International Journal of Heat and Mass Transfer 54 (2011) 5497–5503

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International Journal of Heat and Mass Transfer

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

Low grade heat driven multi-effect distillation technology

Xiaolin Wang a,b, Alexander Christ a,b,c, Klaus Regenauer-Lieb b,c, Kamel Hooman d, Hui Tong Chua a,b,⇑a School of Mechanical and Chemical Engineering, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australiab Western Australian Geothermal Centre of Excellence, 35 Stirling Highway, Crawley, WA 6009, Australiac School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australiad Queensland Geothermal Energy Centre of Excellence, School of Mechanical and Mining Engineering, The University of Queensland, QLD 4072, Australia

a r t i c l e i n f o

Article history:Received 1 April 2011Received in revised form 21 July 2011Accepted 21 July 2011Available online 16 August 2011

Keywords:Geothermal energyLow grade thermal energyMulti-effect desalinationFreshwater yield

0017-9310/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.ijheatmasstransfer.2011.07.041

⇑ Corresponding author.E-mail address: htchua@mech.uwa.edu.au (H.T. Ch

a b s t r a c t

Low grade heat driven multi-effect distillation (MED) desalination has received tremendous attentionrecently. The primary reason is that many countries, such as Australia, are water short and conventionaldesalination technology is energy intensive. If the required energy hails from fossil fuel source, then thefreshwater production will contribute to carbon dioxide emission and consequently global warming. Lowgrade heat sources such as geothermal energy and waste heat from process plants generate minimal car-bon dioxide. This source of energy is generally abundant at a typical temperature around 65–90 �C inmany localities, and matches perfectly with the MED technology which is driven with a maximum tem-perature of about 90 �C. In this paper, we propose a MED design to better harness the low grade thermalenergy. By means of a calibrated simulation model, validated with experimental data of single effectfreshwater generators, we demonstrate that 25–60% improvement to the freshwater yield compared withconventional MED design is possible.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The worldwide scarcity of freshwater is a serious and an urgenthumanity problem. Due to such factors as the continuous popula-tion growth, improved life styles in developing countries and theincreasing pollution of existing natural resources, global waterscarcity is increasing significantly in the past decades. It affects88 developing countries that are home to half of the world’s pop-ulation. In these places, 88–90% of all diseases and 30% of all deathsresult from poor water quality. Furthermore, the number of peopleaffected is expected to increase fourfold over the next 25 years [1].Approximately two billion people are experiencing water scarcityand among them 1.2 billion people are under physical water scar-city [2]. Industrial desalination of seawater or process water is oneof the possible solutions to assist in alleviating this problem.

The desalination processes can be split into two main categories[3–10]: (1) thermal processes including multi-stage flash distilla-tion (MSF), multi-effect distillation (MED), thermal vapour com-pression (TVC), mechanical vapour compression processes (MVC)and low temperature waste heat driven adsorption desalinationand (2) Membrane processes including reverse osmosis (RO). Thegreatest advantages of thermal processes are their ability to be dri-ven by any type of thermal sources and that they consume very lit-tle electricity. Among all these thermal processes that are driven by

ll rights reserved.

ua).

low grade heat, MED enjoys the highest thermal efficiency sincethe thermal energy is utilized multiple times in the form of latentheat to generate freshwater, and the entire process operates closeto thermal equilibrium. Adsorption assisted desalination, on theother hand, has the lowest thermal efficiency since only 60–70%of thermal energy supplied is dedicated to generating the freshwa-ter and this thermal energy cannot be repeatedly used in its pro-cess, unlike the MED technology. In general, the overall energyconsumption for all these mentioned processes is high. The mem-brane process has a high overall efficiency at the expense of con-suming a large amount of electricity. From the energeticviewpoint, all these desalination processes are energy intensive.This leads to two basic requirements for its economic feasibilityand environmental friendliness, viz.: (i) the desalination technol-ogy should be optimized in order to minimize the overall con-sumption of energy; (ii) the use of renewable energy resources orlow grade heat from process plants should be introduced.

