Accepted Manuscript

Heat dissipation effect on a borehole heat exchanger coupled with a Heat Pump

J. Darkwa, W. Su, D.H.C. Chow

PII: S1359-4311(13)00499-7

DOI: 10.1016/j.applthermaleng.2013.07.011

Reference: ATE 4925

To appear in: Applied Thermal Engineering

Received Date: 19 March 2013

Revised Date: 3 July 2013

Accepted Date: 7 July 2013

Please cite this article as: J. Darkwa, W. Su, D. Chow, Heat dissipation effect on a boreholeheat exchanger coupled with a Heat Pump, Applied Thermal Engineering (2013), doi: 10.1016/j.applthermaleng.2013.07.011.

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Heat dissipation effect on a borehole heat exchanger coupled with a Heat Pump

J. Darkwa, W. Su and DHC. Chow

Centre for Sustainable Energy Technologies (CSET) The University of Nottingham Ningbo China

199 Taikang East Road Ningbo 315100, P. R. China

Tel: +86 574 88180255; Fax: +86 574 88180313 Email: jo.darkwa@nottingham.edu.cn

Abstract

Thermal performance evaluation of a ground-coupled heat exchanger has been undertaken to

assess the extent of heat dissipation into the ground and its long term effect on the cooling

performance of a heat pump system. Simulation results were compared with operational data

over a 3-year period and found to be in good agreement. However the annual average energy

being dissipated into the borehole was found to be about 4.5 times more than the amount

being extracted thus raising concern about long term effectiveness of the borehole as a heat

sink. Even though, there was a slight decline in the energy dissipation rate during the third

year, the result does not provide adequate evidence to support creeping soil degradation

process in the borehole over such a relatively short period. Since the performance of each

ground-coupled heat exchanger appears to be influenced by its location, more research is

needed to acquire better and wider understanding of the effect of heat dissipation and soil

degradation processes in borehole systems.

Keywords: Heat dissipation; Borehole Heat Exchanger; Ground Source Heat Pump (GSHP).

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Nomenclature C1ε,C2 Constants cp Specific heat (J/kg K) d Pipe diameter (m) E Total energy (J/kg) Gb Generation of turbulence kinetic energy due to buoyancy (J/s m3) Gk Generation of turbulence kinetic energy due to the mean velocity gradients (J/s m3) gi Component of the gravitational vector in the ith direction h Specific sensible enthalpy (J/kg) hs Specific sensible enthalpy for solid component (J/kg)

Specific sensible enthalpy of species (J/kg)

k Thermal conductivity ( W/m K)

Effective thermal conductivity ( W/m K)

Mass flow rate of fluid (kg/s)

np Number of pipes in boreholes p Pressure (Pa) Prt Turbulent Prandtl number for energy σk, σε Turbulent Prandtl numbers for and models

Qc Maximum cooling load (kW) Qh Maximum heating load (kW) Re Reynolds number S Modulus of the mean rate-of-strain tensor

Sk, Sε, Sij User-defined source terms T Temperature (K) t Time (s) V Velocity (m/s)

v Component of the flow velocity parallel to the gravitational vector (m/s)

u Component of the flow velocity perpendicular to the gravitational vector

ui,uj,uk Velocity for different direction (m/s)

x Length (m)

xi,xj,xk Length in different direction (m)

Yj Mass fraction of species

Greek letter β Thermal expansion coefficient ε Dissipation rate µ Dynamic viscosity (Pa s)

µt Turbulent viscosity (Pa s)

ρ Density (kg/m3)

τ Deviatoric stress tensor

Subscripts

o Outside

su Summer

wi Winter

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1.0 Introduction

One of the energy and environmental issues dominating most global policies is how to

reduce energy consumption in buildings since the sector is responsible for about 40% of total

global energy consumption and carbon emissions [1].

