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Current status and performance of the argonne Hycsos chemical heat pump system
401
Journal of the Less-Common Meials, 74 (1980) 401 - 409 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
CURRENT STATUS AND PERFORMANCE CHEMICAL HEAT PUMP SYSTEM*
OF THE ARGONNE
HYCSOS
IRVING SHEFT, D. M. GRUEN and G. J. LAMICH
Chemistry Division, Argonne National Laboratory, Argonne, 111.60439 (U.S.A.)
The Argonne hydrogen conversion and storage system (HYCSOS), under development since 1975, is a two-hydride concept which operates as a chemical heat pump for the storage and recovery of thermal energy for heating, cooling and energy conversion, The system, designed and constructed to show the scientific feasibility of the concept and to evaluate system components and materials for use as hydrides, has been operational for several years. Hardware for the operation of the system under computer program control is complete and routines for data handling and analysis are available. Transient Wermal and effectiveness rne~~rnen~ on the current coiled-tub~g heat exchangers were made and will be compared with those for an advanced concept tubular heat exchanger with alloy-loaded aluminum foam for heat tmnsfer enhancement. Recent developments of alloy materials and their use in chemical heat pumps will be discussed.
1. Introduction The Argonne hydrogen conversion and storage system (HYCSOS) is a twohydride concept [l -61 which operates as a chemical heat pump for the storage and recovery of thermat energy for heating, cooling and energy conversion. Low grade thermal energy, e.g. from a solar collector, can be used to decompose the metal hydride Ml with the higher free energy of dissociation; the released hydrogen is reabsorbed at an intermediate temperature and stored as the second hydride M2 with the lower free energy of dissociation The heat of reabsorption of the second hydride at the intermediate temperature can be used for space heating. The heat pump mode of the heating cycle is the use of low temperature outdoor heat to decompose the second hydride and to reabsorb the hydrogen at the intermediate temperature as the first hydride. The heat of absorption of the first hydride can now be used for *Paper presented at the ~ternation~ Symposium on the Properties and Applications of Metal Hydrides, Colorado Springs, Colorado, U.S.A., April 7 - 11, 1980.
402
space heating. By rejecting the intermediate temperature heat of hydrogen absorption to the outdoors and withdrawing the low temperature from indoors, the heat pump cycle can be used for space cooling. The HYCSOS system also lends itself to the conversion of thermal energy into useful shaft work. High pressure hydrogen from the high temperature dissociation of a hydride could do work in an expansion engine driving an electric generator before being reabsorbed on another bed of the same hydride at a lower pressure and temperature. The heat pump cycle can also be used to raise the temperature available from an inexpensive flat-plate solar collector or that available as waste heat in many industrial operations.
2. The HYCSOS system
The Argonne HYCSOS system is a demonstration test facility to evaluate materials and components for use in the hydride heat pump concept. The facility and its manual operation have been described elsewhere [ 1 - 6 ] . The instrumentation and the control system will be described. Many variables are followed during the operation of the system. Information from the various sensors, e.g. temperature, pressure, flow and power, is digitized and transmitted by a data logger to a Tektronix dab-h~dl~g system. Important variables are also displayed on a remote graphics panel from which the system is controlled. The instrumentation system can be used to provide (1) a real-time indication of important system characteristics, (2) for the logging of data generated during system operation, (3) a means of processing raw data and (4) a way of automatically controlling the HYCSOS system operation. A decoder and relay drive accepts commands from the computer in the form of ASCII characters, decodes them and activates the appropriate relay to provide the required function. Any operating parameter, e.g. hydrogen flow, heat effects, pressure changes or time, can be used to control the cycle. In addition, the system permits the rapid recall and plotting of previous data for comparisons, trend analysis, record keeping, visual presentation slides etc. Because the hydride reactions are heat transfer limited, design consideration of the heat exchangers containing the alloy powder is important. The current units are tanks with internal heat transfer surfaces in the form of coiled tubing with the heat transfer cooling or heating fluid circulating inside the tubing. The alloy powder is between the loops of the coils such that no powder is more than $ in from a coil surface. Advanced design heat exchangers are being investigated in order to reduce cycle times and to increase the heat transfer in the alloy beds. A heat exchanger containing the alloy powder in the small interstices of an aluminum foam matrix is being developed f7] and constructed for Argonne by Ergenics, a division of Into Ltd. The alloy-loaded ~urn~~rn foam is contained in openended aluminum capsules [ 81 with stainless steel
filters crimped into the ends. Capsuleswith the two alloys, MmNi4rsFee_s5* (Into HYSTOR 209) and LaNi,,Al,s (Into HYSTOR ZO?), are sealed into a stainlesssteel tube (Fig. 1) separatedby a centralempty capsule forming a tubularhydride bed. Ten beds are combined to form a heat exchangerin which the heat transfer fluid enters and exits around the outside of the hydride bed tubes. A centraltube sheet keeps the two fluid paths separate. The shell is designed to have a smallvolume to minimize the thermallosses duringregeneration.The hydrogen flows from one alloy to the other within the bed tubes. This design eliminatesthe need for hydrogen valvesand has high reliabilitydue to the largenumber of independent hydride beds,
7-
Sf PLUG
6 HIGH TEMPERATURE ALLOY CAPSULES
f
CEN~ALEMPTY CAPSULE
6L~TEMPERATURE ALLOY CAPSULES
TUBULAR ELEMENT TYPOF IO
Fig. 1, A schematic diagram of (a) the tubular hydride bed and (b) the element bundle heat exchanger.
