naturally circulated humidifyingdehumidifying solar still with a built-in passive condenser

0011-9164/04/$ See front matter 2004 Elsevier B.V. All rights reserved

Desalination 169 (2004) 129149

A naturally circulated humidifying/dehumidifying solar stillwith a built-in passive condenser

Hassan E.S. Fath, Samy Elsherbiny*, Ahmad GhazyMechanical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt

Tel. +20 (10) 514-2218; Fax: +20 (3) 546-9378; email: h_elbanna_f @ hotmail.com

Received 7 October 2003; accepted 9 December 2003

Abstract

A numerical study has been carried out to investigate the transient thermal performance of a naturally circulatedhumidifying/dehumidifiying solar still. A comparison of forced circulation performance and the influence of differentenvironmental, design, and operational parameters on the still productivity and efficiency were investigated. Thenaturally circulated still shows very similar results to that of forced circulation. This finding is of significant technicaland economic importance. Different attempts have been considered to investigate the effect of partial storage of basinenergy and partial recovery of condensation energy. The results show insignificant changes on still performance. Aneconomic assessment of water production cost was also highlighted and showed that solar stills can challenge othertechnologies for special applications.

Keywords: Desalination; Solar still; Humidification; Dehumidification

1. Introduction

Fresh water is the essence of life and is themost important constituent of the environment.Water is a basic human requirement for domestic,industrial and agriculture purposes. Rapid inter-national developments and population explosionall over the world have resulted in a largeescalation of demand for fresh water. On theother hand, surface water (rivers and lakes)pollution caused by industrial, agricultural and

*Corresponding author.

domestic wastes limits the availability of freshwater in many regions. By the beginning of thiscentury, fresh water shortages and quality becamean international problem confronting humans andcountries. The problem is more apparent in theMiddle Eastern/North African (MENA) countriesdue to their limited natural resources.

According to the International Atomic EnergyAgency (IAEA), an estimated 1.1 billion peoplehave no access to safe drinking water, and morethan 5 million die from water-borne diseases eachyear. Provisions are no better even for the future.

H.E.S. Fath et al. / Desalination 169 (2004) 129149130

It is estimated that more than 2.7 billion peoplewill face severe water shortages by the year 2025if the world continues consuming water at thesame rate per capita and real population growthfits the forecasted trend. The crisis is mainly dueto the mismanagement of existing water re-sources, population growth, and continuousclimatic changes. It is, therefore, necessary that asincere effort be made to face the looming watercrisis and conserve shrinking water supply amidthe rising demand [1].

Seawater desalination has already confirmedits partial potential to resolve the fresh waterproblem in numerous countries. Although itseems more expensive in area of surface orground water availability, it is not so in areas300500 km from surface fresh water availabilityor of deep ground fresh water. For municipalitieswith large water demands, conventional desali-nation technologies such as MSF, MED, RO andVC proved to be technically and economicallysuitable. Considering the fact that areas of verysmall communities exposed to the water scarcityare at the same time characterized by high levelsof solar radiation, appropriate consideration needsto be given to the opportunity of using solarenergy no doubt a bit costly but withoptimal features for its coupling with desalinationprocesses. This is true, especially in isolated andremote areas, having no access to an electricalgrid or fuel supply and with very few technicalcapabilities.

Direct solar distillation, in many respects,might be an ideal solution to small communitiesin many MENA countries due to the following:C Many of these countries enjoy an abundant

solar intensity (annual daily average is be-tween 200300 W/m2) and annual sun hours(30005000 h/y); with an incident energy ofabout 56 kWh/d.

C The diurnal and seasonal fluctuation in solardistillation productivity is intrinsically linkedto the fluctuating water demand

C Direct solar stills involve simple technologythat needs fewer design, manufacturing,operation and maintenance capabilities

C Solar energy is available in almost everylocation and, in addition, is an environmen-tally friendly energy resource (with no CO2emission)

Many researchers have investigated anddeveloped solar stills of different configurationsin order to increase still efficiency and yield (see,e.g., Fath [2,3], Delyannis [4], Hamed et al. [5],Nafey et al. [6,7]). The efficiency of solar stillscan be increased by increasing input energy. Thiscould be partially achieved by (1) having theliquid surface oriented at an optimal inclination toreceive maximum solar radiation and (2) placingthe transparent glass cover of the still parallel tothe water surface to minimize reflection losses.Several methods have been tried, including theinclined stepped still [8], tilted tray, and multipleledge multi-wick [9].

Humidification/dehumidification techniqueshave also been introduced as an effective solardistillation system. Different systems have beenproposed in the literature: (1) with/withoutair heating, (2) with/without water heating,(3) forced/natural air circulation (see, e.g., Dater[10], Fath and Ghazy [11], Nafey et al. [12,13],Younis et al. [14] and Muller-Holst et al. [15,16]).

