David Wallace of Power Parameters Pty Ltd explains the difference between centralised climate control and in-row cooling.
Thousands and thousands of cores going at clock speeds at 4GHz to 8GHz, throwing off some 20 watts per core, demand effective cooling solutions. Apart from computer-based solutions to limit power (more on this further down) a choice needs to be made between centralised climate control and in-row cooling, or a combination.
Energy consumption in data centres is also a serious issue. Electricity tariffs are not likely to come down and therefore monitoring of energy as well as controlling it is important.
Server facilities being mission critical, require back-up. In the first place this is battery supplied. The temperature in the battery bank room needs to be kept under control for best performance.
For property developers and owners of commercial buildings, mandatory energy efficiency disclosure applies to whenever office space with a net lettable area of more than 2000 square metres is offered for sale, lease or sub-lease. The National Australian Building Energy Rating System NABERS released three Energy for Data Centres tools in 2013. At present, they are entirely voluntary. But don't count on this continuing indefinitely. The NABERS energy rating system for data centres is based on three parameters being processing capability in terms of number of server cores multiplied by clock speed in GHz and unformatted storage capacity in terabytes.
Data centres consume something of the order of three terawatt-hours annually (or roughly equivalent to the emission of three million tones of CO2 emissions annually) and energy costs can be as high as 35 per cent or more of the total cost of ownership - a powerful incentive for energy efficiency.
Basically, data centres are designed to handle peak loads but for large periods operate below peak, whereas all the support infrastructure including cooling fans and computer room air conditioning (CRAC) continue to run based on maximum processing power consumption levels.
At its most basic level, a CMOS gate transistor's power consumption is a function of the square of the rail voltage and clock frequency. By simple extension, the most important power consumption indicating parameter for a data centre is to know the number of server cores and clock speeds. Highly sophisticated programs allow adjustment for clock speed and even rail voltage depending on the computing tasks involved.
The fundamentals of energy conservation are in the detailed construction of the data centre rather than in the coefficient of performance (COP) of the CRAC unit or units. The energy consumption per unit area will fluctuate wildly between facilities but typically can be of the order of three kilowatts/m2 and the task is to carry the dissipated energy away as efficiently as possible.
At the most basic level, this involves separating hot and cold areas through the use of so called hot and cold isles, and the intelligent design of plenums, and sometimes double ceilings that serve as return conduits for CRAC units. As important is the design of racks and rack cooling but obviously for a given installation the technology is locked in. Even so, simple strategies such as the use of blanking panels for empty positions in racks (thus preventing mixing of exhaust air with incoming cooler air being drawn in by equipment fans) will increase efficiency.
However, the use of cooling-integrated server racks is becoming increasingly adopted, and can be combined with smaller, more energy-efficient CRAC units. The Citec HDHC integrated server racks incorporating the dual Citec Rack Fan with control display panel are designed for use in high power density computer rooms eliminating 'hot aisles' and by means of the highly efficient rack cooling.
The cool air supply, coming in through the perforated raised computer room floor, is directed to the top of the rack by means of a dual axial fan unit.
The hot air exiting from the server rack is then passed to the computer room cooling equipment. Pressure transmitters within the rack monitor airflow and possible obstructions. Temperature sensors monitor inlet and outlet temperatures and together with the pressure transmitters, activate alarms. The fan unit can include an LCD readout unit, or this can be separately mounted in the Citec server rack.
For very high energy density racks, the new Citec In-Row (CNR) units were developed and are designed to be located close to the heat source for effective heat removal. The units are available for both direct expansion and chilled water systems and can be an alternative or complement to room cooling units as complete cooling solution for high heat density data centre.
Conservation of energy can be achieved by separating cold isles, or hot isles, or/and exhausting hot air to the plenum. In this way, fan speed can be reduced and fan power requirements lowered. As an example, halving fan speed can reduce power by 80 per cent. The obvious way to achieve this is by means of inverter control. Bearing in mind that climate control for a typical data centre represents 30 per cent or often more like 50 per cent of total energy requirements, this is a fruitful area for improving efficiency. By controlling the return air, there is an opportunity to raise the temperature of the cool air and this again improves efficiency.
More efficiency is achieved through the use of the Citec CRAC's Genesis scroll compressor. Because the scroll suction and discharge take place during a full rotation (compared to a quarter rotation for piston-driven compressors) the flow of refrigerant is much smoother (i.e. reduced gas pulsations) with attendant lower noise levels and with very high efficiency (note: the scroll compression process has a volumetric efficiency of close to 100 per cent).
In conclusion, rack configuration is important - servers providing the path of least resistance in terms of air intake. The use of ceiling returns and plenum cold air supply offer the best opportunity for restricting energy cost of operating CRAC units. Of great importance is the under plenum cable and cable tray arrangements. Very frequently, the jumble of cabling under floor provides severe restriction to airflow.
