Backup power DC/inverter approach

Discussion in 'Electrical Forum discussion & Blog' started by alternety, Mar 5, 2008.

  1. alternety

    alternety Like an engineer

    Messages:
    661
    Location:
    Washington
    I am trying to do a backup system for the house. My initial direction was a 15KW propane generator. It used an automobile engine running at 1800 rpm. The problem is propane consumption when it has little load (3600 rpm are even worse as I recollect).

    Looking at the newer inverter based portable generators, they don't have the capacity (but they sure do have a high price).

    I have been trying to find a DC generator whose speed/fuel consumption can be controlled by the DC load, feeding one or more inverters designed to operate from a battery backed solar system. I found one that uses an automobile engine, but it is a step control; full power or low power. The specs did not fully enlighten me. My email was returned with a message that they were busy and may get back to me in a few months. And it is pretty expensive on a $/KW basis.

    Have any of you considered this approach? Any idea of a source of a DC generator like this. Full load capacity in the range of 12 KW. I can use a set of batteries to absorb short term demands, but they are very expensive for meaningful capacity. The target DC voltage is 48 V. This is based on wire size and inverter performance. It, of course, raises the cost of batteries.

    Any thoughts?
  2. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    I've looked into it.

    A thought

    1. Getting a used sine wave UPS.
    I picked up a 3000KVA unit for $500

    2. Use a DC generator to charge the batterys and cycle the engine on an off so that it stays in the peak efficiency range.

    3. Use a generator switch panel to decide what circuits to power.
    I don't need to power the kitchen stove, water heater or sewer lift during a power failure.

    4. If you are using a main breaker switch you can connect the same 120V to both phases to automatically disable 240Vac devices

    -
    The DC generator I got puts out the ~96 volts I need to charge the batterys in my UPS. It's actually a 120 pound, 240VDC servo motor, but it will generate lower DC voltages at lower RPMs.

    I don't have a engine yet, so it's just a large paperweight for now.

    I've also contemplated mounting a 20Hp modern compact brushless DC generator in the car where the A/C compressor used to be.
  3. alternety

    alternety Like an engineer

    Messages:
    661
    Location:
    Washington
    Good idea, but I need more capacity. I would either need to parallel multiple inverters or direct connect a generator. And I need 240V for water. I have a number of fairly robust motors that I need to use even if power is out.

    The Honda inverter/generators are actually designed to be paralleled, but the cost gets seriously excessive to get to the 12 KW range. I could get to about 10 KW with two. And that would probably be enough if it will start the motors. But I would rather not have an air cooled engine.

    I built the house with a subpanel that contains all the things that need to stay powered so switching is fairly easy.

    What I really would like to find is an efficient generator attached to a small auto engine with appropriate controls. I have no idea how much propane would be consumed at low power draw conditions. That would have to be figured out. The controls could be designed and built, but I would rather just get a unit that does what I need.

    If I use DC and add batteries, I have the potential of putting up some solar panels if the prices drop enough to provide some standby power without the generator running at all (future idea). 48 VDC makes the wire requirements lower, and inverters are easy to get at this voltage. Not inexpensive, but easy.

    The commercial 15KW backup system uses an auto engine. I would need about the same engine HP, but the commercial unit has to stay at 1800 rpm. Mine would not.
  4. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    Compare your expected performance to the best that can be achieved with a high quality commercial/industrial unit. That information is available at the links below. You will find fuel consumption at full and partial loads and you can calculate the fuel consumption per kilowatt-hour.

    I suspect that a good commercial/industrial generator set will give you better performance at less cost than a combined generator/battery/inverter system. Also, I suspect that when you do the analysis you will find that fuel cost is not a big factor for a generator that will be used for a backup system and will probably not operate more than 100 hours per year.

    A small diesel engine is probably the most efficient you are going to get. You can also find propane engines and the operating cost will depend on the relative cost of propane and diesel fuel. You can buy non-taxable diesel for stationary engines. In many areas the diesel fuel and home-heating fuel oil are the same fuel, delivered from the same tank.

    If you have a water-cooled engine you can run the heat to your house during the heating season.