The potential of ground source heat pumps (GSHPs) are widely recognised as

efficient technologies that could play an important role in providing heating and cooling for

buildings [2-7]. GSHPs may be coupled with either vertical U-tubes or horizontal heat

exchangers in order to dissipate or extract heat from the earth. The overall thermal

performance of a GSHP is therefore dependent on factors such as the geometric configuration

of the heat exchanger and thermophysical properties of the surrounding soil. To this end a

number of theoretical and experimental studies have been carried out towards determining the

main factors affecting the performances of ground heat exchangers (GHEs). For instance,

Seong-Kyun et al. [8] developed a dynamic model for borehole heat exchangers. The model

was validated on a number of installations and found to be fairly predictable in the

assessment of the performance of GHEs. Yang et al. [9] investigated the effect of fluid

temperature variation and heat transfer rates between two adjacent legs of a U-tube heat

exchanger and obtained a validation error of less than 6%. Oppelt et al. [10] developed a

model for the simulation of borehole filling (grout) of double U-pipe heat exchangers in

relation to location of pipe shanks and achieved an error of 6% and 9% for a minimal and

maximal shank spacing respectively. Georgios et al. [11] developed models for evaluating

complex temperature variation around GHEs. Other theoretical approaches for evaluating the

performance of heat exchangers have also been investigated [12-14] but they all have

limitations in terms of the ground and GHE conditions at different locations.

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Regarding experimental studies, Jun et al. [15] identified soil type and associated

thermal resistance as the dominant factors affecting thermal performance of a ground heat

exchanger. Pahud et al. [16] showed that thermal resistance on a double-U-pipe heat

exchanger could be reduced by about 30% when quartz sand is used instead of bentonite.

Aristodimos et al. [17] also revealed that de-bonding at the interface between borehole pipes

and grouting material do have significant impact on ground heat exchangers. As a result they

recommended a grouting material with low shrinkage properties for overcoming the de-

bonding phenomenon. Studies carried out by Jun et al. [15] further showed that ground heat

exchangers with wider shank spacing have better thermal performance than closely spaced

shanks. Diao et al. [18] also established that the performance of ground heat exchangers

could significantly be affected by groundwater.

There have however been growing concerns about the long term effectiveness of

GHEs due to the relatively higher rates of heat dissipation into the ground as compared with

the extraction rates in cooling dominated situations. This is due to the fact that as heat is

added to or removed from the ground, the surrounding soil temperature changes. If the total

annual heating load does not balance the total annual cooling load, then there will be a net

change in the ground temperature after each year of operation. After many years of operation,

the ground will become a poorer source of heat sink in an unbalanced system, and therefore,

the overall cooling performance of a GSHP system would be affected.

These concerns have led to recent guidelines for tests procedures as reported in

ASHRAE 2007 [19] and by Sanner et al. [20] in an effort to establish effective tests methods

necessary for designing and sizing of ground- coupled heat pump systems. Meanwhile some

heat mitigation techniques have been proposed and evaluated. For example, Li et al. [21]

investigated a novel multi-function ground source heat pump system for mitigating the

excessive heat in the soil and achieved a reasonable level of heat reduction. Wei et al. [22]

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evaluated an integrated solar-earth source heat pump system with a heat storage water tank

and achieved a reduction of about 14.5% in the heat rejection rate as compared with

conventional ground source heat pump. Sagia et al. [23] also showed that a cooling tower

could be used to mitigate the excessive heat load in a Hybrid Ground Source Heat Pump

system. However in all these test and evaluation cases, the results were influenced by factors

such as geological condition, geographical location and the end use of the heat recovered

from the systems thus making each case a unique system. For the benefit of achieving further

understanding and contributing to wider knowledge of heat mitigation concepts in borehole

designs, this study evaluates the thermal performance of a typical cooling dominated ground-

coupled heat pump system.

2.0 Ground Coupled Heat Exchanger System

Fig 1 shows the schematic diagram of a reversible heat pump coupled with the

vertical U-bend close loop pipe heat exchangers. The system was fully commissioned in 2008

in the Centre for Sustainable Energy Technologies (CSET) building at the University of

Nottingham Ningbo China. The installation is also part of an integrated ground source heat

pump (GSHP) system for providing water supply to a radiant panel circuit in the building at

15oC in summer and 45oC in the winter period. The basic specifications for the system are:

• Heating capacity = 65 kW

• Cooling capacity = 55kW

• Electrical power input during cooling = 10.5 kW

• Electrical power input during heating = 15 kW

• Heating coefficient of performance (COP) = 4.3

• Cooling COP = 5.2:

• Depth of each borehole: 70 m;

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• Specific heat factor (winter) for each borehole: 30 W/m

• Specific heat factor (summer) for each borehole : 50 W/m

• Total number of boreholes: 20.

