Free cooling in data centers : Experimental test of direct airside economization with direct evaporative cooling

Abstract: The backbone of the expanding Information and Communication Technology (ICT)-sector are data centers. In these, Information Technology (IT) equipment is housed which provides computational power for e.g. cloud computing and internet services. Data centers consume massive amount of electricity, estimated to 1% of the global demand. All this power is however not used directly by the IT equipment, to keep the operating conditions in the desired range 21-61% of the electricity is used by the cooling solution. This is mainly due to the extensive use of vapor-compression refrigeration systems (VCRS) which provide a dependable cooling solution that works independent of climate conditions. To get around VCRS the concept of free cooling has been utilized in data centers, this can be done in many ways but the main idea is to introduce a natural cooling source without compromising the operating environment. Previous studies have showed that direct airside economization, i.e. using outdoor air directly in the data center have potential to reduce the energy demand of the cooling solution. This is however directly dependent on the outdoor conditions, by combining direct airside economization with direct evaporative cooling and recirculation of hot air from the IT equipment the cooling solution can handle a wider range of weather conditions and still keep the operating environment in desired conditions. Simulations of similar cooling solutions have been been done by Endo et al. and Ham et al. and showed promising results, but no study of an experimental setup have been published. To test how direct airside economization with direct evaporative cooling technology performs and find its characteristics an experimental setup was constructed, coolers with direct airside economziation and direct evaporative cooling was installed in a data center module at RISE SICS North data center ICE. The setup consisted of 12 racks of OCP Winterfell servers in a hot and cold aisle setup with containment, ducts on the ceiling connected the hot aisle to the coolers and made recirculation of hot air possible. A test schedule was developed to test the cooling solution in two of its four operating modes where the IT-load and setpoint temperatures where adjusted in predefined steps. The IT equipment consumed between 60 - 100kW and the facility power varied between 1.5 - 7kW, which results in a power usage effectiveness (PUE) value between 1.02 and 1.08. Compared to traditional VCRS systems these are very low values. By running the coolers in evaporative cooling mode the PUE was consistently lower compared to ventilation mode, the supply air temperature drop was up to 10°C while in cooling mode. The water consumption, and the corresponding water usage effectiveness (WUE) value was not measured or calculated due to limitations of the test rig that made long tests unstable. Direct airside economization with direct evaporative cooling is not the cooling solution for all data centers in all climates. But if the right conditions are present it is a simple cooling solution that without VCRS or heat exchangers (HEX) shows impressive PUE capabilities. Due to the psychical limitations of the system it can not handle high temperature and/or humidity levels, the data center either needs to be shut down, operated in undesirable conditions or complemented with a separate cooling system to operate in these conditions. To find the limits for this system supply air alteration and removal of exhaust air needs to be implemented. Due to the natural limitations of evaporative cooling combined with the ASHRAE guidelines the technology needs to be further researched to find what climate conditions it can handle. The water consumption which according to previous studies can be substantial also needs to be further studied.