ORIGINAL PAPER Determination of aerosol deliquescence and crystallization relative humidity for energy saving in free-cooled data centers L. Ferrero • L. D’Angelo • G. Rovelli • G. Sangiorgi • M. G. Perrone • M. Moscatelli • M. Casati • V. Rozzoni • E. Bolzacchini Received: 22 November 2013 / Revised: 7 May 2014 / Accepted: 23 September 2014 / Published online: 1 October 2014 Ó Islamic Azad University (IAU) 2014 Abstract This study examines an innovative application of the aerosol deliquescence and crystallization determi- nation, for corrosion prevention and energy-saving strate- gies in free-cooled data centers. Aerosol deliquescence and crystallization were investigated by combining standard- ized aerosol sampling techniques (i.e. EN-14907) with the assessment of the electrical effects of aerosol, while varying relative humidity within a specially designed aer- osol exposure chamber. Aerosol samples collected in the Po Valley (Northern Italy) were analysed; a clearly defined hysteresis cycle (deliquescence and crystallization at 60.5 ± 0.8 and 47.9 ± 0.7 % of RH, respectively) was found. Results were applied to a data center designed for the Italian National Oil and Gas Company, making it possible to identify a critical area for direct free cooling at this data center. As a result, aerosol hydration was avoided (thus preventing aerosol from damaging electrical compo- nents) and a large amount of energy saved (using free cooling instead of air-conditioning); the potential energy saving achieved in this way was 79 % (compared to the energy consumption of a traditional air-conditioning sys- tem): 215 GWh of energy was saved, and 78 fewer kt of equivalent CO 2 was emitted per year. Moreover, in order to evaluate whether a real-time estimation of the aerosol hydration state within a data center could be performed, measured deliquescence and crystallization were compared through simulations performed using three different mod- els: two thermodynamic models for deliquescence and a parametric model for crystallization. The results obtained tend to converge in terms of deliquescence, whereas in the case of crystallization, they failed to effectively simulate experimental aerosol behaviour. Keywords Conductivity  Aerosol chamber  Hygroscopicity  Models  Energy  Carbon footprint Introduction The level of aerosol hydration impacts a wide range of aerosol properties (both physical–chemical and optical) and processes (multiphase reactions, corrosion, etc.) and is of fundamental importance in several areas of aerosol research (such as climate forcing, visibility, atmospheric aerosol chemistry, energy saving in data centers, nuclear waste management, etc.) (Ferrero et al. 2011a, 2013; Di Nicolantonio et al. 2009; Randriamiarisoa et al. 2006; Syed 2006; Martin 2000; Martin et al. 2003; Rood et al. 1987). The aerosol hydration level depends on the relative humidity (RH) to which an aerosol particle is exposed in the atmosphere compared to the values of the aerosol’s deliquescence relative humidity (DRH) and crystallization relative humidity (CRH) in function of the RH history at atmospheric temperature (T). In particular, during a humidification process (i.e. an increase in RH), the aerosol is solid until RH reaches DRH: the aerosol then starts to absorbs water, producing a saturated aqueous solution; a further increase in the ambient RH leads to hygroscopic growth due to the continuous condensation of water. The Electronic supplementary material The online version of this article (doi:10.1007/s13762-014-0680-2) contains supplementary material, which is available to authorized users. L. Ferrero (&)  L. D’Angelo  G. Rovelli  G. Sangiorgi  M. G. Perrone  M. Moscatelli  M. Casati  V. Rozzoni  E. Bolzacchini Department of Earth and Environmental Sciences, POLARIS Research Centre, University of Milano-Bicocca, P.zza della Scienza 1, 20126 Milan, Italy e-mail: luca.ferrero@unimib.it 123 Int. J. Environ. Sci. Technol. (2015) 12:2777–2790 DOI 10.1007/s13762-014-0680-2