s 2010132511300011

ENHANCEMENTOFHEATANDMASS TRANSFERIN SOLID GAS SORPTION SYSTEMS

J. K. KIPLAGAT*, R. Z. WANG*,‡, T. X. LI* and R. G. OLIVEIRA†

*Institute of Refrigeration and CryogenicsShanghai Jiao Tong University, 800 Dongchuan Road

Shanghai 200240, P. R. China†Federal University of Santa CatarinaCampus Ararangu�a, 88900-000, Brazil

‡[email protected]

Received 15 March 2011

Accepted 14 September 2011

Published 31 March 2012

Solid gas sorption systems driven by heat have gained much attention due to their energy con-servation and environmental bene¯ts. These sorption machines can be driven by waste heat orrenewable energy source such as solar energy, and can utilize natural working °uids with no GWPand ODP, such as water, methanol, and ammonia. However, poor heat transfer process and slowdi®usion rate of the refrigeration gas in the adsorber have been identi¯ed as the main drawbackslimiting the cooling density performance, and consequently, commercialization of sorption ma-chines. This paper provides a review of techniques that have been applied to enhance heat andmass transfer in solid gas sorption systems. These techniques mainly include the use of materialswith high thermal conductivity, consolidation of adsorbents, and the use of specially designedheat exchangers in the adsorbers. The e®ect of these methods on the coe±cient of performanceand the speci¯c cooling power is also discussed.

Keywords: Adsorption refrigeration; solid�gas reaction; heat transfer; mass transfer; heatexchangers.

1. Introduction

Sorption machines utilize a reversible physisorption

or chemisorption process to deliver useful cooling or

heating, for refrigeration or heat pump applications.

These solid sorption devices contain a porous solid

(the adsorbent) which has the ability to adsorb vapor

(the adsorbate or refrigerant) at ambient tempera-

ture and to desorb it when heated. The operation

principle of sorption system has been discussed

extensively in literature.1,2 The commonly utilized

working pairs are ammoniated salts (chlorides

in connection with ammonia), zeolites (with water),activated carbon (with methanol or ammonia), andmetal hydrides (with hydrogen).3�5

Unlike the conventional compressor-based heator cold producing systems powered by electricity,the use of sorption machines can help in energyconservation and can have direct positive e®ects onthe environment. This is because sorption cycles canbe driven by heat rather than work; hence, they mayreduce CO2 emissions since they can make use ofrenewable energy sources such as solar and biomass,as well as waste heat. In addition, sorption devices

International Journal of Air-Conditioning and RefrigerationVol. 20, No. 1 (2012) 1130001 (16 pages)© World Scienti¯c Publishing CompanyDOI: 10.1142/S2010132511300011

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http://dx.doi.org/10.1142/S2010132511300011

can utilize natural working °uids with no GWP andODP such as water, methanol and ammonia.1

In order to produce useful cooling or heating,sorption system must undergo a thermodynamiccycle where the adsorbent bed is heated duringdesorption phase and cooled to the initial tempera-ture during adsorption phase. Each complete cycleresults in addition and removal of a given amountof heat into/from the adsorption bed as well asadsorption and desorption of refrigerant gas. Themain drawbacks responsible for the low speci¯ccooling power and cooling power density, are slowheat transfer process and poor di®usion of therefrigeration gas within the porous solid reactant.These drawbacks lead to bulky and costly machines.The slow heat transfer occurs due to high thermalresistance along the heat °ow path. There are twomain types of thermal resistances in solid gasadsorption systems.1 The ¯rst one occurs at theinterface between the metal and the adsorbent,which mainly depends on the physical contact ofthe adsorbent material and the wall of the reactor.If the thermal contact between the metal surfaceand the adsorbent materials is not good, a lowcontact heat transfer coe±cient and a high-tem-perature gradient will occur at the interface.

The absence of gas convection at the interfacealso contributes to low heat transfer coe±cient,which in the case of low evaporating pressureworking °uids (such as water or methanol), maybecome extremely low. The second resistance toheat transfer occurs inside the adsorption bed due topoor thermal conductivity of adsorbents. Granularadsorbents have low e®ective thermal conductivitybecause of the high porosity and discontinuity of thesolid material. For example, the e®ective heat con-ductivity of zeolite is about 0:1W=m �K, for ammo-nia salts or active carbon is about 0.3�0:5W=m �K,whereas for metal hydrides is about 1W/m �K.6

Poor mass transfer of the refrigerant gas occursmainly due to low permeability in the reaction bed,which may become more critical in machines utiliz-ing low evaporating pressure refrigerants. The rateof di®usion of the refrigerant gas into and from theadsorbent in°uences the duration needed for adsorp-tion and desorption; and hence the cycle time. It istherefore necessary that an acceptable level of gaspermeability inside the adsorbent bed is attained forhigh system performance.

When evaluating the performance of adsorptionsystems, three performance indicators are usually

considered, viz., the coe±cient of performance(COP), the speci¯c cooling power (SCP), and thecooling power density (CPD). The COP shows howe±cient (according to the ¯rst law of thermodyn-amics) the adsorption machine is in transformingthe heat input into useful cooling or heating output.The SCP indicates how much cooling power can beexpected from a certain mass of adsorbent. LowSCP machines need a large amount of adsorbent toachieve a certain cooling power, which increases thecost of the machine per kW of cooling output. TheCPD is the ratio between the cooling power outputand the volume of the adsorbent. Low CPD ma-chines are bulky, which increases the cost of theadsorber, and limits the installation places.

