Why you may want to consider installing a VSD

Why should I install a VSD?

Motors that are used to operate fans, pumps, and air compressors typically operate at a constant load.  To control air or fluid flow, you can either mechanically throttle the motor—for example, a valve or damper—or you can electronically reduce the speed or frequency of the motor using a variable speed drive (VSD) or a variable frequency drive (VFD).

When you mechanically limit the output of a motor in these applications, the motor is still operating at full load and consuming the same amount of electricity, even though the amount of work that you get from it is being reduced.

However, when you electronically limit a motor, the resulting power consumption of the motor can be drastically reduced, benefiting from favorable scaling offered by fan and pump affinity laws:

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Which can be rearranged to make the calculations easier when you’re trying to calculate the new demand of your motor:

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So, if your existing motor is operating at 90% loading and has an electric demand of 5 kW, reducing the load down to 50% will reduce the motor’s kW down to 0.86 kW:

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This is an 82% savings on electric load over the existing case!  So yes, adding VSDs or VFDs to existing motors has the potential to save you a lot of energy at your facility.


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VSDs on Pumps

Many facilities are interested in replacing their constant speed pumps with either a two-speed pump or a VSD controlled pump.  But which one fits best? To demonstrate the difference between a dual speed pump and a pump with a VSD, the graph below displays a demand curve over a 20 hour period.  Both are providing the same total flow of water (in gallons), but the two-speed pump is cycling between high/low and on/off as a way to control the overall flow, whereas the variable speed pump adjusts its flow automatically according to the system’s demand.

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The Short Answer

Two speed pumps are appropriate if you have large amounts of storage available or your water demands are fairly constant or predictably at a ‘low’ or ‘high’ flow periods.  If you have a varied water demand or little to no buffer storage, a pump equipped with a VSD may be the best solution for you.

The Long Answer

The type of pump and controls that are most effective for your system depends on your water demand profile.  For example, conventional pump stations control their pumps by using high and low level tank controls.  When the tank water level reaches its low set point, the pumps initiate full speed operation to fill the holding tank until the water level reaches its maximum set point.  If there is enough water storage, the pumps may only operate for a few hours a day or intermittently throughout the day.  In this case, the best option may be a two speed pump.  The benefit of operating the pumps is found by working with pump suppliers to determine the make and model with best fit impellor size to find pump curves that maximize the efficiency of the two proposed flowrates with your system curve.  The idea is to operate at the best efficiency points (BEP) of the pump versus the best flowrate for your facility, which is possible if the required flowrates are predictable and there is enough storage room as a buffer.

A pump station that serves a receiving tank with a low amount of storage, or that has demands that range widely may benefit from a VSD-controlled pump.   With a small storage buffer, the pumps have to react quickly to meet demands or slow down to prevent overflow conditions.  The VSD would prevent excessive on/off cycling of the pump motor and decrease power demand (through the affinity fan laws) by automatically varying its flow to meet the requirements of the system.

VSDs on Air Compressors

A lot of the compressed air projects that we look at involve the replacement of a constant speed air compressor with a new comparable VSD unit.  But how do you know if your facility would benefit from a similar replacement project?

The Short Answer

Again, it depends on your compressed air demands.  If your existing air compressor is consistently loaded and your compressed air loads are fairly high and don’t vary much throughout the day/week, then you’re probably better off sticking with your constant speed unit.  If your existing air compressor is frequently switching between being loaded and unloaded, this could indicate that your compressed air loads have a lot of variation throughout the day/week, and switching to a VSD unit may be beneficial to you.  We usually put a data logger on the air compressor in question to see what it does when no one’s watching.

