(K) I
o.1
I
|
0.2 o.~ o.t Pl
(MPa)
(HPa)
(b) kr
(a) N 2 J O00r __
T3=
,-, 6ool- /
_~.~.~...~5ooK
~
200~
~---~------
. . . .
'Jlo
.,,~.-4.00K
. _ ~___--~__./300 K
I;.~ 1~o d5
d0
T I ,(K)
oll
0.2
o'.3 o'.~
Pl (MPa) (c) p2= ....
CH4 8MPa
P2=20MPa
Fig. 5. Effect of TL(p]) and 7"3 on w for three different working fluids; N2, Ar and CH4.
fluid. It is clearly illustrated in this figure that decrease of T~ by ATL and increase of T3 by AT3 both result in an increase of w by Awl and Aw3, respectively. The net amount of work w was then calculated as a function of T~ (P0 and T3 for three different working fluids N2, Ar and CH4, and their results were compared in Fig. 5. It should be noted that temperature 7"1 is automatically determined by expander exit pressure p~, because state (1) is on the saturation line. The figure shows that a larger amount of work w can be obtained by decreasing temperature T~ and pressure pt. Since each fluid has its own saturation line, temperature T~ will be different for each different working fluid even if pressure p~ is kept the same. Thus, the amount of work w among N2, Ar and CH4 under the same expander exit pressure p~ and the same amount of heat input q~ is compared in Fig. 6, where the amount of work for N2 is set at unity and those for the other two fluids are expressed as a fraction. It is clearly shown in Fig. 6 that the amount of work w linearly decreases as temperature 7", increases. The relationship between w and T3 (peak temperature) is also shown in Fig. 5, where higher /'3 results in larger w for all working fluids under the condition of constant 7"1. Figure 7 illustrates the effect of peak pressure p2 on the net amount of work w. As in the figure, ifp2 is increased, w is increased by Aw2, hut is decreased by Aw4 at the
same time. Thus, the net amount of work w for N2 was calculated as peak pressure was varied, and was plotted in Fig. 8. In each case with a different peak temperature T3, a maximum value of w exists. This is because higher peak pressure not only increases the expansion work w2 in the expander, but also increases the compression work w~ that should be given by the pump. For high p2, the increase of w~ surpasses that of w2, as shown in Fig. 8, which results
1.0
ql = c o n s t P2=8MPa Pl =0. IMPa
e N2 "". ~ Ar
T 3 =300K
~ 0.9 "~ CR 4
0.8 I 7
I 87
I 112
T1
(K)
Fig. 6. w Comparison among N2, Ar and CH4 in view of the difference in TI.
152
S. FURUHAMA et al. Constant-Pressure Line of P2+AP i~
T1
500
CII4 40C
CH4
300 I
N2 20~ Ar
"---.--- ~. w 4
I0~
"-®
~.~
21o
1.0
~
• P2= 8MPa • p2 =20MPa TI= 80K T3=300K Pl =0. I MPa
Ar 1
-
G=Ikg .const
&@ N2
310
4.°
210
cp (kJ/kg.K)
I I s1
Fig. 9. Effect on cp of T~, pz, T3 and p2 being varied.
• s4 s
input, the working fluid with the largest c? allows the largest w, as indicated in this figure.
Fig. 7. T - S cycle diagram with p2 being varied.
w
500
It has been demonstrated so far that the net amount of work is expected to increase if a fluid with larger Cp and lower boiling temperature 7", is chosen, and if cycle peak temperature T3 is made higher. However, LH2 fuel whose flow rate is to be determined by the main engine operation is used as the heat sink in this system. Therefore, the amount of heat rejection from the cycle is proportional to the temperature difference between T~ and the LH2 temperature in its tank, which indicates that when Tj is decreased, the flow rate of the working fluid should also be decreased to completely condense the fluid. The power output w of the system was calculated by the following equation in which the working fluid flow rate was taken into consideration:
500K
-- 400
300
3.2. C o m p a r i s o n o f p o w e r output
G=I kg const, T3 =600K
. ~ -
200
/,,
/
/
/
/
/
/
400K
/
300K
I00
T3 = 600 K
w2
]'-!
