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Revue des Energies Renouvelables CISME'08 Sousse (2008) 185 192

Experimental and theoretical study on the effect cooling tower on solar desalination system

H. Marmouch1*, J. Orfi2 and S. Ben Nasrallah1

1 2

LESTE, Ecole Nationale d'Ingénieurs de Monastir, 5000, Monastir, Tunisie

On leave at King Saud University, Department of Mechanical Engineering, Riyadh, KSA

Abstract - The humidification dehumidification desalination process is viewed as a promising technique for small capacity production plants. The process has several attractive features, which include operation at low temperature, ability to utilize sustainable energy sources, i.e. solar and geothermal, and requirements of low technology level. In arid and semi arid regions, the performance of solar distillers using humidification-dehumidification principle is very sensitive to numerous parameters such as the condensation process. This paper presents a theoretical and experimental study on the effect cooling tower on the distillate fresh water production. A general model based on heat and mass transfer balances in each component of the system was developed to predict the mass of fresh water. Keywords: Cooling towers - Solar desalination - Thermal efficiency - Condensation Heat and mass transfer.

1. INTRODUCTION

In many parts of the world, especially in the Middle East, people suffer from lack of fresh water and they live mostly in arid, remote areas and islands. On the other hand, in these regions, abundant of solar energy is available with the large amount of sea or underground saline water. To solve these problems many researchers, proposed several solar water desalination systems using humidification/dehumidification processes. Those systems are economical and environmentally friendly solution to supply small settlements in these locations with enough drinking water. A number of studies on the humidification/dehumidification process can be found in the literature. Most of these studies focus on performance evaluation for systems. J. Orfi et al. (2004) theoretically and experimentally studied a solar water desalination system based on the humidification/dehumidification principle. The system consists of two solar collectors, an evaporator and a condenser. In order to improve the productivity of the system, the authors utilized the latent heat of condensate water vapor in the condenser to preheat the feed water. In addition, they concluded that the global efficiency of the system depends on the efficiency of each component (solar water and air heaters, evaporator and condenser). E. Chafik (2002) presented a new seawater desalination process based on the stepwise heating-humidifying technique to increase the water vapour concentration in the process air. It was reported that, by this way, the airflow rate through the plant can be reduced and low investment and operating costs can be achieved. A theoretical investigation of a humidification/dehumidification desalination system configured by a double-pass flat plate solar air heater presented by C. Yamali et al. (2007). Their results showed that the productivity of the unit increases up to 8 % by

*

[email protected] _ [email protected] 185

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using a double-pass flat plate solar air heater as compared to single-pass flat plate solar air heater and decreases about 30 % without double-pass flat plate solar air heater under the same operating conditions. G. Al-Enezi et al. (2006) studied an experimental system includes a packed humidification column, a double pipe glass condenser, a constant temperature water circulation tank and a chiller for cooling water. They confirmed that the highest production rates are obtained at high hot water temperature, low cooling water temperature, high air flow rate and low hot water flow rate. All of these solar distillers using humidification-dehumidification principle presented the same problem of condensation process especially in remote area. This is attributed mainly to the high value of the cooling water temperature. Thus, adding cooling towers in such desalination systems can be a solution for this condensation problem. The principle of cooling towers is based on heat and mass exchanges using a direct contact between ambient air and hot water through some types of packing. The performance of the cooling tower is generally determined by the ratio of ( KaV / L ) as reported in ASHRAE (1997). This ratio is named the tower characteristic ratio or number of transfer units (NTU). K , a , V and L are respectively the mass transfer coefficient, the surface area per unit volume of the tower, the volume and the water mass flow rate. Milosavjevic et al. (2001) studied the thermal performance of a counter current cooling tower considering evaporation of a quantity of water and assuming that the heat and mass transfer are equivalent (Lewis number Le is equal to unity). Halasz (1998) presented a general model describing all types of evaporative cooling processes. In his model, the author simplified the nondimensional equations by considering the air saturation curve as linear and the water mass flow rate as constant. Jaber et al. (1989) shows how the theory of heat exchanger design may be applied to cooling towers. These authors demonstrated that the definitions of the effectiveness and NTU are in very good agreement with those used for heat exchanger design and are applicable to all cooling tower operating conditions. Several other mathematical models to correlate heat and mass transfer processes occurring in wet cooling towers exist; Such as those proposed and discussed by Khan et al. (2004) and Kloppers et al. (2005). The main objective of this study is to investigate the thermal performance of a counter flow wet cooling tower filled with two types of film packing materials. And predict his effect on the distillate fresh water.

