3kw propane generator for off-grid w/ automatic start

willyjake

I'm trying to find an appropriate generator to use for this home: http://www.solterre.com/concept-cottage

The generator would be used as backup for when the batteries are unable to handle any loads (so the generator would be used to charge 12V DC batteries as well as handle any AC loads). The generator would be used about once or twice per year.

I require an automatic start - I want the charge controller to tell the generator to start once the battery voltage drops too low. We currently use a 2-wire system. (I'm not totally sure what this means... in case you can't tell I'm a novice about this sort of thing!)

3kW will be more than enough to handle our needs. We also need cold start technology.

Any help would be greatly appreciated!

Cariboocoot

Re: 3kw propane generator for off-grid w/ automatic start

Welcome to the forum.

I have a question which is related although it may not seem so at first: Why are you using a 12 Volt system? As a rule they are not very efficient and should be avoided if at all possible.

Second question would be how are you sizing the generator? This may also seem strange but with all the data at hand we may be able to come up with some alternate plan which will widen the generator choice. For example using an inverter with gen support allows the use of a smaller generator as any 'heavy' loads (nearly always intermittent) can be shared between the two power sources. This saves fuel over the old "enough capacity for full charging and full loads" sizing which is largely unnecessary.

A "two wire system" means there are two wires going from the inverter to the gen to signal it to start/stop. As such it requires more technology in the gen itself to control these functions. Basically the inverter just tells the gen "turn on" or "turn off" and it's up to the gen to take that 'command' and activate ignition, engage starter, etc.

If you can provide details about the system (battery bank size/type, load demand, inverter size/make/model, PV & controller specs) we may be able to come up with several options.

BB.

Re: 3kw propane generator for off-grid w/ automatic start

Also, propane is not always a great choice for fuel if you are in a the frozen north.

Gasoline seems to be the best fuel for cold weather--Although it is possible to get diesel and propane to function OK with modest amounts of preheat (in a small shed). (as I understand--The only ice I see around here is in my freezers when they are working correctly).

As Marc (Cariboocoot) says--Work with us on the whole picture and we may be able to help with a more optimal (cost/performance) design.

More or less, you are not going to find very many under 4kWatt gensets that are very amenable to full automatic start. Honda makes a very nice gasoline unit with commercial performance in 4kWatt size (em400sx) that can be hacked pretty easily to auto-start (when paired with generator controller?).

If your expected generator usage is so small (couple days a year)--Why auto-start?

-Bill

willyjake

Re: 3kw propane generator for off-grid w/ automatic start

Cariboocoot wrote: »

Welcome to the forum.

I have a question which is related although it may not seem so at first: Why are you using a 12 Volt system? As a rule they are not very efficient and should be avoided if at all possible.

Second question would be how are you sizing the generator? This may also seem strange but with all the data at hand we may be able to come up with some alternate plan which will widen the generator choice. For example using an inverter with gen support allows the use of a smaller generator as any 'heavy' loads (nearly always intermittent) can be shared between the two power sources. This saves fuel over the old "enough capacity for full charging and full loads" sizing which is largely unnecessary.

A "two wire system" means there are two wires going from the inverter to the gen to signal it to start/stop. As such it requires more technology in the gen itself to control these functions. Basically the inverter just tells the gen "turn on" or "turn off" and it's up to the gen to take that 'command' and activate ignition, engage starter, etc.

If you can provide details about the system (battery bank size/type, load demand, inverter size/make/model, PV & controller specs) we may be able to come up with several options.

Thanks for the reply. I will attempt to provide some clarity.

I was mistaken, it is a 6V system. The batteries are Rolls Deep Cycle Series 5000 Model 6 CS 25P. I believe there are 5 or 6 batteries wired in series. The maximum voltage they get to is about 29.4V (maybe this means it is a 30V system?)

The load demand is very small. Occupancy schedule is less than 50% per year. I would estimate that we use about 5kWh per day maximum.

Both the charge controller and the inverter are Outback (http://www.outbackpower.com/outback-products/charge-controllers/item/flexmax-80) I'm uncertain of the Inverter model but I'm fairly sure it's one of the FX models.

The PV are Kyocera 210 watt panels.

We want a 2-wire (or automatic start capable) gen because we don't want to have to go out into the cold to start the thing ourselves, and because we are going to be renting the cottage out in the future and want our guests as comfortable as possible. What I've gathered is that any gen with an automatic start (like a switch instead of pull) you can basically turn it into a unit with an automatic start.

