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Ammonia-based thermochemical transformers
Heat Recovery Systems & CHP Vol. 13, No. 4, pp. 301-307, 1993
0890-4332]93 $6.00+ .00 © 1993 Pergamon Press Ltd
Printed in Great Britain
AMMONIA-BASED
THERMOCHEMICAL
TRANSFORMERS
B, Spilt,mR CNRS-IMP (Institut de Science et G~nie des Mat~riaux et Proeakt6s), Laboratoire EuroI~n Associ~SIMAP (Science et Ing~nierie des Mat6fiaux et Proc~d~s), Universit6, 66860 Perpignan Cedex, France (Received 5 February 1993)
Almtraet--The implementation of thermochemicat systems based on the reaction between chlorides and ammonia involves certain unusual aspects of thermodynamics and reactivity as well as peculiarities in the evolution of the grain texture and of the beds during the reaction. It is the evolution of the beds that is the most unusual, calling for appropriate solutions involving new implementations of porous beds with optimal characteristics of elasticity and heat and mass transfer. The choice of either multi-effect or multi-salt systems depends on the experimental performances of laboratory pilot systems currently under test or development, and on the real costs derived from experimentation with industrial prototypes.
1. I N T R O D U C T I O N
Thermodynamic systems based on the thermicity of reversible reactions between salts and ammonia are in their first phase of industrial development, mainly in the form of pilot systems, because the chemistry involved is very classic, ammonia is a well known refrigerating fluid which is well controlled at the industrial level and advocated by the International Refrigeration Institute as a substitute for CFCs and HCFCs. The ammonia-NH3 couples were initially used in storage systems and then in continuous refrigeration and/or heat production systems. The 1987 symposium on chemical heat pumps [1] covered the field of current research on solid sorption machines. The particularities, the scientific problems of research on systems using ammonia are studied in greater detail in order to access their up-to-dateness as well as their industrial development. 2. F E A T U R E S
OF
SYSTEMS
BASED WITH
ON THE THERMICITY AMMONIA
OF
REACTIONS
Systems based on the use of salts reacting with ammonia are monovariant under thermodynamic equilibrium. This variance is also observed in systems based on a reaction with water (hydrates such as the Na2S-H20 system), and over a wide range of reactivity in hydrides (the horizontal plateau in the graph of P = f ( X ) at constant temperature): thus in the Clausius-Clapeyron diagram, it is represented by lines marking the boundaries of decomposition reaction zones (on the right) and of synthesis reaction zones (on the left), in contrast with the families of isosteric lines, characteristic of gas-solid adsorption systems [1]. Ammoniates or amino-derivatives (reaction with ammonia derivatives such as monomethylamine or dimethylamine) are many and varied: alkaline, alkaline-earth or metallic halides, double or mixed halides, sulphates, nitrates, phosphates [1-4], They react over very wide ranges of pressure (up to 50 bar), and of temperature ( - 50-300°C), and depending on the number of moles of gas reacting per mole of salt compound, their energy density or storage capacity (J mol-1 or W h kg-i) can be very high (510Whkg -1 in the case of NiCI2 6/2NH3). Generally speaking, these ammoniates are very stable: there are no secondary reactions: it is well known that their chemistry is simple. With such a wealth of reactants, it is possible to implement thermochemical systems for low temperature refrigeration (down to -50°C by evaporation of ammonia), multi-effect systems, systems without condensation/evaporation of ammonia, and finally thermotransformers able to bring the thermal potential of a source up to 250°C. On the other hand ammoniates present significant changes in molar volumes during the synthesis and decomposition reactions: for example, the Mauran et al. [5] study leads to the definition of compaction limits for chlorides. Many research teams consider this feature to be such an obstacle 301
302
B. SPINNER
that they have discarded this type of thermochemical system. However it must be underlined that such molar volume changes are to be found, albeit to a lesser extent, in hydride and adsorption systems [6], and that interesting solutions have been found to deal with this, such as using these variations of molar volume to improve the contact coefficients between grains and binder, and therefore the heat transfer coefficients.
3. S C I E N T I F I C PROBLEMS S P E C I F I C TO A M M O N I A SYSTEMS The features of thermochemical systems based on the implementation of ammoniates lead to the definition of unique scientific problems linked to:
3.1. Thermodynamics and trans[ormation dynamics of solid-ammonia couples The precise positioning of equilibrium lines is hard to obtain experimentally, since it depends on the experimental protocols used: isobar or isothermal scales, enthalpic measurements at a given pressure; there are several degrees of difference in the data depending on the authors. In practice, the exact position of the equilibrium line is not absolutely fundamental: in fact; it lies within a pseudo-equilibrium zone which delimits the practical domain of synthesis and decomposition of the ammoniate considered: depending on the sensitivity of the apparatus used to obtain measurements (thermogravimetry, differential thermal analysis or microcalorimetry), the authors' results are this time very similar [7-9]. The origin of these pseudo-equilibria, whose value in AT, at a given P, can reach more than 10°, is not yet fully understood: it is commonly accepted that, although they have not been identified, low reactive phases exist. The characterization of these domains of pseudo-equilibrium is very important when choosing a pair for a specific system, and for modelling the dynamics of grain transformation and crystal construction, without external limitations on mass and heat transfer. Indeed, it is not easy to choose the structure of this model: the dynamics of grain transformation must take into account the evolution of the reactive surface which decreases as the reaction develops, of the two thermodynamic constraints, P and T, and finally of the density of the solid products and reactants leading to a change in the internal porosity and hence in the diffusivities and conductivities inside the grain as the reaction progresses. Very few authors have tackled this problem which is however fundamental if we are to model the transformation of reactors; the very basic models remain dependent on the nature of the reactant and on the type of reversible reaction [10]; it is possible to take into account successive reactions with a more elaborate model, but this is only valid in given zones of P and T [11]. Finally, a model taking into account the evolution of the grain size during the reaction is satisfactory for several structures of chlorides over a very wide range of P and T [12]; however, it assumes an isothermal transformation of the grain. Thiele's module ~b2, which allows the relative significance of the limitations due to pure chemical dynamics and to mass diffusivity to be characterized, varies during the advancement of the reaction, from a generally low value (pure chemical boundary) to a higher value (mainly diffusional boundary) for synthesis reactions, remaining low for decomposition reactions. These approaches are totally different from those used in adsorption/desorption where it is easier to identify the parameters of models of the Dubinin type for example, since the measurements are obtained under thermodynamic equilibrium. Zones of reduced reaction rates in the desorption dynamics of ammoniates have been observed by several authors [10, 13, 14]. This can be very troublesome in the implementation of thermochemical processes over a pressure range of 0.5-2 bar. A few teams have tried to understand the origin of these zones of non-monotonous evolution of dynamics as a function of pressure and temperature: they cannot be due to the evolution of the conductivity of the gaseous phase, which is believed to be the cause of the decomposition of the hydrates [15]. Neveu and Spinner's thermodynamic model [16] is satisfactory as far as the experimental validity of its parameters is concerned, but not totally predictive of the P and T delimitation of the reduced rate zone. Finally, the use of synthesis reactions of ammoniates whose equilibrium is close to that of ammonia can lead to ammonia adsorption or to the formation of saturated solutions depending on their position on the P, T diagram. There is still a significant lack of such characterizations,
