ASHRAE Journal - December 2021 - 34

TECHNICAL FEATURE
TABLE 4 Input parameters and results obtained using the monthly sizing method for a rectangular bore field.
in the Handbook example. The g-functions are evaluated
with one segment. GHXSizing results in cell C70
show that the borehole length is 99.8 m (327.4 ft),
which is the value reported in the ASHRAE Handbook.
Calculations for these input parameters with the classic
and modified ASHRAE equations (not shown in
this article) give borehole lengths of 100.9 m (331.0 ft)
and 101.4 m (332.7 ft), respectively. Thus, all three
methods give essentially the same results for this
case. However, as shown by Ahmadfard and Bernier,2
the classic ASHRAE equation starts to diverge from
the modified ASHRAE equation and the alternative
ASHRAE method for large annual ground thermal
imbalance.
GHXSizing offers the possibility to account for the
influence of borehole thermal capacity (fluid, pipe and
grout) on sizing. This additional feature, not described
in the Handbook, can be activated in cell A32. When the
feature is used, borehole thermal capacity effects are
calculated using short-term g-functions.18 Considering
borehole thermal capacity reduces the required borehole
length by a few percent, especially if the peak duration
is short.
In these first three sizing models, it is assumed that
the length is required at the end of the design period.
However, in some cases, the required borehole length
may be required in the first months of operation, which
34
ASHRAE JOURNAL ashrae.o rg D ECEM BER 2 0 2 1
may lead to an undersized bore field if these models are
used. One way to circumvent this issue is to use either
L3 or L4 sizing methods. These two methods will now be
described.
Hourly and Monthly Sizing Methods
The governing equations for the monthly (L3) and
hourly (L4) sizing methods are also g-function-based.
They can be derived from Equation 4 and are presented
in the instructions tab in GHXSizing. Much like
Equation 7, these governing equations are solved iteratively
in GHXSizing by updating borehole lengths until
either the maximum or minimum heat pump inlet
temperatures are reached. During these iterations,
g-functions are evaluated dynamically as the solution
progresses. For the hourly sizing method, a multiple
load aggregation algorithm19 is used to limit calculation
time.
The monthly and hourly sizing methods available in
GHXSizing are illustrated here with two geometries
involving 25 boreholes and a relatively high annual
ground thermal imbalance. The geometries are illustrated
in Figure 4. The first geometry is a square bore
field, while the second geometry has a pentagonal
shape. In both cases, borehole spacing is 8.0 m (26.2 ft).
The monthly and hourly ground loads of this problem

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ASHRAE Journal - December 2021

Table of Contents for the Digital Edition of ASHRAE Journal - December 2021

Contents
ASHRAE Journal - December 2021 - Intro
ASHRAE Journal - December 2021 - BB1
ASHRAE Journal - December 2021 - BB2
ASHRAE Journal - December 2021 - Cover1
ASHRAE Journal - December 2021 - Cover2
ASHRAE Journal - December 2021 - 1
ASHRAE Journal - December 2021 - Contents
ASHRAE Journal - December 2021 - 3
ASHRAE Journal - December 2021 - 4
ASHRAE Journal - December 2021 - 5
ASHRAE Journal - December 2021 - 6
ASHRAE Journal - December 2021 - 7
ASHRAE Journal - December 2021 - 8
ASHRAE Journal - December 2021 - 9
ASHRAE Journal - December 2021 - 10
ASHRAE Journal - December 2021 - 11
ASHRAE Journal - December 2021 - 12
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ASHRAE Journal - December 2021 - 14
ASHRAE Journal - December 2021 - 15
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ASHRAE Journal - December 2021 - 17
ASHRAE Journal - December 2021 - 18
ASHRAE Journal - December 2021 - 19
ASHRAE Journal - December 2021 - 20
ASHRAE Journal - December 2021 - 21
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ASHRAE Journal - December 2021 - 25
ASHRAE Journal - December 2021 - 26
ASHRAE Journal - December 2021 - 27
ASHRAE Journal - December 2021 - 28
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ASHRAE Journal - December 2021 - 30
ASHRAE Journal - December 2021 - 31
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ASHRAE Journal - December 2021 - 33
ASHRAE Journal - December 2021 - 34
ASHRAE Journal - December 2021 - 35
ASHRAE Journal - December 2021 - 36
ASHRAE Journal - December 2021 - 37
ASHRAE Journal - December 2021 - 38
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ASHRAE Journal - December 2021 - Cover4
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