# The Magic of Variable Speed Drives

09/09/2014 |

Last week I was teaching the Certified Energy Manager program in Singapore, where the weather is consistently hot and humid. Nevertheless, variable refrigerant flow (VRF) HVAC systems are frequently used there. It got me thinking about applications of variable speed drives (VSDs) in data centers and other areas where the cooling load is more constant than in most buildings.

In this article, I will explain the magic of VSDs in regions where, like in the U.S., weather and cooling loads vary through the year. At the end, I will discuss the economics of VSDs in general as well as in applications where the building loads are more constant.

VSDs, VFDs and How They Work

According to the Energy Information Agency, motors in fans, pumps and compressors use more electricity than any other device within buildings. The majority of these motors are AC induction motors and this technology has not changed significantly in many years. The rule-of-thumb efficiencies are usually above 90%.

Motors, which are simply a collection of wires and magnets (pole pairs), are fixed speed devices. The speed (revolutions per minute or RPM) is determined by the equation below. Most of the variables are fairly constant:

Note that in the U.S., the electrical system frequency is 60 Hz (cycles per second).

Thus, after a motor is constructed, the only practical way to adjust the speed is to adjust the electrical frequency, which we can do with modern electronics known as “drives.” A variable frequency drive (VFD) is essentially a small computer that modifies the electrical system frequency to manipulate the RPM of a motor, which impacts the flow of air in an HVAC system. This is why VFDs are commonly referred to as VSDs.

There are other ways to achieve variable speed flows of air or fluids by using a magnetic clutch, outlet dampers, or inlet vane controls. Magnetic clutches offer similar performance when compared to a VSD. However, outlet dampers and inlet vane dampers are not as efficient as VFDs because using dampers (i.e., throttling) is basically analogous to controlling your car by putting the gas petal to the metal and, at the same time, controlling speed with the brake.

The Efficiencies of VSD

VSDs are magic because they can achieve over 50% energy savings, which is generally more than most energy retrofits! Although initially unbelievable to most people, this is true because the relationship between power and airflow is not linear as indicated by the so-called fan laws or affinity laws. For this article, we will only talk about the most relevant law, which is the 3rd fan law equation:

For example, consider a 100 HP HVAC fan motor that is controlled by a VSD. During a day when the temperature is mild, you can reduce the flow to 50% and still keep occupants comfortable. The equation below shows you what the new HP required would be. In this case, CFMnew = 0.5* CFMold :

Therefore, RPMnew = 0.5* RPMold

If we insert this into the 3rd fan law equation:

HPnew = HPold (RPMnew/RPMold)3
HPnew = HPold (0.5 * RPMold/RPMold)3
HPnew = 100 HP (0.5)3
HPnew = 100 HP (0.125)
HPnew = 12.5 HP

Although this is an incredible 87.5% reduction in power required, we must remember that the above calculation is based on the fan law curve and, like thermodynamics, we won’t achieve 100% efficiency or be exactly on the curve. However, as many experiments have shown, we can get very close to these savings. To be conservative, you can assume that you would achieve 90% of the estimated savings above.

It is important to remember that we only achieve the above savings when the system is operating at 50% flow, which is not year round, so we need to measure how much time we are operating at common load profiles (40%, 50%, 60%, 70%, 80%, etc.). Once we have the profiles, we can use the fan law equation to calculate the savings for each profile and then sum the savings to determine the annual savings. This task can easily be done on a spreadsheet. Software is also available to do this, but I like to check my numbers on a spreadsheet whenever I use software that might overstate the savings.

Paybacks and Cost Savings

Most of the engineers I have worked with have found VSD applications to have paybacks of less than 3 years, and sometimes less than one year if a system is frequently part-loaded. Because of these favorable returns, variable air volume (VAV) HVAC units are the standard design for most buildings today.

As I asked at the beginning of the article, would VSDs be useful in places that have relatively constant HVAC loads year round, such as data centers or buildings near the equator? My logical mind would say probably not. However, according to a study done by Lawrence Berkeley National Laboratory , VSDs achieved a 24% reduction within a data center and a payback of 2 years. So it appears worthwhile to investigate VSDs as an option even in a data center. However, note that maintenance will be higher with a VAV system.

It may also be wise to consider VRF HVAC systems, which offer superior performance at part-load and save energy because they do not bring in outside air (which must be conditioned). These are very popular in Asia, as well as in many data centers, which don’t have human occupants on a regular basis.  In addition, the VRF systems can take heat from a computer room and move it to another area that might need that heat in the winter season. This also has many applications, such as in hospitals, where a lot of equipment heat (MRI, CT and other machines) can be re-used in other areas.

I am sure the VRF trend will continue to spread in the U.S. in the coming years.

Greenberg, S. (2013) “Variable-Speed Fan Retrofits for Computer-Room Air Conditioners”, Prepared for the U.S. Department of Energy Federal Energy Management Program Technology, Case Study Bulletin By Lawrence Berkeley National Laboratory.

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Eric A. Woodroof, Ph.D., is the Chairman of the Board for the Certified Carbon Reduction Manager (CRM) program and he has been a board member of the Certified Energy Manager (CEM) Program since 1999. His clients include government agencies, airports, utilities, cities, universities and foreign governments. Private clients include IBM, Pepsi, GM, Verizon, Hertz, Visteon, JP Morgan-Chase, and Lockheed Martin. In August 2014, he was named to the Association of Energy Engineers (AEE) Energy Managers Hall of Fame.