Again, thanks for the great replies.  I'll answer some of the questions that have arisen.

1.  My house is 40 feet from the water's edge.  The loop will be located another 100 feet from the shoreline in12-18 feet of water.  So the total horizontal run will be approximately 140 feet.  My property is located in a 360 foot-wide cove on a 30,000 acre dammed river.  (you can google-earth Lake Norman to see what I'm talking about).

2.  The vertical lift will be minimal.  Approximately 8 feet when the lake is full, 12 feet when the lake is down.

3.  Surface temperature of the lake this morning is 82*F (heat wave this week, 100*F outside air temps).  And it's pretty close to isothermal all the way to the bottom (13.5 feet deep below my dock).  This isn't unusual.  Lake Norman gets really stirred up due to the boat traffic and wave action.

My questions/comments:

1.  For Alex_in_FL:  you write,

"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. "

I can't find the reference to a "lake unit with a secondary coil."  How does this eliminate concerns about anchors/fishhooks? 

Are thermal conductivity and specific heat the only variables in determining efficient heat transfer?  What other physical properties are involved?

2.  Open-loop questions:

I'm beginning to consider an open-loop system but I'm concerned about the algae and other issues with an open-source system.  What concerns should I have?  Can a pool filter really combat the vast amounts of "crud" in the lake water?  Is the efficiency increase significant with an open-loop? 

Thanks to all for your input!!

Dan Cook

Do you have any neighbors that use lake water for sprinklers? Or boats using fresh water cooling? Either one will provide some data on what the water is like.

Almost everybody on the lake irrigates out of the lake.  We just drop a 2" pvc pipe into the water, cap it with a plastic filter/ball valve and hook it to an irrigation pump.  Is the pumping process similar for open-loop geothermal? 

Geo generally needs less pressure than irrigation. Careful selection of the pump will result in highest system efficiency. Pump power can be a signifcant fraction of system power, so choose well.

Heat transfer is simple to understand but complex to calculate (many, many variables). A coil in a large body of water will see little to no heat buildup as heat can flow via conduction and convection. Heat flow in the ground is almost exclusively via conduction (small amount of convective). Thus the ground near the coil will heat and slow the heat transfer. You'll generally see heat build up to a peak in late August early Sept for horizonal inground systems and a bit earlier for vertical systems.

Key heat tranfer factors: thermal resistance of coil, grout resistance, heat sink (soil/water) factors, heat sink temp, movement of groundwater or lake water. Then it becomes more complex like water flowrate in the coil (faster = better), pipe roughness, quality of contact between heat sink and coil, etc.

Yes, a pool cartridge filter should suffice for an open source but you could always put a higher quality inline filter after it to really keep the water really clean (probably change it annually but clean pool filter monthly if high use).

Making a SWAG (Scientific Wild Ass Guess)
- open source would be lowest initial cost and might be most economical overall
- closed source in the lake coil would likely be the next most economical
- inground horizonal next
- inground vertical next (however likely would be more energy efficient than the horizonal loop).

Exactly.  Here's some data to support your comment.

For my vertical system, my EWT (blue & black lines) peaked 3 weeks ago (late July) and is now very slowly declining.  High was about 83 °F (Dallas climate) and is now running around 79°.

The EWT high for the year matches up very nicely with the avg outdoor ambient (magental line) high for the year.  This is now coming down, thus the amount of heat to be extracted from the structure is less, thus the amount of heat to be 'dumped' into the earth is less, and thus the earth has a little more time now to absorb and dissipate the heat.

Looking back a year (52 weeks ago), the results are the same.

Best regards,

Bill

Irrigation pumping is similar, but most importantly, it gives some idea of the mineral deposits or fouling you might get.

I have a small (second) home on the Lake of the Ozarks (south central Missouri) and have recently begun to think about converting it to a ground source/lake loop heat pump system. My house is 700 sf with a 700 sf walk out basement facing the north, total square feet of 1400.

