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Author Topic: Costs for Geothermal?  (Read 3171 times)

MaineShark

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Re: Costs for Geothermal?
« Reply #15 on: November 08, 2008, 07:41 PM NHFT »

The problem with geothermal is the ground is about 50 degrees (varies though) consequently it is a lot more efficient to cool with geothermal then it is to heat with geothermal, this makes geothermal more ideal for warmer areas where heating isn't as critical as in NH. In order to get a (traditional) geothermal system you would have to drastically oversize the system to meet your heating needs.

There is no "oversizing."  The system is sized to the heating/cooling load.

Often, we will actually undersize geothermal, by sizing it to the cooling load (typically about half the heating load), and using fossil fuel to cover the very coldest part of the year.

I'm not sure about the south facing wall with radiant floor heating/geothermal with floor cooling... cooling through the floor is a poor may to do it, it may sorta work in NH but in most places would just result in a hot, humid house with a constantly damp floor from condensation.

Properly-designed radiant cooling systems incorporate humidity control, which does not allow the floor to get below the dew point.

I don't understand why it has to be a forced air system. Seems like a heat pump could be used to heat or cool liquid for radiators or baseboard systems, or whatever.

The limitation is temperature.  Peak outlet temperature on a water-water geothermal system is 125 degrees, and efficient operation really needs the water temp to be 110 or lower (just using the higher temp for domestic hot water, not space heating).  That's not hot enough for baseboard, which really needs 140-degree water.  Large radiators can be used (sized for 110-degree water, rather than 212-degree steam or 180-degree water, as are typical), but that can cost a lot.  Forced-air uses lower temperatures, so it's often the most cost-effective way to use geothermal.

In-floor radiant can often run with lower water temps (90-110, depending upon the floor construction), so it can be used for heating, at least.  When the humidity is low enough, it can also be used for cooling.  Alternately, wall-mounted blower units can be used - they look sort of like a space heater, but are operated by hot or cold water - that eliminates the need for ductwork, and gives room-by-room control.

I don't see how people come up with idea that geothermal is about the same price as a traditional system... basically you have pretty much all the same expenses PLUS the real big expense of digging around in your yard installing ground loops and don't forget... a backup heating system is pretty much essential.

Unless you are intentionally doing a hybrid system, there's no need for a fossil fuel backup.

Regarding the costs, when folks say that it's similar, it's probably because they had a high-producing well, allowing them to run an open system, eliminating the cost of a second well.  You need about 2 to 3 gallons per minute for every ton of geothermal capacity.  Most houses use three to six tons of geothermal, so a well that can produce somewhere in the 6-18gpm range can be used without needing to drill any additional wells.  Of course, you also need to have a place to discharge the used water, so not all sites are suitable for open systems (if you have downhill neighbors, they probably won't like you dumping the water on the ground, to run down and flood their basement).

Geothermal is, in most cases, more expensive that traditional.  Almost anything "alternative" is going to be.  If it were not more expensive, it would not likely be "alternative" at all.  Fossil fuels are the norm because the equipment is cheap, and the fuel has been cheap for a long time.

Realistically, installed geothermal in NH costs somewhere between $6k and $10k per ton, for most cases, depending upon the specifics of the project.  Given typical residential sizing, that yields a $18k to $60k range.  Operating costs are very low, so the price difference can be paid off pretty quickly, in most cases.  Most geothermal installations will see operating costs in the hundreds of dollars, rather than in the thousands (like is the case with most fossil fuel systems).  There are also rebates from PSNH and other power companies, since they want you to buy their power rather than someone else's oil.

Of course, there is the disadvantage that geothermal is strictly a grid-tied system.  You would need a ridiculously-large generator to power a geothermal system in an emergency.  Most installations get a separate electric service with their own meter, due to the power demands, and because PSNH will often charge a lower rate for the geothermal power.  So, if your goal is to be off-grid, geothermal may be a bad choice.

What it all boils down to is that there is a huge range of options in heating, both in the conventional and alternative markets, and there's no one right answer for all applications.  Some folks will be best off with oil.  Others will be very pleased with wood-fired heating.  Some will love their geothermal.  And still others will beg used fry oil from the local restaurants, and heat their houses using that.

Joe
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ByronB

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Re: Costs for Geothermal?
« Reply #16 on: November 09, 2008, 06:28 AM NHFT »

There is no "oversizing."  The system is sized to the heating/cooling load.

Often, we will actually undersize geothermal, by sizing it to the cooling load (typically about half the heating load), and using fossil fuel to cover the very coldest part of the year.
What I meant is that since the ground is colder then what you want your house to be geothermal is more efficient at cooling a building then heating, therefore if you size a unit to cover all your heating needs you will have a bigger compressor and ground loop then you need for your cooling needs... sizing it by cooling needs like you said makes the most sense if you are trying to get your moneys worth IMO.


Properly-designed radiant cooling systems incorporate humidity control, which does not allow the floor to get below the dew point.
Hmmm, tie in a humidistat to a mixing valve to control the water temp? I have never seen anything like that... I suppose it would work; however on real hot humid days it may cut your cooling most all the way off... unless you know of a better way.

I'm use to having a week or two a summer hitting the 100s every day with the humidity never dropping below 90% so maybe the standards I'm use to are overkill for in NH.

Unless you are intentionally doing a hybrid system, there's no need for a fossil fuel backup.
Maybe I like redundancy too much (or I've fixed WAY to many ACs that crapped out) but I'd personally like to see at least a electric backup for any heat pump I'm depending on to keep my pipes from freezing.