Ammonia-based thermochemical transformers
303
and the transposition of data from phase diagrams to the Clausius-Clapeyron diagram is very hazardous. 3.2. Evolution of the texture of the ammoniate bed during the reaction. Problems regarding the elaboration of models of the transformation of fixed bed reactors The swelling of ammoniate grains during the synthesis reaction of salts rich in NH3 means that there is a major decrease in the permeability of the fixed bed. Returning to a salt without ammonia does not bring back the ~bed's initial texture. Moreover, since in most cases the reaction used involves ammoniates which after desorption contain ammonia molecules fixed on the chloride (for example, CaCI 2 4-2 NH3, or MnC12 6-2 NH3), and whose texture is very difficult to ascertain, it is clearly very hard to fix the initial mass per unit volume of the reactive bed such that the head losses are low during the synthesis and decomposition reactions, while trying to achieve a significant intergrain contact favourable to heat transfers. The use of binders does improve the values of effective permeability and conductivity and consequently the dynamics of reactor transformations, if one is careful to choose binders able to follow the variation in the grains' volume whenever possible. Heat and mass transfer parameters during the reaction are difficult to measure: this can only be done using purpose-built apparatus [13, 17]. Another problem arises from the intricate modelling of the transformation of the reactive bed: during the reaction, the evolution of the size of the grains confined within the reactor leads to an evolution of the heat and mass transfer parameters in the bed and, by interaction, in the grain. Therefore, depending on the bed's degree of compaction, the evolution of the reactive bed may be limited by the transformation of the grain itself (very low internal diffusivity) or by the mass and heat transfers within the bed. Therefore a two-level model has to be devised. The characterization of a binder capable of allowing continual evolution of the grain size and of permitting a high permeability of the bed while leading to a significant effective conductivity is thus of the utmost importance. 3.3. System-analysis and power characteristics of multi-effect systems Since there is a large number of suitable reactants, there is also a large number of different systems available for the implementation of ammoniates in COP and COA processes over ultra-low temperature refrigeration and ultra-high temperature heat production ranges. These systems involve the inter-connection of solid-gas reactors by the gaseous phase or by the heat transfer fluid, by the transformation of reactants of different nature within a single enclosure subjected to P and T, or by the more complex inter-connection of 4 reactors linked by gaseous phases or heat transfer fluids. Whereas inter-connection by heat transfer fluids is fundamental to multi-effect systems in adsorption, inter-connection by the gaseous phase, on the other hand, is new for systems using ammoniates in the reaction. The problem of modelling the evolution of such complex systems lies in the degree of sophistication of sub-system models. Experimental validations are required with each sub-system in order to select the simplifying hypotheses. Moreover, these sub-system models have to be adequately precise if their coupling is to represent the very unsteady evolution of the complex process. 4. PRESENT STATE OF RESEARCH AND DEVELOPMENT 4.1. Thermodynamic and kinetic data The choice of thermochemical systems is based on the use of a large number of couples whose thermodynamic characteristics were established long ago and originated from calorimetric measurements of the reaction enthalpy at a given pressure: all these characteristics have to be reassessed since they are usually incorrect. Recent works are more reliable [7-9 and 14], especially when measurements of reaction dynamics were carded out in wide domains of synthesis and decomposition. Solid-gas pairs other than the usual halides are suggested by Rockenfeller [4] in the low temperature domain, Bougard [18] and Touzain [19] for placing a reactant at a selected position
304
B. SPINNER Time gained in % 50 30 20 1
0,3 0,6
1,2 2,4 4,8 9,6 19,2 38,4 )¢ reference (W/m.°K) Fig. 1. Influenceof conductivityand of grain dynamics.Dark hatching--relativetimegain by multiplying conductivity by 2. Light hatching--relative time gain by increasing the grain transformationrate by a factor of 2. in the Clausius-Clapeyron diagram, and finally Marty [20] for systems based on the crossing-over of ammoniate equilibrium lines in the Clausius-Clapeyron diagram [1, 21]. There have been few in-depth studies of grain dynamics despite the utmost importance of their characteristics and modelling. Hosatte-Ducassy [9] measures the dynamics at the start of the reactions in order to characterize pseudo-equilibria, Goetz and Marty [12] model the transformation of grains subjected to variable P and T using micro-calorimetry: this model is valid for chlorides such as BaC12 8-0 NH2, MnC12 and NiCi 2 6-2 NH 3. 4.2. Implementation of fixed bed solid-gas reactors. Modelling their transformations There have been various attempts at solving the problem raised by the evolution of the size of grains of reactant during the progression of the synthesis and decomposition reactions, and consequently by the evolution of the bed's texture which has still to maintain excellent heat and mass transfer ceofficients: (i) implementation of fixed beds without binder in mobile wall reactors. This solution recommended by Rockenfeller [22] seems difficult to implement and leads only to very low heat and mass transfer coefficient values; (ii) crystallization on the exchanger walls of chlorides reacting with the gas in order to allow good contact with the wall on the one hand, and the continuity of intergrain contacts on the other hand. However, it seems that Doi and Ikeuchi's [23] heat transfer coefficient values are not high; (iii) mixture of chlorides with lithium nitrate which liquefies when reacting with NH3: Woerse-Schmidt's [24] results are of interest but the possibility of creating a pasty or liquid binder remains limited to use at temperatures below 150°C; (iv) implementation of ammoniates intercalated in graphite. Spinner and Touzain [25] are studying this very elegant solution which is valid for different chlorides such as MnCI2, MgCI2, NiC12. These insertion sub-strata take the form of grains with the usual problems of their
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Ammonia-based thermochemical transformers
305