• Distance between boreholes: 4.3 m

• Heat transfer fluid in borehole pipes: water

3.0 Borehole investigation

3.1 Soil particle sizing

In general, textural classification of a soil is a relevant criterion for determining the

type of soil and assessing its thermal performance since smaller size particles do retain heat

better than larger ones [24-25]. Soil particle size distribution information also provides

estimates for properties such as permeability, water holding capacity strength and stability.

According to the USDA soil textural classification system [26] a soil particle size fraction of

less than 2µm is defined as clay; 2µm-50µm is classified as silt; 50µm-2000µm as sand. A

Particle Size Analyser, (Bettersize type 2000), was therefore used to determine the size of the

individual samples extracted at various depths of 10m, 25m, 45m and 70m inside the

borehole. The test results are presented in Fig. 2 and based on the USDA soil textural system

it is clear from the summary in Table 1 that, the soil is mainly of the silt type.

3.2 Soil thermophysical properties

Even though the soil particle sizing procedure indicated that the soil samples are of

the same type it was considered necessary to ascertain their thermophysical properties since

varying soil conditions could affect the performance of the heat exchanger. Therefore a KD2

Pro Thermal Properties Analyser was used to obtain the relevant data from the samples. The

Analyser consists of a handheld controller and sensors that can be inserted into subject soil

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specimen to measure the thermal properties. As shown in Table 2 the results were slightly

different from location to location which could be attributed to soil conditions. However, the

mean values were found to be fairly comparable with the individual results. Normally in the

design of GHEs, effective or average thermal properties over the length of the borehole are

usually sought. Therefore the mean values could be taken as the representative data for the

soil within the borehole. The specifications for both the Particle size Analyzer and the KD2

Pro are presented in Table 3.

4.0 Theoretical analysis

4.1 Physical model of borehole

Figs 3a and 3b show the plan and sectional views of a borehole containing double U-

tube pipes. The pipes are encased in a high conductivity geothermal grouting material

designed to improve the efficiency and performance of the loop systems. This was achieved

by matching the thermal conductivity of the surrounding soil and creating a permanent

flexible seal to prevent aquifer contamination.

4.2 Governing equations

The governing equations associated with each component of the borehole are

expressed as follows:

(i) Solid (Grout, pipe and soil)

Since there is no internal heat generation in the solid components the general energy equation

as defined in FLUENT Software [27] is expressed as;

(1)

Where

(2)

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(ii) Fluid (Water)

Water is an incompressible fluid and therefore the governing equations are based on the

continuity and energy conservation equations [28].

Continuity equation in vector form:

(3)

Energy conservation equation:

The energy equation is dependent on the type of flow in the pipe and can be

determined by Reynolds’ equation as;

Re (4)

From the flow data, Re was calculated as 7640 which makes it a turbulent type of flow since it

is greater than 2000. For this reason the realizable k-epsilon (k-ε) model [28] was selected

being the commonest model used for solving all turbulence models. It also has the advantage

of providing accurate and superior performance for flows involving rotation and boundary

layers under strong adverse pressure gradients. The modelled transport equations for and

are therefore expressed as:

Turbulent kinetic energy (k-equation)

(5)

Dissipation (ε-equation)

(6)

In accordance with Boussinesq hypothesis [29];

(7)

(8)

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(9)

In FLUENT and by default, is taken as zero in the dissipation ε-equation.

Other terms are given as;

; ;

(10)

In this flow model, the buoyant shear layers are perpendicular to the gravitational vector and

therefore, = 0.

The model constants are given as;

, , ,

The dissipation modelled energy equation is obtained as [30];

(11)

Where, [26], and [31] are expressed as:

(12)

(13)

(14)

(15)

(16)

Where

= 298.15 K

Prt = 0.85

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4.3 Simulation

By considering identical conditions for the pipes in the boreholes, a Fluent Software

Version 6.3 was used to simulate the thermal behaviour of the system based on the data in

Table 4 and the following assumptions.

