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Thermal Storage for Data Centers

Most people are aware of the need for uninterruptible power sources (UPS Systems) to assure continuous clean power for critical computing equipment in a data center. However, many people do not realize that if cooling systems in a data center are lost, temperature rise can very rapidly, leading to imminent failure of critical electronic components.

Electronic components do not like heat, and failure rates increase exponentially with linear increases in temperature. And while it is somewhat rare to find yourself without any cooling system capacity, failure to plan for this eventuality can increase your risk of downtime substantially.

Thermal storage is needed to delay the effects of cooling system failure to the point where either the cooling system is repaired, or when it doesn’t matter anymore – such as the case where the UPS system battteries have been depleted and backup generator power is unavailable. There are several methods available to achieve thermal storage, incuding both passive and active systems.

Ceiling Height

The first line of defense is to have a sufficiently high ceiling in the raised floor or computer room environment so that stratified heat has somewhere to go rather lingering down low near the computer systems. A good rule of thumb is that you need 9’-0” of clear ceiling height plus 1’-0” for each 20 watts per sq. ft. of power density. For example a 100 w/sf data center should have a minimum ceiling height of 14’-0” (9’ + (100w / 20w)’). This is the minimum. With each additional foot of vertical clearance, you buy additional time before alarm-level temperatures are reached.

Piping Capacity

If your data center utilizes chilled water to provide cooling through precision air handling cabinets, the stored water in the piping system can provide some level of protection for a short time. Unless the piping is purposely oversized, however, this protection will provide only a moderate delay in temperature rise. For a data center requiring 300 tons of cooling and utilizing 6” chilled water mains at 735gpm, there is approximately 5 minutes of cooling capacity for every 100 lin. ft. available in the pipe itself. But if you have 15 minutes of battery on your UPS, keeping the computers cool for the extra ten minutes might be difficult. This is where more active thermal storage is needed.

Chilled Water Storage Tanks

When a longer duration of chilled water supply is required, a thermal storage tank can provide up to eight hours of cooling, providing not only back-up cooling but also the ability to run chillers on off-peak rates and thereby save energy costs. Thermal storage tanks come in several varieties, but probably most common for data centers is a non-pressurized stratification tank. The tank is provided with sensors at regular intervals – say 12” – to monitor the thermocline (stratification layer that separates colder water from the returning warm water).

As a means of reducing energy costs during the day, the building operator has the option of using the thermal storage tank to cool the building for a period of time and still maintain one hour of reserve capacity. By operating the chillers and cooling towers during cooler evening hours, ambient conditions typically allow for production of cooler condenser water. By shifting chiller usage to off-peak times of the day, the owner can take advantage of lower electrical rates.

Once it’s decided to provide a level of redundancy for the data processing room by implementing a thermal storage system, the question becomes how much additional capacity is desirable. There are disadvantages to having a thermal storage tank that is too small and from having one that is too large.

If the tank is too small, cooling load demand will quickly drain the tank and, once started, the chillers will quickly refill the tank. This event is referred to as “short-cycling.” Space limitations and reduced thermal storage efficiency (due to long storage times) become issues with large storage tanks. Other disadvantages of a large tank include a relatively large first cost and lengthy payback period.

If you would like an analysis of the benefits thermoal storage can provide for your data center, contact Reliable Resources.