implementation in beds: contact resistance and exchange coefficient with the wall. However, their intercalation in fibres [19] is a new and promising solution; (v) mixing the reactant with an expanded graphite binder, with the right elastic characteristics to maintain the contacts between grains of variable size and vermicules of binder [26]. The conductivities, exchange coefficients at the wall, and permeabilities of such mixtures depend on the proportions and conditions of implementation [11, 13, 17]. The impregnation of the reactant in consolidated blocks of expanded natural graphite [27] retains the strongly anisotropic characteristics of natural graphite while keeping a very porous bed with low radial head loss (perpendicular to compression): the radial conductivity of such composite beds is remarkable (> 10 W m-1 K-J), as is their exchange coefficient at the wall (> 500 W m -2 K -l) which can be compared to the solutions adopted in the case of hydrides and adsorbants [28]. Depending on the nature of the solution adopted, the use of these reactants, mixtures or blocks leads to reactors of different forms whose evolution depends on the limitations of one or more of the intrinsic reaction dynamics, intra and inter-grain mass transfers, heat transfers in the grains and in the bed, and exchange coefficients with the wall both on the reactant side and on the heat transfer fluid side. Under the closed protocol, i.e. with interruption of the link between the reactor and the condenser (evaporator), as the evolution of the reactor starts, it is limited by chemical dynamics whatever the set-up: the consumption of the bed's sensible heat produces a quick reaction. Thereafter, the evolution of the reactor only depends on mass and heat transfer [12]. Under the open protocol, i.e. without interruption of the link, only the second evolution is observed: the relative importance of the dynamics of grain transformation and of heat transfers (in this case, the conductivity), in the absence of limitation by mass transfer, has been determined by Goetz and Marty [12] with a model experimentally validated on variable thickness reactors. The use of highly conductive blocks (even over 10 cm thick) with high exchange coefficients at the wall (radial permeability remaining non-limitative) poses the problem of the nature of the external exchangers: the more practical use of heat transfer fluids must be abandoned in favour of heat pipes [29]. Thus, mean power levels of reactor transformation of more than 1 kW kg- ~ of reactant can be reached [12, 28] and exceeded if the deviation from thermodynamic equilibrium imposed between the equilibrium and heat transfer fluid temperatures is high (>30°C); the global exchange coefficient on the reactant side varies from 200 to 4 0 0 W m - 2 C -l for reactants of 5-15 cm in diameter and of variable degrees of consolidation [30]. 4.3. Analysis of multi-effect systems and performance of laboratory pilot systems Beyond the basic system involving two reactors, a condenser and an evaporator for continuous refrigeration and/or heat production, whose possibilities, performances in terms of COP and irreversibility were analysed in ref. [31], of which the performances in terms of COP and COA (in refrigeration at 5°C, COP <0.4) have been measured [32] by experimental investigation using a pilot system of 25 kW of refrigeration (50 kW of heat), three types of systems are considered to increase the performance: (i) single-effect reactor 1/reactor 2 system (without condenser-evaporator). These systems usually use hydrides. The performance of a laboratory pilot system shows that 60% of the theoretical coefficient can be reached very easily. Experimental COP = 0.42 for the couples BaCI2-MnC12 with NH3, which are not the best adapted to refrigeration: what was aimed at was an understanding of how to control the system; (ii) double-and triple-effect systems. Interaction between two reactors exchanging their reaction heat during one of the phases leads to a noticeable improvement in the process performance [34]. The choice of reactants is important if the system is to reach significant deviations from equilibrium, and consequently economically viable power levels per unit mass or volume. Designing a system with three types of reactor [35], requires in-depth modelling and validation of each subsystem (see 3.3). The experimentation of a double-effect system with condenser-evaporator (rather than reactors RI and RI') was carried out with the couples MnCl2 and NiCI2: practical deviations from thermodynamic equilibrium of the order of 30-35 ° are compatible with the operation of a machine with a power output of about 10 kW [30] whose COP when refrigerating at 10°C is 67% of the
306
B. SPINNER
theoretical value, i.e. 0.45; this low value is due to the use of NiCI 2 whose reaction enthalpy is very high (59.2 kJ mol -j NH3). From simulation using the model validated by this experiment and experimentation with the reactor l/reactor 2 single-effect system, in refrigeration at 0°C the COP of the double-effect machine with 4 interconnected reactors described above is of the order of 1-1. l, which indicates a good match between the reactants used and the objective. Following the same principle, but with double recovery of reaction heat (triple-effect) involving the use of three types of salts coupled with a condenser-evaporator (i.e. three interconnected solid-gas reactors), Rockenfeller [36] expects to reach similar COP values (1-1.1); though thermodynamically possible, such a machine will only operate under very low deviations from thermodynamic equilibrium and it will have to be of considerable size. The double-effect system seems to be a realistic limit especially when it associates 3 types of solid reactants. A double-effect system associating three types of solids at three pressure levels is advocated in the case of hydrides. Its use with ammoniates does not seem realistic for the same reason as above. (iii) multi-salt reactor systems. Heat front reactors from adsorption systems [28] inspired Rockenfeller and Kirol [39], Neveu and Castaing [40] to propose a system integrating several salts in a single reactor; two such reactors are alternatively in the synthesis and decomposition phase and are linked together by the heat transfer fluid. The COP of this type of system depends on the practical number of reactants that can be used, and on the quality of the heat front which has nevertheless to be created in each reactant. Nothing has as yet been published about these experiments. Finally, it is worth noting that it is with ammoniates that thermotransformation is particularly promising: such a machine, with two pressure stages, has recently been demonstrated on a pilot system with a power output of 5-7 kW: temperature step-up from ll5 to 165°C, then from 160 to 220°C with the couples MnC12 and NiCl2 [30]. 4.4. Development in industry The field of refrigeration is the main target of industrialists interested in systems using reaction with ammonia. It is in particular the possibility of refrigerating at - 30°C which is aimed at, since the COPs of the new machines are either similar to or higher than the COPs of traditional machines, in terms of primary energy. The advantage is obvious in the case of recycled energy. Industrialists are very interested in the storage function in order to reduce peak demand for air conditioning, or for refrigerated transport. Finally, the simplicity of reactor l/reactor 2 single-effect systems is also an asset for processes intended for widespread distribution. The energy context is not favourable to the development of thermotransformation, though the financial incidence of taxes envisaged for uncontrolled CO2 production may change this. In contrast, simultaneous low temperature refrigeration and high temperature heat production (over 60°C) is of greater economic interest. Some industrial pilot systems are being developed and others are already operational, mostly in France. 5. C O N C L U S I O N There is no scientific or technological obstacle to interfere with t h e d e v e l o p m e n t of ammoniabased thermochemical transformers. The texture variation in the grains and reactive b e d s c a n be used positively to improve heat transfer, the main limiting factor in the evolution of these reactors. The choice of the best s y s t e m s h a s still to b e m a d e : s o m e s o p h i s t i c a t i o n in the c o n t r o l s y s t e m of the machine leads to improved performance but a l s o t o costs which have yet to be determined. Efforts must be continued in the field of research, but a l s o a n d a b o v e all in the field of industrial development. REFERENCES I. B. Spinner, Pompes ~ chaleur bas~es sur la rracfion renversable entre un gaz et un solide ou un liquide ou une solution satur~e. R~cents Progr~s en G~nie des Proc~d~s 2, 222 (1988). 2. R. de Hartoulari, Etude des r~actions entre rammoniac et les sels mrtalliques. Bull. Soc. Chim. F. 10, 1849 (1960).
