ASHRAE Journal - December 2021 - 36

TECHNICAL FEATURE
calculated lengths of 123.0 m (404 ft) and
118.5 m (389 ft), respectively.2
TABLE 5 Detailed results for the third sizing problem using GHXSizing.
METHOD
Concluding Remarks
This article proposes a multilevel approach
for sizing VGHEs. Most of the methods presented
here rely on ground thermal response
factors, also known as g-functions, which are
the subject of a follow-up article. Low-level
approaches use only three ground load pulses
and can be used at an early design stage when
limited information about the ground loads
is available. Higher-level approaches use
monthly or hourly loads to size the bore field.
The sizing equations and the example contained
in the 2019 ASHRAE Handbook-HVAC Applications are
revisited with GHXSizing, a spreadsheet tool that
accompanies this article and can size rectangular and
nonrectangular bore fields. It is shown that the discretization
(number of segments) used in the determination
of the g-functions influences the required
borehole length.
SHORT-TERM
EFFECTS
Monthly Sizing Method L3
Monthly Sizing Method L3
Monthly Sizing Method L3
Hourly Sizing Method L4
Modified ASHRAE Equation L2
Alternative ASHRAE Equation L2
×
×
P
×
×
×
NUMBER OF
BOREHOLE SEGMENTS
1
12
12
1
1
1
BOREHOLE
LENGTH (m)
125.9
122.6
118.1
123.7
128.7
125.5
PERCENTAGE
CHANGE
-
-2.6
-6.2
-1.7
+2.2
-0.3
4. 2019 ASHRAE Handbook-HVAC Applications, Chap. 35, Equations
In one example, the calculated borehole length is
reduced by 2.6% when more precise g-functions are
determined with 12 segments instead of one. It is
shown that short-term or borehole thermal capacity
effects, which are not yet treated in the 2019 ASHRAE
Handbook-HVAC Applications sizing methods, tend to
reduce the required borehole length by a few percent.
Finally, it is observed that low- and high-level methods
give similar results when they use the same load profile
(Table 5).
Supplementary Material
A spreadsheet tool called GHXSizing accompanies this
article. It can be found at on ASHRAE's website at https://
tinyurl.com/mbmdpbv4 and on the author's website
(https://tinyurl.com/GeoSizing).
References
1. Philippe, M., M. Bernier, D. Marchio. 2010. " Sizing calculation
spreadsheet: vertical geothermal borefields. " ASHRAE Journal
52(7):20-28.
2. Ahmadfard, M., M. Bernier. 2019. " A review of vertical ground
heat exchanger sizing tools including an inter-model comparison. "
Renewable and Sustainable Energy Reviews 110:247-265.
3. Carlson, S.W., J.W. Thornton. 2002. " Development of
equivalent full load heating and cooling hours for GCHPs. " ASHRAE
Transactions 108(1):88-98.
36
ASHRAE JOURNAL ashrae.o rg D ECEM BER 2 0 2 1
4 and 5.
5. 2019 ASHRAE Handbook-HVAC Applications, Chap. 35, Equations
24, 25 and 26.
6. Spitler, J.D., M. Bernier. 2016. " Vertical borehole ground heat
exchanger design methods. " In Advances in Ground-Source Heat Pump
Systems, edited by Simon Rees, 29-61. Cambridge, Mass.:Woodhead
Publishing.
7. 1995 ASHRAE Handbook-HVAC Applications, Chap. 32.
8. Kavanaugh, S. 1995. " A design method for commercial ground
coupled heat pumps. " ASHRAE Transactions 101(2):1088-1094.
9. 2019 ASHRAE Handbook-HVAC Applications, Chap. 35, Page 35.9.
10. 2015 ASHRAE Handbook-HVAC Applications, Chap. 34, Table 7.
11. 2019 ASHRAE Handbook-HVAC Applications, Chap. 35, Equation 19.
12. 2019 ASHRAE Handbook-HVAC Applications, Chap. 35, Example 1.
13. Cooper, L.Y. 1976. " Heating of a cylindrical cavity. " International
Journal of Heat and Mass Transfer 19(5):575-577.
14. Hellström, G. 1991. " Ground Heat Storage: Thermal Analyses
of Duct Storage Systems. " Ph.D. diss., Department of Mathematical
Physics, University of Lund, Lund, Sweden.
15. Bernier, M.A., A. Chahla, P. Pinel. 2008. " Long-term
ground temperature changes in geo-exchange systems. " ASHRAE
Transactions 114(2):342-350.
16. 2019 ASHRAE Handbook-HVAC Applications, Chap. 35, Page 35.15.
17. 2019 ASHRAE Handbook-HVAC Applications, Chap. 35, Example 3.
18. Brussieux, Y., M. Bernier. 2019. " Universal short time g*functions:
generation and application. " Journal of Science and
Technology for the Built Environment 25(8):2374-4731.
19. Bernier, M., P. Pinel, R. Labib, R. Paillot. 2004. " A multiple
load aggregation algorithm for annual hourly simulations of GCHP
systems. " HVAC&R Research 10(4):471-487.
20. Spitler, J.D., J.C. Cook, X. Liu. 2020. " A preliminary
investigation on the cost reduction potential of optimizing bore
fields for commercial ground source heat pump systems. " 45th
Workshop on Geothermal Reservoir Engineering.
https://www.tinyurl.com/mbmdpbv4 https://www.tinyurl.com/mbmdpbv4 https://www.tinyurl.com/GeoSizing https://www.ashrae.org/
ASHRAE Journal - December 2021

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