International Journal of Heat and Mass Transfer 54 (2011) 43–51

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International Journal of Heat and Mass Transfer

journal homepage: www.elsevier .com/locate / i jhmt

Study on an advanced adsorption desalination cycle with evaporator–condenserheat recovery circuit

Kyaw Thu a, Bidyut Baran Saha b,⇑, Anutosh Chakraborty c, Won Gee Chun d, Kim Choon Ng a,⇑a Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singaporeb Mechanical Engineering Department, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka-shi, Fukuoka 819-0395, Japanc Department of Mechanical and Areospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapored Department of Nuclear and Energy Engineering, 66 Jejudaehakno, Jejusi, Cheju National University, Republic of Korea

a r t i c l e i n f o

Article history:Received 25 November 2009Received in revised form 20 August 2010Accepted 24 September 2010

Keywords:AdsorptionDesalinationWaste heatHeat recovery

0017-9310/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.ijheatmasstransfer.2010.09.065

⇑ Corresponding authors. Tel.: +81 92 802 3101; faxE-mail addresses: saha@mech.kyushu-u.ac.jp (B.B.

(K.C. Ng).

a b s t r a c t

This paper presents the results of an investigation on the efficacy of a silica gel–water based advancedadsorption desalination (AD) cycle with internal heat recovery between the condenser and the evapora-tor. A mathematical model of the AD cycle was developed and the performance data were compared withthe experimental results. The advanced AD cycle is able to produce the specific daily water production(SDWP) of 9.24 m3/tonne of silica gel per day at 70 �C hot water inlet temperature while the correspond-ing performance ratio (PR) is comparatively high at 0.77. It is found that the cycle can be operational at50 �C hot water temperature with SDWP 4.3. The SDWP of the advanced cycle is almost twice that of theconventional AD cycle.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction industrial-scale desalination units have been installed or con-

Potable water is a necessity for daily consumption and activitiesof human as well as the economic growth through the industrialprocesses. The accessible fresh water on Earth is less than 1% ofthe available water whilst 97% of the available water is either sea-water or brackish water where the total dissolved solids (TDS) aremore than 35 ppt. Within the fresh water inventory, as much as65% is locked in the ice-caps or stored in deep aquifers under theground surface. On the other hand, water consumption by humanswould double in every 20 years as the population growth rate in-creases. In the 2nd UN World Water Development Report, morethan one billion people lack accessibility to safe drinking water,and 40% lack water for basic sanitation [1]. It is noted that overthe past century, the global water consumption level has increasedby tenfold, reaching or exceeding the limits of renewable water re-sources in some water stressed countries, such as in the MiddleEast, city states and North African countries. For instance, freshwater resources are almost completely exhausted in many mid-dle-east region countries [2–5].

Desalination has become a practical solution for the globalwater shortage problem as its economic viability has improveddrastically in recent decades. Worldwide, more than 15,000

ll rights reserved.

: +81 92 802 3125 (B.B. Saha).Saha), mpengkc@nus.edu.sg

structed. These plants account for a total capacity of 8.5 billiongallons of water/day [6]. The process of producing potable waterby desalting of sea or brackish water can be classified into twomain groups; namely (i) thermally-activated systems and (ii) pres-sure-driven systems. The International Desalination association(IDA) data shows that the reverse osmosis (RO) and multi-stage-flash (MSF) desalination processes account for 44% and 40% ofthe global installed desalting capacity, respectively [7]. However,these desalination processes have shortcomings such as high en-ergy intensive, fouling in the separation units (membrane andevaporator) and high operation cost. Being energy intensive, theycontribute to environmental pollution such as global warmingand green house gas emission through the burning of fossil fuels.

Over the last 200 years, a significant contribution by anthropo-genic emission of greenhouse gases from the burning of fossil fuelshas increased the atmospheric temperature by about 1.2–1.4 �F [8],contributing to the increasing melting rate of polar ice caps. Globalwarming has been increasing the melting rate of sea ice. Data fromthe National Ice and Snow Data Center (NISDC) showed that Arcticsea ice has declined dramatically over at least the past 30 years [9].If the ocean surface receives a layer of fresh water, thermohalinecirculation that contributes to a proper temperature balance acrossthe earth can be disrupted [10]. Hence, the re-utilization or theconversion of waste heat to useful effects plays a crucial role inslowing down the global warming. Thus, the development of desa-lination technologies coupled with renewable energy sources orprocess waste heat has become indispensable [11].