Desalination units driven by sustainable energy have beenwidely used in remote areas. State of the art applications of renew-able energy to desalination are summarized by Mathioulakis et al.[11] and Garcia-Rodriguez [12]. Among the different renewable en-ergy resources available, low grade geothermal energy is one of themost promising options for desalination processes. It promises tobe renewable and does not generate any carbon dioxide. For exam-ple, geothermal energy provides a stable stream of hot groundwa-ter round the clock and it is also reliable. Surveying the largesedimentary basins which occupy a large proportion of the world

Nomenclature

BPE boiling point elevation �Ccp specific heat capacity kJ/kg.KL Latent Heat kJ/kg_m Mass flow rate kg/s_Q Heat transfer rate kWT Temperature �CUA UA value kW/KX Seawater concentration ppm

Subscriptsb boilingc cooling water or condenser

e evaporatorf, feed feed of seawaterh hot waterin, inlet effect and condenser inletk effect numbersdm single-effect distillation moduleout, outlet effect or condenser outletv vapourvs saturated vapour

5498 X. Wang et al. / International Journal of Heat and Mass Transfer 54 (2011) 5497–5503

landmass in general and one fifth of the Australia continental land-mass more specifically [13], geothermal energy at typical temper-atures of 65–90 �C is generally abundant at a typical depth of 1.5–2.2 km which can be accessed by means of standard water-drillingtechnology. This source of energy matches extremely well with themulti-effect distillation (MED) technology which is driven with amaximum temperature of about 90 �C. Higher temperatures haveto be avoided to prevent the precipitation of gypsum which will se-verely foul the heat exchanger in the distillation plant. With thissource of geothermal energy, one can alternatively consider usingit to drive an organic Rankine cycle power plant to produce greenelectricity which is in turn used to power a reverse osmosis plant.At the typical groundwater temperatures that are available, how-ever, the resultant net efficiency of the power plant will be verylow (typically 5% [14]). Consequently geothermal reverse osmosistechnology is commercially unviable.

The MED technology has been widely used in seawater desali-nation and in the chemical industry worldwide due to its low elec-trical energy consumption, low operation cost and high thermalefficiency. The concept of geothermal driven MED plant has re-ceived tremendous attention recently [15–17]. In this paper, wefirst describe the standard low grade heat driven standard MEDtechnology, and then we propose our MED scheme to harnessthe low grade thermal energy more effectively. In order to demon-strate the efficacy of this improved scheme, a simulation model isdeveloped, which is validated with experimental data of single ef-fect freshwater generators from a manufacturer [18].

2. Working principle of a geothermal heat driven MED plant

The standard MED plant comprises a set of evaporator effectsand a final condenser. Seawater enters the condenser tubes andthat helps to preheat the feedwater. A great portion of the seawateris rejected back to the sea as cooling water, while the remnantflows into the heat exchanger of each effect. The motive steam orhot water enters the first effect as the heating medium and the en-ergy is transferred to the feedwater. The feedwater warms up to itsboiling temperature and a portion of it evaporates subsequently.The generated vapour goes through demisters and then flows intothe second effect to be condensed. The coincident condensationand evaporation occurs in the second effect. This process is re-peated throughout all the effects. A portion of the generated va-pour in the last effect is condensed in the final condenser. Thefreshwater from each effect is pumped into a storage tank andthe brine is pumped back to sea.

Focusing then on the geothermal distillation technologies, themulti-effect distillation (MED) plant can be powered with either

hot water or steam. Fig. 1 shows the typical process designs topower the distillation plant with hot water which is the main focusin this paper. The geothermal groundwater is directly fed into thefirst effect of the MED plant. If the geothermal groundwater hashigh salinity, a groundwater heat exchanger can be installed to iso-late the groundwater loop from the MED plant. In order to furtherextract energy from the geothermal groundwater before it is re-turned underground, a preheater can be installed to scavenge en-ergy from the hot water leaving the MED plant to preheat thefeedwater. Turning now to the steam driven technology, a separat-ing vessel is required for the groundwater flashing, so that theflashed stream can be used to power the MED plant. The resultantcondensate and the un-flashed groundwater are then used to pre-heat the incoming seawater. This process is almost the same as thehot water driven technology. Fig. 1 serves to illustrate standard de-signs used to couple geothermal energy to MED technology.