*Mm (misch metal) is an unrefined rare earth mixutre of average composition 48 50% Ce, 32 - 34% La, 13 - 14% Nd, 4 - 5% Pr and 1 - 2% other rare earths.
404
3. Hydride materials as chemical heat pumps The selection and availability of desirable pairs of hydrides to function as a chemical heat pump are important to its efficient and economical operation. In recent years, lanthanide-nickel hydride systems have been investigated [9] which have properties that make them attractive candidates for heat pump utilization purposes. These compounds easily absorb and reversibly desorb large amounts of hydrogen with excellent kinetics. Although temperature dependent, the kinetics of the system are rapid, with over 95% of the equilibrium hydrogen pressure attained in a few minutes at room temperature. From a trea~ent [lo] of chemical heat pumps it has been shown that the most efficient heat pump operation (being able to pump from the lowest temperature) occurs when the entropy for the hydriding reaction is the same for the two hydrides. The temperature regime over which a given pair of hydrides can function, however, is determined primarily by the enthalpy for the hydriding reaction. Correlations of free energy changes with cell volumes have been useful in obtaining hydrides with specified properties. Varying the ratio of nickel to lanthanum in LaNi, and partially substituting other lanthanides for lanthanum or other transition metals for nickel has made available a number of alloys with thermodynamic properties that are useful over a considerable amperage range 111-181. The advent of these ternary alloys adds flexibility to the selection of alloy pairs for the opt~ization of engineering design and performance characteristics of the hydride heat pump system. To be useful for chemical heat pump purposes the hydrides must be sufficiently stable to undergo many hydriding-dehydriding cycles without decomposition. Since the binary hydrides of the lanthanide component (which can also be calcium) are very stable, the ternary hydrides tend to be me&table [ 191. The plateau capacity of CaNis, currently used as Ml in HYCSOS systems, has been found [ 201 to be substantially reduced when it is contained in a hydrogen atmosphere for an extended period of time. Figure 2 shows the effect on the CaNi in the HYCSOS system of an exposure of several years to hydrogen. Ex~ination of the CaNi&by X-ray diffraction and magnetic moment me~uremen~ indicated a small increase in the free nickel content and the presence of some CaaNi,. Little change in bulk properties was observed -not enough to account for the substantial reduction in the plateau. The plateau shortening may be due to as yet not understood surface effects. Regeneration of the original alloy can be achieved by annealing the material in a vacuum at a moderate temperature [ 211.
4. HYCSOS system operation The Argonne HYCSOS system was designed and constructed as a facility to evaluate materials and components as candidates for use in the hydride chemical heat pump concept. Since most hydride materials are rel-
405
7/76
167 D.p.C
0179
97 DoQ.C
0.1 0
1
2
3
4 H/mol.e
Fig. 2. Hydrogen desorption metastahility of CaNis.
5
6
allay
from CaNis after various periods of storage to show the
atively expensive, economic co~ide~tions require rapid cycling to obtain maximum thermal benefits. ~e~urement of rapid thermal and pressure transients is thus important for the evaluation of materials and components. In multichannel logging, HYCSOS data can be collected at rates of up to eight channels per second. Successive readings of a single channel can be recorded at up to four readings per second. The heat transfer characteristics of the hydride heat exchangers can be evaluated by comparing the temperature of the hydride bed with the temperature of the heat transfer fluid leaving the heat exchanger. The bed temperature, calculated from the measured hydrogen pressure using the appropriate hydride vapor pressure equation and assuming rapid gas sorption kinetics, is a measure of the lowest bed temperature. In Fig. 3 the bed temperature, normalized to the fluid ~mperat~e at 15 min, is compared with the heat transfer fluid temperature for the absorption of approximately 6 mol (equivalent to 41.33 kcal input) of hydrogen. The large lag of the bed temperature shows the relatively slow transfer of heat between the bed and the fluid. Since film coefficients are large the hydriding reaction is seen to be heat transfer limited, with the slow step being the transfer of heat from the reaction site in the bed to the first wall of the heat transfer fluid tube. The heat transfer of powder beds is limited by the point contact between adjacent particles and can be improved by reducing the path length in the powder. Imbedding the hydride powder in open-pore aluminum foam with 20 pores to the inch reduces the bed half-thickness from 0.12 to 0.02 in. In a single adiabatic experiment with a test module 1.74 mol I& were desorbed. Figure 4 shows the subsequent rapid adiabatic absorption of 1.07
406
I
4
2
6
6
16
12
14
nmlRc6
Fig. 3. A comparison of the hydride bed and the exit heat transfer fluid temperatures for absorption on LaNiB. T(bed) = {-3013/&P - 11.39)}273.1.(T(bed) is normaiized to T(outlet) at 15 min.) Since the apparatus contained 5.74 mol Hz, and AH = 7.2 kcal (mol Hz)-l’, the total input of hydrogen is equivalent to 41.33 kcal. The heat transfer fluid flow is 7.26 1 m-l.