Poor thermal conductivity of an adsorption bedcauses slow heat addition/removal rates, resultingin slower reaction rates, long cycle times, low SCP,and CPD. For instance, in the exothermic reactionbetween a sorbent and a gas, the heat producedmust be dissipated so as to maintain a low tem-perature in the sorbent, which is necessary for higherreaction rate. If the reactive medium is a poor heatconductor, it is not able to quickly release the heatproduced, and the temperature of the sorbent willrise near to equilibrium condition, which can resultto an almost zero reaction rate.

Considerable e®orts have been done by di®erentresearch groups to enhance the heat transfer insorption machines with the aim of improving speci¯cpower performance to a level where they can com-pete e®ectively with conventional electricity pow-ered systems. In the following sections, we reviewvarious techniques that have been developed andapplied to enhance the heat transfer and masstransfer in solid�gas reactor beds, and discuss theire®ects of the performance on sorption machines.

2. Techniques to Enhance Heat andMass Transfer within the Adsorbent

2.1. Use of materials with highconductivity or high porosity

In the absence of any technique for heat transferenhancement, granular adsorbent beds exhibit lowthermal conductivities and wall heat transfer coef-¯cients. Furthermore, some of the adsorbents, suchas metal chlorides, experience poor mass transferdue to agglomeration of the sorbents after a fewadsorption cycles. Thus, to achieve higher SCP,

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alternatives to enhance heat transfer without deplet-ingmass transfer are necessary; and in this section, wepresent various materials that have been suggestedfor use as additives to enhance heat transfer, masstransfer or both, in sorption beds.

2.1.1. Metal additives and metal inserts

Eltom and Sayigh7 added copper powder into acti-vated carbon and lump wood charcoal, andmeasured the conductivity at temperatures rangingfrom 30�C to 95�C, in the absence of adsorbate. Theimprovement in the conductivity for the twosorbents varied from 2%�25%, according to thesorbent temperature. Higher thermal conductivitywas observed at low sorbent temperature, andsharply decreased at relatively high temperatures.Demir et al.8 tested the e®ect of various metaladditives on the heat transfer rate of unconsolidatedsilica gel. The silica gel granules were mixed withfour di®erent metal pieces: (i) aluminum, (ii) copper,(iii) brass, and (iv) stainless steel (Fig. 1). Resultsshowed that the addition of metal pieces in thegranular adsorbent increased considerably the heattransfer rate in the adsorbent and led to a reductionin the cycle time and an increase in the SCP. Thebed with 15% loading of aluminum particles1.0�2.8mm wide had the highest e®ective thermalconductivity and di®usivity, which were respectively

242% and 157% higher than those of the bed withpure silica gel. The increment in conductivity wasin°uenced by three parameters, which are as follows:loading amount, shape of the metal pieces, andsize of the metal pieces. The authors emphasizedthe need for proper selection of shape and size of themetal additives in order to avoid an increase in themass transfer resistance, and optimization ofthe weight fraction of silica-gel�Al mixture.

Wu et al. used copper nanopowder to enhance theheat transfer performance of silica gel. Compositebricks of an adsorbent consisting of silica gel withcopper nanopowders were prepared by the sol�gelprocess and compressed at 2MPa. A 20% increase inthe thermal conductivity was observed, when com-pared to pure silica gel without compression, and thethermal conductivity also varied according to theamount of copper nanopowder in the adsorbent.The breakthrough curves showed an apparentincrease in the mass transfer resistance with themetal additive, however, high water vapor uptakeswere reported which correlated well with the struc-ture of silica gel. The results showed that the heatand mass transfer properties were reproducible andconsistent, and the water vapor uptake, thermalconductivity, and e®ective intra particle di®usivityvalues are suitable for adsorption refrigerationsystems.9

(a) (b)

(c) (d)

Fig. 1. Photographs of metal pieces used to enhance thermal conductivity of silica gel: (a) Copper, (b) aluminum, (c) stainless steel,and (d) brass.8

Enhancement of Heat and Mass Transfer in Solid Gas Sorption Systems

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Khattab studied four approaches to improve thethermal properties of the adsorbent bed of a solarpowered system as follows: (i) Black metallic mesheson both faces of a glass adsorber and granular car-bon inside; (ii) black metallic plates on both facesof a glass adsorber and granular carbon inside;(iii) granular carbon mixed with small pieces ofblackened steel; and (iv) granular carbon bondedwith small pieces of blackened steel. The glassadsorber enhanced the e®ectiveness of solar energyabsorption by allowing the adsorbent to receivesolar energy on both sides. The bed with granularcarbon bonded with blackened steel achieved thebest results among the tested beds, and the machineCOP reached 0.16 with a daily ice production of9.4 kg/m2 of adsorber when the insolation was about20MJ/m2 and the average outdoor temperature was29�C.10

2.1.2. Expanded graphite additives

Expanded graphite can be used as an inert materialto provide a support matrix for reactive materials insolid�gas reactors. Expanded graphite originatesfrom the exfoliation of graphite intercalation com-pounds submitted to a brutal thermal shock.11 Theresult is the creation of worm-like or accordion-likeparticles, which have very low apparent density, inthe range of 6�45 kg/m3, depending on the tem-perature and length of the thermal treatment.12 Themajor elements in expanded graphite are C, S, andO (Table 1).