2. SYSTEM DESCRIPTION

The apparatus shown in figure 1 represents the different parts of the solar distiller. Sea water/ brine is heated in the solar collector (5), and it will be introduced through the water distribution system into the top of the humidifier (2) enters in contact with the air steam. This air can remain in closed loop between the air solar collector and the humidifier before sending it toward the condenser (1). This technique of heating /humidifying improves the air characteristics by loading it with a high amount of humidity and to reduce the volume of circulating air. Chafik (2004) presented a design of plants for solar desalination using the multi-stage heating/humidifying technique. His study shows the optimum number of stages heating/humidifying is equal to 5.

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The next operation consists of condensing this vapor in the condenser cooled by the feed water and recuperates the fresh water. The feed water is in closed loop between the condenser and the cooling tower (3) to decrease the feed water temperature.

3. 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 water or air solar collector, the temperature distribution is given by the analytical solutions for the steady state regime equation: U .I .I (1) T ( x ) = T ( x = 0 ) - Tamb - 0 exp x = Tamb + 0 M .C p U U In the evaporator and cooling tower, we consider a counter flow air-water system. The principal assumptions used to obtain the mathematical model for the humidifier are:

Fig. 1: Schematic diagram of the solar desalination system · The liquid and gas flows are steady state and one-dimensional. · Heat and mass transfer coefficients are constant along the column. · Constant value of Lewis number throughout the tower. · A very thin layer of saturated air exists between the liquid and the gas streams. This layer is supposed at the temperature Ti . The following equations were derived to describe the heat and mass balances in the evaporator or cooling tower:

K .P a = e / co e / co ( sat ( Tw ) - a G x

)

(2)

U w , e / co . Pe / co Tw ( Tw - Ti = x L . Cp w

)

(3)

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U a , e / co . Pe / co Ta = x G C p w + a C p v

(

) ( Ti - Ta )

)

(4)

G

d a h f g = U a , e / co . P e / co ( Ta - Ti ) + U l, e / co . P e / co ( Tl - Ti dx

(5)

The following equations describe the heat and mass transfer in the condenser:

a K .P = c c ( sat ( Tw ) - a x G

)

(6)

U w , c . Pc Tw = ( Tw - Ti x L . Cp w

)

( Ti

- Ta

(7)

U a , e / co . Pe / co Ta = x G C p a + a C p v + m c . C p w

(

)

)

(8)

G

d a h f g = U a , c . Pc ( Ta - Ti ) + U l, c . Pc ( Tl - Ti dx

)

(9)

The value of the heat and mass transfer strongly depends on the type of the packing used in the cooling tower, and on the water and airflow rates. The mass transfer coefficient is often correlated in the form ASHRAE:

KaV L = c G G Where c and n are empirical constants specific to a particular tower design. NTU =

n

(10)

4. RESULTS AND DISCUSSION

The performance of a cooling tower depends on several parameters such as the range of cooling temperature, the inlet water temperature and the L G variation ratio. At given operating conditions, the outlet water temperatures measure tower capabilities. Fig. 3 shows the measured outlet-water temperature variation with L G ratio for different water mass flow rates. The rate of increase in water temperature is quite small at low L G ratios. As the L G ratio increases, a sudden increase in the slopes of the curves is observed. This change occurs at lower L G ratios when the water flow rate is small; it is delayed with an increase in flow rate. Figure 2 shows a good agreement between, the experimental and theoretical study. Figure 3 shows the effect of cooling water on the mass fresh water production. The results showed that the mass production increased with the decreasing of the temperature cooling water.

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Fig. 2: Outlet-water temperatures vs L G for different liquid flow rates

Fig. 3: Effect of cooling water on the mass fresh water production ( Tamb = 25 °C, = 10gkg-1, Tw = 60 °C, L = 0.1 kg/s, G = 0.033 kg/s) The mass of fresh water was less than in the case without cooling tower is about 17 l/day/m2. The effect of cooling tower on the fresh water production is shown on figure 4. The experimental mass collected is about 22 L per square meter of solar collector surface. As observed, the tower characteristic, KaV / L , is influenced by the air and water flow rates. Therefore and in order to derive an equation characterizing the heat and mass exchange through the film type packing, the following correlations, based on our experimental results, are proposed for three inlet water temperatures ( Tw = 30, 40 and 50 °C) and for the following ranges of L and G: L = 200 1000 kg/h and G = 250 2000 m3/h.

