Cost Benefit Considerations for Data Center Economizer Mode Cooling

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In an earlier blog post, we explained when not to use economizer mode for data center cooling and there are metrics available to assist IT and facility management in this process, including cost benefit considerations, which we will cover first. In cases where economizer mode is deemed to be appropriate, it now comes down to the choice of which mode. There are two broad classes; air and water modes, each of which has three sub-classifications.

Air economizer modes use air conditioner bypass with evaporative assist via direct fresh air, via air heat exchanger or via air heat wheel. Water economizer modes use chiller bypass via heat exchanger, packaged chiller bypass via evaporative cooler and CRAC compressor bypass via second coil with evaporative assist – the first two in series with the chiller to allow for partial economizer operation.

At first glance, the capital cost of the entire cooling system favours computer room air conditioning (CRAC) compressor bypass mode at $2 per watt. Coming second is air conditioner bypass with evaporative assist via direct fresh air at $2.2 per watt, while bypass via heat exchanger appears to have the highest capital cost of all the other systems at $3 per watt due to the added cost of the cooling tower, piping, pumps, and custom control system. The design and implementation for such a control system represents a significant cost because most if not all of the individual components are sourced from different vendors which requires custom coding, testing, verification, and tuning to ensure the entire cooling system is reliable and realises the expected savings.

Capital cost includes all the materials, installation labor, design costs, and all project fees associated with the entire cooling system in terms of dollars per watt of IT load. However, there are myriad other considerations that come into play other than capital cost, including annual water consumption, annual maintenance cost of the entire system, total cooling energy, annual hours on full economizer mode, annual hours on partial economizer mode and annualized power usage effectiveness (PUE) which was referred to in the previous blog before last.

The chiller/cooling tower system is a very common cooling system used in data centers and serves as a good benchmark for other cooling system maintenance costs. Annual maintenance cost of the entire system as percent of a baseline traditional chiller/tower cooling system favours air economizer modes via direct fresh air and via air heat exchanger at 75% each, followed by via air heat wheel at 83%. CRAC compressor bypass comes in at 92%, while chiller bypass via heat exchanger and packaged chiller bypass via evaporative cooler is 100%.

The next blog will investigate the other metrics mentioned above, including annual water consumption.

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