In these times of economic development and the era of complicated geopolitical scenarios, it becomes important to take care of the demands and supply of power plants hence there is a need for energy storage systems and methods.
A solar pond seems to be a promising candidate in this regard. Solar ponds or salt ponds are artificially designed salty ponds that maintain a definitive concentration gradient. These ponds can be utilized for collecting and storing solar energy.
Solar ponds combine solar energy radiation and sensible heat storage (stored by heating a liquid or a solid) to collect and store solar energy. A solar pond has three following zones.
1. SCZ – “Surface (homogenous) convective zone”
It is adjacent to the surface and acts as a buffer between environmental fluctuation at the surface of conductive heat transport from the layer below.
SCZ is usually 10-20 cm thick with a low uniform concentration at ambient air temperature.
2. LCG – “Lower Connective Zone”
The bottom-most layer of the pond with high concentrations of salt amounting to high temperatures.
3. CGZ – “Concentration/Intermediate Gradient Zone”
This is the layer that separates SCZ and LCG and provides excellent insulation for the storage layer while transmitting the solar radiations.
To maintain a solar pond in this nonequilibrium stationary state it is important to replace the amount of salt that is transported by molecular diffusion from LCG to SCZ, to achieve this, salt must be added to LCG and freshwater to SCZ, while the brine is removed and recycled.
A plastic grid can be spread over the surface (as major heat loss occurs from the surface of ponds) to prevent the major heat loss due to the disturbance caused by wind, it is a common notion that the disturbed water loses the heat transfer faster than when it’s calm.
One might be wondering that there is a high possibility of salt intrusion into the nearby groundwater or soil to check the infiltration of salt. This linear or preventive measure is responsible for the increase in the cost of solar ponds.
This cost can be lowered if the site has natural soil impermeability (base of natural pond or lake), or at the sites where soil can be made impermeable by compaction or other relevant methods.
There are a few factors that affect the optical transmission properties and related collection efficiency these factors are as follows –
- Salt concentration.
- Quantity of suspended dust or other particles
- Surface impurities like leaves or debris, biological material like bacteria and algae.
- Type of salt.
It is common knowledge that higher efficiencies and storage can be achieved through the utilization of pure salt or refined salt.
Organic Rankine Cycle Turbine (ORC Turbine)
Usually, a solar pond is an effective collector of diffuse as well as direct radiation and can gather useful heat even on cloudy and overcast days.
If ideal conditions prevail, then a solar pond absorbs almost 50% of incoming solar radiation which is subjected to 20% after considering heat losses. When the lower layer of the pond reaches over 60°C the heat generated is drawn off through a heat exchanger and used to drive a low-temperature ORC Turbine, this turbine harnesses the pressure difference that is created when a low boiling point organic fluid or gas is boiled by heat from the pond by a heat exchanger and cooled by a condenser to drive a turbine to generate electricity. An ORC turbine has a conversion efficiency of 5-8%.
The applications of solar ponds include power generation, crop drying, desalination, process heat, and space heating and cooling.
Although solar ponds have their drawbacks like the availability of large land area, salt availability, and availability of water and there is a higher probability of achieving the ideal conditions in the region of sunny climate.
- The first Solar Pond in India was built in the year 1973 in Bhavnagar at the Central Salt Research Institute having an area of 1200 m2.
- The latest solar pond is built in Bhuj, Gujarat having an area of 6000 m2 and provides 90,000 liters of hot water at 80°C as process heat for can-sterilization, the heat stored by this pond is used to generate 150 kW power.
- This pond is the second-largest solar pond in the world (the largest being Beit HaArava pond in Israel, operated up until 1988).
In my opinion, energy storage is important for achieving energy security, we need to collect and store energy from wherever we can hence we should invest time and money in the R&D of solar ponds.
Remember people, knowledge is power so let’s spread it and Save the Planet!
Source and Reference:
R.K Rajput, Non-Conventional Energy Sources and Utilization.