Nomenclature

A total area (m2)cp specific heat capacity (J/kg K)Dso kinetic constant for the silica gel water system (m2/s)E characteristic energy (kJ/mol)Ea the activation energy (kJ/kg)hf sensible heat (J/kg)hfg latent heat of vapourization or condensation (J/kg)M mass (kg)_m mass flow rate (kg/s)

n D–A constant (–)P pressure (Pa)PR performance ratio (–)q fraction of vapour adsorbed by the adsorbent (kg/kg of

silica gel)Q power (W)q0 maximum adsorbed amount (kg/kg of silica gel)q* equilibrium uptake (kg/kg of silica gel)Qst isosteric heat of adsorption (J/kg)R gas constant (J/kg mol)Rp particle radius (mm)SDWP specific daily water production (m3/tonne of silica gel

per day)t time (s)

T temperature (�C)U overall heat transfer coefficient (W/m2 K)v specific volume (m3/kg)s number of cycles per day (–)

Subscriptsabe adsorbateads adsorberchilled chilled watercond condensercw cooling watercycle one cycle with one bed undergoing either complete

adsorption and desorptiond distillatedes desorberevap evaporatorhw hot waterHX heat exchangerin inletout outlets sea watersg silica gelswitching switching period

44 K. Thu et al. / International Journal of Heat and Mass Transfer 54 (2011) 43–51

Several studies have contributed to the desalination technolo-gies that utilize renewable energy sources ranging from solar togeothermal energy [12–17]. The earliest development on adsorp-tion-based desalination was reported by Broughton [18], using anion-retarded resin for the vapour uptake, where a process with athermally-driven two-bed configuration is simulated. Similar the-oretical studies on the adsorption desalination plant were also pro-posed recently by Zejli et al. [19] and Al-kharabsheh and Goswami[20]. Solar heat source was studied as a heat source for the desali-nation plant, combined with an open-cycle adsorption heat pumpusing the zeolite as the adsorbent. Wang and Ng investigated theperformance of the AD cycle using a four-bed regeneration schemeand reported the optimal specific daily water production (SDWP)of 4.7 kg/kg silica gel [21]. The operation strategy of the adsorptiondesalination plant has been reported highlighting the existence ofthe optimal cycle time depending on the regeneration tempera-tures [22]. The same authors have reported the performance oflow temperature waste heat-driven adsorption desalination cumcooling cycles [23–25].

The motivation of the present study is to enhance the perfor-mance of the conventional multi-bed adsorption desalination cycleby incorporating internal heat recovery within parts of the ADcycle, namely the evaporator–condenser units by having a coolant‘‘run-around” circuit. The performance of the retrofitted AD desali-nation cycle is compared with the conventional AD cycle at similarranges of the heat source, coolant and chilled water temperaturesand flow rates as well as varying half-cycle time.

2. Description of the advanced AD cycle with the condenser/evaporator heat recovery circuit

The schematic diagram of the advanced AD cycle that employsthe internal heat recovery between the condenser and the evapo-rator by a heat recovery circuit is shown in Fig. 1. The AD cyclecomprises an evaporator, a condenser, and four adsorber/desorberheat exchangers. The experimental AD plant was designed with theflexibility to operate either two-bed or four-bed operation mode. Inthe present study, the AD plant is investigated under two-bed

operation mode where two physical adsorbers are under adsorp-tion process while the other two beds perform as desorbers. Thede-aerated sea water is evaporated in the evaporator where thewater vapours are adsorbed on the surfaces of the adsorbentpacked in the adsorber bed which is in communication with theevaporator through vapour pipe. The adsorption process is an exo-thermic process and thus, cooling is needed to maintain the tem-perature of the adsorbent so as to maximise the vapour uptake.For potable water conservation, cooling tower may use either thebrackish or the seawater as the medium for heat rejection. Thecoolant used in the AD cycle during adsorption processes is iso-lated from the cooling tower water, namely the brackish or seawa-ter, via an heat exchanger. The water vapours from the adsorberbed which is previously in adsorption mode are removed usinglow temperature hot water, typically below 85 �C. The regeneratedvapours from the desorber are condensed on the surfaces of thecondenser tubes and condensate is collected as potable water.These evaporation–adsorption and desorption–condensation pro-cess are controlled by the cycle time.

