solar pond and its application to desalination - citeseerx

Asian Transactions on Science & Technology (ATST ISSN: 2221-4283) Volume 02 Issue 03

July 2012 ATST-50201035©Asian-Transactions 1

Solar pond and its application to desalination

A.Z.A. Saifullaha*

, A.M. Shahed Iqubala and Anirban Saha

b

a Department of Mechanical Engineering, IUBAT – International University of Business Agriculture and Technology, Dhaka 1230, Bangladesh

bDepartment of Electrical and Computer Engineering, the University of Texas at San Antonio, Tx – 78249, USA

Abstract

This paper discusses the solar pond technology and how it is applied to desalination. A solar pond is a shallow

body of water which acts as a solar collector with integral heat storage for supplying thermal energy. Solar ponds

are mainly two types: convective solar ponds and non-convective solar ponds. The shallow solar pond and the deep

saltless pond are the examples of convective type. There are three types of non-convective solar ponds: salinity

gradient solar pond (SGSP), membrane solar pond and polymer gel layers solar pond. A SGSP is a pool of water

about 1-5 m deep, which contains dissolved salts to establish a stable density gradient. There are three layers in a

SGSP: upper convective zone (UCZ), lower convective zone (LCZ) and salinity gradient non-convective zone

(NCZ) in the middle. Incident solar energy is collected and stored which may be delivered at temperature near

100C. The SGSP is the most eco-friendly and environment-friendly among all the solar energy systems for

electricity generation, desalination, hot water applications in agriculture, green house heating, domestic hot water

production and space heating and cooling of buildings. Nevertheless, a SGSP is more cost-effective since its

collection cost per square meter is only one-fifth of that of a liquid flat plate collector, and cost of 1KWh of

electricity production by a SGSP is only one-fifth of that produced by photovoltaic cells. A solar pond multi-stage

flash distillation system (SPMSF) is very promising for Bangladesh. MSF plants can produce 6-60 L/m2/day,

whereas for typical solar stills it is 3-4 L/m2/day.

Keywords: solar pond, salinity gradient, sodium chloride, desalination, SPMSF

Introduction

Study and research have been made for a low cost

collection and storage system of solar energy in

various countries [1]-[3]. Simultaneous collection

and storage of solar energy is feasible in a purposely

built open water reservoir commonly called as solar

pond [4]. Solar pond is a convenient and effective

means which collects solar radiation and stores its

thermal energy for a relatively longer period of time.

Remarkable research effort and publications started



in 1960’s, mostly in Israel. Then going slowly effort

on research speeded throughout the world after the

energy crisis in 1970’s [5]. Research has been done

on solar pond for about 50 years. It is now used in

Israel, USA, India and Australia [6] – [10]. China

has done remarkable progress in study and

application of solar pond technology to various

applications [11], [12]. Some other countries like

Iran, Turkey and Libya are also actively engaged in

research on solar ponds [18], [26], [32]. Simulation

has been performed for heat and mass transfer in a

SGSP by several researchers [26], [60]-[63].

Besides, experimental research in SGSP is also there

[26], [64].

The thermo-nuclear reaction in the sun originates

solar energy. Solar energy covers the entire

electromagnetic wave spectrum. The surface

receives about 47% of the total energy reaching the

earth. This amount only is the usable energy [13].

Solar energy can be utilized directly by two

technologies – solar thermal and solar photovoltaic.

Solar thermal technology results in solar collectors,

solar water heater, solar passive space heating and

cooling system, solar refrigeration and air-

conditioning system, solar cooker, solar furnace,

solar greenhouse, solar dryer, solar distillation, and

solar thermo-mechanical systems. Solar thermo-

mechanical system includes solar thermal water

pump, solar vapour compression refrigeration and

solar pond [14]. Solar pond is a simple and low cost

solar energy system. Solar pond is now an attractive

means which can be used for electric power

generation, desalination, salt production, grain

drying, fruit and vegetable drying, fruit and

vegetable canning industry, aquaculture, dairy

industry, green house heating, domestic hot water

production and space heating and cooling of

buildings.

The objective of this paper is to describe the

technology of solar pond and its application to

desalination.

