Rack-level study of hybrid cooled servers using warm water cooling for distributed vs. centralized pumping systems

In the wake of ever-growing demand for power and energy across US and worldwide, development of energy efficient solutions has become very important. Considering data center applications, cooling power consumption constitutes significant part of the overall energy usage of the system. In the process of optimizing the energy consumed per performance unit, liquid cooling has become one of the key solutions. In this study, 2OU (OpenU; 1OU = 48mm) web servers are tested in a rack level and the effect of higher inlet temperatures in terms of IT and cooling powers, and internal component temperatures are reported. The study serves as a comparison for two different coolant pumping systems i.e. distributed vs. centralized systems. The cooling set up includes a mini rack capable of housing up to eleven liquid cooled web servers and two heat exchangers that exhaust the heat dissipated from the servers to the environment. Each server is equipped with two cold plates cooling the CPUs while rest of the components are air cooled. The configuration that consists of cold plates with integrated pumps is referred as distributed pumping system. Whereas, the configuration with no integrated pumps at cold plates and only has two pumps placed in series with heat exchanger at the rack is referred as centralized pumping system. To study performance characteristics such as device temperatures and power consumptions of server components, synthetic load has been generated on each server using stress-testing tools. The servers are tested for higher inlet temperatures ranging from 25°C to 45°C which falls within the ASHRAE liquid cooled envelope, W4 [1]. This current work is a follow-up study to the analysis conducted comparing centralized and distributed pumping [2].