Researchers in France are among the pioneers inthe use of expanded graphite to improve the kineticsin solid�gas reactions.14 Various methods of mixingexpanded graphite with reactive adsorbents havebeen developed, which included direct mixing andconsolidation, and impregnation of graphite matrixwith an aqueous solution of reactive salt.

Lin and Yuan15 mixed calcium chloride andexpanded graphite for use in chemical heat pumpwith ammonia as the gas, and obtained a reductionin synthesis and desorption time, from 10 h withoutadditives to 3 h with additive. The authors alsoinvestigated the e®ect of two mixing methods

(direct mixing and mixing by salt solution), andresults showed no signi¯cant e®ect of the mixingmethod on the heat and mass transfer enhancement.

2.1.3. Activated carbon or carbon ¯bers

Activated carbon is produced from a wide variety ofcarbonaceous materials, which have been activatedby chemical or physical means to make them porous.Hence, they have extremely high internal surfacearea within its intricate network of pores. Totalsurface area ranging from 450 to 1800m2/g has beenestimated.16 Activated carbon typically comes inthree general types: Granular or natural grains,pellets, and powders. Graphite ¯bers have very highaxial thermal conductivities which vary with the ¯bertype and quality. Values as high as 500W=m �K forex-pitch carbon P120, and 1100W=m �K for K1100type have been reported.17

Dellero and co-authors17 used carbon ¯bers toimprove the performance of a metallic salt�ammoniachemical heat pump. They compared the perform-ance of the heat pump with di®erent methods ofcombining carbon ¯bers with manganese chloridesalt, viz., simple mixing, impregnation, and inter-calation. The results obtained by the later twomethods were much superior to those obtained bysimple mixing, in that they exhibited faster reactionrates, complete reaction, and avoided the agglom-eration phenomenon. However, some drawbacks ofthe two methods were observed; for the impregnationmethod, the salts appear to attach less well to the¯bers after few reactions, and there is a risk of the saltgrains dropping to the bottom of the reactor. Thedrawback of the intercalation method is the di±cultyin controlling the quantity of salt used, and the lengthof the preparation, which is longer than that by theother methods.

Dellero and Touzain18 studied the e®ect of ¯berdisposition on the reaction kinetics (Fig. 2). Theycompared ¯bers 30 cm and 3 cm long impregnatedwith MnCl2. Results showed that, in the case ofshort ¯bers, the reaction kinetics was faster than inthe long ¯bers case. When using short ¯bers thereaction could be completed after 15min, whereas

Table 1. Chemical analysis of expanded graphite powders.13

Elements C S Ca Mn Cr Al Cu Ti K Si Zn Mg FeP

Remainder

wt% 74.2 2.82 0.16 0.55 0.003 0.56 0.001 0.026 0.13 1.82 0.084 0.15 0.5 19.0


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with long ¯bers, the reaction took 60min to becompleted. They attributed the di®erence to the factthat the short ¯bers could stay in the radial direc-tion, which was the same direction of heat transfer;and thus, providing a heat °ow path, unlike in thecase of the long ¯bers, which tended to wind roundnear the centre of the adsorber.

2.1.4. Other additives

Polyaniline belongs to a family of conductive poly-mers, and has electrical properties similar to those ofsome metals. The advantages of its use as additiveinclude the simplicity to combine with the sorbent,good stability, and high electric conductivity.19

Various researchers19,20 introduced polyaniline intozeolite to enhance the conductivity of the sorbentbed. The composite of polyaniline and adsorbentwas prepared by chemical oxidation and in situpolymerization of aniline onto the surface of adsor-bent particles. A thin thermal conducting layer onthe surface of adsorbent particles was grown. Theprocess of preparing the composite involved fourmaterials which were aniline, zeolite 13X, HCl, andðNH4Þ2S2O8. The results showed that the thermalconductivity of the zeolite�polyaniline compositewas four times higher than that of untreated zeo-lite.19 Hu et al.20 reported a two to three foldincrease in the e®ective thermal conductivity, and anear constant adsorption performance for zeolitecoated with polyaniline.

Veselovskaya et al.21 used expanded vermiculiteas a porous matrix for the composite sorbent(BaCl2/vermiculite) to prevent agglomeration of thesalt and improve mass transfer, due its macroporousstructure. The performance of this composite sor-bent was tested in a lab-scale adsorption chiller withammonia as the refrigerant. Results showed that

the chiller using this material exhibited a COPof 0.54 and SCP ranging from 300 to 680W/kg,when driven by a low temperature heat source of80�C�90�C.

2.2. Consolidated beds

Consolidated adsorbent beds are obtained from thepowders of solid sorbents, which may be combinedby a binder or other materials with higher thermalconductivity. Consolidation can produce continuoussolid adsorbent blocks with reduced void spaces,which provides heat conduction path; therefore, canlead to compact adsorbers with increased thermalconductivity and improved surface heat transfercoe±cient. However, consolidation should be usedwith caution to avoid a situation where improve-ment in the heat transfer is achieved, at the expenseof intolerable reduction in the mass transfer.