There are two main drawbacks to most ofthese single-effect solar stills. The first is the lossof large amounts of condensation energy from thestill. The combined evaporation, convection andradiation energy received by the condensationsurface(s) represents about 4050% of the energylost to the atmosphere. One may consider thepartial recovery of this heat for (1) feed pre-heating, as in Mink et al. [17,18] and Kunze [19],or (2) in an energy storage system for after sun-set energy recovery. Note that a full recovery ofcondensation energy is impossible since the glass

H.E.S. Fath et al. / Desalination 169 (2004) 129149 131

(as a condensing surface and the door of the inletsolar energy) cannot be covered during the day.An additional condenser might be used for addi-tional heat and mass sink as presented by Fath etal. [20,21]. The capital and running cost of anyenergy recovery system, as well as the additionaltechnical complexities, should be justified withadditional yield production. The second drawbackis the sinusoidal trend of the stills temperatureand water productivity. Water is not producedafter sunset and during cloudy periods where theatmospheric temperature is minimum and heatsink is mostly available. High midday tempera-tures lead to greater energy losses and a need formore expensive absorbing materials. This draw-back can, however, be overcome by increasingthe thermal storage capability of the system inorder to store part of the solar energy and re-utilize it after sunset and during cloudy periods.The incorporation of such energy storage systemsshould be justified against a simple still withdaily water production.

For both absorber and condensation energystorage and after sunset recovery, two thermalstorage systems could be used either a sensibleor a latent heat system. The latent heat thermalenergy storage system (LHTESS) has manyadvantages over sensible heat storage, including(1) larger energy storage capacity per unitvolume, and (2) an almost constant temperaturefor energy charging and discharging (see Fath[22]). Paraffin wax and Glaubers salt are typical

phase change materials (PCM) generally used insolar energy storage systems. Table 1 shows themain properties of the energy storage materials:paraffin wax (to be used for basin energy storage)and Glauberts salt (to be used for condensationenergy storage and recovery).

Fath et al. [2] proposed a simple and efficienthumidifiying/dehumidifying distiller (HDD). Themain drawback of this still is the forced circula-tion, which requires a fan. On the other hand, theauthors indicated that, within the parametricvalues studied, a decreasing circulating air flowrate showed an insignificant effect on still pro-ductivity and efficiency. This result leads to thepossible use of natural circulation to drive the airflow rate. Natural air circulation with a small stillis more desirable since the absence of a circu-lating fan means a large reduction of both capitaland operation costs in addition to the eliminationof the technical complexities. How natural aircirculation affects the total productivity and otherperformance parameters of the still (with andwithout energy storage and recovery) is the focusof the present study.

2. System description and analysis

2.1. Description

Fig. 1a illustrates the proposed HDD con-figuration and its main components [2]. The stillbody is a thin rectangular box constructed of

Table 1Thermo-physical properties of some storage materials

Material properties Sand Paraffin wax(Sunoco P116)

Glauberts saltNa2SO410H2O

Melting temperature, C 60 30Latent heat of fusion, kJ/kg 190 251Solid/liquid density, kg/m3 1500 930/830 1460/1330Thermal conductivity, W/m.C 0.29 0.21 2.25Solid/liquid sp. heat, kJ/kg.C 0.798 2.1 1.72/3.3


Fig. 1. Proposed humidifier/dehumidifier solar still.

1.0 mm thick framed aluminum sheeting with a3.0 mm (window-type) glass cover. A centralstepped absorber sheet (connected to the two still

side walls with two slots at the top and bottomside walls) divides the still into two chambers,upper (glass to absorber) chamber and lower


Table 2Thermo-physical properties of the stills main com-ponents [23]

Property Glasscover

Basin/condenser

Insulation

Material Windowglass

Aluminum Fiberglass

Density, kg/m3 2700 2707 24Specific heat, kJ/kg. C

0.754 0.896 0.7

Thermal conductivity, W/m.C

0.78 204 0.038

Absorbitivity 0.1 0.9 0.1Transmissivity 0.9 0.0 0.0

(absorber to condenser) chamber. The dividingstepped sheeting carries a series of insulated andblack coated basins which contain the salinewater. The dehumidifying condenser is made of1.0 mm aluminum sheeting. Air is circulatedbetween the upper chamber (where it is heatedand humidified) and the lower chamber (where itis cooled and dehumidified for water production).The still has a tilted configuration to enhancesolar energy reception. Table 2 summarizes themain physical and thermal properties for theproposed stills component materials.

2.2. Heat and mass balance

Fig. 1b illustrates the energy balance of onebasin (element) of the proposed still. Incidentsolar energy (1) is partially reflected (2) andpartially absorbed (3) by the glass cover. Themain part is transmitted through the glass cover(4) to the saline water and absorber. Part of thetransmitted energy is reflected back from thewater surface (5), and the rest is absorbed bysaline water (6) and the absorber surface (7). Partof the absorber energy is transferred to the sea-water by convection, another part is conducted to

the insulation (8), and the rest heats the absorber.Some of the insulation energy is transferred to thedehumidifier by convection (9) and to the con-denser by radiation (10).