    You are not likely to find an automobile engine that will match the efficiency of a small stationary diesel. They are designed for different purposes and life.

    Stable frequency comes from a good control system operating the engine at constant speed. A clean sine wave comes from a brushless generator with accurate windings and controls, and stable excitation.

    You will always have some frequency droop when you are adding a big load such as starting a large motor. The throttle and engine simply can't respond fast enough.

    Even if you choose something else, you should compare the performance against a standard of a top quality stationary generator.

    http://www.jobsite-generators.com/single_phase_diesel_generators.html
    http://www.jobsite-generators.com/multiquip_deisel_generator_dca_15spx3.html

    http://www.cumminspower.com/www/com...nicaldocument/SpecSheets/Diesel/na/s-1137.pdf

    http://www.cumminspower.com/na/products/generators/sparkignited#results

    http://www.cumminspower.com/www/com...ocument/SpecSheets/Sparkignited/na/s-1522.pdf
  5. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    Why? Are you in an especially hot environment?
  6. alternety

    alternety Like an engineer

    Messages:
    661
    Location:
    Washington
    I have pretty thouroughly looked at the commercial generators. My favorite is the Kohler 15 KW unit. It is the smallest of their units that use a water cooled engine. If I were going to get that sort of generator I want the water cooled engine for a couple of reasons. The engine is much more durable. It is built as an automobile engine. It runs at 1800 rpm rather than the 3600 of all the air cooled engines. This decreases wear but also makes a significant difference in noise levels. The draw back is that it uses about a gallon an hour of propane at 1/4 load. Power has been out for more than a week and a couple of days outage have occurred several times. Propane is expensive and I am able to store only a finite amount. Dealers are somewhat reluctant to keep the tank topped off and charge 10 -30 cents/gal more. I do not wish to be turning the generator on and off to conserve fuel. A week of running would use somewhere around 200 gallons of propane. At $2.40 a gallon that is about $500. A big advantage is that is is completely automatic.

    I chose propane rather than diesel for fuel. All of the major energy consumers in the house except the clothes dryer are propane. I have a boiler rather than a heat pump for a number of reasons. One was the large electrical load if it needed to operate without the grid. Propane also does not get "spoiled" if stored for prolonged periods and does not care if it gets cold. This is all installed.

    Power producing inverters used for off grid or grid-tie solar power systems provide frequency and waveform distortion virtually identical to the power grid.

    This DC based system would not be the least capital cost approach. It would provide the maximum operating time on whatever propane is on site when power fails.
  7. Nate R

    Nate R New Member

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    472
    Location:
    Milwaukee, WI
    Ah, I see now.

    How many hours/year do you plan on running? Do you have frequent outages?
  8. alternety

    alternety Like an engineer

    Messages:
    661
    Location:
    Washington
    I don't have a planned yearly running time other that no matter how long it is that the generator work through the event. We had a storm last year where we could not even get in or out of the lot because of downed trees and wires. That was the one that was out for more than a week. Some areas were out for weeks.
  9. Bill Arden

    Bill Arden Computer Programmer

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    584
    Location:
    MN, USA
  10. alternety

    alternety Like an engineer

    Messages:
    661
    Location:
    Washington
    Nifty devices. But smallest is 30 KW. No pricing but it is aimed at commercial industrial. I sent them an email asking if they contemplate any residential units.

    Maintenance and repair could be a real issue; but interesting.

    I got a response - about $47K
    Last edited: Mar 7, 2008
  11. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    I dunno, for a residential backup, seems overkill to go to the Industrial side of Kohler's line if you're not going to run it a LOT. Why 15KW? Do you NEED to be able to do EVERYTHING during the time the power's out?

    Just my thoughts.

    I work for Kohler, FWIW. I do R&D testing on Kohler engines in the Engine division. (I don't deal w/ Generator stuff)
  12. alternety

    alternety Like an engineer

    Messages:
    661
    Location:
    Washington
    No Nate I probably don't need 15KW. It is just the smallest residential unit that uses a water cooled engine.