In the advanced adsorption desalination (AD) cycle, the energyrejected at the condenser is recovered for the evaporation of thesaline water in the evaporator. This internal heat recovery isachieved by the implementation of the heat transfer circuit thatruns across the condenser and the evaporator of the AD cycle.

In the advanced AD cycles, the advantages over the conven-tional cycle are (i) the recovery of the condensation energy andits utilization for evaporation of the sea water, (ii) saving in thepumping power requirement for the condenser cooling watercircuit, and (iii) the increase in the pressure of adsorption processis a result of a higher evaporator temperature caused by the recov-ery of condenser heat.

3. Mathematical modeling

Several mathematical models have been proposed for adsorp-tion cooling cycles and can be found in open literatures [26–32].The present mathematical modeling of a 2-bed advanced adsorp-tion desalination cycle with the heat recovery between the

Brine discharge

FM

Potable water extraction

Evaporator-condenser heat recovery loop

Col

lect

ion

tank

Hot

wat

er

tank

Condenser

Evaporator

T T T T

T T T T

T

TP

P P PP

FMT

TP

Coo

ling

wat

er

tank

Silica gel SE I SE II SE III SE IV

T

Cooling tower

1-3 kPa

Gas valve “Closed”

Gas valve “Opened”

Open/close watervalve

Water pump

-T Temperature sensor

-P Pressure sensorFM

Flow meter

Spray nozzles

T

4 -7 kPa

T

T

T

Feed sea water

Fig. 1. Schematic diagram of the advanced AD cycle with the evaporator–condenser heat recovery circuit.

K. Thu et al. / International Journal of Heat and Mass Transfer 54 (2011) 43–51 45

condenser and the evaporator is developed based on adsorptionisotherms, adsorption kinetics and energy balances between thesorption elements (adsorber/desorber heat exchangers), the evap-orator and the condenser. Type-RD silica gel is employed as theadsorbent and the physical properties of type-RD silica gel arelisted in Table 1. Dubinin–Astakhov (D–A) equation is used forthe calculation of the water vapour uptake by the silica gel at spe-cific temperature and pressure and is given by

q� ¼ q0 exp � RTE

lnPP0

� �� �n� �ð1Þ

where, q0 is the maximum adsorbed amount, E is the characteristicenergy and n is the surface heterogeneity parameter.

The transient uptake by the silica gel can be obtained usinglinear driving force equation as

dqdt¼

15Dso exp �EaRT

� �R2

p

ðq� � qÞ ð2Þ

where Dso is the kinetic constant for the silica gel water system, Ea isthe activation energy, Rp is the particle radius and q denotes theinstantaneous uptake.

Table 1Physical properties of type-RD silica gel.

Property Value

Pore size (nm) 0.8–7.5Porous volume (cm3/g) 0.37Surface area (m2/g) 720Average pore diameter (nm) 2.2Apparent density (kg/m�3) 700pH (–) 4.0Specific heat capacity (kJ/kg K) 0.924Thermal conductivity (W/m K) 0.198

It is assumed that the adsorbed and desorbed amounts of watervapour during the adsorption and desorption are the same and theoverall mass balance of the cycle is given by

dMs;evap

dt¼ _ms;in � _md;cond � _mb ð3Þ

Here, Ms,evap is the amount of sea water in the evaporator, _ms;in is therate of feed sea water, _md;cond is the mass of potable water extractedfrom the condenser and _mb is the mass of concentrated brine re-jected from the evaporator. The energy balance of the evaporatorin communication with the adsorbers is written as

cp;s Tevap� �

Ms;evap þMHX;evapcpHX� dTevap

dt

¼ hf Tevap� �

_ms;in � hfg Tevap� �

Msgdqads

dt

� �

þ _mevapcp;evap Tchilled;in � Tchilled;out� �

ð4Þ

Similarly, the energy balance of the condenser is given by

cp Tcondð ÞMcond þMHX;condcpHX� dTcond

dt

¼ �hf Tcondð ÞdMd

dtþ hfg Tcondð ÞMsg

dqdes

dt

� �

þ _mcondcp;cond Tcond;in � Tcond;out

� �ð5Þ

In this cycle, _mcond ¼ _mevap since water re-circulating circuit is em-ployed across the evaporator and the condenser for heat recovery.

