Zinc Whiskers: Low-Cost Data Center Energy Efficiency Programs in Action
As a result, companies are looking for operating strategies that target the highest cost items and that give the biggest bang for the buck. In data centers, companies usually try to reduce the energy costs of providing HVAC.
I’d like to suggest some easy and low-cost strategies to help data center operators achieve greater energy-efficiency in their HVAC systems, along with a step-by-step discussion to help them consider as many opportunities as possible. These strategies are most effectively implemented in facilities operating with greater capacity than demand.
1. Controlling your environment allows you to turn equipment off. Simple but prudent changes to equipment operations take little time and effort to implement. For example, turning off CRAC re-heaters and humidifiers can pay off with reductions in both energy and maintenance costs. Some devices may need to be turned back on to remain within ASHRAE recommended limits for data center operations, so it is important to monitor local temperature and humidity readings after these changes are made.
On a larger scale, almost all of today’s legacy data centers can profit from the “miracle” of airflow controls. Heat removal and airflow strategies have improved dramatically over the last few years and are now recognized as the most influential single factor in defining the demand for data center air conditioning. In order to take advantage of the newest airflow strategies, it is important to maintain good control of the IT space and to implement as many of the following progressive improvements as possible.
At a minimum, IT equipment should be laid out in open-air racks with racks and servers in a hot aisle/cold aisle configuration. In this configuration, controls to minimize by-pass air such as plugs in floor penetrations and blanking panels in server racks can be very effective. These controls deliver cooler air to the IT equipment and reduce the volume of air required to remove the heat load. This reduction in chilled air is a strategic improvement that may allow you to turn off some CRAC units. Turning off CRAC units is one of the most aggressive methods of reducing HVAC energy. Not only can the energy needed to power the CRACs be saved, but the chillers and cooling towers that feed the CRACs can be turned down as well. This procedure should be implemented with some caution to assure that the necessary air pressures and volumes are maintained in all areas populated with IT equipment.
Next, damming chilled air in the under-floor plenum spaces below the unpopulated areas of a raised floor can reduce demand further. More CRAC units can be turned off as a result, along with the chiller and tower capacity that supports them. Paneling off vacant areas to contain chilled airflow creates a similar effect in IT spaces with overhead air delivery systems.
Finally, enclosing aisles with strip curtains or panels separates the chilled supply air from the hot server-exhaust air and prevents their mixing. That assures the delivery of air with predictable and uniform temperatures to all the cold aisles so the supply air temperature can be raised to the desired server inlet temperature...often twenty degrees higher than before. This is another significant and strategic improvement as the demand for chilled air is again reduced dramatically; this time allowing for higher air temperatures instead of lower air volumes. As a result of this change, chillers and cooling towers can be turned down even further.
Computational fluid dynamics modeling can be a valuable tool in predicting the changes in airflow temperatures and volumes that you are bound to experience when deploying these new airflow strategies in existing facilities. Monitoring the operating environment during change can confirm what the analytical models predicted.
2. Slow down the fans, control the pumps, and open up the windows. With airflow temperatures and volumes closer to an ideal state, equipment controls can be added to take advantage of the new environmental conditions while providing for more efficient and reliable operations. From the perspective of energy efficiency, variable frequency drives (VF) and economizers are possibly the two best investments that a data center owner can make. These controls systems remove much of the uncertainty inherent in the operating changes suggested above.
VFDs provide several benefits to systems operations. First, they allow equipment to operate at ideal speeds to more precisely meet the demand for cooling, instead of operating some equipment at full speed while other equipment is off. Operating more equipment at lower speeds to achieve the same total flow rates can provide surprising energy savings. And, the lower operating speeds reduce equipment maintenance costs as well as energy consumed. CRAC VFDs also provide a more uniform distribution of static pressure in the cold aisles.
VFDs are simple improvements that offer excellent returns on investment. Usually, they can be easily installed on most rotating equipment including chilled water CRACs, chillers, chilled water pumps, condenser water pumps, and cooling tower fans and recirculation pumps. VFDs and compressor controls are now being retrofitted onto air-cooled and water-cooled Dx CRAC units, as well. This is something that manufacturers often frown upon because of Dx CRAC design limitations, but appropriate safeguards can be taken to avoid problems involving freezing of the refrigerant.
Economizers, on the other hand, use outside air to remove the data center heat load instead of using the re-circulated air cooled by chiller plants. Economizers introduce cool outside air directly into the data center or via a heat exchanger, to remove IT equipment heat loads. In either case, economizer operations sometimes allow an operator to turn off chillers and cooling towers completely, and those energy savings can be huge. Economizers can be expensive propositions in an existing facility. But, when aisles are enclosed and supply air temperatures are elevated, economizers can provide a quick return on investment because they can operate for so many additional hours at higher temperatures with little or no support from a chiller plant.
Two other factors contribute to achieving good returns on investment with economizers. First is the selection of a cost-effective distribution system. Air economizers, for example, can be designed to deliver outside air directly into plenums that are already connected to existing distribution systems so that new metal ductwork is less needed. And last, but not least, electrical utility energy-efficiency incentives can turn an economizer project that might not otherwise considered into a very reasonable investment.
CFRT is a non-profit organization based in the Silicon Valley and dedicated to the sharing of information and solutions amongst our members made up of critical facilities owners and operators. Please visit our Web site at www.cfroundtable.org or contact us at 415-748-0515 for more information.