The Use of Outside Air Economizers In Data Center Environments
August 08, 2007
Appeared in ZDT Magazine
White Paper Synopsis
Compared to most types of commercial buildings, data centers are energy hogs. A large data center, for example, can easily consume as much electrical power as a small city. Consider an average size data center – say a facility with an average demand of 1 megawatt (MW) over the entire 8,760 hours of the year. The cost of driving that 1 MW computer load is $700,000 per year (assuming a cost of electricity of $0.08/KWH). The cooling load associated with the 1 MW load is 285 tons. At an average chiller efficiency of 0.5 KW/ton, the cost of running that chiller is approximately $100,000 per year. Of all the facility’s HVAC equipment, the chiller is easily the largest energy consumer. As such, significant HVAC energy savings can be realized by reducing chiller energy. The fact that the most energy efficient chiller should be selected for the facility goes without saying. However, an even more important issue is that reducing the number of hours of chiller operation can have a larger impact on the reduction of that piece of the building energy pie than simply selecting a more energy efficient chiller.
Syska Hennessy Group has formed a Green Critical Facilities Committee to address these very issues relating to sustainable design as they relate to critical facilities. Of necessity, the members of this committee represent the various industry specialties (engineering design, information technology, commissioning, etc.) in order to assure that a wide range of sustainable design elements are considered for every design. Our need (and especially our clients’ need) to incorporate more sustainable and environmentally responsible design elements – especially in a facility that consumes staggering amounts of energy every single hour of the year – must accept a double-pronged approach toward a cooling system design. On the one hand, more energy efficient equipment must be selected. On the other hand, a method of reducing the hours of operation of the equipment must be incorporated into the cooling system design. Bracketing this double-pronged approach is the absolute necessity to assure that the overall system reliability is never compromised.
There are two types of economizers which can accomplish a reduction in hours of chiller operation – waterside economizers, and airside economizers. Waterside economizers are explained briefly below; the remainder of this paper will concentrate on issues relating to airside economizers.
A waterside economizer uses the building’s cooling towers to cool the chilled water by taking advantage of the hours of the year during which the outdoor wet bulb temperature is sufficiently lower than the chilled water supply set point. In essence, rather than running the chiller during those hours, the cooling tower water is bypassed around the chiller and diverted to a heat exchanger so that it can cool the chilled water directly. This type of economizer has certain advantages and disadvantages, none of which will be addressed in this paper.