Abstract

Theoretical and experimental analysis of performance of a solar desalination pond as a second stage of proposed zero discharge desalination processes is considered in this work. Major purpose of this proposed process is producing salt and potable water. Experiments are conducted for brackish wastewater with different salinity content. The relation between temperature variations of brackish water, glass, and base of solar desalination pond with condensation rate are discussed. Results indicate when brackish water temperature is increasing; the average daily production of solar desalination pond is increased considerably. Results of the mathematical modeling show good agreement with experimental data.

Introduction

Man has been dependent on sea, rivers, lakes, underground water, and raining for his demands. About 97% of the earth water is brackish and just approximately 3% of earth water is potable [1]. Due to sever increase in population and also industrial rapid growth, the consumption of clean water is increasing day by day. Human and animals are suffering a lot with clean water scarcity problem. Apart from this, water pollution is also a major problem for water scarcity [2]. Zero discharge desalination is the best and promising solution for the mentioned problem. Fossil fuels are used to produce potable water in existent zero discharge desalination process. But availability of fossil fuels in the whole universe is very limited [3,4]. High efficiency processes which consume renewable energy are utilized considerably [5–8]. A solar desalination pond has been widely used in solar zero discharge desalination process to produce potable and clean water in lower price [1,9] especially applicable in rural regions [10].

Also, zero discharge desalination process is a feasible solution to decrease or remove the biological problems which are resultant of concentrated brine wastewaters drainage into sea ecosystem.

Salinity content of the effluent wastewater from desalination unit of Mobin petrochemical complex is too high to use reverse osmosis, RO, electro dialysis, ED, and other technologies which are dependent on membrane performance. Also, multi-effect distillation, MED, multistage flash, MSF, and the other same technologies require high level of energy. So, the superiority of this work is introducing a feasible and viable desalination method.

So, in this paper, a proposed solar pond is considered as a second stage of the zero discharge desalination process which is shown in Fig. 1. The effluent wastewater of desalination unit of Mobin petrochemical complex should be passed through the pretreatment unit and a solar desalination pond to produce potable water and high concentrated brine. Performance of this solar pond is investigated experimentally and also a mathematical modeling is presented to predict performance of solar desalination pond.

Fig. 1
The schematic of proposed zero discharge desalination process with a photo of
                        used solar desalination pond
Fig. 1
The schematic of proposed zero discharge desalination process with a photo of
                        used solar desalination pond
Close modal

Materials and Method

Experimental Setup.

Solar desalination pond consists of an airtight basin to trap the solar energy inside the pond and enhance the transmissivity coefficient. This is located according to the latitude of Shiraz and is imposed the most solar radiation. The construction is on a table and the space between the base of solar pond and table is filled with sawdust for insulation purpose. So, these conditions provide maximum solar absorption for brackish wastewater. A photo of used solar desalination pond is shown in Fig. 1. Evaporation technique separates sweet water from the waste water in the solar pond. Vapors condense on the glass roof and slip to the corners of the pond.

Distillates exit from two sides of solar desalination pond continuously so nonequilibrium condition between liquid and vapor remains in system. Table 1 shows the chemical analysis of entrance wastewater to solar desalination pond.

Table 1

Physical characterization of the solar desalination pond and chemical analysis of entrance wastewater to solar desalination pond

Physical characteristics
MaterialGalvanized iron (zinc coated iron to prevent corrosion)
Dimensions100 cm × 170 cm × 46.5 cm
Glass roof thickness4 mm
Inclination29 deg facing south
Height from the ground70 cm
Drainage diameter2 cm
Initial volume of feed100 liter
CompositionUnitBrine outlet line
Ca++ppm as CaCO314616.3
Mg++ppm as CaCO336080
Fe++ppmTrace
Ba++ppmTrace
SO4−−kg/m35.25
HCO3kg/m30.185
Total hardnessppm as CaCO3453
SalinityPercent5.45
Conductivitys/m58666 ×10-4
Silicappm0.1
Specific gravity at 15 c1.06
pH10.43
Viscosity (kinematic)m2/s0.75 ×10-6
TSSkg/m3Trace
Physical characteristics
MaterialGalvanized iron (zinc coated iron to prevent corrosion)
Dimensions100 cm × 170 cm × 46.5 cm
Glass roof thickness4 mm
Inclination29 deg facing south
Height from the ground70 cm
Drainage diameter2 cm
Initial volume of feed100 liter
CompositionUnitBrine outlet line
Ca++ppm as CaCO314616.3
Mg++ppm as CaCO336080
Fe++ppmTrace
Ba++ppmTrace
SO4−−kg/m35.25
HCO3kg/m30.185
Total hardnessppm as CaCO3453
SalinityPercent5.45
Conductivitys/m58666 ×10-4
Silicappm0.1
Specific gravity at 15 c1.06
pH10.43
Viscosity (kinematic)m2/s0.75 ×10-6
TSSkg/m3Trace

Theoretical Analysis.

In this work, theoretical analysis is being made to achieve the temperature variations of brine wastewater in solar desalination pond, glass covers, and base of solar pond at every instant and finally the condensation rate equation is formulated. Amount of condensation rate represents the performance of solar pond.

