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Geothermal - Insufficient water power

Posted by beelzerob , 04 April 2010 · 13295 views

So I guess some background, as I don't think I gave much....

We have a 3200 sq ft house, not counting the unfinished basement, that we want the unit to be able to heat.  We currently have a 4 ton air exchange heat pump, and I'm dis-satisfied with its heating capabilities.  Our duct work is more than sufficient for a new geo furnace, so really it's just a device swap out.

There are 4 basic ways to go with geothermal, and I won't go into great detail here...there's much better sites out there for describing the differences.

Pond/Lake exchange - Can't, there ain't one near.
Horizontal Looops - Can't, not enough cleared land (and I won't kill our trees to accomplish the least efficient method)
Vertical Loops - Definitely a contender
Open Loop (well) - Preferred method.

The main things they say about doing open loop, with your existing water well, is that the water has to be sufficient quality and quantity.  Well, quality basically revolves around if there is iron and/or sulfur in the water.  Having lived with this water for 2 years, if we had an iron problem sufficient to be a problem with geothermal, then we would have seen the red results in our sinks and toilets by now.  So, iron isn't a problem.  With sulfur, the water quality test guy told me "If you can't smell it, then you don't have it".  And we've been rotten egg smell-free, so no issues there either.

As to quantity...from what our well contractor told us, there is PLENTY of water in the ground for the having.  We basically can't pump our well dry through normal usage.

Now, once you pump that water into your house, it's got to go out somewhere.  If you had a lake nearby, you could probably dump it there.  Streams are also a candidate, though I understand you might run into environmental regulations if you do that.  *shrug*  Doesn't really matter, neither of those are an option for me.  So, the other way to get rid of that water is to put it right back into the ground.  You have to dig another well, and hope to heck that at some point in the drilling process, they find a crevasse or cavern....something where their air doesn't return to the surface.  That's where you know you can dump water and it will go away.  It's not completley hopeless, since our well contractor lost his air return when drilling our water well....so we can hope he'll do that again, and not too far down from the surface.   It's still one rather large gamble, though.

Finally, your existing well pump has to be of sufficient size to handle the load.  And you'll usually need a bigger pressure tank.  Turns out we don't need a larger one because we opted to go with a constant pressure pump system, instead of the on/off type.  So, it uses just enough pump power to maintain the pressure, instead of bringing on the full power to reach the set point and cutting off.  So that's good at least.  So the remaining question was....can our pump meet the needs of the geothermal unit at max capacity, AND our normal household needs too.  And today I learned that it is a most highly probably NOT.

First, I wanted to see what our theorhetical max gpm was.  I turned on the outside hose and set it to full open.  That produced just about 12 gpm.  I then went inside and turned on the kitchen sink.  I already noticed a decrease in pressure.  The sink was giving just about 1 gpm.  I went and checked the water pressure, and it was holding steady at 38 psi, and the well pump was steady at 2500 watts.  Our normal water pressure is 55 psi, so I knew the pump was already giving all it could.  So basically, it could do 13 gpm, and at that point, well be noticing a definite decrease in water pressure.

I then wanted to see what the max gpm was at a pressure we would consider reasonable.  So I adjust the hose outside to 8 gpm, and the pump was bouncing between 1800 and 2500, as was the water pressure reading (52 to 55).  So I opened the kitchen sink again, and measured 1.5 gpm.  Pump was still slightly oscillating in power usage, so I opened another sink and got 1.5 gpm out of that too.  Now the pump was going full out at 2500 watts, and the pressure was holding rock steady at 50 psi.  So I decided that was the max rate at a pressure were used to....11 gpm @ 50 psi.

Water Furnace says that for their 4 ton systems, it is just under 2 gpm per ton....so that's 8 gpm if the thing is using both stages (2 tons per stage).  That means, should it be running full speed, that we'll only have 3 gpm left to use in household type situations before we start losing pressure.   That's 2 sinks, or 2 showers, or probably 1 laundry machine....in other words, that's an uncomfortable margin for me.

So this is the first bummer about all this....its bad enough we're replacing a perfectly fine air exchange heat pump (and still paying for it in our mortgage), but if we have to start replacing well pump components too, this is going to spiral out of range real quickly.  If enough gets replaced, then we'll begin to move into the realm where closed loop vertical wells is more cost effective for installation.  I really won't know until the well driller comes out and we discuss the situation.

