Variable frequency drives (VFDs) rank high in the arsenal of energy efficiency improvements recommended by experts. Why? If done right, a VFD can save a significant amount of money over the course of its life. If done wrong, VFDs will produce little or no improvement in system efficiency, or even worse, use more energy than what was there before! One key culprit of misuse is installing multiple VFDs. Most of the time, using multiple VFDs is just plain unnecessary – like having multiple remotes for all of your electronics devices, when one universal remote will do.
VFD Overload
Industrial customers that have chilled water or compressed air systems can often benefit from the installation of a VFD. As with electricity generation, there is often a base load (compressed air being used constantly) as well as a variable load (intermittent use by particular equipment). Installing a VFD to handle the variable portion of the loading (to operate as “trim”) will allow the base load equipment to run at full speed, which usually corresponds to peak efficiency.
Michaels Energy has seen numerous occasions where vendors, or customers, will say, “Yeah, but VFD compressors are so efficient it makes sense to get rid of the “old” equipment and have lots of variable speed compressors.” Not so fast. It’s important to remember that VFDs cost a lot of money, and are actually less efficient than most other technologies when operating near full capacity. Additionally, the savings don’t compound as expected. Having two really efficient part load compressors does not yield twice the savings. In fact, it may be costing more money!
The Proof
The system performance curves shown illustrate why using multiple VFDs does not payoff. The solid gray line shows the system power of two VFD compressors running in parallel (equally loaded). The dashed line shows one base loaded on/off compressor in conjunction with a trim VFD compressor. There is virtually no difference in system performance, but the single VFD system is overall more efficient. Additionally, the single VFD system costs 30-50% less money upfront.
A single VFD will work the best for most applications in most facility types. Just as with having too many remotes, save the headaches, complexity, and money, and just get one.
