Energy and Buildings 50 (2012) 81–92 Contents lists available at SciVerse ScienceDirect Energy and Buildings j ourna l ho me p age: www.elsevier.com/locate/enbuild Minimizing the thermal impact of computing equipment upgrades in data centers Jayantha Siriwardana a,∗ , Saman K. Halgamuge a , Thomas Scherer b , Wolfgang Schott b a Department of Mechanical Engineering, The University of Melbourne, Parkville, Victoria, Australia b IBM Zurich Research Laboratory (ZRL), Rüschlikon, Switzerland a r t i c l e i n f o Article history: Received 11 November 2011 Received in revised form 6 February 2012 Accepted 12 March 2012 Keywords: Data center energy efficiency Data center cooling Hot air recirculation Thermal aware equipment upgrading Load spreading Computational fluid dynamics (CFD) Particle swarm optimization (PSO) a b s t r a c t Upgrading of today’s air-cooled data centers (DCs) with high-performance computing, networking, and storage equipment is a challenging task due to typically higher power needs and more adverse cooling requirements of new equipment. To cope with the increase of the power and heat load in DCs, load spreading is commonly applied. This technique distributes the thermal load of new equipment over mul- tiple racks if the power requirement and heat generation of new equipment exceeds the rack’s capacity. We present a novel load spreading technique that allows upgrading of the computing equipment with minimal thermal impact on the existing optimized DC cooling environment. Our approach is based on an abstract heat-flow model of the DC, whose parameters are determined by performing a measurement campaign in the DC and with support of computational fluid dynamics simulations. The optimum place- ment of the new equipment in the racks of the DC is found by applying a particle swarm optimization technique to this model. The effectiveness of our method was assessed based on experiments performed in a production DC. The results show that our holistic approach for optimizing the placement of the upgraded computing equipment in the DC outperforms the conventional load spreading technique. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The successful operation of an air-cooled data center (DC) requires an efficient cooling environment to ensure that the DC operator can provide its services to customers with maximum availability and reliability at minimal operational cost. An efficient cooling system guarantees that the temperatures at the inlets of all devices in the computer racks of the DC never exceed a given threshold value to prevent device overheating, and achieves this goal with a minimum amount of cooling energy. In today’s DCs, a hot-/cold-aisle cooling concept is usually employed to efficiently cool the computing equipment [1]. For this purpose, the racks are arranged in rows to form hot and cold aisles with alternating airflows. In the cold aisles, chilled air from the computing room air conditioners (CRACs) blown through the raised-floor plenum and perforated floor tiles is directed to the device inlets, while in the hot aisles heated exhaust air from the racks circulates back to the CRACs. To ensure that the inlet temper- atures at the devices never exceed a given threshold value and no cooling energy is wasted, a nominal airflow and temperature dis- tribution is provided in the DC by adjusting various parameters of the cooling system (e.g. supply airflow, supply temperature), ∗ Corresponding author. Tel.: +61 425177770. E-mail addresses: jsat@pgrad.unimelb.edu.au, jay.siriwardana@gmail.com (J. Siriwardana). determining the proper placement of the perforated tiles in the cold aisles, and carefully selecting the best-suited placement of the computing devices in the racks. These steps of setting the DC cool- ing environment to an optimal operating point have to be carefully performed and successfully verified by using, for example, IBM’s Mobile Measurement Technology (MMT) [2] before putting the DC into operation. Due to the increasing demand for supporting new on-line ser- vices such as video on-demand, Internet banking, cloud computing, social networking, etc., DC operators are continuously compelled to upgrade their DC with high-performance computing equipment such as blade servers. These devices are usually smaller and process information at significant higher rates than their predecessors, but typically consume more power and thus dissipate more heat. This additionally generated heat should be efficiently removed from the devices to avoid creating hot spots and other cooling inefficiencies. Hot air exhausted from the rack outlets can recirculate into the cold air stream supplied to the rack inlets, causing the equipment to under-perform, malfunction, or fail all together [3]. To combat the undesired thermal effects of the upgraded equip- ment on the existing optimized cooling environment, various strategies can be applied: The DC operator can lower the sup- ply air temperature of the cooling system. This approach avoids device overheating, but will significantly increase the annual cost to be spent for cooling energy. Another approach, which is com- monly applied in practice, is load spreading [4]. To determine the best placement of the new equipment in the computer racks, the 0378-7788/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2012.03.026