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How Heat and Mass Recovery Strategies Impact thePerformance of Adsorption Desalination Plant: Theoryand ExperimentsXiaolin Wang a , Kim Choon Ng a , Anutosh Chakarborty a & Bidyut Baran Saha ba Mechanical Engineering Department , National University of Singapore , Singaporeb Interdisciplinary Graduate School of Engineering Sciences , Kyushu University , Fukuoka,JapanPublished online: 05 Oct 2011.

To cite this article: Xiaolin Wang , Kim Choon Ng , Anutosh Chakarborty & Bidyut Baran Saha (2007) How Heat and MassRecovery Strategies Impact the Performance of Adsorption Desalination Plant: Theory and Experiments, Heat TransferEngineering, 28:2, 147-153, DOI: 10.1080/01457630601023625

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Heat Transfer Engineering, 28(2):147–153, 2007Copyright C©© Taylor and Francis Group, LLCISSN: 0145-7632 print / 1521-0537 onlineDOI: 10.1080/01457630601023625

How Heat and Mass RecoveryStrategies Impact the Performanceof Adsorption Desalination Plant:Theory and Experiments

XIAOLIN WANG, CHAKARBORTY ANUTOSH, and KIM CHOON NG

Mechanical Engineering Department, National University of Singapore, Singapore

BIDYUT BARAN SAHA

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka, Japan

A prototype adsorption desalination facility is experimentally studied, and the performance tests are conducted with andwithout the heat and mass recovery procedures. The experiments show that practical and yet effective methods could yield asignificant boost to the specific daily water production and performance ratio of the desalination plant by 15.7% and 42.5%,respectively.

INTRODUCTION

Adsorption desalination (AD) is a novel method of produc-ing potable water, despite the adsorption cycle, for cooling ap-plications found in chemical, power and co-generation plants.Hitherto, there are several kinds of commercial-scale desalina-tion plants in many water scarce countries, such as the multi-stage flash (MSF) [1] type; the multi-effect desalination [2] type;the membrane-based reverse osmosis (RO) [3] plants; the hybridplants, which combine the RO and MSF processes [4, 5]; andelectrodialysis (ED) or electrodialysis reversal (EDR) [6]. Allof the mentioned desalination methods are found to be eitherhighly energy-intensive to maintain the processes of desalina-tion or prone to serious erosion and fouling problems in the evap-orating units operating at elevated evaporating temperatures [7].

The AD cycle is proposed to mitigate the shortcomings ofthe conventional desalination methods [8–11]. The advantagesof the AD cycle are that

1. it employs waste heat at low temperatures for the cycle, tem-peratures of 85◦C or lower;

2. the vaporization of saline or brackish water in the evaporatoris kept at a low temperature, typically between 20–25◦C, tomitigate problems of corrosion and fouling; and

Address correspondence to Dr. K. C. Ng, Mechanical Engineering Depart-ment, National University of Singapore, 10 Kent Ridge Crescent, Singapore119260. E-mail: mpengkc@nus.edu.sg

3. the complete elimination of any bio-contamination by des-orption at 65◦C or more where any unwanted aerosol-entrained microbes or cells from the evaporator would bekilled.

The reduced corrosion and fouling rates in the evaporatorimply a low maintenance cost for the AD plants as comparedwith the conventional plants. Other than the evaporator, which isusually made of stainless steel, the other components of the ADplant use a comparatively inexpensive carbon steel. In addition,the AD plants employ low-temperature waste heat from indus-tries that otherwise would have been purged into the ambient,not only saving the primary energy resource but also reducingglobal warming.

Wang and Ng [11] have recently reported the performance ofa four-bed silica gel-water adsorption desalination plant, whereboth the specific daily water production (SDWP) and the plantperformance ratio (PR) are yet to be optimized. The PR is de-fined here as the ratio of the equivalent latent heat associated withthe potable water production to the energy input to the adsorp-tion cycle. Being a batch-operated cycle, the effects of thermalmass from the reactor beds and the mixing of the residual heatof the coolant remaining in the cycle during switching could besubstantial in relation to the total heat input per cycle. One ofthe pioneering works in heat and mass recovery procedures thatwas applied to adsorption cycle employed to provide cooling

