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Introduction
Formally known as a salinity gradient solar pond, solar ponds are an alternative source of
harnessing the suns energy to heat water that can be converted to electricity. This
technology is very basic and easy to use with adequate land space and proper design. For
residential use, ponds need to be at least 12x12 feet and 3 feet deep and for commercial use
ponds must be sized according to their functionality. Solar ponds require lots of sunlight
and salt water. The first solar pond was discovered in the early 1900s in Transylvania and
was naturally occurring. Following this discovery these ponds were replicated and dubbed
solar ponds.
How they work
Solar ponds can be naturally occurring; however, most ponds are man-made. Once the
pond is dug, the pond must be lined with an impermeable lining, preferable one that is
insulating. Then the pond is filled with salty water. Once the sun hits the pond the water
warms and divides into three layers. The top layer, known as the surface zone, is composed
of primarily freshwater due to the fact that salt typically settles at the bottom of water.
The middle layer is known as the insulation zone. The insulation zone has a higher salt
concentration than the surface zone. Crucial to a solar pond is the bottom layer known as
the storage zone. The storage zone is where all the hot water is held and this is what is
converted into electricity. The hot salt water produced is similar in chemical
characteristics to brine.
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Diagram of the different layers of a solar pond
In a typical freshwater pond, when the sun penetrates the water the layers that are heated
up rise to the top of the pond and release the heat into the atmosphere. This is how a pond
maintains a constant temperate. The oxygen in warm water is greater than cold water.
This causes warm water to rise to the top of the water body and this heat is then released.
However, in a solar pond this process does not happen. Instead the water that is warmed is
unable to rise to the top due to the salt concentration. Therefore, the warm water stays at
the bottom of a pond and gets hotter and hotter with the more sunlight it receives. The
bottom layer of a solar pond can reach 178 degrees farenheit.
What allows a solar pond to be used as an energy source is that a pipe is placed at the
bottom of the pond and draws the warm/ hot water out of the pond by a pump and is
circulated through a piping system that utilizes the heat. It is similar to how radiant heat,
or solar hot water heaters use the warm water. Once the water has run through the pipe it
is deposited back into the pond in the storage zone so this water can be heated again. This
system is a close system so is quite efficient in terms of water retention. Typically this is
how a solar pond is used for heating purposes.
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Solar ponds can be used in all climates as Long as there is plenty of sun. Even when a pond
is frozen over, a salient gradient solar pond still produces hot water. Therefore, they can
be used all over the United States and the world.
Using a solar pond
Solar ponds have a number of uses. They are effective for heating facilities such as
industrial complexes, greenhouses, and agricultural building. When used for heat, it is
hard to regulate the temperature; therefore solar ponds are typically used in situations that
the heat temperature does not have to be regulated.
Solar ponds are also used to produce electricity. In this case, the hot water produced isused to spin a turbine which generates electricity.
5Some solar ponds rely on solar
powered pumps to push the water through the piping. This is a renewable and
environmentally friend system for electricity production.
A visual demonstration of how a solar pond is used to generate electricity
http://climatelab.org/Solar_Ponds#ref_5http://climatelab.org/Solar_Ponds#ref_5http://climatelab.org/Solar_Ponds#ref_57/30/2019 Karthik Solar Ponds Report
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Solar ponds can also be used for desalinization. Since the saltiest water separates into the
storage and insulation zone, the top layer of water is fresh, potable water. In fact, the
United States government used solar pond technology 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 beginning 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.
Solar ponds have the above mentioned uses and are valuable for a producing renewable
and environmentally friendly heat and electricity.
Solar ponds in India:
Table enlist the various solar ponds constructed in India. The first solar pond in India was
constructed at Central Salt and Marine Chemicals Research Institute (CSMCRI) in
Bhavnagar, Gujarat in 1971. Extensive studies on heat extraction pattern, effect of rainfall
on salinity gradient and overall variation of temperature profiles were conducted atIndian Institute of Science, Bangalore in 1984. Another pond of 400 m areas was
constructed at Mysore, Karnataka around 1990 with the purpose of meeting the hot water
requirements of a village. A 6000 m area solar pond was commissioned by Government of
India and executed by Gujarat Energy Development Agency (GEDA), Gujarat Dairy
Development Corporation Limited (GDDC) and Tata Energy Research Institute (TERI)
at Bhuj, Gujarat State. Its cost was Rs.3.167 million and could supply 80000 litres of hot
water at 70oC daily for washing, cleaning of aluminium cans, pasteurising and boiler
activity. The pond was successfully established by the end of 1990 and developed liner leak
due to high LCZ temperature of 99C during May 1991. The pond was then operated from
September, 1993 to February, 1995 and thereafter it was dysfunctional up to March 1996
due to lack of funds. The pond has again been commissioned from July 1996 and
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operational till date. A similar project has been sanctioned to Pondicherry Electricity
Board.
The Government of India has evinced keen interest in solar pond research by providingfinancial aid to the ponds constructed at Bhuj and Pondicherry. However, serious efforts
are required to make this technology economically viable.