Solar Pond.

The energy conservation equations are written for the absorber plate (base of solar desalination pond), brackish water, and glass covers to find temperature profiles. Energy gained by the base of solar pond (from sun and convective heat transfer between brackish water and the base of solar pond) is calculated by Eq. (1). Brackish water in the solar pond receives energy from sun and base of solar pond which is equal to the summation of energy lost by convective heat transfer between brackish water and glass covers, evaporative heat transfer between brackish water and glass, irradiative heat transfer between brackish water and glass covers. So, energy gained by brackish water is written as Eq. (2).

Energy gained by glass covers (from sun and convective, irradiative, and evaporative heat transfer from brackish water to glass covers) is equal to the summation of energy lost by irradiative heat transfer between glass covers and sky. So, energy gained by glass is written as Eq. (3).
IαbaAba-Qc,b-ba-Qloss=mbacba(dTbadt)
(1)
IαbAb+Qc,b-ba-Qc,b-g-Qr,b-g-Qe,b-g=mbcb(dTbdt)
(2)
IαgAg+Qc,b-g-Qr,g-s-Qc,g-a+Qr,b-g+Qe,b-g=mgcg(dTgdt)
(3)
where
Qc,b-ba=hc,b-baAba(Tba-Tb)
(4)
Qloss=UbaAba(Tba-Ta)
(5)
Qc,b-g=hc,b-gAb(Tb-Tg)
(6)
Qr,b-g=hr,b-gAb(Tb-Tg)
(7)
Qe,b-g=he,b-gAb(Tb-Tg)
(8)
Qr,g-s=hr,g-sAg(Tg-Ts)
(9)
Qc,g-a=hc,g_aAg(Tg-Ta)
(10)
According to Tamimi and Rawajfeh in 2007, Velmurugan et al. in 2009, and Zurigat and Abu-Arabi in 2004 the related coefficients are hc,b-ba = 135 W/m2K, cba = 473 J/KgK, αba = 0.93, αb = 0.05, cg = 800 J/Kg K, ɛg = 0.88, ɛb = 0.97, αg = 0.048, Uba = 14 W/m2K, mg = 6.2 Kg, mb = 100 kg, and mba = 7.6 Kg [11–13]. Specific heat coefficient of brackish water cb is calculated from Srithar and Mani [14].
cb=A+BTb+CTb2+DTb3
(11)
A=4206.8-6.6197ξ+1.2288×10-2ξ2
(11a)
B=-1.1262+5.4178×10-2ξ-2.2719×10-4ξ2
(11b)
C=1.2026×10-2-5.5366×104ξ+1.8906×10-6ξ2
(11c)
D=6.8774×10-7+1.517×10-6ξ-4.4268×10-9ξ2
(11d)
Heat transfer coefficients are defined as below [13,14].
hc,b-g=0.884{(Tb-Tg)+[Pb-Pg][Tb+273.15](268.9×103-Pb)}1/3
(12)
hr,b-g=ɛeffσ(Tg2+Tb2)(Tg+Tb)
(13)
ɛeff=(1ɛb-1ɛg-1)-1
(14)
he,b-g=16.273×10-3hc,b-g(Pb-Pg)(Tb-Tg)
(15)
hr,g-s=ɛeffσ(Tg2+Ts2)(Tg+Ts)
(16)
ɛeff=(1ɛg-1ɛg-1)-1
(17)
Convective heat transfer between sky and glass covers depends on wind velocity [12]. This relation is presented in Eq. (18).
hc,g_a=2.8+3VW
(18)

Obviously, effect of convective heat transfer between glass covers and ambient is not considerable in the evaporation rate in the used closed solar pond. So, an average value 22 W/m2 K for hc,g_a is adjusted in modeling calculations considering normal geographical conditions [13].

Energy conservations for glass covers, brackish water, and base of solar pond (1, 2, and 3) are solved simultaneously to result the temperature profiles. Also, average values of solar radiation and ambient temperature are used for evaluating the temperature profile of brackish water in the simulation. Finally, calculation for brackish water temperature and glass temperature conducts to the total condensation rate which is presented in Eq. (21) [13].
dmc/dt=he,b-g(Tb-Tg)/hlv
(19)
dmc/dt=he,b-g(Tb-Tave)/hlv
(20)
Tave=0.8Tg+0.2Tba
(21)

Equation (13) is modified as Eq. (14), in order to evaluate precise results near to experimental data. Amount of condensation rate shows the productivity of solar pond as a performance index of solar desalination pond, so calculated values are compared with experimental data in various salinities considering the amounts of solar flux during a day.

Results and Discussion

Both salinity of wastewater and solar insolation rate are considered as the most effective independent variables in condensation rate. Variations of solar radiation values on June 17 and Dec. 30 are represented in Fig. 2, hourly. Also, experiments are carried out with 10 g/kg, 20 g/kg, 30 g/kg, and 40 g/kg salinity in the solar desalination pond. Also, as shown in Fig. 2, productivity of potable water varies during a day, so productivity increases in higher values of brackish water average temperature. Figures 3 and 4 show modeling results and experimental data of brackish water average temperature during a day in mentioned salinity contents on June 17 and Dec. 30, respectively.

