ICETCESD 2012

March 10-12, 2012, Sylhet, Bangladesh

SOLAR DESALINATION USING LOW COST TUBULAR SOLAR STILL (TSS)

Md. Abdur Rab1*, Dr. Kh. Md. Shafiul Islam2, A. K. M. Maminul Islam3, Md. Manjur-A-Elahi4

ABSTRACT: In the world, demand of portable fresh water is increasing day by day because of

population explosion all over the world, greater industrial development, expansion of agricultural

activities and climate change. The available underground and surface water sources are being

affected by salinity, industrial and agricultural wastes, limit the suitability of many fresh water

availability resources. Now it is recognized that freshwater is a scare resources and more country

is converted into water-stressed country due to the scarcity of freshwater resources. Therefore,

this project was undertaken to study the performance of tubular solar still for desalination

purpose. At first, a low cost tubular solar still was designed and constructed using locally

available materials. Field experiment was carried out on constructed unit located on the roof top

of the Civil Engineering Department, KUET, Khulna, Bangladesh from January 01 to April 30,

2011. The hourly output and ambient air temperature data was also collected in some typical days

(2, 3 and 10 May, 2011) to show the effect of air temperature on productivity of tubular solar still.

The analysis shows that the production rate of a tubular solar still is mainly depend on the

intensity of solar radiation and the production rate will be higher in the summer season. It was

observed that average daily distilled output of Tubular solar still increases for the months January

to March except for the month April. It is founded that the average daily production rate of

tubular solar still was 0.302lit/day (1.885lit/m2-day) and the production cost of distilled water

was Tk.0.453/lit. The construction, operation and maintenance of tubular solar still are easier. So,

one can easily construct a tubular solar still using locally available material to produce fresh

water from saline water in the remote, coastal and arid areas to meet small scale fresh water

demand.

1. Introduction

Water is one of the most abundant resources on earth, covering three-fourths of the planet’s

surface. There is an almost unfathomable amount of water on earth: about 1.4 billion km3

(330

million cubic miles) (Barlow and Clark, 2002). About 97% of the earth’s water is salt water in the

oceans and a tiny 3% (about 36 million km3) is fresh water contained in the poles in the form of

ice, ground water, lakes and rivers, which supply most of human and animal needs (Ahmed and

1 Under Graduate student, Civil Engineering Department, KUET, Khulna, Bangladesh. abdur.rab06kuet@

gmail.com 2 Professor, Civil Engineering Department, KUET, Khulna, Bangladesh. khmsislam@yahoo.com 3 Under Graduate student, Civil Engineering Department, KUET, Khulna, Bangladesh. shaikat.engr.ce@g

mail.com 4 Deputy Project Engineer, DOM-INNO Developers Limited, Dhaka, Bangladesh. manjurkuet@gmail.com

Rahman, 2002). Nearly, 70% from this tiny 3% of the world’s fresh water is frozen in glaciers,

permanent snow cover, ice and permafrost. Thirty percent of all fresh water is in underground,

most of it is in deep, hard-to-reach aquifers. Lakes and rivers together contain just a little more

than 0.25% of all fresh water; lakes contain most of it (Kalogirou, 2005). The data relevant to

water requirements shows that around 25% of the total world populations do not have an

adequate fresh water supply, both for quality and quantity (Agha et al., 2005). Water shortages

affect 88 developing countries that are home to half of the world’s population. In these places,

80-90% of all diseases and 30% of all deaths result from poor water quality (Leitner, 1998).

Furthermore, over the next 25 years, the number of people affected by severe water shortages is

expected to increase fourfold (Engelman et al., 2000). Some of this increase is related to

population growth, some is related to the demands of industrialization. Currently, water

consumption doubles every 20 years, about twice the rate of population growth (Barlow and

Clark, 2002). The ground water source is being polluted by organisms, organic and inorganic

compounds due to the ultimate disposal of man-made harmful pollutant into the underground

reservoir (Malik et.al., 1982).

