ASHRAE Journal - December 2021 - 24
TECHNICAL FEATURE
Sizing Vertical Ground
Heat Exchangers
BY MOHAMMADAMIN AHMADFARD; MICHEL BERNIER, PH.D., P.ENG., FELLOW ASHRAE
Sizing vertical ground heat exchangers (VGHEs) remains a difficult task and cannot
be done with a simple rule of thumb, especially for fields with multiple boreholes.
A decade ago, Philippe et al.,1 presented in ASHRAE Journal a spreadsheet tool to size
geothermal bore fields based on the models available at that time. This article introduces
significant enhancements to this new tool based on recent developments in the
field.
The new spreadsheet tool, called GHXSizing, proposes
a multilevel approach in which different methods can
be used depending on the amount of information available
to the designer. This article provides details on the
development of GHXSizing's governing sizing equations
to complete the information given in the 2019 ASHRAE
Handbook-Applications and presents a number of examples
of the use of GHXSizing.
Five Levels of Sizing Models
A multitude of VGHE sizing models are available.
Ahmadfard and Bernier2 characterized these models
into five levels of various complexities, L0 to L4. L0 and
L1 are based on rules of thumb and two peak pulses,
respectively, and will not be discussed here because of
their inherent inaccuracies. As illustrated schematically
in Figure 1, this article presents the use of GHXSizing
with five sizing methods in the L2 to L4 categories. As
shown in the right column of Figure 1, GHXSizing can
also evaluate g-functions, or ground thermal response
factors, which are often used in the methods presented
here and which can also be used to assess the temperature
penalty required by some sizing models. An
upcoming article will address the use and generation of
g-functions more specifically.
Because they use only three ground load pulses (a
minimum of information), L2 sizing models are considered
low-level methods. In these models, peak building
loads are used to calculate peak ground loads using
the estimated heat pump coefficient of performance
(COP); monthly ground loads are obtained using an
estimate of the operating time during the peak month;
and average yearly ground load can be determined
based on equivalent full load hours (EFLH).3 Three L2
methods will be presented: the classic ASHRAE sizing
equation presented in the 2019 ASHRAE Handbook-HVAC
Applications;4 the modified ASHRAE sizing model introduced
by Philippe et al.;1 and an alternative method,
based solely on g-functions, first introduced in the 2019
ASHRAE Handbook-HVAC Applications.5 The classic and
Mohammadamin Ahmadfard is a research associate and Michel Bernier is a professor in the department of mechanical engineering at Polytechnique Montréal.
24
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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
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ASHRAE Journal - December 2021 - Cover3
ASHRAE Journal - December 2021 - Cover4
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