Ammonia-based thermochemical transformers
307
3. M. Balat, A. Roca and B. Spinner, Proctd6 de conduite d'une rtaction d'absorption ou de dtsorption entre un gaz et un solide. Patent FR 8712389 17 September (1987). 4. U. Rockenfeller, System for low temperature refrigeration and chill storage using ammoniated complex compounds. U.S. Patent 4848994 29 February (1988). 5. S. Mauran, D. Bodiot and G. Crozat, Optimisation des densitts 6nergttiques de systtmes de stockage chimique basts sur des rtactions solide-gaz renversables. Rev. Phys. Appl. 18, 107 (1983). 6. I. Park and K. S. Knaebei, Adsorber dynamics: stress associated with swelling and shrinkage. Workshop Adsorption Processes for Gas Separation. Orsay, France, September (1991). 7. Ffirrer, EIR Bericht 392, 8 (1980). 8. A. Maxty, Etude par microcalorimttrie de la rtactivit6 de deux ammoniacates de chlorure de mangantse. J. therm Anal. 37, 479 (1991). 9. S. Hosate-Ducassy, Etude cinttique et modtle de simulation numtrique des rtactions solid-gaz pour les pompes fi chaleur chimiques. Thesis, Universit6 de Valencienne, France, November (1989). 10. M. Moutaabbib, Pompes fi chaleur chimiques et rtfrigtration solaire. Thesis, Universit6 de Bourgogne, France (1986). 11. N. Mazet, M. Amouroux and B. Spinner, Analysis and experimental study of the transformation of a non isothermal solid/gas reacting medium. Chem. Eng. Comm. 99, 155 (1991). 12. V. Goetz and A. Marty, A model for reversible solid-gas reactions submitted to temperature and pressure constraints; simulation of the rate of reactions in solid-gas reactor used in chemical heat pumps. Chem. Eng. Sci 47, (1992). 13. S. Mauran, Flux de chaleur en milieu poreux rtactif dtformable. Relations entre texture, proprittts, mtcaniques et transferts. Thesis, Universit6 de Perpignan, France, December (1990). 14. A. Marry, Cinttiques de transformation de BaCI2 8-0 NH3. Personal communication. 15. G. Bertrand, M. Lallemant, A. Mokhlisse and G. Watelle-Marion, Abnormal variation of the rate of the composition of a solid. J. Inorg. Nucl. Chem. 40, 819 (1978). 16. P. Neveu and B. Spinner. Modtlisation des zones de ralentissement de transformation gaz-phase condenst¢ fi partir de la caracttrisation de ia temptrature 6volutive de la zone rtactionnelle. J. chim. Phys. 87, 1375 (1990). 17. J. L. Ores, Identification des paramdtres thermiques d'un milieu poreux rtactif dtformable. Revue Phys. Appl. 30, 394 (1991). 18. Bougard, Report of CEE JOUE 38 contract, Research of solid-gas reacting media and of intercalt, compounds for thermochemical heat pumps, September (1991). 19. P. Touzain, Report CEE JOUE 38 contract, September (1991). 20. A. Marry, Personal communication, IMP-CNRS. 21. D. Payre, N. Mazet, S. Mauran and B. Spinner: Proc&i6 et dispositif thermochimique de stockage et dtstockage de chaleur. Patent FR 8508408, 4 June (1985). 22. U. Rockenfeller, Method and appartus of achieving high reaction rates in solid-gas reactor system. US Patent 90-01171 8 March (1989). 23. A.Doi and M. Ikeuchi, Reactor. Patent JP 64-51142 21 August (1987). 24. P. Woerse-Schmidt, Some results from the development of a solid absorption refrigerating system. Proc. Absorption Expert Meeting, Paris (1985). 25. B. Spinner and P. Touzain, Applications of exfoliated graphite and graphite intercalation compounds in order to improve the performances of thermochemical energy conversion processes. Workshop Carbone, Paris, July (1990). 26. C. Coste, S. Mauran and G. Crozat, Proctd6 de mise en oeuvre de r~action gaz-solide. U.S. Patent 4595774 15 June (1983). 27. S. Mauran, M. Lebrun, P. Prades, M. Moreau, B. Spinner and C. Drapier, Composite actif et proctd~ de mise en eouvre de processus physico-chimiques gaz-solide ou liquide~gaz utilisant comme milieu rtactionnel un tel composite actif. Patent FR 91-0303 11 April (1991). 28. M. Groll, Reaction beds for dry sorption machines. Heat Recovery Systems & ClIP 13, 341-346 (1993). 29. M. Lebrun, S. Mauran and B. Spinner, Dispositif pour produire du froid et/ou de la chaleur par r~action solide/gaz gtrts par caloducs gravitationnels. Patent FR 8913913 13 October (1989). 30. B. Spinner, Report of ADEME-SNEA-CNRS contract, October (1992). 31. V. Goetz, F. Elie and B. Spinner, The structure and performance of single solid-gas chemical heat pumps. Heat Recovery Systems and CHP 13, 79-96 (1993). 32. P. Neveu, Conception, simulation, dimensioning and testing of an experimental chemical heat pump. ASHRAE Trans. 98, (1992). 33. E. Lepinasse, Mod~lisation et validation exp~rimentale sur pilote de 1-2 kW de thermotransformation chimique solide-gaz sans phase condense. Thesis, Universit6 de Perpignan, France, November (1992). 34. P. Neveu and J. Castaing, Solid/gas chemical heat pumps: field of application and performance of the internal heat of reaction recovery process. Heat Recovery Systems and ClIP 13, 233-251 (1993). 35. M. Lebrun, S. Mauran and B. Spinner: Dispositifs pour la production du froid et/ou de la chaleur par r~action solide-gaz. Patent FR 895268 11 January (1989). 36. U. Rockenfeller, Discrete constant pressure staging of solid-vapor compound reactors. U.S. Patent 5079928 7 June (1991). 37. U. Rockenfeller and T. R. Roose, Gas fired complex compound heat pump. In Proc. lntersoc. Energy Cony. Eng. Conf. 23, 321 (1988). 38. S. V. Shelton, Solid adsorbent heat pump system. U.S. Patent 4610148 9 September (1986); Dual bed heat pump. U.S. Patent 4694659 22 September (1987). 39. U. Rockenfeller and L. D. Kirol, Continuous constant pressure staging of solid-vapor compound reactors. U.S. Patent 9006392 14 November (1989). 40. P. Neveu and J. Castaing: Dispositif pour la production de froid et/ou de chaleur par rtaction solid-gaz. Patent FR9201680 14 February (1992).