It is estimated that approximately one-third of the world’s population use groundwater for

drinking (Nickson et al., 2005). However, in the coastal regions in any country, groundwater

quality patterns are complex because of the input from many different water sources. These

include precipitation, seawater, ascending deep groundwater, and anthropogenic sources such as

wastewater or irrigation return flow (Ramkumar et al., 2010). The main sources of water in

Bangladesh are surface waters in rivers, reservoirs, lakes, canals and ponds, and ground water in

deep and shallow aquifers. In the coastal areas of Bangladesh water supply is facing great

challenges in meeting the rising freshwater demand due to limited water supply from the

available underground and surface water sources, which are affected by salinity. The increase in

salinity in Khulna started after the commencement in 1975 of the Farrakka Barrage operation in

India, which significantly reduced the flow in the Ganges, located at upstream of the Gorai River,

a major source of freshwater to the rivers surrounding Khulna. At present, the principle cause of

salinity intrusion in Khulna region is the drop of hydraulic head during the dry period (November

to May) into both surface and groundwater of the area (Hassan et al., 1998). The major difficulty

in developing the appropriate water supply system for the communities is that the aquifers

containing fresh water are not always found at convenient locations. So, it would be feasible to

address the water shortage problem with seawater desalination. Most of the modern interest in

desalination is focused on developing cost-effective ways of providing fresh water for human use

which typically needs extremely huge amount of energy as well as specialized expensive

infrastructure. But small scale and renewable energy powered desalination plants are also being

taken interest as a particular option where solar power is rich. Solar desalination is the simplest

desalination technique and environmentally friendly compared other methods such as reverse-

osmosis, electro-dialysis and biological treatment.

2. Design, Construction and field experiment of tubular solar still

Tubular solar still consisted of tubular frame covered with a transparent normal polythene paper

and a black rectangular trough for storing saline water. The tubular frame is 1.00 m long, 20 cm

in diameter and is comprised of 2.75 mm thick helical G.I wire. The pitch of the spiral ring is

about 5 cm. The trough of 1.0 m long, 16 cm width and 5 cm depth is made of carton paper

(painted black inside). The schematic diagram of tubular solar still is shown in Figure 1.

Cross section at A-A

Figure 1 Schematic diagram of tubular solar still

2.1. Production principle of tubular solar still

The solar radiation, after transmission through a tubular cover, is mainly absorbed by saline water

in the trough. The tubular cover and trough absorb the remaining small amount of the solar

energy. Thus, the water in the trough is heated and then begins to evaporate. Many types of heat

transfer occur inside the tubular cover and outside, e.g., evaporative heat transfer from the saline

water to the tubular cover, condensate heat transfer between the saline water and tubular cover,

convective heat transfer between the saline water and the tubular cover, the trough and the water,

the trough and the tubular cover and the cover and the atmosphere, radioactive heat transfer

between the water surface and the tubular cover and the tubular cover and the atmosphere. The

evaporative water vapor is transferred to the tubular cover and then finally condensed on the

tubular cover inner surface, releasing its latent heat of vaporization. The condensed water trickles

down at the bottom of the tubular cover due to gravity and is stored in a collection bottle through

a pipe (12 mm diameter) provided at the bottom point of trough. Production principle of a tubular

solar still is illustrated in Figure 1.

2.2. Mass balance and energy balance equations

Figure 2 shows, the mass and energy transfer within and outside of a tubular solar still. The key

point of modeling is the accuracy of the evaporation from the saline water surface and the

condensation on the tubular cover.

Figure 2 Mass and energy transfer of a tubular solar still

2.2.1. Assumptions

The mass and energy balance equations are made up on the following assumptions:

1. Heat and mass transfer in a TSS are formalized using the representative temperature of the

saline water, trough and tubular cover.

2. Water vapour on the water surface is saturated.

3. There is no water vapour leakage across the tubular cover.

4. The absorption of the solar radiation in the humid air is negligibly small.

2.2.2. Mass balance equation

Mass balance equation for saline water in trough is given in equation (1)

w

e v pwm

t

h

(1)

2.2.3 Energy balance equations

Energy balance equations for saline water, trough and tubular cover are given in equation (2), (3)

and (4), respectively.

rwcwewctwww

w QQQQRt

TCV

)( (2)

ctcctwtrtr

tr QQRt

TCV

)( (3)

rcccrwctccwcdccc

c QQQQQQRt

TCV

)( (4)

Where,

C : Specific heat capacity (J/Kg)

hw : Depth of water in the trough (m)

Rw : Solar radiation adsorbed by saline water (J/s)

Rtr : Solar radiation adsorbed by trough (J/s)

Rc : Solar radiation adsorbed by tubular cover (J/s)

mevp : Evaporation flux (Kg/m2-s)

Qcc : Convective heat transferred between cover and atmosphere (J/s)

Qcdc : Convective heat transferred between cover and water (J/s)

Qcw : Convective heat transferred between water surface and cover (J/s)

Qew : Evaporative heat transferred from water surface and cover (J/s)

Qrc : Radiative heat transferred between tubular cover and atmosphere (J/s)

Qrw : Radiative heat transferred between tubular cover and water surface (J/s)

Qctc : Convective heat transferred between trough and cover (J/s)

Qctw : Convective heat transferred between trough and water (J/s)

Rs : Solar radiation (W/m2)

T : Temperature (℃)

ρw : Density of water (Kg/m3)

Vw : Volume of water (m3)

The subscripts a, c, t and w denote atmosphere, tubular cover, trough and saline water,

respectively.