Scope and Limitations

This paper gives a detailed concept on SGSP with its

working principle, thermal behavior, and stability

criterion. Besides, management of a SGSP has been

described with its construction, salt used, site

selection, soil character, salinity gradient formation,

obstruction and remedies and heat extraction. Solar

desalination has been discussed with special

emphasis on SPMSF.

There is no data and recommendations on site

selection, linear selection, salt gradient

establishment, heat extraction, environmental



protection and cost analysis of a SGSP, because no

research project on experimental

solar pond has been operated in Bangladesh until

today.

Solar Ponds

A solar pond is a shallow body of water which acts

as a solar collector with integral heat storage for

supplying thermal energy. There are two types of

solar ponds – convective solar pond and non-

convective solar pond. The shallow solar pond is a

convective solar pond. It consists of a large bag that

prevents evaporation but permits convection. The

bag has blackened bottom with foam insulation

below, and two types of glazing (sheets of plastic or

glass) on top. Solar energy heats the water in the bag

during the day and at night the hot water is pumped

into a large heat storage tank to minimize heat loss.

Another type is the deep, saltless pond. Double

glazing covers deep saltless pond. When solar

energy is not available or at night placing insulation

on the top of the glazing reduces heat loss [15].

A non-convective solar pond is a large shallow body

of water 1 to 5 m deep, but 3-4 m on the average,

which is arranged in a way so that the temperature

gradient is reversed from the normal. This allows

collection of radiant energy into heat (up to 95 C),

storage of heat and transport of thermal energy, at

temperature 40-50 C above normal, out of the system

[15]-[17].

There are three types of non-convective solar ponds

in terms of the methods of maintaining layered

structure. One is SGSP where density gradient is

maintained by salt water. The other is membrane

solar pond which uses horizontal and vertical

membranes. The third one is polymer gel layers solar

pond [16].

A SGSP is a system for solar energy collection and

storage. It uses solar radiation to heat water; stores

sensible heat in dense saline water; establishes

density gradient to prevent convective heat flow and

thus stores thermal energy. Fig. 1 shows the

schematic view of a SGSP [17], [18].

A SGSP has 3 main layers. These are UCZ (Upper

Convective Zone): top layer; NCZ (Non-convective

Zone): middle layer and LCZ (Lower Convective

Zone): bottom layer.

UCZ is of almost low salinity and at close to ambient

temperature. This zone is the result of evaporation,

wind mixing, surface flushing and nocturnal cooling.

Generally this layer is maintained as thin (0.3 – 0.5

m) as possible by the use of wave-suppressing

meshes or by placing wind-breaks near the ponds.



NCZ is a gradient which is much thicker and

occupies 1.5 m or more than half of the depth of the

pond. In NCZ, both salt concentration and

temperature increases with depth. The vertical

salinity gradient in NCZ holds back convection and

thus offers the thermal insulation effect. Temperature

gradient is formed due to absorption of solar

radiation at the pond base. LCZ is a zone of almost

constant relatively high salinity (20-30% by weight)

at nearly constant high temperature. Heat is stored in

LCZ, which should be sized to supply energy

throughout the year. It is almost as thick (usually 1

m) as the NCZ [15], [19]-[21]. This is the heat

collector, heat storage and heat removal medium.

The bottom boundary is a black body [22].

Working Principle

When solar radiation falls on the surface of the

SGSP, most of it penetrates and absorbed at the

bottom of the pond. The temperature of the dense

salt layer thereby increases. If there were no salt, the

bottom layer would become less dense than the top

layer and the buoyancy effect would cause this water

rise up and thus the layers would mix. Heat from the

surface of the pond is then rapidly dissipated to the

surroundings. But the denser salt layer at the bottom

of a SGSP prevents the heat to be transferred to the

top layer of fresh water by natural convection. Due

to this the temperature of the bottom layer may rise

up to 95 C making the SGSP a unique energy trap

with added advantage of built-in long-term heat

storage capacity [15], [19].

Thermal Performance

The thermal performance of a SGSP which is similar

to that of a conventional flat plate solar collector has

been shown by Srinivasan [20]. Assuming steady

state condition,

eau QQQ

where uQ useful heat extracted, aQ solar

energy absorbed and eQ heat losses.

The thermal efficiency of a SGSP is defined as

IQu /

where I is the solar energy incident on the pond.

IQe /0

where IQa /0 optical efficiency of the pond.