    The biggest motor start load I actually need is a 1.5 HP pump. But the house includes an elevator shaft is case we become to frail to do stairs. If that ever gets installed that will be a big start load as well. But motor start loads typically press the capabilities of generators.
  13. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    Well the amount of run time is my other question : If you don't have huge run times OFTEN, why demand the liquid cooled automotive engine if you wouldn't even exceed the life of an air cooled engine?

    Not saying someone needs a good reason for anything. Just making sure you've thought it through. If it's because you'd feel better w/ the liquid cooled, go for it.

    My side of it is that I've seen how the air cooled utility engines do throughout their lifespan. I've seen tear downs after their life. And I've been rather impressed.
  14. alternety

    alternety Like an engineer

    Messages:
    661
    Location:
    Washington
    The air cooled ones all run at 3600 rpm. The water cooled is 1800 rpm. It is significantly quieter.

    Is there somewhere I can find average lifetimes of the air cooled engines? One other aspect of the air cooled that bothers me is how will they hold up to say a few weeks of constant use. The water cooled units should have no problem with that.
  15. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    Not ALL water cooled units are the same. Just want that statement to be carefully made. Kohler makes a Liquid cooled engine, but it's not automotive spec. The one you're looking at is of course automotive spec.

    I will ask someone who would know what the sound difference is at 1800 RPM and 3600 RPM full load on an air cooled twin. I'm curious myself.

    I don't know where you can find average lifespans.
    Air cooled engines are designed around lifespans of 125-2000 hours depending on the size, application, etc. Push mower engines (3.5 HP, etc) are 125 hours. Bigger displacement twins for commercial applications are 2000ish. That doesn't mean they're junk after 2000 hours. It just is the designed life. Most engines I've seen in residential backup generators that are powered by gaseous fuel have been 1500-2000 hour type engines.

    That 2000 hours is often run/based on a cycle of loads that would likely never be seen in a generator application for home backup. Thus the home genny engine would last longer.

    Air cooled would have no problem w/ several weeks of constant use, either. They can run at full load for an indefinite amount of time. The fans on air cooled engines keep them cool enough. Most specs I've seen look at oil temp stabilization in 110 degree ambient temps.

    Noise though? No contest, the liquid cooled automotive engine would by far be the best.
  16. alternety

    alternety Like an engineer

    Messages:
    661
    Location:
    Washington
    The noise difference on the 15 KW Kohler is both the engine and the noise suppression box. I suspect they have more ability to quiet when the cooler is a separate item. The sound level is 61 dBA for the 15 KW and 65 dBA for the 12 KW. three dBA is a doubling of sound level.

    Thanks for your info. It is interesting. Does engine life improve (or just stand a better chance of reaching the upper design target) if synthetic oil is used in the air cooled engines after break-in? I have used that in all my cars for years.
  17. mikept

    mikept DIY Senior Member

    Messages:
    152
    Location:
    CT
    10db is percieved as double the sound level by the human ear though 3db is a doubling of energy. It also depends on the character of the noise.
  18. BrianJohn

    BrianJohn DIY Senior Member

    Messages:
    151
    Location:
    VA
    One issue is the battery, I have not read all the post (just lazy) but have you determined.

    Number of cells
    Type of cells sealed lead acid, wet cells, NiCad, sealed NiCad?
    Amp Hours required.
    Life expectancy, Sealed lead acid batteries with a 10 year life last on average 3-5 years depending on the number of discharges, how deep a discharge is and how good the rectifier is that will recharge the battery.
  19. alternety

    alternety Like an engineer

    Messages:
    661
    Location:
    Washington
    Yes the battery, if used, will be carefully selected.

    But what I need now is a DC alternator, 48 VDC, maybe 8 - 12 KW with good surge characteristics for motor starting, with engine speed dependent on actual load. Using propane or capable of being converted. This is the purpose of the post. The variable speed engine characteristic will probably make surge response less effective than a constant speed generator.

    A larger version of the Honda inverter based 6500 (5 KW) would be considered.
  20. Lakee911

    Lakee911 I&C Engineer (mostly WWTP)

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    1,328
    Location:
    Columbus, OH
    Are you saying that you want a DC generator, or an AC alternator? If you want the former, you can use an DC electric motor. If you want an AC alternator, be aware that you'll need a rectifier to convert the AC to DC.
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