2.3 Field experiment

The field experiment was carried out on the constructed tubular solar still from January 01 to

April 30, 2011. A wooden frame was used to support tubular solar still so that free circulation of

air occurs beneath the TSS. The vessel for distilled water collection was put in a wooden box in

order to collect the distilled water from the tubular solar still accurately. One end of tubular solar

still can be opened to feed raw water into the trough and to remove the accumulated saline into

the trough. The depth of solution into the trough was kept low to increase water temperature and

daily production rate. The daily output of distilled water from the tubular solar still is collected

approximately two hour after sunset. The hourly output and ambient air temperature was

measured in some typical days (2, 3 and 10 May, 2011) to show the effect of air temperature on

productivity. Hourly data of water temperature, humid-air temperature, air temperature and

productivity is also collected in 10 May, 2011. Figure 3 shows, the photograph of the field

experiment. The instrumental set up for the collection of data of water temperature, temperature

between cover and water surface (humid-air temperature) and ambient air temperature are shown

in Figure 4.

Figure 3 Photograph of field experiment

Figure 4 Experimental set up for temperature measurement

2.4. Data collection

The output of distilled water from the tubular solar still is collected everyday approximately two

hour after sunset from 01 January to 30 April, 2011. The hourly output is also collected in some

typical days (2 and 3 May, 2011). Table 1 shows, the daily production of distilled water from the

tubular solar still. Table 2 shows, the hourly production of distilled water for tubular solar still.

Table 3 shows, the water temperature, humid-air temperature, air temperature and distillate output

Thermometer

for measuring

air temperature

Thermometer for

measuring water

temperature

Thermometer for

measuring humid-

air temperature

Tubular cover inside

rectangular trough

Collection Vessel

inside the wooden

box

of tubular solar still in 10 May, 2011.

Table 1 Daily distilled water production of tubular solar still from January to April, 2011

Day

Daily distilled water production (Lit./day)

January February March April

1 0.241 0.293 0.338 0.503

2 0.235 0.237 0.472 0.338

3 0.296 0.212 0.397 0.319

4 0.164 0.233 0.408 0.293

5 0.282 0.306 0.319 0.217

6 0.247 0.293 0.410 0.338

7 0.244 0.202 0.481 0.345

8 0.251 0.303 0.437 0.293

9 0.273 0.306 0.449 0.247

10 0.280 0.358 0.416 0.205

11 0.079 0.360 0.176 0.317

12 0.176 0.265 0.374 0.316

13 0.205 0.286 0.436 0.299

14 0.118 0.326 0.293 0.234

15 0.159 0.355 0.273 0.364

16 0.153 0.303 0.364 0.341

17 0.228 0.287 0.449 0.325

18 0.231 0.241 0.390 0.371

19 0.213 0.286 0.377 0.452

20 0.241 0.111 0.345 0.325

21 0.199 0.039 0.338 0.428

22 0.243 0.341 0.291 0.294

23 0.190 0.347 0.408 0.333

24 0.200 0.356 0.363 0.484

25 0.252 0.391 0.267 0.291

26 0.235 0.381 0.382 0.320

27 0.172 0.289 0.456 0.321

28 0.273 0.429 0.273 0.352

29 0.228 - 0.391 0.425

30 0.216 - 0.346 0.267

31 0.225 - 0.389 -

Table 2 Hourly distilled water production from the tubular solar still

Table 3 Water temperatures, temperature between cover and water surface (humid-air

temperature), air temperature and distillate output of tubular solar still in 10 May,

2011

Time

At 02/05/2011 At 03/05/2011

Air

Tem. (°C)

Hourly

Pro. (Lit/hr.)

Air

Tem. (°C)

Hourly

Pro. (Lit/hr.)

