Solar Ponds

SOLAR PONDS By : Sunil Kumar CH - 6268

CONTENTS       

Introduction Description of Solar Ponds Methods of maintaining layered structure Working of solar ponds General Construction features Applications Hindrances and Remedies

NORMAL PONDS 

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Normal ponds receive sunlight a part of which is reflected at the surface, a part is is absorbed and the remaining is transmitted to the bottom Due to this the lower part gets heated up and the density decreases as a result of which it rises up and convection currents are set up. As a result, the heated water reaches top layer and looses its heat by convection and evaporation.

SOLAR PONDS 

They are large shallow bodies of water that are arranged so that the temperature gradient are reversed from the normal.

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This allows the use for collection and storage of solar energy which may ,under ideal conditions, be delivered at temperature 40-50 `C above normal.

Zones of Solar Ponds A salt-gradient non-convecting solar pond consists of three zones: 1)

UCZ ( Upper Convecting Zone)

: top layer

2)

NCZ ( Non Convecting Zone) : middle layer

3)

LCZ (Lower Convecting Zone)

: bottom layer

Schematic of Solar Ponds

Upper Convective Zone 

This is a zone, typically .3 m thick, of almost low salinity which is almost close to ambient temperature.

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UCZ is the result of evaporation, wind induced mixing, and surface flushing.

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Usually this layer is kept as thin as possible by use of wave suppressing mesh or by placing wind breaks near the ponds.

NON CONVECTING ZONE 

In this zone both salanity and temperature increases with depth.

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The vertical salt gradient in the NCZ inhibits convection and thus gives the insulation effect.

LOWER CONVECTING ZONE 

This is a zone of almost constant, relatively high salinity ( typically 20 % by weight) at high temperature.

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Heat is stored in the LCZ, which should be sized to supply energy continuously throughout the year.

DIFFERENT METHODS OF MAINTAINING LAYERED STRUCTURE 1)

2)

3)

Maintaining Density Gradient by salt water Use of horizontal and vertical membranes. Polymer gel layers.

TYPICALL SALANITY GRADIENT

Working of Solar Ponds Maintenance of salt-gradient •

The concentration gradient that exists in pond lead to diffusion from higher to lower concentration i.e. from bottom to top.

•

Therefore, to maintain stability salt must be added to lower layer and remove from upper layer.

•

Now as the sunlight falls on the pond, the part which is transmitted to the bottom heats the lower layer and as a result inverse temperature gradients are set up.

INVERSE TEMPERATURE GRADIENT 

It is the temperature gradients are reversed from normal i.e. hottest zone is at the bottom of the pond and coldest zone is at the top.

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They are maintained to eliminate convection currents that set due to temperature difference during normal temperature gradient.

TYPICAL TEMPERATURE GRADIENTS

GENERAL CONSTRUCTION FEATURE  

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They are 1-3 m deep. Constructed on level ground by combination of excavation and embankments. Membrane liners are used to make the basin leek proof. Membranes are covered with clay to protect them and improve their durability.

Continued.. 

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Since solar ponds are horizontal collectors sites should be at low to moderate northern latitude and southern latitude i.e. -40 to +40 degree latitude. Evaluation of geological salt character as underline earth should be free from stresses, strains and fissures. Thermal conductivity of soil increases with moisture, so water table of site must be low.

APPLICATIONS    

Electric power generation Desalination process Domestic hot water production For space heating & cooling of buildings

ELECTRIC POWER GENERATION

DESALINATION

MAJOR SALT – GRADIENT SOLAR PONDS (in India) Location

Area (m2)

Depth (m)

Main Objectives

Achievements

Bhavnagar (India)

1210

1.2

Operating experience Max. Temp. 800C in and behaviour of 1972. Worked for two materials years.

Bhavnagar (India)

1600

2.3

Operating experience Getting heated, and applications for designed to supply 20 power production. KW. Rankine cycle turbines.

Pondicherry (India)

100

2.0

Experience, material Built in 1980. behaviour, monitoring & modeling.

Bhuj (India)

6000

3.0

Operating experience, Supplying process heat material behaviour to a dairy and possible applications

PERFORMANCE COMPARISON A= Curve for flat plate collector B= Solar pond with 1m depth of LCZ from surface C= Solar pond with 2m depth of LCZ from surface

Cont… 1)

2)

3)

Difference in intercept represents additional radiation absorption by the additional meter of insulation in the UCZ. Slope of C represents additional meter of information over LCZ. At high operating point the performance of solar ponds are better than flat plate collectors.

HINDERANCES 

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Cleanliness of pond as contaminants can reduce transmittance. Increase in thickness of UCZ ( Upper convective zone) due to surface waves and evaporation. Algae growth. Horizontal temperature gradient caused by salt solution removal and addition.

REMEDIES 

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Contaminants can be removed by filtration and the effect of contaminants can also be minimized by constructing larger solar ponds. Floating nets and wind barriers can reduce surface waves and mixing of UCZ. Algae growth can be minimized by adding 1.5 mg CuSO4 per liter of water. Horizontal temperature gradient can be minimized by injecting and removing salt solutions very slowly.

REFRENCES    

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Tsilingiris 1994 Angeli et al.2006 Nielsen 1976 Solar energy engineering – Soteris Kalogirou Solar engineering of thermal processes – Duffie & Beckman

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