Dynamic behavior of heat and hydrogen transfer in a metal hydride cooling system

Copyright

Pergamon

In.‘. J. Hydrogen Energy. Vol. 21, No. t:~ 1996 International Association

PII: SO360-3199(96)00017-l

DYNAMIC

BEHAVIOR

OF HEAT HYDRIDE

& Environmental

in Great

0360&3199/96$15.00+0.00

AND HYDROGEN TRANSFER COOLING SYSTEM

B. H. KANG,* Air Conditioning

Printed

9, pp. 769-774, 1996 for Hydrogen Energy Elsevier Science Ltd Britain. All rights reserved

IN A METAL

C. W. PARK and C. S. LEE

Control Lab., KIST, P.O. Box I3 I Cheongryang,

Seoul 130-650. Korea

(Receiaedfor publication 5 February 1996) Abstract-An experimental study has been carried out to investigate transient transport processes of hydrogen and heat between two coupled reactors in a metal hydride cooling system. This problem is of particular interest in the design of metal hydride thermal energy conversion systems, such as refrigerators, heat pumps, and thermal storage systems. A pair of hydride reactors are designed to extend the contact surface between hydride materials and flowing hydrogen. for fast kinetics. Dynamic correlations of pressureetemperature and temperature-concentration are investigated in a typical operational condition. The amount of hydrogen gas transferrable between the paired metal hydrides is measured and the optimum value of the charged hydrogen amount is found for the maximum hydrogen transfer. Copyright 6 1996 International Association for Hydrogen Energy

NOMENCLATURE CP H/M m n e T

t t, t2

is applicable to energy conversion devices, such as refrigerators, air conditioners, and heat pumps. The key element of such systems is a reaction bed, in which the absorption and desorption processes take place. The heat and mass transfer inside this reaction bed affects the system performance. The main feature of the metal hydride energy conversion systems is the hydrogen transfer and associated heat transfer between two coupled hydride reactors. The operational characteristics of the coupled hydride reactors are a dynamic process of the heat and mass transfer. Tucher et al. [l] measured hydrogen flow rate at various half-cycle times with water cooled at 286 K and heated at 353 K. The hydrogen transfer between two reactors is investigated experimentally for two pairs of hydrides by Bjurstorm Edal. [2]. The amount of hydrogen gas transferrable between the paired metal hydrides is considered and the optimum operational conditions for the maximum hydrogen transfer rates were obtained [3, 41. Some numerical investigations have also been carried out to predict the dynamic behavior of heat and mass transfer in coupled metal hydride reactors [5-71. Most work focuses on the hydrogen transfer rate and pressure variations. The dynamic behavior of mass transfer is strongly related to the reactor characteristics, such as the heat transfer mechanism and the geometrical configuration of the reactor. The resulting heat transfer along with the hydrogen transfer between two connected hydride reactors is the predominant factor in designing thermal energy conversion systems. However, very little work has been performed on the heat transfer associated

Specific heat (kJ/kg “C) Atomic ratio of hydrogen and hydride Mass flow rate of flow carrier (kg/s) Hydrogen mols (Hz mol) Heat transfer rate (kW) Temperature (K) Time (min) Regeneration period (min) Cooling period (min)

material

Subscripts

h m, mh 1

M

Driving Cooling Cooling

heat source water for LaNi, ,A], 3 reactor water for MmNi, ,SFe,,, reactor

Chilled water Hydride material I. INTRODUCTION

A reactor containing hydride material has an exothermic reaction process when hydrogen is absorbed into the metal hydride reactor whereas it has an endothermic reaction when hydrogen is desorbed from the reactor. A metal hydride system, consisting of two paired reactors,

*Author to whom correspondence should be addressed: Tel: + 82-2-958-5673; Fax: + 82-2-958-5689; E-mail: [email protected]. 769

