Hi All,

I am working to replace an on-site diesel generator that currently charges a bank (9) of 12V car batteries. The batteries mostly power measurement devices on location. I am hoping to replace the diesel generator with an expander/generator combination that uses on-site natural gas as the prime mover. The gas is being produced at 1,200 psi and is cut down to 100 psi to enter into a pipeline. The pressure drop is what spins the expander/generator. The generator connected to the expander is a AC 240V, 2,300 watt.

A big wrinkle in this process is that the prime mover has a variable pressure inlet. Most cases (~90% of the time) it is regulated down to 500 psi and allows the generator to spin at ~55Hz. However, depending on upstream processes the inlet pressure can drop to ~200 psi which causes the generator to spin at 20Hz.

I have two questions:

1) How would I go about using the generator to charge the 12V car battery bank?
2.) Would it be better to charge something else (lithium ion)

I know very little about electronics so I apologize if I am missing any major detail. Always happy to clarify more. Appreciate any help.

 

Do You actually need ~2400-Watts for "measurement-devices" ?

Why do You need ~108 to ~126-Volts DC at high Currents ?

The Batteries and the Generator probably don't care about the Frequency being generated.

The Generator only needs to produce around ~110% of
the actual amount of Power that your equipment uses on average.

How long do your Batteries last with zero Charging ?

What is the "worst-case" amount of time that the available Pressure remains at ~200psi ?
What is the "average" pressure over a period of, let's say, 1-hour, or 4-hours, or 24-hours ?

What Type of Motor is driving the Generator ?
Is it a Positive-Displacement type ?, a Turbine ?, Centrifugal ?, other ?
Was the Motor originally, specifically designed, as a "Power-Take-Off" type of device ?
Or, is this a cobbled-up "Rube Goldberg" type of arrangement.

"" allows the generator to spin ""
under how much Electrical-Load ?
How fast the Generator "spins" with no-Load is not useful information.
The RPM with the Generator under full-Load,
at XXX amount of Pressure-Delta across the Inlet and Outlet of the Motor,
is a very important indicator of the amount of Power available.
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It's possible but a little of the risky side for a person with little experience.

Charging batteries, so just do a AC to DC conversion to circumvent the AC power frequency shifting problems. You can do that directly from the generator output but it requires the proper DC rated switches, breakers and component to make it safe.

A simple rectifier and maybe a ripple filter should work with a output of ~340 VDC peak voltage.





Once you have the DC conversion, a high voltage DC input charge controller can handle the battery charging details.
https://www.morningstarcorp.com/products/tristar-mppt-600v/

PV Input Operating Voltage Range	100V to Voc = 525V

I would use a lithium iron phosphate battery (LiFePO 4 battery) bank of at least 24volts instead of 12volt batteries of any chemistry if you have the budget to make a long term energy investment.

 

Hi LowQCab - thanks for the response.

I don't actually need 2,300 watts. That's just the rating of the motor.

I am not sure where you came up with the 108 to 126 Volts, but I don't think I need a specific range.

The batteries will probably last a few days without charging. Never really tested it.

Worst case at 200 psi is probably 24 hours.
Average pressure is 500 psi for all scenarios. It is rare that upstream drops significantly.
Centrifugal expander is driving the generator
I wouldn't call it a Rube Goldberg. It was built with energy take off in mind, but could be considered a "trial"

At 1 kW of load the generator spins around 35Hz.

 

It's possible but a little of the risky side for a person with little experience.

Charging batteries, so just do a AC to DC conversion to circumvent the AC power frequency shifting problems. You can do that directly from the generator output but it requires the proper DC rated switches, breakers and component to make it safe.

A simple rectifier and maybe a ripple filter should work with a output of ~340 VDC peak voltage.

View attachment 300650



Once you have the DC conversion, a high voltage DC input charge controller can handle the battery charging details.
https://www.morningstarcorp.com/products/tristar-mppt-600v/

PV Input Operating Voltage Range	100V to Voc = 525V

I would use a lithium iron phosphate battery (LiFePO 4 battery) bank of at least 24volts instead of 12volt batteries of any chemistry if you have the budget to make a long term energy investment.

