Micro CoGen.

Ah, this is a grid tie inverter? Connected to batteries? A current limiter would have to switch the dc on and off, this would make crappy dc which might burn the inverter. I'm still not sure what your goal is here.

QuoteUsually the current limiting is done on the load on the inverter, not the supply to it. Can you tell more about what your trying to do?

This class of small Chinese inverters are notorious for drawing unlimited amperage and burning up.
Their input is determined by the size of the PV array feeding it.
As an example:
The inverter can accept 20 - 38 vdc and is rated at 1000 watts.
A 35 volt 900 watt solar PV array puts out a MAX of 30 amps and 900 watts when under load and pulled down to 30 volts.
There is zero chance of killing the inverter because the panels can never produce enough to overpower it.

However if you connect the inverter to a 24 vdc, 400 amp battery bank, the inverter runs right up to full input amps and runs so hot that it cooks the Mosfets. If the battery bank output (inverter supply) was limited to 800 watts just like a PV array, the unit would last longer.
That's what I'm trying to achieve.

Usually the current limiting is done on the load on the inverter, not the supply to it. Can you tell more about what your trying to do?

Can anyone help me with a recommendation for a device that can limit DC current from a battery bank to a device?
In this case the 24 volt battery bank is connected to a 1000 watt grid tie inverter.
When connected, the inverter immediately draws the full 1000 rated watts and runs at it's maximum capacity (and heat) with no restrictions for longer life or safety.
My goal is to restrict the supply current to 800 watts (33.3 amps) by installing some device between the battery and the inverter.
For low wattage circuits a light bulb can sometimes suffice, but for 800 watts, I have no idea what would work.

If a big resistor is a possible solution, I need some help sizing it.

Any recommendations or instruction ?

Thanks

So things have moved on a bit. Some good things, some bad things, kind of at a pause right now.

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Things got off to a bad start when I decided I needed to remove the pulley from the alternator to fit a different size one. Not having a large-enough impact wrench, I gave it to a guy at the local garage/auto workshop, forgetting to tell him the main nut was a left-had thread. I heard a loud snap and a sheepish engineer appeared, holding the alternator in two pieces. No repairing that, so I had to buy another alternator (it turns out Suzuki use exactly the same device, and it's cheaper because there's less demand than for Ford).

The alternators are nicely made - SKF bearing at the rear and the main one says "made in germany" so is likely to be good quality.

I have set up a test rig, driving the alternator using an old washing-machine motor with a speed controller so I can vary the rpm.  I'm using a bench power supply to feed the field coils/rotor, so I can adjust the current, and running the output through a five-phase bridge rectifier (just two three-phase units in parallel) and watching the AC output with an oscilloscope and the DC output with a meter. It's open-circuit output - no load - but I reckoned it would be representative.

I do indeed get very clean DC out of it. However, I'm finding I need to get the RPM up *really* high. Here's a graph at 3600 rpm, showing how output voltage varies with field current :

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Current in Amps is along the bottom (x-axis) and you can see we've pretty much reached saturation by 10 amps - no useful increase beyond that.

So let's fix the current at 10A and then wind up the speed :

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Nice linear relationship, BUT we're at 5,000 rpm before hitting 24 volts, which means to get to 48 volts (or a bit beyond it, if we want to charge batteries) we would need to be at around 11,000 rpm. Hmm.

Looking at the car this unit comes from, the crank pulley looks roughly 2 x the size of the alternator pulley, so in order to charge the batteries, the engine would need to be doing over 5,000 rpm. That doesn't seem right ! How often will it be doing that ? The idea is it does this under braking, capturing waste energy, and you're not going to braking at 5,000 rpm unless you're on a race circuit.

So, where are we ? I could probably get this useful by running it really, really fast, but I don't want to do that as 11K rpm sounds excessive even if the device is rated for that. And I don't see how in it's intended application (the car) it can do anything useful.