07.00 am 30.1 0.000 26.3 0.000

08.00 am 31.1 0.000 31.3 0.000

09.00 am 34.4 0.003 33.0 0.008

10.00 am 35.0 0.004 38.1 0.026

11.00 am 39.1 0.014 38.3 0.025

12.00 am 33.6 0.054 38.7 0.030

01.00 pm 34.7 0.075 37.0 0.033

02.00 pm 35.5 0.042 34.2 0.070

03.00 pm 36.1 0.048 35.1 0.060

04.00 pm 34.7 0.030 32.8 0.024

05.00 pm 32.2 0.028 31.0 0.031

06.00 pm 29.5 0.018 30.0 0.010

07.00 pm 28.1 0.004 29.2 0.008

08.00 pm 27.5 0.004 28.2 0.003

09.00pm 26.3 0.000 27.6 0.000

Time Temperature (◦C) Distillate

output

(Lit/hr.) Water Humid-air Ambient air

07.00 am 28 26 31 0.000

08.00 am 37 33 31 0.005

09.00 am 49 43 33 0.017

10.00 am 58 53 35 0.022

11.00 am 57 52 35 0.047

12.00 am 59 54 36 0.055

01.00 pm 61 55 38 0.053

02.00 pm 56 52 38 0.073

03.00 pm 53 50 36 0.050

04.00 pm 51 47 35 0.048

05.00 pm 47 41 35 0.027

06.00 pm 40 33 33 0.023

07.00 pm 34 29 32 0.006

08.00 pm 33 28 31 0.005

09.00 pm 30 26 29 0.000

3. Data Analysis, results and discussions

3.1. Data analysis

Data of Table 1, 2 and 3 were used to calculate the daily and hourly production rate per unit

surface area of the trough. Table 4 shows, the daily production rate of tubular solar still from 01

January to 30 April, 2011. Table 5 shows, the hourly production rate of tubular solar still for some

typical days (2, 3 and 10 May, 2011).

Table 4 Daily distilled water production rate of TSS for January to April, 2011

Day Daily distilled water production (Lit/m2-day)

January February March April

1 1.507 1.824 2.113 3.146

2 1.47 1.477 2.947 2.113

3 1.854 1.322 2.474 1.986

4 1.027 1.455 2.548 1.824

5 1.765 1.906 1.994 1.359

6 1.544 1.824 2.556 2.113

7 1.529 1.264 3.007 2.149

8 1.567 1.892 2.733 1.824

9 1.707 1.906 2.799 1.544

10 1.751 2.231 2.6 1.286

11 0.495 2.253 1.093 1.98

12 1.101 1.655 2.341 1.972

13 1.286 1.788 2.718 1.869

14 0.738 2.038 1.832 1.463

15 0.989 2.217 1.707 2.275

16 0.961 1.892 2.275 2.127

17 1.418 1.795 2.799 2.032

18 1.448 1.507 2.438 2.311

19 1.33 1.788 2.357 2.829

20 1.507 0.694 2.149 2.032

21 1.242 0.244 2.113 2.674

22 1.521 2.127 1.817 1.84

23 1.19 2.171 2.548 2.083

24 1.248 2.223 2.267 3.021

25 1.573 2.445 1.661 1.817

26 1.47 2.379 2.386 2.002

27 1.071 1.802 2.851 2.009

28 1.707 2.682 1.707 2.201

29 1.426 - 2.445 2.659

30 1.352 - 2.165 1.669

31 1.404 - 2.43 -

Table 5 Hourly distilled water production rate of tubular solar still

Time Production rate (lit/m2-hr)

At 02/05/2011 At 03/05/2011 At 10/05/2011

07.00 am 0.000 0.000 0.000

08.00 am 0.000 0.000 0.031

09.00 am 0.019 0.050 0.106

10.00 am 0.025 0.163 0.138

11.00 am 0.088 0.156 0.294

12.00 am 0.338 0.188 0.344

01.00 pm 0.469 0.206 0.331

02.00 pm 0.263 0.438 0.456

03.00 pm 0.300 0.375 0.313

04.00 pm 0.188 0.150 0.300

05.00 pm 0.175 0.194 0.169

06.00 pm 0.113 0.063 0.144

07.00 pm 0.025 0.050 0.038

08.00 pm 0.025 0.019 0.031

09.00 pm 0.000 0.000 0.000

3.2. Results and discussions

Figure 5 shows, daily distilled output per unit surface area of the trough of tubular solar still from

January 01 to April 30, 2011. In Figure 5, it is evident that daily distilled output increases

gradually approximately from January to mid of March then gradually decreases up to mid of

April and therefore increases again. The average daily production rate of the tubular solar still is

0.302 lit/day (1.885 lit/m2-day). The daily production rate can be expressed by the linear

regression equation (5). This regression equation is only valid for month January to April, 2011.

