Size a chilled water thermal energy storage tank around the cooling shortfall it must cover, the duration of that shortfall, and the temperature range the connected equipment can actually use. Total building cooling demand alone does not establish the required tank volume.

A practical design must satisfy two separate requirements: enough stored cooling energy to last through the peak period, and enough discharge capacity to meet the highest instantaneous shortfall. A tank can meet one requirement and fail the other. Establish both before comparing tank dimensions or quotations.

Define the cooling duty the tank must cover

Start with a cooling-load profile across the intended discharge period. Compare the load at each interval with the cooling plant capacity that will remain available under the planned operating strategy. Storage supplies the difference whenever demand exceeds that available capacity.

Required storage energy = sum of each cooling shortfall × its duration.

Use thermal cooling demand, expressed in kW of cooling, rather than electrical consumption. If the objective is to limit electrical demand, translate that limit into an achievable chiller operating schedule, accounting for plant efficiency and auxiliary power.

For data centres, distinguish between routine peak shaving and cooling continuity during a plant interruption. Peak shaving may require storage to cover only part of the load for several hours. Continuity duty may require a much higher discharge rate for a shorter period while equipment restarts.

If one tank serves both purposes, define the minimum reserve that peak shaving cannot consume. Otherwise, a tank discharged for cost control may have insufficient energy available when an interruption occurs. Also establish whether the design must cover simultaneous events or separate operating scenarios.

Convert the cooling requirement into tank volume

For preliminary sizing with water, the relationship is:

V = E ÷ (1.163 × ΔT × f)

• V: tank water volume in cubic metres.
• E: required delivered cooling energy in kWh of cooling.
• ΔT: usable temperature difference in °C between the charged cold water and the design warm return water.
• f: effective usable-capacity factor, reflecting how much theoretical storage is available before the discharge temperature exceeds the permitted limit.

The coefficient 1.163 approximates the thermal capacity of one cubic metre of water per degree Celsius. Glycol mixtures require the density and specific heat appropriate to their concentration and operating temperature.

The usable-capacity factor should reflect the proposed tank arrangement and operating conditions. It can account for mixing, thermocline development and inaccessible water volume. Establish whether the supplier’s stated capacity already includes these effects before adding further allowances. Heat gain during storage and distribution losses also need a defined treatment so they are neither omitted nor counted twice.

An illustrative calculation

Assume storage must deliver a constant 600 kW cooling shortfall for three hours. The required delivered energy is 1,800 kWh. With an assumed usable temperature difference of 6°C and an illustrative usable-capacity factor of 0.85:

V = 1,800 ÷ (1.163 × 6 × 0.85) ≈ 304 m³.

This is a preliminary water-volume estimate under those assumptions. It does not establish the tank’s external dimensions or include every project allowance. Freeboard, internal components, insulation, access and installation clearances affect the final arrangement.

If the usable temperature difference falls to 4°C, the same calculation gives approximately 455 m³. This explains why confirming operating temperatures early can prevent a substantial change in footprint later.

Use temperatures the cooling system can sustain

The design temperature difference must remain achievable during storage discharge. A scheduled supply-and-return temperature pair is insufficient if actual return water is cooler, bypass flow mixes the circuits, or terminal equipment requires more flow than expected.

In a stratified chilled water tank, cold water occupies the lower region and warmer return water enters above it. Diffusers limit disturbance between those regions. The transition layer, or thermocline, occupies space and changes during operation; the tank does not deliver its initial cold-water temperature indefinitely.

Define the maximum acceptable tank outlet temperature at the point where storage is considered depleted. This limit should follow the requirements of the connected cooling equipment. A tank may still contain cooler-than-return water after that limit is reached, but the remaining energy may no longer be useful to the application.

Where a heat exchanger separates the tank circuit from the facility circuit, include its approach temperature in the temperature budget. For liquid-cooled equipment served through CDUs, trace the required temperatures through each exchanger. The allowable coolant temperature at the equipment does not directly establish an acceptable storage-tank temperature.

Check discharge flow before accepting the volume

Stored energy determines duration; flow and temperature difference determine cooling output. For water, a preliminary discharge-flow calculation is:

Flow in m³/h = cooling output in kW ÷ (1.163 × ΔT).

In the illustrative 600 kW case with a 6°C temperature difference, the required flow is approximately 86 m³/h. If the load profile contains a higher short-duration shortfall, size the discharge path for that higher duty even when the total energy requirement remains unchanged.

Check pump duty, pipe losses, valve capacity, heat exchanger performance and diffuser operating limits at the required flow. Excessive inlet velocity can disrupt stratification and reduce usable storage. Increasing pump size without checking the tank internals may therefore reduce the duration the system can deliver.

Hydraulic arrangement also matters. An atmospheric tank connected to a pressurised circuit requires an appropriate pressure and hydraulic interface. Confirm static pressure, expansion provisions and pump suction conditions alongside tank selection.

Make sure the tank can recharge before the next peak

A tank that meets one discharge event can still fail a repeating daily schedule. During charging, the cooling plant must serve the live load and restore storage within the available window.

Estimate charging time by dividing the energy to be restored, including applicable losses, by the net cooling capacity available for charging. That capacity may be restricted by overnight demand, chiller performance at the required charging temperature, or heat exchanger limits.

Test the operating sequence across consecutive demanding days. Include the starting state of charge, the reserve requirement and any period when charging is unavailable. A larger tank does not solve an ongoing shortage of charging capacity.

Thermal storage can shift cooling production into periods with favourable electricity prices or renewable generation availability. Whether it reduces total electricity consumption depends on charging conditions, heat gains, pumping energy and plant efficiency. Evaluate demand reduction and energy consumption separately.

Specify usable performance in the equipment package

Request a guaranteed delivered cooling duty under defined conditions, supported by a discharge temperature profile. A nominal volume alone leaves too much uncertainty about how the tank will perform when connected to the plant.

The procurement specification should establish:

• Required cooling energy, maximum discharge output and discharge duration.
• Initial charge condition, return temperature, flow range and maximum acceptable outlet temperature.
• How usable volume, thermal losses and any design reserve are accounted for.
• Charging duty, available charging window and required end-of-charge condition.
• Tank geometry, structural loads, access requirements and hydraulic interfaces.
• Control sequences and acceptance criteria for charging, peak shaving and interruption response.

Shandong Liangdi Energy Saving Technology Co., Ltd. supplies data centre cold storage tanks, CDUs, water distribution manifolds and heat exchanger units. When coordinating these components, use one shared temperature-and-flow schedule so each equipment selection supports the same operating duty.

At commissioning, demonstrate discharge performance at the agreed conditions and verify the transition between plant cooling and stored cooling. Temperature sensors at several tank heights can help track stratification and estimate remaining capacity, but the controls must relate those readings to usable cooling energy.

Before fixing the tank footprint, settle the load profile, allowable supply temperature, realistic return temperature and charging schedule. These inputs determine whether additional volume provides useful capacity—and whether the selected system can deliver that capacity when the peak arrives.