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High-Capacity Heat Storage System

Many processes where electricity is produced also release heat into the atmosphere. This potentially important energy resource often simply evaporates into the air without being put to any further use. Thermal energy storage (TES) is a technology that stores thermal energy by heating or cooling a storage medium so that the stored energy can be used for heating and cooling applications and power generation at a later time. Currently, TES systems are mainly employed in buildings and in industrial processes.

Now (2020), an innovative system is being developed at the Argonne National Laboratory that can quickly store heat and release it for use when needed. This system shows greater flexibility and efficiency than conventional storage options. Argonne’s thermal energy storage system, or TESS, was originally intended for capturing and storing surplus heat from concentrated solar power facilities. It can also be used for different commercial applications, such as combined heat and power (CHP) systems, industrial processes, and heavy-duty trucks. It is a form of latent heat storage, where energy is contained within a phase-change material such as molten salt. Such materials are usually poor conductors, e.g. it takes long for them to absorb and release energy. To solve this problem, scientists at Argonne came up with a method to embed phase-change materials within porous, thermally-conductive foam. They subsequently contained the foam inside a module with inert gas, which prevents moisture or oxygen from getting inside and degrading the components. The heat stored inside a unit can then be transferred to water, for example, where it becomes steam that can move a turbine. Researchers have shown that TESS can work in temperatures over 700°C. It has high energy density and is smaller and more flexible than commonly used sensible heat storage systems, which rely on raising and lowering a material temperature.

Scientists have long been looking for methods to improve latent thermal energy storage systems. In 2017, the effect of using different heat transfer fluids (HTF) on the combined system was studied while using a low melting phase change material (PCM) i.e., paraffin wax. The heat transfer fluids chosen were water (low-boiling fluid) and therminol-66 (high-boiling fluid). Heat transfer rates were compared by employing both the heat transfer fluids. At first, water was made to flow during the charging process, followed by the discharging process where different encapsulation materials, such as copper, aluminium and brass were employed. These processes were then repeated for therminol-66 as HTF. Finally, it was concluded that even though therminol-66 improved the latent heat storage capacity, water offered a higher sensible heat storage capacity, making it a better HTF for low melting PCM.

In 2018, scientists researched the discharging and charged energy from and to phase change materials (PCM). To achieve better conjugate heat transfer between the heat transfer fluid (water) and the PCM at liquid state, they simulated what influence the addition of fins to the storage unit with various configurations, including online and staggered fins, might have. Then they looked at the effect of the inlet fluid flow and the fluid inlet temperature on the total PCM freezing time and the effect of the natural convection in the storage unit.

One of the big advantages of the new technology is that it has a modular structure and large storage structures are not needed. The modules have manageable size and can be installed in any quantity required. The system forms a thermal version of a battery, where you charge and discharge heat rather than electricity. Apart from enhancing CHP systems and making desalination and power plants more widely applicable, TESS could someday also convert waste heat to mechanical energy in heavy-duty trucks or to interior heating for electric vehicles. Furthermore, as TESS is a battery for heat, its function could be reversed, and may someday also serve as a cooling option for commercial buildings.

With the help of industry partners, scientists continue to refine TESS technology and have installed an in-house testing facility to test performance with repeated charging and discharging. As TESS can also be tuned to a specific application by selecting different phase-change materials, scientists are convinced that the system will be applicable in many fields of industry.