As soon as the water temperature exceeds thesurrounding air temperature, saline water startstransferring heat and vapor to the circulating airin the humidifying channel. The generated vaporcarries both convection (11) and evaporation (12)energies. Saline water surfaces also radiate someenergy (13) to the glass cover. Part of the en-trained water vapor in the circulating air iscondensed on the glass and leaves its convective(14) and condensation energy (15), while the aircarries the rest of the entrained vapor to the nextelement. As a limiting criterion, the circulating airhumidity cannot exceed the saturation conditionat its temperature. The heated glass cover loses itsheat to the surroundings by convection (16) andto the atmosphere by radiation (17).

When the absorber contains PCM for partialenergy storage, heat is conducted from the basinto the PCM for energy storage. Heat is first stored(charging process) as sensible heat till PCMreaches its melting temperature. Additionalcharging heat is then stored as latent heat, and themelted PCM thickness increases. After completemelting, any additional heat will be stored asliquid PCM sensible heat. When the absorbercools down (after sunset, say), the liquid PCMtransfers heat to the absorber (discharging pro-cess) and its temperature decreases till it reachesthe solidification temperature, after which anydischarged heat will be extracted as latent heat tillthe PCM layer is fully solidified. More extractedheat will be sensible heat to cool the solid PCM.The chargingdischarging processes for thestored energy in the condenser PCM are similar.

2.3. Governing equations

Based on the heat and mass balance discussedabove, the governing equations for the still com-ponents can all be written in the form:


M C dT/dt = E Qin!E Qout (1)EQin and EQout for each component can beexpressed as follows:

C Glass cover:

E Qin = Qsu-g + QRsw-g + QCa-g + QCDa-gE Qout = QRg-sk + QCg-amC Absorber:

E Qin = Qsu-abE Qout = QCab-sw + QKab-iC Saline water:

E Qin = Qsu-sw + QCab-sw E Qout = QCsw-a + QEsw-a + QRsw-gC Basin insulation:

E Qin = QKab-i E Qout = QRi-co + QCi-aC Condenser:

E Qin = QCa-cu + QCDa-cu + QRi-cuE Qout = QCcu-am + QRcu-erthC Humidified air (evaporation chamber):

Ma h2a = Ma h1a + QCsw-a + QEsw-a !QCa-g!QCDa-g(2)

C Dehumidified air (condensation chamber):

Ma h2a = Ma h1a + QCi-a!QCa-cu!QCDa-cu (3)

C PCM/container:Meq Ceq dTeq/dt = E Qin!E Qout (4)

In this equation, the PCM (wax or Glaubertssalt) and its container (absorber, condenser, andinsulation) are combined as one equivalent body(for each element). Based on its temperature, thePCM and its container are combined as thecontainer material equivalent mass (Meq) of heatcapacity equivalent to:C the sensible heat for both the container and

solid PCM (for T Tm)

For example, the equivalent absorber/PCM (wax)heat capacity [Eq. (4)] is calculated as follows:

Meq Cab = [Mab * Cab + Mpcm * C(s)pcm]

for Tab < TmMeq Cab = [Mab * Cab + Mpcm * HsL]

for Tab = Tm + *Meq Cab = [Mab * Cab + Mpcm * C(l)pcm]

for Tab > Tm

Similar equations are written for the condenser/PCM (Glauberts salt) and other combinations.Details of the above terms can be expressed asfollows:

I = Io sin (B t/tsu )Qsu-g = "g I ApQRsw-g = F gsw Ap [(T+273)4sw!(T+273)4g]QCa-g = HCa-g (Ta!Tg) (Ap + Asid)QCDa-g = HCD (Pa!Pg) (Ap + Asid)QRg-sk = F gg (Ap +Asid) [(T+273)4g!(T+273)4sk]QCg-am = HCg-am (Ap + Asid ) (Tg!Tam)QCsw-a = HCsw-a Ap (Tsw!Ta)QEsw-a = HE Ap (Psw!Pa)


Qs-ab = "ab Jsw Jg I ApQCab-sw = HCab-sw Ab (Tab!Tsw)QKab-i = (Ki /Thi /2) Ap (Tab !Ti)Qsu-sw = "sw Jg I ApQRi-cu = F gi Ap [(T+273)4i !(T+273)4cu]QCi-a = HCi-a Ap (Ti !Ta)QCa-cu = HCa-cu (Ap + Asid ) (Ta!Tcu)QCDa-cu = HCDa-cu (Pa!Pcu ) (Ap + Asid)QCcu-am = HCcu-am (Ap + Asid ) (Tcu!Tam)QRcu-erth = F gcu (Ap + Asid)

[(T+273)4cu!(T+273)4am]

ME/CD = QE/CD/Hfg(t)

Tam = Tam-min + Tam-o sin (Bt / tday)Tsk = [0.0552 (Tam + 273)1.5] !273HChot-cold = 0.884 [(Thot!Tcold ) + (Phot!Pcold)