B. H. KANG rfd.

770

Flow of Heat Carrier

@O

(4

Fig. 1. Geometrical configuration of reactors. (a) Previous studies. (b) Present study with hydrogen transfer. Bjurstrom and Suda [8] numerically and experimentally studied the dynamics of hydrogen transfer along with heat transfer for heat pump applications. They found that the heat transfer rate determines the hydrogen flow rate between coupled reactors. In the above literature review, the reactor is employed as a double-tubular type, as seen in Fig. la. The hydrogen is absorbed onto or desorbed from metal hydride through a cylindrical filter tube. The outermost jacket, through which the heat carrier is circulated to heat up or cool down the hydrides during the desorption or absorption process, is surrounded by metal hydride. In the present study, an effort is made to extend the contact surface between metal hydrides and flowing hydrogen for fast kinetics, as seen in Fig. lb. A heat transfer medium is circulated inside the cylindrical tube. Hydrogen is supplied to or withdrawn from the hydrides outside the tube. With this geometric configuration, it is easy to use the finned tube in order to enhance the heat transfer rate between the hydrides and the heat carrier. The transient behavior of hydrogen rate and associated heat transfer rate are investigated experimentally. LaNi,,Al,, is used for high temperature material and MmN& d% s for low temperature material, respectively. The detailed specifications of the present reactors are shown in Table 1. Dynamic correlations of pressuretemperature and temperature-concentration are investigated in a typical operational condition. The amount of total hydrogen transfer between the paired metal hydrides is measured and the optimum value of the charged hydrogen amount is obtained for the maximum hydrogen transfer. 2. EXPERIMENTAL

for the driving heat source at T,,. A water cooling tower is usually used for the medium temperature heat sink at T,,. The low temperature T, is a heat source from the cooling load. The hydrogen flows from A to B to C to D to A for a full cycle as shown in Fig. 2. A detailed description of the operation of a metal hydride chiller has been given in Refs [9, lo]. An essential feature of the experimental setup consists of coupled metal hydride reactors, four isothermal baths, piping of hydrogen between the two reactors and piping of heat carrier between the reactors and the isothermal baths. A schematic diagram of the experimental arrangement is shown in Fig. 3. During the regeneration period (t,), hydrogen is desorbed from Reactor I (LaNi,,Al,,) by a driving heat source at T,, and absorbed to Reactor II (MmNi, ,SFeo,,). Absorption heat in Reactor II is removed by cooling water at T,,,. During the cooling period (tJ, the hydrogen moves from Reactor II, where chilled water at T, is obtained, to Reactor I, where the absorption heat is removed at T,,,. Isothermal baths provide the desired temperatures, r,,. r,,, and T,, through the flow of heat carrier to the metal hydride reactors. Solenoid valves are installed in piping to control the operating cooling period (tJ and regeneration period (t,). Ten copper-constantan thermocouples are employed to measure the temperatures of the hydride materials inside the reactors and the temperatures of the heat carrier. Hydrogen flow rate between the two reactors and the pressure in the reactor are measured by a mass flow controller (Unit, Model UFC-9320) and a pressure transducer (Druck, Model PDCR-330) respectively. Temperature and pressure signals were measured by means of a data acquisition system.

SYSTEM

Figure 2 shows a pressureetemperature diagram of a typical thermodynamic cooling cycle. Low grade heat sources such as solar energy or waste heat can be used

3. EXPERIMENTAL

RESULTS AND DISCUSSION

An experimental study has been carried out to investigate the time-dependent behavior of hydrogen transfer

METAL

HYDRIDE

COOLING

S-YSTEM

771

Table 1, Specification of reactors

Tube type Rleactor size Material

High temperature reactor ( LaNi, ,Al,, 4 Finned tube

Finned tube

~100x200 Vessel: SUS

4100 x 250

Tube: Cu Fin: Al Fin area (m’) Tube area (m’) Reactor weight (kg) Hydride weight (kg)

Low temperature reactor (MmW I5Fe,,x7)

0.587 0.033 13 4.5

Fig. 2. Metal hydride cooling cycle on In p - I ;T diagram

and associated heat transfer for coupled reactors in a metal hydride cooling system. LaNi, ,Al,, ? is used for the high temperature hydride and MmNi, ,,Fe, xs for the low temperature hydridie. A reference operation condition assumed that chilled water at a temperature of 293 K is obtained using cooling water at a temperature of 303 K and driving heat source at a temperature of 423 K. The time for the regeneration process (1,) and the time for the cooling process (z2) are selected to be IO min each. Figure 4 shows transient variations of hydrogen flow rate between two reactors. The trends in the transient behavior of the hydrogen flow rate are very similar for both the regeneration and cooling periods. In the beginning of both processes, the flow rate is rapidly increased to reach a maximum value and then one more peak is observed. After this peak. the flow rate is gradually decreased. When hydrogen is reacted with hydride materials, hydrogen is adsorbed on the material surface and then it penetrates into the material by absorption. During this transition from adsorption to absorption, the hydrogen flow rate is a little decreased. The dotted line in this figure shows the accumulated hydrogen flow dur-

