Computational Fluid Dynamics in TES in District Cooling design

Currently, over 50 % of the global population lives in urban areas. There are all indications that this figure will continue to increase, adding more strain to the environment in terms of energy use and emissions. Due to these developments, energy efficiency is taking center stage in most processes, including district cooling. Managing energy in district cooling through CFD analysis is an effective way of countering these problems. At the past, engineers relied merely on experience to develop the aerodynamic aspects of their designs. Later, they would resort to tunnels, but the process remained expensive and time consuming. Ever since Computational Fluid Dynamics (CFD) emerged in the early 1930s, many industries have made leaps and bounds and numerous innovations have emerged. The flexibility, affordability and precision of this technology have seen ARANER apply it in district cooling designs to the satisfaction of clients.

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Basics of CFD Analysis

Computational Fluid Dynamics (CFD) is a concept of predicting the quantitative result of a fluid flowing, especially under the simultaneous influence of mechanical movement, mass transfer, heat flow and chemical reaction among other factors. This tool is already well established in leading industries with applications such as predictive pollution control and aerodynamic design. ARANER is now using CFD tool to reshape thermal energy storage installations. TES tanks designed through CFD modeling by ARANER are highly sought in the market for reasons such as reliability and cost-effectiveness. Our specialists use the best methods to design diffusers that ensure stable thermocline. By maintaining little or no turbulence, our TES tanks ensure that the cold and hot water remain separate, leading to optimal tank capacity and efficiency. Are you wondering what is CFD? A review of TES may help. Click here to view a short video to see how CFD works.  

Thermal Energy Storage (TES) and Diffusers

Thermal Energy Storage (TES) tank is a thermal accumulator that allows the storage of cooling produced during off-peak time, thus reducing the operational cost and the required capacity of the district cooling plant. The chilled water supplements the chiller, covering for the time that the demand is high, the chiller is restarting or power failure. TES is available in two concepts namely partial storage and full storage, each with its advantages and disadvantages. For simplicity and low cost reasons, natural stratification is the most popular system. In this case, gravity separates the water, with the warmer (less dense) water settling at the top and the colder (denser) water at the bottom.  
  A stratified chilled water Thermal Energy Storage tank comprises of two sets of water distribution; one at the bottom and another at the top. Between these two distributors appears a non-physical boundary layer that is popularly known as thermocline. These sections hold water at different temperatures. Storage capacity depends on the temperature difference, whereby a higher temperature difference yields higher storage capacity for a given water volume.

TES Tank with Stratified Water Fig: TES Tank with Stratified Water

However, the thermocline in the natural stratification setup is at risk of destruction in case of improper design/ omission of CFD analysis, since the design of diffusers affects performance of TES in district cooling. These components play a critical role during both charging and discharging processes. Without diffusers, it would be difficult to achieve stratification at the desired flow rate. Water velocities at the inlet and outlet of the TES tank must remain slow so that inertia does not predominate over buoyancy. For stratification to occur, the diffusers must be designed so that they channel the water gently into the tank in a gravity current, with minimal mixing between cold and warm water.  

The Secret, Computational Fluid Dynamics (CFD)

Often, older engineering tools are inadequate when it comes to supporting a reliable TES system. Owing to this, the team at ARANER uses a number of ingenious approaches, the most popular one being Computational Fluid Dynamics. Natural stratification is highly regarded because of its low cost and simplicity. It may be simple, but it features distribution of water at different temperatures, separated by a non-physical boundary called the Thermocline. The temperature difference is crucial as it determines the capacity of the tank. The need to maintain this boundary makes mixing of the water a complicated process. Specifically, mixer design can be a daunting task in TES systems. CFD comes in handy as a reliable tool for unique designs. Computational fluid dynamics ( CFD) is therefore used in district cooling for thermal design. The very reliable tool involves the use of numerical techniques and computers to solve fluid flow problems. Using high-speed computers, specialists at ARANER are able to predict mass and heat transfer to the finest detail.