Fig. 2
The insolation rate and the productivity rate in different times of a day on
                        June 17 and on Dec. 30, 2011
Fig. 2
The insolation rate and the productivity rate in different times of a day on
                        June 17 and on Dec. 30, 2011
Close modal
Fig. 3
The variations of brackish water average temperature during a day for 10%,
                        20%, 30%, and 40% salinity (June 17, 2011)
Fig. 3
The variations of brackish water average temperature during a day for 10%,
                        20%, 30%, and 40% salinity (June 17, 2011)
Close modal
Fig. 4
The variations of brackish water average temperature during a day for 10%,
                        20%, 30%, and 40% salinity (Dec. 30, 2011)
Fig. 4
The variations of brackish water average temperature during a day for 10%,
                        20%, 30%, and 40% salinity (Dec. 30, 2011)
Close modal

According to the results, higher salinities increase condensation rate since increase the temperature of brackish water. Results of derived theoretical equations show very good agreement with experimental data. Maximum deviation between experimental amounts of condensation rate (solar desalination pond productivity) and theoretical results is approximately 7%.

Sensitivity Analysis.

The average convective heat transfer coefficient is considered according to the regular wind velocity reported in the literature to calculate external convection. Also, sensitivity of ambient temperature to dust and tiny particles is ignored and a fixed transmisivity factor for the ambient is used. The mentioned estimations may result the 7% deviation between mathematical predictions and experimental data.

Lower operating costs in the form of alternative energy source have been found to be key factors in the economic viability of solar desalination ponds. Construction cost of this desalination method is not considerable compared with other desalination method. Solar desalination plants have a mean life time of about 20 yr while the cost of fresh water produced by solar plants ranges from 0.62 US$/m3 to 3.5 US$/m3, depending on the plant and the cost analysis method. It is important to realize that maximum production rate free of energy charge and lower operating costs are main indexes of the performance evaluation of solar pond.

Conclusions

In this paper, the productivity of a solar desalination pond is considered in a proposed zero discharge desalination system as a second stage. Salinity of wastewater and solar radiation as major parameters which affect the productivity rate are investigated experimentally and also theoretically by mathematical modeling. With higher insolation rate on 17 June, the average daily production of distilled potable water has been found to be increased considerably while the brackish water average temperature is increasing by salinity increasing. Salinity of brackish water, insolation rate, wind velocity, and dusty air affect the brackish water temperature values and also evaporation rate. Temperature gradient and productivity are sensitive to the first two parameters, significantly. But wind velocity which leads to external convection from the glass cover and dusty weather distorts the predicted trend of temperature gradient resulted by mathematical modeling. So, deviation between experimental data and mathematical values of brackish water temperature is resulted. In this paper, the average value for convection heat transfer coefficient between glass cover and ambient is considered.

This solar desalination pond uses evaporation method without fuel energy and electrical energy charges and also is environmental friendly. Maintenance cost of this solar pond is not significant compared with the other technologies. The quality of produced water is very high because the evaporation method is utilized. So, this produced water can be consumed for drinking, pharmaceuticals purposes, laboratories, factories, and micro irrigation. This solar desalination pond can be used in parallel for obtaining more potable water production.

Also, theoretical analysis is verified by experimental values of evaporation rate with maximum deviation of approximately 7%. So, the results indicate the validity of modeling equations to forecast the solar desalination performance.

Nomenclature

    Nomenclature
     
  • Ab =

    surface of solar pond base, (m2)

  •  
  • Aba =

    surface of brackish water, (m2)

  •  
  • Ag =

    surface of glass covers, (m2)

  •  
  • A,B,C,D =

    constants of specific heat equation

  •  
  • c =

    specific heat, (J/Kg K)

  •  
  • I =

    solar flux, (W/m2)

  •  
  • h =

    heat transfer coefficient, (W/m2 K)

  •  
  • hlv =

    latent heat of vaporization, (J/Kg)

  •  
  • ζ =

    salinity, (gr/Kg)

  •  
  • mc =

    condensate mass

  •  
  • Q =

    heat flux, (W/m2)

  •  
  • T =

    temperature, (C)

  •  
  • t =

    time

  •  
  • dt =

    time interval, (s)

  •  
  • Ts =

    sky temperature, (C)

Greek Symbols

    Greek Symbols
     
  • ɛ =

    emissivity factor

  •  
  • α =

    absorptivity

  •  
  • δ =

    Stefan–Boltzmann constant, (W/m2K4)

Subscripts

    Subscripts
     
  • A =

    ambient

  •  
  • Ba =

    base (of solar pond)

  •  
  • B =

    brackish water

  •  
  • E =

    evaporative

  •  
  • Eff =

    effective

  •  
  • G =

    glass

  •  
  • R =

    radiative

  •  
  • S =

    sky

  •  
  • C =

    convective

  •  
  • T =

    time

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