Again, )(0 ase TTUQ

where aT ambient temperature

and 0U over-all heat-loss coefficient

Neglecting heat losses from the bottom and sides of

the pond and assuming the temperature of the upper

mixed layer to be the same as the ambient,

bKU w /0



where wK thermal conductivity of water

and b thickness of the gradient zone

A steady state analysis of a SGSP that includes the

effect of solar radiation absorption in the gradient

zone on the temperature profile has been given by

Nielsen [20].

In steady state, the energy equation can be written as

)/()/( 22 dZdIdZTdK

where K thermal conductivity of water

and fraction of solar radiation I

reaching a depth .Z

On integration this equation gives

])/()()()[/()/(21 ZdZdTZZKIdZdT

where 1Z interface between the upper

convective zone and the gradient zone

and 2Z interface between the gradient

zone and storage zone

If is the fraction of the incident solar energy

which is extracted from the system as heat, including

heat losses, then the energy balance of the storage

zone gives

)])()[/()/(2

ZKIdZdT z

Combining the above two equations the temperature

profile in the gradient zone is obtained as

])()[(/()/( ZKIdZdT

The effect of ground-heat losses on the performance

of a SGSP has been analyzed by Hull et al [20], [24].

They have expressed the ground heat-loss coefficient

as

)//1( AbPDKU g

where K ground conductivity, D

depth of water table, P pond perimeter, A

surface area

and b a constant whose value is around

0.9 (depending upon the side slope).

The thermal efficiency of a steady state solar pond

now becomes

I

TU

I

TKdZZ

ZZ

gz

z

w

2

1

)(1

12

where T temperature difference

between the storage zone and the upper mixed layer.

Stability of Solar Pond

A solar pond is said to be statically stable if its

density increases with height from the top. Wind

blowing at the top surface and heating of the side

walls, etc. cause disturbance to a solar pond. The



criterion of dynamic stability is rather tough and is

obtained by perturbation analysis of the basic laws of

conservation of mass, momentum and energy [20].

The stability criterion is written as

1Pr

1Sc

Z

S

Z

TST

where

T

pT

1 thermal

expansion coefficient

SS

1 salinity

expansion coefficient

Pr Prandtl number

Sc Schmidt number

For typical conditions, this result is simplified to

Z

T

Z

S

19.1

where S is in kg/m3 and T in C.

The above criterion has to be satisfied at all points

within the gradient zone in order to prevent

development of internal convective zones within the

gradient zone. A safety margin of 2 is essential as

recommended by Hull et al [20], [25].

Safety margin (SM) is defined as

ltheoriticaactual Z

S

Z

SSM

where

Z

T

ScZ

S

S

T

ltheoritica

1

1Pr

The thickness of the gradient zone can be reduced by

the development of internal convective zones or

erosion of the boundaries of the gradient zone.

Wind-induced mixing is primarily responsible for

the erosion of the gradient zone-surface zone

interface and can be minimized by using floating

plastic nets or pipes. The density and temperature

gradients at the gradient zone -storage zone interface

causes the erosion of the gradient zone-storage zone

interface. The gradient zone -storage zone interface

remains stationary, which was experimentally found

by Nielsen [20], if the salinity and temperature

gradients satisfy the following relationship

63.0ZTAZS

where A = 28 (kg/m4)(m/K)

0.63.

Management of Solar Pond

Typical Construction:

Size of a SGSP ranges from hundreds to thousands

square meters in surface area. These are 1-5 m deep.

Typically these are lined with a layer of sand

insulation and then a dark plastic or rubber

impermeable liner material [17].



Salt used:

Sodium chloride (NaCl) is used normally.

Magnesium chloride (MgCl2), sodium nitrate

(NaNO3), sodium carbonate (Na2CO3), sodium

sulfate (Na2SO4), ammonium nitrate (NH4NO3),

fertilizer salts as urea (NH2CO.NH2) satisfy the

stability criterion and thus considered suitable for a

solar pond [17], [26].

Site selection:

Since solar ponds are horizontal collectors, sites

should be at low to moderate northern latitudes, that

is, latitudes between -40 to + 40 degree [16].

Soil character:

Evaluation of geological soil character is necessary

because the underline earth should be free from

stresses, strain and crack, which could cause

differential thermal expansions, resulting in earth

movement if the structure is not homogeneous [16].

