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Desalination 168 (2004) 151–159

Experimental and theoretical study of a humidification– dehumidification water desalination system using solar energy J. Orfia*, M. Laplanteb, H. Marmoucha, N. Galanisb, B. Benhamouc, S. Ben Nasrallaha, C.T. Nguyend a

Laboratoire des Etudes des Systèmes Thermiques et Energétiques, ENIM, Monastir, Tunisia Tel. +216 (73) 500-511; Fax +216 (73) 500-514; email: [email protected] b Département de Génie Mécanique, Université de Sherbrooke, Sherbrooke, Canada Tel. +1 (819) 821-7144; Fax +1 (819) 821-7163; email: [email protected] c LMFE, Département de Physique, Faculté des Sciences Semlalia, Marrakech, Morocco Fax +212 (44) 43 74 10; email: [email protected] d Département de Génie Mécanique, Université de Moncton, Moncton, NB,Canada Tel. +1 (506) 858-4347; Fax +1 (506) 858-4082; email: [email protected]

Received 16 February 2004; accepted 25 February 2004

Abstract This paper presents the characteristics of a solar desalination system based on the humidification–dehumidification principle. The system which, can be used in an open or closed cycle for air, is a modular one. It has the following independent components: two solar collectors, an evaporator and a condenser. The first part of the study deals with the presentation of the designed and constructed components. Some experimental results are analysed. They present a general picture of the evolution of the temperature and the humidity between the entry and the exit of the solar collector, the evaporator and the condenser. In the second part, a general mathematical model including the mechanism of heat and mass transfer in the various components of the present desalination system was developed. The theoretical results show that there exists an optimum mass flow rate ratio corresponding to a maximum fresh water product. Keywords: Solar desalination; Humidification-dehumidification; Evaporator; Modelling; Heat and mass transfer, Air-water interface *Corresponding author.

Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l’Eau Potable, Marrakech, Morocco, 30 May–2 June, 2004. 0011-9164/04/$– See front matter © 2004 Elsevier B.V. All rights reserved

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J. Orfi et al. / Desalination 168 (2004) 151–159

1. Introduction Solar desalination with humidification–dehumidification processes has proven to be an efficient means of production of fresh water in remote and sunny regions [1]. Numerous solar desalination installations concerned with small and medium production have been developed and studied. Examples of such installations include passive and active basin type solar stills and systems using humidification–dehumidification principle. Goosen et al. [2] presented an overall review of recent developments in solar desalination systems. They reported that the solar stills, known to have simple principle and low operating costs, have however a low efficiency and consequently a low production of fresh water (less than 5 l/m2d). Systems based on the humidification–dehumidification principle produce fresh water at rates higher than those obtained from single solar stills under similar solar radiation [3]. The thermodynamic background for this process is clearly explained by using the psychrometric chart [4,5]. Nawayseh et al.[3] constructed two different units in Jordan and Malaysia. These units consist of two vertical ducts connected end to end to form a closed loop for air circulation. The authors conducted a simulation study in which the effects of various operating parameters such as the air and water flow rates on the fresh water production are analysed. It was shown that the variation of the daily production with the water flow rate exhibits a maximum. On the other hand, the authors also developed appropriate correlations for heat and mass transfer coefficients in the humidifier as well as in the condenser. Al Hallaj et al. [6] studied the performance of a closed air cycle humidification-dehumidification process using solar energy. They reported that the constructed units were also capable of producing a large quantity of saline warm water for domestic use. It is observed that the water flow rate has a strong effect on unit production. Their experiments included results for night operation which can serve to keep the unit at elevated temperature.

