What Is a Solar Pond?
Solar Ponds are solar thermal energy systems that collect and store solar energy, thereby providing a sustainable source of heat and power.
These are typically sizable human-made bodies of water that use the sun's heat as a stable temperature source in areas where traditional cooling technologies cannot be implemented.
Solar ponds differ from other solar thermal energy systems as they store the collected heat instead of transferring it through fluids or devices. Solar ponds may use any number of different fluid heating and cooling mechanisms.
History of Solar Ponds
Around the last century, the solar pond was discovered as a natural phenomenon in the Medve Lake in Transylvania, Hungary, as Kalecsinsky (1902) noted.
In 1963, a pond feasibility study was conducted by Tabor to check power generation. He achieved a temperature for small ponds up to 90°C, however, there were technical problems encountered for larger ponds.
Consequently, Rabl and Nielsen studied the solar pond as a solution to space heating of residences in Ohio and similar areas in 1975.
Rabl calculated that a pond equal in volume to a well-insulated three-bedroom home could meet all of the winter space heat requirements of that home. Nielsen developed a unique salt gradient establishment procedure using a small pool and laboratory models.
Rabl and Nielsen's previous work, a full-scale experimental solar pond designed to meet all winter heat requirements, was built next to the Department of Agricultural Engineering Greenhouse at the Ohio Agricultural Research and Development Center (OARDC).
How It Works
Solar ponds work based on a fundamental principle. When the sun's rays heat the water in an ordinary pond, the heated water becomes lighter and rises upward, losing its heat in the atmosphere. This means that the pond water remains at its atmospheric temperature.
However, the Solar pond prevents this occurrence by dissolving salt in the bottom layer of the pond, which makes the water heavier and unable to rise.
Simply, solar ponds work by using the sun to collect solar radiation and absorb it as heat, which is stored in the top layers of the pond. It is used to provide temperature stability, allowing processes that rely on high temperatures to run continuously without interruption.
Solar Pond Zones
Solar ponds typically have three layers: top, bottom, and gradient.
Top Zone
The top layer is the sun-activated heating portion of the Solar Pond; it consists of a surface liquid layer containing salts that absorb solar energy.
The Solar Pond's top layer also has a deep bottom, which serves as insulation preventing Solar Pond heat from escaping.
Bottom Zone
The Solar Pond's bottom zone is located under the top zone and absorbs Solar Pond heat.
It uses a high melting point substance that can store collected Solar Pond heat until it is ready for use. Solar Pond bottom zones can be almost any material, such as concrete, fiberglass, or plastic.
Gradient Zone
The Solar Pond's gradient zone is located between the top and bottom zones and absorbs energy from the top zone and transports it to the bottom zone.
This portion of Solar Pond technology is a thermal diode; it absorbs heat from the top zone and sends it to the Solar Pond's bottom zone.
Types of Solar Ponds
With the various types of Solar Ponds, there are two classifications for each type, convecting and non-convecting Solar Ponds.
Convecting Solar Pond
In a convecting Solar Pond, the gradient zone is separate from the top and bottom zones, allowing heat to transfer from top to bottom without assistance from the gradient zone.
Solar Ponds of this type are very inefficient, as Solar Pond heat must be transferred from the top zone to Solar Pond's bottom zone before being used. Covering the pond's surface prevents heat loss since no evaporation takes place.
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Shallow Solar Ponds - also known as Solar Dip Ponds, both the Solar Pond top and Solar Pond bottom zones are shallow and separated by the gradient zone. Solar ponds of this type have a top zone that only covers the gradient zone. Solar Dip Ponds quickly reach maximum temperature since heat is not transferred through a thermal diode.
Non-Convecting
A Solar Pond of this type has both Solar Pond top and Solar Pond bottom zones that are shallow, yet the Solar Pond gradient zone is deep.
This reduces heat loss by preventing the transfer of heat from one place to another by the movement of fluids with a concentration of 20–30 percent salt to the bottom level.
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Partitioned Solar Ponds - have gradient zones that are divided, separating the top and bottom zones. Partitioning allows for more gradual temperature changes within the pond, making Solar ponds of this type less likely to fluctuate in temperature.