As thermal conductivity of soil increases greatly

with moisture content the water table of the site must

be at least a few meters below the bottom of the

pond to minimize the heat loss [16].

Forming the Salinity Gradient:

First the storage layer is formed with high

concentration brine solution mixed in bottom. Then

layers (10-20 cm thick) of decreasing salinity

stacked on top of the storage layer using horizontal

diffuser. Lastly fresh water is the final layer pumped

on the surface [17], [27]. Fig. 2 shows the formation

of the salinity gradient.

Maintenance of Salinity Gradient:

The concentration gradient existing in SGSP leads to

steady diffusion of salt from higher to lower

concentration, that is, from bottom to top through the

gradient zone. The transport of salt through the

gradient zone by diffusion is expressed as

bDSSQ ulm /])[(

where b = thickness of gradient zone, D = mass

diffusion coefficient and ul SS , salinity in lower

and mixed layers respectively [20].

So, stability is to be maintained by introducing salt at

the bottom while the top is frequently washed with

fresh water. When solar radiation fall on the pond,

the part which is transmitted to the bottom heats the

lower layer and as a result inverse temperature

gradients are set in. These are temperature gradients

that are reversed from the normal. Inverse

temperature gradients are maintained to eliminate

convection currents that occur due to temperature

difference during normal temperature gradient [16].



Obstructions and Remedies

1. Cleanliness of pond since transmittance

can be reduced due to contaminants.

Filtration can remove contaminants.

Construction of larger ponds can

minimize the effect of contaminants.

2. Increase of UCZ caused by surface waves

and evaporation.

Use of floating nets and wind barriers can

reduce surface waves and mixing of

UCZ.

3. Algae and bacterial growth.

Algae growth can be controlled by

adding bleaching powder. Alternatively,

algae growth can be minimized by adding

1.5 mg CuSO4 per liter of water. If the

water used is alkaline, CuSO4 will not

dissolve.

The pond clarity can be maintained and

the thermal efficiency of the solar pond

can be improved by using a combination

of chemical and biological treatment

methods. Hydrochloric acid could be

used initially as a shock treatment to kill

all the algae and then introduction of

brine shrimps would control the growth

of algal and maintain transparency [49].

Cupricide is found to be more effective

than chlorine and is therefore

recommended chemical for algae control

in solar ponds. Chlorine is more corrosive

than Cupricide due to the acidic effect it

has on the pH [50].

4. Horizontal temperature gradient created

by salt solution and removal.

Injecting and removing salt solutions

very slowly can minimize horizontal

temperature gradient [16], [20], [23].

Heat extraction

For extracting energy stored in the bottom layer, hot

water is removed continuously from the bottom by a

pump, passed though an external heat exchanger or

an evaporator and then returned so as to heat this

water again. Another method of heat removal is

extracting heat with a heat transfer fluid by pumping

it through a heat exchanger placed on the bottom of

the pond [15], [22].

Heat is extracted from a SGSP conventionally by

drawing the heat from the LCZ only. This is done

with the help of an in-pond heat exchanger located in

the LCZ. A heat transfer fluid is circulating in a

closed cycle extracts heat from the internal heat

exchanger and transfers its thermal energy through

an external heat exchanger. Fig. 3 shows this method

of heat extraction used for heating application [28].



The 3000 m2 solar pond at Pyramid Hill, Australia

used this method.

Fig. 4 shows another conventional method of heat

extraction by pumping the hot brine from the top of

the LCZ through an external heat exchanger and then

sending back the brine at a lower temperature to the

bottom of the LCZ. The velocity of the brine

circulated is to be regulated in order to prevent

erosion of the gradient layer [28]. An example of this

type of heat extraction is the sodium chloride SGSP

of 3000 m2 constructed at El Paso, USA in 1983

[29].

A novel system of heat extraction for improved

efficiency is to extract heat from the non-convective

gradient layer of a SGSP as well as, or instead of,

from the lower convective zone (LCZ). This method

has been analyzed theoretically and compared with

the experimental results at Bundoora East, RMIT.

An in-pond heat exchanger made of polyethylene

pipe has been used to extract heat for over 2 months.

Heat extraction from the gradient layer increases the

overall efficiency of the SGSP by up to 55%,

compared with conventional method of heat

extraction solely from the LCZ [28], [30].

