Making Large UPS Systems More Efficient

Maki ng Large UPS
Syste ms More
Effici ent


By Victor Avelar
White Paper #108


Executive Summary
As energy resources become more scarce and more expensive, electrical efficiency is
becoming a more important performance factor in the specification and selection of large
UPS systems. There are three subtle but significant factors that can materially affect a
company's cost of operating a UPS system and particularly the electrical bill. Unfortunately,
the people who specify systems often fail to recognize these factors, which leads to
increased costs to the owner because operational efficiencies are not correctly considered.
This paper discusses the common errors and misunderstandings in evaluating UPS
efficiency. UPS efficiency curves are explained, compared, and their cost implications
quantified.
©2006 American Power Conversion. All rights reserved. No part of this publication may be used, reproduced, photocopied, transmitted, or stored in any retrieval system of any nature, without the written permission of the copyright owner. www.apc.com WP108 Rev 2007-1 2Introduction
The traditional approach to the specification and selection of UPS systems has focused almost solely on system reliability, as represented by the mean time between failure (MTBF) provided by manufacturers and consulting engineers. Two issues are now conspiring to move efficiency, as much as reliability, to the forefront in UPS evaluation: (1) a focus on total cost of ownership (TCO) over the lifetime of the system, and (2) public and private environmental initiatives, as exemplified by "green building" certification programs and demand-side management programs offered by utility companies. There are two major contributors to UPS inefficiency: the inherent losses of the UPS modules themselves, and how the system is implemented (i.e. right-sizing, redundancy). Oftentimes, when specifying UPS systems, the only efficiency value considered is the best case value published by manufacturers. This is misleading and will be explained further. A hypothetical example is perhaps the best way to demonstrate how this practice can have a material effect on a company's electrical expense. Consider two 1 MW UPS systems from two different manufacturers. UPS system 1 and UPS system 2 have identical published efficiencies (93% at full load), are operated in a 2N architecture, use an electrical cost of $0.10 / kW hr, and support a 300 kW load. Many would argue that there would be no difference in the annual electrical cost of operating these two systems. This is a flawed statement except for emergency or maintenance scenarios, UPSs are never operated at a 100% load level in a 2N configuration since each side of the "N" has to be capable of supporting the full load if one side fails. Therefore, the maximum design load on each UPS in normal operation cannot exceed 50%. In reality 2N systems rarely achieve even 50% load on each system. Some field surveys indicate that 2N data centers 1operate at 20-40% of their 2N capacity. For this example, a typical 30% load is assumed, where each UPS supports 150 kW. Each UPS in system 1 incurs an annual electrical cost of $10,470 in power losses vs. $28,322 for each UPS in system 2. Since there are two UPSs in each system, the electrical losses are doubled to $20,940 and $56,644 per year, respectively. These UPS losses manifest themselves as heat which must be removed by the cooling system. Assuming each kW of heat requires 400 watts for the cooling 2system to remove it, an additional $8,376 vs. $22,651 per year is required . In this example, a typical data center lifespan of 10 years, results in a total cost of UPS system losses of $293,165 vs. $793,021 as shown in Table 1. So, how is it that the electrical losses between two seemingly identical UPS systems can differ by almost a factor of three? Table 1 - Two different systems at the same load in a 2N architecture incur different costs
UPS System UPS Loss Cost Cooling Cost Annual Cost of Inefficiency 10 Year Cost of Inefficiency UPS System 1 $20,940 $8,376 $29,317 $293,165 UPS System 2 $56,644 $22,651 $79,302 $793,021
1 A typical UPS load in a data center is about 30... [download for more]