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is the HIJC USA, Inc. [12], where both the water circulationand mass recovery schemes were applied to the beds during theswitching interval. It reported some improvement to the chillercooling capacity. More recently, Wang [13] analyzed in somedetail the energy recovery principle of the adsorption cycle, andLiu et al. [14] attempted to explain the functional aspects ofheat and mass recovery schemes for chillers by considering anopen-loop design for the adsorption cycle, as well as the use ofan external heat source and cooling tower. They concluded thatfor the chiller performance to improve, an independent fluid re-circulation loop is necessary to augment the heat recovery. Sucha process could be operated either in a continuous manner oronly during the switching period [15]. Such an active heat re-covery scheme works best with a high-temperature heat source,which seems incompatible with the novel aim of utilizing low-temperature waste heat. On the other hand, Ng et al. [16, 17]employed only passive heat and mass recovery schemes on afour-bed adsorption plant that was operated as a water desalina-tion plant. Although many possible heat and mass combinationsor schemes could be applied to the adsorption cycle, the authorshave demonstrated two methods that require minimal changesto the plant hardware and yet significantly improve the systemperformance of the adsorption desalination plant.

EXPERIMENTS

The schematic and prototype adsorption desalination plant isshown in Figures 1 and 2, comprising the four beds, condenser,and evaporator. For a two-bed mode, each pair of beds operates intandem, either concomitantly as the adsorber or desorber. Duringthe desorption process, a low-temperature heat source is supplied

Figure 1 A schematic of a two-bed adsorption desalination plant.

to the desorber bed, where the water vapor from the silica gelis expelled and the vapor is allowed to condense on the tubesof the condenser. The heat rejected by the condenser is cooledby circulating water from the cooling tower, and the condensateproduced within the condenser is collected as the potable water.At the same time, the designated adsorber is in heat and masscommunications with the evaporator through the controlled va-por valve, thus causing boiling to occur within the evaporator.As adsorption is an exothermic process, external cooling waterfrom the cooling tower is circulated to the designated adsorber,enhancing the vapor uptake to the adsorbent. Details of the plantswitching and cycle operations have been previously describedin literature [17] and thus will not be elaborated here. It is notedthat the equipment found below the platform is the purpose-builtrating facility, which enables constant supply conditions for thecoolant or heat source temperatures, such as chilled water andcooling and hot water temperatures, to within an accuracy of±0.3◦C.

The saline or brackish water is first pre-treated (e.g., filteringand de-aeration) and feeds to the evaporator, while purging fromthe evaporator is conducted intermittently for salt concentrationcontrol. Potable water is produced in the adsorption cycle intwo steps. First, vapor is evaporated by the thermal load fromthe circulating evaporator-water loop, and the spray evaporationis employed as compared to conventional pool boiling. Vapor up-take or adsorption is maintained by the unsaturated properties ofadsorbent in the designated water-cooled adsorber. Secondly, va-por is purged out from the designated hot-water driven desorberand condensed in the condenser to produce the pure water. Thecondensate is collected in a collection tank and intermittentlypumped out to the ambient.

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Figure 2 A pictorial view of a two-bed adsorption desalination plant.

The test facility is extensively instrumented, and some ofthe measurement points are as marked in Figure 1. In spite ofthe batch operation of the beds, the production of water in theevaporator is continuous but fluctuating and is measured usinga flow meter. All temperature measurements employ the 5 k�-type thermistors with a low time-constant sensor, typically 3s(±0.2◦C, YSI). The small time constant enables these sensorsto track accurately the transient swings during the switching andoperation periods, avoiding any unnecessary cumulative error.Electromagnetic flow transmitters for the flow rate measure-ments (±0.5% of reading, Krohne) are used, and the absolutepressure sensors with an accuracy of ±3.5% of the reading arealso employed. The measurements of the condensate water andthe performance ratio have uncertainties of the order of ±2.5%and ±3.5%, respectively.