2.2.1. Consolidated activated carbon

Consolidation of carbons has been used to produceadsorption beds with high packing density andimproved conductivity. Mauran et al.22 reported avalue of bed thermal conductivity as high as40W=m �K, which is considerably larger than thebest values that have been obtained with unconso-lidated beds. Tamainot-Telto and Critoph23 inves-tigated activated carbon mixed with a polymericbinder, compressed in a die and ¯red to producemonolith of a desired shape, with a density of713 kg/m3 and conductivity of 0:33W=m �K. Thesystem with the consolidated compound had a SCP90% higher than the bed with granular carbon.Cacciola et al.24 prepared carbon bricks using poly-tetra°uoroethylene (PTFE) as a binder and testedthe conductivity and adsorption performance onmethanol. The adsorption capacity of the compoundwas 40% higher, whereas its thermal conductivityranged between 0.13 and 0:20W=m �K. The authorsobserved that the low thermal conductivity ofPTFE (about 0.2W=m �K) lowered the thermalconductivity of solid grain from 1.06W=m �K to0.45W=m �K, and hence a binder with a higherthermal conductivity should be sought.

To reduce mass transfer resistance in con-solidated beds, arteries or mass transfer channelsmay be added within the bed. Wang et al.25 usedsolidi¯ed activated carbon with mass °ow channelsin an adsorption bed of a prototype ice maker using

Fig. 2. Fiber disposition in reactor.18


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methanol as refrigerant (Fig. 3). The maximumcooling power of the prototype was 2 kW, whichcorrespond to a speci¯c cooling power of 17W/kg.

2.2.2. Consolidated activated carbon withexpanded graphite

Biloe et al.26 evaluated the performance of con-solidated activated carbon and expanded graphitecomposite for natural gas storage, and found thatthe thermal conductivity was 30 times higher thanthat of activated carbon packed bed. The additionof graphite reduced the charge time of methane by afactor of 10.

Py et al.27 prepared in situ composites of acti-vated carbon and expanded natural graphite with-out any binder. The material had an adsorptioncapacity up to 80wt%, and presented high thermalconductivities, which ranged from 1 to 32W=m �K,depending on the mass ratio between expandedgraphite and carbon. Recently,Wang et al.measuredthe thermal conductivity, permeability, and theadsorption rate of consolidated activated carbon andexpanded natural graphite composites. The use ofexpanded graphite enhanced both the thermal con-ductivity and permeability of the composite, and thehighest values of each obtainedwere 2.47W=m �K and4:378� 10�12 m2, respectively. However, the authorsreported lower performance of the consolidatedcomposite adsorbent at evaporation temperature

of �10�C, due to reduced mass transfer of therefrigerant at low saturation pressure.28

2.2.3. Consolidated activated carbon þ salt

Wang et al.29 mixed CaCl2 with activated carbonand obtained 35% improvement in the volumecooling density, in comparison to unconsolidatedadsorbent. Furthermore, the use of activated carbonavoided salt agglomeration, and maintained a con-stant adsorption capacity. Lu and coworkers30 alsoused activated carbon to enhance heat and masstransfer of CaCl2 adsorbent (mass ratio 1:4) for usein an adsorption ice maker, and the machineachieved a SCP ranging from 111 to 161W/kg.

2.2.4. Consolidated expanded graphite þ salt

Mauran et al.31 patented a process of manufacturingconsolidated blocks impregnated with metallic salts.The common preparation procedure incorporates anaqueous solution of inorganic salt into expandedgraphite matrices, followed by drying and calcina-tions to deposit the salt inside the pores of expandedgraphite. Han et al.17 studied the thermal andphysical properties of expanded graphite matrixwith bulk densities ranging from 100 to 400 kg/m3,which were fabricated by pressing expanded graphitepowders. The gas permeability of porous graphitematrices was in the range of 10�12 to 10�15m2, andthe thermal conductivity values in the axial andradial directions were between 4.1 and 20W=m �K,and between 4.6 and 42.3W=m �K, respectively.

Han et al.32 measured the e®ective thermalconductivities of expanded graphite�CaCl2 � nNH3

(n ¼ 8; 4; 2), MnCl2 � nNH3 (n ¼ 6; 2), and BaCl2�8NH3. The e®ective thermal conductivities of thegraphite�metallic salt complexes were in the rangeof 10�49W=m �K, which are considerably higherthan the values of 0.1�0.5W=m �K, normally foundin powder beds. The results showed that the e®ec-tive thermal conductivity of a graphite�metallicsalt complex has a strong dependency on the bulkdensity, the weight fraction of graphite, and theammoniated state of salt.

Wang et al.33 measured the e®ective thermal con-ductivity of consolidated composite adsorbent ofCaCl2and expanded graphite at ¯xed pressure andtemperature under an atmosphere of ammonia. Thee®ective thermal conductivity ranged from 7.05�9.2W=m �K depending on the mass fraction ofexpanded graphite, bulk density, and the ammoniated

Fig. 3. Solidi¯ed activated carbon bed with gas channels.25


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state of CaCl2. The thermal conductivity of the con-solidated bed was about 30 times that of an unconso-lidated bed. The CPD and the SCP of the consolidatedadsorbent was 52% and 353% higher than that of pureCaCl2 powder, respectively, under the working con-ditions for ice making.34

In the work conducted by Fujioka et al.,35 theauthors showed that an uncompressed compound ofCaCl2 and expanded graphite in the proportion 15:1had a thermal conductivity 60% higher than that ofpowder CaCl2. However, the same compound whencompressed to reduce the overall void fraction toabout 0.6 reached a thermal conductivity of about1W=m �K, which was 10 times that of powderedCaCl2. The same study also showed that the im-pregnation of CaCl2 in activated carbon ¯bers fol-lowed by compression did not change considerablythe thermal conductivity when compared to that ofthe powdered CaCl2.