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Fig. 4: Effect of cooling tower on the mass fresh water production ( Tamb = 25 °C, = 10gkg-1, Tw = 60 °C, L = 0.1 kg/s, G = 0.3 kg/s)

Tw = 50 °C Tw = 45 °C Tw = 40 °C KaV L = 0.61 . L G KaV L = 0.47 . L G KaV L = 0.46 . L G

- 0.42 - 0.53 - 0.47

For:

For:

For:

Fig. 5: Tower characteristic, KaV / L , vs L G ratio for different inlet water temperatures

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5. CONCLUSIONS

This paper presents a theoretical and experimental study of a solar desalination system with humidificationdehumidification. Correlations expressing the variation of this tower characteristic with the liquid to gas mass flow rate ratio are developed for three inlet water temperatures. The results show the cooling tower effect on the distillate fresh water.

NOMENCLATURE

a

G L V K U

w a sc s

Surface area of water droplets per unit volume of the tower, m2/m3 Air mass flow rate, kg/s Water mass flow rate, kg/s Tower volume (m3) Mass transfer coefficient, kg/m2.s Collector overall thermal loss coefficient, heat transfer coefficient, W/m2.K Greek symbols Absolute humidity, g/kgas Subscripts Water Air Solar collector Saturation

Cp

Specific heat at constant pressure, J/kg.K Temperature, °C Specific enthalpy, kJ/kg Wetted perimeter, m Optic collector efficiency, W/m2.K Solar irradiation Intensity, W/m2

T h fg P 0

I

i ev C Co

Efficiency Interface Evaporator Condenser Cooling tower

REFERENCES

[1] J. Orfi, M. Laplante, H. Marmouch, N. Galanis, B. Benhamou, S. Ben Nasrallah and C.T. Nguyen, `Experimental and Theoretical Study of a Humidification Dehumidification Water Desalination System Using Solar Energy', Desalination, Vol. 168, N°1-3, pp. 151 159, 2004. [2] E. Chafik, `A New Seawater Desalination Process Using Solar Energy', Desalination, Vol. 153, N°1-3, pp. 25 - 37, 2002. [3] C. Yamali and Ismail Solmus, `Theoretical Investigation of a Humidification Dehumidification Desalination System Configured by a Double-Pass Flat Plate Solar Air Heater', Desalination, Vol. 205, N°1-3, pp. 163 177, 2007. [4] G. Al-Enezi, H. Ettouney and N. Fawzy, `Low Temperature Humidification Dehumidification Desalination Process', Energy Conversion and Management, Vol. 47, N°4, pp. 470 - 484, 2006. [5] Book, `ASHRAE Handbook of Fundamentals', Atlanta, GA: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Chapter 36, 1997. [6] N. Milosavljevic and P. Heikkilä, `A Comprehensive Approach to Cooling Tower Design', Applied Thermal Engineering, Vol. 21, N°9, pp. 899 915, 2001.

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[7] B. Halasz, `A General Mathematical Model of Evaporative Cooling Devices', International Journal of Thermal Sciences, Vol. 37, N°4, pp. 245 - 255, 1998. [8] H. Jaber and R.L. Webb, `Design of Cooling Towers by the Effectiveness-NTU Method', ASME Journal of Heat Transfer, Vol. 111, N°4, pp. 837 - 843, 1989. [9] Jameel-Ur-Rehman Khan, B. Ahmed Qureshi and Syed M. Zubair, `A Comprehensive Design and Performance Evaluation Study of Counter Flow Wet-Cooling Towers', International Journal of Refrigeration, Vol. 27, N°8, pp. 914 - 923, 2004. [10] J.C. Kloppers and D.G. Kröger, `A Critical Investigation into the Heat and Mass Transfer Analysis of Counter flow Wet-Cooling Towers', International Journal of Heat and Mass Transfer, Vol. 48, N°3-4, pp. 765 - 777, 2005. [11] E. Chafik, `Design of Plants for Solar Desalination Using the Multi-Stage Heating/Humidifying Technique', Desalination, Vol. 168, pp. 55 - 71, 2004.

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