HEAT AND MASS RECOVERY SCHEMES

Heat and mass recovery schemes are deemed necessary inan adsorption cycle because the cycle operates in a cyclic man-ner, whereas a designated reactor bed is initially heated for thedesorption process and cooled for the adsorption process in thefollowing half-cycle. As the beds comprise components such asthe silica gel and the finned-tube heat exchanger, the residualthermal energy associated with a heating or cooling change overor transient could be substantial in comparison to the amountof energy input over a half-cycle. Although one could recoverthe transient residual heat in many ways [12–16], only the costand performance effectiveness methods, which require minimalhardware alteration, are investigated.

The adsorption cycle is commonly depicted on a P-T-q orDuhring diagram where the sloping lines are the lines of con-stant vapor uptake or isostere, as shown in Figure 3. The isobars,shown by lines 1–2 and 3–4, are the adsorption and desorptionprocesses in a cycle, respectively, while the isosteres, repre-sented by 2–3 and 4–1, refer to the switching intervals duringpre-cooling and pre-heating, respectively. State points 2 and 4are the lowest and the highest temperatures achieved by the ad-sorbent in the cycle. Being time independent, the P-T-q diagramis a simple state diagram for discussing energy transfer wherethe temperature abscissa could provide an indicator as to how re-alistically an energy recovery procedure could be implemented.

One of the simplest heat recovery procedures that could beimplemented in an AD plant is the management of the thermalfronts of coolant flow within the beds during the pre-heating andpre-cooling switching intervals. Owing to the finite volume ofcoolant being retained in the flow passages of a bed, the simul-taneous closure or opening of the inlet and outlet valves of a bedwould result in the unwarranted flow of residual water of the bedinto an incorrect return path or circuits. By introducing a simpletime delay for the activation of the respective outlet valves ofthe beds, the proper segments of hot and cold coolants couldbe directed without thermal degeneration until the arrival of thedesignated thermal fronts, at which time the valves are activatedto switch the flows into the respective circuits. Such heat recov-ery management requires no hardware alteration in the circuitsexcept a timed delay to the operation of outlet valves, but the con-sequences on the performance of AD plants are two-fold. First,the heat input to the plant is reduced as the residual hot/cold waterin the previously designated desorber/adsorber beds (Qresidual) iscorrectly returned and, hence, the plant performance ratio (PR)

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Figure 3 Duhring diagram of the adsorption cycle.

improves. Second, the proper reverting of the residual hot waterof the bed during switching, simply denoted here as valve-delay(VD), reduces the need for a larger cooling tower for heat re-jection. The improvement in the plant performance ratio (PR)could be seen from the smaller denominator contribution due tothe better management of flow, i.e.,

PRwith VD = hfg∫ τ

0 mevap dt∫ τ

0 (Qin − Qresidual) dt(1)

where τ is the half-cycle time and Qresidual refers to the energysaved in the heat source circuit from a timed delay of the outletvalves.

The second heat and mass recovery procedure tested in theprototype AD plant is called the gas pressure equalization (PE),where the pressures of the beds are equalized during a part of theswitching interval. PE is achieved by opening an equalizationvalve linking a pair of the adsorption and desorption beds ofdifferent pressure levels. Using the P-T or Duhring diagram, asshown in Figure 3, the PE enables the Pdesorber or P4 of the des-orber and Padsorber or P2 of the adsorber to be equalized almostinstantaneously to an intermediate pressure, Pequalized, permittingan instant flow of vapor from the beds. Within the desorber, areduction of the bed pressure from P4 to P4A leads to a reduc-tion of the isosteres of adsorbent from q4 to q4A. On the otherhand, the previously designated adsorber experiences a pres-surization of P2 to P2A, leading to an increase in the isosterefrom q2 to q2A and aided by a rush of vapor mass via the equal-ization valve. Hence, there is a corresponding increase in theheat input associated to change of state points 3 to 3′, i.e.,Q3′ →3= hADS Ms,i(q2A − q2), while the increase in coolingcapacity can be estimated by �Qevap,PE = hfg Ms,i[(q4 − q4A)].The combined schemes on the operation of the AD plant would

certainly boost the specific water yield, but in the increase inplant PR, it is not obvious where the latter definition has contri-butions from positive and negative quantities in the denominator,i.e.,

PRwith VD+PE = hfg∫ τ

0 (mevap + mevap,PE)dt∫ τ

0 (Qin − Qresidual + Q3′→3)dt(2)

It is noted that the sensible energy change due to pressureequalization is deemed small by comparison and ignored in thePR definition. The increase in SDWP due to pressure equaliza-tion can be attributed to the recovery of mass or vapor from thedesorber to the adsorber beds during switching.