I'll be very grateful for any more info! And thanks for the welcome

Willem

BB.

Re: 3kw propane generator for off-grid w/ automatic start

We would call that a "24 volt" battery bank with 4x 6 volt batteries in series (2 volts per cell for lead acid battery bank--Just like your 6 cell car battery is called a 12 volt battery).

Note, while it is possible for make for 5 or 6 x 6 volt batteries in series (30 or 36 volt)--Most loads/AC inverters/off the shelf battery chargers are 12 or 24 or 48 volt units. Other voltages are a bit odd ball. Need to know if your bank is a single string of 4x 6 volt batteries in series--Or what.

If this is 4x 6 volt 820 AH batteries in series, then you have a 24 volt @ 820 AH battery bank. And a 25% per day discharge (for 2 days with 50% discharge as the typical maximum--for longer battery life), you would be drawing:

24 volts * 820 AH * 0.25 discharge = 4,920 Watt*Hours per day

Pretty much your 5,000 WH (5 kWH) per day load estimate.

How many Kyocera 210 watt panels do you have? What is the array series parallel configuration (i.e, 2 panels in series * y number of strings)? What brand/model of solar charge controller are you using?

What brand/model of AC inverter (or inverter-charger) are you using?

Have you been monitoring the Battery Bank's specific gravity with a hydrometer? Do you know if the batteries are getting fully charged a few times per week (+/-)? How much water are they using... Roughly, if you fill them every two months you are probably charging them OK. If you are filling them more often than once per month, over charging. And filling them 2x per year or less, probably under charging them.

-Bill

Cariboocoot

Re: 3kw propane generator for off-grid w/ automatic start

willyjake wrote: »

Thanks for the reply. I will attempt to provide some clarity.

I was mistaken, it is a 6V system. The batteries are Rolls Deep Cycle Series 5000 Model 6 CS 25P. I believe there are 5 or 6 batteries wired in series. The maximum voltage they get to is about 29.4V (maybe this means it is a 30V system?)

As Bill said, this would be 24 Volts nominal. There should be four 6 Volt batteries in series. If there are more batteries than that it would be eight, meaning two parallel strings. More than that and you've got a design problem. Surrette 5000's are 546 Amp hours, which is quite substantial. At 25% DOD you'd have approximately 3kW hours AC available.

Both the charge controller and the inverter are Outback (http://www.outbackpower.com/outback-products/charge-controllers/item/flexmax-80) I'm uncertain of the Inverter model but I'm fairly sure it's one of the FX models.

An FM80 with about 1700 Watts of panel would be needed for those batteries. This would also fit with 3-3.5 kW hours power use per day. Probably eight of those KD 210 Watt panels.

We want a 2-wire (or automatic start capable) gen because we don't want to have to go out into the cold to start the thing ourselves, and because we are going to be renting the cottage out in the future and want our guests as comfortable as possible. What I've gathered is that any gen with an automatic start (like a switch instead of pull) you can basically turn it into a unit with an automatic start.

Usually. Sometimes you need some adaptation.
I terms of gen sizing I think you've picked a correct power requirement at 3kW. Now as for finding one that is autostart and propane ... that's a bit more difficult.
So let's go about this the other way 'round: what (brands) can you get locally? I know what is available here, and frankly I'd label them all as "unsuitable" for your purposes. Or mine for that matter.

willyjake

Re: 3kw propane generator for off-grid w/ automatic start

BB. wrote: »

We would call that a "24 volt" battery bank with 4x 6 volt batteries in series (2 volts per cell for lead acid battery bank--Just like your 6 cell car battery is called a 12 volt battery).

Note, while it is possible for make for 5 or 6 x 6 volt batteries in series (30 or 36 volt)--Most loads/AC inverters/off the shelf battery chargers are 12 or 24 or 48 volt units. Other voltages are a bit odd ball. Need to know if your bank is a single string of 4x 6 volt batteries in series--Or what.

If this is 4x 6 volt 820 AH batteries in series, then you have a 24 volt @ 820 AH battery bank. And a 25% per day discharge (for 2 days with 50% discharge as the typical maximum--for longer battery life), you would be drawing:

24 volts * 820 AH * 0.25 discharge = 4,920 Watt*Hours per day

Pretty much your 5,000 WH (5 kWH) per day load estimate.

How many Kyocera 210 watt panels do you have? What is the array series parallel configuration (i.e, 2 panels in series * y number of strings)? What brand/model of solar charge controller are you using?