Examples of Solar ponds

El Paso Solar Pond:

The El Paso Solar Pond project was initiated by the

University of Texas at El Paso in 1983. It is a

research, development and demonstration project. It

has been operating since May 1986 showing

electricity, process heat and fresh water can be

produced successfully in the Southwestern United

States using solar pond technology [55].

Pyramid Hill Solar Pond:

A consortium of RMIT University, Geo-Eng

Australia Pty Ltd and Pyramid Salt Pty Ltd has

finished a project by the use of a solar pond located

at the Pyramid Hill salt works in Northern Victoria.

Its purpose is to capture and store solar energy using

pond water which can go up to 800C [56]. The heat

produced by the pond will be used for commercial

salt production as well as for aquaculture,

specifically producing brine shrimps for stock feed.

Plan is there to generate electricity from the heat

stored in the pond in a subsequent stage [57].

Bhuj Solar Pond:

The first large-scale solar pond in industrial

environment to cater actual user demand is the 6000

m2 solar pond in Bhuj, India. It supplied about 15

million litres of hot water in total to the dairy at an

average temperature of 750C

between September 1993 and April 1995 [58].



Ohio Solar Ponds:

Four SGSP have been designed, built and operated in

Ohio by Ohio State University.. Two solar ponds

were constructed in Columbus for physical studies,

one solar pond was constructed at the Ohio

Agriculture Research and Development Center at

Wooster and one solar pond was constructed in

Miamisburg to heat a community swimming pool

and recreational building. Data and

recommendations have been developed from these

research efforts on site selection, linear selection, salt

gradient establishment, heat extraction and

environmental protection. Sodium chloride was used

as the stabilizing salt for each pond. The costs of

building solar ponds varied from $38/m2 to $60/m

2

[59].

SGSP applied to Desalination

Desalination is termed to be any of several

processes which remove some amount of salt and

other minerals from water. Generally speaking,

desalination refers to extraction of salts and

minerals, as in soil desalination. Desalination of

water is done for converting salt water to fresh water

to make it suitable for human consumption or

irrigation. Sometimes the process gives out table salt

as a by-product. Many seagoing ships and

submarines use desalination. Large-scale

desalination is very costly compared to the use of

fresh water from rivers or ground water as it

typically uses extremely huge amount of energy as

well as specialized, expensive infrastructure. But,

beside recycled water this is one of the non-rainfall

dependent water sources particularly countries like

Australia which traditionally have depended on

rainfall in dams to supply their drinking water. As

scarcity of water has appeared as a major problem all

throughout the world desalination is getting priority

to meet the increasing demands for fresh water. One

of the most important international health issues is

clean potable water. The warm arid countries in the

Middle East and North Africa (MENA) and Southern

Asia within the latitudes 15-350N are the areas with

the severest water shortages. The increase in ground

water salinity and infrequent rainfall characterize

these areas. Simultaneous increase in industrial and

agricultural activities with the increasing world

population growth all throughout the world leads to

the depletion and pollution of fresh water resources

[38], [41]. During the last couple of years

desalination technologies have been significantly

developed. Large energy consumption occurs in the

major commercial desalination processes using oil

and natural gas as heat and electricity, while

emission of harmful CO2. Kalogirou estimated that

the production of 1000 m3/day of fresh water



requires 10,000 of oil/year [51]. This is of high

significance as it needs a recurrent energy expense

which only a few of the water-short areas can afford.

Alternative sources energy, especially, renewable

energy sources have been drawn attraction to sea

water desalination [42]. However, today only 0.02%

of the global desalination is run by renewable energy

systems [46]. Solar distillation presents an

ecologically advantageous means of the use of

renewable energy [39]. Due to the diffuse nature of

solar energy, large-scale desalination plants using

solar energy have the problems of relatively low

productivity rate, the low thermal efficiency rate and

the considerable land area requirement. But the very

nature of solar desalination plants is free energy and

insignificant operation cost. Besides, this technology

is suitable for small-scale production, especially in

remote arid areas and islands, where there is scarcity

of supply of conventional energy [54].

Desalination involves desalting a variety of raw

waters (sea water, brackish ground water or

industrial waste-water) through suitable treatment

and obtaining fresh water for drinking and irrigation.

Solar energy has been used for distillation of

brackish or saline water for a considerably long time.