0..
2 __ MDUX6
SE* !!
8 I,,
t
SttX:lD!r 1.1
I-rs:altai
IS
***a
III
1
I
t
Fig. 4. The hydrogen absorption on LaNi6 in an aluminum foam heat exchanger.
407
mol Hs. Equilibrium absorption pressures are reached in approximately 30 s, showing the temperature of the unit to be uniform. An interesting mode of HYCSOS operation is made feasible by the ability to select alloys with a particular decomposition pressure. By proper choice of pairs of alloys with properties suitable for the available temperature regime, low grade solar energy (such as that which can be obtained from inexpensive flat-plate collectors in northern climes with low levels of insolation) can be enhanced to provide domestic hot water. The addition of heat pump operation to the solar collector system enables longer periods of usefulness with lower available solar temperatures. To cycle from a solar collector temperature as low as 40 “C and to produce a high temperature of 75 “C, ~e~od~amic considerations [lo] of hydride heat pumps suggest a ratio of 1 .l for the enthalpies of hydride formation for two alloys having the same entropy. Using LaNi,, with an enthalpy of 7.2 kcal (mol H,)-l, as M2 and 8.1 kcal (mol Ha)-’ for the enthalpy of the more stable hydride Ml, a heat pump input temperature of 40” C could be enhanced to 75 “C. The same 40 “C input temperature could also be used to complete the cycle at a reject temperature of 4.5 “C. In late spring, summer and early autumn during the time of day when solar collector output temperatures are sufficient, direct water heating can be used. At other times the addition of a heat pump would decrease the need for auxiliary heating. Using LaNis (AH= 7.2 kcal (mot. Ha)-“) as M2 and CaNis (AH = 7.5 kcsl (mol Hz)-l) as Ml, approximately 5 mol of hydrogen was desorbed from LaNi5 at 39 “C and absorbed on CaNis at 66 “C. During the hydrogen desorption from LaNis, the temperature drops to 36 “C before the increased heating power returns it to 39 “C. No heating power to the absorbing CaNi, is required until the temperature is again returned to 66 “C from its peak at 70 “C. Approximately 34 kcal of thermal energy was used to increase the temperature of the heat transfer fluid from 39 to 66 “C. Because these alloys are not optimum, 60 “C, rather than the 40 ‘C assumed available from the solar collector, would be required to return the hydrogen from CaNis, However, using LaNi4.sA&,a to form the more stable hydride would permit the return of hydrogen at 40 “C to the LaNie as the less stable hydride at 4 “C. In many commercial operations, e.g. factories, hospitals and foundries, low temperature thermal energy is rejected to the environment. Because of the wide temperature range available by proper selection of alloy pairs, hydride chemical heat pumps can upgrade the heat to useful temperatures. The HYCSOS system has been operated in a fully automatic mode. For the successful automatic operation of a hydride heat pump at a constant cycle time, the amount of hydrogen transferring in each direction must be the same to prevent the build-up of hydrogen on one alloy. The computer program for automatic operation of the HYCSOS system uses the transfer times to change the low temperature in order to equalize the transfer rates for the selected quantity of hydrogen.
408
During each of the 40 automatic transfers, 19 kcal at 100 “C was used to transfer 2.5 mol Hz from CaNis to La& at 40 “C., At the same time 2.5 mol Hz was transferred from LaNis at 25 “C ;to CaNis at 40 “c. In this process 19 kcal was “pumped” from 25 “C and a total of 38 kcal was delivered at 40 “C.;The temperature regime is determined by the alloy pairs used. The shortened CaNis plateau, due to deterioration, required a higher low temperature than would a more desirable. alloy.
5. Conclusions An independent assessment [ 221 of the Argonne HYCSOS system where design was optimized using an iterative computer program concludes that, whereas a solar absorption cooling system to provide 3 tons of cooling currently retails for $3000 excluding an expensive cooling tower and an indoor heat exchanger, a HYCSOS system to provide 3 tons of cooling would cost $3700 - $3800 including all heat exchangers. HYCSOS also provides heating with a coefficient of performance (COP) greater than unity and electrical power, although it requires a higher solar input than the absorption unit (250 - 280 “F compared with 180 OF)and has slightly lower cooling COPS. Thus this HYCSOS system, which can be packaged similar to a conventional heat pump, compares favorably with a solar absorption coolingdirect solar heating unit.
