Thermal energy storage (TES) tanks do exactly what the name implies: they store chilled water until needed. They are employed in any application with sudden, high demand for cold water. In data centers, they can back up the main chilling system if it fails or supplement it if it can’t keep up with demand. This post explains how TES tanks work, along with their physical characteristics, benefits, and applications.

 

How Does a TES Tank Work?

TES tanks operate on a closed-loop principle. First, water is chilled by a chiller and enters the bottom of the tank. When it’s drawn out, warmer water returns to the top of the tank. The TES tank operates on the principle of stratification. This describes the condition in which colder water remains near the bottom of the tank, while warmer water stays on top. They are separated by a thermal boundary. (To better visualize this, just picture how oil and vinegar remain separate in salad dressing.) When that boundary remains intact, the tank can deliver at full capacity. However, usable capacity diminishes very quickly when warm and cold water mix.

Physical Characteristics

The tanks themselves are large, insulated vessels. Most of them are vertical cylinders ranging from 8 to 10 feet in diameter and about 20 feet or so tall. For height-constrained environments, tanks can be horizontal. TES tanks are typically made from steel. They are insulated with fiberglass or aluminum cladding. This helps minimize heat gain from the surrounding environment. An elaborate piping manifold controls how water enters and exits the tank, and its design is a major factor in the tank’s performance.

Maintaining Stratification

Maintaining stratification is an even bigger challenge. The good news is that designs have come a long way over the years. Earlier, TES tanks struggled with turbulence at the inlet, which caused the cold and warm water to mix. Today’s designs rely on internal baffles to keep the cold and warm water separated. The baffles act as diffuser plates, which slow and spread the incoming water before it can disturb the stratified layers of water in the tank. To better understand how they function, just imagine pouring water into a glass from a foot above, compared to gently pouring it in along the inside of the glass. It’s the same volume, but a significantly different degree of disturbance.

These baffles, which can be perforated or slotted, are typically positioned horizontally near both top and bottom inlet points. Since demand will vary for each installation, internal baffles must be uniquely designed. Flow rate is an important factor. It is determined by a combination of perforation pattern, hole size, spacing, and geometry. To aid in the tank design process, manufacturers will use Computational Fluid Dynamics (CFD) to validate customer-supplied drawings before fabricating the tank.

Usable Gallons

For most water storage systems, tank volume is usually the biggest concern. However, for TES tanks, customers care more about usable gallons for cooling. This parameter is determined by several factors, including flow rate, return water temperature, and the extent to which stratification holds up over a full discharge cycle. For a relatively simple operating principle, specifying a TES tank in practice can get quite complex.

Working The Night Shift

Chilling water requires a good amount of energy. TES tanks have a huge advantage here, as operators can chill the water at night, when energy is often cheaper. It’s then ready to flow when needed during peak hours.

Typical Applications for TES Tanks

As we mentioned earlier, these days data centers are the dominant application for TES tanks. Server racks generate a tremendous amount of heat and require constant cooling to keep them functioning. Most of the time, the main chilling system handles the work, but the TES tank serves as a backup if the main system fails and as a buffer during peak cycles.

Besides data centers, TES tanks can be found anywhere cooling demand is large, time-dependent, and where the economics of off-peak electricity make sense. Applications include commercial office buildings, hospitals, manufacturing facilities, and district cooling networks serving multiple buildings.

Alternative Approaches

While the demand for TES tanks is growing rapidly, they are not the only way to manage cooling capacity. Several alternative methods are available as follows:

Ice-Based Thermal Storage – rather than chilling water, some systems will use ice. Ice has a much higher energy density than water, so the tanks can be smaller. But there’s a big tradeoff. To keep ice frozen, the chiller must operate at lower temperatures, which reduces energy efficiency. These systems are also harder to maintain. They are acceptable for smaller facilities with modest chilling demands. But for larger facilities with greater needs, TES-based systems are more efficient and reliable.

Redundant Chillers – traditionally, operators have relied on spare chillers to keep facilities cool should the primary chilling units fail. This is a costly approach. Especially when considering that many backup chillers spend most of their time idle. TES provides backup capacity at a lower cost and performs at a much higher level than spare chillers.

Mechanical Flywheel/UPS Cooling – some data centers will rely on thermal buffering to assist should a chiller fail. Here, they rely on the thermal mass in the cooling infrastructure to buy time. The problem? It’s not much, perhaps just seconds or maybe minutes. Certainly not the hours that a TES tank provides.

Indirect Evaporative Cooling (IEC) – facilities operating in cooler climates will rely on outside air to reduce or eliminate mechanical chilling. While this is highly efficient, particularly in terms of energy costs, it does not provide the storage capacity needed in the event of a failure. TES is often installed alongside these systems rather than in place of them.

Conclusion

A TES tank is a fairly straightforward concept. It stores cold water and makes it available when needed. However, the engineering behind them is anything but simple.
For data centers in particular, TES isn’t a luxury. It’s a crucial part of a reliable cooling strategy. The right tank, properly engineered for its intended use, will provide the backup capacity needed to keep servers running when primary systems fail.

 

Would you like to learn more about TES tanks or get a quote for your project? RECO has been engineering and fabricating tanks for over a century. We can design and fabricate a TES solution to meet your needs. Contact us today to get more info or a quote.