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Performance tests on a thermally operated hydrogen compressor
International Journal of Hydrogen Energy 33 (2008) 463 – 469 www.elsevier.com/locate/ijhydene
Performance tests on a thermally operated hydrogen compressor P. Muthukumar a,∗ , M. Prakash Maiya b , S. Srinivasa Murthy b a Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India b Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India
Received 19 June 2007; accepted 9 July 2007 Available online 4 September 2007
Abstract A metal hydride based hydrogen compressor using MmNi4.6 Al0.4 is tested at both constant and variable delivery pressures. The effects of operating parameters such as supply pressure and heat source temperature on the compressor performance are investigated. The compressor yields a maximum isentropic efficiency of 7.3% at 95 ◦ C heat source temperature at variable delivery pressure operation mode, and 14.2% at 85 ◦ C heat source temperature at constant delivery pressure operation mode. At both the operation modes, the volumetric efficiency of the compressor is found to decrease at high heat source temperatures. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Metal hydride; Hydrogen compressor; Volumetric efficiency; Isentropic efficiency
1. Introduction Mechanical compression of hydrogen gas poses many technical problems related to compressor construction, balancing, lubrication, maintenance, etc. In addition, such compression consumes electrical energy in contrast to thermal compression, which can be accomplished by waste heat. Moreover, it is desirable to adopt engineering solutions, which eliminate moving parts working under a hydrogen medium. The metal hydride based hydrogen compressor is one of the potential applications of hydride technology which can be tailored to cover a wide range of operating pressures and pressure ratios by selecting suitable alloys [1–3]. These compressors also permit noiseless and vibration free operation, in addition to delivering pure hydrogen. Recently the authors [4,5] investigated the influence of operational and reactor bed related parameters on the compressor performance by analyzing the heat and mass transfer aspects of the hydride bed. In an experimental study using MmNi4.6 Al0.4 , the influences of supply pressure and hot fluid temperature on the compressor performance were investigated for variable
∗ Corresponding author. Fax: +91 361 258 2699.
E-mail address: [email protected] (P. Muthukumar).
delivery pressure using a constant volume storage cylinder of 1 l capacity. This paper presents the performance studies of a hydrogen compressor using MmNi4.6 Al0.4 at both variable and constant delivery pressure modes, and also studies the influence of different storage volumes. 2. Experimental studies 2.1. Operating principle The operating principle of the compressor is explained in an earlier publication by the authors [5]. The operation of the hydride compressor consists of four processes namely: Process AB: Absorption of hydrogen at low temperature (Tc ) and low pressure (Ps ). Process BC: Sensible heating accomplished by compression from cold fluid temperature (Tc ) to hot fluid temperature (Th ). Process CD: Desorption of compressed hydrogen at hot fluid temperature (Th ) and high pressure (Pd ). Process DA: Sensible cooling from Th to Tc . Process CD can be of two cases: In Case 1, compressor starts to discharge hydrogen when the hydride bed reaches the specified hot fluid (heat source) temperature. Storage pressure increases with time and becomes equal to hydride equilibrium pressure at the end of the desorption process. In Case 2,
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.07.019
464
P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
Nomenclature Cf Cr M ma mf P Q Ru T Vs W
specific heat capacity of heat transfer fluid, 4.182 kJ kg−1 K −1 clearance ratio molecular weight of hydrogen, 2.01 kg kmol−1 mass, kg mass flow rate of heat transfer fluid, kg s−1 pressure, bar heat input, J universal gas constant, 8314 J gmol−1 K −1 temperature, K time, s storage volume, l compressor work, J
concentration, H /M ratio index for isentropic compression, 1.405 compressor efficiency
x Υ c Subscript ab c d de f1 f2 h s 1, 2, . . . , 4
end of the absorption cold fluid desorption end of the desorption heat transfer fluid inlet heat transfer fluid outlet hot fluid supply sensor locations (Fig. 2)
compressor starts to discharge hydrogen when the hydride equilibrium pressure exceeds the preset value of the delivery pressure. In this case, the storage pressure is constant at the preset value of the delivery pressure.
equation (Eq. (1)), the pipe volume v1 (V4 –MF–V5 –V6 –V3 –V2 –V1 ) is calculated.
2.2. Description of the test rig
where Tat and v0 are the atmospheric temperature and correction volume, respectively. Then valve V1 is opened and argon is allowed in the reactor. After reaching pressure equilibrium, the fall in pressure (P ) is calculated. From P , the void volume of reactor and the pipe volume till valve V1 (vc ) is obtained:
The experimental setup shown in Fig. 1 is configured to achieve the above processes. The reactor is made of seamless stainless steel (SS-316) tube of wall thickness of 3 mm and the filter is of sintered SS-316 of 2-m pore size [5]. Eight copper fins of 1 mm thickness are used to enhance the effective thermal conductivity of the hydride bed and also to keep the hydride powder in place during the repeated absorption and desorption processes. One end of the reactor is closed with the brazed assembly of four grounded metal-sheathed “K” type thermocouples (sensitivity 0.1 ◦ C) and the other end of the reactor is flanged. Other instrumentation and sensors are shown in Fig. 1. Two constant temperature baths (±0.1 ◦ C) are used for supplying hot and cold fluids at flow rates ranging from 1 to 3.5 l/min, which correspond to the overall heat transfer coefficient ranging from 500 to 1250 Wm−2 K −1 . With the aid of the basic screening procedure deduced by the authors [6] and considering practical aspects such as absorption at near ambient temperatures, low supply pressure and ready availability at low cost, MmNi4.6 Al0.4 is selected for the present experimental study. 2.3. Experimental procedure After charging 400 g of MmNi4.6 Al0.4 into the reactor, the whole system is evacuated down to 10−3 mbar. Then, the reactor is isolated from the setup by closing the valve V1 . Argon gas at Pi bar is supplied to the system and the mass flow meter reading is observed. Then valve V4 is closed. After reaching the pressure equilibrium, the amount of argon transferred (mAr ) is obtained from the mass flow meter. By using the real gas
v1 =
vc =
mAr Ru Tat + v0 , Pi
mAr Ru T − v1 . P
(1)
(2)
Activation of MmNi4.6 Al0.4 is carried out at 27 ◦ C and 80 bar. Before applying hydrogen gas to the reactor, the whole system is evacuated down to about 10−3 mbar. Then it is flushed three times with hydrogen gas. It is evacuated again and then hydrogen gas at 80 bar is allowed to the reactor via the mass flow meter. During the first cycle of operation, a hydrogen uptake of 0.57 wt% is measured within 30 min. Then the reactor is heated to 60 ◦ C and the hydrogen is completely desorbed to atmosphere. During the next 10 absorption/desorption cycles the storage capacity gradually increased to a maximum of 1.43 wt%. At the start of the cycle, cold fluid is circulated such that the equilibrium pressure of the hydride bed falls below that of the supply pressure. Later, valves V1 –V4 are opened and hydrogen is allowed to flow into the bed. Absorption of hydrogen is continued till the hydride equilibrium pressure becomes equal to the supply pressure, which completes process AB. Heat of absorption is rejected to the cold fluid. Then valves V1 –V4 are closed and the bed is heated by circulating hot fluid till the bed temperature is in equilibrium with the hot fluid temperature, thereby completing process BC. Valves V1 , V2 and V6 are opened to allow hydrogen to flow into the storage cylinder. Desorption of hydrogen is continued till the storage cylinder pressure is in equilibrium with the hydride pressure which
P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
465
Data logger
D.C. Constant voltage source
V1 T1-4
P1
V2
Vs1 S
Reactor
M.F
V3
V5
Tf2
Tf1
V4 V6
T5
P2
SS- sinter 2M filter
T6
Ps High Temperature Thermostatic Bath
Low Temperature Thermostatic Bath
Packless bellow seal valves
H2 Storage
H2 Supply
S
Ball valves
One way valves
T1 -T6 Metal sheathed "K" type thermocouples
P1-P2 ressure transducers
Tf1-Tf2 Water inlet and out let temperatures
M.F Mass flow meter
Heat Transfer fluid line
Electrical wire
Hydrogen line
Fig. 1. Setup for testing metal hydride based hydrogen compressor.
