Thermal Storage

Thermal energy storage is an essential part of a solar thermal systems including power plant and process heat recovery units[1]. The storage system plays a vital role in energy balancing between the power supply and demand cycles. 

Among several options of thermal energy storage, namely, the latent heat, chemical energy, the sensible heat storage stands out as one of the widely accepted system because of the ease of operation and wide range of operating temperatures.  There are a number of configurations proposed in literature for sensible energy storage. A two tank system is the most commonly used sensible heat storage system in solar thermal power station[1]. However, it is acknowledged that the two tank system is more difficult from the aspect of temperature control than the single tank storage systems. The single tank storage systems are typically conventional packed bed with ceramic packing materials having high specific heat. The solid filler materials act as heat storage medium. These types of storage tanks are known as dual media single tank storage system. The temperature gradient zone inside the tank in termed as the thermocline. The extent of the thermal stratification decides the performance of the single tank storage system. Lower the thickness of the thermocline zone better is the performance of the storage system. Recent research concludes that single media single tank storage systems i.e., vessels without any packing material, perform better than a dual media single tank system[2]. 

 Many heat transfer fluids including water can be used in single media single tank storage systems, however, molten salts are the most common heat transfer fluid used for high temperature operation. The wide range of operating temperature of molten salts makes them popular as heat transfer fluid in thermal storage systems, especially for high temperature applications.

A pilot scale single media single tank thermal energy storage system is designed and developed in-house. The storage tank is equipped with a vertical flow distributor with a porous wall. A 1:4 reduced scale replica of the large scale model is also fabricated. A large number of experiments are carried out with water as heat transfer fluid. The range of the operating temperature is selected such that the relevant properties of water expressed in scaled form, are in close agreement with those of the molten salts commonly used in solar thermal power plants. Flow visualization experiments are also performed to substantiate the inference made based on the temperature measurement. It is demonstrated that the flow distributor plays an important role in achieving thermal stratification in the storage tank and the design is scalable [3,4].

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