Where can I find aproximate ground temperatures for example in Eureka Nevada?
Bruce

You looking for data for 5' below grade, or 100' below, i.e., for a horizontal ground loop, or vertical? If the latter, I would think well driller reports on file with your state water resources dept is probably about a good a place as any. Some drilling reports record water temp during the well performance test. Look for wells drilled close to the depth you would be drilling.

Attached are a couple of chart thats might help.   The appendix shows deep ground temps.

Thanks guys! That helps a bunch. It looks like I am looking at an average of 50 to 53 in Eureka.
I am contemplating lateral field. I have about a half acre to work with. The unusual variable is that I
believe that there is a fair amount of white rock in the soil. Don't know for sure until I start digging. As I am sure you know
white rock is an insulator, so that tends to complicate things. I am guessing that I may have to go down instead
lateral if this is the case. I welcome any thoughts on this.
Bruce

Looking at the mpas, I'm suprised there are such a large swings in underground temperatures. Where Fairbanks Alaska has 0 degree F and Bangkok has 85 degree F. I always thought that below the frost line, the ground temperature stays pretty much consitant, around 50 degree F.  I would guess these numbers are for temeratures within 25 or 30 feet of the surface, I would be very much suprised that these number hold true for depths deeper than that.

My guess is that the 0° for Fairbanks is a mistake. With the other low temps in the 40's, 0 for Fairbanks is just out of place.

The 50 degree constant ground temp is a common misconception. The ground temps vary by location and reflect the average annual temperature of the surface. For example the average air temp over the course of 1 year might be 65 F in the south and so  the average ground temperature will be about 65. In a northern clime with an average air temp of 45 F the average ground temp will be 45F.

Of course the actual ground temp throughout the year will vary depending on how deep you are measuring. Near the surface will vary more and deeper will vary less.

The ground temps at 25 to 30 feet will be very close to the average annual surface temp and vary little. (If you go spelunking in the north and the south this will be obvious as the temperature of deep caves reflect the average surface temp for that area.)

If you go deep enough you will have effects from the earths molten core. In most areas this is many thousands of feet deep. In a few areas it is much closer to the surface (Yellowstone park for example).

So your claiming that temperatures 500 feet underground are very close to the average surface temperature? I find that difficult to beleive.  Ruling out molten lava from the core heating up the rock around it, I wouldn't think the surface would have that much affect on the temperature deeper underground. I've heard that the temperature in Manhattan is hotter the farther you go down underground, but I assume it from the massive heat buildup from the power and heating requirements of the skyscrapers above, and only to a point where the heat is disappated enough by a larger area of rock.

Assuming the temperature averages 50 degrees in an area, and the surface temperature becomes colder than 50 degrees, wouldn't the underground heat affect the surface temperature as it gave off its heat above?  There a Mine fire in Columbia County, PA that has burned for the last 34 years with temeratures ranging from 617 to 772 Fahrenheit. I would think if the effects of the sun heating underground temperatures up the reverse would be true, the high underground temperatures should heat everything above it up to the point of bursting in flames, since this hasn't happened I would assume the transference of heat does not conduct thru more than a few hundred feet as most. Another point, if it is indeed true that the surface temperatures can heat thousands of feet of rock up below it, than is it safe to assume that the molten core measuring thousands of degrees in temperature can heat thousands of feet of rock above it making the surface of the planet uninhabitable? I would need evidence to convince me that surface temperatures affect underground temperatures so deep down, and still there must be a point where the affects disappate otherwise the molten core below us should be able to heat the surface to lethal levels.    

link
To PA Mine Fire

The oil storage caverns of the Strategic Petroleum Reserve along the Gulf Coast are nominally about 2000' deep and 2500' to 4000' below ground surface. This means the bottom of the caverns are on the order of 4500' to 6000' deep. The stable temperature of the oil, when there is no oil moved in or out for 10 to 15 years, is on the order of 120F to 130F. These are in salt domes which reach 10s of thousands of feet deep so very well may be conducting more heat upward than does the usual soil and rock formations of the crust.

Have they looked into using those as a source of heat? Seems like a huge oil bath like that would be perfect to provide heat for an entire community.

I retired from the SPR so have no influence now. Specific heat of oil isn't real great so you'd have to pump a lot of it to retrieve the Btus you'd want. It's not feasible to pump the oil in and out simultaneously in a cavern due to the way the wells are configured. The oil is at a high pressure at the well head, on the order of 700 to 900 psi so moving it around willy-nilly is not wise.

On top of all the technical problems is the political issue of competition. If Uncle were to suck heat out of the caverns and sell it to the local community they would be directly competing with public utilities. That idea has about as much chance of sailing as an ice cube on a bath of molten steel!!


But it's still an intriguing idea!!

This may be a little offtopic, but I read somewhere that it's currently only possible to pump 1/3 of what an oil field has, and newer extraction methods like injecting high-pressured steam can improve that amount marginly. Can you explain why this is and if it was possible to completely pump a well completely dry, could the void inthe earth cause a massive sinkhole?
  

