G e o t h e r m a l | ||
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In-Situ Geothermal Testing vs. Predictive | ||
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Determination of the thermal properties of the earth is a major challenge facing designers of Ground Source Heat Pump (GSHP) systems. The number of boreholes and borehole depth depend upon these thermal properties. The thermal properties are directly related to the geologic formations beneath the project site. Geologic formations change in character, both vertically and horizontally, even on the same site.
It is well known that the cost of a geothermal wellfield is highly dependent upon the geologic formations’ drilling conditions and geothermal properties.
See the success of Predictive vs In-situ Thermal Conductivity testing here
Typically, most designers of GSHP systems drill a test hole at the proposed site. The test hole determines drilling conditions and somewhat of a driller’s log showing the borehole geology. A 1.0 to 1.25 inch high-density polyethylene geothermal loop is installed in the borehole, which is grouted from the bottom to the top. About a week later, an in-situ thermal conductivity test is conducted on this test hole to determine the thermal properties of the earth.
Companies selling “test holes” generally claim that this is the only way to really know what the thermal properties of the earth are at your site. That may have been true in times past, but now there is a much more cost-effective solution. The problem with test holes is time (which is money), and money. Drilling and installation of a typical 300 ft test hole can cost about $3,000.00 to $6,000.00. This cost does not include the thermal conductivity test, which will cost another $3,500.00 to $4,500.00. So the cost alone for one test hole can be over $10,000.00. Now what about time?
In reality, it takes at least one week (+1) to find and schedule a driller. Once the borehole is completed (geothermal loop and grout installed), it takes about one week (+1) to equilibrate. The earth has been disturbed, so this extra week is necessary. After this week goes by, the thermal conductivity test can be conducted. In general one day is needed to mobilize and setup the test, two days to run the test (collect data), and possibly a 4th day to demobilize. In reality, this is another week (+1). On top of this you should allow another week (+1) to obtain the Thermal Conductivity Test Report. So from conception to receiving the actual report is about one month.
Some Owners may not want to spend this extra time or money. Other Owners may want a more inexpensive ‘look’ to even see if he wants to pursue a GSHP system. And now they don’t have to. There exists another solution, with proven results and backed by science. There is a plethora of geologic data (maps, well logs, cross sections, geothermal information) all across the U.S. This information has been used to successfully predict the geologic conditions and thermal properties of the earth.
How It Works The subsurface earth parameters (thermal properties of the earth) every geothermal designer needs are the: (1) average thermal conductivity; (2) average thermal diffusivity; and (3) average earth temperature. Geologist, Roger Sigler will provide his conservative predictions of these thermal properties to you for pennies on the dollar.
This will give you enough information to conservatively design a GSHP system. During the construction phase, you can run a thermal conductivity test on one or more actual geothermal boreholes (part of the actual GSHP system) to verify the predictive values given during the design phase. Thus, no time will be lost.
* The high thermal conductivity (TC) value at Pittsford is probably due to rapid groundwater flow conditions. It has been found that groundwater flow increases the effective conductivity (Spitler, Rees, & Chiasson). The design (based on a TC of 1.13) of 125 boreholes x 400’ in depth could have been decreased to 105 boreholes based on the measured TC of >2.0. However, since groundwater conditions can change seasonally or over time, the engineer installed all 125 boreholes. | |
Know the Geology beneath your feet. | ||
Copyright © 2005 R. Sigler | Contact Information: Roger Sigler Ph: (281) 450-0557 Fax: (281) 492-0910 eMail: RGSIGLER@sbcglobal.net |  |
