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01 June 2003

Maximize energy buck with efficient bang

By Alan Katz

Today's volatile energy costs preclude ignorance to high-efficiency UPS.

Facility managers are wondering what this summer will bring us. Are we again facing electricity shortages and compromised control equipment?

Our insatiable appetite for energy is not going to go away. Safeguarding equipment against the damaging effects of power anomalies is only part of the equation.

Selecting an energy-efficient uninterruptible power supply (UPS) not only will help protect one's facility from damaging power fluctuations—but will save money as well. The value of energy savings from using a high-efficiency UPS instead of a conventional one often equals the value of the UPS in as little as three to five years.

UPS Efficiency

SOME AMOUNT OF ENERGY ESCAPES

Energy efficiency of a UPS is the difference between the amount of energy that goes into a UPS versus the amount of useful energy that comes out of the UPS and actually powers loads.

In all UPS systems, some energy is lost as heat when it passes through the internal components of the UPS (including transformers, rectifiers, and inverters). The amount of energy lost between the input and output can be significant when one considers how much the wasted energy costs.

Energy efficiency advantages of as little as 1% between one UPS and another can translate to thousands or tens of thousands of dollars saved per year, depending on the size of the UPS.

Ten kilowatts of lost power may not seem like a lot of power; however, UPS loads operate continuously three hundred sixty-five days a year. Ten kilowatts (kW) of lost energy now equals 87,600 kilowatt-hours (kWh) of power each year.

At a typical utility rate of $0.10/kWh, this equates to $8,760 of energy wasted by the UPS and an additional $2,600 in extra energy costs just to cool the heat rejected by the UPS—a total of $11,360 in wasted energy.

No matter what UPS system you select, there will be some energy lost between the utility and the output, but high-efficiency UPS systems can dramatically limit the energy loss, resulting in substantial cost savings.

If the same UPS discussed was 95% efficient instead 90% efficient and losing only 5 kW of power as heat, the difference in energy savings would be more than $5,500 per year.

Considering that a typical 100 kW UPS costs approximately $25,000, the energy savings would pay for the UPS in five years.

GETTING A HIGH-EFFICIENCY UPS

When comparing the energy efficiency of UPS vendors, their published efficiency specifications may seem very similar, leaving one to wonder whether there is any difference between manufacturers.

An efficiency test is like test-driving a car to measure fuel efficiency. Just as cars get radically different mileage when driving on the highway versus a windy mountain road, UPS efficiency also can change with the type of load it powers.

Today almost all UPSs power 100% nonlinear loads—computers, servers, motors, and electronic equipment. Oftentimes though, manufacturers test their UPSs using linear loads.

UPS efficiency is often much higher when powering linear loads. Thus, it is wise to seek an eyewitness test on a load similar to the one the system will serve.

The second major factor to influence UPS efficiency is the power level at which the efficiency is measured. As a car has its best mileage operating at about 60 miles per hour, most UPSs typically have their best efficiency operating at 50% to 100% load level.

In the real world, most UPS systems operate at 25% to 60% of their nominal load—not fully loaded. To determine accurate efficiency, the UPS should demonstrate efficiency at loads between 25–50%, where most UPSs will likely be operating, especially if operating in redundant configurations.

A PERFECT SINE WAVE

The principal area where UPSs waste energy is in switching losses in the inverter and transformers.

To minimize switching losses, a digitally based power management system optimizes the switching characteristics of the UPS inverter for specific load types and load levels by precisely controlling every pulse of the switching cycle.

This results in the most efficient switching patterns with the least loss, outperforming older-style systems with fixed-switching patterns.

The system operates by creating the output waveform from hundreds of small but tightly controlled pulses. The inverter output is constantly compared to a computer-generated theoretical sine wave, and the pulses make corrective microfrequency adjustments to keep the output waveform to within a few percent envelope of a perfect sine wave.

By actively contouring the output to the load conditions, the need for power hungry output filters is eliminated, maintaining a higher efficiency and delivering a perfect sine wave output.

Other energy saving features are high-efficiency transformers that meet or exceed national standards for high-energy efficiency as required for the Energy Star certification.

High-efficiency transformers can offer a 2–3% overall efficiency advantage over a generic lower-efficiency transformer. IT

Behind the byline

Alan Katz is a manager at Costa Mesa, Calif.-based MGE UPS SYSTEMS. Write him at alan.katz@mgeups.com.