HRS 13/4--C
0890-4332]93 $6.00+ .00 © 1993 Pergamon Press Ltd
Printed in Great Britain
AMMONIA-BASED
THERMOCHEMICAL
TRANSFORMERS
B, Spilt,mR CNRS-IMP (Institut de Science et G~nie des Mat~riaux et Proeakt6s), Laboratoire EuroI~n Associ~SIMAP (Science et Ing~nierie des Mat6fiaux et Proc~d~s), Universit6, 66860 Perpignan Cedex, France (Received 5 February 1993)
Almtraet--The implementation of thermochemicat systems based on the reaction between chlorides and ammonia involves certain unusual aspects of thermodynamics and reactivity as well as peculiarities in the evolution of the grain texture and of the beds during the reaction. It is the evolution of the beds that is the most unusual, calling for appropriate solutions involving new implementations of porous beds with optimal characteristics of elasticity and heat and mass transfer. The choice of either multi-effect or multi-salt systems depends on the experimental performances of laboratory pilot systems currently under test or development, and on the real costs derived from experimentation with industrial prototypes.
1. I N T R O D U C T I O N
Thermodynamic systems based on the thermicity of reversible reactions between salts and ammonia are in their first phase of industrial development, mainly in the form of pilot systems, because the chemistry involved is very classic, ammonia is a well known refrigerating fluid which is well controlled at the industrial level and advocated by the International Refrigeration Institute as a substitute for CFCs and HCFCs. The ammonia-NH3 couples were initially used in storage systems and then in continuous refrigeration and/or heat production systems. The 1987 symposium on chemical heat pumps [1] covered the field of current research on solid sorption machines. The particularities, the scientific problems of research on systems using ammonia are studied in greater detail in order to access their up-to-dateness as well as their industrial development. 2. F E A T U R E S
OF
SYSTEMS
BASED WITH
ON THE THERMICITY AMMONIA
OF
REACTIONS
Systems based on the use of salts reacting with ammonia are monovariant under thermodynamic equilibrium. This variance is also observed in systems based on a reaction with water (hydrates such as the Na2S-H20 system), and over a wide range of reactivity in hydrides (the horizontal plateau in the graph of P = f ( X ) at constant temperature): thus in the Clausius-Clapeyron diagram, it is represented by lines marking the boundaries of decomposition reaction zones (on the right) and of synthesis reaction zones (on the left), in contrast with the families of isosteric lines, characteristic of gas-solid adsorption systems [1]. Ammoniates or amino-derivatives (reaction with ammonia derivatives such as monomethylamine or dimethylamine) are many and varied: alkaline, alkaline-earth or metallic halides, double or mixed halides, sulphates, nitrates, phosphates [1-4], They react over very wide ranges of pressure (up to 50 bar), and of temperature ( - 50-300°C), and depending on the number of moles of gas reacting per mole of salt compound, their energy density or storage capacity (J mol-1 or W h kg-i) can be very high (510Whkg -1 in the case of NiCI2 6/2NH3). Generally speaking, these ammoniates are very stable: there are no secondary reactions: it is well known that their chemistry is simple. With such a wealth of reactants, it is possible to implement thermochemical systems for low temperature refrigeration (down to -50°C by evaporation of ammonia), multi-effect systems, systems without condensation/evaporation of ammonia, and finally thermotransformers able to bring the thermal potential of a source up to 250°C. On the other hand ammoniates present significant changes in molar volumes during the synthesis and decomposition reactions: for example, the Mauran et al. [5] study leads to the definition of compaction limits for chlorides. Many research teams consider this feature to be such an obstacle 301
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that they have discarded this type of thermochemical system. However it must be underlined that such molar volume changes are to be found, albeit to a lesser extent, in hydride and adsorption systems [6], and that interesting solutions have been found to deal with this, such as using these variations of molar volume to improve the contact coefficients between grains and binder, and therefore the heat transfer coefficients.
3. S C I E N T I F I C PROBLEMS S P E C I F I C TO A M M O N I A SYSTEMS The features of thermochemical systems based on the implementation of ammoniates lead to the definition of unique scientific problems linked to:
3.1. Thermodynamics and trans[ormation dynamics of solid-ammonia couples The precise positioning of equilibrium lines is hard to obtain experimentally, since it depends on the experimental protocols used: isobar or isothermal scales, enthalpic measurements at a given pressure; there are several degrees of difference in the data depending on the authors. In practice, the exact position of the equilibrium line is not absolutely fundamental: in fact; it lies within a pseudo-equilibrium zone which delimits the practical domain of synthesis and decomposition of the ammoniate considered: depending on the sensitivity of the apparatus used to obtain measurements (thermogravimetry, differential thermal analysis or microcalorimetry), the authors' results are this time very similar [7-9]. The origin of these pseudo-equilibria, whose value in AT, at a given P, can reach more than 10°, is not yet fully understood: it is commonly accepted that, although they have not been identified, low reactive phases exist. The characterization of these domains of pseudo-equilibrium is very important when choosing a pair for a specific system, and for modelling the dynamics of grain transformation and crystal construction, without external limitations on mass and heat transfer. Indeed, it is not easy to choose the structure of this model: the dynamics of grain transformation must take into account the evolution of the reactive surface which decreases as the reaction develops, of the two thermodynamic constraints, P and T, and finally of the density of the solid products and reactants leading to a change in the internal porosity and hence in the diffusivities and conductivities inside the grain as the reaction progresses. Very few authors have tackled this problem which is however fundamental if we are to model the transformation of reactors; the very basic models remain dependent on the nature of the reactant and on the type of reversible reaction [10]; it is possible to take into account successive reactions with a more elaborate model, but this is only valid in given zones of P and T [11]. Finally, a model taking into account the evolution of the grain size during the reaction is satisfactory for several structures of chlorides over a very wide range of P and T [12]; however, it assumes an isothermal transformation of the grain. Thiele's module ~b2, which allows the relative significance of the limitations due to pure chemical dynamics and to mass diffusivity to be characterized, varies during the advancement of the reaction, from a generally low value (pure chemical boundary) to a higher value (mainly diffusional boundary) for synthesis reactions, remaining low for decomposition reactions. These approaches are totally different from those used in adsorption/desorption where it is easier to identify the parameters of models of the Dubinin type for example, since the measurements are obtained under thermodynamic equilibrium. Zones of reduced reaction rates in the desorption dynamics of ammoniates have been observed by several authors [10, 13, 14]. This can be very troublesome in the implementation of thermochemical processes over a pressure range of 0.5-2 bar. A few teams have tried to understand the origin of these zones of non-monotonous evolution of dynamics as a function of pressure and temperature: they cannot be due to the evolution of the conductivity of the gaseous phase, which is believed to be the cause of the decomposition of the hydrates [15]. Neveu and Spinner's thermodynamic model [16] is satisfactory as far as the experimental validity of its parameters is concerned, but not totally predictive of the P and T delimitation of the reduced rate zone. Finally, the use of synthesis reactions of ammoniates whose equilibrium is close to that of ammonia can lead to ammonia adsorption or to the formation of saturated solutions depending on their position on the P, T diagram. There is still a significant lack of such characterizations,
