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 know 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 a brine.¹

Diagram of the different layers of a solar pond

Source: unknown. Author: unknown. Permission: http://matse1.mse.uiuc.edu/energy/prin.html.

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. ²  

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.³

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.

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.⁴

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 is used to spin a turbine which generates electricity.⁵   Some 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

Source: www.powerfromthesun.net/Chapter6/Chapter6.htm. Author: unknown. Permission: www.powerfromthesun.net/Chapter6/Chapter6.htm.

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. (http://www.green-trust.org/solarpond.htm)

Solar ponds have the above mentioned uses and are valuable for a producing renewable and environmentally friendly heat and electricity.

Benefits of a solar pond

An obvious benefit to a solar pond is it produces heat or electricity with little to no carbon emissions.  The emission depends on the type of pump used to push water through a turbine or piping.  Some solar ponds use gravity to their advantage when power a turbine.  For desalinization, no energy is required to produce potable water, instead the clean water is a result of the separation of water according to salt concentration.  

According to NASA research, solar ponds are not only an effective way for producing solar heat for industrial processes, but also quite cost effective:

Solar ponds offer perhaps the simplest technique for conversion of solar energy to thermal energy, which can be used for industrial process heat. It is unique in its capability in acting both as collector and storage. Further, the cost of solar pond per unit area is less than any active collectors available today. Combination of these economic and technical factors make solar ponds attractive as a fuel saver in industrial process heat (IPH) applications.⁶

Another benefit is that solar ponds are relatively low cost.⁷   Aside from the labor costs of constructing a pond there are few reoccurring costs to maintain pond.  "The payback period depends on the transparency of the pond, and for the conditions considered in the study, it varies between 3 and 4.5 years."⁸   This payback was determined using a solar pond for heating a catering company in Singapore; the water temperature used for heating was approximately 60 degrees Celsius.

Barriers and limitations

The primary barriers that limits the wide spread use of solar ponds is the fact that solar ponds require a large am mount of land.  To make solar ponds cost effective they need to be built on cheap land.  This makes the use of solar ponds in cities and residential areas quite limited because it is simply not cost effective to build these ponds.  

Heat produced by solar ponds is not easily controlled and therefore using the heat output is constrained to agricultural and industrial situations.  In these settings the precise temperate of the heat is not of much importance.  However, if solar ponds were used for home or office heat a major drawback is the fact that a thermostat can not be used to set the temperature.

Also, solar ponds are not the most efficient way of capturing the sun's heat and turning it into energy.  Typically, only 30% of the sun's heat penetrates deeper than two feet in water.  Although solar ponds do produce heat, the heat produced is not nearly as efficient as other forms of heat or electricity production.

Once a solar pond is built, the productivity of a solar pond cannot be increased.  Therefore, for commercial use, as a company expands and demands more heat and energy, a solar pond reaches a threshold and can no longer produce the quantity of heat or energy demanded.

Also, since solar ponds lay on a horizontal axis, they only receive direct sunlight for a limited number of hours daily, and this number changes by season. However, studies have proved that adding reflectors to solar ponds can dramatically increase their productivity. "It is found that the use of reflector having area only 5% of the total area of the pond can increase the radiation gain by more than 10%. This can further be enhanced by increasing the area of the reflector, considering the economic feasibility."⁹

Current uses of solar ponds

Solar ponds have the potential to produce large am mounts of energy at relatively cheap prices. This is beneficial in the use of municipal energy supply. During the 1970s when oil prices were increasing with no sign of a decrease insight, the Israel government strategically built solar ponds to lessen their dependence on oil. As a result, the country has many solar ponds and utilizes them to produce electricity throughout the country. This solar pond (pictured above) in Israel is capable of producing up to 150 kilowatts of electricity and is able to produce at a price comparable to traditional forms of electricity production.

In the United States solar ponds are not as widely used due to the relatively high cost of land.  The only place feasible to place solar ponds are barren areas such as deserts.  Logistically, placing solar ponds in such areas is not practical because industries and agricultural businesses are not usually located in the desert.  To connect a solar pond in the desert to a residential electricity grid would cost far too much money. 

Studies are being funded by the United States government to research solar ponds for desalinization purposes.  However, the cost of potable water from this process is extremely high and therefore not cost effective.

Summary

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.

Footnotes

1. http://matse1.mse.uiuc.edu/energy/8.gif

2. http://www.springerlink.com/content/...ext.pdf?page=1

3. http://www.motherearthnews.com/uploa...184-01-im2.jpg

4. http://www.green-trust.org/solarpond.htm

5. http://www.solarponds.com/

6. http://adsabs.harvard.edu/abs/1979siph.confR....B

7. http://www.financialexpress.com/old/.../fco13049.html

8. http://www.informaworld.com/smpp/con...7975545&db=all

9. http://papers.ssrn.com/sol3/papers.c...act_id=1413673