Cooling systems in large buildings tend to work hardest exactly when electricity is most expensive: on hot afternoons, when occupancy and outdoor temperatures peak at the same time. That mismatch between demand and cost has pushed engineers to look for ways to decouple cooling production from cooling consumption. Thermal energy storage (TES) is one of the most effective answers to that problem, and it has become a standard feature in many large commercial and district cooling projects across hot climates like the UAE and the wider GCC.
1. What Thermal Energy Storage Is and Why It Matters
Thermal energy storage is best understood as a battery for cooling. Instead of storing electricity, a TES system stores “cold” — produced during off-peak hours and held in reserve until the building actually needs it. Chillers run overnight, when ambient temperatures are lower, electricity is cheaper, and equipment operates more efficiently. The cooling they generate is captured in insulated tanks and then released during the day, easing the load on the chiller plant precisely when demand and utility rates are at their highest.
This is not a new idea, but the economics behind it are compelling. Many utilities charge more for electricity during peak demand windows, and TES gives building operators a way to shift consumption away from those expensive hours without sacrificing comfort. The strategy works best in facilities with predictable, repeating occupancy patterns — office towers, universities, airports, hotels, and hospitals are all common candidates, since their daily cooling curves are easy to plan around.
Every TES system, regardless of design, operates through two distinct phases: a charging phase, where chillers produce and store cooling energy, and a discharging phase, where that stored energy is released to meet the building’s actual demand. The approach also aligns with efficiency-focused standards such as ASHRAE 90.1, which encourages load-shifting strategies of exactly this kind. The technology used to store that cooling — typically ice or chilled water — differs from project to project, but the underlying logic of “make it cheap, use it later” stays the same.
2. Ice Storage: Compact, Efficient Use of Space
Ice storage systems take advantage of a simple piece of physics: when water changes from liquid to solid, it releases a large amount of energy, known as the latent heat of fusion. That phase change means ice can hold far more cooling capacity in a much smaller volume than chilled water can — roughly 144 BTU per pound during melting, compared with about 1 BTU per pound per degree Fahrenheit for chilled water. For projects where plant room space is tight, that difference can dramatically shrink the footprint needed for storage, which is one reason ice storage is popular in dense urban developments.
Ice systems generally use one of two configurations. In an internal melt design, ice forms around submerged tubes, and during discharge a warmer glycol solution circulates through those tubes to melt the ice from the inside out. In an external melt design, water instead flows over the outer surface of ice-coated coils, picking up cooling energy as the ice melts away. Both approaches are widely used, and the choice usually comes down to project-specific design and maintenance preferences.
Glycol plays a critical supporting role in either configuration. Because pure water would freeze solid inside the circulating loop long before enough ice could be produced, engineers add a glycol-water mixture to lower the fluid’s freezing point, allowing chillers to run at the low temperatures ice production requires. Companies with deep regional chiller and HVAC expertise, including Daikin, work with building owners across the Middle East to weigh these design trade-offs — since producing ice does come at a cost. Lower evaporator temperatures reduce chiller efficiency, and glycol itself reduces the heat-transfer performance of the circulating fluid. The space savings are real, but they need to be balanced against that efficiency penalty during system design.
3. Chilled Water Storage: Simpler, but Larger
Chilled water storage takes a different route to the same goal. Rather than relying on a phase change, it uses sensible heat storage — cooling energy is held simply through a temperature difference, with large volumes of water typically chilled to between 4°C and 5.5°C and kept in insulated tanks until needed.
What makes this approach elegant is a natural phenomenon called stratification. Cold water is denser than warm water, so within a single tank, chilled water settles toward the bottom while warmer water stays near the top, forming a distinct thermal boundary known as a thermocline. During discharge, cold water is drawn from the bottom of the tank to cool the building, while the warmer water returning from occupied spaces is redirected to the top. A well-designed thermocline stays stable over time, preserving the effectiveness of the stored cooling.
The trade-off is size. Because chilled water storage doesn’t benefit from a phase change, it needs considerably more volume than ice to deliver the same cooling capacity — typically around 15 cubic feet of storage per ton-hour, compared with 2 to 4 cubic feet per ton-hour for ice. That larger footprint can be a real constraint on tightly developed sites, even though chilled water systems tend to be simpler to operate day to day, since they avoid glycol management and low-temperature ice production altogether.
4. Choosing Between Systems and the Business Case for TES
Neither ice nor chilled water storage is universally “better” — the right choice depends on site constraints, utility tariff structures, cooling loads, and long-term operating priorities. Ice storage tends to suit space-constrained urban sites, where a smaller tank footprint outweighs the efficiency penalty of freezing water. Chilled water storage tends to suit campuses, industrial sites, and large commercial developments where land is more available and operational simplicity is valued.
Whichever technology is chosen, the underlying business case is similar. TES lets facilities avoid sizing chiller plants for the building’s absolute peak cooling load, since storage helps absorb that peak instead. That can reduce capital spending on chillers, pumps, and electrical infrastructure, on top of the ongoing operating savings from shifting cooling production to cheaper, more efficient overnight hours. Buildings with cooling capacities above roughly 100 tons generally see the strongest returns, and the case becomes even more compelling in hot, high-demand climates with a large gap between peak and off-peak electricity rates — conditions common across much of the GCC.
There are broader sustainability benefits too. By flattening demand curves, TES eases strain on electrical grids during peak periods and complements the growing share of renewable generation, which doesn’t always align neatly with when buildings need cooling most. For organisations working toward net-zero or broader carbon-reduction targets, thermal energy storage offers a way to cut operating costs and improve efficiency at the same time — a rare case where the financial and environmental incentives point in the same direction.