I know we focus in insulation and HVAC here, but I thought I'd mix it up a bit. I'm trying to make a policy recommendation to a town where about 85% of the homes have private wells of 75' to 175' deep. Electricity is hugely expensive ($0.40 /kWh and rising). I'm looking for info on what power savings might be expected by using a VFD driven well pump vs. a conventional pump. The town is very energy conscious and is considering requiring new and replacement pumps be VFD's but before doing that they want to know if VFD's provide a reasonable payback. Also, any drawbacks to the VFD's like pump life, maintenance, initial high cost? Any experiences, info or sources would be great. As always, thanks to all the great contributors on this forum - one of the best!

Never heard of so looked up VFD and came across this site - very interesting info - maybe something here to help provide a comparison for your customers?

http://www.pumped101.com/

I doubt there is much savings potential for typical amounts of water usage. I'd put the money into a larger pressure tank and set a smaller range on the high/low pressure limits. Less cycling and less peak pressure (equals less power).

Or just ban SUVs :-).

JonR
Thanks. I like both suggestions! Any idea where I could find some hard data on what a VFD performance might be - for better or worse? At our electric rates normal paybacks are cut by about 70% so even stuff that doesn't make sense elsewhere makes sense for us.
Once we get our off shore wind farm that'll change :-) .

Flow of  a centrifugal pump increases linearly with speed, however, the power draw of a centrifugal pump increases as the cube of speed; i.e. twice the speed requires 8 times the power.  Therefore, if your process only requires 90% of full flow, and your pump motor is driven by a VFD, it will only consume 73% of the power to deliver water at that flow rate.  At 80 % flow it is 50% of full power.

In a house, most demands will be for much less than maximum capacity, so the VFD driven pump makes sense.  You have to use a pressure transducer and set up the VFD to maintain a constant line pressure.

Drawbacks: requires a three phase motor, so you can't pop one of these on an existing pump.  There are some Mickey Mouse single phase speed controllers out there but don't use them, single phase motors aren't designed for variable speed operation.

Advantage:  in a contradiction, the three phase motor lasts a lot longer and often costs less than the single phase motor, so, in new construction, it's a better choice.
A typical industrial VFD can replace an "Add a Phase" i.e., can convert single phase to three phase power, you have to double the size of the VFD (use a 2 HP on a 1 HP motor) because the internal capacitors in the VFD are sized for three phase input power, and need to be bigger to work on single phase, due to something called "DC Bus Ripple".

Another advantage is that the VFD greatly reduces inrush, which is why the lights in your house flicker when your well pump starts.  You can start a 1 HP VFD driven well pump with a 1000 watt generator, whereas you would need a 3750 watt generator to start it normally due to inrush.

I sold a lot of VFD's in Georgia for use on well pumps, mostly on larger farms, but many ended up on houses.  It's difficult for me to believe that you could save that much money, but power wasn't $0.40 per KWH there.  Most people wanted them to get away from single phase pump motors.  One guy wanted one well to supply both his house and his catfish pond, so we ran a lot slower when he wasn't trying to top the pond off.

Depending on the depth of the well, you may require a long lead filter for the motor leads.  Also, be aware that many pump manufacturers size their pumps to run into the service factor of the motor, which is a 15% fudge for over current.  Using a VFD usually reduces a motors service factor from 1.15 to 1.00.  I usually just set the maximum speed of the motor at 58 hz instead of 60 hz, which was 99% of maximum flow but dropped the current a bit, enough to keep it out of trouble and the owners never knew the difference.

http://www.reliance.com/prodserv/standriv/appnotes/d7737.pdf

I hate to refer you to these guys, but this little white paper has a chart in it that will tell you how much power you can save.  There are better choice$ than this brand, though.

Most of the analysis of cost savings compares a VFD to a continuously running pump that is throttled. But that doesn't apply when a pressure tank is used and the pump is cycled. In fact, with a large enough tank, the pressure and flow could always be very close to the optimal efficiency values - ie, efficiency could be better than a VFD and pump with no pressure tank.

Tigerfan
Thanks for all the info - as a non-engineer I'll have to chew on that for a bit. Real quick, though - do I understand correctly that if 3P isn't available at the house a VFD is pretty much out of the question, or would it be OK with an up-sized VFD? Electric service here is not only expensive but it's bad quality, too. Lucky us. Lots of voltage drops and spikes, and I suspect frequency waivers a bit too - can VFD's handle that? For a residential installation, what would an "order of magnitude" all-in up charge be for VFD over a comparable quality "conventional" set up? I understand the VFD requires a much smaller pressure tank so there must be a bit of savings there.......
Again, many thanks. Geez, this is a great forum!

jonr: First off, I said that I doubted you'd save much, but if it takes 100% power to pump at 100% flow, but it only takes 50% power to pump at 80% flow, then simple mathematics tells you that for a given volume, if the 80% flow rate is acceptable, then you'd save 37.5% power. Whether or not this is significant on a well pump is debatable. My instincts say no, but it is there.

