Energy efficiency - Part 7 (How to calculate)

How to calculate energy and cost savings

Our series on energy efficiency continues. The need for more energy efficient systems emerges day by day driven by increasing environmental concerns. Today’s post will guide you through calculation of energy and cost savings when utilizing a VFD in flow control.

Energy savings and minimization of my carbon footprint is important to me. However, not everyone shares such mindset. In order to make energy saving technologies widely used they shall ideally bring cost benefits as well. And that is what VFDs do and it is one of the reasons for their popularity. Therefore, let’s have a closer look how to calculate energy and cost savings.

It was already well explained why and how the user saves energy and cost when utilizing VFDs in the drive systems (click HERE to quickly access previous posts on Energy Efficiency). Applications with quadratic torque load, such as fans, pumps, compressors and others heavily profit from reduced power consumption by even slight decrease of speed.

In order to get the funding for energy efficiency driven projects one needs to demonstrate the business case, expected energy and cost savings and eventually also reduction of carbon footprint. There are many god tools nowadays that allow you to quantify the savings potential in a transparent way. Let’s walk through it step by step!

1. Data of actual system

When comparing the existing system with the upgraded one you shall collect all available information about the actual system and the way it is operated. The quality of the input data directly impacts the accuracy of your calculation. The more mid/long term measurements and trends you have the more accurate is the calculation of energy saving potential.

If your system is hydraulic (pump), then you would enter information about the liquid and its density, volumetric flow as well as static and dynamic head. Pump efficiency shall be entered as well.

System data for pumps

– Type of liquid and its corresponding density

– Nominal system total head

– Nominal system static head

– Nominal system flow

– Pump nominal efficiency

System data for fans

– Type of gas and its corresponding density

– Nominal system total pressure

– Nominal system static pressure

– Nominal system flow

– Fan nominal efficiency

2. Operating profile

In order to quantify the energy and cost savings on annual base you need to enter information about operating profile. You shall know the amount of hours that the system is operational per year. If you don’t have exact data (e.g. records from past year) use approximate value or qualified guess. Of course, application running 24/7 likely has much more potential for energy savings than a system operating 3-4 hours per day or a system that does not even operate every day.

Next step is to distribute the total operating time according to flow condition. Such figure already gives a hint about the savings potential. The wider the control range of flow and the more time the application operates below nominal point the more energy savings can be expected. Example of the operating profile is shown below.

3. Reference control method

How is the flow controlled today? In this step you define the currently used method of flow control that the variable frequency control will be compared to (CFC = Compared Flow Control). Again, the better you know your actual the more accurate comparison you obtain.

Typical flow control methods for pumps (apart from VFD) are:

– Bypass (recirculation)

– Throttling valve

– On / Off control

– Hydrodynamic (fluid) coupling [2]

Typical flow control methods for fans (apart from VFD) are:

– Inlet guide vanes

– Inlet damper

– Outlet damper

– On / Off control

– Hydrodynamic (fluid) coupling [2]

4. VFD system parameters

In this step you will insert the parameters of considered variable frequency drive system. Important is the efficiency of the VFD. Either you know only the nominal efficiency and the calculation tool uses an estimation algorithm to get efficiency for other load points as well. Or you get the efficiency curve as function of load from the manufacturer and insert it in form of look-up table. The latter method is of course little bit more accurate. The VFD might have an integrated transformer [3] and the efficiency figures already include transformer losses. For VFDs with external transformer a common way is to enter the no-load losses and load losses for the nominal point. Based on that transformer efficiency and losses for arbitrary load point can be estimated.

Note that motor nominal power might be different for variable frequency control and for compared flow control.

Note also that motor efficiency in VFD duty might be slightly lower than in direct on-line operation. Professional tools allow you to consider all these details so that the calculation is as accurate as possible.

5. Economical data

Once we are done with technical inputs we start filling in the economical data. These inputs are used to calculate yearly cost savings and payback time of your variable frequency drive system.

Needed inputs are:

– Total investment cost

– Additional operating cost

– Energy cost

– Interest rate

– Service lifetime

The tool shall allow you to calculate two different cases:

A. Existing installation

You already have an installation with flow control (e.g. pump with a throttling valve) and your aim is to calculate the annual savings and payback time when installing a variable frequency drive system.

B. New installation

You consider a new installation and your aim is to compare the economy of the different solutions. In this case the payback of VFD can be especially short as the energy savings need only to cover the investment difference of the two solutions being compared.

Based on above inputs the tool calculates the annual savings and payback of your VFD system.

Note that VFD might not only save energy cost, but often requires less maintenance as well. This aspect can be considered under “additional operating cost” as mentioned above.

Results: energy and cost savings

Finally we come to energy and cost savings. The results include technical results (energy savings in kWh or MWh, savings of carbon dioxide etc) and economical results (annual cost savings, payback time, net present value etc).

Payback time is often in the range 2 – 5 years which is a very interesting result considering the lifetime of equipment to be 20 years or more. The total energy and cost savings over the whole life cycle can be significant. As mentioned, flow control with VFDs usually also saves a portion of maintenance cost as the system is to a large extent maintenance free. Modern VFDs support predictive maintenance strategies that ultimately increase the uptime and optimize the service intervals. Finally note that besides energy savings (kW) VFDs often help to reduce the consumption of reactive power (kVAr) as well. Refer to [4] for more information.

The results can be visualized in different ways to provide more insight. Figure above shows savings of VFD pump system compared to throttling (left side) and bypass (right side).

Summary

Topic of energy efficiency steadily gains more attention. Systems with high efficiency are not only good for the environment. They result in interesting energy savings and increased competitiveness. Not every system benefits from variable frequency control. However, most of the process flow systems do benefit from variable frequency control. We have shown you how to calculate energy and cost savings in flow control with VFDs. If you consider installing a VFD and are not sure about the economics, use one of the tools available or contact the VFD manufacturer. Shall you have more questions about energy and cost saving calculation please use below contact form.