As will be shown later, the standard or typical designs can com-petently extract geothermal energy for the production of freshwa-ter; however they are not optimum in terms of energy utilization.It is very important to appreciate that for geothermal energyexploitation, the more energy we could extract from a geothermalenergy source to derive our desired commodity, be it freshwater,electricity or even cooling energy, the more efficient is the geother-mal energy utilization. This is in stark contrast with the usual par-adigm where the energy source is charged in terms of the amountof energy used. If geothermal energy is charged in terms of theamount of energy we extract or equivalently the extent of temper-ature drop for a given set groundwater flow rate, then the usualparadigm applies. In reality, geothermal energy is not charged interms of the amount of energy one extracts. Hence, once the dril-ling is done the geothermal energy source can be effectively con-sidered as a waste heat source. Hence the exploitation ofgeothermal energy calls for a paradigm shift and mandates anew approach toward designing energy intensive plants. Given aset of groundwater flow rate and temperature, for a fixed amountof exploitable energy relative to ambient temperature, the keyobjective is to maximize the freshwater yield.

Fig. 2 shows a new design scheme of geothermal driven MEDplant. The geothermal hot water is first introduced into the first ef-fect heat exchanger as the driving source for the primary MEDplant. The hot water leaving the heat exchanger is still very hotand it will be wasteful to immediately inject the spent water backunderground. Therefore, this water is pumped into a seawater dis-tillation module (SDM) where a portion of vapour is generated.This generated vapour is judiciously fed into an appropriate effectwithin the primary MED plant. Obviously, the size and seawaterfeed rate of the following effects have to be increased accordinglyto cater for the boost in capacity. It is obvious that this process

Geothermal Hot water, Thi

Geothermal return water, Thr

Thmi

Thmr

Tbs1 Tbs2 Tbs3 Tbs4 Tbs5 Tbs6

Tbv1

Tb2 Tb3 Tb4 Tb5 Tb6Tb1

Tbv2 Tbv5Tbv4 Tbv6Tbv3

Sea water feed, Tw-feed

Sea waterTci

Brine water, Tbj

Return sea water

Generated Fresh water

Fig. 1. Schematics of a geothermal/low grade heat multi-effect distillation plant.

Geothermal Hot water, Thi

Geothermal water, Thmi-booster

Thmi

Thmr

Generated Fresh water

Tbs1 Tbs2 Tbs3 Tbs4 Tbs5 Tbs6

Tbv1

Tb2 Tb3 Tb4 Tb5 Tb6Tb1

Tbv2 Tbv5

Tbv4

Tbv6Tbv3

Sea water feed, Tw-feed

Sea water, Tci

Brine water, Tbj

Return sea water

Geothermalreturn water, Thmo-booster

Tbv-boosterTb-booster

Sea water feed, Tw-feed

Brine water, Tb-booster

Fig. 2. Schematics of the improved geothermal/low grade heat multi-effect distillation plant.

X. Wang et al. / International Journal of Heat and Mass Transfer 54 (2011) 5497–5503 5499

substantially boosts the freshwater yield and results in the maxi-mal exploitation of the available ground source energy. If the mul-ti-effect evaporators reduce to a single effect evaporator, this newdesign can be simply represented by a cascaded two single effectdistillation plant. Therefore, the fundamental basis of such a designcan be demonstrated on a cascaded single effect scheme based onthe manufacturer’s data. This new design scheme also applies toany other conceivable types of low grade heat source.

3. Numerical analysis of the MED system

The main focus in this paper is to demonstrate the efficacy ofthis proposed new design rather than the simulation model itself,a simple mathematical model was adopted directly from the

literature [19]. The model was developed based on the steady statemass and energy balances coupled with the heat transfer equationsfor each individual effect and conjoining them with the ratio be-tween the mass of feedwater to that of produced freshwater.

