cook90
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| 07/23/2009 11:15 PM |
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I live on Lake Norman, NC and am considering a closed lake-loop, GSHP for a new construction home. Lake Norman is a large (>30,000acres), dammed river North of Charlotte, NC. The lake surface temperatures fluctuate from a high of 90 degrees F in the summer to a low of 45 degrees F in the Winter (http://www.lakenormansweb.com/weather/temperature_water_2008.php ). The lake bed temperatures will range somewhere in between these limits. I have 15-18 feet water depth where I plan to place the loops and there is a slight current.
Here are my questions: Is the above situation an efficient use of geothermal? How would it compare to a ground loop where soil temps and ground water are 62 degrees? Can the efficiency of the lake loop, compared to a ground loop, be accurately calculated (assuming all GSHP equipment is the same)?
I'm guessing that heat transfer to the lake water is significantly greater than heat transfer to soil. How much greater?
Thanks,
Dan
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engineer
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| 07/24/2009 7:51 AM |
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Mark Custis is our resident pond loop guru.
My guess is that your year-round efficiency would be somewhat lower than with a vertical well system, perhaps on the order of 10-20%. However, installation cost may be substantially lower, well justifying the choice. Efficiency can be calculated by reference to manufacturers' extended performance tables. Power consumption, unit capacity, EER and COP are all tabulated for a wide variety of entering water temperatures and flows.
Summer might turn out to be the worst case scenario - 90 degree lake water (though it might be considerably lower at 15-18') likely translates to 100 degree loop water, and performance with that will be no better than, and possibly worse than high end air source units.
Two other issues spring to mind: Are you allowed to deploy loops in Lake Norman? I know that permitting agencies, especially the Corps of Engineers, can be a real PITA. Also, how will you keep the loops safe from fishermen's hooks and anchors. Envision some nitwit snagging their anchor and attempting to free it by tying off to the boat and powering away. Imagine everyone's surprise seeing a bass boat towing a couple hundred yards of slinky tube...
There is a more compact and possibly more durable pond heat exchanger called a Slim Jim - check it out.
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Without data, you only have an opinion. |
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joe.ami
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| 07/24/2009 9:19 AM |
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Posted By engineer on 07/24/2009 7:51 AM
Are you allowed to deploy loops in Lake Norman?
That's the real question. In my area public property can't be permenantly built on for private use.
In many cases, open loop is perfectly okay however.
Good luck,
Joe |
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Just a Mechanic;
Geothermal; Savings Underfoot |
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jonr
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| 07/24/2009 9:47 AM |
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Around here, lakefront property owners typically own the bottom lands. Ie, I can moor a boat in front of my property but others can't. I would guess that sinking a coil would be similar.
But why use closed loop (vs open loop)? You might also take a thermometer and get a reading of temperature at depth. That is needed for an accurate comparison. And remember, ground loops may start at 62F, but they swing widely - probably more than the lake surface temperatures do.
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joe.ami
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| 07/24/2009 10:06 AM |
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Posted By jonr on 07/24/2009 9:47 AM
Around here, lakefront property owners typically own the bottom lands. Ie, I can moor a boat in front of my property but others can't. I would guess that sinking a coil would be similar.
"Guessing"? Usually you at least find some obscure report to read before you disagree with the pros.
J |
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Geothermal; Savings Underfoot |
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jonr
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| 07/24/2009 10:23 AM |
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Joe gets so upset when he is wrong - eg "public property" applying to loops on lakefront bottom lands in his area.
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Bergy
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| 07/24/2009 11:39 AM |
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Posted By jonr on 07/24/2009 9:47 AM
Around here, lakefront property owners typically own the bottom lands. Ie, I can moor a boat in front of my property but others can't. I would guess that sinking a coil would be similar.
jonr,
The ONLY time I've seen ownership of the "bottom lands" is on a private lake. Public waterways are just that...PUBLIC. As such, the land owner owns the property to the normal high water mark. Past the high water mark is PUBLIC. Lakefront property owners can put a dock in the water but that dock MUST be removed for winter. (I can only assume than warmer climes allow docks year-round.) I can drop anchor in front of private property, I CAN NOT dock my boat there.