(Thot + 273) / (268,900!Phot)]0.33

HCD/E = 0.016 HChot-coldHam = 5.7 + 3.8 VwHCab-sw = 0.54 (Ksw/Lab) (Gr Pr)1/4

If Gr Pr < 8*106

= 0.15 (Kwtr/Lab) (Gr Pr)1/3

If Gr Pr > 8*106

HCi-a = 0.27 (Ki/Li) (Gr Pr)1/4

P = 0.14862 T!0.0036526 T2 + 0.0001124 T3Hfg(t) = [2503.3!2.398 T(t)]*103

2.3.1. Natural circulation air flow rateThe naturally developed air mass flow de-

pends on the balance between both driving andresisting forces (pressures). The driving force

depends on: (1) the difference of average airdensity in both humidification and dehumidi-fication channels (which varies along with airtemperature and humidity) and (2) the stillsheight. The resisting force varies with circuitresistance and flow velocity. The air flow rate canthen be calculated as follows:

ma = ()PD/)PR)0.5 (5)where (see Ghazy [24]):

)PD = (Ddh!Dh) g L cos (2))PR = )PR1 + )PR2 + )PR3)PR1 = 0.018 L/2 Dh Dh A2a )PR2 = 0.018 L/2 Ddh Ddh A2a )PR3 = (0.3/2 A2a) (1/Ddh + 1/Dh)Dhyd = 4 Aa/perimeterAa = L (B!Thi)/2Perimeter = 2 [L + (B!Thi) /2]

2.3.2. Productivity and efficiencyThe stills productivity and instantaneous and

daily average efficiencies are calculated asfollows:

Productivity = Mpw-g + Mpw-cu = EME(t) )t (6)

0inst = (Mpw-g + Mpw-cu) Hfg(t) /(I Ap) = [ME(t) Hfg(t)] / [I(t) Ap] (7)

0av = [ME(t) Hfg(t)] )t /[E I(t) )t Ap] (8)

A computer program was developed to solvesimultaneously the above equations. The stillscomponent transient temperatures, heat transfercomponents, still productivity, instantaneous anddaily average efficiencies are then calculated.


3. Results and discussionTable 3 summarizes the environmental, de-

sign, and operational parameters and their studiedvalues for the parametric study.

3.1. Circulating air flow rateAs indicated above, the circulating air flow

rate depends on the balance between the drivingand resisting forces (pressures). Both forces

depend on air conditions along the chambers andchannel configurations in the still. The predictionof circulating air flow rate requires an iterativeapproach for the solution of Eq. (5) simultane-ously with Eqs. (1)(4). Fig. 2 shows the changesin the air flow rate with time for the referencevalues. Air flow rate increases sharply in the first3 h after sunrise, then remains almost uniform forthe next 7 h (of about 0.0138 kg/s ), then dropsnear sunset. Comparing with Fath et al. [2],

Table 3Range of parameters for 2.0 m2 projected area still

Parameter Min. value Reference (base) value Max. value

Environmental: Solar intensity at mid-day, Io (W/m2) 600 800 1000 Min./max. ambient temperature, C 10/20 20/30 30/40 Wind speed (Vw), m/s 0 2 5Design: Basin absorbitivity ("ab) 0.5 0.9 1.0 Condenser absorbitivity ("co) 0.5 0.9 1.0 Condenser/projected area ratio 1 1.5, 2.0 Evaporation/projected area ratio 1 1.5, 2.0 Basin (glass wool) insulation thickness, mm 10 50 100Operational: Basin water mass (Mb), kg 12 15 20 Initial water temperature, C 20 30 Still tilting angle (from horizontal) 10 30 60

Fig. 2. Naturally circulating air flow rate.


the results indicate that natural air circulationgives a very close value of air flow rate to that ofthe forced circulation most of the effective hoursof the day. A forced air flow rate of 0.01 kg/s isalmost the average value of the natural circulationrate.

3.2. Energy balanceFig. 3 illustrates the still overall energy

balance for the reference value at midday. Thefigure shows that for an incident solar intensity of800 W, 80 W are absorbed on the glass cover,72 W are reflected from the seawater basin, and

Fig. 3. Overall performance of still.


56.5 W are reflected from the absorber surface.The rest is mainly absorbed in the absorber(518.4 W) and the seawater (72 W). Very little ofthe absorbed energy in the seawater is radiated tothe glass cover (16.3 W), and the large amountforms evaporation heat (525.2 W), in addition toconvection heat of 33.5 W, transferred from theseawater to the circulating air in the humidifyingchannel. The circulating air carries the addedenergy to its large recirculating enthalpy of3995.74 W (note here that the values of theenthalpy are not drawn to the same scale of thestill energy components, but are just descriptive).

The air enthalpy (4181.26 W) is mainly recir-culated to the dehumidifying channel where it ispartially transferred to the condenser as con-vection (30.6 W) and condensation (309.95 W)heat. Condensation heat produces the condenserwater (after dividing by the latent heat of conden-sation). The total heat transferred to the con-denser is dumped into the environment byconvection to the atmosphere (238.8 W) andradiation to the earth (109.5 W).