The energy balance of the adsorber/desorber is given by

Msgcp;sg þMHX;adscp;HX þMabecp;a� �dTads=des

dt

¼ �Q st Tads=des; Pevap=cond� �

n�Msgdqads=des

dt� _mcw=hwcp;cw=hwðTads=desÞðTcw=hw;in � Tcw=hw;outÞ ð6Þ

Table 2Values of the parameters used in the simulation program.

Parameter Value

q0 0.592 (kg/kg of silica gel)E 3.105 (kJ/mol)n 1.1 (–)Dso 2.54 � 10�4 (m2/s)Ea 4.2 � 10�4 (kJ/kg)Rp 0.4 (mm)Thw,in 50–85 (�C)Tcw,in 30 (�C)Msg 36 (kg)Abed 41.7 (m2)MHX,adscp,HX 184.1 (kJ/K)MHX,evapcp,HX 25.44 (kJ/K)MHX,condcp,HX 12.62 (kJ/K)Uads 250 (W/m2 K)Udes 240 (W/m2 K)Aevap 3.5 (m2)Acond 5.08 (m2)cp,sg 924 (J/kg K)tcycle 600 (s)tswitching 40 (s)_mcw; _mhw 0.83–2.08 (kg/s)_mchilled ¼ _mcond 1.42 (kg/s)

46 K. Thu et al. / International Journal of Heat and Mass Transfer 54 (2011) 43–51

where n is the number of adsorber or desorber bed in the cycle. Inthis analysis, the value of n is 2 since a pair of two reactors is underadsorption process whilst the other pair is under desorption mode.The specific heat capacity cp is calculated as a function of adsorbertemperature (Tads) or desorber temperature (Tdes) during adsorptionor desorption process, respectively. Here Qst stands for the isostericheat of adsorption which is calculated [33,34] as follows,

Q st ¼ hfg þ E � lnq

qm

� � �1=n

þ Tvg@P@T

� �g

ð7Þ

where vg is the specific volume of the gaseous phase, hfg is the latentheat.

The outlet temperature of the water from each heat exchangeris estimated using log mean temperature difference method and itis given by

Tout ¼ T0 þ ðTin � T0Þ exp�UA

_mcpðT0Þ

� �ð8Þ

Here T0 is the temperature of the heat exchanger assuming uniformtemperature across the reactor heat exchanger. In the advanced ADcycle where the heat recovery between the evaporator and the con-denser is achieved by the heat recovery loop that circulates thewater in between them. In this case, the outlet water from the evap-orator is channelled to the inlet of the condenser whilst the outletwater from the condenser is sent to the evaporator.

The energy required to remove water vapours from the silicagels, (Qdes), can be calculated by using the inlet and outlet temper-atures of the heat source supplied to the reactors, and this is givenby,

Q des ¼ _mhwcp;hwðThwÞðThw;in � Thw;outÞ ð9Þ

where _mhw indicates the mass flow rate and cp,hw defines the specificheat capacity of heating fluid. Concomitantly, the energy rejected tothe cooling water during the adsorption process is estimated by theinlet and outlet temperatures of cooling fluid supplied to the otherreactor and this is written as,

Q ads ¼ _mcwcp;cwðTcwÞðTcw;out � Tcw;inÞ ð10Þ

where _mcw and cp,cw indicate the mass flow rate and the specificheat capacity of cooling fluid. At the same time, the heat of evapo-ration (Qevap), and the condensation energy (Qcond) rejected at thecondenser are given by

Q evap ¼ _mevapcp;evapðTevapÞ Tchilled;in � Tchilled;out

� �ð11Þ

Q cond ¼ _mcondcp;cwðTcondÞ Tcond;out � Tcond;in

� �ð12Þ

It is noted that the roles of the reactors for adsorption or desorptionare switched in a half-cycle time.

