11-18-10, 10:20 PM | #361 | |
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Actually, as I look at this graph again, it actually confirms what I mean. In this test you were adding heat with a coffee cup heater, right? The graph shows the temperature leveling off... Think about it, if the borehole wasn't able to keep up with sinking the heat, the T would be shooting straight up and your water would be boiling! Instead, as it reaches a higher delta T, it is leveling off, so the borehole is actually keeping up and reaching equilibrium. But as you pointed out, whether that would remain true for weeks or months is certainly a different story. My point is simply that the rate of heat transfer through the soil seems like it should depend on the delta T, not just be static. |
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11-18-10, 11:57 PM | #362 | ||||||
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I think the problem here is that you might be reading the "Temp F" part of the chart and thinking that it means "delta T". It really is the temperature of the water in the test box. This test is usually done by measuring the temp of the water going into the loop and the water coming back from the loop, averaging them mathematically and plotting that.
I made the simplifying assumption that since the water coming back from the loop was splashing and swirling around in the test box, it was physically averaging itself, so there was no need for separate measurements and an averaging computation. Quote:
To be really correct, the slope of the curve is getting progressively smaller, in a logarithmic fashion. Quote:
Do you really mean to say, "shooting straight up"? This would take an infinite amount of heat. I only had 250 watts. I think you need to tell me what you actually mean by "delta T"... As I consider the graph, in the first part of the test, the delta T (difference between the temp of the water in circulation and the temperature of the earth surrounding the pipe), the delta T is highest at the beginning of the test and more heat is transferred, as reflected in the lower temp in the box. Then because heat travels so slowly through earth, the delta T becomes gradually smaller, and there is a temperature rise of the water. Quote:
The thing that amazed me about the test is that the borehole did not reach equilibrium. I expected it to do so in about six hours. The test only ran 157.3 hours, enough for my purposes, which was to determine how much heat I could expect to pull out of the hole, on an ongoing basis. But in 157.3 hours it never reached a balance. Quote:
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-AC_Hacker |
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11-19-10, 01:22 AM | #363 | |
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GSHP payback
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In your particular case I would buy portable heat pump(like portable AC). Because you do not need much heat and when you decide to move you will be able to take it with you to your next garage. GHSP have very long payback period. |
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11-19-10, 01:40 AM | #364 | |
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The funniest thing in whole thread LOL
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11-19-10, 03:11 AM | #365 | |||||
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The temperature difference (delta T or dT) is the difference between the ground temperature and the water temperature (or in a more general sense the temperature difference between hot and cold, causing heat transfer). It is what causes the transfer of heat across the boundary of the tubing. I assume you started with tap water at roughly the same temp as the ground, so the original delta T would be 0. There is no difference. So if you just let the water sit there without heating or pumping, it would remain at 54 degrees because with delta T of 0, no heat transfer will take place. Q=mCdT If dT=0, then Q=0. But, if you look at the data, at 4 hours you have 79 degrees in your water. You don't have a reading for the ground temperature surrounding the tubing, but say for the sake of argument it was raised to 60 degrees (it should start to rise due to the warm water nearby). Now your delta T is 19 degrees. Then, at 7.5 hrs, your water is at 85. If your ground were at 65 degrees, your delta T would then be 20. Since you are adding heat to the water, you are creating a difference in temperature, causing heat to transfer from the water, across the tubing walls, into the soil. The soil is soaking up the heat you are adding due to the temperature difference. And that transfer of heat is directly proportional to the temperature difference. The point I was trying to make is that even though you are adding a constant (linear) amount of heat energy from the resistive coil of the coffee heater, your rise in temperature of the water is not linear. In the first half hour your water heats up 6 degrees, but at the end it requires 74 hours to heat the water 6 degrees. That means the ground is catching up with the heater coil. The only way that can happen is by a growing difference in temperature, allowing more and more heat per unit time to transfer through the tubing. Eventually, because of the temperature difference, the rate of energy absorbed by the soil will equal that of the energy being added by the heater. Here is what I mean graphically: Quote:
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For example, put that pump and heater in an insulated box with water and no outlet or inlet and plot the temperatures again. Of course you would expect to see a reasonably straight line with a positive slope. Now, imagine if you connected the ground loop and your soil was absorbing the heat energy at a constant rate. Then you would still have a straight line, the slope would be smaller. Quote:
Steady State I recognize oversimplification of a complex process. Last edited by pick1e; 03-23-11 at 12:09 AM.. Reason: image is working now |
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11-19-10, 06:44 AM | #366 | |
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* * * I hope your work on your project goes well. Don't forget to share photos. There may be a different site for this kind of work in the future. Do you wish to be notified when it comes on line? -AC_Hacker |
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11-19-10, 07:09 AM | #367 | |||
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If you knew how easy, fast, cheap, strong and long-lasting HDPE welding was, you just might change your approach to this project. Quote:
When you consider the difficulty of installation, it might be easier and cheaper to just dig another hole, and install more pipe. If you are really space challenged, then gaining the extra % could be worth it. There may be a different site for this kind of work in the future. Do you wish to be notified when it comes on-line? -AC_Hacker Last edited by AC_Hacker; 11-19-10 at 07:13 AM.. |
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11-19-10, 11:16 AM | #368 |
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Bummer
I think it's a bummer that there is so much good and helpful information here, and yet some bad information that you have no interest in correcting. So I guess you are more interested in showing off than helping other DIYers.
How many people are going to follow what you have done, and waste their time graphing useless information and calculating what they think is thermal conductivity from an erroneous equation that was grabbed from some kid's failing school paper instead of trying to understand some thermodynamics? (You could ask the paper's author how they arrived at that equation but they didn't even put their name on the paper.) Instead of all that work and getting a "k" value of 0.569 (which should be between 1 and 2 for soil, even the bad paper says that on page 5.) you could have saved a lot of effort by simply using the thermal conductivity of HDPE pipe, 0.50, since it is your bottleneck anyway. Last edited by pick1e; 11-19-10 at 11:17 AM.. Reason: Forgot a definite article |
11-19-10, 08:03 PM | #369 | |
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The great thing about DIY is that you now get to build your system your way and demonstrate by the performance of your system, the superiority of your approach. And as to the testing methodology, you also get to develop a truly relevant method. Not only I, but the whole GSHP industry stands to benefit from what you will do. I am very interested in the results of your efforts. We are very fortunate to have someone of your discerning perception working with us. Don't forget the photographs... Best Regards, -AC_Hacker Last edited by AC_Hacker; 11-19-10 at 08:06 PM.. |
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11-19-10, 10:48 PM | #370 |
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AC,
My qualms with some of your methods aside... You have a lot of good resources linked here and there throughout this huge thread, have you considered compiling them- for example at the bottom of the first post? |
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air conditioner, diy, gshp, heat pump, homemade |
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