The recirculated air enthalpy to the humidi-fying channel (4181.26 W) is partially transferredto the glass cover by convection (18.64 W)and (condensation (200.38 W) and the rest(3995.74 W) is recirculated to repeat the cycle.Condensation heat on glass produces the glasscover water production (after dividing by thelatent heat of condensation). Heat transferred toand absorbed by the glass is dumped to theenvironment by convection to the atmosphere(289.8 W) and radiation to the sky (23.5 W). Notethat at midday a very small amount of energy isstored in the glass cover (1.95 W), seawater(5.03 W), insulation material (0.21 W) and con-denser (0.75 W).

Fig. 4 shows the stills main overall perfor-mance parameters. The figure shows very closeresults between the present naturally circulatedair and that of forced circulation [2]. With naturalcirculation, the still daily average efficiency (0av)is about 56% and still productivity is 5.1 kg/

Fig. 4. Overall still performance.

d.m2. In both cases, water production ends almost12 h after sunset due to the limited system heatcapacity. Water production at the glass cover isabout 2.15 kg/d.m2 (42 % of the total), while thecondenser produces 2.95 kg/d.m2 (58% of thetotal).

Fig. 5 shows that the still temperature profilefollows the sinusoidal trend following the inputsolar intensity. Fig. 5 also shows the gradualchanges in still components temperature along itslength. As was expected, the absorber has thehighest still temperature followed by (with a verysmall difference) the saline water, and then theflowing air (humid and dehumid), glass cover,and condenser, respectively. The saline watertemperature in the basins reaches its maximumvalue of about 69C at the highest (top) nodes,almost half an hour after midday. The differencein temperatures between the first and last nodes isin the range of 35C. Humidified and de-humidified air temperature difference is in therange of 0.52.0C.


Fig. 5. Temperature distribution of stills components.

3.3. Circulating air conditions

Similar to the forced case of Fath et al. [2], aircondition lies always on the saturation line of thepsychrometric chart (Fig. 6). During daylighthours, air is heated and humidified till middayand its temperature reaches about 57C at thehumidifier exit and 53C at the dehumidifier exit.After midday, air starts to cool till sunset, and itstemperature almost reaches ambient conditions.

3.4. Parametric study

The effect of environmental, design, andoperation parameters (Table 3) on the stillsproductivity was studied. Very similar resultswere obtained to that of forced circulation asshown. The results indicated that increasing thesolar intensity, ambient temperature, basinabsorbitivity, and initial saline water temperatureincreases the system productivity. On the other

Fig. 6. Naturally circulated air conditions.

hand, increasing wind velocity, basin insulationthickness, evaporation and condensation surfaceareas, condenser emissivity, and saline watermass have little effect on the productivity.

The natural circulation operating parameterthat influences the system circulating airflow rateis the stills tilting angle. Fig. 7 shows thatincreasing the tilt angle of the still increases theair flow rate (higher driving force). However,within these values, the air flow rate does notinfluence the stills yield. Increasing airflow ratehas two effects: the first is to increase the capa-bility of flowing air to entrain and carry morevapor; and the second is to reduce the heat


Fig. 7. Influence of the tilt angle on performance of thestill.

content of the hot water (and therefore theevaporation rate). These two effects seem tobalance out.

3.5. Energy storage and condensation energyrecovery

Different cases have been considered to inves-tigate the effect of both the partial basin energystorage and partial recovery of condensationenergy at the condenser. These cases cover(1) only partial storage of basin energy for aftersunset reuse, (2) only partial recovery of con-densation energy at the condenser, and (3) a com-bination of partial basin energy storage andpartial recovery of condensation energy.

3.5.1. Partial basin energy storageFig. 8 shows the case of only partial basin

energy storage for overnight reuse. The firstattempt assumes 2.0 cm of paraffin wax (melting

temperature of 60C) to be contained within thehigh temperature absorber. The figure shows acomparison of overall still performance with andwithout energy storage. With paraffin wax (solidlines), the components temperatures are flatterduring both the wax melting/solidification period(hour 5 to hour 12), after which they graduallydrop till hour 24. The absorber temperature ismaintained almost constant at 60C due to waxmelting (energy charging period continues tillalmost midday) and solidification (energy dis-charges from midday to about sunset). By thesunset hour, the absorber/wax temperature startsto drop and heat is discharged from the absorber/wax as sensible heat. Similarly, the air and glasstemperatures follow approximately the sametrend. For the still without wax (dotted lines),these parameters follow the sinusoidal trend ofthe solar energy (i.e., no energy is available aftersunset; Fig. 5). Fig. 8 also shows a small reduc-tion in both the stills daily average efficiencyand yield. The productivity of the still is about9.6 kg/d.2m2 as compared to 10.2 kg/d.2m2without wax.

