Project Report
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1.0
Introduction
1.1
Background
1.2
Purpose
1.3
Deliverables
2.0
Project
information
2.1
Technical Review
2.2
Environmental Review
2.1
Discussion of Technology Application
2.2
Test
Results Discussion
·
Range of Results
·
Open System Consideration
·
Design Resources
3.0
Conclusions
4.0
Recommendations
5.0
Disclaimer of Warranties and Limitation of Liabilities
1.0 INTRODUCTION
1.1
Background
According to the EPA, geothermal heat pumps are the
most energy-efficient, environmentally clean, and cost-effective space
conditioning system available.
In a report prepared for the DOE, the Arthur D. Little
Company said, "The use of geoexchange systems
(geothermal heat pumps) results in increased electric load, improved (utility)
asset optimization through higher annual load factors, and slower peak load growth".
TVA
has been heavily promoting the use of geothermal heat pumps in schools and
other commercial buildings since 1995.
The Customer Services and Marketing (CS&M) and the Public Power
Institute (PPI) groups working together have promoted the technology and
facilitated market transformation using an integrated approach. Prior to TVA’s effort there was essentially no use of geothermal heat pumps in the
commercial market.
A
majority of the applications have been in schools and use vertical closed-loop
ground heat exchangers for the geothermal component.
1.2
Purpose
Much
work has been done in developing software and design procedures to properly
size the vertical ground heat exchanger based on building loads, the thermal
conductivity of the earth and the grouting/backfill methods used to install the
pipe. One important aspect of system
design is determining the thermal conductivity (TC) of the earth. This study documents the TC testing done in
the Tennessee Valley and overlays the sites on a geology map in order to assist
the design engineer in the preliminary system design.
1.3
Deliverables
The
final report is the project deliverable.
The project report includes TC test results for 89 sites throughout the
Tennessee Valley overlaid on a geology map.
The drill logs and TC test reports as available are included in the data
contained on the attached CD ROM.
2.0 PROJECT
INFORMATION
2.1
Technical Review
The
American Society of Heating, Refrigerating and Air-Conditioning Engineers
(ASHRAE) has sponsored research on the best practices
associated with performing TC tests. The ASHRAE recommendations were for test
durations of 36 to 48 hours. The tests
performed in this report range in duration from 7 hours to 49 hours. The ASHRAE work also defines other
recommended test parameters. It should
be noted that some of these tests were conducted prior to the ASHRAE research
project.
It
is beyond the scope of this report to interpret the accuracy and/or validity of
the tests procedures used by the different TC testing contractors.
2.2
Environmental Review
This
project was a paper study of pre-existing TC testing. No new test holes were drilled as part of the
project.
Overall, the application of geothermal heat pumps should
have a positive environmental effect.
According to the EPA, geothermal heat pumps are the most
energy-efficient, environmentally clean, and cost-effective space conditioning
system available. In a report prepared
for the DOE, the Arthur D. Little Company said, "The use of geoexchange systems (geothermal heat pumps) results in
increased electric load, improved (utility) asset optimization through higher
annual load factors, and slower peak load growth".
One concern with the installation of vertical ground
heat exchangers is the protection of groundwater. It is necessary to prevent the injection of
contaminated surface waters into clean aquifers, and to prevent the
cross-contamination of aquifers. The
Geothermal Heat Pump Consortium (GHPC) funded a multi-disciplined task force
headed by the National Ground Water Association (NGWA) to address the
requirements for the installation of vertical ground heat exchangers (to
provide good thermal performance and protect the groundwater.) The product of the task force was the NGWA
printing of Guidelines for the
Construction of Vertical Boreholes for Closed Loop Heat Pump Systems. This document is available from the NGWA and
represents the industry’s consensus of the best practices for installation of
vertical ground heat exchangers. TVA staff participated in both the regional
and national workshops in the development of the document.
2.3
Discussion of Technology Application
Determining
the TC of the earth is one of the important design considerations in optimizing
the design of the vertical ground heat exchanger. The design procedure also requires an
accurate load calculation with various data inputs depending on the analysis
software being used.
Oversized
ground heat exchangers perform well thermally, but run up the system first
costs. Undersized systems are cheaper to
install, but suffer from extreme loop temperatures which make the systems less
efficient, reduce the heating/cooling capacity, and in extreme cases cause the
heat pump units to trip out on head pressure which means they can not meet the
heating or cooling loads. This is most
common during the cooling cycle as commercial buildings tend to be internally
load dominated, and the heat rejection to the ground loop must include both the
building load and the heat of compression.
2.4 Test Results
Discussion
Range of
Results
TC
test results conducted in the Valley ranged from a 1.03 to 2.63. Of the 89 TC Tests included, the mean TC
value was 1.63 and the median was 1.60.
There were several instances where it appeared that improper backfill
resulted in a lower than expected TC value.
An example is site ST-17 where two test holes were completed in similar
drilling conditions. One test hole
performed at 1.90 where a second test hole was at 1.1. The higher TC number was used in the
reporting. Both holes were backfilled
with cuttings. It is most likely that
the lower hole had some bridging during the backfill where some of the loop was
not in good contact with the earth. This
illustrates the concern of proper backfill for good thermal performance.