What brand/model of AC inverter (or inverter-charger) are you using?

Have you been monitoring the Battery Bank's specific gravity with a hydrometer? Do you know if the batteries are getting fully charged a few times per week (+/-)? How much water are they using... Roughly, if you fill them every two months you are probably charging them OK. If you are filling them more often than once per month, over charging. And filling them 2x per year or less, probably under charging them.

-Bill

Great reply, thank you.

9 panels, 3x3.

Inverter / CC are Outback

We have not been using a hydrometer, but we are using a WELserver to monitor the battery voltage. Basically whenever it is sunny out they go to about 95% charge, so yes, they are getting fully charged about 2-3 times per week. I think they are being charged properly, with bi-monthly refills.

Willem

BB.

Re: 3kw propane generator for off-grid w/ automatic start

I am not sure if they are 820 or 546 AH batteries (at 20 hour rate)--Marc (cariboocoot) certainly knows more about them than I...

Anyway, you have 9*210 Watt panels (3x3 should be just fine array configuration for your setup) = 1,880 Watt array.

Based on 5% to 13% typical rate of charge for lead acid battery banks (better battery life, practical day to day system usage), your solar array would optimally charge:

• 1,880 Watt array * 0.77 panel+controller derating * 1/0.05 rate of charge 1/29 volt charging = 998 AH @ 24 volt maximum battery bank
• 1,880 Watt array * 0.77 panel+controller derating * 1/0.10 rate of charge 1/29 volt charging = 499 AH @ 24 volt nominal battery bank
• 1,880 Watt array * 0.77 panel+controller derating * 1/0.13 rate of charge 1/29 volt charging = 384 AH @ 24 volt minimum "cost effective" battery bank

Or looking at it from the battery point of view:

• 820 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,544 Watt array minimum
• 820 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 3,088 Watt array nominal
• 820 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 4,014 Watt array "cost effective" maximum

So--depending on your needs and the exact battery bank AH capacity--You might want to revisit the array size.

If full time, I would suggest 10% or larger array... If part time, not much in winter, the 5% array would be the absolute minimum I would suggest.

-Bill

willyjake

Re: 3kw propane generator for off-grid w/ automatic start

BB. wrote: »

I am not sure if they are 820 or 546 AH batteries (at 20 hour rate)--Marc (cariboocoot) certainly knows more about them than I...

Anyway, you have 9*210 Watt panels (3x3 should be just fine array configuration for your setup) = 1,880 Watt array.

Based on 5% to 13% typical rate of charge for lead acid battery banks (better battery life, practical day to day system usage), your solar array would optimally charge:

• 1,880 Watt array * 0.77 panel+controller derating * 1/0.05 rate of charge 1/29 volt charging = 998 AH @ 24 volt maximum battery bank
• 1,880 Watt array * 0.77 panel+controller derating * 1/0.10 rate of charge 1/29 volt charging = 499 AH @ 24 volt nominal battery bank
• 1,880 Watt array * 0.77 panel+controller derating * 1/0.13 rate of charge 1/29 volt charging = 384 AH @ 24 volt minimum "cost effective" battery bank

Or looking at it from the battery point of view:

• 820 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,544 Watt array minimum
• 820 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 3,088 Watt array nominal
• 820 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 4,014 Watt array "cost effective" maximum

So--depending on your needs and the exact battery bank AH capacity--You might want to revisit the array size.

If full time, I would suggest 10% or larger array... If part time, not much in winter, the 5% array would be the absolute minimum I would suggest.

-Bill

Properties
-The cottage has a photovoltaic array of 9 Kyocera 210-watt panels (arranged 3x3) for a total array wattage of 1890W.
-The battery bank is 4x Rolls Deep Cycle Series 5000 Model 6 CS 25P wired in series for a nominal battery voltage of 24V. These batteries have a 20hour capacity of 820amp hours. (Although this is for a specific gravity of 1.28… Keith has said that in past measurements, the hydrometer returns values of about 1.25)
-The inverter is an Outback VFX3524. This inverter is rated at 82 amps (for continuous battery charge output), with a continuous RMS AC output of 29.2A (@120VAC)


Calculations
The minimum charge rate for the batteries is (820AH / 20Hours) = 41amps
However, an ideal charge rate (minimizing generator run time and allowing PV to contribute without exceeding max charge) would be about (820AH / 10Hours) = 82amps
This works out perfectly as the inverter is specified to provide a continuous output of 82amps.
The PV array provides (1890W/29.2V (battery max charge)) = 64.7amps
The AC input charging current to get to full charge rate (from manufacturer specs): 29.2A @ 120VAC = 3504W
This gives us an excellent place to start for sizing a generator.