The Long Answer

VSD air compressors do save you energy—but only up to a certain point.  Below I’ve graphed the average electric demand for two air compressors: one 200 HP VSD compressor, and one 200 HP constant speed compressor.  They both have the same max flow rate (1,115 CFM), but the constant speed compressor must switch between operating at full load and then idling to meet any compressed air demands that are less than the maximum rated flow, whereas the VSD compressor is capable of easily modulating itself to meet lower compressed air demands.  As you can see, at very high loads, the VSD air compressor actually consumes a little bit more energy than the constant speed air compressor.  This is why if your compressed air loads are already near the peak flow capacity of the air compressor, you’re better off sticking with your constant speed unit.

Many facilities choose to have a constant speed air compressor as their base-loaded unit, with an additional VSD unit being used to meet their swing demand.  This arrangement capitalizes on the advantages that are offered by both the constant speed and VSD units and has the potential to save them a significant amount of energy.

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Need Help?

Energy efficiency projects such as VSD installations are some of the easiest ways to reduce your facility’s overall energy consumption.  To date, ANTARES has performed over 100 energy audits for a wide range of commercial and industrial clients.  The results of our studies have helped clients to identify areas of their facility that could benefit from efficiency projects, and help them prioritize their project implementation.

Climate Impacts of CNG Trucks

This press release showed up in my news feed today:  Natural gas versus diesel: Examining the climate impacts of natural gas trucks (http://www.eurekalert.org/pub_releases/2015-05/acs-ngv052015.php).  We have worked with fleets considering conversion to compressed natural gas (CNG) and have also ventured into lifecycle analysis, so I had to take a look.

The paper itself had not been published yet, but the authors found that switching big fleets of diesel vehicles to CNG may not have quite the greenhouse gas benefits that we usually hear about.  Why?  Because of energy consumption and methane (natural gas) releases during the extraction and production of natural gas.  As methane is a much more potent greenhouse gas than carbon dioxide, even small releases of methane have a large impact on the relative greenhouse gas intensity of fueling a vehicle with CNG.

Without reading the paper itself, I can’t say whether the authors’ assumptions and scenarios make sense, but the finding is very plausible, so  we will be keeping an eye on this discussion.  If you have any questions about greenhouse gas reduction or CNG as a transportation fuel, please don’t hesitate to contact us.

Using Ice Storage to Offset Peak Demands

Last year Southern California Edison solicited a proposal for 2,221 MW of new energy generation systems and 264 MW energy storage systems.  The utility is looking for ways to offset the peak electric generation capacity that they’re going to be losing by retiring some of their older nuclear and gas-fired generation plants.

The winning energy storage bids included a mix of battery storage and ice storage systems.  Ice storage systems use chilled water to build up banks of ice at night when electricity prices and electric demand are low, and then discharge the ice during the day to reduce a facility’s peak electricity consumption.  The facility benefits from lower electricity and demand prices, and the utility benefits from lower peak electric demands during their busiest times, effectively offsetting the need to replace all of those generation plants that they’re retiring.

We’re in the middle of a study involving ice storage as a method of demand reduction, so this article seems especially timely.

Considering an anaerobic digester with electrical generation? Some thoughts on ADG.

For the past year and a half, I worked as part of an Antares team to offer technical assistance to help pretty much anyone developing or operating an anaerobic digester project in New York State.  We were working for the New York Power Authority, and the goal of the effort was to help sites with potential digester projects “over the hump” by providing them with technical assistance at no cost to their projects.  It was a great assignment and the sheer breadth of the assistance we provided meant that I helped answer questions such as:

  • Does ADG make economic sense for my farm/manufacturing site/wastewater treatment plant/landfill?
  • Why, oh why, won’t my digester stop foaming?
  • How do I find off-farm materials to feed to my farm digester? What tipping fee can I get? What permits do I need?
  • What rate should I be valuing my electrical generation at? What about my demand charge—will it go away?
  • Two vendors are telling me two different things about connecting my generator to the grid, and the utility doesn’t agree with either of them! Help!

[Read more…]

The Virginia State Wind Forum

The American Wind Energy Association (AWEA) organized the Virginia State Wind Forum, which was held on June 3rd 2014, at James Madison University. This event brought together a variety of people with backgrounds including education, policy, business, and technical aspects of wind energy project development. The result was a comprehensive overview of the wind energy industry in Virginia.