Ammonia-based thermochemical transformers
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and the transposition of data from phase diagrams to the Clausius-Clapeyron diagram is very hazardous. 3.2. Evolution of the texture of the ammoniate bed during the reaction. Problems regarding the elaboration of models of the transformation of fixed bed reactors The swelling of ammoniate grains during the synthesis reaction of salts rich in NH3 means that there is a major decrease in the permeability of the fixed bed. Returning to a salt without ammonia does not bring back the ~bed's initial texture. Moreover, since in most cases the reaction used involves ammoniates which after desorption contain ammonia molecules fixed on the chloride (for example, CaCI 2 4-2 NH3, or MnC12 6-2 NH3), and whose texture is very difficult to ascertain, it is clearly very hard to fix the initial mass per unit volume of the reactive bed such that the head losses are low during the synthesis and decomposition reactions, while trying to achieve a significant intergrain contact favourable to heat transfers. The use of binders does improve the values of effective permeability and conductivity and consequently the dynamics of reactor transformations, if one is careful to choose binders able to follow the variation in the grains' volume whenever possible. Heat and mass transfer parameters during the reaction are difficult to measure: this can only be done using purpose-built apparatus [13, 17]. Another problem arises from the intricate modelling of the transformation of the reactive bed: during the reaction, the evolution of the size of the grains confined within the reactor leads to an evolution of the heat and mass transfer parameters in the bed and, by interaction, in the grain. Therefore, depending on the bed's degree of compaction, the evolution of the reactive bed may be limited by the transformation of the grain itself (very low internal diffusivity) or by the mass and heat transfers within the bed. Therefore a two-level model has to be devised. The characterization of a binder capable of allowing continual evolution of the grain size and of permitting a high permeability of the bed while leading to a significant effective conductivity is thus of the utmost importance. 3.3. System-analysis and power characteristics of multi-effect systems Since there is a large number of suitable reactants, there is also a large number of different systems available for the implementation of ammoniates in COP and COA processes over ultra-low temperature refrigeration and ultra-high temperature heat production ranges. These systems involve the inter-connection of solid-gas reactors by the gaseous phase or by the heat transfer fluid, by the transformation of reactants of different nature within a single enclosure subjected to P and T, or by the more complex inter-connection of 4 reactors linked by gaseous phases or heat transfer fluids. Whereas inter-connection by heat transfer fluids is fundamental to multi-effect systems in adsorption, inter-connection by the gaseous phase, on the other hand, is new for systems using ammoniates in the reaction. The problem of modelling the evolution of such complex systems lies in the degree of sophistication of sub-system models. Experimental validations are required with each sub-system in order to select the simplifying hypotheses. Moreover, these sub-system models have to be adequately precise if their coupling is to represent the very unsteady evolution of the complex process. 4. PRESENT STATE OF RESEARCH AND DEVELOPMENT 4.1. Thermodynamic and kinetic data The choice of thermochemical systems is based on the use of a large number of couples whose thermodynamic characteristics were established long ago and originated from calorimetric measurements of the reaction enthalpy at a given pressure: all these characteristics have to be reassessed since they are usually incorrect. Recent works are more reliable [7-9 and 14], especially when measurements of reaction dynamics were carded out in wide domains of synthesis and decomposition. Solid-gas pairs other than the usual halides are suggested by Rockenfeller [4] in the low temperature domain, Bougard [18] and Touzain [19] for placing a reactant at a selected position
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0,3 0,6
1,2 2,4 4,8 9,6 19,2 38,4 )¢ reference (W/m.°K) Fig. 1. Influenceof conductivityand of grain dynamics.Dark hatching--relativetimegain by multiplying conductivity by 2. Light hatching--relative time gain by increasing the grain transformationrate by a factor of 2. in the Clausius-Clapeyron diagram, and finally Marty [20] for systems based on the crossing-over of ammoniate equilibrium lines in the Clausius-Clapeyron diagram [1, 21]. There have been few in-depth studies of grain dynamics despite the utmost importance of their characteristics and modelling. Hosatte-Ducassy [9] measures the dynamics at the start of the reactions in order to characterize pseudo-equilibria, Goetz and Marty [12] model the transformation of grains subjected to variable P and T using micro-calorimetry: this model is valid for chlorides such as BaC12 8-0 NH2, MnC12 and NiCi 2 6-2 NH 3. 4.2. Implementation of fixed bed solid-gas reactors. Modelling their transformations There have been various attempts at solving the problem raised by the evolution of the size of grains of reactant during the progression of the synthesis and decomposition reactions, and consequently by the evolution of the bed's texture which has still to maintain excellent heat and mass transfer ceofficients: (i) implementation of fixed beds without binder in mobile wall reactors. This solution recommended by Rockenfeller [22] seems difficult to implement and leads only to very low heat and mass transfer coefficient values; (ii) crystallization on the exchanger walls of chlorides reacting with the gas in order to allow good contact with the wall on the one hand, and the continuity of intergrain contacts on the other hand. However, it seems that Doi and Ikeuchi's [23] heat transfer coefficient values are not high; (iii) mixture of chlorides with lithium nitrate which liquefies when reacting with NH3: Woerse-Schmidt's [24] results are of interest but the possibility of creating a pasty or liquid binder remains limited to use at temperatures below 150°C; (iv) implementation of ammoniates intercalated in graphite. Spinner and Touzain [25] are studying this very elegant solution which is valid for different chlorides such as MnCI2, MgCI2, NiC12. These insertion sub-strata take the form of grains with the usual problems of their
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/
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e
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Ammonia-based thermochemical transformers
305