Thanks Nsaspook.

To make things as simple (and safe). Is there any of the shelf equipment you might recommend to solve the DC conversion?

 

Thanks Nsaspook.

To make things as simple (and safe). Is there any of the shelf equipment you might recommend to solve the DC conversion?

The only things that's close, off the shelf, is 600VDC solar combiner boxes with breakers and disconnects. A big one should have the space for a decent bridge rectifier.

I would highly recommend getting some local qualified help for this instead of random answers from the internet.

 

"" I don't actually need 2,300 watts. That's just the rating of the motor. ""
Motor ? or Generator ?
Motors are sometimes rated in Kilo-Watts, but usually in Horsepower.
Generators are always rated in Volts X Amps = Watts or Kilo-Watts,
and "may" have a Horsepower requirement listed.


I was assuming a series Battery connection ............
9-Batteries X 12-Volts = 108-Volts DC,
9-Batteries X 14-Volts = 126-Volts DC, ( when the Batteries are being Charged ).
Or, do You have all 9-Batteries connected in parallel ?

What is the actual Part-Number and Brand-Name of the Batteries ?

If You have 9-Batteries connected in Parallel,
then why have You done this ?,
Large numbers of Batteries in parallel can easily become problematic.

You need to actually measure
the maximum amount of Current that your "measurement-devices" require.
On average, and during peak-demand-usage.
Do the "measurement-devices" have a Specification-Plate listing the Power requirements ?

Do You have multiple "measurement-devices",
each with their own self-contained 12-Volt-Battery ?

What is the longest distance between the Batteries and the "measurement-devices".

Please describe, in detail, the environmental conditions that the Batteries are subjected to.

There may be a simple, and relatively inexpensive solution,
BUT ALL OF THE DETAILS MATTER.
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The only things that's close, off the shelf, is 600VDC solar combiner boxes with breakers and disconnects. A big one should have the space for a decent bridge rectifier.

I would highly recommend getting some local qualified help for this instead of random answers from the internet.

I certinaly plan on getting an electrician involved. I am just looking to learn before hand.

 

Why wouldn't something like this work?

https://www.amazon.com/MEAN-WELL-LR...pcontext&ref_=fplfs&psc=1&smid=A19U4HLR14OT9T

 

Why wouldn't something like this work?

https://www.amazon.com/MEAN-WELL-LR...pcontext&ref_=fplfs&psc=1&smid=A19U4HLR14OT9T

If you could find one that can handle a wide variation in frequency (DC to 65Hz range would be nice) , voltage 88-264 AC and outputs sufficient voltage (higher than the nominal battery voltage) and current (power rating close the generator output) to fully charge the batteries (most are designed to provide a stable output voltage that can be used to supply a multi-stage battery charger) it might work as the DC converter. It's pretty cheap to run a smoke test on one.

 

Thank again for the input.

Agreed on the price.

I have had other recommend this, but maybe try and get the ducks in a row first.

 

The output will usually charge a virtually flat 100 amp hour battery to 40% or so within 6-8 hours. The DC output varies according to the generators RPM. The generator won't "cut back" when the battery is nearly full, so you can't risk leaving the battery charging too long.

 

Thanks Barnett. Is that in regard to the Victron Multiplus? If so, is there something I can put inline after it to have it cut back/out when battery is charged?

 

Nobody knows what You are trying to accomplish yet.
Everyone is just "assuming", but assuming what I don't know.
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Haha!! I agree with LowQCab..

 

Apologies All.

I thought I answered all of LowQCab questions, but let me summarize what I am trying to do.

I have an expander with natural gas as the prime mover. It goes in at 500 psi out at 100 psi with 500 watt load.

This spins an AC generator around 55 Hz. The Ac generator is 240 V, 2,300 W, single phase. At 55Hz is produces around 220 V.

Currently there are solar panels and diesel generator charging a bank of 9 12V car batteries. I would like to swap out those things for the expand/generator using natural gas as the prime mover.

