The Drive for Energy Efficiency
Part 1: Variable-Frequency Drives Match System Output, Load Requirements
Part 2: New-Generation VFDs Offer Constant Power Factor
Part 3: Fans, Centrifugal Chillers Common VFD Applications
Part 4: Variable-Frequency Drives Reduce Maintenance, Extend Motor Life
Variable-Frequency Drives Match System Output, Load Requirements
By James Piper, P.E. - March 2009
Variable-frequency drives (VFD) are one of the most effective tools managers can use to control a facility’s energy use. As organizations struggle to curtail costs in every possible way, VFD applications offer some of the best available returns on investment. Simple payback for a VFD typically falls between six months and three years.
For years, managers have used drives successfully for a range of applications, such as reducing energy use in fans, pumps, chillers, and cooling towers by up to 50 percent. While VFDs have proven effective in reducing energy use, technology advances are making them even more appealing.
Making a Match
A building’s energy-using systems traditionally have enough capacity to meet the estimated peak load for that building, even though peak load typically occurs for no more than 1 percent of the annual hours of operation.
When a system operates at less than capacity, HVAC systems traditionally use a number of different devices to reduce the flow of air or water. This method of throttling output to match load requirements is effective in controlling the flow but is not the most energy-efficient strategy.
VFDs offer an alternative means of matching system output to load requirements by slowing the motors that drive HVAC-system components. Unlike throttling, where a motor’s energy use decreases only slightly with the decreasing system load, slowing a motor to match a decreased load results in a rapid drop-off in a motor’s energy requirements.
Comments
ydimitrov wrote re: Variable-Frequency Drives Match System Output, Load Requirements
on 8/4/2009 3:35:27 AM
What you say is interesting and seems like a fairly experienced point of view. I've never had an expirience with vfd chillers and have always thought that they are the better solution in general only because I've read meny articles about the vfd benefits
polyeng wrote re: Variable-Frequency Drives Match System Output, Load Requirements
on 8/2/2009 8:57:10 AM
This article is very misleading - the comment that chiller plants typically operate a full design load 1% of the time is true but if you have multiple chillers they can be staged so that they run 20%+ of the time at full load (for each individual chiller). Chillers running at full load, or close to, with a vfd, are less efficient than if they had no vfd (@ full load vfd's do nothing but suck up extra power - hence they make the machine less efficient). Vfd's do provide a "soft start" but so do wye-delta starters. One other thing about vfd's that is not mentioned is the usable life. Centrifugal chillers can run up to and beyond 30 years and the typical life of a vfd is MAYBE 7 years or so (MAYBE 10 years at best). So in the life of the chiller you will have to replace the vfd's 3 or 4 times. I am not totally against vfd's because in certain situations they do make sense, however, to make blanket statements that putting vfd's on chillers will greatly improve your energy savings is simply just not always justified for the end user. Every situation is different and all facts need to be weighed before making the decision of whether or not a vfd make sense.