The energy balance in the first effect is expressed as

_Qe1 ¼ _mhcp;hðTh;inlet � Th;outletÞ¼ _mfeed;1cp;feedðTb;1 � Tf 1;inÞ þ _mv;1Lv;1 ð1Þ

The energy balance equation for the kth effect is written as

_Qek ¼ _mv;k�1½cp;vðTb;k�1 � Tvs;kÞ þ Lv ;k�1�¼ _mfeed;kcp;feedðTb;k � Tfk;inÞ þ _mv;kLv;k ð2Þ

The energy balance equation for the condenser can be expressed as

5500 X. Wang et al. / International Journal of Heat and Mass Transfer 54 (2011) 5497–5503

_Q c ¼ _mv;n½cp;vðTb;n � Tvs;cÞ þ Lv;n� ¼ _mccp;cðTc;out � Tc;inÞ ð3Þ

n is the total number of effects, Cp,v is the constant pressure specificheat capacity of vapour, Cp,feed and Cp,c is the constant pressure spe-cific heat capacity of the brine which depends on water tempera-ture and salinity. Lv is the latent heat of evaporation whichdepends on the boiling temperature and can be calculated by thevapour saturation temperature Tvs as

Lv ¼ 2499:5698� 2:204864Tvs � 1:596� 10�3T2vs ð4Þ

This vapour saturation temperature Tvs is a function of the pressurein the evaporator vapour space. This temperature is less than theboiling temperature Tb by the boiling point elevation (BPE):

Tb ¼ Tvs þ BPE ð5Þ

The BPE at a given pressure is the increase in the boiling tempera-ture due to the salts dissolved in the water. It can be calculated fromthe following empirical formula [19]

BPE ¼ XðBþ CXÞ � 10�3 ð6Þ

with

B ¼ ð6:71þ 6:34� 10�2Tb þ 9:74� 10�5T2bÞ � 10�3 ð7Þ

and

C ¼ ð22:238þ 9:59� 10�3Tb þ 9:42� 10�5T2bÞ � 10�8 ð8Þ

where X is the salt concentration in parts per million [ppm] and theboiling temperature (Tb) in �C. This formula is valid for 20,000 < X <160,000 ppm and 20 < Tb < 180 �C.

Table 1Single-effect distillation (SED) plant performance data available from the Alfa Laval Marin

Freshwater Prod. Th,in Th,out _mh _Qe;1_mc

m3/d �C oC kg/s kW kg/s1 65 55.2 0.83 34 1.16

68 57.3 0.69 31 1.1670 58.1 0.72 36 1.1675 59.9 0.55 35 1.1678 61.3 0.50 35 1.1680 63.8 0.47 32 1.1685 68.8 0.47 32 1.1690 70.5 0.42 34 1.16

2 65 54.3 1.38 62 2.1868 56.1 1.24 62 2.1870 57.0 1.14 62 2.1875 59.7 0.97 62 2.1878 61.6 0.92 63 2.1880 63.0 0.88 63 2.1885 66.9 0.83 63 2.1890 68.8 0.72 64 1.16

3 65 53.5 2.22 107 3.3268 56.6 1.94 93 3.3270 53.4 1.58 110 3.3275 58.3 1.50 105 3.3278 59.9 1.38 105 3.3280 63.3 1.33 93 2.1885 66.7 1.33 102 2.1890 65.9 0.97 98 2.18

4 65 54.4 2.76 123 4.3468 56.1 2.48 124 4.3470 57.0 2.66 145 4.3475 61.3 2.21 127 3.3278 61.9 2.04 138 3.3280 62.5 1.71 126 3.3285 66.4 1.60 125 3.3290 66.8 1.36 132 3.32

Tc,in 32 �C X 35000 ppm

The vapour generated in the evaporator is superheated steam atthe outlet of effects. Vapour pressure does drop when this gener-ated vapour flows from an upstream effect to a downstream effect.This pressure drop includes the pressure loss caused by the demis-ter, friction loss in the connecting pipes and pressure drop duringthe condensation in the evaporator. However, these are typicallyvery small and their effect on the vapour temperature drop is neg-ligible. Therefore, the temperature of the vapour is nearly the sameas the boiling temperature from the outlet of an upstream effect tothe inlet of a downstream effect.

The feedwater inlet temperature (Tfk,in) to each effect is thesame as the cooling seawater outlet temperature (Tc,out) becausea portion of the cooling seawater is used as the feedwater to re-cover waste heat.