My questions are:
1) How would I go about using the generator to charge the 12V car battery bank?
2.) Would it be better to charge something else (lithium ion)

Going through LowQCab's questions below:

How long do your Batteries last with zero Charging ? I would assume a few days, but haven't tested or want to test that out.
What is the "worst-case" amount of time that the available Pressure remains at ~200psi ? Worst case at 200 psi is probably 24 hours.
What is the "average" pressure over a period of, let's say, 1-hour, or 4-hours, or 24-hours ? Average pressure is 500 psi for all scenarios. It is rare that upstream drops significantly.
What Type of Motor is driving the Generator ? Centrifugal expander is driving the generator
Was the Motor originally, specifically designed, as a "Power-Take-Off" type of device ? Yes
Or, is this a cobbled-up "Rube Goldberg" type of arrangement. I wouldn't call it a Rube Goldberg. It was built with energy take off in mind, but could be considered a "trial"
Or, do You have all 9-Batteries connected in parallel ? Series
What is the actual Part-Number and Brand-Name of the Batteries ? Not sure right now, but can find out.
You need to actually measure the maximum amount of Current that your "measurement-devices" require. Peak usage is around 200 Watts, but average is more like 80 watts
Do the "measurement-devices" have a Specification-Plate listing the Power requirements ? Probably, can find out.
Do You have multiple "measurement-devices"? yes
each with their own self-contained 12-Volt-Battery ? No, they feed from the bank.
What is the longest distance between the Batteries and the "measurement-devices". 50 yards
Please describe, in detail, the environmental conditions that the Batteries are subjected to. They are in an enclousure built specifically for batteries. I can find out more if this is important.

I hope this help. Thanks again for the support and patience

 

Finally we are allowed to know what all this was about and I called it right. Glad to see this one starting off the right way this time.

Now that there is actual information to work with, it sounds to me like the best way is to rectify the AC output into a capacitor bank and use a buck/boost converter to take any DC voltage out of the capacitor bank from ~60V to ~300+ and turn it into a stable/consistent output voltage appropriate for charging the batteries.

 

Haha - apologies - proprietary stuff at the time. I wasn't allowed to share exact details. I do appreciate you working with me originally as best you could.

Is there something off the shelf that can do what you suggest?

 

Is there something off the shelf that can do what you suggest?

If I were you I would start down the path illuminated by @nsaspook . The specs of the specific model he posted seem pretty close but there may be better fits. Your application is essentially the same as a wind power application so I would be looking at wind turbine charge controllers. They ought to handle the rectification, boost/buck, and battery charging algorithms all in a single unit.

 

The Buga-Boo with this project is finding or creating a High-Voltage-Battery-Charger
that will still operate with acceptable stability,
with a substantially varying AC-Input-Frequency and Voltage.
A ~130-Volt Battery-Charger is not a common "off-the-shelf" item.

The Batteries must not be overly discharged for long periods of time,
so the Charger must be able to supply substantially more Charging-Current, on average,
than the "measurement-devices" consume.

Every time that Power is converted from one form to another, there will be inevitable losses.
Sometimes these losses can be simply estimated
when there is plenty of "overkill" designed into the system.
Just how much "overkill" is available, must be determined by measurement and math-formulas,
unless experience indicates that there is a very good chance that there will be "substantial-overkill".
Vague descriptions need not apply.

Every specification, on every piece of interactive equipment, is a factor in a Math-Formula.
No specifications means no Math-Formulas.
No Math-Formulas means no definitive answers, just guesswork and estimations.

People who want to help others here will generally try to provide
guesses and estimations just to get a conversation going.
This is useful because the experience level of a Thread-Starter is usually a big-fat unknown.
It's quite common in these Forums that people think that
they have a "World-beater" Idea that must be kept "secret",
but "secret" ends the conversation, and results in disappointments all around.
This has gotten to the point that it is virtually a running-joke with
the regular contributors here, sometimes almost an expectation.

If anything about your project must be kept "secret", the result will be only "off-hand-guesses".
All specifications, of all related equipment, must be either,
documented,
measured,
or estimated as closely as possible,
to receive any sort of really useful answers, or recommendations.
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