Finally, the performance of the advanced adsorption desalina-tion cycle is assessed in terms of specific daily water production(SDWP) and the performance ratio (PR) as follows,

SDWP ¼Z tcycle

0

Q condshfgðTcondÞMsg

dt ð13Þ

PR ¼Z tcycle

0

_mwaterhfgðTcondÞsQ des

dt ð14Þ

In this cycle, tcycle is the duration of the adsorption and desorptionprocesses whilst tswitching is the time for which the adsorber anddesorber beds change their roles. The mathematical modeling equa-tions of the advanced adsorption desalination are solved using theGear’s BDF method from the IMSL library linked by the simulationcode written in FORTRAN Power Station, and the solver employs adouble precision with tolerance value of 1 � 10�6. Table 2 furnishesthe parameters used in the simulation program.

4. Results and discussion

The performance of the advanced adsorption cycle with theinternal heat recovery between the evaporator and the condenseris experimentally investigated. The pilot AD plant has been con-structed and the heat recovery circuit is implemented betweenthe condenser and the evaporator. Fig. 2 depicts the pictorial viewof the advanced AD plant showing the condenser–evaporator heatrecovery circuit.

The transient temperature profiles of the major components ofthe advanced AD cycle that incorporates the heat recovery schemebetween the condenser and the evaporator are shown in Fig. 3. Inthis analysis, the hot water inlet temperature was maintained at70 �C while the cycle time (tcycle) and the switching time (tswitching)are kept 600 s with 40 s, respectively. It is noted that the experi-mental temperature measurements are subjects to the time con-stant of the sensors. As can be observed from Fig. 3, thesimulated results of transient temperature agree well with thoseof the experimental data thus the simulation is being validatedby the experiment.

The hysteretic found in the condenser temperature is resultedfrom the location of the sensor which is placed near the desorbervapour pipe. Thus, it fails to capture the temperature drop by thecold front from the evaporator at the beginning of the cycleoperation.

The temperature profiles of the inlet and outlet water of theheating and cooling fluids for 55 �C hot water inlet temperatureare shown in Fig. 4. It is found that there is a large fluctuation inthe hot and cooling water outlet temperatures during the switch-ing period where the pre-heating of the adsorber and the pre-cool-ing of the desorber reactors are taking place.

It can also be observed from Fig. 4 that the temperature differ-ences between the evaporator outlet and the condenser inlet, andthat between the condenser outlet and the evaporator inlet arenegligibly small resulted from the good thermal insulation of thepipes involved in the heat recovery circuit between the condenserand the evaporator.

The parametric study of the advanced AD cycle has been con-ducted to investigate the performance of the cycle under variousoperating conditions. The cycle is studied under different hot waterinlet temperatures, selectively between 50 �C and 70 �C, which isthe practical available temperature range that can be recovered

Fig. 2. Pictorial view of the AD cycle with the condenser–evaporator heat recovery circuit.

0

10

20

30

40

50

60

70

0 100 200 300 400 500 600

Tem

pera

ture

(°C

)

Time (s)T_Ads_Experiment T_Des_Experiment T_Evap_Experiment T_Cond_ExperimentT_Ads_Simulation T_Des_Simulation T_Evap_Simulation T_Cond_Simulation

Desorber

Adsorber

Condenser

Evaporator

Fig. 3. The temperature profiles of the major components of the advanced AD cycle.

15

20

25

30

35

40

45

50

55

60

0 200 400 600 800 1000 1200

Tem

pera

ture

(°C

)

Time (s)

T_Cool In

T_Cool Out

T_Chilled In

T_Chilled Out

T_Cond In

T_Cond Out

Hot water inlet

Hot water outlet

Condenser outlet & evaporator InletCondenser inlet & evaporator outlet

Fig. 4. The temperature profiles of the inlet and outlet of the heat transfer fluids measured experimentally.