P = 1.3817 + 0.00844 RD (5)

Where,

P = Productivity rate of distilled water (Lit/m2-day)

RD = Rank of the day in which 1 is for January 01 and 120 for April 30.

The variations of daily average productivity for different months for tubular solar still are shown

in Figure 6. It is observe that the average daily production rate is maximum for the month March

and minimum for January. The average daily production rates of tubular solar still were 1.361,

1.813, 2.317 and 2.074 lit/m2-day, respectively for the months January to April.

Figure 5 Variation of daily productivity (lit/m2-day) for tubular solar still

Figure 6 Variation of average daily productivity in different months for tubular solar still

Figure 7 shows, the variation in hourly production rate for different ambient air temperature in

some typical days (2, 3 and 10 May, 2011). From Figure 7, it is evident that the hourly production

becomes maximum after one or two hour of maximum ambient air temperature. It was also

observed from the figure that all temperatures and hourly production slopes are steeper in the

morning than that of afternoon.

January February March April0

0.5

1.0

1.5

2.0

2.5

3.0

Ave

rage

pro

duct

ivity

(lit/

m2 -d

ay)

Month

0 30 60 90 1200

1

2

3

4

P= 1.3817+0.00844RD

January February March April

Dis

tille

d ou

tput

(lit/

m2 -d

ay)

Time (Day)

Data collection from January 01 to April 30, 2011

Figure 7 Hourly variations of productivity and ambient air temperature for the TSS

Figure 8 shows, the variation of water temperature, humid air temperature, air temperature and

hourly production rate in 10 May, 2011. From the figure, it is observe that all air temperature rise

rapidly from the morning to noon, peak approximately at 1.00 pm and then decline gradually. The hourly

production rate is also rise gradually from morning to noon then decline gradually.

Figure 8 Hourly variations of production rate and tem. parameters for the TSS in 10 May, 2011

7 8 9 10 11 12 13 14 15 16 17 18 19 20 210

10

20

30

40

50

60

70

80

90

Tem

pera

ture

(°C

)

pmamTime of Day (hr)

Water temperature oC

Humid air temperatureoC

Air temperatureoC

0

0.2

0.4

0.6

0.8

1.0

1.2

Dis

tille

d ou

tput

(lil/

m2 -h

r)

Distilled output (Lit/m2-hr)

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

20

25

30

35

40

45

50

Air temp. Productivity

Air

tem

pera

ture

(0 C)

Time of Day (hour)

7 8 9 10 11 12 13 14 15 16 17 18 19 20 210

0.2

0.4

0.6

0.8

1.0

pmam

Pro

duct

ivity

(lit/

m2 /h

r)

May 02

May 03

May 10

3.3 Cost Analysis

Table 6 Cost estimation of tubular solar still

SL. No. Item Unit Rate Quantity Amount (Tk)

1 G. I. wire m 4.5 12 54

2 Trough nos. -- 1 16

3 Polythene

30

4 Wooden frame 40

Total = Tk. 140.00

If we consider design life of tubular solar still is 4 years and in a year 70% time one can get the

calculated average daily output (i.e. 0.302lit/day) then,

The production of water in the design life =0.302×365×0.70×4 =308.64 Lit.

Production cost of water = (140 /308.64) =Tk. 0.453 /lit.

4. Conclusions:

It is concluded that the design and construction of tubular solar still is very simple and the

construction cost is very low. The technology involved in desalination of saline water using solar

energy is relatively simple, the fabrication of component is very simple and maintenance can be

carried out by households. The analysis shows that the production rate of a solar still is mainly

depend on the intensity of solar radiation and the production rate will be higher in the summer

season. All temperatures and hourly production slopes are steeper in the morning than that of

afternoon. The hourly production rate is also rise gradually with air temperature from morning to

noon then decline gradually. It is founded that average daily distilled output of Tubular solar still

increases for the months January to March except for the month April. The average daily

production rates of tubular solar still were 1.361, 1.813, 2.317 and 2.074 lit/m2-day, respectively

for the months January to April. It is founded that the average daily production rate of tubular

solar still was 0.302lit/day (1.885lit/m2-day) and the production cost of distilled water was

Tk.0.453/lit. Since the construction, operation and maintenance of tubular solar still is easier so,

one can easily construct a tubular solar still using locally available material to produce fresh

water from saline water in the remote, coastal and arid areas to meet small scale fresh water

demand.

Acknowledgements

The authors wish to express their acknowledgements to the Department of Civil Engineering,

Khulna University of Engineering & Technology (KUET), Khulna, Bangladesh for extending all

sorts of supports for this study.

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