This work was performed under the auspices of the Division of Energy Storage Systems, U.S. Department of Energy. References 1 D. M. Gruen and I. Sheft, Metal hydride systems for solar energy conversion and storage. In Proc. NSF-ERDA Workshop on Solar Heating and Cooling of Buildings, Charlottesville, Virginia, 1975, American Society of Heating, Refrigeration and AirConditioning Engineers, New York, 1975, p. 96. 2 D. M. Gruen, F. Schreiner and I. Sheft, A thermodynamic analysis of HYCSOS, a hydrogen conversion and storage system. In Proc. 1st World Hydrogen Energy Conf., Miami Beach, Florida, March 1976, Vol. II, University of Miami, Coral Gables, Florida, 1976, paper 8B, p. 73. D. M. Gruen, F. Schreiner and I. Sheft, Znt. J. Hydrogen Energy, 3 (1978) 303 - 310. 3 D. M. Gruen, R. L. McBeth, M. Mendelsohn, J. M. Nixon, F. Schreiner and I. Sheft, Proc. 11th Zntersociety Energy Conversion Engineering Conf., State Line, Nevada, 1976, American Institute of Chemical Engineers, New York, 1976, p. 681. 4 I. Sheft, D. M. Gruen, G. J. Lamich, L. W. Carlson, A. E. Knox, J. M. Nixon and M. Mendelsohn, HYCSOS: a system for evaluation of hydrides as chemical heat pumps. In Proc. Znt. Symp. on Hydrides for Energy Storage, Geilo, 1977, Pergamon, New York, 1977, p. 551.
409
5 D. M. Gruen, I. Sheft, G. Lamich and M. Mendelsohn, HYCSOS: a chemical heat pump and energy conversion system based on metal hydrides, Argonne National
Laborer Rep. ANL-77-39,19??. 6 I. Sheft, D. M. Gruen and G. J. Lamich, HYCSOS: a chemical heat pump and energy conversion system based on metal hydrides, Argonne National Labomtory Status Rep. ANL-79-8, 1979. 7 J. S. Horowitz, P. A. Nelson and C. A. Blomquist, Engineering development of a HYCSOS chemical heat pump. In Proc. 2nd Miami Znt. Conf. on AZternatiue Energy Sources, Miami, FZorida, 1979, University of Miami, Coral Gables, Florida, 1979, p. 371. 8 U.S. Patent 4,135,621 (January 23,19?9), to Into Incorporated. 9 J. N. H. van Vucht, F. A. Kuijpers and H. C. A. M. Bruning, Philips Res. Rep., 25 (1970) 133 - 140. 10 D. M. Gruen, M. H, Mendelsohn and I. Sheft, Metal hydrides as chemical heat pumps, Sol. Energy, 21 (1978) 153. 11 K. H. J. Buschow and H. H. van Mat, J. Less-Common Met., 29 (1972) 203. 12 H. H. van MaI, K. H. J. Buschow and A. R. Miedema, J. Less-Common Met., 35 (1974) 65. 13 M. H. Mendeisohn, D. M. Gruen and A. E. Dwight, Nature (London), 269 (1977) 45. 14 J. C. Achard, A. Percheron-Guegan, H. Diaz, F. Briancourt and F. Denany, 2nd Znt. Congr. on Hydrogen in Metals, Paris, 1977, Pergamon, New York, 1977, p. lE12. 15 T. Takesbita, S. K. MaIik and W. E. Wallace, J. Solid State Chem., 271 (1978) 23. 16 D. M. Gruen, M. Mende~ohn and A. Dwight, Adu. Chem. Ser., 173 (1978) 279. 17 D. M. Gruen, M. MendeIsohn, I. Sheft and G. Lamich, Proe. 2nd World Hydrogen Energy Conf., Zurich, 1978, Pergamon, New York, 1978, p. 1931. 18 G. D. Sandrock, Proc. 2nd World Hydrogen Energy Conf., Zurich, 1978, Pergamon, New York, 1978, p. 1625. 19 K. H. J. Buschow and A. R. Miedema, Hz absorption in rare earth intermetaiiic compounds. In Proc. Znt. Symp. on Hydrides for Energy Stomge, Geilo, 1977, Pergamon, New York, 1978, p. 235. 20 I. Sheft, D. M. Gruen, G. J. Lamieh, M. H. Mendelsohn and G. Sandrock, to be published . 21 R. Cohen, K. W. West and K. H. J. Buschow, Degradation of hydrogen-absorbing rare earth intermetaIIics by cycling, Solid State Commun., 25 (1978) 293. 22 R. Gorman and P. S. Mortiz, Metal hydride solar heat pump and power system (HYCSOS). In Proc. AZAA/ASERC Con& on Solar Energy: Technology Status, Phoenix, Arizona, November 1978, American Institute of Aeronautics and Astronautics, New York, 1978, paper no. 78-1’762.
