Thermal management in legacy air-cooled data centers: An overview and perspectives

Depletion of fossil fuel reservoirs, greenhouse gas emissions' impact on global warming, and rising energy costs are pushing the data center sector to reduce energy use. This paper reviews strategies for improving the energy performance of air-cooled systems in datacom facilities and enhancing temperature and flow distribution in white space by analyzing different airflow delivery architectures (hard floor and raised floor designs), eliminating cold and hot air mixing by incorporating cold/hot aisle containment or exhaust chimneys, and potential energy savings achievable by utilizing evaporative cooling systems. It was found that the optimal ventilation system is hard-floor architecture with locally-ducted supply and return air. Hard floor is less complicated than raised floor design, and overhead supply air can minimize hot spots at the top of racks. Airflow management strategies including cold and hot aisle formation, aisle containment, and exhaust chimneys can reduce annual cooling energy usage by 10–50% in conjunction with air-side and water-side economizers, minimize hot spots, and enhance thermal performance during cooling system failure. Hot-Aisle Containment or exhaust chimneys provide better thermal and energy performance than open and/or cold-aisle containment, but they require new ducting in traditional data centers. Depending on the climate and geographical location, evaporative cooling can reduce annual cooling energy usage by 20–70% and lead to Power Usage Effectiveness as low as 1.06. Evaporative coolers are more suitable for dry climates, but this limitation can be ameliorated by incorporating desiccant wheels and thermal energy storage.