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Battery Charging using Variable speed generator
2

Battery Charging using Variable speed generator

Battery Charging using Variable speed generator

(OP)
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.

RE: Battery Charging using Variable speed generator

2
Trade the generator for solar panels.
Joking aside,
If the generator is only spinning at 20Hz to 50 Hz at no load, I doubt that you can get much energy out of it.
Replace the generator with an automotive alternator and voltage regulator.
Using an AC generator on a variable speed drive poses several challenges.
Those challenges were solved over 50 years ago with automotive alternators.
And, surprise surprise, the output voltage is optimum for charging lead acid batteries.
If you belt drive the alternator from the turbine, you will be able to better optimize the efficiency by changing ratios.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Battery Charging using Variable speed generator

To add to waross's comment, there are brushless alternators that will give you industrial level duty/reliability/service life. The Delco Remy 40si alternator exceeds your current system ratings.

RE: Battery Charging using Variable speed generator

How are you dealing with the drop in temperature of the gas?

But yes an alternator was my first thought.

Our just cut out the charging for the 10% of the time it isn't at 500 psi.

Normally you just convert ac to dc using a set of battery chargers.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Battery Charging using Variable speed generator

Some challenges to overcome with a conventional generator:
The Automatic Voltage Regulator should hold the voltage up, regardless of the speed, but it won't.
At one time, AVRs did hold the voltage up, regardless of frequency but:
If the operator ran the set at reduced speed, the old AVRs had a tendency to let the smoke out.
The old AVRs were switchable and the operator was supposed to keep the AVR off until the set was up to speed, and turn the AVR off before stopping the set.
Then Under Frequency Roll Off was developed. UFRO is universal on small AVRs now and I have never seen an AVR with defeatable UFRO.
As the frequency drops, the AVR will drop the voltage proportionally.
So at 20 Hz, you will have about 40% of rated voltage. (You can set the UFRO to 50 Hz.)
So, you are saying to yourself, how can I get an AVR without UFRO.
Well if you are able to get an AVR without UFRO (possibly off of a very old generator set, but most of the old pre UFRO AVRshave let out the smoke.) you may be able to force the field to hold the voltage up at 50 Hz.
Then you will find the next issue, Volts per Hertz.
If you feed rated voltage at 20 Hz to a battery charger rated at either 50 Hz or 60 Hz you will quickly overheat and burn up the transformer.
In equipment such as transformers and motors where the current is limited by the inductive reaction, you will be reminded that inductive reaction and magnetic saturation are related to frequency. The lower the frequency, the lower the voltage that may be tolerated before the core goes into magnetic saturation and the windings overheat and self destruct.
If the atmosphere near your generator location is corrosive, then pay the extra money for the brushless alternator recommended by TugBoatEng.
For normal conditions, you may get good life out of an automotive alternator.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Battery Charging using Variable speed generator

(OP)
Thanks for the background all.

Obviously you have much more experience than me. I should note that I don't plan on doing anything myself. I will hire an electrician. I am on here only to learn.

Unfortunately, I am working with an AC generator, but not opposed to switching out in the future. Curious what makes the alternator different than an AC generator. I thought they were the same?

Initially I was expecting to simply convert form AC to DC using something like this: https://www.amazon.com/MEAN-WELL-LRS-350-12-Switch...

Then implementing a simple charger like this: https://www.amazon.com/dp/B07W3QT226?maas=maas_adg...

Could you help me understand the shortcomings of this?

RE: Battery Charging using Variable speed generator

(OP)
This is on the more expensive side. Thoughts?

https://powerstore.com/catalog/product/view/id/885...={ad}&utm_term=&gclid=CjwKCAjwivemBhBhEiwAJxNWN33enwLGDxbNQkftUUcvcOgrLBCPhS9jJKHZMnyJJdnB5PFyfN1JCRoC8A0QAvD_BwE

RE: Battery Charging using Variable speed generator

Comparing an automotive alternator to a 240 Volt alternator:
Design speed.
Automotive alternator; Designed to withstand high speed, possibly as high as 10,000 RPM.
240 Volt alternator; Designed to withstand 1800 to 3600 RPM.

Minimum speed for full output,voltage and current;
Automotive alternator; less than 1000 RPM
240 Volt alternator; close to 1800 RPM or 3600 RPM.

The Automotive alternator, has a field designed to start generating at as low a speed as possible.
The 240 Volt alternator has a field designed to generate at rated speed.

Once an alternator reaches a speed where it is capable of producing full output, it may be turned faster.
If the alternator is turned faster both the voltage and the frequency will increase.
The voltage regulator will reduce to field so that the voltage does not rise with the speed.
The upper limit on the speed is mechanical, that is what centrifugal force will the rotor withstand.
For many uses, the frequency should remain relatively constant.
In the automotive alternator the AC is run through an internal bridge rectifier and the output is DC. The frequency is unimportant.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Battery Charging using Variable speed generator

(OP)
Thanks again for your input Waross. That makes sense to me. I probably should have started with an automotive alternator. That being said. Do you think this Victron unit could be a solution? If not, what are the potential issues?

https://powerstore.com/catalog/product/view/id/885...

RE: Battery Charging using Variable speed generator

You still have not answered the most important question. You said at 200 psi your generator is only spinning at 20hz. It's not producing power at this time as your automatic voltage regulator must be in under frequency roll-off mode. This means now power is being generated and your expander is still not able to maintain speed. This means there is NO power available to charge batteries under those conditions.