Bergy
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cook90
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| 07/24/2009 2:16 PM |
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Thanks to all who have responded. I'll answer several of the questions that have arisen:
Lake Norman is owned by Duke Power. I have received regulatory approval for a lake loop (as long as the loop remains under my pier/dock). Anchors and fish hooks do pose a problem. I am familiar with the Slim Jim. However, area GSHP installers question the efficiency of the Slim Jim after it gets "gunked up" with summertime algae.
I spent some time researching the Waterfurnace website and have learned that 100 degree F entering water temperatures (EWT) result in an EER of 12.6 - 15.6, depending on the geothermal unit selected. Not great, but that's a worst-case scenario and will occur only when outside air temps exceed 100*F for an entire week. How can Air Source Heat Pumps (at an OAT >100*F) be more efficient than a GSHP (at a lake temp of 90*F)?
I'm having dificulty comparing apples to oranges. Does anyone have a sure-fire way of comparing different systems (GSHP ground loop, GSHP lake loop, ASHP) under similar operating conditions?
Thanks!
Dan
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engineer
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| 07/24/2009 9:09 PM |
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An ASHP can be more efficient rejecting heat into a 100 degree environment if its outdoor fan uses less power than a geo unit's pump(s)
More importantly, if the lake temperature rises to 90 and stays there for several weeks, the geo unit is fighting that 24/7, whereas the ASHP is only dealing with high OAT for a few hours per day.
Despite the forgoing, I'd be surprised if Lake Norman is ever 90 degrees at 15-18' depth.
An earlier poster mentioned Open Loop. I have one and it works well, but in many cases open loop pump power is much higher, often eclipsing the savings from marginally more favorable EWT. There is also the potential for increased maintenance owing to clogging, mineral scaling, or bio fouling.
The 'sure-fire' comparison comes from a competent local contractor running the numbers for various design alternatives for your site-specific conditions. It is unlikely to happen here on the internet. |
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joe.ami
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| 07/25/2009 8:25 AM |
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Posted By cook90 on 07/24/2009 2:16 PM
Thanks to all who have responded. I'll answer several of the questions that have arisen:
1) Lake Norman is owned by Duke Power. I have received regulatory approval for a lake loop (as long as the loop remains under my pier/dock). Anchors and fish hooks do pose a problem.
2) I am familiar with the Slim Jim. However, area GSHP installers question the efficiency of the Slim Jim after it gets "gunked up" with summertime algae.
3) I spent some time researching the Waterfurnace website and have learned that 100 degree F entering water temperatures (EWT) result in an EER of 12.6 - 15.6, depending on the geothermal unit selected. Not great, but that's a worst-case scenario and will occur only when outside air temps exceed 100*F for an entire week. How can Air Source Heat Pumps (at an OAT >100*F) be more efficient than a GSHP (at a lake temp of 90*F)?
4) I'm having dificulty comparing apples to oranges. Does anyone have a sure-fire way of comparing different systems (GSHP ground loop, GSHP lake loop, ASHP) under similar operating conditions?
Thanks!
Dan
1) I don't know how big your pier is but this won't be a tiny loop. This also implies that you would be close to shore not in the deep end....
2)Loops would gum up with the same algae. Silt may also get on lake loops. As long as it's a saturated coating there will be little change to heat transfer rates.
3) ASHP's do not have a high SEER with high EAT. But a GSHP and properly designed earth loops might stay under 100*
4) Operating cost software can make these comparisons for you, but the truth is that the difference is generally modest from one to the next as we select a min/max EWT and design loops accordingly. In that way verts, horizontals and pond loops all perform about the same . The auto size on my Geo design software defaults to a 30* minimum loop and a 95*. The only thing that would change for you is a larger pond loop due to lower Delta T (brine temperature-surrounding water temperature).
I wasn't a fan of lake loops in your application and am less so now.
By the way the reason most folks go with a pond loop is to save money on excavating. It's not much of a savings, and you still have to tear up the yard to get there.
J |
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jonr
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| 07/25/2009 9:37 AM |
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> in many cases open loop pump power is much higher
This is true, but not in the cases where the distance is reasonable and you put the water back at the same level that it came from. That's a zero net lift case, just like closed loop.
