How a VFD saves money and energy.

Drives are coupled with motor systems for one of two basic reasons: 1.) affinity laws promise greater energy efficiency when full speed operation isn’t required, or 2.) something about the process demands variable operation and control. Sometimes the needs are combined and the drive, or drives, become very important elements in a machine, building, water or factory system. This article will focus on energy efficiency.

Since the drive came into view as a green technology -- a tool to lower consumption and costs associated with energy -- the economics of drives have improved. For example, since 2001, the base costs of a drive, purchased, installed and operating, are flat or slightly down, while the electricity price paid by businesses is up.

As long as these two numbers move in the directions that they have been moving, more businesses will become drive owners, whether the people in them know what a drive is or not. It would seem to be a matter of necessity.

However, as competition increases, project budgets tighten, and economic trends remain volatile, small differences in energy performance matter. More importantly, service advantages among drive suppliers can be the difference between a selecting a smart, green system to start with, or regressing to full throttle operation in the interest of stretching limited construction funds. Drives are not ubiquitous, yet. In the U.S., 84% of motors are not yet drive controlled and in the E.U., 76% of motors are not drive controlled.

To try to understand why, we asked energy consultants to explain the challenges that they face specifying drives. The top two reasons: effort to predict energy ROI, and complex or custom application work.

Interestingly, those same energy consultants tell us that they struggle to prove energy payback concretely. While it is common practice among consultants to rely on drive suppliers to offer pre-sale predictions of energy performance and payback, it is rare that an installed drive is proven, through audit or performance testing, to be delivering on its economic promise.

More than 50% of consultants say that they have no idea if their projects payback and another 20% say that the payback estimate is proxy for compliance (but it shouldn’t be.)

So we think that 2012 will be the year of the audit. Watch as suppliers visit job-sites, parametric data, calculators and kW meters in hand, to show why drives are, and will be, crucial to reducing energy consumption, costs, and to lowering carbon footprint. And watch as consultants begin to demand that such services are included by, even required of, accepted suppliers in winning contracts.

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*We interviewed more than 190 mechanical and HVAC consultants and users of DASH Energy ROI Software in North America for this data.

In my last post, I took a look at the results of some research I did regarding variable speed drive efficiency and motor efficiency.  In this post, I’ll take a look at why designers, in particular, building system designers, are driven to apply variable speed technology in their systems.  This, combined with the previous post, will lay the foundation for the next post, where I will look at what happens when you apply a VFD to motor serving a centrifugal machine in an HVAC system.  Its mostly good news, but there are a few surprises.

Variable Speed and Building System Loads
For folks like me in the building industry, VSDs in general and VFDs in particular are just about always applied to a centrifugal machine like a fan or a pump or a centrifugal chiller.  To some extent, the ability to vary the speed of a centrifugal machine as the demand on it changes is “God’s gift” to the industry because for HVAC and other building systems, just about everything varies all of the time.  Here is an example.

What you are looking at is the daily and seasonal load profile for a facility in Southern California that we developed using the building trend data.

We were going to add chiller capacity and instead of doing a model, we let the building tell us what the best chiller size would be and what “sweet spot” we should target on its performance curve by picking hourly flow and temperature data (which we knew was reliable) and doing the math.