Solar ponds
Location Year Area SaltusedBhavnagar,Gujarat 1971 1200 Bittern
Pondicherry 1980 100 Sodiumchloride
Bhavnagar,Gujarat 1980 1600 Bittern
IIT,Kharagpur 1983 16 Sodiumchloride
IISc,Bangalore 1984 240 Sodiumchloride
Bhuj,Gujarat 1990 6000 Bittern
Masur,Karnataka 1990 400 Sodiumchloride
ScopeofSolarPonds:
The solar ponds are widely considered as the low temperature energy storage devices
havinguseinwiderangeofprocessapplications.Thefollowingsectiondealswithscopeof
the applicationsofsolarpondheatadoptedinvariousprocesses.
Greenhouseheating
SokolovandArbel demonstrated theuseoffreshwatersolarpondforgreenhouse heating
purpose.Thepondcomprisedanexcavation intheearthwithlinerandathintopcover.
Thewaterwasusedasaheattransferringfluidduringperiodsofsolarradiation.Energy
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was deliveredtothegreenhousebypumpinghotwaterfromtheupper layerofthepond
througha heatexchanger.Thewaterreturnedafterheatextractiontothebottomofsolar
pond. In another study, Arbel and Sokolov studied different collector materials havingdifferentmaterial properties andconcludedthattheuseofappropriatematerialimproves
the solar pond performance. Riva studied a 20 m2
solar pond for two years before
constructingabiggerpond of140-160m2
area.Theenergyefficiencywasfoundtobe10
to 20percentduringpreliminary testing.The energywas intended for airheating in a
dryerof40-50m2
area.
Processheatindairyplants
ThehotwaterrequirementsforsterilisationandpasteurisationinadairyplantatBhujof
KutchdistrictofGujaratStateisbeingmetfromasolarpondof6000m2
areas.Thehot
watertemperaturewasintherangeof84to95
o
Cduringthepond operationperiod.
Desalination:
Desalinationinvolvestheprocessofobtainingfreshwaterfordrinkingandirrigation from
eitherbrackishorsalinewateraftersuitabletreatment. Thesolarenergyhas beenutilized
fordistillationofbrackishorsalinewaterforaverylongtime.Asolarpondmulti effect
distillation (SPMED) system as shown inFig comprises a set of evaporative condensers
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and heat exchanger extracting heat from the solar pond. The fresh water is produced
through repetitive cycles of evaporation and condensation, using low temperature heat
fromthesolar ponds.
Schematicdiagramofsolarponddesalination
1. Multistageflashpan,2. Freshwater3.Brackishwater 4.HotBrine 5.Heatexchanger
6.Coldbrine 7.Solarpond 8.Diffuser
Tabor(1975)showedthatapondof1/3km2areacouldoperateamulti-effectdistillation
unit,withanannualmeanoutputof4000m3
/dayatarateofUS$0.67/m3
.Hefurther
remarked that a solar pond desalination plant produces about 5 times the quantity
produced from simple tray typesolarstill.A20000m2
solarpond inItalywasused for
desalinationofseawaterto produce120toffreshwater/day.
Powerproduction
Some of themajor solarpondpowerplants are listed inTable 2.3. In theseplants, the
solution from the lower convective zone is pumped to a heat exchanger that acts as
evaporator for an organic Rankine cycle. Trieb et al made a comparative analysis of
different solar electricity generation options and found that solar pond produces
electricityatacostof0.254 GermanMarks (DM)/kWhasagainst1.198GermanMarks
(DM)/kWhforphotovoltaiccells.
Prominentsolarpondsusedforelectricitypowergeneration
Name/site Power,kW Pondarea,m2 Operationperiod
EinBoqek,Israel 150 6250 1979-1986
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BeithHaArava,Israel5000 250000 1984-1989
AliceSprings,15 1600 1985-1989
ElPaso,UnitedStates70(Electricity)
3350 1986-tilldate
Hotwaterapplicationsinagriculture:
Manyoftheagriculturaloperationsinvolvehotwaterapplicationfordifferentpurposes.
Some of them includepaddy soaking inparboiling, sugarcane sett treatment, vegetable
blanching, washing of cans in dairy industry and domestic hot water consumption.
Traditionally, parboiling process involves soaking of rough rice in water at ambient
temperatureinmasonrytanksfor3daysandsteamingofdrainedpaddy.Themethodwas
later improvedtosoakthepaddyinhotwaterataround70o
Cforfewhoursdepending
upon the type of parboiling method. This method could eliminate unwanted odours
associatedwithtraditional methodandreducethesoakingtimefromafewdaystoafew
hours.
Heat therapy of sugarcanesetsbeforeplanting is desirable to raise the crop free
from seedpiecediseasesandcertaininsectpests.Conventionallythesetsaretreatedinhot
waterata temperatureof50o
Cfor2hoursandat54o
Cfor4hoursinhumidhotair.
It is clear that the solarpondshave agreat scope in agricultural applicationswith low
temperature requirements. It is equally important to understand the practical aspects
involved withtheoperationandmaintenanceofsolarpondssothattherealsituationsolar
pondscanbe properlymanaged.