RESULTS AND DISCUSSION

Table 1 summarizes the typical test conditions employed forthe adsorption desalination cycle. As there are no available stan-dards for the testing of the adsorption desalination cycle, therating conditions were set arbitrary at Thot = 85◦C, Tcond =29.4◦C, and T in

evap = 12.2◦C, the conditions similar to those used

Table 1 Rating conditions for an adsorption desalination plant with two-bedoperation

Half cycle time (s) 180–720Switching time (s) 40Hot water inlet temperature (◦C) 85Cooling water inlet temperature (◦C) 29.4Evaporator water inlet temperature (◦C) 12.2Hot water flow rate (L/min) 48Cooling water to bed flow rate (L/min) 38Cooling water to condenser flow rate (L/min) 125Chilled water flow rate (L/min) 46

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Figure 4 Performance of the basic two-bed adsorption desalination plant atassorted cycle time.

for chiller testing. In addition to the mentioned rating conditions,the cycle and switching time intervals are equally important inthe operation of the AD plant. Figure 4 presents the adsorp-tion desalination performance, conducted under a two-bed modewith a changing cycle time on the abscissa. It is observed that thespecific daily water production (SDWP) of the AD plant peaksgradually with the cycle time changing from 400s to 600s. Theoptimal SDWP is found to be around 4 kg of water/kg silica gel,and the corresponding maximum PR is 0.32. Increasing the cy-cle time leads to the asymptotic increase in the PR but results in aslight reduction in the SDWP. This is attributed mainly to the sat-uration phenomenon of the adsorbent at the rating temperatures.On the other hand, a shorter cycle time has the negative effect ofnot fully utilizing the adsorption potential of silica gel, leadingto a large reduction in both the SDWP and PR of the AD cycle.

The efficacy of the heat recovery scheme, demonstrated dur-ing the switching interval by the implementation of a valve clo-sure delayed (VD), is appropriately depicted in Figure 5. Using

Figure 5 Temperature profile of the cooling and heat source at the systeminlet and outlet with and without heat recovery scheme. Symbols: Hotwater inlet, Hot water outlet with heat recovery scheme, Hot wateroutlet without heat recovery scheme, Cooling water inlet, Coolingwater outlet with heat recovery scheme, Cooling water outlet without heatrecovery scheme.

Figure 6 Effects of the heat recovery scheme only on a two-bed adsorptioncycle.

temperature-time traces from the sensors that are placed in thesupply and return pipes of both the hot and cold circuits, the roleof valve timing delay in energy savings can be estimated by theshaded area enclosed by superimposing two temperature traces:Path “A” is obtained before the implementation of VD, Path “B”is derived with VD applied, and the shaded area between them isa measure of the amount of energy saved (i.e., Qresidual). Typ-ically, a delay time spanning from 40s to 70s has been inves-tigated and the optimal delay is close to 70s for a two-bedmode. Figure 6 shows the PR at different cycle time with orwithout the heat recovery scheme. The percentage improve-ment in the PR increases by as much as 30% when the cycletime is short; however, at a long cycle time, a lesser improve-ment in PR is observed, decreasing to about 16%. At a longcycle time of 600s and in two-bed mode, the AD plant PRwith VD is 0.41, as compared with a PR of only 0.34 with-out time delay. Thus, the simple management of the residualheat in the beds during switching improves the PR, and thiscould be instituted simply via software modification in the ADplant, as the heat recovery scheme has insignificant effect on theSDWP.

