16 Solar Pond

Dr. Dhanya M.S M.S

1. Introduction

The solar pond collects and stores the solar energy. Due to the prevention of convection currents the solar ponds store the heat energy from the sun in more effective manner than that of a pond. The solar ponds are considered cost effective over solar collectors in solar energy collection and storage.

2. What is a solar pond?

A natural and artificial body of water for collecting and absorbing radiation energy and storing it as heat is a solar pond.

A solar pond of shallow depth with a radiation absorbing (e.g., black plastic) bottom is an example. The heat loss to the ground is reduced with the help of a bed of insulating materials. The solar radiation is entered through a cover made of transparent fiber glass placed over the pond inhibiting radiation and convection losses. The water in the shallow pond gains the temperature uniformly but only to a fewer degrees. The energy is stored in low grade (60 to 100°C) thermal form.

It is defined as artificially constructed pond in which significant temperature rises are caused to occur in the lower regions by preventing convection. These are also known as ‘shallow solar pond’.

3. Types of solar ponds:

The main categories of solar ponds are:

Non-convecting ponds and Convecting ponds

3.1 Non- convecting ponds reduce heat loss by preventing convection from occurring within the pond.

There are two main types of non-convecting ponds.

Salt gradient ponds and A salt gradient pond has three distinct layers of brine (a mixture of salt and water) of varying concentrations

Membrane ponds.

3.1.1 Salt gradient ponds: The salt gradient solar pond is a body of saline water in which the concentration increases with depth, from a very low value at the surface to near saturation at the bottom. The stratification of concentration is achieved by dissolving a salt at different salinity levels.

The salt gradient solar pond is a body of saline water in which the concentration increases with depth, from a very low value at the surface to near saturation at the bottom. The stratification of concentration is achieved by dissolving a salt at different salinity levels.

3.1.2 Membrane ponds inhibit convection by physically separating the layers with thin transparent membranes.

3.2 Convecting ponds: This type of pond prevents the evaporative losses with a cover over the surface of the pond. In this shallow solar pond, pure water is enclosed in a large bag that allows convection but hinders evaporation. The bag has a blackened bottom, has foam insulation below, and two types of glazing (sheets of plastic or glass) on top. The sun heats the water in the bag during the day. At night the hot water is pumped into a large heat storage tank to minimize heat loss. Excessive heat loss when pumping the hot water to the storage tank has limited the development of shallow solar ponds.

4. Characteristics of a salt used in Salt gradient ponds

The salt used in a solar pond for creating density gradient should have the following characteristics :

High solubility

No variation in solubility with temperature.

Its solution must be adequately transparent to solar radiation.

It must be environmentally benign, safe to handle the ground water.

It must be available in abundance near site so that its total delivered cost is low, and It must be inexpensive.

Stability of non-convective layer in the presence of a temperature gradient can be maintained by creating, a sufficiently steep salt concentration gradient. In order to generate the initial salt concentration gradient required to prevent convection, the usual practice is to fill the pond with several layers of salt solution with the successive layers changing stepwise in concentration from near saturation at the bottom to fresh water at the top.

For a typical pond of 1 metre deep, one might use six or eight layers. Less layers can be floated successively on top of the existing ones, or successively, more dense layers can be put in at the pond bottom flowing in as under currents and gradually lifting the whole pond. Because of the turbulent mixing and molecular diffusion, which occur during filling, the staircase type concentration profile evolves into a smooth nearly uniform concentration gradient. Typical value of salt concentration at the top surface is 20kg/cubic metre, increasing to 300 and 260 kg/cubic metre, for magnesium chloride and sodium chloride respectively, at the bottom. It is necessary to add periodically concentrated solutions at the bottom, and wash the surface with fresh water to maintain the concentration gradient in the presence of diffusion effects.

5. Zones of Non-convective solar pond

The solar pond has plastic lining at the bottom.

Plastic liner: Butyl rubber, black polyethylene and hypalon reinforced with nylon mesh.

The salt gradient solar pond uses salts: magnesium chloride, sodium chloride or sodium nitrate. The commonly used salt is sodium chloride.

The stratification of solar pond is

Surface convective zone or upper convective zone, UCZ (0.3-0.5 m), salinity < 5%. Non-convective zone, NCZ 1 to 1.5 m, salinity increases with depth.

Storage zone or lower convective zone, LCZ 1.5 to 2 m, salinity of 20%.

5.1 The surface convective zone usually has a small thickness, around 10 to 20 cm with uniform concentration (<5%) and a uniform temperature near to the ambient air temperature.

5.2 The non-convective zone is much thicker and occupies more than half the depth of the pond. In this zone both salt concentration and temperature increase with depth. This zone acts like insulator and prevents the heat loss in upward direction.

5.3 The storage zone is at the bottom 1.5-2 m depth with concentration and temperature is nearly constant. This is the main heat collection and storage point. The storage of heat increases with the depth.

6. Principle of Non-convective solar pond:

6.1 Collection and storage: The collection area for the solar energy in solar pond with a plastic liner at bottom is the water present (1 – 2 meters deep) in it. The salt gradient non-convective ponds maintain the density gradient with the dissolved salts. The portion of the incident solar radiation reaching pond is absorbed by the water and the rest penetrated absorbed by black bottom. In the fresh water pond, the expansion and rising to surface occurs after heating up of bottom layers. The convective mixing and heat loss create very low increase in temperature in the pond.

This can be excluded by salt concentration gradient in the pond that overcomes convection and thereby thermal expansion in lower layers. The salt concentration ranges from 20 – 30 per cent at the bottom to 5 percent or even less to zero at the top. This creates a density gradient with heavier layer at bottom and lighter layer at the top. The saline water with low conductivity acts as insulator and so develop high temperature in bottom layer.

