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Free Electricity, Maybe GeoThermal Is The Way To Go?

cheftim_2cheftim_2 Member Posts: 14
I've gotten a lot of great advice from the wall over the years. Converted the house 12 years or so ago from electric and coal to a Buderus oil fired and radiant throughout and couldn't have done it without the help here.

I had 15kw of solar installed last year and it's providing more than enough power throughout the year for us. When I installed the Buderus, I put in some valving to possibly accommodate solar thermal input assistance. Given my abundance of electricity, I'm wondering if a geothermal heat pump with a heat exchanger interface to the current radiant system might be a worthwhile investment. Use up the solar and stop burning the oil. We don't have NG available.

I'm on Cape Cod in Massachusetts. My delivery temps on the floors runs anywhere from 95 to a max of 115.

Anyone out there can walk me through the pros and cons?

Thanks as usual.

Comments

  • pecmsgpecmsg Member Posts: 1,510
    The only heat pumps I’ve found that stand a chance on solar are mini splits. Without an inver driven compressor and small one at that a standard compressor will draw more inrush current then your system can provide. 
  • Pughie1Pughie1 Member Posts: 129
    John Siegenthaler has a whole series of articles I believe in the publications of Radiant & Hydronics and PMEngineer including his own personal experience designing and installing the system in his daughters house. I don't know how to tell you how to find them, perhaps someone here could tell you how.
  • SuperJSuperJ Member Posts: 586
    edited October 13
    I assume you're net metered and not off grid? If so geothermal could work but has a relatively high first cost. For the cost of the loop you might be able to install another 10kw of solar.
    I would be tempted to install an air to water heat pump, and possibly an electric boiler, depending on how much excess production you have. The heat pump will give more than double the BTU per kwh but if you have a lot of excess generation the electric boiler is cheap to install (and expensive to run).
    You could even skip the hydronics and just put some electric heaters in the house to use up your kwh credit. Hopefully your utility allows you to see usage on a day to day (or hour by hour) basis so you don't end up with a month of over usage though.

    If the goal is to run the equipment off grid via solar and batteries that's a whole different discussion.
  • Jamie HallJamie Hall Member Posts: 13,418
    There is a fundamental problem with solar PV: the sun doesn't always shine. In fact, when you need heat, it is quite likely that the sun isn't shining. Therefore... unless you also have a large battery bank and a big enough array to charge it, you have to be looking at net metering -- or not use PV for heat, even by a heat pump.

    This is not to say that it can't be done, although for new construction it is much better and more efficient to use solar heat gain and thermal mass storage directly, rather than losing the efficiency through the conversion to electricity. It's only to caution that if you want to use solar PV for heat, do your math very carefully A couple of rather general rule of thumb numbers to start you off on the math, in New England -- including the Cape -- the general rule is that one can expect three hours of usable sunlight, on average, in a day, but, also on average, you can expect no usable sun at all for three days at a time. Therefore, to simplify, you need to be able to store enough kilowatts to run for three days with no input, and enough solar array to recharge that battery capacity in no more than nine hours. If you are using net metering, and your electricity rate is the same for use from and sale to the grid, then the math is a little different: you need to be able to generate in 3 hours the total electricity you use in 24.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ethicalpaulethicalpaul Member Posts: 1,867
    Regardless of solar, I like ground source heat pumps better than oil. The up front cost is an issue for sure, but the operating cost is equivalent to cheap natural gas in my experience. Here's my story:

    I had a vertical well. It was a 2000 sq ft cape cod two story house in Connecticut and my engineer sized it for a single 500 foot well.

    You do a regular heat loss calculation on the building, and then size the heat pump and the well to that BTU rating. You do want to have (or eventually install) good insulation. I purposely had him size mine a little small with a plan to increase insulation (which I did).

    The well I contracted out to a driller. His quote included putting in the plastic loop, then sealing the well with the thermal grout, and running the loop into my basement terminated with fittings.

    I got the heat pump from an internet supplier that I can recommend called Ingram. I got a slightly older version of this unit: https://iwae.com/shop/3-ton-variable-speed-climatemaster-central-air-handler-multiposition-ha12537.html

    So that's the "heat pump". It heats and cools. Then you also need a pump (flow center) to circulate the ground loop. I went with this one: https://iwae.com/shop/geo-pulse-non-pressurized-flow-center-single-pump-ha12100.html

    It is an "atmospheric" pump meaning the ground loop is not under pressure. I chose it due to its simplicity...you don't have to "purge" and pressurize the loop with this kind of flow center. I ran a little food-safe antifreeze in the loop, but I never got the loop water down to 32dF.