The effectiveness of mass recovery by pressure equalization(PE) in boosting AD plant performance is shown by the continu-ous lines of Figure 7, where the intercept axis indicates the spe-cific water production (SDWP) and the abscissa is the PE periodwithin the switching interval. At a cycle time of 300s, the dailySDWP increases from 3.5 kg of water/kg silica gel to 4.05 kg ofwater/kg silica, while the PR increases marginally from 0.235to 0.298. Over the PE intervals investigated, the maximum per-centage increase of SDWP and PR are 15.7% and 26.8%, re-spectively. It is noted that when the gas pressure equalizationtime interval is short, the adsorption and desorption potentialswithin the beds are not fully utilized, and both the SDWP andPR would plummet. Conversely, a long PE time interval inter-feres with the normal cycle operation and decreases the SDWP.Hence, there is an intermediate time interval of about 30s wherethe PE between the beds would yield an optimal SDWP. Thedashed-line in Figure 7 shows the combined contributions fromthe two schemes that were applied onto the AD cycle, where the

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Figure 7 Effects of the energy recovery schemes on a two-bed adsorptioncycle at assorted pressure equalization (PE) time but fixed half-cycle time of300s. Points A and C represent the PR and SDWP, which were obtained withoutheat recovery scheme. The solid line shows the effects of the mass recoveryonly with the increase of PE time intervals. Point B represents the effect of heatrecovery scheme (VD) on the performance ratio, and the dotted line shows thecombined effects of both the heat & mass recovery (VD + PE) with the increaseof PE time intervals.

cycle time is held constant at 300s but the PE intervals change.It is observed that the PR of AD plant improves from 0.24 of thebasic AD cycle to 0.34, an increase of 42.5% with the mentionedschemes.

As the temperature at the inlet to the evaporator increases,the SDWP performance of an adsorption desalination plant in-creases accordingly. As opposed to cooling, the chilled watertemperature requirement of 14◦C or lower can now be dispensedwithin the adsorption desalination plant, where the evaporatortemperature can be raised to an optimal level. The regenerativefeature is part of the claims cited in [13]. Table 2 summarizes thekey performance results (but with constant cycle and PE timesof 480s and 30s, respectively) at four evaporator-inlet tempera-tures, namely T in

evap = 12.2◦C, 15◦C, 20◦C, and 25◦C. They showsignificant improvements to both SDWP and PR.

Table 2 The effect of raising water temperature at inlet to the evaporator forthe basic AD cycle, as well as the comparisons with cycles with pressureequalization (PE) and valve-closure delay (VD) of an AD plant

Modified cycle with heat and massrecovery (VD + PE) with

PE = 30s, Thalf−cycle = 480s

T inevap

Basic cyclePE = 0s andTcycle = 480s

12.2◦C 12.2◦C 15◦C 20◦C 25◦C

SDWP (m3/tonne or kg ofwater/kg of silica gel perday) ± 0.14

3.95 4.32 5.09 6.30 7.60

Performance ratio (PR) ±0.011

0.315 0.429 0.455 0.567 0.599

Note: All other parameters of AD plant are as per Table 1.

CONCLUSION

The performance of a prototype adsorption desalination planthas been tested successfully using a purpose built rating facility.Under the basic two-bed mode, the optimal specific daily waterproduction (SDWP) is measured to be about 4 kg of water/kgsilica gel, where the corresponding PR is about 0.32. With theimplementation of the management of the residual heat in thebeds using valve closure delay (VD) and the pressure equaliza-tion (PE) between the adsorber and desorber beds, the percentageimprovements in SDWP and PR are 15.7% and 42.5%, respec-tively. As cooling is no longer the objective for an AD plant,the evaporator inlet temperature could be raised, resulting in atwo-fold increase in the plant SDWP and PR.

ACKNOWLEDGMENTS

The authors wish to thank Jonathan M. Capel, C. H. Lim Pe-ter, and Lim Yee Sern for gathering part of the experimental data.The authors also gratefully acknowledge the financial supportof an A*STAR SERC grant.

NOMENCLATURE

hADS heat of adsorption, kJ/kg of adsorbenthfg latent heat of evaporation, kJ/kg of waterM mass of adsorbent or coolant, kgP pressure, PaPR performance ratioq vapor uptake to adsorbentQ heat transfer rate, WSDWP specific daily water production, kg of water/kg of

adsorbent or m3 of water per ton of adsorbentt time, sT temperature, ◦C or K

Greek Symbol

τ half-cycle interval, s

Subscripts

ads adsorptionads start start of the adsorption processads end end of the adsorption processc, cond condensercold cold coolant streamdes desorptiondes end end of the desorption processdes start start of the desorption processevap, e evaporatorhot heat source stream

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i bed numbering of a multi-bed systemin heat inputin-sw heat input during switching periodmax maximummin minimumout-sw heat removed during switching periodPE pressure equalization timeresidual residual heat capacity of coolant in beds silica gel

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[7] Ehrenman, G., From Sea to Sink, Mechanical Engineering,vol. 126, no. 10, pp. 38–43, 2004.

