What's more efficient for generator?High current/short time or low current/long time?

My system should be in the signature below. The diesel generator's specs can be found here:

http://www.xtremeatv.com/resources/WENGeneratorPg3.aspx

I'm off the grid and when there are a couple of real cloudy days in a row and no rain I don't get solar or hydro power. (We're in a drought and the tiny stream is almost non-existent now)

So I can run the generator, but don't know if I should run it full out at about 30 amps (110 volts) for a shorter charge or dial the charger back to say 10 amps and let the generator run longer.

I'd run it during peak usage several hours during the evening. Typical load on the inverter is 150 - 300 watts during the day and evening and maybe 30 - 50 watts at night. (200 watts at night when the refrigerator runs)

It seem that if I run it at maybe 500 watts for several hours it would feed the loads directly so I wouldn't have to deal with the charger/battery/inverter inefficiencies. Any excess power would go to the batteries to slowly charge them. I have eight Surrette six volt 460 (if I remember correctly) batteries wired for a 24 volt system. I realize that this may not give the batteries sufficient charge current to do this long term, but I expect sunny days in a day or two. If not, I'll give them a high current charge anyway.

Is a typical generator more efficient running full out or somewhere in the middle of its range?

Thanks for any help!

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: What's more efficient for generator?High current/short time or low current/long t

    Hmmm... You may end up having to provide part of the answer--but some basics:

    Your batteries, assuming they are flooded cell, are very roughly around 80% energy efficient (between charging and discharging). And your inverter is around 85%-95% efficient (depending on model and load)... To assume the average worst case, around Eff running off of battery is Ebat=80%*85%=68%...

    So, just for the pleasure of having quiet electricity 24x7, your are loosing around 32% or 1/3 of your power (don't get me wrong, this is not bad, as you will see shortly).

    For your generator set--if this was the typical "cheap" 5-10kW gasoline driven generator, you would find that at anything less than ~50% electrical load, that the fuel flow would probably not be reduced. So, if you had a 5kW gasoline genset running at anything between 0 and 2,500 watts--it would still be using a gallon of fuel every xx hour (assume around 5.5 kWhrs per gallons of gasoline, 5.5kWhrspergallon/2.5 kW=2.2 hours per gallon).

    I don't have very good numbers for a diesel genset (let alone yours), I looked up a 6.5 kW air cooled diesel generator and found that it runs at 1/2 load (which is 1/2 of 5.5kW) 0.34 gallons per hour or:

    1/2 * 5.5 kW / 0.34 gallons per hour = 8 kWhrs per gallon of fuel

    Here is a nice water cooled Isuzu 6kW generator that uses:

    0.81 gph at 100%
    0.58 gph at 75%
    0.37 gph at 50%

    or:

    6kW / 0.81 gph = 7.4 kWhrs per gallon
    75% * 6kW / 0.58 gph = 7.76 kWhrs per gallon
    50% * 6kW / 0.37 gph = 8.1 kWhrs per gallon

    Here is another vendor; Onan 6kW liquid cooled generator.:

    No Load (Gal/h) 0.13
    Half Load - 50% (Gal/h) 0.4
    Full Load (Gal/h) 0.7

    6kW / 0.7 gph = 8.5 kWhrs per gallon
    50% * 6kW / 0.4 gph = 7.5 kWhrs per gallon

    And, assuming above number of say, ~7kWhr per gallon:

    7kWhr per gallon * 0.13 gallon per huor = 0.91 kW =

    This gives us, very rounghly a 910 watt minimum load or so--where we are not "wasting fuel" just to turn the generator and make noise.

    So, at least from the first two models from the first vendor, the GPH usage from 50% to 100% is pretty linear (more power, more fuel--and with these numbers, they claim these gensets are a bit more efficient at 50% load vs 100% load--not quite sure I believe it--The Onan numbers look a bit more reasonable).

    Based on the above, lets assume that your gph usage for a kWhr of power generated is reasonably efficient down to 25% load. Or,

    25%*5.5kW=1.375 kW = 1,375 watts minimum useful load on your genset.

    And, given that you have ~68% efficiency on your battery/inverter conversions, your "decision point" between genset vs battery would be a load of:

    1,375 watts * 68% = 935 watt household load

    So, above ~935 watt load, start the genset and below ~935 watts, use your battery/inverter system.

