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A Quick Dip in a Solar Pond

Image of Bhuj solar pond provided by ResearchGate

A rather interesting concept of harnessing energy from the sun is through the use of solar ponds. This electricity generation method is one that requires two perhaps obvious main ingredients, being the sun and an isolated body of water, and uses their properties to convert heat generated from absorbed solar waves into electricity.

This concept is not as common as photovoltaic cells due to its lack of practicality when the two are compared, especially when it comes to space usage. However, it is still a promising topic that has become quite popular in many countries around the world.

1. How Solar Ponds Work

Let’s begin by discussing how these systems actually work. The first step is to have a shallow area where water can be added to. The reason the area must be shallow is to ensure that the rays of sunlight reach the bottom section of the water, as this is essential for the inner workings of a solar pond. It is also important that the pond has a black colored impermeable liner to absorb sunlight while also preventing water from draining out of it.

Image of the El Paso Solar Pond provided by ResearchGate

After this, a layer of water must be added to the pond, and in this case, the water added must as be as salty as possible. In order to achieve this, the water must be boiled while salt is continuously added to it, until it reaches a point where no more salt can dissolve into it. At this point, the saltwater is super-saturated, with about 20% of it being salt. Once the super-saturated saltwater is added, the next step is to add a layer of freshwater on top of it. This process requires some precision to ensure the water doesn’t mix, with the best method of doing so being a gradual process, where a small amount of freshwater is added in over and over again, over a period of time. When this is done, the freshwater added will have produced a middle layer in the pond known as a gradient layer, where the salinity level gradually decreases from about 20% to 0%, and effectively allows the top layer to be comprised entirely of freshwater, as the less dense freshwater will float atop the other layers naturally. Now that the solar pond has a bottom, gradient and top layer, it is able to create a continuous convection current when the black lining at the bottom of the pond heats up from the sunlight. In the bottom layer, when the saltwater is heated up, the hotter water is naturally obligated to travel upwards, and the cooler water travels downwards, creating a convection current. However, due to the gradient layer having each individual part of it being less dense as it approaches the layer of freshwater, convection does not occur. This property allows the gradient layer to act as an insulator, and essentially traps the convection current in the bottom layer, where it continuously cycles as the hotter saltwater attempts to travel upwards.

Diagram of solar pond by Lenntech

2. Electricity Generation

The functionality of a solar pond is useful for when it comes to electricity generation, as the pond is taking in a large amount of thermal energy from the sun, and this energy can be easily converted into electricity using some simple and well-known methods used with various other electricity generation methods. By connecting a pipe system, the hot saltwater is able to easily travel through the pipe. Adjacent to the pipe, a tank full of refrigerant is situated, and when the hot saltwater travels by the tank in the pipe, the heat radiated from the saltwater causes the refrigerant to boil and produce steam which spins a turbine that generates electricity as a result. This process relates to renewable energy ventures like hydroelectric dams, which instead rely entirely on the kinetic energy of water flowing through to spin a turbine that generates electricity.

Rankine Cycle electricity generation diagram illustrated by ScienceDirect

3. Heat Usage and Storage

Another way that the properties of a solar pond can be used is similar to the methods discussed with electricity generation. Once again, if a pipe system is connected to the hot saltwater, the saltwater is able to travel through the pipe. In this case; however, the hot water passes through radiators in a building where the thermal energy in the water is used to heat up rooms during colder weather. It is worth noting that because the heat in the saltwater as a result of solar radiation cannot escape due to the gradient layer preventing it from doing so, the saltwater in the bottom layer will remain hot even on cloudy days or in colder weather (to an extent), which makes it quite reliable. This also allows solar ponds to act as an ideal energy storage system, as it not only collects heat emitted from the sun, but traps it.

References:

[1] https://energyeducation.ca/encyclopedia/Solar_pond

[2] https://www.nrel.gov/docs/legosti/old/617.pdf

[3] https://youtu.be/Ob9XouO-SZU

[4] https://www.sciencedirect.com/

[5] https://youtu.be/9TxLPnMX5gY

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