implementation in beds: contact resistance and exchange coefficient with the wall. However, their intercalation in fibres [19] is a new and promising solution; (v) mixing the reactant with an expanded graphite binder, with the right elastic characteristics to maintain the contacts between grains of variable size and vermicules of binder [26]. The conductivities, exchange coefficients at the wall, and permeabilities of such mixtures depend on the proportions and conditions of implementation [11, 13, 17]. The impregnation of the reactant in consolidated blocks of expanded natural graphite [27] retains the strongly anisotropic characteristics of natural graphite while keeping a very porous bed with low radial head loss (perpendicular to compression): the radial conductivity of such composite beds is remarkable (> 10 W m-1 K-J), as is their exchange coefficient at the wall (> 500 W m -2 K -l) which can be compared to the solutions adopted in the case of hydrides and adsorbants [28]. Depending on the nature of the solution adopted, the use of these reactants, mixtures or blocks leads to reactors of different forms whose evolution depends on the limitations of one or more of the intrinsic reaction dynamics, intra and inter-grain mass transfers, heat transfers in the grains and in the bed, and exchange coefficients with the wall both on the reactant side and on the heat transfer fluid side. Under the closed protocol, i.e. with interruption of the link between the reactor and the condenser (evaporator), as the evolution of the reactor starts, it is limited by chemical dynamics whatever the set-up: the consumption of the bed's sensible heat produces a quick reaction. Thereafter, the evolution of the reactor only depends on mass and heat transfer [12]. Under the open protocol, i.e. without interruption of the link, only the second evolution is observed: the relative importance of the dynamics of grain transformation and of heat transfers (in this case, the conductivity), in the absence of limitation by mass transfer, has been determined by Goetz and Marty [12] with a model experimentally validated on variable thickness reactors. The use of highly conductive blocks (even over 10 cm thick) with high exchange coefficients at the wall (radial permeability remaining non-limitative) poses the problem of the nature of the external exchangers: the more practical use of heat transfer fluids must be abandoned in favour of heat pipes [29]. Thus, mean power levels of reactor transformation of more than 1 kW kg- ~ of reactant can be reached [12, 28] and exceeded if the deviation from thermodynamic equilibrium imposed between the equilibrium and heat transfer fluid temperatures is high (>30°C); the global exchange coefficient on the reactant side varies from 200 to 4 0 0 W m - 2 C -l for reactants of 5-15 cm in diameter and of variable degrees of consolidation [30]. 4.3. Analysis of multi-effect systems and performance of laboratory pilot systems Beyond the basic system involving two reactors, a condenser and an evaporator for continuous refrigeration and/or heat production, whose possibilities, performances in terms of COP and irreversibility were analysed in ref. [31], of which the performances in terms of COP and COA (in refrigeration at 5°C, COP <0.4) have been measured [32] by experimental investigation using a pilot system of 25 kW of refrigeration (50 kW of heat), three types of systems are considered to increase the performance: (i) single-effect reactor 1/reactor 2 system (without condenser-evaporator). These systems usually use hydrides. The performance of a laboratory pilot system shows that 60% of the theoretical coefficient can be reached very easily. Experimental COP = 0.42 for the couples BaCI2-MnC12 with NH3, which are not the best adapted to refrigeration: what was aimed at was an understanding of how to control the system; (ii) double-and triple-effect systems. Interaction between two reactors exchanging their reaction heat during one of the phases leads to a noticeable improvement in the process performance [34]. The choice of reactants is important if the system is to reach significant deviations from equilibrium, and consequently economically viable power levels per unit mass or volume. Designing a system with three types of reactor [35], requires in-depth modelling and validation of each subsystem (see 3.3). The experimentation of a double-effect system with condenser-evaporator (rather than reactors RI and RI') was carried out with the couples MnCl2 and NiCI2: practical deviations from thermodynamic equilibrium of the order of 30-35 ° are compatible with the operation of a machine with a power output of about 10 kW [30] whose COP when refrigerating at 10°C is 67% of the
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theoretical value, i.e. 0.45; this low value is due to the use of NiCI 2 whose reaction enthalpy is very high (59.2 kJ mol -j NH3). From simulation using the model validated by this experiment and experimentation with the reactor l/reactor 2 single-effect system, in refrigeration at 0°C the COP of the double-effect machine with 4 interconnected reactors described above is of the order of 1-1. l, which indicates a good match between the reactants used and the objective. Following the same principle, but with double recovery of reaction heat (triple-effect) involving the use of three types of salts coupled with a condenser-evaporator (i.e. three interconnected solid-gas reactors), Rockenfeller [36] expects to reach similar COP values (1-1.1); though thermodynamically possible, such a machine will only operate under very low deviations from thermodynamic equilibrium and it will have to be of considerable size. The double-effect system seems to be a realistic limit especially when it associates 3 types of solid reactants. A double-effect system associating three types of solids at three pressure levels is advocated in the case of hydrides. Its use with ammoniates does not seem realistic for the same reason as above. (iii) multi-salt reactor systems. Heat front reactors from adsorption systems [28] inspired Rockenfeller and Kirol [39], Neveu and Castaing [40] to propose a system integrating several salts in a single reactor; two such reactors are alternatively in the synthesis and decomposition phase and are linked together by the heat transfer fluid. The COP of this type of system depends on the practical number of reactants that can be used, and on the quality of the heat front which has nevertheless to be created in each reactant. Nothing has as yet been published about these experiments. Finally, it is worth noting that it is with ammoniates that thermotransformation is particularly promising: such a machine, with two pressure stages, has recently been demonstrated on a pilot system with a power output of 5-7 kW: temperature step-up from ll5 to 165°C, then from 160 to 220°C with the couples MnC12 and NiCl2 [30]. 4.4. Development in industry The field of refrigeration is the main target of industrialists interested in systems using reaction with ammonia. It is in particular the possibility of refrigerating at - 30°C which is aimed at, since the COPs of the new machines are either similar to or higher than the COPs of traditional machines, in terms of primary energy. The advantage is obvious in the case of recycled energy. Industrialists are very interested in the storage function in order to reduce peak demand for air conditioning, or for refrigerated transport. Finally, the simplicity of reactor l/reactor 2 single-effect systems is also an asset for processes intended for widespread distribution. The energy context is not favourable to the development of thermotransformation, though the financial incidence of taxes envisaged for uncontrolled CO2 production may change this. In contrast, simultaneous low temperature refrigeration and high temperature heat production (over 60°C) is of greater economic interest. Some industrial pilot systems are being developed and others are already operational, mostly in France. 5. C O N C L U S I O N There is no scientific or technological obstacle to interfere with t h e d e v e l o p m e n t of ammoniabased thermochemical transformers. The texture variation in the grains and reactive b e d s c a n be used positively to improve heat transfer, the main limiting factor in the evolution of these reactors. The choice of the best s y s t e m s h a s still to b e m a d e : s o m e s o p h i s t i c a t i o n in the c o n t r o l s y s t e m of the machine leads to improved performance but a l s o t o costs which have yet to be determined. Efforts must be continued in the field of research, but a l s o a n d a b o v e all in the field of industrial development. REFERENCES I. B. Spinner, Pompes ~ chaleur bas~es sur la rracfion renversable entre un gaz et un solide ou un liquide ou une solution satur~e. R~cents Progr~s en G~nie des Proc~d~s 2, 222 (1988). 2. R. de Hartoulari, Etude des r~actions entre rammoniac et les sels mrtalliques. Bull. Soc. Chim. F. 10, 1849 (1960).