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ManagementofSolarPonds:
Solarpondsneedspecialtechniquesofoperationandregularmaintenance.Followingare
afewpracticalaspectsofoperatingasolarpond.
Operational
The operational aspects essentially involve the methods of filling a solar pond in the
beginningandmanagingitforcontinuoususe.
Fillingthepond
Thefillingofpondassumessignificanceasitinvolvestheestablishmentofdensity gradient
alongthesolarponddepth.Zangrando developed atechniquethatwaswidely adaptedin
other ponds. This technique involves the filling of solar pond initially with high saline
solution to a depth equal to depth of lower convective zone + half the depth of no
convective zone.Later thedilution ismade starting from the interfaceof the two zones.
Thismethodof fillingthepondiswidelypractised.
Maintenanceofbrinetransparency
Algal growth mainly affects brine transparency. Chlorination provides the solution to
control algal growth.Scientists reported similar measure for ammonium sulphate solar
pond.
Topzoneflushing
Due to salt diffusion from lower layers to the top layers, the flushing of top zone is
necessary to maintain the salt gradient stability in the solar pond. Top zone flushinginvolves the processofremovalof topsaline layerand injecting freshwater to thepond
surface.Shermanand Imberger suggestedfrom theirsimulationstudythat nowashing
was required if the salinity in the top zone was less than 3 per cent and therewas no
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tangiblebenefittomaintain the topzoneat less than2percentsalinity. Otherworkson
topzoneflushinginvolvedtopzone flushingatregularintervals.
Heatextraction
Anexternaltubularheatexchangerforheatextractionfromthesolar pond was also used.
However the0.1kWpumpused topump thesaltsolution from lowerconvectivezone to
theheatexchangerdevelopedshaftsealfailureafterafewhoursofoperation.Solarpond
experiments were conductedwith threeheat exchangersviz.,a titaniumheat exchanger
(external),acopperheatexchanger(immersed),andaplasticheatexchanger(immersed).
Outof these the copperheat exchangerwas found tobemost reliable.Hull etal (1985)
demonstrated polypropyleneheatexchangerthoughitseffectiveness is lessthanthatofa
copperheat exchanger.Shellandtubeheatexchanger with aheat transferareaof36.1m2
foraheatdutyof341000kcal-hr-1
was used forhotwater supply to thedairyplant in
Bhuj.Theheatexchangercouldoperateforalmosttwoyearswithoutanymajorproblem.
Further,thesteelpipesusedforpipingoftheheatexchangercorrodedseverelyinayear
and werereplacedbypolypropylenepipes.Useofsubmergedheatexchangerswasruled
outforlarge ponds.
Problemsfrequentlyencountered:
Solarpondsbeingrelativelyanewdevelopmentstillencountera fewpractical problems
during its operation.The following sectiondealswith some of the frequently occurring
problems experiencedinthesolarpondoperation.
Linerleak
Linerleakisoneofthemostcommonproblemsreportedinthecontextofsolarpondsin
India. Srinivasan (1992) reported salt leak due to liner failure in a solar pond
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constructed at Masur, Karnataka. The leak forced to abandon the solar pond. The
damage to the liners occurred in the regions where the temperature exceeded 75oC.
Though the liner leak in thispond is attributed torecycledplasticsused in liner, people
felt that the leakcould have stilloccurreddue to the lowoven lifeofLDPEmaterial.A
similarsaltleakwasreportedat Bhuj,GujaratonaccountoflinerfailurewhentheLCZ
temperature increased to99o
C.Solar ponds constructedatPondicherryandHublialso
facedtheproblemoflinerleakforcingthem to beabandoned.
Corrosionofmaterial
Scientists reported the possibility of rapid corrosion of copper metal by ammonium
sulphate.Intheabsenceofchlorine,ammoniahasarelatively lowcorrosiononsteeland
aluminium.Some of themreportedoffrequentshutdownofthetitaniumheatexchanger
becauseofseal failure in thebrinepump.Studies were done on thecorrosionofcopper
tubes immersed in storagezone.About1% ofdecreaseofmassofcopper inayearwas
estimated.
Conclusion
Overall, solar ponds are an effective source of renewable and environmentally sustainable heat
and energy. However, widespread adaptation of this technology has not been successful do to
the limited uses of solar ponds. The main constraint for solar ponds is the amount of land they
require. This issue makes developing solar ponds in many part of the world not cost
effective. On the upside, solar ponds can complement commercial electricity production nicely
as showcased in Israel. As the use of oil and coal for electrical and heat production faces more
scrutiny, solar ponds will be a nice addition to a diverse portfolio of renewable and
environmentally friendly energy sources.
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REFRENCES1.http://edugreen.teri.res.in/explore/renew/pond.htm
2.http://edugreen.teri.res.in/explore/renew/solar.html 3.http://www.eere.energy.gov/consumerinfo/factsheets/aa8.html 4.http://www.rmit.edu.au/browse/Our%20Organisation%2FFaculties%2FEngineer
ing%2FSchools%20and%20Departments%2FAerospace,%20Mechanical%20and
%20Manufacturing%20Engineering%2FResearch%20and%20Development%2FS
olar%20Pond/
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