5001'[
/
400K 300K
N = (ql - q2) X G = (ql - q2) x
'2.00
Wl b,,
L'-T'~
l
20
30
= (ql Iq2 -
410
)
1) X
c? dT x C~,
(4)
/6
Fig. 8. Effect of P2 on w for N2.
in a decrease of the net amount of work w. However, the effect of peak pressure p2 is relatively small, especially for P2 higher than 10 MPa where it is nearly constant. The relationship between w and specific heat at constant pressure Cp can be expressed by the following equation: W = ql -- q2
where C_m represents the consumption rate of hydrogen. Operating conditions of the main hydrogen engine listed in Table 1 were used for this calculation and the results shown in Fig. 10 are power output N and flow rate G obtained for N2 with 7', being varied and p2 being kept constant. Four different temperature levels, at 300, 400, 500 and 600 K, were chosen as the constant T3 and in each case there was
Table 1. Main hydrogen engine and its operating conditions
cpdT1"2
cp d T x Cmlq2
7"6
__
I
10
P2 (MPa)
=
(l
cpdT+
Tt(s5
-
sj)
(3)
T5
The effect of cp was calculated with Ti, pl, T3 and p2 being fixed and results for At, N2 and CH4 are shown in Fig. 9. Since a larger Cp results in a larger amount of heat
Displacement Rotation frequency Excess air ratio Power output NE H2 consumption rate GH
3839 cm 3 2000 rpm 1.57 60 kW 2.0 )< 10 -3 kg s -1
153
RANKINE CYCLE ENGINES UTILIZING LH2 CAR FUEL P2= 8MPa ------ p2=2OMPa
2.5
~'~'~'---~
2.0
4.0
[02= 8MPa
,.,
~
t"
/
\ 600K
/
3.5
/
3.(
J.5
~
"
"
~
300K
v.
¢3--" × 10"* 300Kl10
i
i
65
~0
75
2.5
/
soo,,:/
..... i
z
.o,,.], ; i
i
i
80
8.5
90
/
/ w / / / / / ~ //~ / // .//
400 K
CH4
/ Ar
/ /
\ ',~>+
/
/
/
/
/CH
4
~r
~
N2
2.0
,
95
T I (K)
0/02 0?05 0:t
o.'2 9:3 o'.4
1.5
Pl (MPa) 1.0
Fig. 10. Effect on G and N of 7"1 being varied for N2.
I
300
1
I
4.00
500
I
600
T 3 (K)
Fig. 11. Effect on N of 7"3 being varied at optimum value of TI. a maximum value of N. This was because two opposite effects appear when T~ is decreased. The first effect is an increase in the net amount of work for a unit mass of work- If, on the other hand, the exhaust gas of the main engine ing fluid and the second is a decrease in working fluid flow is used as the heat source, i.e. T3=600K, the ratio rate. For high TI, the first effect surpasses the second and becomes 6.2%. power output N increases until its maximum as TI is lowered. However, for low Tt, the second effect becomes dominant and further decrease of Ti results in smaller N. 4. EXPERIMENTAL STUDY The dotted line in Fig. 10 was drawn by connecting these Since the theoretical investigation presented so far sugmaximum values of N. A similar calculation was also carried out for Ar and gested that the Rankine cycle system could be feasible, an CH4. Figure 11 compares the theoretical power output N expander and a heat exchanger were fabricated for its which can be obtained for the optimum value of 7"1. It is experimental study. demonstrated in this figure that CI-L, allows the largest 4.1. Test expander value of N among the three working fluids. Figure 12 shows the dependence of N on p2. The effect Figure 13 shows a drawing of the assembled expander. of P2 can be recognized to be more significant in this The inner diameter, stroke, volume and expansion ratio of figure than in Fig. 8, where the relationship between w and the expander are 50 ram, 70 mm, 137 cm 3 and 10, respecp2 is shown. There is no maximum value of N in Fig. 12. tively. A commercial engine crank-case was utilized and its It can be pointed out that the reason for this is that higher piston was used as a cross-head, which was connected to p2 decreases entropy s4 (see Fig. 7) and rejected heat q2 for the expander piston by means of a piston-rod. The pistoncondensation, and that as a result, the working fluid flow rod was long enough to minimize heat inflow through it, rate G can be increased. and a thermal insulation layer was installed on the cylinderWhen the hydrogen main engine is operated under the liner. The P - V diagram shown in Fig. 14 was obtained for conditions listed in Table 1, the theoretically required p2=8 MPa and T3=300K. A supply valve was opened at power for driving the LH2 pump, Np, is 0.22 kW. Thus, top dead center (TDC) and was kept open from (a) through the net power output which can be used as supplement to (b) to (c) for about 14 ° crank-angle (CA), during which the main engine power NE Can be evaluated by subtracting high-pressure working fluid was taken into the expander. Np from N. If the atmosphere is used as the heat source, After the supply valve was closed, the fluid expanded and i.e. T3=300 K, the ratio of the supplementary power is pressed down the piston. During this process, work was calculated to be 2.8% from the following equation: done and extracted through a crankshaft. An exhaust valve was opened at bottom dead center (BDC) and as the piston (N - Np)INE x 100 = 2.8%. went up to TDC, low-pressure and low-temperature work-
154
S. FURUHAMA et al.
T3=
3.5
600K
/ 3.0
A
500K
6
2-
400K 2.