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3. M. Balat, A. Roca and B. Spinner, Proctd6 de conduite d'une rtaction d'absorption ou de dtsorption entre un gaz et un solide. Patent FR 8712389 17 September (1987). 4. U. Rockenfeller, System for low temperature refrigeration and chill storage using ammoniated complex compounds. U.S. Patent 4848994 29 February (1988). 5. S. Mauran, D. Bodiot and G. Crozat, Optimisation des densitts 6nergttiques de systtmes de stockage chimique basts sur des rtactions solide-gaz renversables. Rev. Phys. Appl. 18, 107 (1983). 6. I. Park and K. S. Knaebei, Adsorber dynamics: stress associated with swelling and shrinkage. Workshop Adsorption Processes for Gas Separation. Orsay, France, September (1991). 7. Ffirrer, EIR Bericht 392, 8 (1980). 8. A. Maxty, Etude par microcalorimttrie de la rtactivit6 de deux ammoniacates de chlorure de mangantse. J. therm Anal. 37, 479 (1991). 9. S. Hosate-Ducassy, Etude cinttique et modtle de simulation numtrique des rtactions solid-gaz pour les pompes fi chaleur chimiques. Thesis, Universit6 de Valencienne, France, November (1989). 10. M. Moutaabbib, Pompes fi chaleur chimiques et rtfrigtration solaire. Thesis, Universit6 de Bourgogne, France (1986). 11. N. Mazet, M. Amouroux and B. Spinner, Analysis and experimental study of the transformation of a non isothermal solid/gas reacting medium. Chem. Eng. Comm. 99, 155 (1991). 12. V. Goetz and A. Marty, A model for reversible solid-gas reactions submitted to temperature and pressure constraints; simulation of the rate of reactions in solid-gas reactor used in chemical heat pumps. Chem. Eng. Sci 47, (1992). 13. S. Mauran, Flux de chaleur en milieu poreux rtactif dtformable. Relations entre texture, proprittts, mtcaniques et transferts. Thesis, Universit6 de Perpignan, France, December (1990). 14. A. Marry, Cinttiques de transformation de BaCI2 8-0 NH3. Personal communication. 15. G. Bertrand, M. Lallemant, A. Mokhlisse and G. Watelle-Marion, Abnormal variation of the rate of the composition of a solid. J. Inorg. Nucl. Chem. 40, 819 (1978). 16. P. Neveu and B. Spinner. Modtlisation des zones de ralentissement de transformation gaz-phase condenst¢ fi partir de la caracttrisation de ia temptrature 6volutive de la zone rtactionnelle. J. chim. Phys. 87, 1375 (1990). 17. J. L. Ores, Identification des paramdtres thermiques d'un milieu poreux rtactif dtformable. Revue Phys. Appl. 30, 394 (1991). 18. Bougard, Report of CEE JOUE 38 contract, Research of solid-gas reacting media and of intercalt, compounds for thermochemical heat pumps, September (1991). 19. P. Touzain, Report CEE JOUE 38 contract, September (1991). 20. A. Marry, Personal communication, IMP-CNRS. 21. D. Payre, N. Mazet, S. Mauran and B. Spinner: Proc&i6 et dispositif thermochimique de stockage et dtstockage de chaleur. Patent FR 8508408, 4 June (1985). 22. U. Rockenfeller, Method and appartus of achieving high reaction rates in solid-gas reactor system. US Patent 90-01171 8 March (1989). 23. A.Doi and M. Ikeuchi, Reactor. Patent JP 64-51142 21 August (1987). 24. P. Woerse-Schmidt, Some results from the development of a solid absorption refrigerating system. Proc. Absorption Expert Meeting, Paris (1985). 25. B. Spinner and P. Touzain, Applications of exfoliated graphite and graphite intercalation compounds in order to improve the performances of thermochemical energy conversion processes. Workshop Carbone, Paris, July (1990). 26. C. Coste, S. Mauran and G. Crozat, Proctd6 de mise en oeuvre de r~action gaz-solide. U.S. Patent 4595774 15 June (1983). 27. S. Mauran, M. Lebrun, P. Prades, M. Moreau, B. Spinner and C. Drapier, Composite actif et proctd~ de mise en eouvre de processus physico-chimiques gaz-solide ou liquide~gaz utilisant comme milieu rtactionnel un tel composite actif. Patent FR 91-0303 11 April (1991). 28. M. Groll, Reaction beds for dry sorption machines. Heat Recovery Systems & ClIP 13, 341-346 (1993). 29. M. Lebrun, S. Mauran and B. Spinner, Dispositif pour produire du froid et/ou de la chaleur par r~action solide/gaz gtrts par caloducs gravitationnels. Patent FR 8913913 13 October (1989). 30. B. Spinner, Report of ADEME-SNEA-CNRS contract, October (1992). 31. V. Goetz, F. Elie and B. Spinner, The structure and performance of single solid-gas chemical heat pumps. Heat Recovery Systems and CHP 13, 79-96 (1993). 32. P. Neveu, Conception, simulation, dimensioning and testing of an experimental chemical heat pump. ASHRAE Trans. 98, (1992). 33. E. Lepinasse, Mod~lisation et validation exp~rimentale sur pilote de 1-2 kW de thermotransformation chimique solide-gaz sans phase condense. Thesis, Universit6 de Perpignan, France, November (1992). 34. P. Neveu and J. Castaing, Solid/gas chemical heat pumps: field of application and performance of the internal heat of reaction recovery process. Heat Recovery Systems and ClIP 13, 233-251 (1993). 35. M. Lebrun, S. Mauran and B. Spinner: Dispositifs pour la production du froid et/ou de la chaleur par r~action solide-gaz. Patent FR 895268 11 January (1989). 36. U. Rockenfeller, Discrete constant pressure staging of solid-vapor compound reactors. U.S. Patent 5079928 7 June (1991). 37. U. Rockenfeller and T. R. Roose, Gas fired complex compound heat pump. In Proc. lntersoc. Energy Cony. Eng. Conf. 23, 321 (1988). 38. S. V. Shelton, Solid adsorbent heat pump system. U.S. Patent 4610148 9 September (1986); Dual bed heat pump. U.S. Patent 4694659 22 September (1987). 39. U. Rockenfeller and L. D. Kirol, Continuous constant pressure staging of solid-vapor compound reactors. U.S. Patent 9006392 14 November (1989). 40. P. Neveu and J. Castaing: Dispositif pour la production de froid et/ou de chaleur par rtaction solid-gaz. Patent FR9201680 14 February (1992).
HRS 13/4--C