completes process CD. Then the valves V1 , V2 and V6 are closed, and the bed is cooled down to initial absorption temperature by circulating cold fluid, thereby completing process DA. Thus the system completes the first compression cycle. The dynamic history of bed temperatures (T1 –T4 ), pressures (P1 and P2 ) and hydrogen flow rate are recorded at 100 ms intervals by the data acquisition system. The heat transfer fluid temperatures at inlet and outlet of the reactor and the hydrogen gas temperatures at the exits of supply cylinder and storage cylinder are also recorded simultaneously. It should be noted that at the end of the first cycle, the bed is not completely dehydrided. The valves V1 –V4 are opened and additional gas is absorbed by circulating the cold fluid. Absorption is continued till the bed equilibrium pressure becomes equal to the supply pressure. Then desorption is repeated followed by heating of the bed after opening the appropriate valves till the bed equilibrium pressure equals the storage pressure (P2 ). Then the valves V1 , V2 and V6 are closed and the bed is cooled down to cold fluid temperature. Thus the system completes the second compression cycle. As will be discussed later, for 1 l storage volume, further cycles are not beneficial for achieving higher pressures and in fact there can be a fall in efficiency. Hence, the compression process is stopped after the second cycle, whereas for 3.8 l storage cylinder five cycles are needed. For Case 2, i.e. constant delivery pressure condition, the valve V6 is replaced by a pressure relief valve. The delivery pressure is varied by varying the cracking pressure of the pressure relief valve.
2.4. Performance parameters The three important performance parameters for the compressor are isentropic and volumetric efficiencies, and hydrogen throughput. The isentropic efficiency is given by c =
Isentropic compression work W = , Heat input to compressor Q
(3)
where
(−1)/ Ru Pd W= mH Tab −1 . ( − 1) 2 MH2 Ps
(4)
Heat supplied during the compression process is estimated as follows: de Q = m f Cf (Tf1 − Tf2 )j. (5) ab
Volumetric efficiency of a hydrogen compressor is expressed in terms of void fraction and pressure ratio as given by Pd 1/ vol = 1 − cr −1 , (6) PS where Cr is the ratio of void (vc ) to the total volume of the reactor (vr ) (analogous to the clearance ratio), and Pd and Ps are the delivery and supply pressures of the compressor, respectively. An uncertainty analysis made taking into account the sensitivities and accuracies of the various sensors shows that the
P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
3. Results and discussion Fig. 2 shows the variation of hydrogen storage pressure with time at various hot fluid temperatures superimposed on the PCT diagram. As mentioned earlier, desorption is initiated after the bed temperature has reached the specified hot fluid temperature. Initially, hydrogen pressure in the storage cylinder increases steeply due to higher desorption rates caused by high difference between hydride equilibrium (delivery) and storage pressures. Subsequently, the pressure increases gradually and attains a steady state (marked X) when the reactor completes the first compression cycle. It may be noted that at the end of compression, the hydride equilibrium pressure is equal to the storage pressure. The state points A1 , A2 and A3 represent the bed concentration at the end of the compression processes corresponding to the hot fluid temperatures of 75, 85 and 95 ◦ C, respectively. The second cycle of compression starts after hydrogen is absorbed by cooling the bed. The initial compression rate of the second cycle is lower than that of the first cycle due to low difference between hydrogen delivery and storage pressures. Desorption is continued and the hydrogen gas is let in to the storage cylinder, thereby completing the second cycle. The bed concentrations at the end of the second compression cycles are represented on the PCT curves at B1 , B2 and B3 , respectively, for the hot fluid temperatures of 75, 85 and 95 ◦ C. It is also observed from Fig. 2 that for the given supply pressure, hydrogen delivery and storage pressures increase with hot fluid temperature in both cycles 1 and 2. This is due to the increase in equilibrium pressure of the hydride bed, with bed temperature as per the van’t Hoff’s equation. This also leads to higher hydrogen throughput due to the lower final concentration. For instance, about 55% of hydrogen is compressed during the first cycle and only up to about 30% of hydrogen is compressed in the second cycle at hot fluid temperature of 95 ◦ C.
0
0.1
Hydride concentration (H/M) 0.2 0.3 0.4 0.5 0.6
0.7
0.8
50 45
B3
Pressure (bar)
40
MmNi4.6Al0.4 Tc = 20˚C, Ps=5 bar ma = 0.4 kg
35 30
85˚C
X
20
X A2
X
B2
75˚C
X B1 A1 Interpolation of final concentration
Interpolation of storage pressure
10
X
X
A3
25 15
95˚C
Storage pressure Equilibrium pressure
5 0 0
50
100
150
200 250 Time (s)
300
350
400
Fig. 2. PCT illustrations for various operating temperatures.
450
12
70
Hydrogen storage pressure
60
10 105˚C
50
MmNi4.6Al0.4 Tc= 20˚C, Ps =10 bar Vs= 1 litre, ma =0.4 kg U = 1000 W/m2K
95˚C 85˚C
40
75˚C
8 6
30
105˚C 95˚C
20
4
85˚C 75˚C
Compression rate (g/min.)
maximum errors in the estimated values of compressor work, isentropic efficiency and volumetric efficiency are ±3.54, 3.96 and ±4.08%, respectively.
Hydrogen storage pressure (bar)
466
2
10
Compression rate
0 0
100
200
300 400 Time (s)
500
600
0 700
Fig. 3. Effect of hot fluid temperature on hydrogen storage pressure and compression rate (Vs = 1 l).