Other experimental results also showed that theuse of expanded graphite improves the mass transferperformance and can control the phenomena ofswelling and agglomeration in reactive salts.12,36�38

In the experiments conducted by Oliveira andWang,12 the pressure drop across the bed made of35% of expanded graphite and 65% of CaCl2 wasnegligible, but the heat transfer although improvedwhen compared to the CaCl2, still was the limitingfactor for the reaction rate. Further studies from thesame research group showed that the reaction ratewas improved when the mass fraction of expandedgraphite increases up to 50%.37

Li et al.39 produced a consolidated compositesorbent made from manganese chloride and expan-ded graphite for sorption deep-freezing processes.Experimental results showed that the SCP of themachine varied from 200W/kg to 700W/kg whenthe evaporation temperature ranged from �35�C to0�C. The COP of the machine employing a basicsorption thermodynamic cycle reached 0.34 at thegeneration temperature of 180�C, the heat sinktemperature of 25�C, and the evaporation tem-perature of �30�C.

2.2.5. Consolidated expanded graphite þsilica gel

Eun et al.40,41 reported the quantity of water adsor-bed, thermal conductivity, and gas permeability forconsolidated composites of expanded graphite andsilica gel prepared with various compositions and

molding pressure. Expanded graphite in the compo-site blocks had no e®ects on the equilibrium adsorp-tion amount of water on silica gel and increased therate of adsorption. The composite blocks of 20% to30% of graphite mass fraction, and under the moldingpressure in the range of 4 to 40MPa showedpermeability of 3� 10�12 to 40� 10�12m2. Thepermeability increased with graphite fraction at con-stant molding pressure. Thermal conductivity of thecomposite blocks ranged between 10�20W=m �K,depending on the graphite bulk density in the block.These values are much higher than the value of0.17W=m �K usually found for silica-gel packed bed.The highest SCP obtained with the composites wasabout 36W/kg in a 20min cycle, whereas thatobtained from silica gel bed was about 21W/kg in a60min cycle.

2.2.6. Consolidated compoundswith zeolite

Poyelle et al.42 used a consolidated compound ofexpanded graphite and zeolite in a prototype airconditioner, and achieved an SCP four times higherthan that obtained using zeolite pellets. Guilleminotet al.43 studied a consolidated compound made froma mixture of zeolite and metallic foam (copper foamand nickel foam). The consolidated adsorbent wasproduced by ¯rst suspending zeolite powder in asilico�aluminate gel to obtain a paste, which was¯lled in a metallic mesh. The resulting compoundwas compressed at some tens of MPa and heated at1000�C for 3 h. The composite with copper foam hada thermal conductivity of 8W=m �K, which was 22times higher than that of the consolidated zeolite.The wall heat transfer coe±cient also increasedsigni¯cantly to a value of 180W/m2 �K. The ther-mal conductivity of the compound manufacturedwith nickel foam was only 1.7W=m �K, whichindicates that the metallic material property playsan important role in the extent of the heat transferenhancement. Although good thermal conductivityvalues were obtained, compression of the materialsat high temperature and pressure resulted in lowporosity in the zeolite, which a®ected the masstransfer. Lang et al.44 enhanced the heat and masstransfer in compact zeolite layers by using a metallicmatrix with a honeycomb-like structures, and a poreforming material. The hexagonal aluminum honey-comb structure was clamped to the inside of acylinder and ¯lled with zeolite to enhance heat


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transfer. An amount of 5wt% ofmelamine and 5wt%of tartaric acid were used to increase the macroporefraction of the compacted zeolite layers. The resultsshowed that this approach improved the adsorptionrate by up to 110%.

2.3. Sinterized metal foam beds

Heat exchangers based on open cell metal forms havehigh surface area to metal mass ratio, which canresult in high adsorbent to metal mass ratio.Bonaccorsi et al.45 developed a process to obtainmetal foams for thermal applications. The authorsproposed a heat exchanger made of copper metalfoam coated pipes on which zeolite 4A was depositedby a direct hydrothermal synthesis process. Thecopper foam obtained had a ¯nal density of 0.5 g/cm3

with an open porosity close to 70%, and a maximumpore size of 0.5mm (Fig. 4). In another work ofBonaccorsi et al.,46 the authors coated zeolite 4A oncopper foams, and zeolite Y on aluminum foams. Thecompounds were characterized by SEM, XRD anal-ysis, and the water equilibrium sorption curves weremeasured by a thermogravimetric method. In both ofthe cases, the zeolite layer had good bond propertiesto the metal substrate, and up to 15%�17% (byweight) of zeolite was deposited by a double syn-thesis process. The authors reported that theadsorption capacity of the zeolite/foam compositeswere comparable with those of commercial zeolites.

3. Reactors with Enhanced Heatand Mass Transfer

3.1. Specially designed heat exchangers

Specially designed heat exchangers with non-conventional geometry and shape have been used inadsorbers of solid gas systems to enhance heat

transfer. These heat exchangers were designed withextended surfaces to increase the heat transfer areas,and granular adsorbents were distributed uniformlyinto the heat exchanger.

Plate type heat exchanger (Fig. 5) with ¯ns hasbeen applied in an adsorption bed of a solar ice makerusing activated carbon and methanol as the workingpair.47,48 The use of ¯ns and thin wall in the adsor-bent improved the performance of the ice maker,when compared to a previous prototype. Exper-imental results showed that 4 to 5 kg of ice could beproduced by the ice maker with an area of 0.75m2,when the insolation ranged between 14 and 16MJ/m2,whereas 7 to 10kg of ice could be produced whenthe insolation ranged between 28 and 30MJ/m2.

