The data center is a demanding environment, with power and cooling issues a perpetual concern for managers.

It is perhaps unsurprising then that there has been scepticism over the ability of containerised configurations, given their compact and confined nature, to compete with, or provide an alternative to, traditional brick-and-mortar data centers.

The two primary concerns for any data center are power availability and cooling reliability. Without consistency in these areas, down time becomes inevitable. It is somewhat straightforward to control these factors in a traditional data center environment, utilising standard electrical infrastructure which also powers large-scale HVAC systems.

But how are the crucial considerations of power and cooling handled in much more compact containerised data centers?



Rackable, now known as SGI

Power
Given that at containerised data centers are portable, flexibility is important in terms of power.

For example, SGI’s system can be configured for a number of different power delivery methods including:
• AC feeds with the power converted from AC to DC at the rack level.
• AC feeds with the phase/voltage conversion via internally mounted transformer, then converted from AC to DC at the rack level.

By far the most energy efficient means of providing power is via 480/277 VAC 3-phase 5-wire wye (3 phases + neutral + ground). This is the preferred power source since 480 VAC 3-phase splits into three 277 VAC circuits without requiring a supplemental PDU. One or more 277 VAC circuits are then rectified to -48 VDC at the rack level and distributed to servers.

Beyond that of the servers, power draw for a container will serve three primary systems – fans, rectifiers and lighting.

The cooling fans for a 40’ 24-rack container allow all servers to operate fanless and consume less than 5000 Watts. This represents a large power saving as an equivalent number of servers in a traditional data center typically require 40KW or more of fan power. Within the SGI ICE Cube, rectifiers are approximately 93% efficient, which results in additional power draw beyond the connected individual servers and fans. However, this is offset with the use of DC powered servers – which are between four and eight percent more efficient than AC powered equivalents. Power draw from overhead lighting is negligible in comparison (~=50 W).

Given that containerised data centers are an ideal solution for providing compute in hostile or isolated environments, UPS capabilities are essential. A rack level UPS system with sufficient hold-up time for an alternate power source such as a generator to fully come online is the preferred option within the ICE Cube. This system is tightly integrated into the DC power system and provides backup power to servers, network, cooling and other support equipment. At the rack level, it consists of one or more 2U battery trays populated with four VRLA 12v cells with a three-year service life. This can provide a baseline holdup time of 120 seconds or as much as ten minutes. Because the UPS system does not include an AC-DC-AC conversion as is typically the case in brick-and-mortar facilities, there is close to no server efficiency loss from the SGI UPS design.

Cooling
The compact nature of containers means that cooling represents just as much of a challenge as power. Fortunately this can be overcome with innovative water-based solutions. For example, using a closed-loop system - the water source is either a chiller loop (including reservoir tank) or any untreated source, as long as it goes through a heat exchange separation loop. The water should contain a simple organic algaecide and contain as little air as possible (a simple air breather vent can be installed high in the loop or a centrifugal air separator can be added). The cleaner and more air-free it is, the longer the cooling infrastructure will last.

Water is supplied to the container via 2.5” grooved connections, with the stub leaving the container being a male end, with a supply and a return line on each side, or end, of the container.

Cooling load is, of course, dependent on server power draw. An estimate is approximately 300 tons of cooling on the chiller side per 1 megawatt of power draw. A typical deployment will require about 900 hours chiller usage in an average year when water side economisers are used. The water circulates radiators at 10.8 PSI with a flow rate of 244 GPM at in-let temperatures of 60F – 75F.

Impeller fans are incorporated into heat exchangers and generate and maintain constant airflow. Each heat exchanger assembly serves a pair of racks, and airflow is drawn through the servers into a common plenum area behind the rack, then drawn through the heat exchanger and expelled as cool air into a central aisle. This arrangement is designed to maintain the temperature swing inside the container, cold aisle and server environment at about 5F with an inlet temperature between 75–85F.

Operating at as high a temperature as the servers support is recommended, as this increases the number of free cooling hours per year and reduces condensation issues. Performance demands on the containerised environment far exceed those placed on conventional data centres.

The compact size and rugged construction requirements present a number of technical challenges that can directly threaten the operational integrity of the unit. However, these issues can be overcome with the correct approach and containerised data centers can present a valid and attractive solution for a wide range of applications.

Author: Geoffrey Noer, senior director of product marketing, SGI

Keywords: Power, cooling, container, fans, data center, AC/DC, chiller,