Airside Economizer or Chiller Plant? What is the Best Solution?

Date posted: August 1, 2012  Posted by

Airside economizer systems are very popular these days and for good reason. Applied in the right design they can be very efficient. However, are they the perfect solution for all clients? No.

Likely, the first thing that comes to mind when data center managers evaluate a cooling solution is the climate. True climate has a big effect on airside economizer systems, but also on chiller systems. In our opinion, climate is a secondary factor in the decision process…the first is understanding your business. Of course everyone wants an efficient data center, but we first need to understand what the compute environment requirements are.

For example, are you an enterprise user building a redundant compute data center where failures are not critical and taking advantage of maximum server tolerances while pushing ASHRAE TC9.9 guidelines? Are you a financial institution where failures are not acceptable and the cost to run your data center is negligible compared to the cost of downtime? Maybe you are a colocation builder and need the flexibility to meet the requirements of both clients?

We do not advocate for either an airside economizer or chiller plant, but rather want to educate you on a few overlooked insights of both systems to help in making an informed decision.

Where do airside economizers shine? The obvious answer is in cool, dry climates, but as discussed earlier it starts with the data center environment. What climate range would you like to achieve in your data center? Are you building to tight tolerances of the recommended ASHRAE TC9.9 class A1 and A2? Or does your business allow for more risk tolerance and higher failure rates of class A3 and A4? Airside economizer systems optimally perform with warmer air delivery temperatures, but humidity is often overlooked. Even if your data center can tolerate elevated supply temperatures, managers need to pay attention to humidity levels as well. Keep in mind dehumidification doesn’t happen until you bring the air below saturation (mid 50s).

Also, note that in economization mode, fan energy is a big consumer. Even with elevated supply temperatures, if you keep the delta T (difference between supply and return temperatures) constant you have not saved any fan energy. You have only opened up the opportunity for greater number of free cooling hours. Or have you? With elevated temperature comes elevated humidity. For example, if you elevate your supply air from 75 degrees to 85 degrees, this might open up 500 hours, but most or all of those hours might also be too humid.

We as a community also tend to focus on cooling season. We make the assumption that we only need to focus on temperatures at or above our desired supply temperature. This could lead to a costly mistake. With cooler OA temperatures also comes lower humidity levels, resulting in the need for expensive humidification systems.

The major advantage of chilled water systems is that we can recirculate air in the data center and have tight control of the environment without having to worry about the outside climate. But they can’t take advantage of elevated temperatures or free cooling. Or can they? It still amazes me that in the same discussion of air or water we go from talking about delivering 75-85 degree air with an economizer then switch to discussing a chiller system with 45 degree water. A standard CRAH unit can deliver 75 degree air with 68-70 degree chilled water. So why are we making 45 degree water? With higher water temperatures your chiller plant is ultra-efficient and you have opened up your free cooling hours to close to that of your airside economizer.

The final overlooked area for saving is in delta T. As discussed above, because delta T of the air is controlled by server fans there is little energy savings to be had on fan hp. But in chilled water systems you can achieve the same air temperature delta T on the air coil with wider and wider delta Ts on the water temperature as long as you lower the supply temperature. For example, if you lower the entering water temperature by one degree you can increase your delta T of the water by two degrees. Higher delta Ts on the water mean less flow. In the water side economizer mode, taking advantage of colder available supply temperatures is still free and you can reap significant pumping savings as well. Also keep in mind that reducing the flow has a cubed reduction of pumping energy.

Therefore, if you are a user looking to take advantage of expanded AHSRAE guidelines and have a higher tolerance for failures, airside economization may be a good fit. However, if you have less flexibility in your data center environment or are building in either a humid or dry climate, chilled water plants with water side free cooling may be the better option.