Recently, Chafik [4] presented a procedure which consists of several steps (stages) of air heating and humidification and leads to high vapour concentration in the airflow. The present work deals with a study of a water desalination system by solar energy using the humidification–dehumidification principle. The proposed system is a modular one and has the following independent components: two solar collectors, an evaporator and a condenser. The study has two parts. The first one concerns the presentation of the characteristics of the designed and constructed components of the desalination system. Some experimental results are analysed. In the second part, a general mathematical model for the system is presented. It includes the models of the different components of the developed system from the governing heat and mass transfer equations. 2. Presentation of the studied system The studied solar desalination system has separate evaporator and condenser. It comprises air and water solar collectors, a humidifier and a condenser. Fig. 1 presents the configuration of this modular system. Sea water which is preheated in the condenser and heated in the solar collector enters in contact with a heated air stream in the evaporative chamber. Because of the heat and mass exchange between the hot water and the air in the evaporator, the latter is loaded by humidity. An efficient process in the humidifier is to obtain at its outlet saturated moist air with a high amount of vapour. The next operation consists of condensing this vapour in the condenser cooled by the feed water and recuperates the fresh water. This system offers the following principal characteristics: • The air cycle can be open or closed one. • The latent heat of condensation is recovered and used to preheat the feed water. • The moist air at the outlet of the evaporator can be returned to the solar collector, heated


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Fig. 1. Schematic diagram of the solar desalination system.

and humidified two or more times. Thus the air can be charged with an important amount of vapour. • The global efficiency of the unit depends on the efficiency of each component. Consequently, it is important to design and construct performent components.

Fig. 2a. Schematic of the constructed solar collector.

3. Experimental study The experimental part of this project concerns the design and the construction of the principal components of the above described solar desalination system. The constructed air solar collector consists of a single glass plate with 3 mm of thickness and 2 m2 of surface (Fig. 2a). The evaporator which is horizontal and has a rectangular cross section is constructed with wood. The fibber glass is used for insulation (Fig. 2b). In order to improve the heat and mass exchange, five parallel plates made with wood and covered with textile (cotton) are fixed in the evaporator. The heated air enters in contact with the hot water by different ways. The horizontal surface of the evaporator is covered by the hot water. The vertical plates are wetted by capillarity and finally the water is sprayed by means of tubes with small holes set in them. These tubes are

Fig. 2b. Schematic of the evaporator.

placed on the vertical plates. The feed water and the air are counter current. In the experimental set-up, the water solar collector has been replaced by an electric water heater. The condenser consists of a chamber with a rectangular cross section. It contains two rows of long cylinders made of copper in which the feed water flows. Longitudinal fins were soldered to

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the outer surface of the cylinders. The condenser is characterized by an exchange surface, 1.5 m2 and 28 m as total length. Various variables characterizing the present problem were measured. Thus, the temperature of water and air and the humidity at the inlet and the outlet of each component and the water and air mass flow rates are measured. The wind and radiation data were taken for the town of Monastir from [7]. Table 1 gives an example of the experimental results obtained for a typical day of July at Monastir, in Tunisia. It presents a general picture of the evolution of the temperature and the humidity between the entry and the exit of the solar collector, the evaporator and the condenser. These results indicate the important effect of the feed water temperature on the quantity ω3 – ω4 which is proportional to the produced fresh water. Fig. 3 presents the time variation of the humidification efficiency of the evaporator, ew, for a typical day of July in Monastir. This efficiency is defined as: ew =

( win − wout )

(1)

win − win′

where win and wout are respectively the humidity of air at the inlet and the outlet of the evaporator. w′in is the saturation humidity corresponding to the actual humidification process. This definition

Fig. 3. Time variation of the efficiency of the evaporator.

of the efficiency, also used by Chafik [4], is analogous to that used in the evaporative coolers [8]. 4. Theoretical study The elaboration of a general mathematical model is based on the conservation principles of mass and energy in each component of the desalination unit. For the solar collectors, analytical solutions for the steady state regime giving the distribution of the temperatures of air (or water) along the collector were obtained [9,10]. In the evaporator and the condenser, we consider a counter flow air–water system. The

Table 1 Measured values at the inlet and the outlet of each component of the system for July 21, 2003 in Monastir (air mass flow rate = 0.037 kg/s; water mass flow rate = 0.043 kg/s) Time