Extending the number of cycles beyond 2 will not be beneficial because progressively less amount of hydrogen is compressed in subsequent cycles at the cost of higher energy inputs. Hence, even though higher pressures can be achieved with more number of compression cycles, the overall compression efficiency will be reduced. Four dotted lines are drawn by joining the storage pressures and concentrations at the end of each cycle. From these lines, one can predict the final condition of the bed and the storage pressure at the end of each compression cycle at any temperature between 50 and 125 ◦ C. The performance of the compressor is also studied at different storage volumes. Fig. 3 shows the variation of hydrogen storage pressures and compression rates in both the compression cycles using 1 l storage cylinder. It is observed that the hydrogen compression rate increases due to high driving potential at the initial stage, which later approaches to zero due the increase of storage pressure. As time progresses, bed desorbs hydrogen against the gradually increasing storage pressure. This leads to lowering of desorption rate. The increase in compression rate in the initial few seconds is more at higher hot fluid temperature. This is due to higher pressure difference between the storage and hydride equilibrium pressures, which is the driving potential for mass transfer. It is also observed from Fig. 3 that the storage pressure increases sharply at the initial stage of compression due to peak compression rate and reaches the maximum at the end of compression cycle. Due to high reaction rate at the initial stage, about 60% of hydrogen is compressed within 10 s and the balance takes about 200 s depending on the temperature of the hot fluid. For a given supply pressure, hydrogen storage pressure increases with hot fluid temperature due to the higher amount of hydrogen desorbed. A maximum storage pressure of 61 bar is obtained at a supply condition of 105 ◦ C hot fluid temperature and 10 bar supply pressure. It is seen from Fig. 4 that initially bed temperature decreases sharply and then increases gradually to the preset value of the heat transfer fluid temperature. This is due to the poor thermal conductivity of the hydride bed. The required amount of
P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
45
95
95˚C
85
85˚C
MmNi4.6Al0.4 Tc= 20˚C, Ps= 10 bar Vs= 1 litre U= 1000 W/m2K ma= 0.4 kg
75 75˚C 65
MmNi4.6Al0.4 U = 1000 W/m2K Tc = 20˚C Ps = 10 bar Th = 85˚C Vs = 3.8 litre
40 35 30
16
Hydrogen storage pressure
14 12 10
25 8 20 Compression rate
15 10
4
5
2
0
55 0
100
200
300 400 Time (s)
500
600
6
0
700
200
400
600
800 1000 Time (s)
1200
1400
Compression rate (g/min)
105˚C
105
Hydrogen storage pressure (bar)
Average bed temperature (˚C)
115
467
0 1600
Fig. 6. Cyclic performance of hydrogen compressor (Vs = 3.8 l).
Fig. 4. Variation of bed temperature with hot fluid temperature (Vs = 1 l).
6 6
Absorption Hydrogen compressed (g)
Hydrogen compressed (g)
Absorption 5 4 3
Cycle 1
2 Cycle 2
1
MmNi4.6Al0.4 Tc = 20˚C, Ps = 10 bar Th = 85˚C, Vs = 1 litre U = 1000 W/m2K ma = 0.4 kg
5 Cycle 1
4 3
Cycle 2 Cycle 3
2
Cycle 4 1
Cycle 5
MmNi4.6Al0.4 Tc = 20˚C, Ps = 10 bar Th = 85˚C, Vs = 3.8 litre U = 1000 W/m2K ma = 0.4 kg
0 0
0 0
100
200
300 Time (s)
400
500
600
100
200
300 Time (s)
400
500
600
Fig. 7. Stage wise compression of hydrogen (Vs = 3.8 l).
Fig. 5. Stages of hydrogen compression (Vs = 1 l).
heat energy is not getting transferred into the bed at the initial stage of rapid desorption and hence the hydride bed is taking the heat from itself, resulting in a sudden fall of bed temperature. The sudden fall in bed temperature is more pronounced at higher hot fluid temperature due to rapid desorption kinetics. Absorption processes are not shown in Figs. 3 and 4, as the absorption is independent of heat source temperature. Similarly, the sensible heating and cooling processes are also not included, as their times are nearly the same for all supply pressures and also do not vary much with temperature as compared to absorption/desorption processes. Fig. 5 shows that about 50% hydrogen is compressed during the first cycle of compression and about 20% of hydrogen is compressed during the second cycle at 85 ◦ C hot fluid temperature. Rest of the hydrogen demands higher operating temperature of above 85 ◦ C. It is also observed from Fig. 5 that first compression cycle takes about 250 s for yielding a hydrogen throughput of 0.61 g/min while the second cycle takes only about 150 s due to the lesser amount of hydrogen compressed yielding a lower hydrogen throughput of 0.45 g/min.
Figs. 6 and 7 together reveal that the compressor needs about five cycles for reaching a maximum storage pressure of 41.5 bar, when the gas is received in a 3.8 l cylinder. As discussed earlier, number of compression cycles reduce the compressor efficiency and hence the volume of the storage cylinder is selected based on the hydrogen storage capacity of the reactor (mass of alloy charged), which decides the maximum amount of hydrogen compressed or the maximum desorption capacity of the compressor. In most of hydrogen filling stations, mechanical compressors are used for compressing hydrogen gas to the storage pressure of above 150 bar using expensive electrical energy. Hence, metal hydride based hydrogen compressor operates under Case 1 may be the ideal replacement of mechanical compressor for such applications. Under Case 2, the performance of the compressor is studied by keeping the delivery pressure constant. Fig. 8 shows that for a given hot fluid temperature of 85 ◦ C and supply pressure of 10 bar, the rate of hydrogen compressed increases from 0.61 g/min (end of the first cycle in variable delivery pressure condition) to 0.72 g/min when the delivery pressure is
P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
35
Hydrogen delivery pressure 20 bar
4
30 30 bar
25 Hydrogen compressed
20 15
40 bar
MmNi4.6Al0.4 Tc = 20˚C, Ps = 10 bar Th = 85˚C, U = 1000 W/m2K
1 0 0
100
200
300 Time (s)
400
500
10 5
18
14
16
13 Compressor work Compressor efficiency
14
12
12
11
10
10
MmNi4.6Al0.4 Tc = 20˚C, Ps = 10 bar ma = 0.4 kg, Th = 85˚C
8
9
6
0 600
15
20
Compressor efficiency (%)
40
2
15 Delivery pressure = Constant
5
3
20
45
Hydrogen absorbed
Compressor work ( kJ)
6
Hydrogen delivery pressure (bar)
Hydrogen absorbed (g) / compressed (g)
468
25 30 35 Delivery pressure ( bar)
40
8 45
Fig. 8. Effect of delivery pressure on hydrogen throughput.