Journal of the Less-Common Meials, 74 (1980) 401 - 409 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
CURRENT STATUS AND PERFORMANCE CHEMICAL HEAT PUMP SYSTEM*
OF THE ARGONNE
HYCSOS
IRVING SHEFT, D. M. GRUEN and G. J. LAMICH
Chemistry Division, Argonne National Laboratory, Argonne, 111.60439 (U.S.A.)
The Argonne hydrogen conversion and storage system (HYCSOS), under development since 1975, is a two-hydride concept which operates as a chemical heat pump for the storage and recovery of thermal energy for heating, cooling and energy conversion, The system, designed and constructed to show the scientific feasibility of the concept and to evaluate system components and materials for use as hydrides, has been operational for several years. Hardware for the operation of the system under computer program control is complete and routines for data handling and analysis are available. Transient Wermal and effectiveness rne~~rnen~ on the current coiled-tub~g heat exchangers were made and will be compared with those for an advanced concept tubular heat exchanger with alloy-loaded aluminum foam for heat tmnsfer enhancement. Recent developments of alloy materials and their use in chemical heat pumps will be discussed.
1. Introduction The Argonne hydrogen conversion and storage system (HYCSOS) is a twohydride concept [l -61 which operates as a chemical heat pump for the storage and recovery of thermat energy for heating, cooling and energy conversion. Low grade thermal energy, e.g. from a solar collector, can be used to decompose the metal hydride Ml with the higher free energy of dissociation; the released hydrogen is reabsorbed at an intermediate temperature and stored as the second hydride M2 with the lower free energy of dissociation The heat of reabsorption of the second hydride at the intermediate temperature can be used for space heating. The heat pump mode of the heating cycle is the use of low temperature outdoor heat to decompose the second hydride and to reabsorb the hydrogen at the intermediate temperature as the first hydride. The heat of absorption of the first hydride can now be used for *Paper presented at the ~ternation~ Symposium on the Properties and Applications of Metal Hydrides, Colorado Springs, Colorado, U.S.A., April 7 - 11, 1980.
402
space heating. By rejecting the intermediate temperature heat of hydrogen absorption to the outdoors and withdrawing the low temperature from indoors, the heat pump cycle can be used for space cooling. The HYCSOS system also lends itself to the conversion of thermal energy into useful shaft work. High pressure hydrogen from the high temperature dissociation of a hydride could do work in an expansion engine driving an electric generator before being reabsorbed on another bed of the same hydride at a lower pressure and temperature. The heat pump cycle can also be used to raise the temperature available from an inexpensive flat-plate solar collector or that available as waste heat in many industrial operations.
2. The HYCSOS system
The Argonne HYCSOS system is a demonstration test facility to evaluate materials and components for use in the hydride heat pump concept. The facility and its manual operation have been described elsewhere [ 1 - 6 ] . The instrumentation and the control system will be described. Many variables are followed during the operation of the system. Information from the various sensors, e.g. temperature, pressure, flow and power, is digitized and transmitted by a data logger to a Tektronix dab-h~dl~g system. Important variables are also displayed on a remote graphics panel from which the system is controlled. The instrumentation system can be used to provide (1) a real-time indication of important system characteristics, (2) for the logging of data generated during system operation, (3) a means of processing raw data and (4) a way of automatically controlling the HYCSOS system operation. A decoder and relay drive accepts commands from the computer in the form of ASCII characters, decodes them and activates the appropriate relay to provide the required function. Any operating parameter, e.g. hydrogen flow, heat effects, pressure changes or time, can be used to control the cycle. In addition, the system permits the rapid recall and plotting of previous data for comparisons, trend analysis, record keeping, visual presentation slides etc. Because the hydride reactions are heat transfer limited, design consideration of the heat exchangers containing the alloy powder is important. The current units are tanks with internal heat transfer surfaces in the form of coiled tubing with the heat transfer cooling or heating fluid circulating inside the tubing. The alloy powder is between the loops of the coils such that no powder is more than $ in from a coil surface. Advanced design heat exchangers are being investigated in order to reduce cycle times and to increase the heat transfer in the alloy beds. A heat exchanger containing the alloy powder in the small interstices of an aluminum foam matrix is being developed f7] and constructed for Argonne by Ergenics, a division of Into Ltd. The alloy-loaded ~urn~~rn foam is contained in openended aluminum capsules [ 81 with stainless steel
filters crimped into the ends. Capsuleswith the two alloys, MmNi4rsFee_s5* (Into HYSTOR 209) and LaNi,,Al,s (Into HYSTOR ZO?), are sealed into a stainlesssteel tube (Fig. 1) separatedby a centralempty capsule forming a tubularhydride bed. Ten beds are combined to form a heat exchangerin which the heat transfer fluid enters and exits around the outside of the hydride bed tubes. A centraltube sheet keeps the two fluid paths separate. The shell is designed to have a smallvolume to minimize the thermallosses duringregeneration.The hydrogen flows from one alloy to the other within the bed tubes. This design eliminatesthe need for hydrogen valvesand has high reliabilitydue to the largenumber of independent hydride beds,
7-
Sf PLUG
6 HIGH TEMPERATURE ALLOY CAPSULES
f
CEN~ALEMPTY CAPSULE
6L~TEMPERATURE ALLOY CAPSULES
TUBULAR ELEMENT TYPOF IO
Fig. 1, A schematic diagram of (a) the tubular hydride bed and (b) the element bundle heat exchanger.