K. Thu et al. / International Journal of Heat and Mass Transfer 54 (2011) 43–51 47

48 K. Thu et al. / International Journal of Heat and Mass Transfer 54 (2011) 43–51

from the low temperature waste heat or solar energy. Fig. 5 givesthe specific daily water production (SDWP) and the performanceratio (PR) of the advanced AD cycle while operating at differenthot water inlet temperatures. The experimental results show thatthe SDWP linearly increases with the hot water inlet temperatureand is about 9.24 m3/tonne of silica gel per day at 70 �C hot waterinlet temperature, whilst the SDWP reaches 14.2 m3/tonne of silicagel per day at 85 �C hot water inlet temperature.

The increase in the SDWP with the increase in the hot watertemperature is attributed to the enhanced desorption–condensa-tion process at higher regeneration temperatures resulting in theincrease in SDWP. It should be noted that the advanced AD cycle

0

2

4

6

8

10

12

14

16

45 55 65

SDW

P (m

3 of

pot

able

wat

er p

er to

nne

of

silic

a ge

l per

day

)

Hot water inlet te

Fig. 5. The SDWP and PR of the advanced AD cycle with assorte

0

1

2

3

4

5

6

7

8

22 24 26

SDW

P (m

3 pe

r ton

ne o

f sili

ca g

el p

er d

ay)

Cooling water inle

Hot water inlet temperature 70°C

Hot water inlet temperature 50°C

Fig. 6. The performance of the advanced AD cycle

is capable to operate at substantially low hot water inlet tempera-ture, i.e., 50 �C at which conventional AD cycle fails to operate andthe produced SDWP is 5.2 m3 of potable water per tonne of silicagel per day.

It can be seen from Fig. 5, the performance ratio (PR) of theplant remains relatively constant with the regeneration tempera-ture due mainly to the resilience of silica gel with water vapour.At 85 �C hot water inlet temperature, the SDWP of the advancedAD cycle is found to be 13.5 which is twice that achieved by theconventional AD cycle [24].

The effect of the cooling water inlet temperature on the perfor-mance of the advanced AD cycle is shown in Fig. 6. The SDWP of

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

75 85

PR (-

)

mperature (°C)

Simulation

Experiment

Simulation

Experiment

d hot water inlet temperatures ranging from 50 �C to 70 �C.

28 30 32 34

t temperature (°C)

tcycle = 600stsw = 20s

at different cooling water inlet temperatures.

0

2

4

6

8

10

12

0.0 0.5 1.0 1.5 2.0 2.5

SDW

P (m

3 pe

r ton

ne o

f sili

ca g

el p

er d

ay)

Hot water flow rate (kg/s)

Cooling water flow rate 50 LPM

Cooling water flow rate 75 LPM

Cooling water flow rate 100 LPM

Cooling water flow rate 125 LPM

Hot water inlet temperature = 70°C

Fig. 7. Specific daily water production of the advanced AD cycle for assorted hot water flow rates.

K. Thu et al. / International Journal of Heat and Mass Transfer 54 (2011) 43–51 49

the cycle for two sets of hot water inlet temperatures, i.e., 70 �Cand 50 �C is presented while varying the temperature of the cool-ant from 26 �C to 32 �C. Higher water production rate is achievedat lower coolant temperatures while the production rate decreasesas increased in the coolant temperature.

The performance of the advanced AD cycle under different flowrates of the heating and cooling fluids is presented in Figs. 7–9. It isfound that the SDWP of the cycle increases with the increase in theflow rate while the temperature of heat source and cooling tem-peratures are held at 70 �C and 30 �C, respectively. The perfor-mance improvement is attributed to the better heat transfercoefficient at higher flow rates.

The effect of hot water flow rates on SDWP for four differentflow rates of cooling water is depicted in Fig. 7. SDWP increasesonly moderately with the increase in hot water flow rates.

0

2

4

6

8

10

12

0.0 0.5 1.0

SD

WP

(m3

per t

onne

of s

ilica

gel

per

day

)

Cooling wate

Hot water inlet temperature = 70

Fig. 8. Specific daily water production of the advance

Fig. 8 gives the SDWP of the advanced AD cycle for assorted cool-ing water flow rates at specific hot water flow rate and constant hotwater inlet temperature at 70 �C. The results showed that the waterproduction rate increased at higher cooling flow rates. This is due tothe improvement in the heat transfer coefficient at higher flow rates.It is also noted that the increase in the SDWP becomes less signifi-cant at flow rates higher than 1.67 kg/s due to finite amount ofadsorption and desorption capacities of the adsorbent.