Fig. 9. Effects of delivery pressure on compressor work and efficiency.
maintained constant. This is because hydrogen does not get compressed against the pressurized gas as in the case of variable delivery pressure. It is observed that for a given delivery pressure of 20 bar about 80% of hydrogen is compressed in a single cycle while only 28% of hydrogen is utilized for compression when the delivery pressure is increased to 40 bar. This is due to sloped nature of plateau pressure on the PCT curve. Hence, one can observe that the alloy selection plays a crucial role in the performance of the compressor and hence, the operating pressure and storage volume of the cylinder are selected based on the hydrogen storage capacity and mass of the alloy. In general, hydrogen liquefaction systems demand continuous supply of hydrogen at constant supply pressure in the range of 50–150 bar at near atmospheric temperature. Usually hydrogen for liquefaction process is compressed by using a specially designed isothermal compressor with minimum frictional loss, which consumes enormous electrical energy. Further, leakage of hydrogen between the moving parts, stringent maintenance procedure and mixing of lubricant with hydrogen are the additional problems encountered in isothermal compressors. Hence, metal hydride based hydrogen compressor operates under Case 2 may be the ideal replacement of such isothermal compressor. Fig. 9 shows the effect delivery pressure on isentropic efficiency and work done by the compressor at 85 ◦ C hot fluid temperature and 10 bar supply pressure under Case 2. It is observed that the compressor isentropic efficiency increases with delivery pressure, reaches a maximum value and then decreases. At high delivery pressure, compressor efficiency decreases due to substantial decrement in the amount of hydrogen compressed than due to the increase in pressure ratio, leading to lower efficiency. A maximum efficiency of 14.2% is observed when the compressor operates at constant delivery pressure of 30 bar at 85 ◦ C hot fluid temperature. Table 1, which shows the effects of hot fluid temperatures and supply pressures on compressor isentropic and volumetric efficiencies, reveals that the isentropic efficiency increases with hot fluid temperature, while volumetric efficiency decreases with hot fluid temperature for variable pressure condition. From the
Table 1 Effects of operating parameters on compressor performances (variable delivery pressure mode) Hot fluid temperature
Supply pressure 5 bar
10 bar
15 bar
75 ◦ C
c vol
4.2 88
3.3 91
2.2 95
85 ◦ C
c vol
5.6 83
4.5 89
2.8 93
95 ◦ C
c vol
7.3 78
5 87
3.5 91
Note: c , Isentropic efficiency; vol , volumetric efficiency.
definition of isentropic efficiency, it is observed that for a given absorption conditions, the compressor work depends on the amount of hydrogen desorbed and pressure ratio, both of which increase with hot fluid temperature leading to higher efficiency. However, the increase in hot fluid temperature also increases the sensible heating of the reactor, which leads to an increase in total heat supply. Therefore, increase in hot fluid temperature improves the isentropic efficiency only by a marginally. It is also observed from Table 1 that for a given hot fluid temperature, the isentropic efficiency increases with decrease in supply pressure. For lower supply pressures, the pressure ratio is more, which leads to higher isentropic efficiency. Hence, one can conclude that for achieving higher isentropic efficiency, supply pressure should be low but above the threshold limit required for complete absorption. For a given hot fluid temperature, the amount of hydrogen compressed is low when the compressor operates under variable delivery pressure mode. This is due to compression taking place against the pressurized gas. Maximum isentropic efficiency of 7.3% is obtained at a pressure ratio of 8.8 (43.8/5.0 bar) at a hot fluid temperature of 95 ◦ C. In case of volumetric efficiency, high hot fluid temperature reduces the volumetric efficiency of the compressor. From the definition of volumetric efficiency given by Eq. (6), it is clear that the efficiency is maximum when there is no compression.
P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
At higher hot fluid temperatures, the delivery pressure is higher, resulting in lower volumetric efficiency. But the main motive of the hydrogen compressor is to achieve higher pressure ratios at possible lower thermal inputs. Since the volumetric efficiency becomes lower at higher pressure ratios, volumetric efficiency may not be a proper parameter for expressing the performance of the hydrogen compressors. 4. Conclusions Performance tests on a MmNi4.6 Al0.4 based hydrogen compressor are carried out by varying the operating parameters such as supply pressure and heat source temperature under constant and variable delivery pressures with different storage volumes. Hydrogen storage pressure increases with supply pressure and heat source temperature. The increase in supply pressure has a negative effect on the efficiency. The increase of the hot fluid temperature above 125 ◦ C also is not beneficial. For a given hot fluid temperature at variable delivery pressure mode, the effect of supply pressure on hydrogen delivery pressure and amount of hydrogen compressed is limited to 10 bar when the hydrogen is stored in 1 l storage cylinder. For a given operating conditions, compressor performance is found to increase when the delivery pressure is constant. A maximum isentropic efficiency of 7.3% is obtained at a pressure ratio of 8.8 (43.8/5 bar) at 95 ◦ C hot fluid temperature for variable delivery condition, while the corresponding figures are 14.2%, 3 (30/10 bar) and 85 ◦ C, respectively, for constant delivery pressure condition. For both
469
the cases volumetric efficiency of the compressor varied from 70 to 93%. Acknowledgements The authors express their sincere thanks to Prof. O.N. Srivastava, Banaras Hindu University, for supplying the alloy. The authors also thank the Ministry of Non-Conventional Energy Sources, Government of India for financial support. References [1] Shmal’ko YuF, Ivanovsky AI, Lototsky MV, Karnatsevvich LV, Milenko YuYa. Cryo-hydride high pressure hydrogen compressor. Int J Hydrogen Energy 1999;24:649–50. [2] Ivanovsky AI, Kolosov VI, Lototsky MV, Solovey VV, Shmal’ko YuF, Kennedy LA. Metal hydride thermosorption compressors with improved dynamic characteristics. Int J Hydrogen Energy 1996;21:1053–5. [3] Shmal’ko YuF, Ivanovsky AI, Lototsky MV, Kolosov VI, Volosnikov DV. Sample pilot plant of industrial metal-hydride compressor. Int J Hydrogen Energy 1999;24:645–8. [4] Muthukumar P, Prakash Maiya M, Srinivasa Murthy S. Parametric studies on a metal hydride based single stage hydrogen compressor. Int J Hydrogen Energy 2002;27:1083–92. [5] Muthukumar P, Prakash Maiya M, Srinivasa Murthy S. Experiments on a metal hydride based hydrogen compressor. Int J Hydrogen Energy 2005;30(8):879–92. [6] Muthukumar P, Abraham K, Rajendra Prasad UA, Prakash Maiya M, Srinivasa Murthy S. Screening of metal hydrides for engineering applications. In: 16th international conference on efficiency, costs, optimization, simulation and environmental impact of energy systems (ECOS). Copenhagen, Denmark; June 30–July 2, 2003.