Besides all that I've read on the Web, I have my neighbor's experience to further add to the long list of questions I'm trying to find answers to. They built an approximately 2500 sf home in 1995, about 1300 sf up and 1200 sf on the lower level, a walkout basement. The installed an open looop heat pump (pump & dump) system. It worked great up until about a year ago when well issues forced them to make changes. Most years they would report to me that their total electric bill for heating, cooling, water heating and water was $1,000. Not bad They, like me, have a relatively shallow well, about 90 foot deep.

Last fall they decided their best alternative would be to put a loop in the lake and make the necessary modifications to their system becuase of that change. (As a point of interest, the Lake of the Ozarks drops about 5 feet during the winter to provide spring time flood control supervised by the Army Corp of Engineers) My neighbor and I live on the deep side of the channel. I have a thirty foot walkway out to my dock. On the shore side of my dock it's about 35' deep, on the lake side of the dock it's about 40'deep. My neiighbor's dock is probably 300' down the shoreline from my dock, so I realize his place could be more or less deep than mine.

This is what's curious to me. Last night I called him and he told me he thought his loop was in water that during the winter period was about 25-35 foot deep. He should have a fair idea, he helped the installer put it in. He fishes the area quite regularly. In the winter he began to think that his new system was not heating very effieciently. As a matter of fact, he tells me his worst bill was $250 for the coldest month. But what stumps me and him is he dropped a thermometer into the lake (I'm supposing where he thought the coil was) during that cold period. His reading he got was 35 degrees F. That was colder than he was expecting. Myself, I would have guessed 40 to 45 degrees at that depth, even in the dead of winter.

So this is my thought. In summer the loop would be throwing off heat into the lake and that heat would rise toward the surface. My guess is the lake would be in the 50 degree area at 25 foot down. (I'll measure that my next trip down there, the lake is presently at it's hottest point of the year and has been around 82 or 83 at the surface the last couple of weeks.) I can't scientifically explain that, but my guess is the heat would move toward the surface. Now in the dead of winter the loop is trying to throw off cold. My guess is the cold would stay put on the bottom of the lake surrounding the loop. What do the rest of you think? Is that why my neighbor measured such a low temperature last winter? There is not much noticable current where we live. A big log will travel very slowly if left afloat in the channel.

Please give me your opinions. His news that the water temp was 35 in the winter has made me a little disappointed about the potential for a water loop.

Thanks.

a weird characteristic of water is that it is at its densest at 39 deg F. Water cooled below 39 will rise, not sink.

System can still be quite efficient at source temps in the 30s or a bit less.

How does the increased pressure of being 25 to 30 feet down effect the temperature and density of water? How does that effect the weirdness you mentioned?

Temps down low are more stable. No effect on density since water is essentially incompressible

Your well should be around 57F - I would use that.

I would gladly use the well, except I think I could have the same issue that my neighbor had. It got to the point that he was having to replace his water filter every three or four days because of some kind of sediment/sand/or something. He couldn't afford to continue on. I have a type of small black sand in my well water. We've raised the pump in the well with no improvement. My neighbor considered drilling a new deeper well, but was warned by the drillers that he could still have the same problem. That's why he went with the loop in the lake.

I suspect that you would like to get rid of the sand even for other uses - you might look into a settling tank or centrifugal sand separator.

I'll have to look into your suggestions. A couple of years ago I had to remove my water filter and have not yet replaced it. I'd thought of reinstalling it without the filter and putting a screen in it, hoping the sand would fall to the bottom of the filter holder. I've never gotten around to trying that. Yes, things would be better without having to remove the little screens on the faucets every so often, to get the sand out. Thanks for the ideas.

The 10 - 25x flows imposed on a well by an open loop system verses normal domestic uses would greatly exacerbate any sediment problem

Some grit to sediment could also impact lifespan of heat exchanger.
j