Vessel: SUS Tube: Cu Fin: Al O.?l

0.040 19.5 5:’

ing the process. In the present experimental conditions, the total hydrogen amount transferred between coupled reactors is about 520 NI during IO min, where NI is an estimated volume of hydrogen at the conditions 0°C and I atm. The dynamic T-C correlations obtained under the reference experimental conditions are illustrated in Fig. 5, where closed loops are observed. The temperature of the hydride materials is the averaged value measured at 3 positions in Fig. 3. Each point in the curve is a measured value every IO s. It is noted here that most of the hydriding and dehydriding processes in the MmNi, ,SFe,,,, reactor occur at isothermal conditions while the temperature of the LaNi,,Al, 1 reactor is rapidly changed during both processes. Figure 6 shows the dynamic relation between pressure and temperature inside the reactors under the reference operation conditions. It is important to compare this figure with the static equilibrium condition in Fig. 2 and a large deviation is seen between dynamic and static PT relations. Pressure under dynamic conditions is restricted so that the hydrating and dehydrating processes occur under isochoric conditions in the actual cycle in this figure. Total hydrogen transfer between two coupled reactors is affected by the hydrogen amount charged initially. If there is only one reactor, the hydrogen transfer through the reactor during hydration and dehydration should be increased with the increase in the hydrogen amount charged. However, when two reactors are coupled, there must be an optimum amount of hydrogen to be charged. The effect of the initial hydrogen amount to be charged in coupled reactors on the total hydrogen transfer is shown in Fig. 7. Maximum hydrogen transfer is observed in the range of 3.54.0 mol Hz/kg alloy, where hydride materials contained in both reactors are accounted for. The present experimental data are compared with that of Nagel et al [4]. Very similar trends are observed for both results. Figure 8 shows the transient variation of heat carrier temperature at the inlet and the outlet of the two reactors at a steady periodic state. Temperature of the hydride materials is also plotted in this figure. In Fig. 8a, the temperature difference between heat carrier and hydride materials is decreased gradually as time elapses.

B. H. KANG et al.

172

Reactor I (taNb.vAAln~, Skg ), Reactor Ii (MmNi&&m,

O-0 Thermocouples

8 Solenoid valve

O-63 Flow meter

m

@

6kg)

Pressuretransducer Mass Flow Controller @ Plane Fig. 3. Experimental

However, in Fig. 8b, the temperature of the hydride materials is lower than that of the heat carrier at the beginning of the regeneration period (t,) while the reverse happens at the beginning of the cooling period (tJ. Therefore. at the beginning of both processes, the heat carrier

arrangement.

does not flow until the material temperature is higher than the heat carrier temperature during the regeneration period, i.e. extracting heat from the LaNi,,,Al,, reactor, and until the material temperature is lower than the heat carrier temperature during the cooling period, 200

-

750

Flow rate

160

- - - - - - Integral flow

500

4

250

z = 73 b 2 =

LaNi .7Al 01

120 & I-

t2

L h

80 MmNi. ,,Fe085

40 t1

\ 0

0 0

5

10 t[min]

15

20

Fig. 4. Temporal variation of the hydrogen flow rate between two hydride reactors at steady periodic state.

t1

0

TV -_ I 0.2

k

12

I

I

I

0.4

0.6

0.8

1

H/M

Fig. 5. Dynamic correlation between temperature and hydrogen content in hydride alloys for coupled hydride reactors.

713

METAL HYDRIDE COOLING SYSTEM

-

I t

LaNi,,7A10,

MmNi, ,SFeO~

TI

160

120 E I80

2

E!.5

3

3.5

4 t [min]

lOOO/T[l/K] Fig. 6. Dynamic correlation between pressure and temperature for coupled hydride reactors. i.e. obtaining a cooling effect in the MmNi, ,sFeoss reactor. The transient variation of the heat transfer rate is strongly related to the hydrogen flow rate since heat is generated or removed by hydrogen absorption or desorption in the reactors. Heat transfer rate in the LaNi,,,Al,,, reactor is much higher than that in the MmNi, ,sFeo85 reactor because the temperature difference between the hydride material and the heat carrier in the LaNi,,Al, 3 reactor is larger than that in the MmNi, ,SFe,,,, reactor. Here, & is the input heat to drive the metal hydride cooling system and e, is the cooling effect to obtain. em. and Ck, are the heat removed from the reactors by

40 r

1 i t1 10

0

< 5

I 10

>

t2

I 15

i

20

t [min] Fig. 8. Temporal variation of temperatures of the heat carrier and the hydride materials at steady periodic state. (a) LaNi, ,Al, 3 reactor, (b) MmNi, ,5Fe, 85reactor. T,, r, Inlet temperature of heat carrier; 7;. r, outlet temperature of heat carrier; T,,, average temperature of hydride materials in the reactor.