Design process by ARANER

Design is done with the full knowledge that there are several types of diffusers, for instance octagonal and radial types. An important parameter that emerges during CFD simulation is Reynolds number, which is the ratio of inertia to kinematics. Other equally crucial measurements include Aspect Ratio, Diffuser Type, chilled water temperature and warm water temperature. Others are diffuser size and flow rate. ARANER understands the importance of designing diffusers such that stratification correctly achieved. The two most popular diffuser styles are radial disk diffuser and octagonal pipe diffuser. The radial disk diffuser is shown in the figure below. 

A radial diffuser Fig 2: A radial diffuser

The radial disk diffuser is sturdy in construction, so it reduces de installation works. It can withstand water surges and hammers. This is unlike the octagonal type, which requires more installation but might be more cost-effective depending on the case and also can be adapted for different tank shapers. We maintain both streams (inlet and outlet) at low velocities so that buoyancy is higher than inertia. This maintains a density current over the top or bottom of the tank. Why is it so important to use this tool? The basis of this approach is the fact that every TES customer comes with unique needs. To get the exact cooling requirements, the designed diffuser geometry must be subjected to real conditions, thus testing its performance before erection is strongly advised.  

Importance of CFD for TES

Poorly designed internal diffusers are a recipe for a system that does not meet requirements. Customers may not get the cooling capacity needed. TES for district cooling is a major project that should be handled with certainty. Application of ARANER’s CFD tools is the surest way of getting optimum performance from a TES system. Any omissions or mistakes in the project are rectified early to avoid money and time losses. The performance of a chilled water TES tank depends on a number of factors including:

  • Tank aspect ratio
  • Delta t or temperature difference
  • Flow rates
  • Diffuser design

For enhanced performance, TES tank design must take account of all these factors and be such that the thermocline is as thin as possible, during both operations (charging and discharging). Applying CFD software to perform simulations, ARANER is able to solve numerous flow problems for the TES tank. High-speed computers enable the specialists to predict mass and heat transfer to the finest detail. The intricate heat transfer rarity of TES tanks involves natural convection, solid conduction and so forth, but CFD tool is capable of analyzing all these. Results from CFD analysis and experimental measurements are compared to create a simplified analysis, leading to the establishment of optimum working conditions and parameters. Computational Fluid Dynamics enables ARANER engineers to optimize the configurations of our tailor-made diffusers and increase the performance of the TES tank. This simulation software enables the engineers to ensure successful product development.  The engineers develop a digital prototype of the TES tank and run simulations to the interaction between the product and fluid behaviors. By using computational fluid dynamics approach, ARANER is able to confirm product conformity to client requirements early in the initial stages of the design cycle. The product development process becomes more effective in the process.  

Commitment to the Design Process

ARANER uses software for optimum diffuser design. The software package is also important in the determination of maximum discharge rates and storage capacity of the thermal energy storage tank. The aim is to design diffusers that produce water at low velocities to reduce chances of water mixing during the charging and discharging processes. From experience, CFD design optimizes the useful stored energy through thermal stratification. While CFD for TES in district cooling is a critical tool in the design process, it is important to have an idea of what the final should look like because operating conditions can vary greatly. The most important considerations in this case include upstream and downstream flow conditions. ARANER understands that customers have different needs in terms of tank geometries and cooling demand. For ideal stratification, the thermocline must be maintained within the proper limits, and this calls for different diffuser parameters for each client. With accurate diffuser geometry, the water in the TES tank will stratify evenly and provide the required district cooling extra capacity.  


District cooling is the ultimate solution for indoor comfort and ease, whether you are talking about an industrial or commercial setting. However, it can eat a big portion of the power budget, if efficiency is overlooked. Since diffuser design affects overall performance of the district cooling system, it is only worthwhile to take time in designing this component, even as you ensure that all stages of TES system design are flawless. The success of such a design is a factor of many things including the specific user’s project requirements. While it is possible to install a system based on previously set parameters, the use of CFD analysis eliminates assumptions that may hinder effective performance. Considering the fundamentals of Computational Fluid Dynamics, it becomes clear that only experienced CFD companies can give reliable help. ARANER provides CFD solutions through its experienced professionals. The company provides comprehensive CFD model for airflow and temperature, leading to accuracy of energy profile.  Contact our experts for further information.

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