The principle of hot and cold aisles in the data center is simple. If you mix the cold air coming into a room with the hot air going out of a room, you waste a lot of energy. You have to pump more air round to achieve the same cooling, with increased energy costs for air movement (fans) and chillers.
With hot and cold aisles cold air is pumped directly into a cold aisle, which ideally is separated (curtains can be used) to prevent any movement of air into other parts of the data center. The air in the cold aisle is sucked by the fans in the equipment on both sides of the aisle through the equipment (equipment properly positioned!), and is blown out of the equipment into the hot aisles on both sides of the cold aisles. The hot aisles are also separated as well as possible to the extent that fire regulations and practicality permit. The air returns direct into the return plenum.
With small amounts of instrumentation ($75 x 4 for each aisle and few more to measure heat differentials between returning air and expelled air) any hot spots can be identified and air flow and air temperatures can be optimized.
Advantages:-
- Cooling is more efficient – less air needs to be pumped – lower energy costs
- The returning air is hotter, which makes it easier to use outside air (more efficient and more hours/year) which reduces energy costs a lot
- The average heat level of the computer room can be raised, which lowers energy costs even more (consistent temperature is important, not cold temperature)
- Can pay for itself in weeks if cooling capacity in the data center has been reached (of course, not the solution that Liebert sells – one customer of PG&E ordered a $4m dollar solution and then used hot & cold aisles to achieve the same results for $200k with no disruption
- Can be eligible for a utility rebate (e.g., PG&E in California)
Disadvantages:-
- The air blows hard on IT workers in the cold aisle – can be chilly and uncomfortable.
- It is much hotter in the hot aisle for IT workers
- Changes often opposed by IT workers, fire regulators and Liebert salesmen (talk to the first two)
Action Item: The three most effective ways to reduce budget and save energy costs in the data center:-
- Use outside air for cooling
- Improve air movement
- Virtualization
Plan to implement and instrument hot & cold aisles in 2008. Think green; Save green.
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