Minoru Yoshikawa

Seeking an energy-efficient modular data center: impact of pressure loss on the server fan power

With the recent development of cloud computing, the amount of information to be processed in data centers has increased in recent years, which promotes the construction of new data centers. Modular data center, which is often packaged in standard shipping formats, has been introduced to construct and deploy data centers more quickly and less expensively than conventional data centers. To conserve cooling costs in modular data centers, a hot aisle/cold aisle arrangement of the IT racks is employed by dividing the top of the racks using the partition. However, dividing the top of the racks increases the pressure loss of the server fans, leading to the increase of the server fan power. To examine the impact of this effect on annual cooling power, we perform simulations of power consumption in modular data center that introduces outside air and louvers on the top of the racks. By varying the opening of the louver as a function of the outside air temperature and rack power, an operation to minimize the power consumption of the outside air fans and server fans is performed. We find that total cooling power in case where the louver is operated so as to minimize the total cooling power can be reduced than that in case where the louver is fully closed by about 5% (10%) for 10kW (20kW) rack power. We discuss that the popular method for calculating energy efficiency in data centers, i.e. Power Usage Effectiveness (PUE), defined by the ratio of total amount of energy used to the energy used in IT equipment, is probably incomplete metric. Therefore, we finally propose that a revised PUE that removes the server fan power from IT equipment energy, and it should be used to estimate the energy efficiency to avoid the wrong interpretation.

Feasibility study of two-phase immersion cooling in closed electronic device

Feasibility of two-phase immersion cooling in a closed electronic device such as a microwave transmitter is experimentally investigated. Two-phase immersion cooling directly cools multiple heat sources with various shapes which enables simple and flexible thermal design by fulfilling the existing electronic device with a coolant. The challenge is to reduce internal thermal resistance from heat sources to the chassis surface by effective thermal diffusion induced by two-phase immersion. The study covers selection of coolant and effect of coolant filling ratio. Then heat dissipation performance by single and multiple heat sources as well as enhancement by two-phase immersion with conduction materials are experimentally tested. Leaf spring and coil spring, both made of phosphor bronze, are used as the conduction materials and compared with rigid copper block in two-phase immersion. Experiment setup contains a vertical heating plate and an outer wall with heatsink separated by a gap space. Thermal resistance of basic two-phase immersion and enhancement by conduction materials are compared to provide design guideline for two-phase immersion application in a closed device.

Liquid-gas heat exchanger for low pressure refrigerant application

Liquid-gas heat exchanger has been developed for subcooling-superheating process of vapor compression refrigeration cycle. Refrigeration cycle utilizes low pressure refrigerant, which facilitates in easy piping installation and spill-proof operation. Experimental and numerical studies have been performed in order to quantify and understand thermal-hydraulic behavior of subcooling-superheating (liquid-gas) heat exchanger. As a result of low pressure refrigerant application, low volumetric flow rate in liquid domain can result in dominant natural convection and thus, poor heat exchanging performance. Also, high volumetric flow rate of low pressure gas refrigerant can increase gas side pressure drop. Both phenomenon were observed in conventional shell and tube heat exchanger application and resulted in system COP (Coefficient of Performance) degradation. This paper deals with development of efficient heat exchanger providing high heat transfer, designed for specific application, composed of multiple-fin and microchannel tubes type heat exchanger installed in shell structure. CFD (Computational Fluid Dynamics) methods are used for design optimization; leading toward improved system performance along with size reduction in comparison to shell and tube heat exchanger.

Phase change cooling with evaporative condenser at data centers

Chiller-less data centers are energy efficient but only available in cold climate regions. This is due to high thermal resistance between heat sink and ultimate sink (ambient air) and heat dissipation rate relies on environment temperature. The purpose of this study is to extend availability of chillerless data center with a rack rear-door heat exchanger in warm climate regions. Evaporative condensers with natural/forced circulation cooling system by phase change heat transfer were experimentally studied to reduce thermal resistance at the condenser. Furthermore, impact of heat exchanger configuration and effect of atomized water spray were studied. Experiment verified that thermal resistance reduction in the external side of the heat exchanger by water vaporization failed to appear when horizontal heat exchanger configuration was used. Approximately 40% reduction of thermal resistance of condenser was observed on the data center test facility by spray of atomized water particles, produced by pneumatic spray nozzle, on the vertical heat exchanger at wet bulb temperature of 15°C. The results suggest that energy savings can be achieved by the appropriate evaporative condenser usage that reduces running cost of data centers in warm climate regions.

Phase change cooling for energy-efficient ICT systems

Data center businesses are growing rapidly, along with their power consumption. Approximately one half of the power consumed at a data center is used to maintain the temperatures of ICT equipment and server rooms. Increasing the energy efficiency has been one of the most critical issues when building and operating these energy consuming infrastructures important for the modern society, and effective management of the airflow in data center holds the key to increasing the efficiency. The present study reviews phase change heat transfer and a cooling module for 1U servers developed to minimize airflow through these most commonly used type of equipment at data centers. Using the thickness of about 44mm, the phase change cooling module allows natural circulation of its coolant with gravity. The cooling module is so efficient that it reduces more than 30 percent of airflow than conventional technology, thereby reducing the cooling power by 70 percent. The application of this cooling module is verified numerically for a large-scale data center to reduce the airflow, and the power consumption is estimated to be reduced by more than 20 percent.

Management of thermal interfacial resistance for a reparable thermal interface

Thermal interfacial resistance is characterized with state-of-the-art thermal interface materials (TIMs), including a grease, a phase change material (PCM) and a graphite-enhanced sheet (GES). A test bench has been developed to control the holding pressure with a built-in heating and cooling system. Pressures and temperatures are measured at multiple locations along the interface to be able to examine the effects of imbalance on the thermal resistance. Results indicate that the GES has a low thermal resistance of about 26 K-mm2/W at 330 kPa while the phase change material has a value of 34 K-mm2/W at a comparable pressure. The grease has a low value of 20 K-mm2/W, but the interface would not be reparable. The PCM and the GES have a metal foil on one of the surfaces, allowing them to construct a reparable interface. The effect of imbalance in holding pressure has also been examined. An imbalance in temperature grows with pressure, and a mechanical inclination of binding surfaces relative to each other is suggested as a cause of the temperature variation.