The solar ponds trap 10 to 20 per cent of the solar energy hitting its surface. Hence, each square metre of pond surface area can supply one half to two giga joules of thermal energy per year at temperatures from 40°C to 80°C. The highest temperature ever recorded in a solar pond is 108°C, set in the summer of 1980 at the University of New Mexico, in Albuquerque. A flat plate collector of the same area would be twice as efficient but cost is ten times as much.

6.2 Extraction of Thermal Energy. Energy is stored in low grade thermal form of the lower convective zone. Convection in the zone is due to the process of heat extraction, accomplished by hot brine withdrawal and cool brine return. It is not practical to cover the bottom of the pond with an array of pipes acting as a heat exchanger due to two reasons.

(i) The cost will increase sharply in the case of large ponds of the size of about a square km; and

(ii) Unless there is convection to around the pipes heat transfer from the stationary hot brine to the fluid in the pipes will be very poor.

The thermal energy stored in the lower layers of the water need to be extracted. Hot water can be extracted from a solar pond without disturbing the concentration gradient. For this, water outlet at the same height as the water inlet was installed. Hot brine can be, withdrawn and cool brine returned in a laminar flow pattern because of the presence of density gradient. For small or model ponds, heat exchangers consisting of pipes can be placed in the hot lower layers, but this entails not only the initial installation cost but the continued pumping losses associated with the heat transfer fluid.

Thermal energy from solar pond is used to drive a Rankine cycle heat engine. Hot water from the bottom level of the pond is pumped to the evaporator where the organic working fluid to vaporized. The vapor flows under high pressure to the turbine and thereby expanding through the turbine wheel and the electric generator linked to it. The vapour then travels to the condenser where cold water from the cooling tower condenses the vapour back it to a liquid. The liquid is pumped back to the evaporator where the cycle is repeated. A 2000 sq. m solar point equipped with a 20 kW engine has been constructed in Australia.

7. Advantages of solar ponds

The saline gradient non convective solar pond is not affected by

– diurnal changes

– seasonal changes

– cloud cover

– ice cover

8. Factors influencing heat trap in solar ponds

– location,

– water clarity and

– temperature

9. Applications of Solar Ponds.

The solar ponds are used in thermal applications (heating an dcooling), desalination, salt extraction and power generation .Some of the main applications of a solar pond are discussed below:

9.1 Heating and Cooling of Buildings. The solar ponds are capable of storing large amount of heat energy in its lower convective zone. The operation of solar ponds is simple and is comparatively less expensive. Some researchers reported that solar pond technology offers heat at a cost of about $0.010 per kW-h. Researchers found solar pond house heating systems effective if that pond is used to supply thermal energy to many buildings.

9.2 Power generation

The electricity generation from the heat trapped in solar pond was either by driving a thermo-electric device or an organic Rankine cycle engine. In this process a turbine is powered by evaporating an organic fluid with a low boiling point. This power generation is very useful in areas with levelled land, receiving enough solar radiation with soil factors suitable for solar pond construction and operation. The solar pond power plant (SPPP) uses halo-carbons (like Freons) or hydrocarbons (such as propane) as the fluids. Tundee et al. (2013) reported significant potential for electric power generation from small solar ponds through a simple and passive device incorporating thermosyphons and thermoelectric cells. They reported electricity production of 36.25 mV from solar pond with water as working fluid and 234.25 mV when R134a is used as a working fluid in salinity gradient solar pond with a surface area of 7 m2 and a depth of 1.3 m at Rajamangala University of Technology Isan Khon Kaen Campus,Thailand.

9.3 Industrial process heat (IPH): The thermal energy required for different industrial process is directly utilized from the solar ponds. This facility was used in drying of crop products, vegetables, paper industry, etc. It is used mainly as a fuel saver in the IPH applications.

9.4 Desalination

Multi-flash desalination along with a solar pond is useful in the purification of saline water and making it potable for drinking purposes. The distilled water can be produced from multi-flash desalination plant operating below 1000C coupled with solar pond. It has been estimated that about700 m3/day-distilled water can be obtained from a pond of 0.31 km2 area with a multi-effort distillation unit. The water produced by this technology is five folds than conventional solar still.

9.5 Salt production

Solar ponds are found helpful in the recovery of salts from brine water.

9.6 Heating the animal houses

10. Solar ponds in India

The artificial solar pond was started in Israel in 1958. Central Salt and Marine Chemical Research Institute at Bhavnagar under the leadership of Dr. G.C. Jain (1973) has designed a solar pond for use in production of salt. Bhuj solar pond at Gujarat constructed in collaboration TERI, the Gujarat Energy Development Agency, and the GDDC (Gujarat Dairy Development Corporation Ltd) was the first solar pond in India to have connected itself to an industrial process. It was supplying heat to the Kutch Dairy from an area of 6000 m2. The construction was started in 1987 and completed in 1993. It used locally mined clay and plastics were used as lining to reduce the cost. The Bhuj solar pond successfully supplied 80,000 litres of hot water daily to the plant. But today it is not operational due to financial supports and government policies. The El Paso Solar Pond, University of Texas and Beit HaArava pond, Israel are examples of other famous solar ponds in the world.

10. Limitations

The solar-to-electricity conversion is not sufficient. The conversion is generally less than 2%. Shallow solar pond is capable to heat up only a few degrees of temperature.

The variations in temperature also occur in deeper ponds.

The shortcomings of the conventional gradient solar pond has been overcome by Solar gel pond :The non- convective salt gradient layer is replaced by a transparent polymer gel layer, which floats on a NaCl solution (Sea water) used in the storage zone and Saltless solar pond : It is a convective type

Conclusion

The solar pond is an eco-friendly method of collection and storage option for solar energy in rural areas that need to be improved with techniques which are more efficient.