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

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

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

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