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http://doi.org/10.1080/10789669.2014.974478
http://hdl.handle.net/10251/88512
Taylor & Francis
Cullin, J.; Spitler, JD.; Montagud Montalvá, CI.; Ruiz Calvo, F.; Rees, S.; Naicker, S.;
Konecny, P.... (2015). Validation of vertical ground heat exchanger design methodologies.
Science and Technology for the Built Environment. 21(2):137-149.
doi:10.1080/10789669.2014.974478
Cullin, J.R., J.D. Spitler, C. Montagud, F. Ruiz-Calvo, S.J. Rees, S.S. Naicker, P. Konečný, and L.E. Southard.
Published in: Science and Technology for the Built Environment. 2015, 21(2):137-149.
Validation of Vertical Ground Heat Exchanger Design
Methodologies
This work presents a validation of two common methods for designing vertical ground heat exchangers.
Both a simulation-based design tool and the ASHRAE Handbook design equation are used to find design
lengths for four different real systems, using actual experimental data, including building loads as well as
physical parameters, as inputs. The measured minimum and maximum ground heat exchanger exiting fluid
temperatures were used as the design constraint. The simulation-based design tool predicted the borehole
length to within 6% in all cases, while the Handbook design equation yielded systems with errors from -21% to
167%. Most of this error can be explained by the way loads are represented in the Handbook equation, with
differences in the borehole thermal resistance also playing a smaller part. The Handbook equation relies on a
very simple load representation; though this allows it to be used as a simple hand calculation, it also precludes
it achieving acceptable accuracy. It does not appear to be possible to revise the Handbook equation so as to
both significantly improve its accuracy and allow its use in a simple hand calculation.
Introduction!
Ground source heat pump (GSHP) systems are among the most efficient heating and cooling systems
currently available (EPA, 2013). These systems utilize a ground heat exchanger (GHX) to extract heat from,
and, in temperate climates, reject heat to the soil on a seasonal basis. In this manner, thermal energy is
effectively stored in warm months for later usage during cooler months. Since GSHP systems are becoming
more and more widely utilized, it is of extreme importance to have a reliable method for sizing the GHXs;
additionally, any such method must be straightforward enough to achieve accurate results in a minimum of
computation time.
At present, there are perhaps three types of methods for sizing the ground heat exchanger for a GSHP
system design. The first method is to use some type of rule-of-thumb relating peak cooling capacity or peak
heating capacity to a required depth. Particularly for non-residential systems, however, the ratio of capacity to
depth varies widely (Underwood and Spitler, 2007; Spitler and Cullin, 2008). Therefore reduction of the sizing
algorithm to a fixed borehole length per unit of peak capacity is unlikely to give satisfactory results, and the
rule-of-thumb approach will not be further considered here.
The second type of method is based on computer simulation of the ground heat exchanger (Eskilson, 1987;
Hellstršm et al., 1997; Spitler, 2000; Cullin and Spitler, 2011), whereby the necessary system parameters
Cullin, J.R., J.D. Spitler, C. Montagud, F. Ruiz-Calvo, S.J. Rees, S.S. Naicker, P. Konečný, and L.E. Southard.
Published in: Science and Technology for the Built Environment. 2015, 21(2):137-149.
(borefield geometry, borehole completion, thermal properties, etc.) are used as inputs to a simulation tool that
generates entering and exiting fluid temperatures as a function of time. These temperatures can then be
compared to the desired temperature constraintsÑusually placed on the heat pump entering fluid temperatureÑ
and the GHX depth iteratively adjusted until those constraints are met, a process which is typically done
automatically by the software. Many design and energy analysis tools (e.g., Hellstršm et al., 1997; Fisher et al.,
2006; Liu and Hellstršm, 2006) rely on the g-function approach first developed by Eskilson (1987); one of
these tools (Spitler, 2000) will be analyzed here. This tool, also described in some detail by Cullin (2008), is a
ground heat exchanger simulation tool that operates on a hybrid time stepÑmonthly average plus hourly peak,
as described by Cullin and Spitler (2011)Ñand is widely used for system design due to its quick computations
compared to other hourly simulation methods.
The other method for GHX system design, and the method currently presented by ASHRAE (2011), is that
of Kavanaugh and Rafferty (1997). They give an equation derived from a cylinder-source model to compute a
required heat exchanger length both for heating and for cooling (with the larger value, obviously, being the one
required for the design). This method also utilizes tables of correction factors to adjust for both borehole-to-
borehole interference and thermal short-circuiting; however, the development of these factors is unclear, and
other researchers (Bernier et al., 2008) have failed to reproduce the tabulated borehole resistances and short-
circuiting factors with any sort of accuracy. Others, however, have proposed alternative techniques for
determination of the penalty value (Fossa, 2011; Capozza et al., 2012). This method has also been integrated
into a software tool that automates much of the computation (Kavanaugh, 1995), and an enhanced version into a
calculation spreadsheet (Philippe et al., 2010).
Shonder et al. (2000) performed a parallel testing of five commercial design programs for vertical ground
heat exchangers, and found differences of 7% for a cooling-dominated site and 16% for a heating-dominated
site. However, these results were for simulated buildings, and thus did not use experimental data. Additionally,
while Kurevija et al. (2012) compared the resulting sizes of the Eskilson g-function method to the ASHRAE
cylinder source-derived method for one building in Croatia, showing somewhat lower sizes produced by the
ASHRAE method for a 30-year design, there has to date been no retrospective assessment of these methods
using actual GHX data.
