ASHRAE Journal - December 2021 - 25

TECHNICAL FEATURE
modifi ed ASHRAE sizing equations both use a
temperature penalty to account for borehole
thermal interaction, but they evaluate it with
different methods.
The monthly and hourly methods (L3 and
L4) are higher-level sizing methods. The fi rst
is based on 36 ground loads (12 monthly average
loads and two sets of 12 peak loads, one for
heating and one for cooling), while the second
is based on hourly ground loads.
Depending on the information available, L2
to L4 methods can be used with GHXSizing.
Three-pulse methods are more appropriate at
the early design phase when limited building
load information is available, while hourly
methods could be used in the fi nal design
stage. As might be expected, the required
effort and calculation time are greater for
higher-level methods. However, as will be
shown, simple low-level methods can give
accurate results when used correctly.
Basic Theory on Borehole Sizing
The important design parameters are shown
in Figure 2. In this fi gure, D is the borehole
buried depth, H is the borehole length, B
is the borehole spacing, rb is the borehole
radius, rp is the radius of the U-tube pipes and
S is the center-to-center pipe distance in the
borehole. The ground heat exchangers are
usually linked in parallel, the total fl ow rate
is assumed to be divided equally among all
boreholes, and each borehole has the same
inlet temperature, Tin. Furthermore, the heat
pump inlet temperature, Tin,hp, is assumed to
be equal to the average of the outlet temperatures
of all boreholes, Tout. Tb is borehole wall
temperature, and Tm is mean fl uid temperature
in the borehole.
Borehole sizing involves both fi nding a borehole
length that will ensure that Tin,hp remains within
certain limits to avoid heat pump operational problems
and limiting installation costs.
The ground loads shown in Figure 3 are used here to
demonstrate the use of L2 to L4 methods based on the
loads. The top portion of the fi gure shows hourly ground
loads. Superimposed on these ground loads are 24
D
H
FIGURE 1 Flowchart of sizing and simulation tools provided by GHXSizing.
GHXSizing
Level 2
Three-Pulse Methods
Using Yearly,
Monthly and Hourly
Ground Loads
1. Classic
ASHRAE Equation
2. Modifi ed
ASHRAE Equation
3. Alternative
ASHRAE Equation
This Paper
FIGURE 2 A typical ground-source heat pump system.
Building
Heat Pump
Tin,hp
Tout,hp
Pump
Tb
U Pipes
Boreholes
B
horizontal lines representing 12 monthly average loads
and 12 monthly peak hourly loads calculated from the
hourly loads. The three ground load pulses based on
these hourly values are shown on the bottom portion of
the fi gure, where the annual peak load, qh, the monthly
average load, qm, and the yearly average load, qy (i.e.,
the annual average heat transfer to ground), are shown.
D E C E M B E R 2 0 2 1 ashrae.o rg ASHRAE JOURNAL
25
rb
Segments
Soil
Fluid
Grout
rp
S
Borehole Wall
Pipe Wall
Tm
Tin
Tout
Borehole Cross Section
Upcoming Paper
Level 3
Monthly Pulse Methods
4. Using Monthly
Average and Peak
Ground Loads
Level 4
Hourly Pulse Methods
5. Using Hourly
Ground Loads
g-Functions
Temperature
Penalty, Tp
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ASHRAE Journal - December 2021

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