Performance tests on a thermally operated hydrogen compressor P. Muthukumar a,∗ , M. Prakash Maiya b , S. Srinivasa Murthy b a Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India b Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India
Received 19 June 2007; accepted 9 July 2007 Available online 4 September 2007
Abstract A metal hydride based hydrogen compressor using MmNi4.6 Al0.4 is tested at both constant and variable delivery pressures. The effects of operating parameters such as supply pressure and heat source temperature on the compressor performance are investigated. The compressor yields a maximum isentropic efficiency of 7.3% at 95 ◦ C heat source temperature at variable delivery pressure operation mode, and 14.2% at 85 ◦ C heat source temperature at constant delivery pressure operation mode. At both the operation modes, the volumetric efficiency of the compressor is found to decrease at high heat source temperatures. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Metal hydride; Hydrogen compressor; Volumetric efficiency; Isentropic efficiency
1. Introduction Mechanical compression of hydrogen gas poses many technical problems related to compressor construction, balancing, lubrication, maintenance, etc. In addition, such compression consumes electrical energy in contrast to thermal compression, which can be accomplished by waste heat. Moreover, it is desirable to adopt engineering solutions, which eliminate moving parts working under a hydrogen medium. The metal hydride based hydrogen compressor is one of the potential applications of hydride technology which can be tailored to cover a wide range of operating pressures and pressure ratios by selecting suitable alloys [1–3]. These compressors also permit noiseless and vibration free operation, in addition to delivering pure hydrogen. Recently the authors [4,5] investigated the influence of operational and reactor bed related parameters on the compressor performance by analyzing the heat and mass transfer aspects of the hydride bed. In an experimental study using MmNi4.6 Al0.4 , the influences of supply pressure and hot fluid temperature on the compressor performance were investigated for variable
∗ Corresponding author. Fax: +91 361 258 2699.
E-mail address: [email protected] (P. Muthukumar).
delivery pressure using a constant volume storage cylinder of 1 l capacity. This paper presents the performance studies of a hydrogen compressor using MmNi4.6 Al0.4 at both variable and constant delivery pressure modes, and also studies the influence of different storage volumes. 2. Experimental studies 2.1. Operating principle The operating principle of the compressor is explained in an earlier publication by the authors [5]. The operation of the hydride compressor consists of four processes namely: Process AB: Absorption of hydrogen at low temperature (Tc ) and low pressure (Ps ). Process BC: Sensible heating accomplished by compression from cold fluid temperature (Tc ) to hot fluid temperature (Th ). Process CD: Desorption of compressed hydrogen at hot fluid temperature (Th ) and high pressure (Pd ). Process DA: Sensible cooling from Th to Tc . Process CD can be of two cases: In Case 1, compressor starts to discharge hydrogen when the hydride bed reaches the specified hot fluid (heat source) temperature. Storage pressure increases with time and becomes equal to hydride equilibrium pressure at the end of the desorption process. In Case 2,
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.07.019
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P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
Nomenclature Cf Cr M ma mf P Q Ru T Vs W
specific heat capacity of heat transfer fluid, 4.182 kJ kg−1 K −1 clearance ratio molecular weight of hydrogen, 2.01 kg kmol−1 mass, kg mass flow rate of heat transfer fluid, kg s−1 pressure, bar heat input, J universal gas constant, 8314 J gmol−1 K −1 temperature, K time, s storage volume, l compressor work, J
concentration, H /M ratio index for isentropic compression, 1.405 compressor efficiency
x Υ c Subscript ab c d de f1 f2 h s 1, 2, . . . , 4
end of the absorption cold fluid desorption end of the desorption heat transfer fluid inlet heat transfer fluid outlet hot fluid supply sensor locations (Fig. 2)
compressor starts to discharge hydrogen when the hydride equilibrium pressure exceeds the preset value of the delivery pressure. In this case, the storage pressure is constant at the preset value of the delivery pressure.
equation (Eq. (1)), the pipe volume v1 (V4 –MF–V5 –V6 –V3 –V2 –V1 ) is calculated.
2.2. Description of the test rig
where Tat and v0 are the atmospheric temperature and correction volume, respectively. Then valve V1 is opened and argon is allowed in the reactor. After reaching pressure equilibrium, the fall in pressure (P ) is calculated. From P , the void volume of reactor and the pipe volume till valve V1 (vc ) is obtained:
The experimental setup shown in Fig. 1 is configured to achieve the above processes. The reactor is made of seamless stainless steel (SS-316) tube of wall thickness of 3 mm and the filter is of sintered SS-316 of 2-m pore size [5]. Eight copper fins of 1 mm thickness are used to enhance the effective thermal conductivity of the hydride bed and also to keep the hydride powder in place during the repeated absorption and desorption processes. One end of the reactor is closed with the brazed assembly of four grounded metal-sheathed “K” type thermocouples (sensitivity 0.1 ◦ C) and the other end of the reactor is flanged. Other instrumentation and sensors are shown in Fig. 1. Two constant temperature baths (±0.1 ◦ C) are used for supplying hot and cold fluids at flow rates ranging from 1 to 3.5 l/min, which correspond to the overall heat transfer coefficient ranging from 500 to 1250 Wm−2 K −1 . With the aid of the basic screening procedure deduced by the authors [6] and considering practical aspects such as absorption at near ambient temperatures, low supply pressure and ready availability at low cost, MmNi4.6 Al0.4 is selected for the present experimental study. 2.3. Experimental procedure After charging 400 g of MmNi4.6 Al0.4 into the reactor, the whole system is evacuated down to 10−3 mbar. Then, the reactor is isolated from the setup by closing the valve V1 . Argon gas at Pi bar is supplied to the system and the mass flow meter reading is observed. Then valve V4 is closed. After reaching the pressure equilibrium, the amount of argon transferred (mAr ) is obtained from the mass flow meter. By using the real gas
v1 =
vc =
mAr Ru Tat + v0 , Pi
mAr Ru T − v1 . P
(1)
(2)
Activation of MmNi4.6 Al0.4 is carried out at 27 ◦ C and 80 bar. Before applying hydrogen gas to the reactor, the whole system is evacuated down to about 10−3 mbar. Then it is flushed three times with hydrogen gas. It is evacuated again and then hydrogen gas at 80 bar is allowed to the reactor via the mass flow meter. During the first cycle of operation, a hydrogen uptake of 0.57 wt% is measured within 30 min. Then the reactor is heated to 60 ◦ C and the hydrogen is completely desorbed to atmosphere. During the next 10 absorption/desorption cycles the storage capacity gradually increased to a maximum of 1.43 wt%. At the start of the cycle, cold fluid is circulated such that the equilibrium pressure of the hydride bed falls below that of the supply pressure. Later, valves V1 –V4 are opened and hydrogen is allowed to flow into the bed. Absorption of hydrogen is continued till the hydride equilibrium pressure becomes equal to the supply pressure, which completes process AB. Heat of absorption is rejected to the cold fluid. Then valves V1 –V4 are closed and the bed is heated by circulating hot fluid till the bed temperature is in equilibrium with the hot fluid temperature, thereby completing process BC. Valves V1 , V2 and V6 are opened to allow hydrogen to flow into the storage cylinder. Desorption of hydrogen is continued till the storage cylinder pressure is in equilibrium with the hydride pressure which
P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
465
Data logger
D.C. Constant voltage source
V1 T1-4
P1
V2
Vs1 S
Reactor
M.F
V3
V5
Tf2
Tf1
V4 V6
T5
P2
SS- sinter 2M filter
T6
Ps High Temperature Thermostatic Bath
Low Temperature Thermostatic Bath
Packless bellow seal valves
H2 Storage
H2 Supply
S
Ball valves
One way valves
T1 -T6 Metal sheathed "K" type thermocouples
P1-P2 ressure transducers
Tf1-Tf2 Water inlet and out let temperatures
M.F Mass flow meter
Heat Transfer fluid line
Electrical wire
Hydrogen line
Fig. 1. Setup for testing metal hydride based hydrogen compressor.
