Cooling Systems for High-Power-Density Data Centers

There has been ongoing talk since 2002 that high-power-density data centers would replace low-power-density data centers. The theory is that a higher density will increase efficiency while reducing energy bills, but with these benefits also comes the risk of cooling failure. Today, a data center where each cabinet consumes more than 10 kW is considered high power density. The density can also be measured by the amount of energy consumed per square foot, which is why many high-power-density data centers are built up rather than out. As rack densities continue to grow, data center manufacturers and designers are having to come up with more-efficient cooling solutions to offset energy consumption.

The traditional data center design is unable to cool these higher-density data centers, which has led to the development of cooling solutions such as CRAC units; racks featuring water-chilled, rear-door cooling units; and aisle-containment structures. Unfortunately, more often than not, simply expanding an infrastructure and adding CRAC units (large computer-room air conditioners) is not enough.

Modern variable-speed CRAC units increase energy overhead when running at a reduced load. A CRAC unit that runs constantly is one of the most efficient and traditional cooling systems around. Unfortunately, running a CRAC unit to keep a data center cool, let alone several, is not cost efficient and will rack up quite the energy bill. Rear-door cooling units as well as hot- and cold-aisle containment structures are the most popular and efficient cooling solutions that will not consume excessive amounts of energy.

Effective airflow management is a successful solution that prevents a data center from overheating, and it’s also cost efficient. A rear-door cooling unit employs liquid cooling technology to exchange hot air for cold air. The rear door holds cold water in a closed loop system, offsetting the heat generated by higher-density racks. The efficiency of this approach can be greatly improved by using chilled or refrigerated water at the lowest temperature possible. Doing so guarantees sensible cooling the entire time, and it offsets any accidental dehumidification. Designing a data center without a raised floor and overhead mechanical and structural electrical services is considered a best practice for rear-door cooling systems.

The aisle-containment structure is a prefabricated system that incorporates all the components of aisle-containment design in a freestanding unit. It comprises a base unit frame that is an aisle-containment platform and is used for cabinet anchoring. Both hot- and cold-aisle containment help improve the efficiency of a data center’s traditional cooling system, because an aisle-containment structure facilitates higher allowable temperatures. Higher-temperature data centers save more energy owing to lower fan speeds, increased temperatures in chilled water and the use of free cooling. Unlike the rear-door cooling unit that exchanges hot air for cold air, hot- and cold-aisle containment structures minimize the amount of hot and cold air that mixes together. This feature in turn can increase the capacity and efficiency of the cooling infrastructure.

Free air cooling is another system used in high-power-density data centers. A data center that has the temperature of 86 degrees Fahrenheit can be cooled by external air, if that air remains below 77 degrees Fahrenheit, without additional mechanical cooling. One drawback to this approach is that external air can carry dust into the data center. The Kyoto wheel helps ameliorate this issue by acting as a filter. It consists of a metal wheel that rotates through a two-part space. The center’s hot air flows through one space while the cold air enters through the second. When the hot air flows through one space, the heat gets transmitted into the metal of the wheel, then that air flows back to the data center while cold outside air absorbs the heat from the metal.

The purpose of cold-aisle containment is to contain cold air while keeping hot air out. It involves installing aisle ceilings, row doors at either end or overhead vertical wall systems, which allows the cold air to be redirected into server air intakes and prevents hot-air recirculation. Cold-aisle containment is also energy efficient. A cold-aisle containment structure guarantees that the IT equipment is cooled to the manufacturer’s specifications. Cold-aisle containment structures guarantee greater operating efficiency by creating an unchanging airflow that removes hot spots. As a result, more space is available throughout the data center.

Hot-aisle containment surrounds the rows in which data servers dump their heated exhaust. Like cold-aisle containment, its role is to prevent cold and hot air from mixing; hot aisles, however, face the air-conditioner ducts and send hot air out to be recycled into cold air. Doing so requires having a separate path for the hot air to reach the AC intake point. In some cases, hot air that hits the AC coil at the highest temperature can double the cooling tonnage. Hot-aisle containment can double the cooling capacity of a CRAC unit, cutting costs and increasing efficiency.

As the data center industry continues to explode with the increasing demand for data, high-power-density data centers will continue to grow in popularity. Operation managers are now tasked with reducing power consumption and increasing efficiency. Having a high-power-density data center increases performance and output, but having an efficient cooling system in place as well greatly reduces a company’s energy bill.

Leading article image courtesy of Intel Free Press under a Creative Commons license

About the Author

As the director of engineering for DAMAC, Erich Hamilton provides support for accounts from the ground up. From concept and design to troubleshooting and marketing, he is the cornerstone for all things racks and containment. After graduating from Cal Poly Pomona with a degree in mechanical engineering, Erich worked as a design engineer for Northrop Grumman, overseeing various contracts. When he is not developing innovative designs for DAMAC, he spends his time hiking California trails, doing yoga and enjoying great vegetarian meals.