300K
1
T~ -J
I
x I 0 "' 300K l0 A
_ _ _ _ _ _ ~ ~
600K
5
z I0
210
i
30
-180
(TDC) IB0 (o C A ) (a) p-@ Diagram
@ ©
.-W v L9
I
40
P2 (MPa ) Fig. 12. Effect on N ofp2 being varied.
2i
TDC ~ Exhaust Cam
w
@
9 (b) p-V Diagram
BDC
Cam Fig. 14. Pressure diagram of cylinder of expander.
Exhaust Valve Exhaust Port Adiabatic Space Piston Ro(
,g Gas Port n
QN~QH N 2 ConstantPressure Line Cll4 Boiling / Point r ..... / r IMPa
Commercia Engine
t Fig. 13. Assembly drawing of experimental expander.
Fig. 15. T - S diagram of H2 and N:.
155
RANKINE CYCLE ENGINES UTILIZING LH2 CAR FUEL
G N2 Heat Exchanger To Flow Meter . u.~, ~ ~ LN2
Table 3. Results for power output and thermal efficiency Power output w LN2 consumptionrate Carnot cycle theoretical thermal efficiency 7/c Rankine cycle theoretical thermal efficiency T/R Experimental thermal efficiency 7/
~Dynamometer
ing fluid was exhausted. The pressure inside the cylinder was kept almost the same as that in the exhaust line during this process from (d) to (a). 4.2. Experimental set-up and results For the experimental study, CH4 was used as working fluid and LN2 as the heat sink instead of LH2. Their T - S plots are shown in Fig. 15. The pressure of LN2 was set at 1 MPa and its flow rate was controlled so that the amount of transferred heat QN from CH4 to LN2 was equal to the amount Q, that would have been expected at the heat exchanger (d) in Fig. 1 if LH2 had been used. It should also be noted that the upper limit of the heat sink temperature for both LN2 and LH2 was assumed to be the CH4 boiling temperature. The experimental set-up for the present study is illustrated in Fig. 16 and the results of the temperature measurement are tabulated in Table 2. The presence of CH4 in its storage tank was set at 0.15 MPa, where the saturation temperature was I17K. Since the measured temperature at the heat exchanger exit was 115K, it was believed that the working fluid condensed almost completely at the heat exchanger. The power output of the cycle and LN2 consumption rate are shown in Table 3. The experimentally obtained thermal efficiency is also compared with the theoretical efficiencies in this table. The measured net power output in Table 2. Results of temperature measurement
N2
1. Expander inlet 2. Expander exit 3. Heat exchanger inlet 4. Heat exchanger exit 5. Heat exchanger inlet 6. Heat exchanger exit
26.0%
Rate of expander power output to enthalpy difference Wexp/Ah Power transmissionefficiencyof LCH4 pump Rate of design amount of heat transfer to experimental for heat exchanger
Fig. 16. Experimentalset-up.
CH4
41.4%
Table 4. Performanceof each component
~LCHgTank
Measuring point
0.89 kW 12.9 × 10 -3 kg s -I 63.5%
Temperature 315 K 169 K 178 K 115 K 83 K 125 K
Pressure of Cl-h in its storage tank was set at 0.15 MPa, where the saturationtemperature was 117 K.
62 % 55% 86%
this system is 0.89 kW, which corresponds to 1.5% of the main hydrogen engine output. When the inlet and exit temperatures are 315K and lI5K, respectively, Carnot cycle efficiency is 63.5% and the ideal CH4 Rankine cycle efficiency is 41.4%. On the other hand, the experimental result shows that the efficiency of the present system is 26.0%. The performance of each component is tabulated in Table 4, which suggests that the improvement of the expander and the LH2 pump would allow the system efficiency to increase toward its theoretical value; such an improvement requires future work. 5. CONCLUSION The present studies, both theoretical and experimental, lead to the following conclusions. (1) As a working fluid, CH4 allows the largest amount of power output among the three examined fluids, N2, Ar and CH4, because the largest value of cp of the above for CH4 enables heat input to the fluid to be a maximum. (2) Under a fixed amount of heat input, the output work linearly increases with respect to temperature difference (T3- T0. (3) The amount of power output increases as peak temperature T3 increases. (4) The amount of power output also increases as peak pressure increases. (5) The theoretical study and preliminary experiment demonstrated that if a Rankine cycle with an independent working fluid was integrated with the LH2 supply system, a supplementary power output of several percent was obtainable. REFERENCES
I. S. Furuharna, T. Matusita, T. Nakajima and K. Yamaura, Hydrogen injection spark ignition engine with LH2-pump driven by high pressure hydrogenexpander, Hydrogen Energy Progress VII, Vol. 3, pp. 1975-1987 (0000).