Tchernev and Emerson49 utilized serpentine °at-pipe heat exchanger (Fig. 6) which winds its waybetween consolidated sorbent bricks in a prototypeof a regenerative heat pump. The °at pipes werefabricated from two thin sheets of metal foil (0.1mmthick) that were laid over one another and joined atthe edges. The use of consolidated bricks showedhigher thermal conductivity than a poured bed ofspherical adsorbent pellets, although the low por-osity of the bricks degraded the performance,the prototype yielded a high COP of 1.1 with 70%regeneration.

Wang et al.50 developed a spiral plate heatexchanger and applied it in a regenerative adsorptionrefrigerator using activated carbon methanol pair(Fig. 7). Improved heat transfer in the adsorbersallowed cycle time of 40min. The distinct advantagesof this type of heat exchanger include the compactsize, the small temperature di®erence between thesorbent and the heat transfer °uid, and a more uni-form temperature distribution within the bed.

Wang et al. applied a plate-¯nned tubes adsorberin an activated carbon�methanol adsorption air

(a) (b) (c)

Fig. 4. SEM images of copper foam (a) without adsorbent (X 35), (b) without adsorbent (X 400), and (c) after two depositions withzeolite 4A (X500).45


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conditioner. Tubes with a diameter of 9.5mm wereused for heat transfer from the thermal °uid to theadsorption bed, and aluminum plates were attachedoutside the tubes to increase the heat transfer area.

The adsorbent bed with the heat exchanger tubeswas packed inside a metallic mesh, and the voidspace between the mesh and the shell was used as agas °ow channels to enhance the mass transfer(Fig. 8). This heat exchanger exhibited a low ther-mal capacity associated to the metallic parts. Theadsorber heat capacity ratio (ratio of adsorber ma-terial to adsorbent) was 1.36, whereas the thermal°uid heat capacity ratio (thermal °uid to adsorber)was 1.5 for water and 0.53 for oil.51

Kanamori et al.52 proposed a disk-module-typeadsorber with 1.35mm consolidated carbon adsor-bent made of activated carbon with carbon ¯ber. Itwas found that the cycle period was about 10 times

shorter than that of the packed-bed-type adsorber ofactivated carbon without carbon ¯ber.

Linder et al.53 presented experimental analysis ofa metal hydride sorption system using a reaction bedwith a capillary tube bundle heat exchanger (Fig. 9).

Fig. 5. Plate heat exchanger type adsorber with ¯ns.47

Fig. 6. Serpentine °at pipe heat exchanger: 1. Fluid °owchannel. 2. Embedded adsorbent.49

Fig. 7. Spiral plate type adsorber: 1. Spiral plate, 2. vaporoutlet, 3. solid adsorbent, 4. °uid °ow passage, 5. thermal °uidoutlet, 6. thermal °uid inlet, and 7. support rod.50

Fig. 8. Plate-¯nned tubes absorber.51

Fig. 9. Capillary tube bundle reaction bed of a metal hydridesorption system (length: 135mm, diameter (max.): 65mm).53


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It consisted of 372 small tubes with an inner diam-eter of 1.4mm made of stainless steel. The heatcarrier °owed through the tubes that were sur-rounded by the metal powder. Due to the high heatand mass transfer rates within the adsorber, cycletime around 200 s were realized, and leading tospeci¯c cooling power up to 780W/kg depending onthe temperature boundary conditions.

3.2. Coated heat exchangers and othermethods to decrease contactresistance at the reactor�adsorbentinterface

Anothermethod used to increase thewall heat transfercoe±cient is the adhesion of the adsorbent on the wallof the heat exchanger tube or plate. Various coatingmethods leading to uniform surface have been applied.They include slip coating, spray coating, curtaincoating, coil coating, and electrically assisted coat-ing.54 Pretreatment of themetal surface before coatingwith adsorbent is necessary in order to improvethe wetting of the metal surface by synthesis mixture,and to promote physical and chemical interactionsbetween growing crystals and metal surface.55

Dunne56 proposed carousel-type heat exchanger con-sisting of tubes coated with zeolite monolayer crystal(about 5mm thick) for air conditioning or with waste-heat recovery systems. Good contact between theheat transfer °uid and the adsorbent was achievedand improved cooling performance ranging from1400�1800W/kg was reported.

Restuccia et al.57 used a thin adsorbent coating ofzeolite around a metal tube (Fig. 10). The prep-aration process involved several steps. Initially, anaqueous solution of zeolite powder and an inorganicbinder (alumina gel precipitated in situ) was hom-ogenized. Afterwards, the slurry was layered on themetal support, properly pretreated, to obtain ahomogenous coating of desired thickness. A ¯nalthermal treatment in air allowed the stabilization ofthe adsorbent. This technique of coating can beapplied to di®erent types of heat exchangers (e.g.,tube and shell). The authors reported a slightincrease of the adsorbent thermal conductivity anda large increase in the metal�adsorbent heattransfer coe±cient, due to the metal/adsorbent ad-hesion. The improvement in the global heat transfercoe±cient resulted in a system with higher speci¯ccooling power. Calculated results showed that thebed with coated adsorber could have a speci¯c

power 20 times higher than that of the bed withsorbent grains.