Your next step is to determine how much battery capacity is required to bridge the gap between power availability. You'll also need to size your charging system to be able to fully charge the batteries within the time power is available.

Is your expander governed?

RE: Battery Charging using Variable speed generator

Quote:

That being said. Do you think this Victron unit could be a solution? If not, what are the potential issues?
Inappropriate, unsuitable.
That is a UPS. It is designed to produce AC power from an internal battery.
A typical UPS has an internal battery charger to charge the internal battery.
This unit makes the charger available to charge external batteries.
This is a nice feature for an industrial UPS as it permits you to add external batteries to extend the time available.
You will be paying a lot of money for features that you may not need.
Useability;

Quote (Spec Sheet)

Input voltage range: 95-140 VAC Input frequency: 45 – 65 H
Using your 230 Volt generator, you will have 92-230 Volts at 20-50 Hz.

How much energy will you have available from the turbo-expander?
If your chosen generator puts too much load on the turbo-expander it may slow down.
This may impact your process.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Battery Charging using Variable speed generator

(OP)
TugboatEng,

Sorry - I don't follow. What is the most important question? I am happy to provide as much detail as possible. Currently The batteries only require about 100 watts a day to charge. Most days its about 75W there are one or two days a week where it requires up to 175W. The expander is governed via regulator for pressure input, but not governed by speed. It can only goes as high as 55 Hz with the input psi set to 500.

Waroos,

Thanks for the reply. Could I not pair the multiplus to something like this? https://www.victronenergy.com/chargers/automotive-...

I could further limit the pressure inlet and drop the Hz to achieve something closer to 120V

Something I completely forgot to mention is that the 500 psi inlet at 55Hz had a 500 watt load connected to it. (heater)

Any alternative suggestions are much appreciated!



RE: Battery Charging using Variable speed generator

100 watts a day is not a valid quantity. Could you please clarify?

It does sound like you have sufficient power available to tap off of but need a generator that works over a wide range of speeds. This falls back to the automotive type alternator. As I said before, the Delco Remy 40SI sounds ideal for your application. It costs the same as that Victron inverter you linked earlier.

RE: Battery Charging using Variable speed generator

(OP)
***100 Watts on an Average day***

I am confident I will have sufficient power, but I am still struggling with my original question. Ideally with we use the AC generator as it is an easy option.

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)

RE: Battery Charging using Variable speed generator

This will drastically limit the output from your generator but if you have the generator installed, it may be the easiest option.
If the turbine is controlled by exhaust pressure, then that will automatically adjust for power extracted from the turbine.
Feed the generator output into a bridge rectifier and use a voltage regulator to control the output to 13.8 Volts DC for battery charging.
2300 Watts at 240 Volts is 9.58 Amps.
That is your current limit and will be able to charge at about 9 Amps total.

Watts a day charging?
Watts would be the rate of charging.
Watt hours would be the amount of charge.
We are not sure what you mean by "Watts per day".

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Battery Charging using Variable speed generator

We are slowly finding extra pieces to add to the puzzle.
500 Watt load?
Why?
Is this load important to the process?

If the turbo is not governed, then it may well spin up above 50 Hz with less load.
If the turbo does speed up with less load, that opens up a few options.
Tell us about the load, please.
How important?
Is the heater part of the process or part of the pressure reduction scheme?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Battery Charging using Variable speed generator

(OP)
Sorry I am not very clear. When I said 100 watts on an average day, I mean that there is a total of 2400 watt hours during an average day From the 12v batteries connected in series.

Is there any piece of equipment that would satisfy the bridge rectifier and the voltage regulation that I could purchase off the shelf? If not is there two separate off the shelf units I could buy?

RE: Battery Charging using Variable speed generator

Do you already have the expander/generator installed and it isn't working?

There are numerous discrepancies and omissions that make a determination difficult.

There is a requirement to absorb enough energy to reduce from 1200 psi to 100 psi, but then there is only 500 psi available sometimes? Or it might only be 200 psi.

Where does the excess energy go?

Why is a generator that is not suitable for the application the "easy" option. Why would a UPS with a minimum A/C input of 45 Hz be considered when the source drops as low as 20 Hz?

"the 500 psi inlet at 55Hz had a 500 watt load connected to it. (heater)"

The 500 psi inlet to what?

RE: Battery Charging using Variable speed generator

(OP)
Dave - thanks for the questions.

The generator and expander are installed and currently power a 500 watt heater.

I am happy to clear up and discrepancies or omissions. Just let me know.

The well is produced 1200 psi. The pressure is then regulated to 500 psi and is what the expander see 90%+ of the time. Sometimes there is an upstream process (ethane separation) that causes the inlet of the expander to only see 200 psi. Not much way around this.

I was considering the UPS because it is not usual the Hz drops below 55Hz and there is plenty of energy being generated to supply the battery bank.

When I say inlet I mean inlet to the expander that is coupled to the generator.

Happy to look at any other piece of equipment you might recommend.

RE: Battery Charging using Variable speed generator

From your description your battery bank has a capacity of ~7500W-hr provided you don't have to bridge more than 24 hours without the generator functioning the battery bank has sufficient capacity.

Next, you need to be able to charge your batteries quickly enough so that they are fully charged before the next draw down. If you have 2300 watts available you should be able to replenish the batteries from a 50% draw down in around an hour.

If these are acceptable conditions, then your system should be satisfactory as is.

RE: Battery Charging using Variable speed generator

I give up.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

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