The dimensions of the evaporators and condenser are deter-mined by their UA values. The UA value for each component is cal-culated from the amount of transferred heat Q and the logarithmicmean temperature difference (LMTD). In the first effect, the energyis transferred from the geothermal hot water to the feedwater. Insubsequent effects, the energy is transferred from the vapour com-ing from the upstream effect to the feedwater. In the condenser,the vapour from the last effect of evaporators is condensed bythe cooling seawater. Therefore, the UA values for all effects andcondenser are determined by:

For the first effect:

UAe;1 ¼_Q e1

ðTh;inlet�Tf 1;outletÞ�ðTh;outlet�Tf 1;inletÞ

lnTh;inlet�Tf 1;outletTh;outlet�Tf 1;inlet

� �24

35

ð9Þ

e & Diesel product catalogue [18].

Freshwater Prod. Th,in Th,out _mh _Qe;1_mc

m3/d oC �C kg/s kW kg/s5 65 57.7 5.00 153 6.24

68 57.9 3.61 153 7.0970 57.5 2.91 153 5.6775 60.6 2.76 167 4.3478 61.2 2.27 160 4.3480 62.9 2.21 159 4.3485 66.0 2.04 163 3.3290 68.2 1.77 162 3.32

7 65 57.3 6.65 215 8.5168 59.6 6.07 214 6.2470 60.8 5.54 214 6.2475 60.9 3.60 213 7.0978 60.7 2.90 211 5.6780 62.8 2.91 210 5.6785 66.7 2.77 213 4.3490 68.4 2.49 226 4.34

10 65 57.2 9.37 307 10.2168 59.5 8.57 306 10.2170 60.9 7.98 305 8.5175 64.5 6.89 304 8.5178 66.7 6.36 302 6.2480 68.3 6.11 300 6.2485 65.3 3.59 297 7.0990 70.4 3.60 296 7.09

15 65 57.1 13.86 460 15.8868 60.6 14.64 455 13.8970 60.8 11.85 458 13.8975 64.4 10.22 455 10.2178 66.6 9.44 452 10.2180 67.9 8.86 450 10.2185 71.8 8.05 446 8.5190 71.7 5.79 445 8.51

0

2

4

6

8

10

12

14

16

2 4 7 10 15

Wat

er p

rodu

ctio

n (m

3 /day

)

Original water production (m3/day)

Original water yieldExtra yield @ 90 °CExtra yield @ 85 °CExtra yield @ 80 °CExtra yield @ 78 °C

52%

49%

55%

62%

49%

50%

58%

57%

52%

67%

70%

67%

67%

67%

Fig. 4. Projected boost in freshwater yield using our proposed technology tocascade two single-effect freshwater generators at different heat source temper-atures and a coolant temperature of 32 �C [18].

70

80

90

100

70

80

90

100

ter y

ield

(%)

n [m

3 /day

]

X. Wang et al. / International Journal of Heat and Mass Transfer 54 (2011) 5497–5503 5501

For the kth effects:

UAe;n ¼_Q ek

ðTb;k�1�Tb;kÞ�ðTvs;k�Tfk;inÞ

lnTb;k�1�Tb;kTvs;k�Tfk;in

� �24

35

ð10Þ

For the condenser:

UAc ¼_Q c

ðTb;n�Tc;outÞ�ðTvs;c�Tc;inÞ

lnTb;n�Tc;outTvs;c�Tc;in

� �24

35

ð11Þ

For the SDM in the proposed design:

UAsdm ¼_Q sdm

ðTsdm;inlet�Tfsdm;outletÞ�ðTsdm;outlet�Tfsdm;inletÞ

lnTsdm;inlet�Tfsdm;outletTsdm;outlet�Tfsdm;inlet

� �24

35

ð12Þ

The total freshwater generated, mv, is the sum of the vapour gener-ated in each effect:

mv ¼Xn

k¼1

mv;k ð13Þ

0

10

20

30

40

50

60

0

10

20

30

40

50

60

72 77 82 87 92

Proj

ecte

d bo

ost o

f fre

shw

a

Fres

hwat

er p

rodu

ctio

Heat source temperature [oC]

Fig. 5. The efficacy of our proposed scheme as applied to a four-effect distillationplant. s, freshwater yield in a standard four-effect distillation plant at a coolingwater temperature of 25 �C; h, freshwater yield of a standard four-effect distillationplant at a cooling water temperature of 32 �C. e, freshwater yield and improvementby using our proposed design at 25 �C; �, freshwater yield and improvement byusing our proposed design at 32 �C.