Is it possible the big chunk of management electronics I took off this is doing something very clever like boosting lower voltages up to enough to charge a 48v battery ? Some sort of boost converter? I am next going to try wiring it back into the circuit and trying things, although I have a feeling it may need some sort of control input to tell it to go into charge mode.

Any thoughts appreciated, as usual, and I will let you know what transpires...
Richard

The exhaust will flow through the small tubes and the water will flow around them.

Alright! Is exhaust going through the shell or tubes?

Naturally I haven't taken a finished photo yet but hopefully soon You cannot view this attachment.

id love to see bob chime in on this. Ive seen lots of talk of what he has come up with but yet to actually see any picture?

Im currently working on building a SS shell in tube exhaust heat exchanger. Hopefully ill be posting pictures soon

Hi Bob.  Not too many postings here of late, but I do try to check in once a week.   For now I have a Deutz 3 cylinder genset from a lighting trailer like used on road construction.  Its about 5kW and water cooled, 1800rpm.  Good candidate for CHP except for the fact that its not a very common engine in the US.  Long term support could eventually become a problem.

I also have 2x Kubota Z482 motors, one of which is in a Semi truck APU package.  I believe that they belt drive the alternator in that configuration and run faster than 1800rpm, but I have yet to dig into it much.

The "subdivision" that I bought in, in Custer County Colorado sits a few miles off the highway and runs from 8000ft up to over 10k ft in elevation.  No utility power.  County population in summer about 3500 and in winter closer to 2000.  Too few people to pay for expanded utility infrastructure.  They have been fortunate that fiber internet was run along highways and this has enabled wireless internet providers to put up towers serving most of the area and thus providing relatively fast wireless internet access.

Its a good location for solar PV systems and also a good location for CHP since its a primarily heating based climate.  No need for AC, low humidity etc based on the altitude alone.  Its not unusual to be snowed in for weeks at a time, high drifts are common with wind that may be over 100mph a few times a year.

Im relocating from Michigan to Custer County this year.  Have a lot of basic work to do before I even get to building, fencing the property, putting in a gate and a driveway.  There are free range cattle in the area, over 350 of them so will have to be able to exclude them from the building site lest they trample everything.   Will be getting a septic put in, rules change means it now has to be done by a licensed installer.  Then after that I can set up a temporary bathroom / washroom with a temporary connection to the septic system.  At that point I could return the rented chemical toilet that is otherwise required by the sanitation dept.   

After that point and assuming I have made the 3 or 4 trips back and forth to MI to move all my equipment I will finally be able to begin with the building foundation.  Im hoping that I can get the slab poured prior to snow flying in October/November.  Then next spring I can frame and close in the main structure.

Perhaps you could recompile your list of what you have available in the way of low speed engines.  I would favor a couple of smaller gennys, which would give me a longer run time when they are actually needed, when the solar is not producing.  So far I have bought about 1.6kW worth of PV panels and I have 4x 75Ah 24V Lipo batteries.   So 7.2kWh of battery storage. This was intended to be the "construction phase" system.  Of course I have the Deutz 5kW genny as a backup in this phase.  I think I will be buying 12 more 370W panels to use in the system, that should give me another 4.4kW of solar generation and will no doubt force me to add significantly to my battery system.  My main battery load will be the solar refrigerator and freezer which need to run 24/7 and some lighting at night.

Im fairly confident that the solar PV array will support my daytime construction activities.  I plan to have a hot water storage tank for the CHP system providing between 1000 and 2000 gal of storage.  Heating in the final building will be via radiant pex pipes and doesnt require a very high water temperature to operate.  Main heating load is likely to be through a propane boiler, with any running of the CHP adding to that.  I will have vaporizing type diesel stoves for the living space, likely later adding a wood stove.   Once I have run the radiant heat system through a winter I will have a much better idea of the heat demand of the building and can then more appropriately size the hot water holding tank.

I could bring an engine hoist to help load things.  I dont have a forklift, but i do have a backhoe although that tends to consume most of the space on my gooseneck trailer when I move it.  Im sure we could work something out.