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Viscosity Stability Solar Ponds - for this type, the gradient zone is made up of a mixture of salt and water with high viscosity and density. This Solar Pond's gradient zone provides Solar Pond heat for more extended periods by preventing Solar Pond heat from escaping from the bottom and top zones.
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Membrane Stratified Solar Ponds - have two Solar Pond layers that create a gradient zone where the top and bottom zones are separated by a membrane with extremely thin pores, allowing for efficient heat transfer and retention over extended periods. They transfer heat through convection currents created by Solar Pond water flowing through the membrane.
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Membrane Viscosity Stabilized Solar Ponds - have gradient zones stabilized using a chemical compound that prevents heat from transporting from the bottom to the top. Solar ponds of this type are not as efficient as other types, but these Solar Ponds often have a lifespan of 30 – 50 years due to their high durability.
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Salt Gradient Solar Ponds (SGSP) - have gradient zones made up of a mixture of water and salt. Solar pond heat transfers through the gradient zone due to convection currents created by Solar Pond's salinity. These Solar Ponds do not require dividers, as the heat can still be prevented.
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Saturated Solar Ponds- have top and zones that are shallow and separated by the gradient zone. Solar ponds of this type hold heat over extended periods due to the high viscosities of the water.
Applications
Solar Ponds have several applications.
Process Heat
Studies have indicated unprecedented scope for process heat applications when a large amount of hot water is needed.
Hot air for industrial uses such as dryers in agricultural produce, timber, fish chemicals, and space heating.
Desalination
The scarcity of water is one of the challenges of the world. Solar ponds can provide energy derived from a solar pond. It is also cost-effective.
Desalination costs in these places work out to be between 0.5 US$ and 1.5 US$/m3, which compares favorably with the current costs incurred in the reverse osmosis or electrodialysis/desalination process.
Refrigeration
Refrigeration applications have tremendous scope in a tropical country like India.
Perishable products like agricultural produce and drugs like vaccines can be preserved for long periods in cold storage using solar pond technology in conjunction with an ammonia-based absorption refrigeration system.
Benefits and Drawbacks of Solar Ponds
The following are the benefits of using a solar pond.
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It can be constructed at a minimal cost.
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The built-in thermal energy storage allows it to be used all year round, regardless of season and weather.
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An alternative to fossil fuel technologies.
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Energy derived from a solar pond is more cost-effective.
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It is safe for the environment since it does not emit harmful toxins and pollutants while conserving traditional energy resources.
However, using solar ponds also has drawbacks.
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Since it requires a large piece of land to be constructed, it may be unsuitable for populated areas.
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It requires a high level of solar energy input.
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It needs to be supplied with an extensive reservoir of water.
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Constant maintenance is required even if any qualified engineer can construct it.
Final Thoughts
A solar pond is an artificial pond that uses solar energy to provide heating, cooling, or desalination for industry, water treatment, or agriculture. It is an efficient way of harvesting solar energy.
Solar ponds are generally more cost-effective than flat-plate solar water-heating systems commonly used in homes. They are considered an excellent alternative to fossil fuel technologies in rural areas.
Although any qualified engineer can build it, it requires constant maintenance and requires a large area of land and a lot of saltwater to maintain.
FAQs
1. What Is a Solar Pond?
A Solar pond is an artificial Solar Pond that creates usable energy through solar energy. Solar Ponds can provide heating, cooling, or desalination for industry, water treatment, or agriculture.
2. How do Solar Ponds Work?
Solar Ponds work by Solar Pond's convection currents created due to Solar Pond's salinity. These Solar Ponds do not require dividers, as the heat can still be prevented.
3. What are the Different Types of Solar Ponds?
There are two types of Solar Ponds: convecting and non-convecting solar ponds.
4. What are Solar Ponds Used for?
Solar Ponds have several applications, such as providing process heat, desalination, or refrigeration.
5. Are Solar Ponds Beneficial?
Solar Ponds can be constructed at a minimal cost and having the built-in thermal energy storage, allows it to be used all year round. They are considered an excellent alternative to fossil fuel technologies in rural areas.
6. Are There Any Drawbacks of Solar Ponds?
Solar Ponds have a few drawbacks like requiring a large piece of land, needing a high level of solar input, and requiring constant maintenance.