Generally
the lower numbers were characterized as mostly clay. Medium numbers were generally some soil with
various limestone mixtures. The higher
numbers were typically a mix of limestone and shale for most of the depth. The lowest TC value of 1.03 was in Paducah,
KY where the drill log revealed a mix of sand and clay. There was only 1 foot of rock encountered in
the entire 310 foot borehole. The
highest TC value of 2.63 occurred in West Knoxville where the drill log
indicated clay and gravel for the first 53 feet, with the remainder of the 300
foot bore hole being limestone. Moving
ground water may have contributed to the high TC value at this site.
Open System Considerations
In
some cases a significant amount of groundwater was encountered during the test
drilling. The option of using an open
system should be considered if an adequate groundwater resource is found. Open systems have far lower first costs, however they typically have higher maintenance
costs. They also have the risk
associated with the well running dry.
Proper disposal of water and the necessary permitting must be
considered. One good application is to
use the water for lawn and grounds watering.
The relative advantages and disadvantages of using open systems can be
discussed with the owner/operators if a large enough groundwater resource is
found during the test boreholes. If an
open system is being considered, it may be more appropriate to perform a draw
down test than a TC test.
Design
Resources
Design
of both open and closed systems is discussed in detail in the ASHRAE
publication Ground-Source Heat Pumps –
Design of Geothermal Systems for Commercial and Institutional Buildings –
Kavanaugh and Rafferty.
An
additional reference entitled Geology
and Drilling Methods for Ground-Source Heat Pump Installations: An Introduction for
Engineers was developed as a joint effort of ASHRAE, the GHPC and
TVA. It is available through ASHRAE. The
document was written by Dr. Harvey Sachs, former technical director of the GHPC
and reviewed by ASHRAE’s Geothermal Utilization Technical Committee. It is meant to provide good background
information on drilling and geology as it relates to geothermal heat pumps.
3.0
CONCLUSIONS
The
need to determine the earth’s TC in the optimum design of large vertical ground
heat exchangers has been established by the industry. The thermal conductivity mapping data will
help the design engineer with at least a good cut at conceptual design and
budget estimates. For large projects, or
sites with limited area it may still be best to perform drilling tests and TC
tests. Drillers are reluctant to give
fixed cost prices without test bores.
Some may still want to add casing clauses. If forced to bid fixed costs without a test
borehole, drillers will usually increase the price to cover the unexpected.
The design engineer must ultimately decide on whether
to require a test borehole and TC test.
As designers gain experience, they may be more comfortable in just doing
a test bore and estimating the TC. For
smaller systems it may be acceptable to design around an estimated TC and
verify the performance when the drilling starts (by reviewing drill logs and/or
requiring a TC test.) For larger the systems
there is a greater need to accurately determine the TC. As the geothermal industry matures, this
should be a routine part of the design process.
As designers gain experience in systems in a specific geographic area,
they may be able to accurately estimate TC.
However if the engineer handles the design by estimating a low TC, and
increases the ground heat exchanger size accordingly, it could unnecessarily
raise the owner’s cost. In that case
the owner would be much better off to have a test borehole and TC test done for
every large job.
4.0
RECOMMENDATIONS
(A) Continue to promote geothermal heat pumps as a high
performance all-electric heating and cooling system.
(B) Encourage design professionals to review available
information such as well drilling records, geology maps, and this report when
determining feasibility of potential geothermal heat pump sites.
(C) Encourage design professionals to use TC tests as part
of the design process for large geothermal jobs.
(D) Encourage that TC tests be performed according to the
ASHRAE recommendations.
(E) Keep accurate records of future TC tests for future
references.
(F) Include GPS coordinates of any future TC sites.
(G) Include a copy of the drill log in the TC report.
(H) Investigate the possibility of using an open system if
a significant groundwater resource is found during test borehole drilling.
5.0
DISCLAIMER OF WARRANTIES AND LIMITATION OF
LIABILITIES
THIS
DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED
OR COSPONSORED BY THE TENNESSEE VALLEY AUTHORITY (TVA). NEITHER TVA, ANY
DISTRIBUTOR OF TVA POWER, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY
PERSON ACTING ON BEHALF OF THEM:
(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER ,
EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION , APPARATUS,
METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR
INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY’S INTELLECTURAL
PROPERTY, OR (III) THAT THIS DOCUMENT IS
SUITABLE TO ANY PARTICULAR USER’S CIRCUMSTANCE;
OR
(B)
ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER
LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF TVA OR ANY
TVA REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES)
RESULTING FROM YOUR SELECTION OR USE OFTHIS DOCUMENT OR ANY INFORMATION,
APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT.
ORGANIZATION(S)
THAT PREPARED THIS DOCUMENT
TVA
Pubic Power Institute
THE
FOLLOWING SUB-CONTRACTORS PERFORMED THERMAL CONDUCTIVITY TESTS INCLUDED IN THIS
REPORT:
(A) Adams-Brown Services, Inc.
(B) Earth Energy Engineering, Inc/Ewbank
and Associates
(C) Georgia Geothermal, LLC/Ewbank
and Associates
(D) Geothermal Resource Technologies, Inc.
(E) Ted Wynne Engineering Contractors, Inc.
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