Birdman: You do not need three phase power to run a VFD. A VFD can convert single phase to three phase. You DO require a three phase motor though. Variable frequency on most single phase motors will reduce torque (and there are other issues), while a three phase motor produces constant torque up to its rated frequency. A VFD can handle a tremendous amount of variation on the input side, you can also add things like noise filters and line reactors on the input side. Frequency variances are no problem within reason, voltage spikes might be, but probably aren't. A simple solution is an isolation contactor on the input side that opens if the VFD faults. I used to program them to automatically clear a fault and attempt to restart.

In reality, if someone can't program a VCR or use satellite TV, they aren't a candidate for this. I doubt it would do anything for their power bill anyway. Many power companies push for them because they eliminate inrush issues from motors starting across the line. I doubt you'd save anything noticeable unless you were irrigating a large plot. That being said, if I had a well pump it would be on a VFD, but I can get them cheap and keep them running.

PS: They would probably make a lot more sense on a geothermal ground loop, in particular if you ran the VFD in PID mode and varied pump speed to control loop temp. Since ground loops run for longer periods of time, you'd realize more savings. All this assumes the use of centrifugal pumps. The characteristics of a positive displacement pump are different. With a positive displacement pump, flow and power are related linearly, so there is no power saving advantage by reducing speed.

Centrifugal pumps are most likely pump style to provide a favorable return based on energy savings when applied with a variable-frequency drive (VFD). While most centrifugal pumps operate at the fixed flow established by the hard piped "free system" needs,many systems require variable flow to meet changing process demands.


__________
SIPs

it only takes 50% power to pump at 80% flow

People often refer to this, but the math isn't quite that simple. It is only true if the pressure is also allowed to drop. But of course lower pressure isn't useful in the case being discussed (household water needs to always be around ~50 psi or there will be complaints).

Depends on the pump curve. If the pump can overcome static head at 80% speed, the savings are there. If it can't, you can't get any water out of your well and you will also typically burn up your pump motor because they usually require water flow for cooling, but let me restate: you probably will never be able to detect any energy savings on your power bill unless you fill your swimming pool daily. However, if your lights blink every time your pump kicks in, or water hammer is driving you crazy, you might be happy with a VFD anyway.

From what I'm hearing it sounds like the answer is "No." Just to be sure - the question was essentially this: In a town that is very energy conscious and most homes use private wells - would it be reasonable for the town to REQUIRE VFD's for new construction or pump replacement? It sounds as though the overall energy consumption reduction would not be large enough to make this a reasonable public policy. The town is considering a ban on the installation of irrigation systems for non-food plants. All this sounds draconian, I know, but this is a small island with a limited power infrastructure and a small sole-source aquifer. I estimate the private wells on the island pump about 90,000,000 gallons a year from an average depth of about 120' - What would be useful would be to know what total electrical power that used and what portion (if any) of that energy might be saved. Payback is really a secondary issue, although if it's ridiculous then the policy becomes too onerous.

Total should be around $60K per year at your electric rate. Electric savings for a typical residence - I'll guess low single digit %.

I suspect that solar panels or wind generators would have a much better payback than VFD pumps in residential use.

Thanks, Jon. Kinda what I suspected. I think my recommendation will be that Big Brother not get into this issue and focus on other , more fruitful measures.
Thank to both you and Tigerfan.

I am glad you finally came to this conclusion without me jumping in. I am very biased on this subject. Having studied electronics extensively, I was using VFD’s over 20 years ago. Back then we always used a valve as a bypass for the VFD. So when the VFD failed, we could continue providing variable flow rates by using the control valve. These systems could easily be switched back and forth from VFD to Valve control. The first thing we noticed was the amp draw or power consumption was virtually the same between VFD and Valve control. It seems the amp draw of a pump will naturally reduce basically the same when restricting a full speed pump with a valve, as when slowing it down with a VFD. So a VFD is just trying to trick a pump into doing something it already does naturally.

The second thing we discovered was that these systems were running on Valve control most of the time, because the VFD’s were not reliable. The fact that you have bad power in your area should be enough to throw out the entire idea of using a VFD.

The power saving theory of a VFD is a myth-understanding. Anytime you reduce the RPM of a pump, you are using more energy per gallon. Forget the cube rule, because you lose head by the square of the speed. So you can’t slow a pump down enough to save energy, or it won’t even get water to the surface, much less build the pressure a house needs.

