Cross-section of a thermal energy storage tank illustrating a thin thermocline.

Thermal Energy Storage: Is Your Tank Ready for the Future?

Mira Elwood in Climate & Environment December 2025 • 4 min read.

"Unlock the secrets to efficient energy storage and discover how optimizing your thermal energy storage tank can lead to significant cost savings and a greener footprint."

In an era where energy efficiency and sustainability are paramount, thermal energy storage (TES) systems are gaining increasing attention. These systems act as a buffer between energy supply and demand, storing thermal energy for later use. Think of it as a giant rechargeable battery, but instead of electricity, it stores heat or cold.

One crucial application of TES is in district cooling plants, where energy is stored during off-peak hours and utilized to provide cooling during peak demand. This not only reduces energy costs but also alleviates strain on the electrical grid. The heart of such a system is the thermal storage tank, and its performance is key to achieving optimal efficiency.

Two primary metrics used to evaluate the efficiency of a thermal energy storage tank are thermocline thickness and the half-cycle figure of merit (FOM). These indicators provide valuable insights into how well the tank is performing and where improvements can be made. Understanding and optimizing these factors can lead to significant cost savings and a more sustainable energy footprint.

Understanding Thermocline Thickness and Half-Cycle Figure of Merit

Cross-section of a thermal energy storage tank illustrating a thin thermocline.

To effectively manage a thermal energy storage tank, it’s important to understand the key factors that determine its efficiency. Two of the most critical criteria are thermocline thickness and the half-cycle figure of merit (FOM). Let's break down what these mean and why they matter.

Thermocline thickness refers to the boundary layer between the hot and cold water within the storage tank. Ideally, you want this layer to be as thin as possible. A thinner thermocline means less mixing of hot and cold water, leading to better stratification and more efficient energy storage. The goal is to keep the hot water hot and the cold water cold, maximizing the usable energy in the tank.

Ideal Scenario: Achieve the thinnest possible thermocline.
Impact: Minimizes mixing, enhances stratification.
Benefit: More efficient energy storage.

The half-cycle figure of merit (FOM) measures how effectively the tank stores and releases energy over a half-cycle, typically a charge or discharge period. It’s expressed as a percentage, and the higher the percentage, the better. Aim for a FOM greater than 90 percent to ensure minimal energy loss during storage and retrieval. A high FOM indicates that the tank is doing a good job of maintaining the quality of the stored energy.

Optimizing Your Thermal Energy Storage for a Sustainable Future

Thermal energy storage systems offer a practical approach to improving energy efficiency and reducing costs. By understanding and actively managing factors like thermocline thickness and half-cycle FOM, businesses and communities can harness the full potential of TES. As technology advances and energy demands evolve, prioritizing the optimization of TES tanks will be a crucial step toward a more sustainable and resilient energy future.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1051/matecconf/201713101010, Alternate LINK

Title: Evaluations Of Thermocline And Half Cycle Figure Of Merit Of A Thermal Energy Storage Tank

Subject: General Medicine

Journal: MATEC Web of Conferences

Publisher: EDP Sciences

Authors: Mohd Amin Abd Majid, Looi Kar Kin

Published: 2017-01-01

Everything You Need To Know

What is thermal energy storage (TES) and why is it important?

Thermal energy storage (TES) systems store thermal energy, either heat or cold, for later use, acting as a buffer between energy supply and demand. TES is important because it enhances energy efficiency, reduces energy costs, and lessens the strain on electrical grids, particularly in applications like district cooling plants. By storing energy during off-peak hours and using it during peak demand, TES contributes to a more sustainable energy footprint.

How do thermocline thickness and the half-cycle figure of merit (FOM) relate to the efficiency of a thermal energy storage tank?

Thermocline thickness and the half-cycle figure of merit (FOM) are key metrics for evaluating the efficiency of a thermal energy storage tank. Thermocline thickness refers to the boundary layer between hot and cold water; a thinner layer indicates better stratification and less mixing, leading to more efficient storage. The half-cycle FOM measures how effectively the tank stores and releases energy over a half-cycle; a higher percentage indicates minimal energy loss and better energy quality. Optimizing both factors leads to significant cost savings and a more sustainable energy footprint.

What does 'thermocline thickness' mean in the context of a thermal energy storage tank, and why should it be minimized?

Thermocline thickness refers to the boundary layer between the hot and cold water layers inside a thermal energy storage tank. Ideally, this layer should be as thin as possible. Minimizing the thermocline thickness is crucial because a thinner thermocline reduces the mixing of hot and cold water. This enhances stratification, ensuring that the hot water remains hot and the cold water remains cold, which maximizes the usable energy stored in the tank. Efficient stratification optimizes the tanks performance.

What is the 'half-cycle figure of merit (FOM)' and what target value should one aim for when evaluating a thermal energy storage tank?

The half-cycle figure of merit (FOM) measures how effectively a thermal energy storage tank stores and releases energy during a half-cycle, such as a charge or discharge period. It is expressed as a percentage, and you should aim for a FOM greater than 90 percent. This indicates that the tank is doing a good job of maintaining the quality of the stored energy with minimal loss. A high FOM is essential for ensuring the tank's efficient and effective operation.

How can optimizing thermal energy storage tanks contribute to a more sustainable energy future, and what are the broader implications of focusing on factors like thermocline thickness and half-cycle FOM?

Optimizing thermal energy storage tanks, by focusing on factors like thermocline thickness and half-cycle FOM, leads to improved energy efficiency and reduced energy costs for businesses and communities. By actively managing these metrics, TES systems can reach their full potential. As energy demands evolve, optimizing these systems helps reduce reliance on traditional energy sources. In doing so, communities enhance the overall resilience and sustainability of energy systems, thereby supporting a greener energy future by minimizing energy waste and promoting efficient energy use.