100

.

Present study Nagel et al.[4]

0

0 0 0 .

.

l

0 .

cooling water. Thus, the cooling performance is defined as the following:

0 l .

. .

1

1

I

I

4 2 3 Amount of hydrogen charged [H, mol / kg alloy]

5

Fig. 7. Effect of the charged hydrogen amounts on the total hydrogen transfer.

In the present case, the COP obtained is about 0.2 which is low, compared with a commercialized absorption chiller. The absorption chiller has an internal heat exchanger and uses the optimized absorber and regenerator to improve performance while the present system is just designed to investigate the internal characteristics of the hydride reactors during absorption and desorption processes in a metal hydride chiller. The results obtained here would be useful not only for the design of a metal hydride cooling system but also in

B. H. KANG et (11

714

3.5-4.0 mol H,/kg alloy. It is also found that heat transfer rate at the LaNi, ,Al,, 1 reactor is much higher than that at the MmNi, ,sFeoHjreactor for the present cooling cycle. The experimental results obtained lead to a better understanding of the underlying transport mechanisms. 2 Ach-nonk~~~!yen?mt~The authors acknowledge the financial support provided by the Ministry of Science and Technology of Korea for this work.

Fo .&

REFERENCES

-2

1. E. Tucher, P. Weinzierl and 0. J. Eder, Dynamic chardcteristics of single- and dual-hydride bed devices. J. LessCommon Metals95, 171~179(1983). 2. H. Bjurstrom, Y. Komazaki and S. Suda, The dynamics of hydrogen transfer in a metal hydride heat pump. J. Less-

-4 -6

Common Metals 131,225%234 (1987).

t [min] Fig. 9. Temporal variation of the heat transfer rate between the heat carrier and the hydride material at steady periodic state. understanding the operating characteristics inside metal hydride reactors. 4. CONCLUSIONS Transient transport processes of hydrogen and heat between two coupled reactors in a metal hydride cooling system have been investigated experimentally. A pair of hydride reactors are designed to extend the contact surface between hydride and flowing hydrogen for fast kinetics. LaNi,,Al,, is used for high plateau pressure and MmNi4 ,sFeO.ssfor lower plateau pressure. Dynamic correlations of pressure-temperature and temperatureconcentration are investigated in a typical operational condition. The amount of hydrogen gas transferrable between the paired metal hydrides is measured and the optimum value of the charged hydrogen amount for the maximum hydrogen transfer is seen to be in the range of

3. M. Nagel, M. Komazaki and S. Suda, Dynamic behaviour of paired metal hydrides: I. Experimental method and results. J. Less-Common Metal.? 120.45-53 (1986). 4. M. Nagel, M. Komazaki, Y. Matsubara and S. Suda, Dynamic behaviour of paired metal hydrides: II. Analytical survey of the experimental results. J. Less-Common Metals 123, 47-58 (1986). 5. U. Mayer, M. Groll and W. Supper, Heat and mass transfer

in metal hydride reaction beds: Experimental and theoretical results. J. Less-Common Metals 131, 235-244 (1987). 6. M. Gambini, Metal hydride energy systems performance evaluation. Part A: Dynamic analysis model of heat and mass transfer. Int. J. Hydrogen Energy 19, 67-80 (1994). 7. M. R. Gopal and S. S. Murthy, Prediction of metal-hydride refrigerator performance based on reactor heat and mass transfer. Int. J. Hydro,gen Energy 20, 607T614 (1995). 8. H. Bjurstrom and S. Suda, The metal hydride heat pump: Dynamics of hydrogen transfer. ht. J. Hydrogen Energy 14, 19-28 (1989). 9. M. Nagel, M. Komazaki, M. Uchida, S. Suda and Y. Mat-

subara, Operating characteristics of metal hydride heat pumps for generating cooled air. J. Less-Common Metals 104, 307-318 (1984).

10. T. Nishizaki, K. Miyamoto and K. Yoshida, Coefficients of performance of hydride heat pumps. J. Less-Common Metals 89, 559-566 (I 983).