Apr 04 2010 08:12 AM

I have never heard of the well system until you mentioned it I have heard of the other 3. I always thought the horizontal or vertical ones were perfered
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One option, is to have a seperate storage tank, say 600 to 1200 gallons for household use. Fill that tank at 1-2 gallon per minute, pressure doesn't matter.
You'll need another pressure tank and 3/4 hp pump, but now your household use has nothing to do with the well and heat exchanger use.

We have this exact storage system, but that's because our well is only capable of 3 gpm.
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Well, the horizontal and vertical loops are perhaps more popular, mainly because of the water quantity and quality issues prevent it from being viable. That, plus most people's systems aren't setup to handle that water demand (as I'm finding mine isn't also), so the cost of upgrading the home water system, plus the "waste" well makes any price savings you'd normally achieve with open loop start to disappear.

In order of system efficiency, horizontal loops is the lowest. That's because being so close to the surface, the loops tend to deplete their heat reserve during long cold winters. That means that over time, the temperature of the water in the loops is going to drop, meaning there's less heat available to it to replenish the heat extracted in the furnace. Vertical loops are next most efficient because the much greater depth of the loops means that the temperature is MUCH more stable than the horizontal loops. So more heat can be extracted and will be replaced more easily by the natural earth temp. *However*....even those loops will see temperature drop over the course of winter. Not as drastic as horizontal loops, but it will still start to happen. That's the reason I really don't want to go with closed loop systems if I can avoid it...at some point, you may STILL have to rely on your auxillary electric heaters, and I think that's just ridiculous to have to do when you're spending that much on a fancy geothermal system.

Open loop is the most efficient. The reason is because it uses the ground water, extracts the heat, and then dumps the water outside of the system. So, your source water isn't getting colder as a result of extracting heat from it...it always comes up at the same temp (which for us is right around 50 deg). That's why I want it...I don't want to have to worry about the loop temp dropping as the cold winter goes on and on and on.

I appreciate the suggestion of a larger storage tank. I'm open to all kinds of remedies for this, and I expect to hear quite a few options when the well driller comes over. I have high confidence in him, he did great work on our water well and just about every HVAC contractor that has come over has nothing but praise for him. So we'll see what our options are.
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What type of well pump do you have? What are your well details (there should be a drilling report that has depth, pump depth, max flow, static level, ...)? I had a similar issue when I installed my 5ton Waterfurnace. I designed my well pump system to provide >20gpm at 50PSI at the static level and reduce to 16gpm at the pump height (in case the draw down brought the water level to the pump, my max flow is 16gpm). I did everything myself, so the cost of changing the well pump was less the $350 ( my pump is fairly shallow, only 80ft). If you have any questions, PM me.
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Ya, that well report is somewhere....

Off the top of my head....the well is about 310 or so feet deep. The water is at like 290ish. The pump is a 1 1/2 hp goulds constant pressure pump. The well driller puts max water capacity in the well at 100 gpm.

From what the well driller told me, to upgrade the pump itself might not cost a lot, but I guess the controller box on the wall in my basement is the really pricey part (part of the constant pressure system I guess), and it will only handle so much of an upgrade in pump before you'd need another box, which could be a lot of $$$.

If your water is at 80 ft, I take it you don't put the water back into the ground via a return well?
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beelzerob... is this going to be for heat only or for heat and AC? The reason I ask is becusae in my research I read that with REALLY deep vertical systems that are AC and Heat, you can "store energy." Basically, all summer long, the heat exchanger deposits heat in the shaft, which - as you stated - dimisishes performance as the season progresses, but then, come winter, a good bit of that heat energy is stored in the immediate area (the surrounding rock is a little warmer than the average temp of the rock in the area). As you switch from AC to heat, you begin cooling the surrounding rock & the processs starts over come summer.

My brain wants to reject any of this being true, becuase it seems like the heat would even out over time, but I guess if you're pumpin heat in/out for months at a time, it has to build up, and it doesn't just magically diffuse all over the place.

I've also read that the open system is the highest maintenance of all the types, since you have to worry about mineral fouling of your equipment (which it sounds like you're aware of).

I suppose, you could have the guy dig the second well without commiting to either an open system or a closed vertical system, and then - if the well is not suitable for an open system, just have them install the vertical system in the hole (I think they drop the pipes/hx in the hole and fill it with some sort of highly conductive material). You're stuck diggin a small, deep hole either way, right?
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Ya, you've got a lot of it right.

If I go with open loop, and they start digging the "waste" well, and get to, say, 200+ ft without hitting a crevasse or something where they can dump the water (like, say, a small community of dwarves), then at that point I'll have to decide if I want to go closed loop instead. Because if they hit ground water (which my current well puts at about 300 ft), then the jig is up for that particular hole. Annoyingly, they could move over 10 ft and find exactly what I need 100 ft down....well drilling is so completely unpredictable.