The current leading desalination process is thermal

desalination which includes multistage flash

distillation (MSF) and Multi-effect distillation

(MED). Thermal desalination is an energy-intensive

process. During the last several years substantial

amount of research work has been done on

desalination using solar energy. For sea water

desalination there are mainly two approaches for

solar energy utilization. The solar distillation plant

may consist of integrated or separated systems for

the solar collector and the distiller. Integrated

systems are termed as direct solar desalination which

involves different types of solar stills. Separated

systems are called indirect solar distillation. The first

approach is suitable for small production systems,

such as, utilization of green house effect to evaporate

salty water in a simple solar still [42]. Solar stills are

used in regions where the fresh water demand is less

than 200 m3/day [52]. The other approach often

involves more than one subsystem: one for energy

collection, another for energy storage and the third

subsystem for energy utilization in the desalination

process. The desalination process may be MSF,

MED, vapor compression (VC), reverse osmosis

(RO), membrane distillation (MD) or electrodialysis

[38], [40]. One significant problem that affect the

still performance is the direct contact between the

collector and the saline water which causes corrosion

and scaling in the still and thus reduces the thermal

efficiency [43]. The water desalination with

humidification-dehumidification (HD) uses air as the



working fluid, which eliminates this problem. The

overall efficiency of the desalination plant increases

by combining the principle of humidification-

dehumidification with solar desalination, and thus

seems to be the best method for water desalination

by solar energy [44]. A HD system consists of a

compact unit containing two heat exchangers for

evaporation and condensation respectively. The

constructions are lightweight usually and

inexpensive, and work at atmospheric pressure. As

desalination capacity is relatively low, the system

performance must be improved to make it

economically competitive [42], [44]. Solar ponds

and parabolic troughs are the most frequently used

solar thermal technology for desalination [53].

Thermal desalination using SGSP is one of the most

promising solar desalination technologies. Solar

ponds generated thermal energy to drive a

desalination plant has been investigated by Tabor

[33], Tleimat and Howe [34], [35], Guy and Ko [36]

and Posnansky [37]. The United States government

used solar pond technology especially for this

purpose. The Water Desalination Research and

Development (DesalR&D) Program was authorized

by Congress under the Water Desalination Act (Act)

of 1996. The Act authorized program funding began

in October 1997 for a six year period. To start the

program, funding was appropriated at $3.7 million

for fiscal year 1998… The act is based on the

fundamental need in the US and world-wide for

additional sources of potable water.

SGSP enables the most convenient and least

expensive option compared to other solar

desalination technologies to store heat for daily and

seasonal cycles. For steady and constant water

production, this is very important from the view

point of operational advantage and economic benefit.

The heat storage enables SGSP to power desalination

during night time and cloudy days. SGSP used

desalination for a 24-hour a day operation needs only

half the size to produce same quantity of water

compared to other solar desalination options. For

desalination SGSP can make the use of reject brine

as a basis to build it. This advantage is very

important when SGSP is considered for inland

desalting for fresh water production or brine

concentration to be used in salinity control and

environmental clean-up applications.

At present the most common and simple technique

for large-scale desalination is MSF, which produces

fresh water a total amount of about 10 million

ton/day globally [42].

A solar pond multi-stage flash distillation system

(SPMSF) comprises a set of evaporative condensers

and heat exchanger for extracting heat from the



SGSP. Repetitive cycles of evaporation and

condensation using low temperature heat from the

SGSP produces fresh water. Fig. 5 shows the

schematic diagram of solar pond desalination [15],

[22].

Several medium scale MSF desalination plants using

solar energy have been recently implemented.

Block’s findings show that MSF plants can produce

6-60 L/m2/day, whereas for typical solar stills it is 3-

4 L/m2/day [45].

A SGSP is one of the most common types of solar

collectors. The SGSP driven desalination plant in

Margarita de Savoya, Italy, has a capacity of 50-60

m3/day and that in El Paso, Texas has the capacity of

19 m3/day. Parabolic trough collector is another

frequently occurring source for solar energy which is

used in a MSF plant in Kuwait with a production rate

of 100 m3/day [46].

Desalination using solar troughs was tested mainly in

the USA. Small-scale units are commercially

available. These combine the MSF process with

steam generating parabolic troughs. A typical plant

produces 450 L/day in three stages by using 48 kW.

