Geothermal / generator question

Discussion in 'General Discussions' started by geome, Mar 15, 2011.

  1. geome

    geome Member Forum Leader

    From an operational cost standpoint in heating mode, is it better to use an LP electric standby generator to power a 3-ton Envision unit with 36F EWT, or to use propane gas logs at near 100% efficiency?

    Considering a 20kW generator that runs
    1/2 load at 1.89 gallons of LP/hour
    Full load at 2.9 gallons of LP/hour
    Last edited: Mar 16, 2011
  2. moondawg

    moondawg Member

    Need to know the BTU output of the gas logs, methinks.

    Unless you're talking "theoretical" gas logs that put out the same BTU as your 3Ton envision.

    One pound of propane = 21,600 BTU

    So at half load, (plenty to run your Envision) you're consuming 1.89lb/h x 21600BTU/lb = 40,800 BTU/h.

    Looking solely at the heat produced, it is likely cheaper (by a little) to run your gas logs.

    Looking at a "total solution" ... you can't keep your freezer from dethawing or draw a hot bath using just your gas logs.
  3. geome

    geome Member Forum Leader

    Thanks moondawg.

    Logs are 28,000 btu's on low, and 39,000 btu's on high. I prefer low.

    I understand I can't eliminate the need for the generator, especially since we want the fireplace blower to operate, but we do have a choice to either run the geothermal system or the logs (or a combination to help the temperature at the ends of the house) in the winter during a power outage. I just want to make the smarter cost choice.

    Generator sizing for the geothermal system shouldn't be an issue since we would have capacity for it by alternating loads with the electric water heater. Of course, we would need IntelliStart for the Envision.
  4. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Three operational considerations

    The gas log input rating likely differs widely from the heat it actually delivers to the room.

    If the genny is already running, the incremental fuel use increase arising from added load is fairly low compared with fuel use just keeping genny spinning.

    If geo has DSH, that's an additional point in favor of running it for heat
  5. geome

    geome Member Forum Leader

    Even for vent-free?

    (Please start another thread should someone wish to discuss vent-free safety issues.) Thank you. :)

    Thanks Curt. Good point on water heating.
  6. Masoud

    Masoud New Member

    Above, moondawg wrote:

    "One pound of propane = 21,600 BTU

    So at half load, (plenty to run your Envision) you're consuming 1.89lb/h x 21600BTU/lb = 40,800 BTU/h."

    But, geome had said the generator drinks LP by the gallon, not by the lb.


  7. Altnrgy

    Altnrgy Member

    In order to do these sorts of comparisons I normally figure out how much it costs to make 100,000 BTU’s with the sources I’m comparing. That way it’s closer to being an apple to apple comparison.

    One gallon of propane = 91,000 BTU's. (or pretty close) Feel free to call me on this but a vent free unit is going to be 100% efficient since there’s no flue, right? So if you put 1g of propane into your vent free gas logs you get 91,000 BTU’s out. Around here propane never got above $1.50 gallon this winter but let’s use a safer number of $2.50 gallon.

    Propane Vent Free Logs = $2.74 per 100,000 BTU’s

    Assuming a COP of 3 for your GEO unit it should make 100,000 BTU’s out of 33,333 BTU’s which is equivalent to almost 10 kWh’s. There are probably plenty of hairs to split but I’m just trying to paint the general picture here. Since you provided the specs of your generator we can readily do that math. At half load your generator would need to run for one hour to make 10 kWh. In hour it would need 1.89 gallons of propane which would cost $4.73 (1.89g x $2.50)

    GEO Unit on Utility Power = $1.00 per 100,000 BTU’s (@ 10c per kWH) I know you didn’t ask that question but it was easy to answer while I was doing the math anyway and I think it’s important.

    GEO Unit on Generator Power = $4.73 per 100,000 BTU’s

    So there you have it…assuming I did the math right the gas logs are cheaper to run but there’s much more to the equation. IE; What are the indoor air quality issues of running the vent free logs for more than a few hrs., the vent free logs are only heating one area of your house, your generator is probably already on anyway, etc.

  8. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I hate ventless, as it kills

    I would add a propane fired tankless and a hot water coil to the duct system before I would go ventless.

    Geome, get with me and I will fix you up with an idea or two better than ventless.

    Last edited: Mar 23, 2011
  9. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Before going vent free please name me in your will.
  10. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

  11. Geo on standy generator

    It almost looks like the question is if using a generator and geo heat pump would use less power than a propane fireplace. The generator probably runs at about 60% efficiency, and propane is high, so if heating is the only concern the ventless propane would do better. Still not a good idea. Even ventless units need combustion air, and should have at least a decent CO monitor near the unit. Still, in theory, it is good question. If we have a COP of 5, then we are producing 5 units of heat for every unit of electrical input. Question would be, does the energy we recover from the ground make up for the energy loss of the generator?
  12. Masoud

    Masoud New Member

    Perhaps not possible!
    If my memory is still good, the vent-free has been in place for a while.
  13. geome

    geome Member Forum Leader

    Would contributors to this thread please honor my original request regarding starting another thread for a fireplace safety discussion? Thank you. :)
  14. geome

    geome Member Forum Leader

    Does this make sense?

    1 gallon of propane = approx 91,500 BTU's = 26.8 kW

    At 1/2 load, a 20kW generator produces 10kW, and consumes 1.89 gallons of propane then,

    1.89 gallons of propane = approx 172,935 BTU's = 50.7 kW

    So the generator efficiency at 1/2 load is 10 kW / 50.7 kW = 19.7% ?
    _ _ _ _ _

    At full load, a 20kW generator produces 20kW, and consumes 2.9 gallons of propane then,

    2.9 gallons of propane = approx 265,350 BTU's = 77.7 kW

    So the generator efficiency at full load is 20 kW / 77.7 kW = 25.7% ?
  15. moondawg

    moondawg Member

    Yes, that makes sense. Thermodynamic efficiency of internal combustion engines is not very high. The best diesels are a little over 50% under just the right conditions. And engines are generally more thermodynamically efficient at rated load.
  16. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Sounds about right to me as well.

    Little gennies are fairly inefficient - optimized for low first cost, not economical operation.

    Going to an 1800 RPM 4 pole genny bumps the efficiency up a bit, but higher first cost may preclude that option.

Share This Page