First answer is yes, this is for heating and cooling. It's replacing our current air-exchange heat pump, which handles heating and cooling, only it exchanges heat with the outside air. Since that can fluctuate from 0 deg to 100 deg over a year, then the efficiency of the unit fluctuates wildly also. That's where the stable temps of the earth make geo units much more efficient, since the earth temp of around 50 deg is just about perfect for dumping heat into and getting heat from.

Having said that...the idea of "storing heat" really isn't a big player in all this. I'm not sure what the heat transfer rate of rock 200 ft down is, but it has to be somewhat reasonable because otherwise, you'd run out of heat during winter probably before long is using a closed loop system. The heat from the earth has to replenish what you've taken or your loop temp will just keep getting colder and colder until the system can't give you sufficient heat anymore.

Now, *in theory*, after a long winter, those vertical loops are then simply RIPE for switching from heat to A/C and cooling the house down, because you're now using really really cold water in the loops (and cold surrounding rock) to cool your house. And the same would be true switching from summer to winter. So sure, there'd be some increased efficiency in that. But keep in mind...there are usually several weeks if not more from the last time you used your system for heat and when you'll want to cool the house down, just because Spring/Fall is generally very mild. So I would just guess that by the time you switch over to Cooling, the temps down in the well have evened out quite a bit. And like I said, that's no a problem. You WANT the earth to transfer that heat around, it'll serve you better in the winter when you need that heat replenished.

Water quality *is* a major issue with open loop systems, but as far as corrossion, sulfer and iron are the two worst issues. If you don't have those issues (and we don't), then your next issue is hard water, since you are pumping a LOT of water through the heating/cooling exchanger. What all geo units I've seen do for this is offer you a "cupre-nickel" coil instead of the normal copper one. I think this is similar to your "self-cleaning" shower heads. Basically, the cupre-nickel coils expand and contract at a different rate than the deposits do, which breaks them free, and thus they get flushed out of the system. Geothermal has been around for quite a while, as as hard water, so I'm not too worried that maintenance is an unsolved problem. Even if maintenance meant a system cleaning with vinegar once a year, I'd still be willing to live with that.
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Is there a specific unresolved issue/question?

The discussion contains several assertions based on a qualitative evaluation for which a definitive answer requires a quantitative calculaton. Eg: pumping water up 290 feet to land surface incurs a significant energy cost unless it is discharged to a substantially lower elevation than land surface.

Work performed = (elevation head + pressure head+ friction head) * volume pumped * density of water. Energy cost = Work * wire-to-water efficiency * $/energy unit.

The geothermal industry has most folks bamboozled into thinking that even the parts of the ground-source heat pumps (GSHP's) that go iinside the home ( IOW, not including loops and associated pumps) are somehow more complicated than conventional heat pumps. Conceptually a water-to-air GSHP is the equivalent of a conventional air-to-air heat pump in which a water coil is substituted for the air coil in the outside unit, put in a more compact package, and moved inside.

A conventional split heat pump or air conditioner could be upgraded into ground-source water-to-air by simply replacing the outside unit with a GSHP of the same tonnage and refrigerant. No need to change ducts or air handler or anything else inside the house if the existing tonnage is adequate, And a very competent tech could even convert your existing air-source outside unit to ground source by replacing the air coil with a water coil and adding a Thermostic Expansion Valve (TXV).

I contemplated building a domestic hot-water heater out of our existing 3-ton HP but opted instead to build a 5-ton water-to-water ground-source heat pump from scratch using individual components and a purpose-built case (yet another work in progress) .

Whether the ground heats up or cools down in the long term depends on which process (heating or cooling) moves more heat. In heat-only usage, the ground will cool down, and the system will become less efficient. This is readily simulated with models and well documented in (eg) schools that are not used in the summer. What the discussion seemed to be centered on is on what, if any, gain in "efficiency" there is during the transient period of startup when switch from heat to cool and vice versa. As a practical matter, it averages out, so unless you pay the utility bill one month and your roomie the next, the differences average out to a Emily Litella/Gilda Radner "Never Mind".

There are of course sophisticated subsurface heat transport models that can simulate these transient conditions, but the average installer is going to use a ft/ton rule-of-thumb or at most a simple steady-state model. Given the uncertainty in specifying the pertinent physical properties of the subsurface needed in the models, in most cases that's all that is warranted anyway.

... Marc
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