completes process CD. Then the valves V1 , V2 and V6 are closed, and the bed is cooled down to initial absorption temperature by circulating cold fluid, thereby completing process DA. Thus the system completes the first compression cycle. The dynamic history of bed temperatures (T1 –T4 ), pressures (P1 and P2 ) and hydrogen flow rate are recorded at 100 ms intervals by the data acquisition system. The heat transfer fluid temperatures at inlet and outlet of the reactor and the hydrogen gas temperatures at the exits of supply cylinder and storage cylinder are also recorded simultaneously. It should be noted that at the end of the first cycle, the bed is not completely dehydrided. The valves V1 –V4 are opened and additional gas is absorbed by circulating the cold fluid. Absorption is continued till the bed equilibrium pressure becomes equal to the supply pressure. Then desorption is repeated followed by heating of the bed after opening the appropriate valves till the bed equilibrium pressure equals the storage pressure (P2 ). Then the valves V1 , V2 and V6 are closed and the bed is cooled down to cold fluid temperature. Thus the system completes the second compression cycle. As will be discussed later, for 1 l storage volume, further cycles are not beneficial for achieving higher pressures and in fact there can be a fall in efficiency. Hence, the compression process is stopped after the second cycle, whereas for 3.8 l storage cylinder five cycles are needed. For Case 2, i.e. constant delivery pressure condition, the valve V6 is replaced by a pressure relief valve. The delivery pressure is varied by varying the cracking pressure of the pressure relief valve.
2.4. Performance parameters The three important performance parameters for the compressor are isentropic and volumetric efficiencies, and hydrogen throughput. The isentropic efficiency is given by c =
Isentropic compression work W = , Heat input to compressor Q
(3)
where
(−1)/ Ru Pd W= mH Tab −1 . ( − 1) 2 MH2 Ps
(4)
Heat supplied during the compression process is estimated as follows: de Q = m f Cf (Tf1 − Tf2 )j. (5) ab
Volumetric efficiency of a hydrogen compressor is expressed in terms of void fraction and pressure ratio as given by Pd 1/ vol = 1 − cr −1 , (6) PS where Cr is the ratio of void (vc ) to the total volume of the reactor (vr ) (analogous to the clearance ratio), and Pd and Ps are the delivery and supply pressures of the compressor, respectively. An uncertainty analysis made taking into account the sensitivities and accuracies of the various sensors shows that the
P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
3. Results and discussion Fig. 2 shows the variation of hydrogen storage pressure with time at various hot fluid temperatures superimposed on the PCT diagram. As mentioned earlier, desorption is initiated after the bed temperature has reached the specified hot fluid temperature. Initially, hydrogen pressure in the storage cylinder increases steeply due to higher desorption rates caused by high difference between hydride equilibrium (delivery) and storage pressures. Subsequently, the pressure increases gradually and attains a steady state (marked X) when the reactor completes the first compression cycle. It may be noted that at the end of compression, the hydride equilibrium pressure is equal to the storage pressure. The state points A1 , A2 and A3 represent the bed concentration at the end of the compression processes corresponding to the hot fluid temperatures of 75, 85 and 95 ◦ C, respectively. The second cycle of compression starts after hydrogen is absorbed by cooling the bed. The initial compression rate of the second cycle is lower than that of the first cycle due to low difference between hydrogen delivery and storage pressures. Desorption is continued and the hydrogen gas is let in to the storage cylinder, thereby completing the second cycle. The bed concentrations at the end of the second compression cycles are represented on the PCT curves at B1 , B2 and B3 , respectively, for the hot fluid temperatures of 75, 85 and 95 ◦ C. It is also observed from Fig. 2 that for the given supply pressure, hydrogen delivery and storage pressures increase with hot fluid temperature in both cycles 1 and 2. This is due to the increase in equilibrium pressure of the hydride bed, with bed temperature as per the van’t Hoff’s equation. This also leads to higher hydrogen throughput due to the lower final concentration. For instance, about 55% of hydrogen is compressed during the first cycle and only up to about 30% of hydrogen is compressed in the second cycle at hot fluid temperature of 95 ◦ C.
0
0.1
Hydride concentration (H/M) 0.2 0.3 0.4 0.5 0.6
0.7
0.8
50 45
B3
Pressure (bar)
40
MmNi4.6Al0.4 Tc = 20˚C, Ps=5 bar ma = 0.4 kg
35 30
85˚C
X
20
X A2
X
B2
75˚C
X B1 A1 Interpolation of final concentration
Interpolation of storage pressure
10
X
X
A3
25 15
95˚C
Storage pressure Equilibrium pressure
5 0 0
50
100
150
200 250 Time (s)
300
350
400
Fig. 2. PCT illustrations for various operating temperatures.
450
12
70
Hydrogen storage pressure
60
10 105˚C
50
MmNi4.6Al0.4 Tc= 20˚C, Ps =10 bar Vs= 1 litre, ma =0.4 kg U = 1000 W/m2K
95˚C 85˚C
40
75˚C
8 6
30
105˚C 95˚C
20
4
85˚C 75˚C
Compression rate (g/min.)
maximum errors in the estimated values of compressor work, isentropic efficiency and volumetric efficiency are ±3.54, 3.96 and ±4.08%, respectively.
Hydrogen storage pressure (bar)
466
2
10
Compression rate
0 0
100
200
300 400 Time (s)
500
600
0 700
Fig. 3. Effect of hot fluid temperature on hydrogen storage pressure and compression rate (Vs = 1 l).