80

90

100

110

120

100110120130140150

ater

yie

ld (%

)

ion

[m3 /d

ay]

4. Results and discussions

We calibrate our model using single-effect distillation (SED)plant performance data available from the Alfa Laval Marine & Die-sel product catalogue [18]. These are tabulated in Table 1. The cal-ibrated model is then used to demonstrate the ameliorationachieved by our proposed design. It is noteworthy that this pro-posed design simplifies to a cascaded single effect distillation plantwhen the number of effects is reduced to one. This enables us tovalidate the fundamental basis of our proposed design using theAlfa Laval SED plant data [18]. The proposed design is then appliedto four and six effects distillation plants under different operationconditions which are the most common on the market.

Fig. 3 depicts a comparison between predicted freshwater yieldfrom the simulation and actual freshwater yield from SEDs re-ported in the Alfa Laval single effect freshwater generators cata-logue [18]. In total, 48 cases in the catalogue stemming fromdifferent heat source temperatures, heat source flow rates andfreshwater generator capacities are studied. It is evident that thepredicted data match the actual data very well for all the 48 cases,with the prediction error being within 5% for most cases.

0

2

4

6

8

10

12

14

16

18

0 2 4 6 8 10 12 14 16

Sim

ulat

ed F

resh

wat

er Y

ield

[m3 /d

ay]

Actual Freshwater Yield [m3/day]

Fig. 3. A comparison between our predicted freshwater yield and actual freshwateryield derived from Alfa Laval single effect freshwater generators [18].

0

10

20

30

40

50

60

70

0102030405060708090

72 77 82 87 92

Proj

ecte

d bo

ost o

f fre

shw

Fres

hwat

er p

rodu

ct

Heat source temperature [oC]

Fig. 6. The efficacy of our proposed scheme as applied to a six-effect distillationplant. s, freshwater yield in a standard four-effect distillation plant at a coolingwater temperature of 25 �C; h, freshwater yield of a standard four-effect distillationplant at a cooling water temperature of 32 �C. e, freshwater yield and improvementby using our proposed design at 25 �C; �, freshwater yield and improvement byusing our proposed design at 32 �C.

When the number of effects is one, our proposed scheme sim-plifies to a cascaded single effect distillation plant. The efficacy ofthe proposed design can then be directly demonstrated using

Table 2The operating conditions for the four and six effect distillation plants.

Th,inlet Th,outlet _Qe;1 Tsdm,inlet Tsdm,outlet _Qsdm_mh

�C �C kW �C �C kW Kg/s

MED without SDM75 65 450.0 – – 10.780 68 450.0 – – 9.085 70 450.0 – – 7.290 70 450.0 – – 5.4MED with SDM75 65 450.0 65 57.1 355.5 10.780 68 450.0 68 59.5 318.8 9.085 70 450.0 70 60.8 276.0 7.290 70 450.0 70 60.8 207.0 5.4Cooling temperature, Tc,in 25 �C / 32 �CSalt concentration X 35000 ppm

Table 3A typical simulation results for a given condition at heat source temperature 80 �C and seawater temperature 25 �C.

Operating Conditions

Input information Th,inlet 80 �C Tsdm,inlet 68.0 �C Tc,in 25.0 �CTh,outlet 68 �C Tsdm,outlet 59.5 �C_Qe;1 450 kW _Qsdm

318.8 kW X 35000 ppm

ResultsEffects Item Unit 1 SDM 2 3 4 5 6 Condenser4 Effect MED without SDM _mf = _mc kg/s 0.31 NA 0.295 0.284 0.276 NA NA 7.98

Tb �C 56.9 NA 53.2 49.6 45.6 NA NA -UA kW/K 16.7 NA 48.8 53.3 58.5 NA NA 29.3Freshwater production 59.1 m3/day