    And, for charging, I would suggest a range of load on your generator of between 25% to 50% load minimum (1,375 to 2,750 watts) for efficient running of a battery charger (plus loads) to charge your battery bank (and, ideally, the house hold loads should be powered from the genset instead of the inverter--save the 89%-95% conversion efficiency of the inverter).

    So your question of 30amps at 110 volts (=3,300 watts) vs 10 amps at 110 volts (=1,100 watts), from the generator point of view--30 amp load at 110 volt should be more efficient charging.

    Now, looking at it from the battery side--your most efficient charging is going to be at a rate of 5-13% of the AmpHour rating of your battery bank (using 20 Hour rating). The more you go above 5%, the more your charging current gets turned into waste heat (if you look at your battery rating, you will see there is less energy available if you discharge the battery at c/5 or c/10 vs discharging it slower at c/20 or c/100). Generally for charging, a c/10 rate would be pretty good compromise.

    So, if you have a 48 volt battery bank (guess, you can change the numbers), then a 30 amp at 110 volt (assuming 80% charger efficiency).

    30A * 110 V * 80% / 56 volt charge = 47 amps

    Battery bank size range for 47 amps:

    47 amp / 5% = 940 AH battery bank
    47 amp / 13% = 362 AH battery bank

    And, lastly, of course, your battery bank will start to accept less current as it nears its full charge--so you are either going to loose diesel fuel efficiency in the last hour or so, or you can run some other optional loads (well pump; freezer/fridge, A/C, etc.) at this time, or you can get a smaller than average solar array to finish off the charging (and save fuel/noise)--depending on your needs.

    Probably not the A/B answer you are looking for--but this is how I would approach the problem.

    And, I would put some sort of totalizing Watt*Hour meter (like a used home utility power meter) or even just keep track of the amperage*hours used, along with your fuel used... And you can enter your own "real numbers" and find your "sweet spot" for the best fuel efficiency for your needs.

    Without knowing your kWhrs generated and fuel quantity used--you are a "bit in the dark" and guessing at what is the best answer. If you can measure your kWhrs and fuel used--and compare it to the ~7-8kWhr per gallon number--you will be able to see at a glance how efficient your current genset usage pattern is.

    -Bill

    Sorry, I forgot you have some solar panels too... So, one recommendation would be to run your genset and "heavy" loads in the morning (before 9-10 am) and then let the solar panels do the taper charge the rest of the day (assuming they are large enough to finish up the charge to 100% by sundown/evening usage).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • James Wilson
    James Wilson Registered Users Posts: 28 ✭✭
    Re: What's more efficient for generator?High current/short time or low current/long t

    Wow! That's some good information, it'll take some digesting though! :)

    Overall, with the batteries as low as they are to cause me to want to start the generator I think that they probably could be charged at a higher current than I'm using right now (10 amps) Interesting that the generators generally need to run above 50% load to be efficient - that would be 30 amps!

    Thanks for all of the information and ideas!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: What's more efficient for generator?High current/short time or low current/long t

    Hope I was clear (and accurate) in my post... If you see anything strange, I probably made a mistake :roll: .

    50% as a floor of efficiency is for those $300 5kW specials you see... Better gensets should probably be pretty efficient down to ~25% load (such as the Honda euX000i series with the internal inverters--the motor can run slower, less fuel wasted spinning things fast).

    Hopefully, the better built diesels are similar in their fuel usage pattern.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: What's more efficient for generator?High current/short time or low current/long t

    diesel engines do not like unloaded conditions. They don't get hot enough internally, and start to carbon up. 50% load is a good point to start at, so I'm guessing you have WAY TOO much generator. If you have a lot of hours of light loads (more than 200hours) you may want to pop the head off (have spare gaskets ready, and other parts to replace associated with head removal) and see what the carbon load is like. Maybe pulling the exhaust manifold/pipe can show you about the same.
    I'd say getting a small genset like the Honda EU series (or clones) that have the "auto throttle" and inverter output would be a more efficient use of fuel, or run an electric heater off the generator part of the time to provide a load. It's good to let the genset run no load for a couple minutes prior to shutdown, to cool off the electrical generator.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • James Wilson
    James Wilson Registered Users Posts: 28 ✭✭
    Re: What's more efficient for generator?High current/short time or low current/long t

    I just ran the generator at 30 amps for an hour, and yes, the fuel consumption seems to be about the same as it was at 10 amps. It sure got the batteries good and charged! This should help with the blowing out the carbon also.