Extending the number of cycles beyond 2 will not be beneficial because progressively less amount of hydrogen is compressed in subsequent cycles at the cost of higher energy inputs. Hence, even though higher pressures can be achieved with more number of compression cycles, the overall compression efficiency will be reduced. Four dotted lines are drawn by joining the storage pressures and concentrations at the end of each cycle. From these lines, one can predict the final condition of the bed and the storage pressure at the end of each compression cycle at any temperature between 50 and 125 ◦ C. The performance of the compressor is also studied at different storage volumes. Fig. 3 shows the variation of hydrogen storage pressures and compression rates in both the compression cycles using 1 l storage cylinder. It is observed that the hydrogen compression rate increases due to high driving potential at the initial stage, which later approaches to zero due the increase of storage pressure. As time progresses, bed desorbs hydrogen against the gradually increasing storage pressure. This leads to lowering of desorption rate. The increase in compression rate in the initial few seconds is more at higher hot fluid temperature. This is due to higher pressure difference between the storage and hydride equilibrium pressures, which is the driving potential for mass transfer. It is also observed from Fig. 3 that the storage pressure increases sharply at the initial stage of compression due to peak compression rate and reaches the maximum at the end of compression cycle. Due to high reaction rate at the initial stage, about 60% of hydrogen is compressed within 10 s and the balance takes about 200 s depending on the temperature of the hot fluid. For a given supply pressure, hydrogen storage pressure increases with hot fluid temperature due to the higher amount of hydrogen desorbed. A maximum storage pressure of 61 bar is obtained at a supply condition of 105 ◦ C hot fluid temperature and 10 bar supply pressure. It is seen from Fig. 4 that initially bed temperature decreases sharply and then increases gradually to the preset value of the heat transfer fluid temperature. This is due to the poor thermal conductivity of the hydride bed. The required amount of
P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
45
95
95˚C
85
85˚C
MmNi4.6Al0.4 Tc= 20˚C, Ps= 10 bar Vs= 1 litre U= 1000 W/m2K ma= 0.4 kg
75 75˚C 65
MmNi4.6Al0.4 U = 1000 W/m2K Tc = 20˚C Ps = 10 bar Th = 85˚C Vs = 3.8 litre
40 35 30
16
Hydrogen storage pressure
14 12 10
25 8 20 Compression rate
15 10
4
5
2
0
55 0
100
200
300 400 Time (s)
500
600
6
0
700
200
400
600
800 1000 Time (s)
1200
1400
Compression rate (g/min)
105˚C
105
Hydrogen storage pressure (bar)
Average bed temperature (˚C)
115
467
0 1600
Fig. 6. Cyclic performance of hydrogen compressor (Vs = 3.8 l).
Fig. 4. Variation of bed temperature with hot fluid temperature (Vs = 1 l).
6 6
Absorption Hydrogen compressed (g)
Hydrogen compressed (g)
Absorption 5 4 3
Cycle 1
2 Cycle 2
1
MmNi4.6Al0.4 Tc = 20˚C, Ps = 10 bar Th = 85˚C, Vs = 1 litre U = 1000 W/m2K ma = 0.4 kg
5 Cycle 1
4 3
Cycle 2 Cycle 3
2
Cycle 4 1
Cycle 5
MmNi4.6Al0.4 Tc = 20˚C, Ps = 10 bar Th = 85˚C, Vs = 3.8 litre U = 1000 W/m2K ma = 0.4 kg
0 0
0 0
100
200
300 Time (s)
400
500
600
100
200
300 Time (s)
400
500
600
Fig. 7. Stage wise compression of hydrogen (Vs = 3.8 l).
Fig. 5. Stages of hydrogen compression (Vs = 1 l).
heat energy is not getting transferred into the bed at the initial stage of rapid desorption and hence the hydride bed is taking the heat from itself, resulting in a sudden fall of bed temperature. The sudden fall in bed temperature is more pronounced at higher hot fluid temperature due to rapid desorption kinetics. Absorption processes are not shown in Figs. 3 and 4, as the absorption is independent of heat source temperature. Similarly, the sensible heating and cooling processes are also not included, as their times are nearly the same for all supply pressures and also do not vary much with temperature as compared to absorption/desorption processes. Fig. 5 shows that about 50% hydrogen is compressed during the first cycle of compression and about 20% of hydrogen is compressed during the second cycle at 85 ◦ C hot fluid temperature. Rest of the hydrogen demands higher operating temperature of above 85 ◦ C. It is also observed from Fig. 5 that first compression cycle takes about 250 s for yielding a hydrogen throughput of 0.61 g/min while the second cycle takes only about 150 s due to the lesser amount of hydrogen compressed yielding a lower hydrogen throughput of 0.45 g/min.
Figs. 6 and 7 together reveal that the compressor needs about five cycles for reaching a maximum storage pressure of 41.5 bar, when the gas is received in a 3.8 l cylinder. As discussed earlier, number of compression cycles reduce the compressor efficiency and hence the volume of the storage cylinder is selected based on the hydrogen storage capacity of the reactor (mass of alloy charged), which decides the maximum amount of hydrogen compressed or the maximum desorption capacity of the compressor. In most of hydrogen filling stations, mechanical compressors are used for compressing hydrogen gas to the storage pressure of above 150 bar using expensive electrical energy. Hence, metal hydride based hydrogen compressor operates under Case 1 may be the ideal replacement of mechanical compressor for such applications. Under Case 2, the performance of the compressor is studied by keeping the delivery pressure constant. Fig. 8 shows that for a given hot fluid temperature of 85 ◦ C and supply pressure of 10 bar, the rate of hydrogen compressed increases from 0.61 g/min (end of the first cycle in variable delivery pressure condition) to 0.72 g/min when the delivery pressure is
P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
35
Hydrogen delivery pressure 20 bar
4
30 30 bar
25 Hydrogen compressed
20 15
40 bar
MmNi4.6Al0.4 Tc = 20˚C, Ps = 10 bar Th = 85˚C, U = 1000 W/m2K
1 0 0
100
200
300 Time (s)
400
500
10 5
18
14
16
13 Compressor work Compressor efficiency
14
12
12
11
10
10
MmNi4.6Al0.4 Tc = 20˚C, Ps = 10 bar ma = 0.4 kg, Th = 85˚C
8
9
6
0 600
15
20
Compressor efficiency (%)
40
2
15 Delivery pressure = Constant
5
3
20
45
Hydrogen absorbed
Compressor work ( kJ)
6
Hydrogen delivery pressure (bar)
Hydrogen absorbed (g) / compressed (g)
468
25 30 35 Delivery pressure ( bar)
40
8 45
Fig. 8. Effect of delivery pressure on hydrogen throughput.