    Ingram has lots of good information and product manuals to look over.

    I was able to plumb everything with pex and wire it myself even though back then I knew a lot less about heating and cooling systems.

    I previously had an oil burning forced air furnace in place that I removed with a dolly and slid this unit right in its place. In a house with a water boiler, I would of course get a water-to-water unit. I was done in a day (It was cold out, so I ran the oil furnace to get the house nice and warm and then went for it.

    In more southern climates I think this system would be perfect. Up north in the winter, to be honest, it was a little slow to heat the house. It always did heat the space, but you really don't want to use a setback because it just can't blast the house with hot air like a burner can. So my wife wasn't always happy with it, but it's really just an adjustment of expectations. Even during bitter stretches it could heat the house fine. You can monitor the temperature of your loop water incoming and outgoing to see how it changes during the seasons.

    The operating cost was similar to today's cheap natural gas (which was not available at my location), and the air conditioning was super cheap because it is using constant 50 degree water and it can dump a lot of heat out of the house easily into unlimited 50 degree water.

    Feel free to ask any questions you think of
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
  • pecmsgpecmsg Member Posts: 1,510

    Regardless of solar, I like ground source heat pumps better than oil. The up front cost is an issue for sure, but the operating cost is equivalent to cheap natural gas in my experience

    Ill agree but the energy required to get that Ground Source Heat Pump up and running with solar? Not going to happen. It is possible to fire the oil gun or Gas controls!
  • Jamie HallJamie Hall Member Posts: 13,418
    Now if someone would come up with a ground source heat pump which would boil water...
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • hot_rodhot_rod Member Posts: 13,850
    Air to water HP are getting more and more common. I suspect if you are grid tied you get little to any money for excess, may as well use as much as possible. GEO does have a small efficiency over air to water, but installation of the loop field can be $$.
    The make or break is the SWT required, you might be in a good position to use a A2WHP.

    Some more reading

    https://www.caleffi.com/sites/default/files/file/idronics_27_na.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
    Zman
  • hot_rodhot_rod Member Posts: 13,850
    edited October 14
    Air to water HP are getting more and more common. I suspect if you are grid tied you get little to any money for excess, may as well use as much as possible. GEO does have a small efficiency over air to water, but installation of the loop field can be $$.
    The make or break is the SWT required, you might be in a good position to use a A2WHP.

    Some more reading Google Idronics 27, can't seem to attach a file tonight.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • hot_rodhot_rod Member Posts: 13,850
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • george_42george_42 Member Posts: 89
    I did ground well for 3000 sq ft home with 4ton geothermal. Average electric use at .11 per kw is $75 summer or winter I live in PA
    ethicalpaul
  • ethicalpaulethicalpaul Member Posts: 1,867
    pecmsg said:

    Regardless of solar, I like ground source heat pumps better than oil. The up front cost is an issue for sure, but the operating cost is equivalent to cheap natural gas in my experience

    Ill agree but the energy required to get that Ground Source Heat Pump up and running with solar? Not going to happen. It is possible to fire the oil gun or Gas controls!
    I agree with that @pecmsg but I read his post as "I am generating a lot of electricity and want to stop buying oil" not "I am off the grid and want to heat exclusively with my generated power"

    If he was saying the latter, then yeah it's not going to work. :)
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
  • cheftim_2cheftim_2 Member Posts: 14
    Thanks for the comments all.

    I am in a net metering situation. So from late march through June, I'm getting 90-100kwh/day and then around 80kwh/day through September when it drops down to around 50.

    In MA we also have buyback credits. On average I see checks per month around $300. In the winter months, I use more electricity than I produce but I have excess in the bank. It's also keeping the wife's Hyundai EV charged.

    My thinking was around consuming some of that excess electricity to power the HP. I'd interface that to a water2water heat exchanger for the radiant loops and maybe the domestice HW. I have the interface plumbed in on the manifold as I had originally planned on putting evac tubes in place. But the MA solar deal was way better. I paid 42k for 15kw of solar and got $17k rebate so I'm currently projected to pay back the cost of it in year 4 based on reduction cost and credits.

    ethicalpauls experience is interesting. Hard and costly to get work done here on the Cape but I might look into seeing what I could get for a price on the drilling, sealin and running the tubes into the house. I can handle the rest of the install from there.

    george_42s figures looks like around 75kwh/day average to run the HP which is fairly minimal given my situation.
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