UPS (uninterruptible power supply) units are critical to keeping your data center up and running, but how should you choose among the myriad configuration options? As with most design decisions, the first step is to determine your goals and requirements for your facility, then to know what design options are available.
Last week, the Data Center Journal (“Basics of a UPS System”) discussed the elements of UPS systems and how they provide short-term power backup and (in some systems) power quality improvement. The article highlighted the inability of utility companies to provide consistently “clean” and uninterrupted power, thus leaving to data centers and other mission-critical facilities the task of ensuring that the power supplied to sensitive IT equipment be free of interruptions, spikes, sags and other deficiencies.
UPS systems break down into three major categories: standby (or offline), line interactive and double conversion (or online). Of course, different vendors will offer different variations of these major design types, depending on their target customers, applications and feature sets, as well as their desire for product differentiation. In addition to these major configurations, UPS systems differ in their energy storage approach, breaking down into two major camps: battery storage and flywheel storage. Batteries—something you’re probably quite familiar with—store energy in chemical form, whereas flywheels store it in energy of motion (a spinning wheel).
Given these options, how should you go about choosing the right UPS configuration?
Chances are you won’t make the “wrong” choice of a UPS system configuration. If you don’t do a little research beforehand, though, you may end up with a less than ideal configuration, however. Thus, the question of how to choose the right UPS configuration is better understood as how to choose the system most appropriate for your needs—and you can’t make a good decision if you don’t know what those needs are. Each option involves tradeoffs relative to the other options, and what works wonderfully for one company may be wholly inadequate for another. Here are some considerations to be aware of when you’re evaluating different UPS candidates.
- Budget. Although this is first on the list, it shouldn’t be your only concern. Know what your budget will allow for your UPS system, keeping in mind both capital and operational costs. UPSs, owing to less than perfect efficiency as well as the need to store some energy in the batteries or flywheel, will cost money to operate, and they will also (like all of your equipment) require maintenance. But don’t forget the adage “you get what you pay for”; skimping to save a buck or two now can cost you much more later on in damaged equipment and data center down time.
- Availability. Does your data center need to be running every millisecond of every day? Does even a minute of down time translate into tens or hundreds of thousands of dollars lost? If so, then your choice of a UPS system will be different than that of a company able to tolerate a couple hours of down time here and there, whether planned or unplanned. Your choice of a UPS configuration should align with your availability requirements, and you should set your budget according to the potential losses associated with a data center outage.
- Cooling infrastructure. Depending on your choice of a UPS system, you’ll be adding to your facility’s cooling load. For large data centers, even a percent or two of UPS inefficiency can translate into a lot of heat that must be removed to protect equipment. Can your existing infrastructure handled the load, or will your UPS necessitate an upgrade?
- Power quality. All UPSs provide temporary power backup in the case of an outage or sag, but does your equipment need more protection? What about power spikes, surges and other events? Not all UPS systems protect against these types of power quality problems. You can count on your utility providing less than ideal power, so if your equipment is highly sensitive, you should also consider power quality features in your UPS configuration choices.
- Space. UPS systems take up precious floor space in your facility, so make sure that your choice of a configuration won’t require you to add more room to your building.
UPS Design Considerations
If you know what you expect from your UPS, you’re in a better position to make an informed design choice. The above-referenced article covers the differences between standby, line interactive and double conversion UPS systems, as well as a few issues related to batteries versus flywheels. Here are some other design aspects to consider when evaluating your options.
- Redundancy. You have a temporary backup system (UPS) for your power supply, so why not a backup for your backup? If availability is a critical consideration for your design, then redundancy is a necessity. By adding backup UPS units, you are avoiding a single point of failure, thereby increasing the reliability of your power system. One common backup configuration is N+1 (if you need six UPS units to run your data center, for instance, than an N+1 design involves seven units); others include 2N (double the needed units), 2N+1 and so on. More redundancy generally improves reliability/availability (up to a point, anyway) but at the cost of more equipment (higher capital expenses), more floor space and (depending on the configuration) lower efficiency.
- Energy efficiency. Energy prices are high and getting higher. Right now, efficiency just makes good business sense; in the near future however, it may become a regulatory mandate. So, consider efficiency when selecting your UPS system. If you need the utmost protection, you may be willing to sacrifice a few percentage points of efficiency to gain higher reliability, but remember that for a double conversion system, all incoming power to your facility goes through your UPS. Thus, each percent of inefficiency wastes a major portion of your consumed energy—and it converts that energy into heat, which your cooling system must remove. You may need to strike a careful balance, therefore, between efficiency and protection.
- Environmental impact. If your company focuses on protecting the environment as part of doing business, then you might consider flywheels as an alternative to lead-acid batteries (the energy storage option for most UPS systems). Flywheels—which are nothing more than mechanical wheels that store energy by spinning—are a lower-impact option, and they also provide other potential benefits over batteries, such as less maintenance.
- Design complexity. Simpler designs usually tend to be less prone to human error and to independent failure, but they may also lack some of the features you’d prefer to see in your UPS system. For instance, the switch in a line interactive UPS is a potential point of failure not present in, say, double conversion designs. Furthermore, complex designs may require more maintenance (or simply higher maintenance costs) compared with simple designs.
- Power quality improvement. If you’re just looking for a little time in the event of a power failure or sag to shut down your systems properly or to switch to a more long-term backup system (e.g., diesel generators), then a simple standby UPS might be sufficient for your needs. But if you need protection against power surges and spikes, you need a UPS system that offers power quality features. A double conversion system, for instance, converts all incoming power to DC, then converts it back to AC for transmission to the load. This design essentially eliminates all power quality problems—an important feature for mission-critical systems or applications that involve sensitive IT equipment.
- Modularity. If you’re expecting your IT needs to grow, then you should consider a modular approach. “Buying forward”—purchasing more equipment than you need right now—will cost you in capital expense, storage space and potentially operational expense (depending on what you use). A modular approach allows you to add infrastructure as you need it, enabling growth without the waste of growing ahead of demand.
These are some of the broad considerations for purchasing a UPS for your data center. Of course, you will also have to navigate the various designs offered by vendors, the particular features of a given product and the cost of each model. But the above considerations—both for evaluating your needs and for evaluating UPS systems—will help guide you to the best choice for your situation.
Photo courtesy of Kecko.