Quote:
Originally Posted by randen
From Wikipedia direct exchange geothermal:The boreholes are drilled to a length of 50, 75 or 100 ft (15, 22 or 30 m) with a diameter of 3 inches (76 mm). A total of 100 feet (30 m) to 140 feet (43 m) of drilling is needed for each ton (3.5 kWth) of system capacity. At 40' from your home a 60' line set is doable.
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Randen, very good info there!
I was just looking on a table of
Thermal Conductivity values of some common Materials and Gases. These values are to be used in equations to calculate the quantity of heat flow, when you know delta-T, and other factors. But I think it is useful to know what these values are, to guide intuitive judgment.
Here are some of the values:
- Carbon Steel - 43
- Aluminum - 205
- Copper - 401
- Concrete, stone - 1.7
- HDPE - 0.42 - 0.51
- Water - 0.58
It's not surprising that aluminum would be a much better conductor than carbon steel. And that copper is much better than aluminum.
I have listed concrete & stone, because I have seen many tables that give the heat transfer of dry earth, damp earth, wet earth, etc. And the value of concrete and stone are very close to the cluster of values found in bore hole heat transfer conditions.
I was just about to make this entry, to suggest that the loops in water could be made much shorter than the loops in earth, but the thermal conductivity value for water really surprised me.
I would imagine that this value would be more favorable when the system is actually in use, because there would be at least some modest flow of water past the tubing due to temperature differences, whether they are copper or HDPE, but it would certainly not be anything like water that is driven through a HX by a pump.
Also, of interest is that when heat flow is calculated, the various factors that would inhibit thermal flow are all added together, for instance the resistance of flow from the refrigerant itself, the resistance of the boundary layer of refrigerant, the resistance of the transfer tubing (copper or HDPE), the resistance of the boundary between the tubing and water, and the resistance of water itself, there could be other terms added in like resistance due to moss, etc.
So the equation for the overall resistance would look something like:
TOTAL RESISTANCE = Rref + Rbdy1 + Rtube +Rbdy2 + Rwtr
So, although the heat transfer index for copper is a big number, and the heat transfer index of HDPE is a very small number, and the ratio of one to the other is something like 800 to 1, when the terms all get summed up and put into the equation, the difference in overall thermal transfer efficiency is nothing like 800 to 1, it is far more modest.
For example...
Gary, over at
Build It Solar did has done a huge amount of testing on many aspects of solar heating, but there was one test that he did that I thought could relate to MEMPHIS91's current project...
HERE is the page that detailed the construction and test setup.
HERE is the page that detailed the test itself, and the results. The conclusion odf the test are quoted below:
Quote:
Commercial Copper Collectors
Commercial copper collectors offer known high performance, long life, and good resistance to high temperature stagnation temperatures for all tilt angles. But, they are expensive (about $25 per sqft), and shipping is costly and can be very frustrating (it took three tries to get an undamaged set of commercial absorber plates for my Solar Shed collectors).
PEX Tubing -- Aluminum Fin Collector
From a cost effectiveness point of view, the PEX collector does very well. If you are willing to put the labor in, you can build the PEX collector for about 1/6 th the cost of a good commercial collector, and only suffer a 15% loss in performance. This makes the PEX collector 5 times as cost effective as a commercial collector on a BTU per dollar basis. In most cases, the loss in performance can be made up for just by making the collector a bit larger. You can literally build the PEX collector for about what it costs to ship a commercial collector to your house!
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Make of it what you will...
Best,
-AC