The performance of the advanced AD cycle under different flowrates of the heating and cooling fluids is presented in Fig. 9. Theliner increment in the SDWP is observed while the temperatureof heat source and cooling temperatures are held at 70 �C and29 �C, respectively.

It has been reported that the cycle time at which the AD cycleprovides the optimal potable water production varies with the

1.5 2.0 2.5r flow rate (kg/s)

Hot water flow rate 50 LPM

Hot water flow rate 75 LPM

Hot water flow rate 100 LPM

Hot water flow rate 125 LPM

°C

d AD cycle for assorted cooling water flow rates.

0

2

4

6

8

10

12

0.0 0.5 1.0 1.5 2.0 2.5

SD

WP

(m3

per t

onne

of s

ilica

gel

per

day

)

Hot and cooling water flow rate (kg/s)

Experiment

Simulation

Thot water = 70°C

Tcooling water = 29°C

tcycle = 600s

Fig. 9. Performance of the advanced AD cycle using different flow rates of heating and cooling fluids at constant source temperatures.

50 K. Thu et al. / International Journal of Heat and Mass Transfer 54 (2011) 43–51

available hot water inlet temperatures [23]. The optimal cycletimes for the advanced AD cycle have been experimentally evalu-ated for two different temperature levels of hot water and areshown in Fig. 10. The results showed that the optimal cycle timesexist at the half-cycle time 570 s and 600 s for 70 �C and 65 �C,respectively whilst the corresponding SDWPs are 9.39 and 8.3.The optimal cycle time for the advanced AD cycle is shorter thanthat of the conventional AD cycle [23].

This is because the evaporation–adsorption and desorption–condensation processes in the advanced AD cycle become muchfaster due to the pressurization of the adsorption and the lowerin the condensation temperature resulted from the heat recoveryprocess between the condenser and the evaporator. The pressuri-zation of the adsorption process is achieved since the relativelyhigher temperature water from the condenser is utilized for the

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

200 300 400

SDW

P (m

3 of

pot

able

wat

er p

er to

nne

of s

ilica

gel p

er d

ay)

Half-c

H

H

Fig. 10. The performance of the advanced AD cycle showing the opti

evaporation. On the other hand, the temperature of the outletwater from the evaporator that is channelled to the condenser isaround 25–27 �C and this cold front lower condensing temperatureimparts better desorption process. These effects are more signifi-cant at the beginning of the operation mode after switching pro-cess since the adsorption and desorption rates at maximum atthat period.

5. Conclusions

A significant improvement in the performance of the advancedAD cycle is recorded with internal heat recovery between the con-denser and the evaporator. These findings are also predicted toagree well with the experiments. For this configuration, the maxi-mum specific daily water production (SDWP) of the AD cycle is

500 600 700 800ycle time (s)

ot water inlet temperature = 70°C

ot water inlet temperature = 65°C

Optimal cycle time

mal cycle times at hot water inlet temperatures 70 �C and 65 �C.

K. Thu et al. / International Journal of Heat and Mass Transfer 54 (2011) 43–51 51

about 9.34 m3 of potable water per tonne of silica gel per day at aregeneration temperature of 70 �C and the corresponding perfor-mance ratio is 0.77 which implies that the AD cycle is an efficientcycle. The experimental results showed that it can operate even ata low hot water inlet temperature as low as 50 �C and yet is deliv-ering a respectable SDWP of 5.5. The optimal cycle time of the ad-vanced cycle has been evaluated to be shorter than that of aconventional AD cycle.

Acknowledgements

The authors’ gratefully acknowledge the financial support givenby grants (No. R33-2009-000-101660) from the World Class Uni-versity (WCU) Project of the National Research Foundation,(R265-000-286-597) from King Abdullah University of Scienceand Technology (KAUST) and (R265-000-287-305) from ASTAR.

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