Nationally, wind energy has experienced great growth. By the first quarter of 2014, the U.S. had an installed wind capacity of 61,327 MW, with another 13,000 MW currently under construction. In 2013, it contributed just over 4% of the total U.S. electric power generation, up from 1% of during the 1990’s. The Mid-Atlantic region of the country currently has over 2,000 megawatts of installed capacity; at the end of 2013, the regional rankings were as follows: Pennsylvania is leading the way with 1,340 MW installed, followed by West Virginia with 583 MW, Maryland with 120 MW, New Jersey with 9 MW and Delaware with 2 MW. In stark contrast, Virginia has yet to install a single utility scale project, although estimates by NREL rank it between West Virginia and Maryland in wind power potential. Below is a map of the wind resource in Virginia at 80 meters, which is the typical height at which utility scale projects are installed. Areas with wind speeds of 6.5 meters per second and higher are considered to have potential for development.

VA 80m Wind Map

Developers in Virginia have been working on wind projects in the state for over 10 years now. Programs like the State Based Anemometer Loan Program, administered by JMU, have provided valuable data that has been used to characterize the wind resource across much of the state. Developers have also instrumented their own wind resource monitoring campaigns at their targeted project sites. However, progress in actually building wind facilities has been slow.

Themes seemed to arise as speakers at the forum discussed obstacles to wind development in the state, as well as their perceived solutions to overcoming these barriers. Speakers at the forum found that one of the greatest obstacles to wind development in Virginia has been public opposition. A lack of communication between project developers and communities makes it difficult to dispel misconceptions about wind projects. Communities have opposed small scale wind projects for a variety of reasons, including issues related to viewsheds, birds, and noise. While these concerns may or may not prove to be valid in particular circumstances, without education about the success and benefits of wind projects, the community may fail to appreciate why this type of project is a worthy investment.

The other obstacle noted was a lack of policy incentives at the state level. Virginia currently has an un-enforceable Voluntary Renewable Energy Portfolio Goal. As a result, utilities in the state are choosing to purchase wind power from out-of-state as their best option. Until this changes, constructing utility-scale projects within the state will remain challenging.

Virginia has a very strong offshore wind resource, as shown in the map below. Several companies have publicly expressed interest in developing it, and received federal grants to support development. However, these projects will take time to develop. For example, Dominion’s Virginia Offshore Wind Technology Advancement Project (VOWTAP) recently received $47 million from the Department of Energy for the installation two 6 megawatt offshore turbines. Dominion predicts the project will be in operation by 2017. The end goal of that project, is in part, to determine how to economically provide offshore wind energy to customers.


Dominion will be using the research accomplished through VOWTAP for the development of a large scale offshore project. In September 2013, Dominion won a bid to lease a 112,800 acre site located three miles off the coast of Virginia Beach. There, the company plans to develop a wind farm with a capacity of up to 2,000 megawatts. Dominion will be submitting a plan for site characterization studies within the next year. Once approved, the plan will be carried out over a 3 year period. Within five years, Dominion will present a design plan to the Bureau of Ocean Energy Management for a National Environmental Protection Act Review. According to Robert Hare, a Dominion Business Development Manager who spoke at the Virginia Wind Forum, Dominion’s goal is to begin the building of the turbine within the next 20-25 years.

The obstacles to wind energy development in Virginia are certainly not insignificant. However, that does not make them insurmountable. There are multiple ways to make Virginia more attractive for wind development. The consensus at the Virginia Wind Energy Forum is that gaining public support is likely to be the most important step in moving small-scale projects forward. The next step will be to streamline the onerous permit-by-rule permitting process, which currently includes duplicative steps.


Special thanks to Ashleigh Cotting for attending the Virginia State Wind Forum and contributing to this blog post. Ashleigh is interning in the Harrisonburg office, and is a rising senior at James Madison University, with a major in Integrated Science and Technology.