Look at it this way. A 10 GPM, 1 HP submersible will deliver 10 GPM per horsepower. Reducing the speed with a VFD you might be able to drop the power consumption by half, but the flow rate decreases by 90%. So now you have a 1 HP pump, pulling a ½ HP load and only producing 1 GPM. This means a VFD is causing 500% more energy used per gallon than when the pump is running at full speed.

In the last 20 years we have made a successful business replacing VFD’s with Valves. It usually takes about 3 to 5 years, after several pump and VFD replacements, before a pump installer or the owner of a VFD realizes the mistake they made. Even though they have had many problems, they have also learned to like the constant pressure compared to the high/low pressure from a pressure tank only system. So a constant pressure valve is a good alternative to the VFD.

VFD’s provide “planned obsolescence” for the manufacturers. VFD’s are expensive equipment, that don’t last very long, are not repairable, and are hard on motors. The fact that the rumor mills make you think a VFD will save energy, and reducing end rush will make a motor last longer, is just icing on the cake for the manufacturers. A Constant Pressure Valve makes pumps last longer and reduces the size of pressure tanks, which is why pump companies are promoting VFD’s instead of Constant Pressure Valves. I am dedicated to providing these facts to prevent people from being taken. However I am still amazed at how many people do not believe that companies would do things like design in a short fail date for their equipment. Look around people. Everything you buy has a plastic gear or something designed to make it fail in short order, so you have to purchase another. The VFD maybe the best marketing gimmick anyone ever came up with.

Birdman, I'm curious about what other energy-saving strategies are typically employed when electricity rates are $0.40/kWh.

Residents here are pretty resigned to high bills for everything. Gas is over $4.00 this week. Oil runs about $0.40 a gallon more than the mainland and propane is much higher too (can't remember the latest price.) Not just energy but everything else, too - food etc. It also costs over $60 to "drive" to the next town ($50 one way on the boat for a car and $12 per passenger)

To answer your question, compact fluorescents are everywhere, and have been for years, folks are highly selective with appliances, using efficient fridges, gas stoves, ovens and dryers (and clothes lines) etc. and they are generally more conscientious in their habits. Scooters and bikes are popular and practical except in dead of winter. There is a high conservation ethos here - 48% of the land is protected in some way, people pay the added costs because they love it here and enjoy walking the beach in February with only the seals for company. That said, there are two communities here - year round and summer. The summer community tends to be, to be a bit crass, those who can afford to waste. Homes here are very expensive (starting at $800,000 for a minimal cottage that "needs work") and summer rents are very high ($2,000/week and up). Many of the summer folks "spend energy" like drunken sailors. Many of the renters are blissfully unaware as utilities are built into the rent. Having utilities in the rent is like an open bar - and everyone drinks more at an open bar.... Most of the construction is for summer homes and to my thinking, woefully inadequate from an energy standpoint. Often the view from a house is the primary driver that results in the ubiquitous "wall of windows". Once the price tag hits the owner (figure 25% to 35% over mainland costs plus the add for hurricane windows) they usually don't care to hear about additional insulation. Many summer folks keep their houses heated all winter even though empty except for a few weekends - which drives me nuts. Typical houses are "code compliant" for zone 5 (although Montauk Point 16 miles to the west is zone 4)

Winter temperatures are somewhat milder (5 to 10 warmer) than mainland as we're tempered by the ocean but the wind is killer - days on end at 25 - 30 are not unusual. I suspect that if you had to pick between air sealing or insulation, but not both, you'd do better with air sealing. Our Building Official is pretty good about insisting on it - to the extent the code requires it...... I'm not sure most homeowners realize what a favor he does them.

We are hoping (at least most of us are) to have an offshore windfarm shortly (first in US). The farm would have a capacity of 8 MW and the peak summer load is about 5 (winter can be under 1). So annualized we'd be 100% wind powered. There is a misalignment, of course, as th best winds are in the winter during the lowest load - but annually we'd be an energy exporter. Rates would probably go down to the $0.25 range - not mainland but better than $0.45 (we hit $0.68 when oil hit $140/bbl).

When the windfarm comes on I suspect we'll experience a bit of the "rebound effect" and conservation habits will "slip" a bit. But time will tell on that.....

Rates would probably go down to the $0.25 range - not mainland but better than $0.45

I was going to say that if that happened, then everyone could afford to "splurge" again, but you dealt with that in the very next sentence.

What about LED lighting? Does everyone chat about the latest or hasn't it hit the radar screens yet?