*Mm (misch metal) is an unrefined rare earth mixutre of average composition 48 50% Ce, 32 - 34% La, 13 - 14% Nd, 4 - 5% Pr and 1 - 2% other rare earths.
404
3. Hydride materials as chemical heat pumps The selection and availability of desirable pairs of hydrides to function as a chemical heat pump are important to its efficient and economical operation. In recent years, lanthanide-nickel hydride systems have been investigated [9] which have properties that make them attractive candidates for heat pump utilization purposes. These compounds easily absorb and reversibly desorb large amounts of hydrogen with excellent kinetics. Although temperature dependent, the kinetics of the system are rapid, with over 95% of the equilibrium hydrogen pressure attained in a few minutes at room temperature. From a trea~ent [lo] of chemical heat pumps it has been shown that the most efficient heat pump operation (being able to pump from the lowest temperature) occurs when the entropy for the hydriding reaction is the same for the two hydrides. The temperature regime over which a given pair of hydrides can function, however, is determined primarily by the enthalpy for the hydriding reaction. Correlations of free energy changes with cell volumes have been useful in obtaining hydrides with specified properties. Varying the ratio of nickel to lanthanum in LaNi, and partially substituting other lanthanides for lanthanum or other transition metals for nickel has made available a number of alloys with thermodynamic properties that are useful over a considerable amperage range 111-181. The advent of these ternary alloys adds flexibility to the selection of alloy pairs for the opt~ization of engineering design and performance characteristics of the hydride heat pump system. To be useful for chemical heat pump purposes the hydrides must be sufficiently stable to undergo many hydriding-dehydriding cycles without decomposition. Since the binary hydrides of the lanthanide component (which can also be calcium) are very stable, the ternary hydrides tend to be me&table [ 191. The plateau capacity of CaNis, currently used as Ml in HYCSOS systems, has been found [ 201 to be substantially reduced when it is contained in a hydrogen atmosphere for an extended period of time. Figure 2 shows the effect on the CaNi in the HYCSOS system of an exposure of several years to hydrogen. Ex~ination of the CaNi&by X-ray diffraction and magnetic moment me~uremen~ indicated a small increase in the free nickel content and the presence of some CaaNi,. Little change in bulk properties was observed -not enough to account for the substantial reduction in the plateau. The plateau shortening may be due to as yet not understood surface effects. Regeneration of the original alloy can be achieved by annealing the material in a vacuum at a moderate temperature [ 211.
4. HYCSOS system operation The Argonne HYCSOS system was designed and constructed as a facility to evaluate materials and components as candidates for use in the hydride chemical heat pump concept. Since most hydride materials are rel-
405
7/76
167 D.p.C
0179
97 DoQ.C
0.1 0
1
2
3
4 H/mol.e
Fig. 2. Hydrogen desorption metastahility of CaNis.
5
6
allay
from CaNis after various periods of storage to show the
atively expensive, economic co~ide~tions require rapid cycling to obtain maximum thermal benefits. ~e~urement of rapid thermal and pressure transients is thus important for the evaluation of materials and components. In multichannel logging, HYCSOS data can be collected at rates of up to eight channels per second. Successive readings of a single channel can be recorded at up to four readings per second. The heat transfer characteristics of the hydride heat exchangers can be evaluated by comparing the temperature of the hydride bed with the temperature of the heat transfer fluid leaving the heat exchanger. The bed temperature, calculated from the measured hydrogen pressure using the appropriate hydride vapor pressure equation and assuming rapid gas sorption kinetics, is a measure of the lowest bed temperature. In Fig. 3 the bed temperature, normalized to the fluid ~mperat~e at 15 min, is compared with the heat transfer fluid temperature for the absorption of approximately 6 mol (equivalent to 41.33 kcal input) of hydrogen. The large lag of the bed temperature shows the relatively slow transfer of heat between the bed and the fluid. Since film coefficients are large the hydriding reaction is seen to be heat transfer limited, with the slow step being the transfer of heat from the reaction site in the bed to the first wall of the heat transfer fluid tube. The heat transfer of powder beds is limited by the point contact between adjacent particles and can be improved by reducing the path length in the powder. Imbedding the hydride powder in open-pore aluminum foam with 20 pores to the inch reduces the bed half-thickness from 0.12 to 0.02 in. In a single adiabatic experiment with a test module 1.74 mol I& were desorbed. Figure 4 shows the subsequent rapid adiabatic absorption of 1.07
406
I
4
2
6
6
16
12
14
nmlRc6
Fig. 3. A comparison of the hydride bed and the exit heat transfer fluid temperatures for absorption on LaNiB. T(bed) = {-3013/&P - 11.39)}273.1.(T(bed) is normaiized to T(outlet) at 15 min.) Since the apparatus contained 5.74 mol Hz, and AH = 7.2 kcal (mol Hz)-l’, the total input of hydrogen is equivalent to 41.33 kcal. The heat transfer fluid flow is 7.26 1 m-l.