The second attempt assumes replacing theinsulation material with the 2.0 cm paraffin waxso that the wax acts as both basin insulator andenergy storage medium. The results show asimilar trend as the case presented above witheven more reduction in productivity and dailyaverage efficiency. A reversed air circulationflow took place after hour 20, as explained inother cases below.

These two attempts clearly indicated thatstoring water produced during the day is moreeffective and economical than storing the energyfor overnight reuse. Storing energy is technicallymore complicated by adding wax (or other PCM)within the still than storing water in the producttank.

3.5.2. Partial condensation energy recoveryFigs. 9 and 10 show the second case of only

partial recovery of condensation energy (no basin


Fig. 8. Effect of partial storage of the basin energy on still performance.

energy storage); 2.0 cm of Glauberts salt (melt-ing temperature of 30C) was assumed to becontained within the low temperature condenser.

The figure shows a comparison of the stillsoverall performance with and without Glaubertssalt. With Glauberts salt (solid lines), there are


Fig. 9. Effect of partial recovery of condensation energy on still performance.

Fig. 10. Effect of partial recovery of condensation energyon still temperature and air flow rate.

Fig. 11. Air conditions with partial recovery of con-densation energy.


no significant changes in the effective heat gained(Qeff), average daily efficiency and total stillproductivity. The water productivity increased onthe condenser at the expense of glass cover pro-ductivity. The circulating air flow rate shows areversed circulation after sunset (Fig. 10). Thereason is that the condenser temperature risesabove the glass temperature, and consequently theair in the dehumidification channel becomeshotter (and lighter) than air in the humidificationchannel. This is more apparent in the temperaturedistribution where the dehumidified air temp-erature is lower than the humidified air tillhour 12, after which the case is reversed. Fig. 10also shows that the condenser temperatureincreases till hour 3 where its temperature reachesthe melting temperature of the built-in Glaubertssalt at 30C. Condenser temperature is maintainedconstant till all the Glauberts salt is melted, atabout hour 8, after which the condenser temp-

erature increases. By about hour 10, the con-denser temperature reaches its maximum andstarts to drop again till the solidification temp-erature of 30C, where it maintains constant(during solidification process) till hour 21 whereit is fully solidified and the temperature dropsagain. It is clear that at about hour 11, thecondenser temperature starts to be higher than theglass temperature, and so is the dehumidified airabove the humidified air which causes air circula-tion to reverse. The reversed circulated air doesnot cause any significant vapor to condense onthe glass cover since the air condition departsfrom the saturation conditions after hour 13, asshown in the psychrometric charts of Fig. 11.

3.5.3. Combined energy storage and recoveryFigs. 1214 show the third case of combining

the partial storage of basin energy for overnight

Fig. 12. Effect of combined absorber energy storage and condensation energy recovery on still performance.


Fig. 13. Effect of combined absorber energy storage andcondensation energy recovery on still temperature andcirculating air.

Fig. 14. Effect of combined absorber energy storage andcondensation energy recovery on air conditions.

reuse and the partial recovery of condensationenergy at the condenser. 2.0 cm of paraffin wax(melting temperature of 60C) is assumed to becontained within the high temperature absorber.

In addition, another 2.0 cm of Glauberts salt(melting temperature of 30C) is also assumed tobe contained within the low temperature con-denser. Fig. 12 shows the overall performance ofthe still with and without these modifications.With these modifications (solid lines), the figureshows a small reduction in both still daily averageefficiency and productivity. Similar to Fig. 9, thecondenser water productivity increased at theexpense of glass cover productivity. Fig. 13shows the circulating air flow rate with reversedcirculation after sunset between hours 13 to 21.However, there is a shift for the starting reversedair flow than that shown in Fig. 10 due to partialenergy storage in the basin. For the same reason,and after hour 21, the humidifying channel startsto get hotter than the dehumidifying channel,which causes the air to recirculate in the normaldirection again. The figure also shows that be-tween hours 5 to 10 the temperature differencebetween the humidifying channel and the de-humidifying channel is constant (melting processof paraffin wax at 60C and Glauberts salt at30C). This causes a constant air circulation flowrate during this period. The almost reversed con-stant flow rate is due to the same reason but inreverse. Fig. 14 shows that the air condition ismaintained at saturation with water vapor up tohour 14, after which it is far from saturationconditions.

One last attempt to combine the partialrecovery of condensation energy and reuse theenergy for basin heating is shown in Fig. 15. Inthis case, 2.0 cm of Glauberts salt (meltingtemperature of 30C) is assumed to be attached tothe condenser as side by side strips for conden-sation energy storage. When the basin is cooleddown and its temperature is reduced to 30C, the2.0 cm of Glauberts salt strips are moved fromthe condenser side and placed under the basins to


Fig. 15. Condensation energy recovery is placed under the basin.

heat it with the recovered stored energy. Thefigure shows a comparison of the overall stillperformance with and without this modification.With the modifications (solid lines), the figureshows a very small improvement in the stillsproductivity. Also, the circulating air flow rateshows a reversed circulation after sunset andcontinues till hour 24. The air condition is

maintained at saturation level till hour 12, afterwhich it shifts away from saturation.