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engineer
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| 07/26/2009 7:07 AM |
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| Sounds good in theory, but as a practical matter one is stuck with the pumping power needed to get the water up to the unit with no recovery of power going back down the hill. During the off cycle the open loop drain line fills with air. |
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jonr
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| 07/26/2009 9:51 AM |
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Intakes and outlets beneath the water surface leave no opportunity for air to get in. But even if you did do it wrong and allowed air in, you would only consume the extra power for a short period (while the air was pushed out).
Bergy: you might want to read http://www.mwai.org/WATER_LAW.htm
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CVB Geo
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| 08/04/2009 10:27 PM |
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Dan,
I am a geothermal installer from the Denver side of Lake Norman. I am getting ready to install a closed pond loop system in the Troutman area of Lake Norman and am currently working through some of these questions with them. I would like to compare notes with your project as well as discuss the logistics of installing the closed pond loop in the lake. Send me an e-mail to carolinavision@bellsouth.net if you are interested in discussing further.
Thanks, -Bill |
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Alex_in_FL
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| 08/09/2009 12:00 PM |
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Re air in line: agree with jonr. Also a simple check/pressure valve could solve the problem but since both ends are submerged it should not be a problem.
Lake temp: The 90 F temp is the surface temp (maybe even a foot deep). Your temp at depth will be far less. I believe there will be a thermocline at 4-10 feet (based upon exp fishing and swimming in lakes and such). But why guess, just go weight a thermometer and drop it in the lake and measure the temp at 10 feet and then at the bottom. I am guessing the temp at depth will be in the 65-70 range (but this is guess I don't know the flow rate, lake usage pattern, shading nearby etc).
Thermal conductivity (BTU/ft-hr-F ) is about: Water 0.346 ,Wet Sand 0.95 , Dry Sand 0.157, Wet Clay 0.87 , Dry Clay 0.138. However the specific heat of water is about 4 x that of soil so it heats slower. This means that the overall coil length is similar for land or water but the water coil can be placed in a more compact area due to the wter being able to move freely (think natural convection).
The water coil should be far cheaper (very limited drilling or trenching). The lake source temps will be more stable than the horizonal install and less subject to soil moisture issues.
See http://www.geo4va.vt.edu/A2/A2.htm for a lake unit with an secondary coil. This eliminates concerns about the anchors and similar.
It seems a lake coil or even an open source coil using a copper nickel (or titanium) coil should be cost effective and efficient in your area. Add a cheap inline pool filter if you want to make sure of keeping efficiency up and avoiding clogging issues.
Best of luck.
Alex
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engineer
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| 08/10/2009 7:55 AM |
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If elevation change is greater than about 34' (theoretical max, upper 20s as a practical matter) , open loop will not stay filled owing to vacuum in upper section of line. A check valve solves the problem in the suction line but not the return.
The most powerful pump in the world can't suck water up from a source open to atmosphere much past 25'.
I agree that temp at depth will be much more favorable. Need a slow acting thermometer or one with a long probe to verify |
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1923mack
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| 08/10/2009 9:06 AM |
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| There is a table somewhere on the internet that gives temperatures of a whole bunch of lakes. They have charts that give water temp from top to bottom of the lakes. They also chart temps at various times of the year. Do a google search and you should find it. I found it by accident while checking on soil temperatures. I looked at a few lakes in California that I was familiar with. This table will give a general idea of depth vs temperature change. Making your own table is still an good idea. Depth vs water temp. |
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jonr
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| 08/10/2009 10:24 AM |
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A couple of points:
Lifting water in water doesn't count towards the lift limit and if you have a lift > 25' from the lake to the house, you need to put the pump closer to the lake.
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engineer
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| 08/10/2009 10:12 PM |
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| I agree with both points, but if the house is 100' above the lake the water going back downhill in an open loop system WILL NOT pull the new water up the 100' hill. This issue is avoided in a closed loop system. |
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Without data, you only have an opinion. |
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jonr
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| 08/11/2009 10:37 AM |
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I agree. It would be interesting to compare 100' of pumping costs to the efficiency loss of closed loop.
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