Natural Refrigerants in Data Center Cooling with Thermosiphon Application

Abstract: Ever since the computer was invented, there has been a need of data storage and the demand has strictly grown since. This has resulted in a huge amount of data centers and the trend has shown no signs of changing. The data centers are powered by electricity and in 2010 the electricity consumption for data centers stood for 1.3% of the world’s electricity usage. The most energy consuming part of a data center is the servers themselves, but the second largest energy consuming part is the cooling system which, in a normal data center, stands for two fifths of the energy usage. Besides the energy consumption, the cooling systems are in most cases a cooling machine using HCFC and HFC refrigerants. These refrigerants are all bad for the environment since HCFCs have high ODP and GWP values and HFCs have high GWP values. The purposes of this work is: A) Find a way to make the cooling systems more efficient. Previous work has shown that using free cooling from the ambient air is an effective method of reducing the yearly electricity demand. Further the systems use a two-phase thermosiphon to move heat from the servers to the ambient, which means that there is no need of pumping power. B) Find solutions using natural refrigerants that have no ODP and very low or zero GWP. C) Evaluated if there is a possibility to recover the waste heat from the data center to e.g. an office building. This work contains two systems being mathematically modeled with the software Engineering Equation Solver: a direct R744 system and an indirect system running with R290 and R744. Both systems are using a thermosiphon application, connected to a condenser, to use free cooling up to a certain set point temperature and the rest is covered with a vapor compression cycle. These systems are then matched to temperature profiles for five cities, Stockholm, Paris, Phoenix, Tokyo and Madrid, to see how many hours of the year are covered by free cooling. The systems are then evaluated considering both energy consumption and cost. To be able to compare these systems to a present cooling system, a reference system is modeled which uses R22 as refrigerant, that is the most commonly used refrigerant in the world today for the data center cooling application. The results show that a direct R744 system or an indirect system with R290/R744 with a thermosiphon application have both energy and economical savings compared to the reference system. The energy savings are up to 88% and the total annual cost savings are up to 69%. The Power Usage Effectiveness is reduced with up to 6% and up to 8% if only cooling is considered. These savings are for an optimized condenser with a 2000 m2 fin area and 6 fans with a set point temperature of 22°C. The indirect R290/R744 system is the best in all cities considering energy efficiency. Both systems are also well suited for use with heat recovery. The Seasonal Performance Factor for the heat recovery is between 8.3 and 15.2, which is a consequence of the high evaporation temperature and low supply temperature to the heating system.