The current cost of the collectors (about 45 m2) is

about US $ 10,000, which translates into production

costs of 7.9 US$/m3 (5% interest, 20 years life time,

annual O & M equivalent to 3% of the investment

costs, 85% plant factor) [47].

Szacsvay et al. has describes a desalination system

consisting of a solar pond as the heat source and an

Atlantis autoflash multistage desalination unit. The

Atlantis Company developed an adapted MSF

system called “Autoflash”, since the standard MSF

process was not able to operate couple to any heat

source. The basis of autoflash process is multistage

flash process concept. Computer simulation and

experimental results of a small-size solar pond and

desalination subsystem in Switzerland which had

been in operation for 9 years were done for

performance and layout data. It was found that the

cost of distillate could be reduced from $ 5.48/m3 for

small desalination system with a capacity of 15 to

2.39 m3/day for desalination systems with a capacity

of 300 m3/day [48].

The average daily solar energy incident in India is 5

kWh/m2. India is in advancement in solar pond

research and applications [2], [20]. Like India

Bangladesh is also in the tropics. It is located

between 20.30-26.38 North Latitude and 88.04-92.44

degrees East Latitude. This is an ideal location for

solar energy utilization. The daily average solar

radiation varies between 4 to 6.5 kWh/m2. Solar

pond appears to be highly promising for Bangladesh



[31]. Bangladesh has 68 districts in total. The 9

districts, namely, Khulna, Satkhira, Bagerhat,

Barguna, Patuakhali, Bhola, Pirozpur, Jhalkathi and

Cox’sbazar lie in the coastal belt. The ground water

available there for drinking purpose is salty. This

water must have to be desalted in order to make

fresh water for drinking supplies. For a developing

country like Bangladesh SGSP used desalination

method is the most suitable.

Advantages

1. Diffuse radiation (cloudy days) can be

used.

2. It is a unique energy trap with built-in

long term energy storage capacity.

3. For low grade heat (below 100 C)

collection cost/m2 of collector area of

SGSP is 1/5th

that of flat plate collector

[15].

4. 1 kg of salt as salt-water concentrate can

produce energy 3 times more than the

heat produced by burning the same

amount of coal in the combustion

chamber [15].

5. Pollution free.

6. In Germany, cost of producing 1kWh of

electricity by a SGSP is only 21% of that

produced by photovoltaic cells [23].

Conclusions

1. Solar pond is an efficient source of

renewable heat energy.

2. Solar pond is environmentally sustainable.

3. The great advantage of solar pond it

possesses built-in long term thermal energy

storage, which no other

solar collection device match.

4. Solar pond can be economically

constructed if there is plenty of inexpensive

salt, flat land and

easy access to water.

5. A great factor in the future of solar

pond operation is the implementation of

an acceptable means of

salt recycling. This is a major point

when a solar pond is to be used on a

farm or private land. On-farm use will

predominate in the more northerly

latitudes, because of availability of

land, machinery and labor.

6. Solar pond can complement the use of

fossil fuel in industries to generate thermal energy

and

commercial electricity.

7. Solar pond is highly promising for the

tropics and the lower latitudes for

electric power generation.

8. Solar pond is eco-friendly.



9. Solar pond desalination is the most

effective for Bangladesh.

10. A solar pond multi-stage flash

distillation system appears to be

promising for the coastal districts of

Bangladesh for fresh water production.

11. Solar pond projects are yet to be started

in Bangladesh. Therefore, an

experimental project on solar pond

needs to be developed for data and

recommendations on site selection,

linear selection, salt gradient

establishment, heat extraction,

environmental protection and cost

analysis.

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Figure Captions

Figure 1 Schematic view of a SGSP

Figure 2 The formation of the salinity gradient

Figure 3 Heat extraction using an internal heat exchanger by conventional method

Figure 4 Heat extraction using an external heat exchanger by conventional method

Figure 5 Schematic diagram of solar pond desalination



Figure 1 Schematic view of a SGSP



Figure 2 The formation of the salinity gradient



Figure 3 Heat extraction using an internal heat exchanger by conventional method



Figure 4 Heat extraction using an external heat exchanger by conventional method



Figure 5 Schematic diagram of solar pond desalination

solar pond and its application to desalination - citeseerx

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