Further research of this same team58 tested ex-perimentally a lab-scale adsorption chiller, in whichthe adsorber was a shell and tube heat exchanger,with ¯nned tubes coatedwith a zeolite layer (Fig. 11).The machine achieved a speci¯c power between 30and 60W/kg of adsorbent (binder included), oper-ating under a cycle 15�20min cycle. Amachine usingthe same type of zeolite, but in the form of pellets hadto operate under a cycle 60 to 120min longer, anddelivered a speci¯c cooling power of about 10W/kg.No major improvements in the COP were observed,although the COPobtainedwith the coated adsorbedwas about 0.02 higher.

Tatlier and Erdem-Senatalar59 studied the e®ectof zeolite coating thickness and wall thickness of astainless steel heat exchanger on the performance ofan adsorption heat pump. They showed that thereexists an optimum zeolite layer thickness that

Fig. 10. Finned tubes heat exchanger coated with zeolite.57

Fig. 11. Photograph of the metal-bound zeolite layer.58


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maximizes the output heating power of an adsorptionheat pump. Moreover, the authors observed a generaltrend whereby very high power and quite low cycleCOP values are obtained when zeolite coatings aresynthesized onmetal heat exchanger. In another studyby the latter research group,60 the authors proposed ascheme to decrease the temperature and concentrationgradient of a solar adsorption heat pump. The schemeincluded the use of zeolite 4A synthesized on stainlesssteel wire gauzes, which were placed vertically in thecollector, to form a ¯rm contact with the absorberplate. The authors estimated that a zeolite 4A coatingthickness value of 50�100�mwas su±cient to removethe adverse e®ects originating from the metal mass inthe collector.

Maier-Laxhuber and Engelhardt61 described anadsorbent bed coating process for use on metallicsurface of lamella heat exchangers. The method useda high temperature adhesive and enabled goodadherence of the adsorbent (preferably a zeolite) tothe heat exchanger during very rapid temperatureincrease and decrease. Chung et al.62 developed aconsolidated copper nanopowder/silica gel layer andcoated directly onto the surface of a metal heatexchanger in the adsorption cooling system. Thethermal conductivity of the silica gel was improvedby the addition of copper nanopowder and a binder,and was further improved by compression of theconsolidated copper nanopowder/silica gel layer.

The German company SorTech AG developed andpatented two innovative techniques of pasting adsor-bent on the heat exchanger surface. The ¯rst techniqueenabled silica gel to be pasted directly on the surface ofthe adsorber heat exchanger with the aid of epoxyresin (Fig. 12(a)).63 Based on this technology, SorTechAG was able to manufacture a compact adsorptionchiller with a nominal cooling capacity of 5.5 kW foruse in solar air-conditioning in private homes andsmall o±ces. The second technique was based on adirect crystallization of zeolite on the surface of theheat exchanger (Fig. 12(b)). Compared to the epoxyresin coating, the direct crystallization technology hasthe following advantages:

. Highmechanical stability of the coated layer due tostrong binding forces at the zeolite–metal interface.

. Enhanced adsorption kinetics, because of good heatand mass transport caused by the very compactlayers containing only active material.

. Reduction of material synthesis and coating to aone-step process.

Zhu and Wang64 investigated the e®ect on theinterface contact resistance caused by exertingpressure and using high thermal conductive (HTC)adhesive. The e®ect of pressure was determined byuse of di®erent weights. The HTC adhesive wasmade from epoxy, butyl gum, ethylated resin, andsinterized copper powder, and it was spread at theinterface between the adsorber and the adsorbent.Experimental results showed that the contact re-sistance decreased by 86% when the pressureapplied increased from 0 kPa to 50 kPa. However, toavoid problems caused by high mass transfer re-sistance, the applied pressure should be optimized.The spreading of the HTC adhesive over the inter-face between the adsorbent granule and the adsor-ber reduced the contact resistances by 40% and by30% at the interface between the adsorbent blockand the adsorber. These experimental results showthat exerting certain amount of pressure or spread-ing adhesive on the interface can reduce the contactresistance signi¯cantly.

3.3. Heat pipes

When thermal resistance within adsorbent�reactoris small, the thermal resistance between the heattransfer °uid and the reactor wall may become the

(a)

(b)

Fig. 12. (a) Photo of a silica gel coated tube and ¯n adsorberheat exchanger. (b) REM picture of the cross-section of analuminum ¯n coated with zeolite by direct crystallization. Theoverall thickness of the ¯n is about 500 microns.63


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limiting factor. Meunier1 suggested the applicationof heat pipe technology in the adsorber to reduce thethermal resistance in the adsorber, because of thehigh heat transfer coe±cient that could be obtainedduring the evaporation and condensation of the heattransfer °uid in the heat pipe. Heat pipes cantransfer a considerable amount of heat using a smallarea because the heat transfer coe±cient in theevaporator and condenser zones of these devicesranges from 103�105W=m2 �K, whereas the ther-mal resistance usually range from 0.01�0.03K/W.65

Vasiliev66 proposed a multi-channel pulsatingheat pipe for adsorption systems (Fig. 13), wherebypropane was used as the heat transfer °uid. Wanget al.67 applied a split heat pipe type in an adsorbercontaining a compound adsorbent for an adsorptionice maker (Fig. 14). During desorption phase, thelower part of the split heat pipe, which was con-nected to the heating boiler, served as evaporator,and the adsorber served as condenser. During cool-ing phase, the adsorber served as evaporator for theupper part of the split heat pipe.

