ASHRAE Journal - December 2023 - 32

FEATURE
FIGURE 7 Thermal conductivity of moist, porous soils with heat addition and
extraction.10
Drying
Wetting
0.1
1.0
Moisture Content (%)
phenomena will limit the modes of heat transfer and
long-term thermal performance of cold climate GHXs.
The impact of thermal performance of GHXs' grouts
and backfills below the freeze point of water is critical to
understand and deserves additional investigation.
Conclusions
 Heat transfer in formations surrounding GHXs
occurs in three interconnected modes of conduction,
convection and phase change (evaporation, freezing).
 GHX models that assume conduction heat
transfer is the primary and dominant mode will
be inaccurate with the uncertainty dependent on
porosity, moisture content and water migration/
percolation.
 The impact of HVAC system design quality,
especially the relative amount of fan and pump power
and energy use, affects the short-term and long-term
performance for GHXs and GSHP systems.
 Economic and performance analysis of GSHPs
using GHX models that do not adjust for long-term
field data measurements and HVAC system details will
be not be credible.
 Unacceptable long-term temperature change
in GHXs will be more likely to occur when the heat
removal imbalances are dominant compared to heat
rejection dominant systems.
Recommendations
The inability of the awardees of 1817-TRP to locate
sufficient long-term data to perform precise evaluations
indicates this information is not available. An alternative
to modify the stated goals of the project is to collect GSHP
system details, energy consumption, hydro-geological,
GHX dimensions and temperature data from many sites
that have operated successfully for many years. This
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7.4%
8.2%
2
1
approach would also provide additional information to
improve GSHP HVAC system and GHX design. These
would include determination of:
 Optimal HVAC systems with low demand and
energy consumption;
 Geological conditions amenable to smaller or to
larger GHXs;
 Best practices for energy and low demand
outdoor and indoor pump/pipe networks and;
 Efficient equipment types and components with
long service life and low maintenance.
Recommendations for Regenerating Ground Heat
Exchangers
The GSHP industry has dealt with overheated GHXs
using fluid coolers or cooling towers with isolation
heat exchangers. These devices have been used in the
original design stage (hybrid GSHPs) or to regenerate
overheated loops. The use of solar heating systems is
recommended in lieu of boilers or water heaters to
regenerate cold/frozen GHXs. Both solar thermal and
photovoltaic (PV) systems can be applied. Additional
information from current and future efforts using
solar regeneration should be compared with current
" hybrid " systems using fossil fuels. Elimination
of boilers and water heaters with solar is suited to
decarbonization efforts.
References
1. Lohrenz, E. 2013. " Energy Modeling for GCHP Systems. "
Thermal Dynamics, Inc. https://www.youtube.com/
watch?v=WcdEJGpteHsGLD
2. ASHRAE. 2020. " 1817-TRP, Long-Term Temperature Change of
Ground Heat Exchangers (GHXs). " Research Proposal. ASHRAE.
3. Dong, B., E. Bogucz, Y. Li, Z. O'Neill, A. Bandyopadhyay, et al.
2021. " Phase 1 Report to Project Monitoring Subcommittee, ASHRAE
1817-RP. " ASHRAE.
4. Kavanaugh, S.P., J.S. Kavanaugh. 2012. " Long-term commercial
GSHP performance, part 3: loop temperatures. " ASHRAE Journal 54(9).
5. U.S. Climate Data. Undated. " U.S. Climate Data, Weather
History Jackson-August 2010. " https://tinyurl.com/y6fszbub
6. Kavanaugh, S.P. 2016. " Ground source heat pumps in Texas
school district. " ASHRAE Journal 56(10).
7. EIA. 2018. " 2018 Commercial Building Energy Consumption
Survey Data. Consumption & Expenditures, Table C4. " U.S. Energy
Information Administration. https://tinyurl.com/mry35pjw
8. Im, P., X. Liu, H. Henderson. 2016. " Case Study for the ARRAFunded
Ground-Source Heat Pump Demonstration at Ball State
University. " ORNL/TM-2016/644. Oak Ridge National Laboratory.
9. 2019 ASHRAE Handbook-HVAC Applications, Chap. 35.
10. Farouki, O.T. 1981. Thermal Properties of Soils: CRREL Monograph
81-1. U. S. Corps of Engineers. Cold Regions Research and
Engineering Laboratory.
Thermal Conductivity (Btu/f·ft·°F)
https://www.youtube.com/watch?v=WcdEJGpteHsGLD https://www.youtube.com/watch?v=WcdEJGpteHsGLD https://tinyurl.com/y6fszbub https://tinyurl.com/mry35pjw http://www.ashrae.org
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