Ta

T1

T2

ϕ1, % ω1, g/kg T′2

T3

T7

T8

ϕ3, %

ω3, g/kg T4

ω4, g/kg ω3–ω4

8:30 9:30 10:30 11:30 12:30 13:30 14:30 15:30

30 30 30 31 33 33 34 33

36 36 38 41 42 43 43 42

40 41 43 48 49 49 49 47

58 58 55 55 43 46 46 44

37.5 38.8 35.8 36.2 37.4 38.9 35.3 34.9

60 60 42 45 40 45 50 52

50 51 38 40 38 39.5 42 45

93.6 91.7 69.7 68.3 66.4 72.6 82.4 81.6

39.42 41.57 26.22 26.26 27.31 32.64 30.34 29.35

29.67 33.20 22.90 22.59 23.11 25.35 26.48 24.20

15.49 15.49 14.67 15.15 13.57 14.54 15.40 13.90

39 40 42 46 46 47.5 48 46

35 37 34 35.8 37 37 34.5 33

9.75 8.37 3.32 3.67 4.20 7.29 3.86 5.15


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One can note that the solution of Eq. (1) is straightforward: m l ( x ) = m 5 + m a ( ω − ω3 )

(7)

The boundary conditions corresponding to the above equations are: m l ( l ) = m 5 , ω ( 0 ) = ω2 , Tl ( l ) = T7 , Ta ( 0 ) = T2 (8)

Fig. 4. Sketch of the evaporator.

model can also be applied for parallel flow with minor modifications. The principal assumptions used to obtain the mathematical model for the humidifier are (see Fig. 4): • The liquid and gas flows are steady state and one-dimensional. • A very thin layer of saturated air exists between the liquid and the gas streams. This layer is supposed at the temperature Tf. The following equations were derived to describe the heat and mass balances in the evaporator:

dm l dω = m a dx dx

m a

(2)

dω = U m ,ev Pev ωsat _ at _ T f − ω dx

(

dTl = dx

U l ,ev Pev (Tl − T f

)

m l C pl

dTa U a ,ev Pev (T f − Ta ) = dx m a ( C pl + ωC pv )

m a

)

(3)

(4)

(5)

dω ivl = Ul ,ev Pev (Tl − Tf ) + U a ,ev Pev (Ta − Tf ) (6) dx

To describe the heat and mass exchanges between the air and the feed water which are not in contact direct in the condenser, equations are obtained in a similar manner to that of the evaporator.

dm c dω = −m air dx dx m air

(9)

dω = U m ,cond Pcond ωsat _ at _ T f − ω dx

(

dTl U l , cond Pcond (Tl − T f = dx m 5C pl

)

U a ,cond Pcond (T f − Ta ) dTa = dx m air ( C pa + ωC pv ) + m c C pc

m a

dω ivl = U l ,cond Pcond (Tl − T f ) dx + U a ,cond Pcond (Ta − T f )

)

(10)

(11)

(12)

(13)

Eq. (9) gives: m c = m a ( ω3 − ω ) The above equations are subjected to the following boundary conditions: m c ( 0 ) = 0, ω ( 0 ) = ω3 , Ta ( 0 ) = T3 , Tl ( l ) = T5 (14)

The evaluation of the different coefficients characterizing the heat and mass transfer in the

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evaporator and the condenser is an important task in the simulation problem. Based on the experimental results, the values of the heat transfer coefficients are as follows:

data, found in [11], of the pressure and temperature at the saturation of the salt water. A general computer program including the mechanism of heat and mass transfer in the various components of the present desalination system was developed. Fig. 5 gives a general picture of the flow chart of the computer model. In order to evaluate the accuracy and the precision of this general model, several tests have been performed. These tests were concerned for example with: • The application of the global balance of heat and mass transfer on each component and also on the entire cycle. • The comparison with results of Ibrahim et al. [12], who studied a similar problem in that of the evaporator.

Ua,ev = 11.2 W/m2K, Ul,ev = 640 W/m2K, Ua,cond = 4360 W/m2K. The following expression was used to evaluate the mass transfer coefficient:

Um =

Ua ( C pa + ωC pv ) × Le2/3

where Le is the Lewis number. The latent heat of vaporization was evaluated by using the Clapeyron relation from a tabulated For the open system, ω1 = ωamb T1 = Tamb

For the closed system ω1 = ω4 , T1 = T4

T1, ω1 Procedure for the air solar collector

T2, ω2=ω1

Initialisation of T6 Procedure for the water solar collector

T7 T8 Feed water temperature

Procedure for the evaporator

T3, ω3

T5

Procedure for the condenser

T4, ω4

T6

Fresh water Fig. 5. Flow chart for the entire system.


