By Mike Robertson, Summit Gear and Project Specialist
It seems that everywhere you look you can find different products that mention energy savings. How about discovering a tried and true method that uses data and independent testing to prove true energy savings? It sounds too easy, right? Let us take a look at variable speed drives (VFDs) and how they cannot only save energy but pay for themselves.
The first place to start is to examine the total amount of energy consumed and break down where the energy is being used. Looking at the total amount of energy consumed in the chart below, the United States uses about 20% of the world’s energy. Out of the 20% of the total energy used, 41% is consumed in the buildings sector (residential and commercial) and 30% in the industrial sector.1 Per the U.S. Department of Energy, 68% of electrical use is by motors and motor-driven systems.2
The majority of the buildings in the U.S. will have loads, which are motor-driven. Most of these loads are heating and cooling, which use motors to circulate both air and fluids. Per the U.S. Department of Energy Data Book (2011), 65% of the total energy consumed is by fans, blowers, compressors, and pumps. Since customers are concerned with the rising electrical cost of operating motors, it makes sense to use energy savings to help with the cost. One proven method to save energy and cost is to use VFDs for fans and pumps.
VFDs provide a technique to match load demand to speed. The by-product is reduced cost and increased productivity for the customer. VFDs control the speed of the motor and the horsepower needed to operate the fans, pumps, compressors, and blowers. Not only will the customer experience energy savings, but additional benefits as well, including longer motor life, increased power factor, and less audible noise.
The energy savings from using VFDs are based on the Affinity Laws (Fan Laws or Pump Laws) for fans/pumps and indicate that the air or fluid provided varies directly with the pump or fan rotational speed. The Affinity Law for centrifugal load state is:
N = Speed
Q = Flow
P = Pressure
HP = Horsepower
FLOW is proportional to motor speed Q2/Q1 = N2/N1
PRESSURE is proportional to motor speed squared P2/P1 = (N2/N1)2
POWER is proportional to the motor speed cubed HP2/HP1 = (N2/N1)3
Therefore, by reducing the speed from full speed to half speed, the power consumption reduces to 88%. Now let us look at the energy savings by using a drive.
In this example, we examine a 25HP fan working three shifts a day for five days in a week for one year. The cost of operating the fan at full speed for one year is:
25HP * 7.46 kW/HP * 6240hrs * 0.075 kWh = $8,728.00 per year at full speed
Let us say the fan does not need to run full speed (most of the time fans do not) but runs:
- 20% of the time at 100% speed
- 60% of the time at 80% speed
- 20% of the time at 60% speed
The cost of operating the fan with a VFD is:
25HP * 0.746 kW/HP * 1248hrs * 0.075/kWh *(1.00)3 = $1,746
25HP * 0.746 kW/HP * 3744hrs * 0.075/kWh * (0.80)3 = $2,681
25HP * 0.746 kW/HP * 1248hrs * 0.075/kWh * (0.60)3 = $377
TOTAL = $4,804
The annual savings for the customer = $8,728 – $4,804 = $3,924
The VFD by using the Affinity Laws provides the customer a method of energy savings and reduced electrical bills. The savings from the VFDs can pay for itself. A simple payback is calculated by:
Simple payback = initial investment cost/savings
Let us assume an enclosed 25HP VFD with bypass at 480V costs $3,263.00. Simple payback is:
$3,263.00/$3,924 = 0.83 years for simple payback for the VFD
Additional customer savings may be available with the local electric utility in the form of rebates by installing VFDs in the customer’s facility. Contact your local Summit representative to help you discover energy savings with VFDs.
 Nasir Khalid, “Efficient energy management: is variable frequency drives the solution,” Malaysia 2014.
 U.S. Department of Energy Buildings Data Book (2011), Chapter 1: Buildings Sector, retrieved from http://buildingsdatabook.eren.doe.gov.