Finally, Staiti (2014) has performed a comparison of the ASHRAE method and the same simulation-based
design tool utilized herein, for a variety of cases using both a residential building and an office in Italy. For
these case studies, which used design (i.e., not actual monitored) data for the buildings as inputs, the ASHRAE
Cullin, J.R., J.D. Spitler, C. Montagud, F. Ruiz-Calvo, S.J. Rees, S.S. Naicker, P. Konečný, and L.E. Southard.
Published in: Science and Technology for the Built Environment. 2015, 21(2):137-149.
method sizes the GHX by an average of 16% higher compared to the simulation-based method for a single U-
tube. For double U-tubes, the two methods produce roughly equivalent sizes, though this is due to an
underprediction of the borehole thermal resistance in the ASHRAE method. While this work does show that the
ASHRAE method may tend to produce different results from a simulation-based design method, there has still
been no assessment of these methods against monitored data.
To assess the performance of both of these methods, they will be validated against data from several
monitored GHX facilities, as described in the next section. Traditional validation efforts typically involve using
a simulation to determine fluid temperatures, which are then checked against experimental values. However, for
this work, the design tools are used to size the GHX for the system, with the measured peak heat pump entering
fluid temperatures serving as the design constraints. This approach provides insight into the accuracy of the
ground heat exchanger sizes computed by the two commonly-used design approaches.
To check the suitability of the equation-based method, relevant information including total and peak load
values, as well as maximum temperatures, will be entered into the design equation. The resulting Òdesign
lengthÓ is then compared to the actual installed GHX length, to see how well the equation predicts GHX length
requirements. This value can also be compared to the length obtained with the simulation-based design tool, so
that the relative performance of the two techniques can be assessed.
It may be the case that the ASHRAE Handbook design equation was intended to use concepts such as block
loads and equivalent full-load hours in its sizing of GHXs, and that the method is designed to produce a Òworst
caseÓ estimate if the design month, and peak block load, were to follow a prolonged annual load imbalance. In
practice, this is not likely to be the case, as the ground temperature will experience some seasonal rise and fall.
In any case, utilizing hourly measured data represents the best possible way to compare the performance of this
method to other design techniques, such as that used by the simulation-based design tool. Thus, while a direct
comparison of the two methodologies as performed herein may not be perfect due to the respective
developments of those two methods, it remains the most feasible option for analyzing the relative performance
of these methods.
Measured Data
Yavuzturk and Spitler (2001) identified criteria for field tests that would be ideal for use in experimental
validation of ground heat exchanger simulations: (1) independent measurement of ground thermal properties (2)
carefully calibrated and monitored data acquisition including, at least, measurement of entering and exiting
fluid temperatures and flow rates (3) continuous data collection from the beginning of the ground heat
Cullin, J.R., J.D. Spitler, C. Montagud, F. Ruiz-Calvo, S.J. Rees, S.S. Naicker, P. Konečný, and L.E. Southard.
Published in: Science and Technology for the Built Environment. 2015, 21(2):137-149.
exchanger operation, and (4) well characterized borehole geometry, backfill material properties, and heat
transfer fluid properties. These same criteria apply for validation of ground heat exchanger design methods. To
these criteria, we might also add that it is desirable to have multiple years of continuous data Ð the more the
better Ð and, if possible, it would also be ideal to have a range of system sizes and climates as well as a range of
system parameters such as number of boreholes, borehole spacing, borehole depths, backfill materials, etc. Such
data sets have been in remarkably short supply.
This paper brings together results from four different GSHP facilities, selected to meet the above criteria as
closely as possible. Two are located in the United States: one is an experimental facility in Stillwater, OK, and
the other is at the ASHRAE Headquarters in Atlanta, GA. The remaining two are in Europe: one in Valencia,
Spain, and one in Leicester, United Kingdom. For each system, there will be by design some "inactive" length
at the top of each borehole, where the U-tube connects to the manifold piping. Cullin and Spitler (2013)
explored the effect of any present horizontal piping on the performance of a vertical GHX, and found that it
could be accounted for as an ÒeffectiveÓ length of vertical piping. For the purposes of this work, then, a
corresponding equivalent length of connecting horizontal piping is considered, so that the actual active length is
equal to the installed vertical design length. This is done in absence of any knowledge of the inactive and
horizontal piping portions of the four GHXs, so that these two secondary effects cancel each other out. As
designers typically neglect both these secondary effects, this mirrors what is done by designers. Each system
will now be discussed in detail.
Stillwater, OK. Hern (2002) designed and constructed a hybrid ground source heat pump test facility at
Oklahoma State University in Stillwater OK. The ground heat exchanger consists of three vertical boreholes,
each 114mm (4.5in) in diameter and averaging about 75m (246ft) in length, spaced 6m (20ft) apart and
connected in parallel. The cooling capacity of the ground loop is supplemented by a three-ton evaporative
cooling tower, connected to the loop via a plate heat exchanger in order to maintain a closed-loop system. Two
water-to-water heat pumps with a nominal cooling capacity of 11 kW (3 tons) are used in the facility; one is
configured to operate in cooling mode, while the other operates only in heating mode. For the majority of the
experiment, only one heat pump is in operation; later in the experiment, the two are run simultaneously for a
short time.
Hern (2002) measured the thermal properties in each borehole, finding a narrow range of values for each
parameter. On average, the conductivity of the soil around the boreholes was 2.55 W/m-K (1.473 Btu/hr-ft-¡F),
with an average borehole thermal resistance of 0.162 m-K/W (0.280 hr-ft-¡F/Btu); the mean undisturbed ground