4 Effect MED with SDM _mf = _mc kg/s 0.31 0.22 0.297 0.499 0.486 NA – 14.08Tb �C 55.4 54.2 51.2 48.6 45.0 – – -UA kW/K 16.3 17.9 48.8 93.6 112.3 – – 54.4Freshwater production 92.0 m3/day

6 Effect MED without SDM _mf = _mc kg/s 0.306 – 0.286 0.270 0.258 0.248 0.243 7.04Tb �C 63.5 – 59.8 56.1 52.5 48.7 43.6 -UA kW/K 19.6 – 39.4 41.0 43.6 47.3 47.3 31.0Freshwater production 82.0 m3/day

6 Effect MED with SDM _mf = _mc kg/s 0.307 0.219 0.288 0.274 0.473 0.458 0.448 13.0Tb �C 62.2 56.3 57.7 53.3 51.0 47.4 43.0 -UA kW/K 18.9 19.3 37.4 41.0 84.2 94.2 101.0 61.0Freshwater production 126.5 m3/day

5502 X. Wang et al. / International Journal of Heat and Mass Transfer 54 (2011) 5497–5503

experimental data provided in the Alfa Laval specifications for thesingle effect freshwater generators at different heat source temper-atures. Based on the Alfa Laval catalogue data [18], Fig. 4 shows theprojected boost in freshwater yield by cascading two single-effectfreshwater generators at a coolant temperature of 32 �C. It is palpa-ble that the projected boost in freshwater yield is at least 50% bycascading two single effect distillation freshwater generators.

The typical commercially available multi-effect distillationplant is four to seven effects at a heat source temperature between75–90 �C. Figs. 5 and 6 show the improvement of the proposed de-sign applied respectively to a four- and a six-effect distillationplant at different heat source and cooling water temperatures.The corresponding operating conditions are listed in Tables 2 and3 shows a set of typical simulation results for different effects ata heat source temperature, 80 �C and seawater temperature,25 �C. The proposed design improves the freshwater yield by 25–60% depending on the heat source and cooling water temperatures.It is observed from the figures that the improvement using theSDM design decreases with the heat source temperature. Withhigher heat source temperature, more high quality energy is uti-lized in the first effect of the primary plant and less energy isrecovered from the separate seawater distillation module (SDM).Therefore, the proportion of recovered energy is lower than thatfor a lower temperature heat source and hence the improvementto freshwater yield diminishes. The improvement drops with

cooling temperature as well. At a lower cooling water temperature,the whole MED plant can operate at a relatively lower temperaturein each of the effects and hence the vapour generated in the singleseawater distillation module can be fed into a higher effect, there-by generating more freshwater through the various effects down-stream. For example when the cooling water temperature is32 �C, the vapour generated in the SDM can only be fed into the lasteffect of a four effect distillation plant and the fourth effect of a sixeffect distillation plant. However, when the cooling water temper-ature is 25 �C, the whole MED plant operates at a relatively lowertemperature in each effect. The vapour generated in the SDM cannow be fed into the third effect of the four effect distillation plantand the third effect of the six effect distillation plant. The recoveredenergy is then used to generate one more portion of freshwater.Therefore, the improvement at a lower cooling temperature ishigher as shown in the figures.

5. Conclusion

A new design for low grade heat driven MED plants is proposedand its efficacy is firstly demonstrated by two cascaded singlestage freshwater generators, based on experimental performancedata provided in the technical specifications of an Alfa Lavalcatalogue [18]. The projected boost in freshwater yield ranges from

X. Wang et al. / International Journal of Heat and Mass Transfer 54 (2011) 5497–5503 5503

50% to 70%. A simulation code, validated by experimental datafrom the technical specifications of Alfa Laval freshwatergenerators, was developed to theoretically study its impact onmulti-effect distillation plants. Based on our simulation, the ame-lioration offered by the new design varies with cooling water andheat source temperatures. Our proposed design can improve thefreshwater yield by 25–60%.

Acknowledgment

We gratefully acknowledge the financial support from the Wes-tern Australian Geothermal Centre of Excellence and the NationalCentre of Excellence in Desalination.

References

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