Fig. 9. Effects of delivery pressure on compressor work and efficiency.
maintained constant. This is because hydrogen does not get compressed against the pressurized gas as in the case of variable delivery pressure. It is observed that for a given delivery pressure of 20 bar about 80% of hydrogen is compressed in a single cycle while only 28% of hydrogen is utilized for compression when the delivery pressure is increased to 40 bar. This is due to sloped nature of plateau pressure on the PCT curve. Hence, one can observe that the alloy selection plays a crucial role in the performance of the compressor and hence, the operating pressure and storage volume of the cylinder are selected based on the hydrogen storage capacity and mass of the alloy. In general, hydrogen liquefaction systems demand continuous supply of hydrogen at constant supply pressure in the range of 50–150 bar at near atmospheric temperature. Usually hydrogen for liquefaction process is compressed by using a specially designed isothermal compressor with minimum frictional loss, which consumes enormous electrical energy. Further, leakage of hydrogen between the moving parts, stringent maintenance procedure and mixing of lubricant with hydrogen are the additional problems encountered in isothermal compressors. Hence, metal hydride based hydrogen compressor operates under Case 2 may be the ideal replacement of such isothermal compressor. Fig. 9 shows the effect delivery pressure on isentropic efficiency and work done by the compressor at 85 ◦ C hot fluid temperature and 10 bar supply pressure under Case 2. It is observed that the compressor isentropic efficiency increases with delivery pressure, reaches a maximum value and then decreases. At high delivery pressure, compressor efficiency decreases due to substantial decrement in the amount of hydrogen compressed than due to the increase in pressure ratio, leading to lower efficiency. A maximum efficiency of 14.2% is observed when the compressor operates at constant delivery pressure of 30 bar at 85 ◦ C hot fluid temperature. Table 1, which shows the effects of hot fluid temperatures and supply pressures on compressor isentropic and volumetric efficiencies, reveals that the isentropic efficiency increases with hot fluid temperature, while volumetric efficiency decreases with hot fluid temperature for variable pressure condition. From the
Table 1 Effects of operating parameters on compressor performances (variable delivery pressure mode) Hot fluid temperature
Supply pressure 5 bar
10 bar
15 bar
75 ◦ C
c vol
4.2 88
3.3 91
2.2 95
85 ◦ C
c vol
5.6 83
4.5 89
2.8 93
95 ◦ C
c vol
7.3 78
5 87
3.5 91
Note: c , Isentropic efficiency; vol , volumetric efficiency.
definition of isentropic efficiency, it is observed that for a given absorption conditions, the compressor work depends on the amount of hydrogen desorbed and pressure ratio, both of which increase with hot fluid temperature leading to higher efficiency. However, the increase in hot fluid temperature also increases the sensible heating of the reactor, which leads to an increase in total heat supply. Therefore, increase in hot fluid temperature improves the isentropic efficiency only by a marginally. It is also observed from Table 1 that for a given hot fluid temperature, the isentropic efficiency increases with decrease in supply pressure. For lower supply pressures, the pressure ratio is more, which leads to higher isentropic efficiency. Hence, one can conclude that for achieving higher isentropic efficiency, supply pressure should be low but above the threshold limit required for complete absorption. For a given hot fluid temperature, the amount of hydrogen compressed is low when the compressor operates under variable delivery pressure mode. This is due to compression taking place against the pressurized gas. Maximum isentropic efficiency of 7.3% is obtained at a pressure ratio of 8.8 (43.8/5.0 bar) at a hot fluid temperature of 95 ◦ C. In case of volumetric efficiency, high hot fluid temperature reduces the volumetric efficiency of the compressor. From the definition of volumetric efficiency given by Eq. (6), it is clear that the efficiency is maximum when there is no compression.
P. Muthukumar et al. / International Journal of Hydrogen Energy 33 (2008) 463 – 469
At higher hot fluid temperatures, the delivery pressure is higher, resulting in lower volumetric efficiency. But the main motive of the hydrogen compressor is to achieve higher pressure ratios at possible lower thermal inputs. Since the volumetric efficiency becomes lower at higher pressure ratios, volumetric efficiency may not be a proper parameter for expressing the performance of the hydrogen compressors. 4. Conclusions Performance tests on a MmNi4.6 Al0.4 based hydrogen compressor are carried out by varying the operating parameters such as supply pressure and heat source temperature under constant and variable delivery pressures with different storage volumes. Hydrogen storage pressure increases with supply pressure and heat source temperature. The increase in supply pressure has a negative effect on the efficiency. The increase of the hot fluid temperature above 125 ◦ C also is not beneficial. For a given hot fluid temperature at variable delivery pressure mode, the effect of supply pressure on hydrogen delivery pressure and amount of hydrogen compressed is limited to 10 bar when the hydrogen is stored in 1 l storage cylinder. For a given operating conditions, compressor performance is found to increase when the delivery pressure is constant. A maximum isentropic efficiency of 7.3% is obtained at a pressure ratio of 8.8 (43.8/5 bar) at 95 ◦ C hot fluid temperature for variable delivery condition, while the corresponding figures are 14.2%, 3 (30/10 bar) and 85 ◦ C, respectively, for constant delivery pressure condition. For both
469
the cases volumetric efficiency of the compressor varied from 70 to 93%. Acknowledgements The authors express their sincere thanks to Prof. O.N. Srivastava, Banaras Hindu University, for supplying the alloy. The authors also thank the Ministry of Non-Conventional Energy Sources, Government of India for financial support. References [1] Shmal’ko YuF, Ivanovsky AI, Lototsky MV, Karnatsevvich LV, Milenko YuYa. Cryo-hydride high pressure hydrogen compressor. Int J Hydrogen Energy 1999;24:649–50. [2] Ivanovsky AI, Kolosov VI, Lototsky MV, Solovey VV, Shmal’ko YuF, Kennedy LA. Metal hydride thermosorption compressors with improved dynamic characteristics. Int J Hydrogen Energy 1996;21:1053–5. [3] Shmal’ko YuF, Ivanovsky AI, Lototsky MV, Kolosov VI, Volosnikov DV. Sample pilot plant of industrial metal-hydride compressor. Int J Hydrogen Energy 1999;24:645–8. [4] Muthukumar P, Prakash Maiya M, Srinivasa Murthy S. Parametric studies on a metal hydride based single stage hydrogen compressor. Int J Hydrogen Energy 2002;27:1083–92. [5] Muthukumar P, Prakash Maiya M, Srinivasa Murthy S. Experiments on a metal hydride based hydrogen compressor. Int J Hydrogen Energy 2005;30(8):879–92. [6] Muthukumar P, Abraham K, Rajendra Prasad UA, Prakash Maiya M, Srinivasa Murthy S. Screening of metal hydrides for engineering applications. In: 16th international conference on efficiency, costs, optimization, simulation and environmental impact of energy systems (ECOS). Copenhagen, Denmark; June 30–July 2, 2003.