All of the above attempts for absorber energystorage and condensation energy recovery showan insignificant contribution in the stills product-ivity. A simple still without these modificationsis, therefore, more efficient both technically andeconomically.


3.6. Economical assessmentThe water production unit cost is the sum of

both initial and running costs contribution. Theinitial cost covers all expenses starting from theprojects conception until its commissioning. Therunning costs cover all the O&M expenses (staffsalaries, energy and its conversion, chemicals,overheads, etc.) [25].

The proposed solar still configuration wasmanufactured for testing as shown in Fig. 16. Itcosts 500 Egyptian pounds (about US $75). Theexpected lifetime is 10 years and its yield is about4 L/m2.d. For a 1.0 m3/d unit (1000 L/d),250 units are required; 10% of the stills are addedto guarantee the continuous production of waterin case of failure or malfunction of some stills.Table 4 summarizes the cost calculations for a1.0 m3 water production unit through a farm ofstills. The table shows that 1.0 m3 of water costs

45 EP ($8). The contribution of the capital costrepresents more than 87% of the water productioncost. Therefore, if the still cost is reduced from500 EP to 100 EP, then 1.0 m3 will cost only25 EP ($3.0). Still costs could be reduced throughthe selection of inexpensive materials and massproduction.

In general, solar stills cannot challenge thelower cost of large units (MSF, RO, etc.) whichproduce water at a cost of less than US $1/m3.However, for special applications such as (1) verysmall communities demand, (2) unavailability oflocal or nearby drinking fresh water supply (re-mote areas), (3) un availability of energy (fuel orelectric grid) and (4) unavailability of technicalcapabilities within the community (for O&M ofhigh-technology systems such as RO), the solarstill seems to be the only technically and eco-nomically competing alternative.

Table 4Typical case study for 1.0 m3/d solar distillation unit (US $1. = 5 EP)

Parameter Normal case Cheaper -1- Remarks

Still area, m2 1Still productivity, L/m2.d 4Unit no. of stills 275 275 Assumed 10% extra stillsUnit area, m2 250Annual production, m3 350 300* *Assumed less efficient stillStill cost, EP 500 100* *Inexpensive material and mass

productionUnit cost, EP 137,500 27,500Annual O&M 2,000 2,000 One operator for 10 units; 500 EP/

month + 800 EP spare partsCapital installment, EP 13,750* 5,500** *10 years, 0% interest

**5 years, 0% interestTotal annual cost, EP 15,750 7,500Cost of 1.0 m3, EP 45 ($9) 25 ($5)


Fig. 16. Manufactured HDD solar still.

4. Conclusions

1. The thermal performance of a naturallycirculated HDD solar still was investigated. Thestill has a simple design with a tilted configura-tion. The results show still productivity andefficiency of about 5.1 kg/m2.d, similar to that ofa forced circulation model [2]. In addition to itssimplicity, natural circulation is more economicaland technically less complex than a forced circu-lation still.

2. The influence of different environmental,design, and operational parameters shows thatincreasing solar intensity (high input energy) andambient temperature (less energy loss) improvesthe stills productivity. This is an advantage ofsolar desalination as water production increasesunder summer conditions, which are consistentwith the water demand during this season.

3. Different attempts to partially store basinenergy and/or recover condensation energy werealso studied. These attempts cover (1) only partialstorage of basin energy for overnight reuse,(2) only partial recovery of condensation energyat the condenser, and (3) a combination of energystorage and partial recovery. The results show aninsignificant improvement in the stills pro-ductivity. It is more economical, therefore, tostore water rather than store energy.

4. An economical assessment of water pro-duction costs shows that 1.0 m3 of product watercosts 45 EP ($9). However, if the still cost isreduced to 100 EP, then 1.0 m3 will cost only25 EP ($5). In general, solar stills cannot chal-lenge the lower cost of large units (MSF, RO,etc.). However, for special applications of (1) verysmall communities demands, (2) unavailability offresh drinking water in remote areas, (3) unavail-ability of energy (fuel or electricity) and(4) unavailability of technical support within thecommunities, solar stills seem to be the only(technically and economically) competingalternative.