    It's a shame that the inverter's fan has to run so long after a charge like this, it just starts the batteries discharging immediately... I guess if I brought the charge current down for the last fifteen minutes this wouldn't be a problem, but that sure is a lot of manual fiddling.

    Thanks for all the help!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: What's more efficient for generator?High current/short time or low current/long t

    James,

    You also need to look at the size of the battery bank... Too small of charge current, and you will not mix the electrolyte properly (especially during equalization--assuming you have flooded cell batteries).

    I just noticed that your battery bank is probably 24 volts... So, just double the AH ratings I did for the 48 VDC bank... And your bank (assuming 30 amp at 110 volts charge rate) should be (roughly) between 724 AmpHours and 1,880 AmpHours.

    You generally do not want to go outside of the above rule of thumb (as always, check the battery manual for proper charge settings).

    Also, you should check the battery voltage too during charging... You don't want it to go too high--it would indicate you are pumping in too much current (charger set points may be wrong), and/or your batteries may be sulfating and near the end of their lives.

    Take a look at the battery FAQ for general information on how to take care of your battery bank.

    And, if you do not have a hydrometer and thermometer--you should pick a set up. And, assuming your set of batteries is not cheap--I highly recommend a battery monitor--the closest you can get to a "gas gauge" for your battery capacity (especially useful if you have AGM/Sealed batteries as you cannot measure specific gravity). For the battery monitor, look at the Trimetric for and lower cost unit. And look at the Xantrex LinkPro for the highend (I like the battery temp sensor and alarm contacts you can program to turn off the loads or turn on an alarm if the batteries are being drained too far).

    Lastly, if you do not have a remote battery temperature sensor hooked up to your inverter/charger--you should find it/purchase one--Your batteries will be charged better (typically closer to 100% capacity) and, if you are dumping a lot of current into your battery bank (~13%+ of 20 Hour rate)--you could experience thermal run-a-way... Basically, you pump a lot of current into the battery, its temperature rises, but its charging voltage falls--without remote battery sensor, it is possible for the charger to be confused and try to charge the battery even more (as its temperature and voltage continues to fall).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: What's more efficient for generator?High current/short time or low current/long t

    Mike had is right re: loading up the diesel.

    When I used to run my lister 5 kw, we used to run 2000 watts of dump load heating elements, just to give it some load. Fuel consumption was about the same regardless of load.

    A also suggest that you have way more generator vs charger than you need. A larger charge (or second) would help. BB's number about battery size relative to charge size is a good rule of thumb.

    Tony
  • James Wilson
    James Wilson Registered Users Posts: 28 ✭✭
    Re: What's more efficient for generator?High current/short time or low current/long t

    Yes this is a large generator for the size of the batteries, but it sure was nice to have one of this size during construction.

    When I need to run it to charge the batteries I run it at about half load - 30 amps. I get about a day's worth of charge in an hour and use about the same amount of fuel as I was using when it charged at 10 amps.

    I'm not so concerned with making sure that I'm getting a full charge because this is just a stopgap between cloudy days. If I have a week of cloudy days I'll be sure that the batteries float from the generator charge.

    I really appreciate the advice!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: What's more efficient for generator?High current/short time or low current/long t

    Your inverter, if properly setup, should charge the batteries without you needing to fiddle with the 30 vs 10 amp charge rates.

    Also, does that inverter have a remote temperature sensor, and do you have one installed? If not, that would probably help too (especially at higher charging currents).

    Your solar charge controller should also have a remote battery temperature sensor too...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • James Wilson
    James Wilson Registered Users Posts: 28 ✭✭
    Re: What's more efficient for generator?High current/short time or low current/long t

    Yes, the Trace 4024 does have a charge controller built in and I do have the battery temperature sensor.

    My original premise was that I wanted to run the generator for a long time with reduced charge current. The theory was that fuel wouldn't be used as quickly and things would be more efficient since more power would be used directly instead of having battery charge, storage, and inverter losses.

    So I don't have to fiddle with the charge rates, but I was limiting them manually.

    Now that I know that the generator's fuel usage doesn't decrease with loads under about 50% I'm just limiting the charge current to 50%.