For redundancy, almost all mission-critical facilities such as data centers are fitted with more air condition units than required. These units are most of the time heavily underutilized, where the fans within the units are still consuming energy circulating air without actually providing cooling. In more modern facilities such fans are equipped with variable frequency drives, which can reduce substantially the energy consumption if proper controls are implemented. While there have several solutions for controlling and optimizing such variable frequency drive operated air conditioning units, control systems without variable frequency drives (discrete on/off ACU controls) have not been addressed thoroughly. In this paper, we present a practical, distributed and automatic control method for such discrete air conditioning units. The technique includes several safety features and is based on dense environmental sensing and events like hotspots or device failures. We discuss this approach by way of example of a case study.

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