Generator
Since most generators are incapable of operating at maximum output, and since we need about 3.5kW for our ideal charge, we should size one about that will run 3.5kW @ 85% output.
3504 / 0.85 = 4122.35W
This is probably an over estimation of the size we need, as there are very few instances where the generator needs to be used (part of occupancy rates / habits and the passive solar design)
I would say a generator capable of producing 3.5kW would be ample. However, part of the product specifications that Keith is looking for is:
- the capability to start in cold weather
- automatically start (from the charge controller)
- propane fuel

How does this look to you guys?

Willem

Cariboocoot

Re: 3kw propane generator for off-grid w/ automatic start

Well that array is too small for those batteries. That's one problem.
The SG being 1.250 indicates they are not being properly charged.
Full output from the VFX is barely sufficient for charging them.
You would need a generator of 2.5 kW just to meet the maximum charge demand and it would be working hard to do so. I think I would lean towards 4kW as your calculations indicate. Don't forget about needing to power average loads at the same time.

Define "cold weather". Up here we would have trouble with propane and cold weather. You get into the need for pre-heating the fuel just to get it to vaporize. This is why gasoline is preferred.

Honda does make a 4kW propane unit but I don't believe it is available in electric start (I can not find such a version anyway). You may need to size up to accommodate that.

This is an example of a 3.5kW propane gen. Note it is not electric start nor is it a quality brand. Beware of such things. http://www.princessauto.com/pal/en/Generators/3500W-Propane-Generator/8190365.p

BB.

Re: 3kw propane generator for off-grid w/ automatic start

willyjake wrote: »

Calculations
The minimum charge rate for the batteries is (820AH / 20Hours) = 41amps

Many folks run power during the daytime too... So, if you have a base power load (say 120 Watts for refrigerator, 400 watts for irrigation/pumping when sun is up, 30 watts for laptop computer, 20 watts for AC inverter=580 Watts base load), you should add this to a minimum array output:

• 580 Watts daytime base load * 1/0.77 panel+controller losses * 1/0.85 AC inverter eff = 886 Watts of additional solar panels for base load

This is why for full time off grid homes/cabins, we tend towards 10% minimum solar array recommendation--That allows people to use power during the day without having to worry about scheduling/managing daytime loads (i.e., charge till 1pm, when batteries full, turn on loads).

However, an ideal charge rate (minimizing generator run time and allowing PV to contribute without exceeding max charge) would be about (820AH / 10Hours) = 82amps
This works out perfectly as the inverter is specified to provide a continuous output of 82amps.

For a "generic" non-commercial genset, the actual generator rating to "reliably" drive 82 amps into your battery bank:

• 82 amps * 29.2 volts charging * 1/0.80 typical AC charger efficiency * 1/0.80 genset derating = 3,741 Watt minimum genset rating

Many (most/all?) Outback inverter chargers are usually programmable on their AC draw/DC charging via a "Mate" interface/controller. So, you can always dial back the AC input current draw if you have a bit smaller Genset.

You may have other generator deratings... ~2-3% for using propane. But if you are using the genset at 80% of rated output anyway, you probably do not need to do this (and altitude) deratings against the fuel burning motor.

Another issue you have to worry about with propane is drawing fuel from the tanks. If you are drawing propane vapor (common for smaller gensets), the tank needs to evaporate the propane--Which cools the tank and lowers pressure. Too small of tank in cold weather, you may not have a high enough vaporization rate to runt the genset (you may run for an hour, then "freeze" the tank). If you draw liquid propane, you have to have a vaporizer on the engine (and possibly pre-heat/alternate heat source for cold weather).

Check with your local propane supplier and see what they recommend... For example, a 420 gallon tank running a 4kW genset at 100% output will run down to about +10F continuously (as tanks become near empty, their vaporization rates fall).

• The PV array provides (1890W/29.2V (battery max charge)) = 64.7amps

I usually like to derate the solar panel output power by 77% (in sub freezing temperatures, solar panels can meet/exceed their rated output):

• 1,890 Watts * 0.77 panel+controller derating * 1/29.2 volts battery charging = 48.8 amps typical max. charging current

Solar/power systems have lots of deratings--We try not to derate the deratings and get too conservative--These rules of thumbs we use seem to work for the vast majority of readers on our forum. You will most likely meet these minimum values. In cold weather you will likely exceed these numbers--But in the far north 20% more power for 2 hours a day of useful sun is still not very much more power--So I generally do not bother with additional fudge factors for cold winter weather, etc... You will still need to use the genset to make up for missing solar power (when needed).

