Introduction

During recent years, information communication and internet technologies are undergoing a dramatically fast development [1]. Thus the increasing demand for data processing caused a rapid growth in the data centers containing IT equipment, rack servers and related devices. It is reported that the total energy consumption in data centers doubled from 2005 to 2010 [2]. The IT devices are often of huge heat emissions and show high demands for temperature and humidity control in data centers, so that the cooling and air conditioning system is indispensable. According to a recent survey, the refrigeration and air conditioning system accounts for about 40 % of total electricity usage for data centers [3]. Therefore, in order to reduce the energy consumption of air conditioning is of great importance in energy efficiency for the whole data center. There are many effective ways to save cooling energy usage in data centers, such as indoor air distribution optimization, heat transfer enhancement for rack servers, thermal performance improvement of chillers and so on [4]. Zimmermann [5] applied the hot water system to refrigeration system in IT rooms and established the energy model. Ebrahimi [6] introduced different cooling systems according to working conditions and found that energy efficiency could be increased substantially through low grade energy recovery. Marcinichen [7] proposed the two phase cooling technologies and put forward the method to evaluate heat recovery ratio. In addition, free cooling is a novel and promising technology that can decrease the load ratio of electrical chiller and save cooling energy consumption accordingly, through making full use of natural free cooling source [8]. Because of the high efficiency and low emissions, free cooling technologies utilized in data centers causes more and more attentions during recent years. In this paper, four typical free cooling technologies and corresponding air conditioning systems are analyzed and compared, to show their mechanisms, main features, energy saving effects and applicable situations respectively. This work is of significance in guiding the design of refrigeration system with free cooling technologies for practical data centers.