• The capacity of the model to predict easily the results corresponding to the limiting cases. More details concerning the validation of the model are found in [9]. Laplante [9] has shown that the most relevant non dimensional parameters that characterize the system are: N1 =

U A m 5 A A , N 2 = ca , N 3 = cl , N 4 = a ,ev ev , m a Aev Aev m a Cpa

N5 =

U , cond Acond , N 6 = ω3 − ω4 m a Cpa

Fig. 6 shows the different states in the psychrometric chart that moist air experiences respectively for the cases of closed system and open one. These results are obtained for the values of 0.05, 0.05, 8 and 24 respectively for the non-dimensional parameters N2, N3, N4 and N5. The mass flow rate ratio, N1 takes the values of 0.8, 1.6, 2.4 and ∞. The fresh water production is obtained by

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means of the state variation of air in the evaporator and in the condenser (i.e. ω3 – ω4). One can observe that for a case of a closed system, the air is loaded with an important amount of vapour at the outlet of the humidifier. These figures show that the closed system is more efficient than the open system. It is also of interest to note the strong effect of the normalized liquid flow rate on the performance of the system. For the limiting case, corresponding to a very large liquid mass flow rate, the water temperature in nearly constant in the circuit and the potential of evaporation and condensation is nearly zero. Also, a very low mass flow rate of the feed water is not beneficial to increase the productivity of the system. Fig. 7 indicates the variation of the quantity ω3 – ω4 with the change of the normalized mass flow rate of the seawater. It shows that there exists an optimum mass flow rate ratio corresponding to a maximum fresh water production. The ambient conditions also have effects on the performance of the desalination system. Fig. 8

Fig. 6. Evolution of the states of the humid air in the open system (left) and the closed system (right).

158


Fig. 7. Effect of the water flow rate on the production rate of the fresh water.

demonstrates the impact of the external conditions on the fresh water production and on the optimum non-dimensional liquid flow rate. It is observed that the latter varies from the value of 1.2 to almost 2 for the different cases considered in this figure. 5. Conclusion The present work concerns an experimental and a theoretical study of a water desalination system by solar energy. The system based on the humidification-dehumidification principle is a modular one and has the following independent components: two solar collectors, an evaporator,

and a condenser. The first part of the study deals with the presentation of the designed and constructed components. Some experimental results are analysed. They show the evolutions of the temperature of air and water and the humidity at the inlet and the outlet of each component of the system. In the second part, a general mathematical model based on the heat and mass balances for air and water is presented. The theoretical results show that there exists an optimum mass flow rate ratio corresponding to a maximum fresh water product. Acknowledgment The financial support of the AUF, Agence Universitaire de la Francophonie is acknowledged. Symbols A Cp D e i I L Le

— Surface area, m2 — Specific heat at constant pressure, J/(kg.K) — Mass diffusion coefficient, m²/s — Humidification efficiency — Specific enthalpy, kJ/kg — Solar insolation rate, W/m² — Length, m — Lewis number = α/D

0.07

2.1 2

0.06 1.9

0.05

1.8 10°C 20°C 30°C 40°C

1.6

ω 3-ω 4

m5/ma

1.7

1.5 1.4

0.04

10°C 20°C 30°C 40°C

0.03

0.02

1.3

0.01

1.2 1.1 0.002

0.004

0.006

0.008

0.01

IA/maivl

0.012

0.014

0.016

0.018

0 0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

IA/maivl

Fig. 8. Effect of the ambient conditions on the optimum normalized water flow rate and on the fresh water production.


l m P T U V x α ω

— Width, m — Mass flow rate, kg/s — Wetted perimeter, m, pressure, Pa — Temperature, K, °C — Heat (or mass) transfer coefficient, W/m²K (or kg/m²s) — Wind velocity, m/s — Coordinate in the flow direction, m — Heat diffusion coefficient, m²/s — Specific humidity, kgvap/kgdryair

Subscripts a c ca cl cond ev f g l m sat v

— Air — Condensate — Air collector — Water collector — Condenser — Evaporator — Vapor film — Mixture (air+water vapor) — Liquid — Mass — Saturation — Vapor

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