[8] Broughton, D. B., Continuous Desalination Process, USPO4,447,329, May 8, 1984.

[9] Zejli, D., Benchrifa, R., Bennouna, A., and Bouhelal, O. K., ASolar Adsorption Desalination Device: First Simulation Results,Desalination, vol. 168, pp. 127–135, 2004.

[10] Al-Kharabsheh, S., and Goswami, D. Y., Theoretical Analysis of aWater Desalination System Using Low-Grade Solar Heat, Journalof Solar Energy Engineering, Transactions of the ASME, vol. 126,no. 2, pp. 774–780, 2004.

[11] Wang, X. L., and Ng, K. C., Experimental Investigation of anAdsorption Desalination Plant Using Low-Temperature WasteHeat, Applied Thermal Engineering, vol. 25, pp. 2780–2789,2005.

[12] Waste Heat Adsorption Chiller—Function, HIJC USA, Inc.,P.O. Box 820307, Houston, Texas 77282-0307, USA. Availableat: http://www.adsorptionchiller.bigstep.com/homepage.html.Accessed October 2004.

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[16] Ng, K. C., Gordon, J. M., Chua, H. T., and Anutosh, C., Electro-Adsorption Chiller: A Miniaturized Cooling Cycle with Appli-cations from Microelectronics to Conventional Air-Conditioning,USPO no. 6,434,955, August 2002.

[17] Ng, K. C., Chua, H. T., Wang, X. L., Kashiwagi, T., and Saha, B. B.,Prototype Testing of a Novel Four-Bed Regenerative Silica Gel-Water Adsorption Chiller, Proceedings of International Congresson Refrigeration, Washington D.C., USA, 2003.

Xialin Wang obtained his B. Eng. and Ph.D. fromXi’an Jiaotong University in P.R. China in 1994 and1999, respectively. He is currently working in theDepartment of Mechanical Engineering of the Na-tional University of Singapore. His areas of researchare cooling engineering, chiller testing and modeling,electro-adsorption chiller, and renewable energy.

Chakraborty Anutosh obtained the B.Sc.Eng fromBangladesh University of Engineering and Technol-ogy in 1997 and M.Eng from the National Univer-sity of Singapore in 2001. He worked one year atthe Power Development Board in Bangladesh. He iscurrently a Ph.D. candidate at the Mechanical En-gineering Department in the National University ofSingapore. His areas of research are solid-state micro-cooling devices, electro-adsorption chiller, and ad-sorption thermodynamics.

Kim Choon Ng obtained his BSc. (Hons.) and Ph.D.from Strathclyde University in Glasgow, UK, in 1975and 1980, respectively. He worked briefly at the Bab-cock Power Ltd. in Renfrew prior to joining the De-partment of Mechanical Engineering of the NationalUniversity of Singapore in 1981, and he is now atenured professor. His areas of research are two-phaseflow, chiller testing and modeling, electro-adsorptionchiller, and renewable energy. To date, he has writ-ten more than 70 peer-reviewed journal articles, four

patents, and co-authored a book, Cool Thermodynamics, printed by CISP (UK).He is a member of the IMechE (UK) and the Institution of Engineer Singapore,a Chartered Engineer (UK) and a registered professional engineer (S) and anassociate editor of Heat Transfer Engineering.

Bidyut Baran Saha obtained his B.Sc. (Hons.) andMEng. degree from Dhaka University in Bangladeshin 1987 and 1990, respectively. He received his Ph.D.in 1997 from Tokyo University of Agriculture andTechnology, Japan. He is currently working as an as-sociate professor at the Division of Advanced DeviceMaterials, Institute for Materials Chemistry and Engi-neering, Kyushu University, Japan. His main researchinterests are thermally powered sorption systems, heatand mass transfer analysis, and energy efficiency as-

sessment. He has published more than 90 articles in peer-reviewed journals andproceedings.

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