Regarding the costs, when folks say that it's similar, it's probably because they had a high-producing well, allowing them to run an open system, eliminating the cost of a second well.  You need about 2 to 3 gallons per minute for every ton of geothermal capacity.  Most houses use three to six tons of geothermal, so a well that can produce somewhere in the 6-18gpm range can be used without needing to drill any additional wells.  Of course, you also need to have a place to discharge the used water, so not all sites are suitable for open systems (if you have downhill neighbors, they probably won't like you dumping the water on the ground, to run down and flood their basement).
Ok, I see how the costs could be similar, thanks for the info, I was thinking of a closed system...

I've always wanted to cool with water from a well, only run it straight through a heat exchanger instead of using it for condenser cooling... we use straight city water for cooling at the hospital I work at in some rooms for a backup when something in our chilled water system goes down, I had to use it once for our MRI room... couldn't tell there was a problem even though it was summer.
« Last Edit: November 09, 2008, 06:31 AM NHFT by ByronB »
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dalebert

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Re: Costs for Geothermal?
« Reply #17 on: November 09, 2008, 07:44 AM NHFT »

I'm use to having a week or two a summer hitting the 100s every day with the humidity never dropping below 90% so maybe the standards I'm use to are overkill for in NH.

Ah, yeah, definitely. Cooling is almost icing in NH, really. I used to live in GA so I know the type of climate you speak of. Ugh. NH summers are really mild and pleasant. We get hots days, but they're rarely relentless and ongoing. We usually get plenty of breaks and nights are rarely anywhere near as warm as days so we get breaks from the heat, unlike GA where the humidity traps all the heat from the day and it stays miserably hot all night long.
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MaineShark

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Re: Costs for Geothermal?
« Reply #18 on: November 09, 2008, 12:20 PM NHFT »

What I meant is that since the ground is colder then what you want your house to be geothermal is more efficient at cooling a building then heating, therefore if you size a unit to cover all your heating needs you will have a bigger compressor and ground loop then you need for your cooling needs... sizing it by cooling needs like you said makes the most sense if you are trying to get your moneys worth IMO.

Cooling is more of a convenience thing, around here.  Regardless of your energy source, the cooling load will be lower.

Solar energy (when available) can be used to boost the ground water temperature before it is sent to the heat pump, thereby improving the efficiency during the heating season.

Properly-designed radiant cooling systems incorporate humidity control, which does not allow the floor to get below the dew point.
Hmmm, tie in a humidistat to a mixing valve to control the water temp? I have never seen anything like that... I suppose it would work; however on real hot humid days it may cut your cooling most all the way off... unless you know of a better way.

Actually, a humidity sensor, with either a standalone PID mixing control, or tied into a computer that manages the system.

Radiant cooling has significant limits.  It's a supplemental cooling option, even here in NH.  It's extraordinarily efficient, so you run it as much as you can, but you still need something else if you want cooling on the really hot/humid days.  An air handler or ductless fan convector can be set up with a chilled-water coil, to operate when there is demand above what the radiant can delivery.

In new construction, or major remodels, we can also install tubing in the ceiling and even the walls.  A larger surface area can provide the same cooling at a smaller temperature differential, thereby reducing the amount of days when the cooling load exceeds the ability of the radiant to deliver without condensation.

Unless you are intentionally doing a hybrid system, there's no need for a fossil fuel backup.
Maybe I like redundancy too much (or I've fixed WAY to many ACs that crapped out) but I'd personally like to see at least a electric backup for any heat pump I'm depending on to keep my pipes from freezing.

It takes at least a day for a house to freeze.  Usually a couple.  Plenty of time to get a repair done.  Geothermal is no less reliable than fossil-fuel.  I've replaced more failed electric elements on heat pumps, than anything in the actual heat pump, itself.

If you want a real backup, it should be something with little or no electrical demand; if you're concerned about having something to fall back on, you want something that will operate if there is a power failure.

Or, you can go the other direction, and install a wood-fired system for primary heat (properly-designed systems can be operated on a generator, or even a battery bank), and then install a heat pump (probably air-source, due to cost) to provide cooling in the summer, and supplemental heat in the winter, if you go away for a few days and aren't home to load wood.  Air-source heat pumps are expensive to operate in the dead of winter (when they are mostly running on resistive coils), but if it only operates a few days out of the year, the operating cost becomes trivial.

I've always wanted to cool with water from a well, only run it straight through a heat exchanger instead of using it for condenser cooling... we use straight city water for cooling at the hospital I work at in some rooms for a backup when something in our chilled water system goes down, I had to use it once for our MRI room... couldn't tell there was a problem even though it was summer.

I have a stream on the property, and I'm considering doing the same thing.  I'm planning on a cistern filled by a solar-powered pump when the sun is out, and drawn from whenever the there is demand (with the warmed water discharged, not returned to the cistern).  Alternately, since the stream can get pretty low during summer, I may try and use a pair of larger cisterns - pump water from one, use it for cooling, and dump it into the other, where it cools back down due to the ground temp, then reverse the cycle.  There's no way that the ground will cool the water enough to make that work, in the time it will sit in a regular cistern (low surface area/volume ratio), so I'm thinking of a few short ground loops that either operate based upon pumps or even natural convection.

But, as you noted, cooling is not a high demand here in NH, so that's pretty far down on the list of priorities, due to the amount of calculation needed to plan it.  I keep hoping I'll get a client who wants something like that designed, and I can get paid to design something that I want to design anyway.

Joe
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