Guaranteed thermal capacity is possible by using CFD in the design of your thermal energy storage. Obtain an efficient and cost-effective solution with undisturbed thermal layers during operation.

Surplus heat: From waste to resource with thermal energy storage

Storing surplus heat from industrial processes is a challenge that when solved, creates economic benefits both for industrial process plants and combined heat and power plants. Moreover, the environmental benefits for society due to reduced over-production and minimized waste is substantial. This is where thermal energy storage, also known as heat storage tanks, make a difference.
Construction of a FORCE Technology designed heat storage tank in Liaoyuan, China
Construction of a FORCE Technology designed 30,000 m3 thermally stratified thermal energy storage in Liaoyuan, China.
Finalised tank
The finalised Liaoyuan thermal energy storage. The tank can store and deliver 1188 MWh of thermal energy, which is used to supply district heating.

For industrial processes, thermal energy storage allows saving waste energy, for later reintroduction into the system when needed. This reduces the need for auxiliary equipment to re-generate energy that has already been produced at an earlier stage. For combined heat and power plants, thermal storage allows closing the time-gap between production and use of district heating and electricity. Heat generated at specific hours of the day, as by-product of electricity production, can be stored and used later when district heating demand rises. 

Meeting the designed capacity of the thermal energy storage

Proper functioning of a stratified water thermal energy storage is directly related to its ability to meet the designed thermal capacity, as well as to preserve high temperatures inside the tank. 

To control the latter, special attention must be paid to the thermal layer, which is the layer that naturally separates cold water at the tank’s bottom from hot water at the top. A good design implies a thin thermal layer, meaning that there is only little mixing between hot and cold water.

Simulation snapshot of water flow lines and coloured temperature during tank operation
Cross sectional view of a tank simulation. Lines are depicting water movement, and colours are depicting temperature. 
Simulation snapshot of water flow lines and coloured temperature during tank operation
The tank performance is simulated fully in 3D, ensuring no loss in flow and thermal information caused by 3-dimensional effects. The thermocline can be easily tracked in real time during full charge and discharge cycles.

How to minimise the thermal layer

Hot and cold water enters and leaves the tank during charging and discharging. This process induces turbulence and mixing inside the tank, disturbing the thermal layer thus increasing its thickness. Such disturbance creates permanent thermal losses and reduced tank capacity, which cannot be recovered. This leads to continuous economic losses for the end user.

In order to preserve an undisturbed thermal layer, reduce thermal losses, and maintain a high thermal capacity, water must enter and leave the tank through diffusers that create as little turbulence and mixing as possible. 

Diffuser optimisation
Every tank goes through a numerical diffuser optimisation. Each tank is slightly different in shape, size and capacity. Therefore, individual diffuser optimisations are necessary, to ensure the thermal stratification stays stable during maximum operating conditions.
Diffuser optimisation – from simulation to finalised product
Finalised construction of a specialised diffuser. Steel material usage was cut by more than 60% compared to conventional designs, while keeping thermal performance even higher. The finalised diffusers are a collaboration between the fluid mechanical institute, the construction design institute, and the EPC company.

Using Computational Fluid Dynamics (CFD) to guarantee high thermal capacity and minimum thermal layer

We use CFD to design and validate stratified water thermal energy storage that operate optimally. Our services cover design and optimisation of internal piping systems and diffusers, ensuring optimal functioning from a flow and mechanical perspective. 

Our CFD design procedure ensures:

  • high thermal capacity
  • thin thermal layer
  • minimum thermal losses by reducing turbulence, mixing effects and optimal design of flow in pipes
  • reduction of diffusers size and weight compared to conventional design methods
  • design flexibility, upon feedback from construction teams.

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Full 3D simulation of discharge cycle. All information of flow, thermal performance and delivered capacity can be mapped.
Full simulation of discharge cycle
Performance test simulation of a discharge cycle. The tank initially flushes any large thermocline by filling the tank completely with hot water, leaving a flushed-tank thermocline height of about 0.35 m. The full discharge cycle hours show the thermal layer peaks out with about 1.1 meters height, over the total cycle of 11 hours.

Through CFD, we are able to predict flow behaviour inside the tank. This is crucial for its proper functioning. Such detailed overview provides valuable information that would otherwise only be available once the thermal energy storage is constructed and tested. Since we simulate the process inside the entire tank, we can guarantee the tank’s performance, heat output and thermocline thickness, prior to its construction.