Variable Speed Drives: Is it a viable energy saving solution for commercial and industrial users?

Introduction

Variable Speed Drive (VSD), also known as inverters or Variable Frequency Drives, is the term that describes the equipment used to regulate the rotational speed and hence torque of an electric motor. Basically VSDs are electronic devices that can be attached to a motor to fluctuate its speed through a control mechanism, such as temperature or pressure.

The concept is not new. Initially VSDs were developed for achieving better process control in the industrial sector and recently they have further developed as a successful solution for smaller industries and building systems due to their potential to save significant amounts of energy. It is possible to couple them with any motor exhibiting a variable load, but the most usual applications are pumps and fans that operate in industrial processes or as part of heating, ventilation and air-conditioning systems (HVAC).

Millions of motors are used by commercial and industrial users around the world, which, according to ABB Group, account for more than 65% of industrial electricity demand. During the design phase of a mechanical project, the exact motor loads is often unknown, this in turn leads to motor selection being oversized to safely meet the maximum system requirements. This leads to energy wastage as the motor operates outside its optimum zone. In order to prevent this and to achieve more control over operations and processes, many commercial and industrial users turn to airflow control vanes, two speed drives and other solutions. These solutions however are inefficient compared to VSDs from an energy saving perspective. In the following lines we shall describe, how VSDs can save energy and money for commercial and industrial users.

VSDs operation and energy saving principles

A VSD can reduce energy consumption of a motor by as much as 60%. This is due to the fact that they control the speed of the motor by altering the frequency and therefore the power supplied to it. Even a small reduction in the rotational speed can give significant savings in the energy consumed by the motor.

The next step is to understand in simple terms how altering the rotational speed of a motor can save energy. In order to do so we take a closer look to the so called affinity laws which are used in hydraulics to express relationships between the variables involved in the operation and performance of rotary machines such as pumps and fans. The following formulas apply to both axial and rotary flows, and are used to express the relationship between head, volumetric flow rate, shaft speed, and power. If we consider that the diameter of the impeller stays the same we have the following:

Where

  • Q is the volumetric flow rate (e.g. CFM, GPM or L/s),
  • N is the shaft rotational speed (e.g. rpm),
  • H is the pressure or head developed by the fan/pump (e.g. ft or m), and
  • P is the shaft power (e.g. W).

Now let’s come back to the declaration that only a small reduction in the rotational speed can significantly reduce the energy consumed by the motor. Let’s assume that the rotational speed, N1, of an industrial pump is reduced by 20%. This should mean that:

If we combine relationships (3) and (4) we get the following expression:

Consequently, a 20% reduction of the rotational speed leads to a 49% power requirement reduction. The explanation for the aforementioned relationships and therefore the energy savings achieved by VSDs lies in the pressure difference across the impeller. When less pressure is produced, less acceleration of air or fluid across the impeller is required. It is the simultaneous reduction of acceleration and pressure that multiplies the savings.

At this point we should clarify to the readers that a VSD does not constrain the rotational speed of a motor to a certain level in order to achieve energy savings. This should mean that the power input would be insufficient at several times. The main advantage of a VSD is that it can alter the rotational speed of a motor so that the power input can match the duty required and this way diminish energy wastage. In the graph below we can see in a simplified way how a system changes its operation after the deployment of a VSD.

In the case where the system operates without the use of a VSD, the power input remains constant regardless of changes in the load output over time, because the controlling device is a throttle or damper. When a VSD is used we the input power is tailored to suit the output duty. The throttle or damper is eliminated with savings in maintenance.

As we have already mentioned VSDs are not the only way to control an operation, therefore the reader may wonder why an industrial or commercial user should prefer this solution over others. The following graph shows how much more energy is saved by VSDs compared to that saved by traditional flow control methods that do not vary rotational speed.

Figure 1 Energy saved by VSD compared to traditional flow control methods

Building system saving example

In the following table we give an example for the energy and cost savings that could be achieved by using a typical VSD for a fan operating in the HVAC system of a hospital. We assumed that the fan is working for 15 hours a day and 7 days a week. The cost of electricity is considered to be £0.075 per kWh. Finally, we assumed that the savings achieved were around 25% which is similar to the savings observed by the HVAC system of Charing Cross hospital after the installation of VSDs.

From the above table it becomes evident that for large buildings where motors operate frequently the payback period could be really short.

Common applications and Summary

VSDs can provide significant energy savings in applications for Industrial users, HVAC systems and Leisure and Commercial buildings. Listed below are some typical applications for each sector:

Typical applications for Industrial users

  • Primary and secondary air fans
  • Boiler feed, chilled water, river water pumps

Typical HVAC applications

  • Variable air volume – air conditioning systems
  • Supply  fans
  • Exhaust air systems, such as dust extraction, paint shop exhaust, and fume cupboards
  • Heating and chilled water pumping, duty/ standby pump sets
  • Refrigeration systems
  • Some modern compressors and chillers

Typical applications for leisure and commercial buildings

  • Swimming pool pumps and ventilation
  • Sports halls, gymnasiums and dance studios
  • Fountains
  • Ice rinks

To wrap up, VSDs are one of the most cost efficient solutions that can reduce energy consumption, carbon emissions and electricity bills. Insulating a building gives a thirty year return of investment, while a VSD usually pays back an investment in less than two years. At this point it should be clarified that the good operation of a VSD is highly depended on the controls and sensors used, however this should be the issue of a future blog.


[1] ABB Drives and Motors Catalogue 2011,  ABB standard drives for fans and pumps

[2] The calculation has been made using the online GE motors calculator, http://www.gemotors.com.br/calculator/index.asp#f

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