The Energy Savings of Variable Speed Drives

A few weeks ago, I was sitting in a Certified Energy Manager training program discussing the popularity of variable refrigerant flow (VRF) HVAC systems in Singapore, where the consistently hot and humid weather is ideal for this specific variable speed drive (VSD) application. It got me thinking about other applications of VSDs and the conditions that impact their effectiveness, including demand profiles, building systems characteristics, and even regional climate. Regardless of these factors, the “magic” of VSDs is that they have applications with energy-saving capabilities to fit almost any situation and help protect the bottom line.

Understanding VSDs and Their Applications

Motors in fans, pumps and compressors use more electricity than any other device in a building. These fixed speed devices are designed to operate at a constant speed that’s determined by the electrical system frequency, which is 60 Hz (cycles per second) in the United States. Once a motor is constructed, the only practical way to adjust its speed is to change the electrical frequency, which can be done with modern technology such as VSDs. VSDs are devices that modify electrical frequency to vary speed and can be applied to any equipment or system component driven by a motor. 

In HVAC systems, VSDs can be applied to manipulate air or water flow. While there are other ways to achieve variable flows by using outlet dampers or inlet vane controls, they are not as energy efficient as VSDs. Using an outlet damper, or throttling, is like controlling your car by putting your foot on the gas and using your brake at the same time. You consume the same amount of gas only to go slower. VSDs, on the other hand, adjust energy consumption and system speed to match output demands. This is where the magic happens.

Variable Speed for Consistent Savings

I say that VSDs are like magic because they regularly achieve over 50% energy savings, which is generally more than most energy retrofits. Most of the retrofit projects I’ve worked on have found VSD applications to have paybacks of less than three years – and sometimes less than one year if a system is part-loaded frequently. Perhaps, even more important, is the fact that most buildings in variable climates operate their systems at part load for the majority of the year, which is when significant savings can be achieved.

Even buildings with relatively constant loads, such as data centers and hospitals, could benefit from VSD applications, including the VRF HVAC systems that are popular in Singapore. Instead of adjusting based on load demand, VRF systems use variable speed compressors to take heat from one area with a constant cooling load and move it to another space or process that requires heat.

To determine potential annual savings from VSD applications, we first need to estimate how much time is spent operating at common load profiles. Once we have the profiles, the affinity laws can be used to determine the annual savings. 

Conserve Resources to Protect the Bottom Line

As your business operating costs continue to increase, new ways must be found to protect the bottom line. Energy costs will continue to increase, but new technology and innovative applications of it can help manage these costs and perhaps even come out ahead. Adding VSDs is a cost-effective and profitable way to conserve resources.

Justin Hunt, PE, LEED AP, CEM, Mechanical Engineer

Justin Hunt, PE, LEED AP, CEM 
Mechanical Engineer

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