4. E®ects on the COP, Speci¯c CoolingPower, and Cooling Power Density

4.1. Use of materials with highthermal conductivity

Materials with high thermal conductivity that havebeen used as additives to enhance thermal conductivity

of granular beds include metallic pieces or metalpowders, °akes of expanded graphite mixed or im-pregnated with salt, carbon and carbon ¯bers, poly-aniline, and vermiculite. The use of these materialshas various e®ects on the performance of adsorptionmachines; however, they introduce additional coststhat could reach about 10 times the conventionalmaterial cost.68

Some of the additives such as metallic pieces,expanded graphite, and polyaniline do not take partin the adsorption process, and therefore increasesthe dead mass in the reactor. Cacciola et al.69 stu-died the e®ect of increased amount of metal in theadsorbent bed on the COP and on the speci¯ccooling power. They observed that such additivesincreased the ratio of dead mass to the adsorbentmass and reduced the SCP; thereby, requiring moremass of adsorbent and expensive adsorbers.

4.2. Consolidated beds

The use of consolidated beds have been successful inenhancing the heat transfer especially in highpressure machines, such as those using salts andammonia as the working pair. However, for low-pressure systems, such as those using water andmethanol as working °uid, consolidation can haveserious e®ects on mass transfer which in turn, mayreduce the performance of the machine. For ex-ample, the water vapor pressure in a typical cycle ofan adsorption refrigerator working with water asrefrigerant is about 8 to 12mbar during the coolingperiod. Thus, a small increase in the mass transferresistance of the bed caused by the consolidationcan greatly compromise the adsorption capacity.Fig. 13. Multi-channel pulsating heat pipe type adsorber.66

Fig. 14. Adsorption bed with application of split heat pipe.67


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However, in a study using silica-gel as sorbent, Eunet al.40 showed that when the molding pressure onthe block and the proportion between expandedgraphite and silica-gel are carefully chosen, theconsolidated block can lead to speci¯c cooling powerup to 75% higher than that obtained with non-consolidated silica-gel.

Aristov et al.70 compared the speci¯c coolingpower and the global heat transfer coe±cient of theadsorber with pelletized zeolite, consolidated zeolite,and consolidated silica-gel impregnated with CaCl2.The consolidation of the zeolite produced a four-foldincrease in the cooling power and a six-fold increasein the global heat transfer coe±cient. Because theconsolidated compound of silica-gel impregnatedwith CaCl2 had thermal conductivity higher thanthat of the consolidated zeolite, the global heattransfer coe±cient of the former compound couldreach 130W=m2 �K and the calculated speci¯ccooling power was 480W/kg.

Klein and Groll71 showed that consolidated com-pound beds with metal hydrides in the proportion20:1 to expanded graphite can be an economicallycompetitive alternative to metal-hydrides�Al-foamsbeds. The metal hydrides�expanded graphite con-solidated beds tested by these researchers had ther-mal conductivity higher than 8W=m �K and wallheat transfer coe±cient ranging between 1500 and3000W=m2 �K.

The consolidation process also showed to bee®ective to enhance the speci¯c cooling power ofactivated carbon�ammonia systems. The thermalconductivity of the consolidated carbon block devel-oped by Tamainot-Telto and Critoph23 was twice theoriginal thermal conductivity of the carbon withoutconsolidation, and could lead to an improvement inthe cooling power of about 90%.

4.3. Reactors with enhanced heatand mass transfer

Reactors with enhanced heat and mass transferinclude specially designed heat exchangers for theadsorber, use of coated heat exchangers and appli-cation of heat pipes. Mazet and Lu72 showed thatspecially designed reactors as those presented bythese authors can help reduced mass transfer pro-blems at low pressure, and enhance the speci¯ccooling power in the range of 100% to 200%. In theirwork, they studied an adsorber whereby the heatexchanger and the gas di®user were a single element.

The authors analyzed numerically several con¯gur-ations of reactors employing such element, andreported that it is possible to expect cooling pro-duction at�60�C,with ammonia reacting at 0.2 bars.

Specially designed heat exchangers may use sev-eral ¯ns, and the major drawback of this approach isthat they increase the thermal capacity of theadsorber due to the increase of the metallic mass. Aheat recovery process between adsorbers is thereforenecessary to reduce the heat input load; otherwise,the energy consumed by the sensible heat will betoo large, and the COP of the system will be severelya®ected. Wang51 recommended a thermal capacityratio between the metallic mass of the heat exchan-ger and the adsorbent mass up to 5, ensure reason-able COP.

5. Conclusion

Various techniques have been employed to enhanceheat and mass transfer in sorption beds, whichinclude use of materials with high conductivity,consolidation of adsorbents, and use of speciallydesigned adsorbers. Most of these techniques haveresulted to great improvements in the heat and masstransfer and increase in speci¯c power of the sys-tems. However, more e®orts and new innovativetechniques are necessary to further enhance coolingpower performance of sorption machines, and leadto more compact machines that can compete e®ec-tively with conventional electricity powered re-frigerators. These e®orts should be guided to theoptimization of the proportion and density betweenhigh thermal conductivity inert materials and highadsorption capacity adsorbents, in order to achieveadsorbent beds with little mass transfer and heattransfer resistances.

Acknowledgment

This work was supported by the Key Project of theNatural Science Foundation of China under con-tract No. 50736004.

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