0..
2 __ MDUX6
SE* !!
8 I,,
t
SttX:lD!r 1.1
I-rs:altai
IS
***a
III
1
I
t
Fig. 4. The hydrogen absorption on LaNi6 in an aluminum foam heat exchanger.
407
mol Hs. Equilibrium absorption pressures are reached in approximately 30 s, showing the temperature of the unit to be uniform. An interesting mode of HYCSOS operation is made feasible by the ability to select alloys with a particular decomposition pressure. By proper choice of pairs of alloys with properties suitable for the available temperature regime, low grade solar energy (such as that which can be obtained from inexpensive flat-plate collectors in northern climes with low levels of insolation) can be enhanced to provide domestic hot water. The addition of heat pump operation to the solar collector system enables longer periods of usefulness with lower available solar temperatures. To cycle from a solar collector temperature as low as 40 “C and to produce a high temperature of 75 “C, ~e~od~amic considerations [lo] of hydride heat pumps suggest a ratio of 1 .l for the enthalpies of hydride formation for two alloys having the same entropy. Using LaNi,, with an enthalpy of 7.2 kcal (mol H,)-l, as M2 and 8.1 kcal (mol Ha)-’ for the enthalpy of the more stable hydride Ml, a heat pump input temperature of 40” C could be enhanced to 75 “C. The same 40 “C input temperature could also be used to complete the cycle at a reject temperature of 4.5 “C. In late spring, summer and early autumn during the time of day when solar collector output temperatures are sufficient, direct water heating can be used. At other times the addition of a heat pump would decrease the need for auxiliary heating. Using LaNis (AH= 7.2 kcal (mot. Ha)-“) as M2 and CaNis (AH = 7.5 kcsl (mol Hz)-l) as Ml, approximately 5 mol of hydrogen was desorbed from LaNi5 at 39 “C and absorbed on CaNis at 66 “C. During the hydrogen desorption from LaNis, the temperature drops to 36 “C before the increased heating power returns it to 39 “C. No heating power to the absorbing CaNi, is required until the temperature is again returned to 66 “C from its peak at 70 “C. Approximately 34 kcal of thermal energy was used to increase the temperature of the heat transfer fluid from 39 to 66 “C. Because these alloys are not optimum, 60 “C, rather than the 40 ‘C assumed available from the solar collector, would be required to return the hydrogen from CaNis, However, using LaNi4.sA&,a to form the more stable hydride would permit the return of hydrogen at 40 “C to the LaNie as the less stable hydride at 4 “C. In many commercial operations, e.g. factories, hospitals and foundries, low temperature thermal energy is rejected to the environment. Because of the wide temperature range available by proper selection of alloy pairs, hydride chemical heat pumps can upgrade the heat to useful temperatures. The HYCSOS system has been operated in a fully automatic mode. For the successful automatic operation of a hydride heat pump at a constant cycle time, the amount of hydrogen transferring in each direction must be the same to prevent the build-up of hydrogen on one alloy. The computer program for automatic operation of the HYCSOS system uses the transfer times to change the low temperature in order to equalize the transfer rates for the selected quantity of hydrogen.
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During each of the 40 automatic transfers, 19 kcal at 100 “C was used to transfer 2.5 mol Hz from CaNis to La& at 40 “C., At the same time 2.5 mol Hz was transferred from LaNis at 25 “C ;to CaNis at 40 “c. In this process 19 kcal was “pumped” from 25 “C and a total of 38 kcal was delivered at 40 “C.;The temperature regime is determined by the alloy pairs used. The shortened CaNis plateau, due to deterioration, required a higher low temperature than would a more desirable. alloy.
5. Conclusions An independent assessment [ 221 of the Argonne HYCSOS system where design was optimized using an iterative computer program concludes that, whereas a solar absorption cooling system to provide 3 tons of cooling currently retails for $3000 excluding an expensive cooling tower and an indoor heat exchanger, a HYCSOS system to provide 3 tons of cooling would cost $3700 - $3800 including all heat exchangers. HYCSOS also provides heating with a coefficient of performance (COP) greater than unity and electrical power, although it requires a higher solar input than the absorption unit (250 - 280 “F compared with 180 OF)and has slightly lower cooling COPS. Thus this HYCSOS system, which can be packaged similar to a conventional heat pump, compares favorably with a solar absorption coolingdirect solar heating unit.