The AC input charging current to get to full charge rate (from manufacturer specs): 29.2A @ 120VAC = 3504W
This gives us an excellent place to start for sizing a generator.

Yep--Close enough for Solar work.

Generator
Since most generators are incapable of operating at maximum output, and since we need about 3.5kW for our ideal charge, we should size one about that will run 3.5kW @ 85% output.
3504 / 0.85 = 4122.35W
This is probably an over estimation of the size we need, as there are very few instances where the generator needs to be used (part of occupancy rates / habits and the passive solar design)
I would say a generator capable of producing 3.5kW would be ample. However, part of the product specifications that Keith is looking for is:
- the capability to start in cold weather
- automatically start (from the charge controller)
- propane fuel

If you do not one yet, I would suggest installing a battery monitor (Victron is another good brand). A few of them have programmable outputs that you could connect to an alarm (50% battery capacity--flashing red light--Start the genset, use to turn of optional loads, etc.). Victron has one with a programmable output--There may be others.

Battery monitors are not perfect--But they are much easier for guests/kids/spouses to use vs a hydrometer (over 80% SOC, everthing good. Less than 80%, start watching loads. Less than 50% SOC start genset. Over 80% SOC, turn off genset).

Automatic gensets--For a part time/non-owner occupied cabin/home with little expected generator run time--I am not sure they are worth the time and money to setup. One of the things that you learn (the hard way)--The more you automate, the more that can go wrong that you never prepared for. Plus more expenses when things fail (generator fails to start--to cold, preheat failed, out of propane, too low of propane pressure, starter fails, generator battery fails. Nobody there to turn off loads -- security system, heating system to keep pipes from freezing, then battery bank taken dead which kills battery bank, then need to add an inverter shut down based on battery state of charge, emergency notification for caretaker to come out and shut down water, drain pipes, fix genset, etc.).

We have posters here that have run their systems for years--Then the adult kids/friends come for a couple of weeks... Plug in the hair driers, leave all the lights/tvs/computers on, run water like there is no tomorrow--And the parents come bank and find a killed-in-action battery bank and many thousands of dollars to replace.

Installers/friends have even installed power cutoff systems when the batteries are in danger of being killed--The safety systems are bypassed and the batteries are killed (even by owners of the system)--They would just rather have any power vs going without when the system is on its knees. It is just human nature.

Making a "fool proof" off grid power system for renters/guests to use--Is difficult. Some times, it is better just to keep it simple and less expensive to repair if things do go south.

If automation is something you enjoy learning about and having on your system--It will give you a warm feeling to see it all work as planned.

-Bill

Cariboocoot

Re: 3kw propane generator for off-grid w/ automatic start

BB. wrote: »

For a "generic" non-commercial genset, the actual generator rating to "reliably" drive 82 amps into your battery bank:

• 82 amps * 29.2 volts charging * 1/0.80 typical AC charger efficiency * 1/0.80 genset derating = 3,741 Watt minimum genset rating

Many (most/all?) Outback inverter chargers are usually programmable on their AC draw/DC charging via a "Mate" interface/controller. So, you can always dial back the AC input current draw if you have a bit smaller Genset.

A couple of notes about this.
The Outback is indeed programmable to limit both maximum AC in and maximum current to charge (although it is read in AC Amps so requires some approximate conversion - on a 24 Volt system it's X5). They will start out at low charge and ramp up, allocating current first to loads and second to charging.

Fortunately their chargers do not have a poor power factor. In fact the PF gets worse as current goes down, but then it matters less. It is also extremely unlikely that you will be drawing 82 Amps at 29 Volts; maximum charge current occurs at minimum Voltage. By the time you get the batteries up to 29 Volts they better not be drawing 82 Amps!

mike95490

Re: 3kw propane generator for off-grid w/ automatic start

how cold is "cold start" going to be ?

Robin Subaru RGX4800E Electric Start Industrial gasoline (should be convertible to propane)
Robin Subaru Inverter RG3200iS Electric Start gasoline (should be convertible to propane)
Robin Subaru Inverter RG4300iS Electric Start gasoline (should be convertible to propane)
Robin Subaru RGD3300H Electric Start Diesel (only good to 0c, or it needs cold start preheater, not quiet - I have one)