5. Symbols

A Area, m2B Still width, mC Specific heat, J/kg CGr Grashof numberh Specific enthalpy, J/kgHfg Latent heat of evaporation, J/kgH Film coefficient of heat transfer,

W/m2 CI Solar intensity, W/m2K Thermal conductivity, W/m.CL Characteristic length, mM Mass, kgm Mass rate, kg/sP Pressure, PaPr Prandtl numberQ Heat transfer rate, Wt Time, sT Temperature, CTh Thickness, mU Overall coefficient of heat transfer,

W/m2Cv Velocity, m/sV Volume, m3w Specific humidity, kgwater/kgair

Greek

" Absorbtivity) Differenceg Emissivity0 Efficiency2 Tilt angleD@ DensityD ReflectivityF Radiation constantE SummationJ Transmissivity

Subscripts

a Airam Ambient

av Averageab Absorberb BasinC ConvectionCD Condensationco CondenserD DriveDh DehumidificationE Evaporationerth Earthg Glassh Humidificationhyd Hydraulici Insulationin InputK Conductionout Outputp Projectedpw Product waterR RadiationRe Resistingsid Sidesk Skysu Sunsw Saline waterw Wind1 Inlet2 Outlet

References

[1] G. Fiorenza, V.K. Sharma and G. Braccio, Techno-economic evaluation of solar powered waterdesalination plant, Energy Conversion Management,in press.

[2] H.E.S. Fath, S. El-Sherbiny and A. Ghazy, Transientanalysis of a new humidification dehumidificationsolar still, Desalination, 155 (2003) 187203.

[3] H.E.S. Fath, Solar desalination: a promising alter-native for water provision with free energy, Simpletechnology and a clean environment, Desalination,116 (1998) 4556.

[4] A.A. Delyannis and E. Delyannis, Solar desalination,Desalination, 50 (1984) 7181.


[5] O.A. Hamed, E.I. Eisa and W.E. Abdalla, Overviewof solar desalination, Desalination, 93 (1993) 563579.

[6] A.S. Nafey, M. Abdelkader, A. Abdelmotalip andA. Mabrouk, Parameters affecting solar still pro-ductivity, Energy Conversion Management, 41(2002) 17971809.

[7] A.S. Nafey, M. Abdelkader, A. Abdelmotalip andA. Mabrouk, Solar still productivity enhancement,Energy Conversion Management, 42 (2001) 14011408.

[8] A. Radhwan, Transient analysis of a stepped solarstill for greenhouse heating and humidification,Desalination, 161 (2004) 8997.

[9] M.A.S. Malik, G.N. Tiwari, A. Kumar and M.S.Sodha, Solar Distillation, Pergamon Press, 1982.

[10] A. Dater, Development of humidificationde-humidification technique for water desalination inarid zones of India, Desalination, 5 (1968) 5563.

[11] H.E.S. Fath and A. Ghazy, Solar desalination usinghumidification dehumidification technology, Desali-nation, 142 (2002) 119133.

[12] A.S. Nafey, H.E.S. Fath, S.O. El-Hlaby and A.M.Soliman, Solar distillation using a single stagehumidificationdehumidification process: (I) Nu-merical investigation, Energy Conv. Mangmt., 45(2004) 12411261.

[13] A.S. Nafey, H.E.S. Fath, S.O. El-Hlaby and A.M.Soliman, Solar distillation using a single stagehumidificationdehumidification process: (II) Ex-perimental study, Energy Conv. Mangmt., 45 (2004)12631277.

[14] M.A. Younis, M.A. Darwish and F. Juwayhel,Experimental and theoretical study of humidi-ficationdehumidification desalting system, Desali-nation, 94 (1993) 1124.

[15] H. Muller-Holst, M. Engelhardt and W. Scholkopf,

Small scale thermal seawater desalination simulationand optimization of system design, Desalination, 122(1999) 255262.

[16] H. Muller-Holst, M. Engelhardt and W. Scholkopf,Multi-effect humidification seawater desalinationusing solar energy or waste heat: various imple-mentations of a new technology, Proc. MediterraneanConf. on Policies and Strategies for Desalination andRenewable Energies, Santorini, Greece, 2000.

[17] G. Mink, M. Aboabboud and E. Karmazsin, Air-blown solar still with heat recycle, Solar Energy,62(4) (1998) 309317.

[18] G. Mink, L. Horvath, G. Evseev and A. Kudish,Design parameters, performance testing and analysisof a double glazed, air-blown solar still with heatrecycle, Solar Energy, 64 (1998) 265277.

[19] H. Kunze, A new approach to solar desalination forsmall and medium size use, Proc. AQUA-TECHConf., Cairo, 2001.

[20] H.E.S. Fath and S. El-Sherbiny, Effect of adding apassive condenser on solar still performance, Int. J.Solar Energy, 11 (1998) 7389.

[21] H.E.S. Fath and H.M. Hosny, Thermal performanceof a single sloped basin still with inherent built-inadditional condenser, Desalination, 142 (2002) 1927.

[22] H.E.S. Fath, Solar thermal energy storage tech-nologies: Technical note, Renewable Energy,14(14) (1998) 3540.

[23] J.P. Holman, Heat Transfer, 8th ed., McGraw-Hill,New York, 1997.

[24] A. Ghazy, Transient analysis of a new humidificationdehumidification solar still, M.Sc. Thesis, Alexan-dria University, Egypt, 2004.

[25] H.E.S. Fath, Economical assessment of desalinationtechnologies, ADST 2nd International Workshop,Alexandria, 2001.

naturally circulated humidifyingdehumidifying solar still with a built-in passive condenser

Documents