This work was performed under the auspices of the Division of Energy Storage Systems, U.S. Department of Energy. References 1 D. M. Gruen and I. Sheft, Metal hydride systems for solar energy conversion and storage. In Proc. NSF-ERDA Workshop on Solar Heating and Cooling of Buildings, Charlottesville, Virginia, 1975, American Society of Heating, Refrigeration and AirConditioning Engineers, New York, 1975, p. 96. 2 D. M. Gruen, F. Schreiner and I. Sheft, A thermodynamic analysis of HYCSOS, a hydrogen conversion and storage system. In Proc. 1st World Hydrogen Energy Conf., Miami Beach, Florida, March 1976, Vol. II, University of Miami, Coral Gables, Florida, 1976, paper 8B, p. 73. D. M. Gruen, F. Schreiner and I. Sheft, Znt. J. Hydrogen Energy, 3 (1978) 303 - 310. 3 D. M. Gruen, R. L. McBeth, M. Mendelsohn, J. M. Nixon, F. Schreiner and I. Sheft, Proc. 11th Zntersociety Energy Conversion Engineering Conf., State Line, Nevada, 1976, American Institute of Chemical Engineers, New York, 1976, p. 681. 4 I. Sheft, D. M. Gruen, G. J. Lamich, L. W. Carlson, A. E. Knox, J. M. Nixon and M. Mendelsohn, HYCSOS: a system for evaluation of hydrides as chemical heat pumps. In Proc. Znt. Symp. on Hydrides for Energy Storage, Geilo, 1977, Pergamon, New York, 1977, p. 551.
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5 D. M. Gruen, I. Sheft, G. Lamich and M. Mendelsohn, HYCSOS: a chemical heat pump and energy conversion system based on metal hydrides, Argonne National
Laborer Rep. ANL-77-39,19??. 6 I. Sheft, D. M. Gruen and G. J. Lamich, HYCSOS: a chemical heat pump and energy conversion system based on metal hydrides, Argonne National Labomtory Status Rep. ANL-79-8, 1979. 7 J. S. Horowitz, P. A. Nelson and C. A. Blomquist, Engineering development of a HYCSOS chemical heat pump. In Proc. 2nd Miami Znt. Conf. on AZternatiue Energy Sources, Miami, FZorida, 1979, University of Miami, Coral Gables, Florida, 1979, p. 371. 8 U.S. Patent 4,135,621 (January 23,19?9), to Into Incorporated. 9 J. N. H. van Vucht, F. A. Kuijpers and H. C. A. M. Bruning, Philips Res. Rep., 25 (1970) 133 - 140. 10 D. M. Gruen, M. H, Mendelsohn and I. Sheft, Metal hydrides as chemical heat pumps, Sol. Energy, 21 (1978) 153. 11 K. H. J. Buschow and H. H. van Mat, J. Less-Common Met., 29 (1972) 203. 12 H. H. van MaI, K. H. J. Buschow and A. R. Miedema, J. Less-Common Met., 35 (1974) 65. 13 M. H. Mendeisohn, D. M. Gruen and A. E. Dwight, Nature (London), 269 (1977) 45. 14 J. C. Achard, A. Percheron-Guegan, H. Diaz, F. Briancourt and F. Denany, 2nd Znt. Congr. on Hydrogen in Metals, Paris, 1977, Pergamon, New York, 1977, p. lE12. 15 T. Takesbita, S. K. MaIik and W. E. Wallace, J. Solid State Chem., 271 (1978) 23. 16 D. M. Gruen, M. Mende~ohn and A. Dwight, Adu. Chem. Ser., 173 (1978) 279. 17 D. M. Gruen, M. MendeIsohn, I. Sheft and G. Lamich, Proe. 2nd World Hydrogen Energy Conf., Zurich, 1978, Pergamon, New York, 1978, p. 1931. 18 G. D. Sandrock, Proc. 2nd World Hydrogen Energy Conf., Zurich, 1978, Pergamon, New York, 1978, p. 1625. 19 K. H. J. Buschow and A. R. Miedema, Hz absorption in rare earth intermetaiiic compounds. In Proc. Znt. Symp. on Hydrides for Energy Stomge, Geilo, 1977, Pergamon, New York, 1978, p. 235. 20 I. Sheft, D. M. Gruen, G. J. Lamieh, M. H. Mendelsohn and G. Sandrock, to be published . 21 R. Cohen, K. W. West and K. H. J. Buschow, Degradation of hydrogen-absorbing rare earth intermetaIIics by cycling, Solid State Commun., 25 (1978) 293. 22 R. Gorman and P. S. Mortiz, Metal hydride solar heat pump and power system (HYCSOS). In Proc. AZAA/ASERC Con& on Solar Energy: Technology Status, Phoenix, Arizona, November 1978, American Institute of Aeronautics and Astronautics, New York, 1978, paper no. 78-1’762.