Sorry, I can't give you a definitive answer. Two people I'm not, a geologist or a petroleum engineer.

But I will say this. Think of a sponge soaked with mineral oil inside a hard shell. Then put a straw in it and suck the oil out. The shell can't collapse to squeeze the sponge, and the oil migrates slowly from the edges of the sponge to the straw. When the pressure in the sponge drops there is nothing to push the oil to the straw. You pump water down and pressure up the sponge, but there's no easy way to make the oil, rather than the water, move to the straw. Simplistic example maybe, but should give you some idea of the process.

About collapsing. It's my understanding most oil is in nooks and crannies in rock and sand thousands of feet deep, like 10,000'+. When you remove the oil the physical structure remains and is self supporting. It's kind of like sand and water. The sand doesn't collapse when you remove the water. Also, being so far down any collapse of rock and sand would mostly cause nothing more than a slight decrease in the density of the formations between it and the surface.

Geodean - I really liked the PDF files with the groundtemp info.

So if 30 feet is how deep you have to go to reach constant temps - then horizontal loops should be more and more efficient the deeper they are until you get down to 30 feet? After all - according to the graph if my loop is 5 feet down - then in January the return temp on the loop will be cooler.

So this begs 2 questions -

1. Should I get my installer to put my horizontal loop like 10 feet down instead of the 6 feet that he will likely suggest? (I know this will cost more)

2. Are vertical loops more efficient (i.e. have a higher COP) due to the fact that in January the dirt around the loop will be warmer than it would be on a horizontal loop?

Todd

Good Questions.  It depends on a couple of things, mostly how much pipe you have in the ground.

It is true that ground temps are more constant the deeper you go until about 30'
It is also true that the warmer the water is coming into the heat pump, the higher the COP.
As heat pumps draw heat out of the ground,  the temperature of the ground around the loop pipes has to drop.  The key is to have enough pipe in the ground so that the temps don't drop too much.

Just going deeper isn't the cure all.

There are a lot of systems out there both vertical and horizontal that have been "short looped" by the installer.  Meaning there is not enough pipe in the ground.  As the heating season progresses, the loop temp keeps dropping until the incoming water temp is below the capacity of the heat pump and then the unit locks out.  Then when the cooling season starts, the loop temps keep climbing until it is higher than the capacity of the heat pump and the unit locks out.  Needless to say, these folks are not happy with their systems.

My house has a 4.5 ton heat pump.  I have 6000' of slinky pipe buried in my yard.  I dug trenches 9' deep by 1' wide and dropped the slinkies in.
I did not at any time work in a 9' deep trench.  It is now the end of January and my loop temp is 42°.  The loop temp in October after heating the ground all summer was 55°.   Since my heat pump can handle incoming temps of 30°  and 90° I am doing quite well.  The swing from 55° to 42° is largely affected by surface temps since the top of my loops have 5 - 6' of cover.

So having said all of that ...is it better to put your loop deeper ?  Maybe, you have to consider the cost and the safety factor.  It is against the law (OSHA) to work in a trench deeper than 5' with out some kind of trench protection.  Or you can taper the sides of the trench.

The graphic ( see above post: ground temps2.pdf) shows that for 5' deep the temps swing 20° and at 10' deep the temps swing 10°.  So by going 5' deeper your loop should be 5° warmer in the winter.  How much is the increase in COP for 5° warmer water?  I don't know,  but not a lot. In looking at my heat pump manual, COP at 40° is 3.84 and 30° is 3.45.  This is a difference of .39 COP for 10° warmer water.  So 5° would give you an increase of about .2 COP or about 5%.

So is digging 5' deeper for a 5% more efficiency worth it?  Let me know what you think.

Hope this helps.

Thanks for the reply - I appreciate the numbers for the horizontal loop depth. Let me try the same logic for vertical loop - and lemme know if you agree.

Well I just read the a pdf file I got of the forum that said that horizontal well cost about 700 per ton and vertical wells about 1100 per ton. Say I have a 4 ton system - that would be a 1600 dollar price difference for well drilling.

If a vertical well is really constant temp then one should get a 10% improvement - if our logic holds -

10% drop in ones heating bill - say year round bill of 1500 for heating a cooling with geo - and it would take 10 years to make it up - very roughly.

I plan to live in this home for 40 years - so maybe I make out all-right doing vertical wells provided I don't live on bedrock that's real hard to drill through.

Todd

Temps in a vertical bore field are not constant unless you really oversize it.  The ground temps will still vary summer to winter as you take heat out of the ground and put it back in.  How much swing depends on the thermal conductivity of the surrounding formation.  Your reasoning is sound,  I just want to give you accurate info.

Don't count too much on pricing you might find on the internet,  you really need bids from local contractors.

Let us know what you decide.

Vertical well temps are not constant. Prce is higher per foot of pipe. A good designer will design within the limits of the heaatpump. You will get improved performance if you oversize the vertical well to keep constant temperatures. You will probably see similar improvement if you oversize the horizontal loop. Choose your premium.