Two-Phase On-Chip Cooling Systems for Green Data Centers

Abstract

Cooling of data centers is estimated to have an annual electricity cost of 1.4 billion dollars in the USA and 3.6 billion dollars worldwide. Currently, refrigerated air is the most widely used means of cooling data center’s servers. According to recent articles published at the ASHRAE Winter Annual Meeting at Dallas, typically 40% or more of the refrigerated airflow bypasses the server racks in data centers. The cost of energy to operate a server for 4 years is now on the same order as the initial cost to purchase the server itself, meaning that the choice of future servers should be evaluated on their total 4-year cost, not just their initial cost. Based on the above issues, thermal designers of data centers and server manufacturers now seem to agree that there is an immediate need to improve the server cooling process, especially considering that modern data centers require the dissipation of 5–15 MW of heat, and the fact that 40–45% of the total energy consumed in a data center is for the cooling of servers. Thus, the manner in which servers are cooled and the potential of recovery of the dissipated heat are all more important, if one wishes to reduce the overall CO2 footprint of the data center. Recent publications show the development of primarily four competing technologies for cooling chips: microchannel single-phase (water) flow, porous media flow, jet impingement cooling and microchannel two-phase flow. The first three technologies are characterized negatively for the relatively high pumping power to keep the temperature gradient in the fluid from inlet to outlet within acceptable limits, i.e., to minimize the axial temperature gradient along the chip and the associated differential expansion of the thermal interface material with the silicon created by it. Two-phase flow in microchannels, i.e., evaporation of dielectric refrigerants, is a promising solution, despite the higher complexity involved. The present chapter presents the thermo-hydrodynamic fundamentals of such a new green technology. Two potential cooling cycles making use of microchannel evaporators are also demonstrated. A case study was developed showing the main advantages of each cycle, and a comparison between single-phase (water and brine) and two-phase (HFC134a and HFO1234ze) cooling is given. Finally, an additional case study demonstrating a potential application for the waste heat of data centers is developed. The main aspects considered were reduction of CO2 footprint, increase of efficiency (data centers and secondary application of waste heat), and economic gains.