• Fluid temperature variation was considered along the pipe length only.

• There is no phase change taking place in the fluid or significant radiation in the pipe

• Conduction was the main heat transfer mode in the pipe, grout and soil.

• Convection was considered in the fluid

Boundary and initial conditions

The simulation was carried out for July and January being the hottest and coldest

periods respectively for the location of the borehole. Based on investigation by Darkwa et al.

[32] the ground temperature was taken as 15.6℃. The initial fluid inlet temperature for July

was fixed at 27℃ and for January at 10.5℃.

5.0 Results and Discussions

Fig. 4 represents the static temperature variation along the full length of the heat

exchanger pipe. Analysis of the curves show an average differential temperature of 4o C

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between the inlet and outlet fluid for July (summer period) and about 2.5o C for January

(winter period). Now using these results and data from Tab. 4, the theoretical heat dissipation

and extraction rates may be obtained as follows:

Heat extraction (Qe) rate in January;

Qe = x np x Cp x ∆Twi (16)

= (0.12 x 40)*4.2 x 2.5 = 50.4 kW

Heat dissipation (Qd) rate in July;

Qd = x np x Cp x ∆Tsu (17)

= (0.12 x 40)*4.2 x 4 = 80.6 kW

The above results show about 38% imbalance between heat dissipation and extraction rates.

For the purpose of comparison, the theoretical heat dissipation and extraction rates were also

calculated based on the National Chinese Design Guide [33] and data from the GSHP [34] as

follows:

Heat extracted (Qe);

(18)

= 65* = 49 kW

Heat dissipated (Qd)

(19)

= 55* = 66 kW

The above results show 26% imbalance between heat dissipation rate and extraction rates.

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In order to validate the theoretical results, selected operational data of the integrated

GSHP system for the coldest (January) and the hottest (July) periods for the years 2009-2011

were used. The data obtained showed similar trends in all the years for the inlet (Ti) and

outlet fluid temperatures (To) as presented in the examples for the year 2009 in Figs. 5 & 6.

Analysis of the results gave the mean differential temperatures in January/July as 3o C/ 4.2o C

for 2009, 3o C/ 4.17o C for 2010 and 2.9o C/ 3.87o C for 2011. These are fairly comparable

with the simulated data which was obtained as 2.5o C/ 4o C.

Based on these data and the fluid flow rate, the corresponding mean heat

extraction/dissipation rates were also obtained as 60.5 kW/84.6 kW for 2009, 60.5 kW/84.1

kW for 2010 and 58.5 kW/78 kW for 2011. By comparison, they do not appear to be too far

off from the simulated value of 50.4 kW/80.6 kW but slightly higher than the calculated

value of 49 kW/66 kW obtained with the National Chinese Design Guide. The percentage of

heat imbalance between heat dissipation and extraction rates ranges from 25% - 28% which is

less than the simulated value of 38% but matches well with the calculated result of 26%. Figs.

7 - 9 show the profiles of the cumulative energy extracted and dissipated for the monitored

period based on an 8-hour daily operation. By taking the annual operating heating period to

be 60 days and cooling period as 200 days for the GSHP system, the yearly energy dissipated

and extracted were obtained as summarised in Table 5 and presented in Fig. 10.

The results clearly indicate that a considerable higher proportion of energy is

dissipated into the borehole as against the amount being extracted. By using the operational

data, the corresponding specific heat factor per meter for each borehole for January (winter)/

July (summer) periods were also obtained as 43.2 W/ 60.4 W for 2009, 43.2 W/60 W for

2010 and 41.8 W/ 55.7 W for 2011. These are not too far off from the design data given as 30

W/m for winter and 50 W/m for summer. However the energy analysis shows that the annual

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average energy being dissipated into the borehole is about 4.5 times more than the amount

being extracted. The concern is that this level of energy dissipation intensity could reduce the

heat storage capacity of the surrounding soil in the borehole and in turn impact on the

effectiveness of the ground heat exchangers and the cooling performance of the GSHP

system.

6.0 Conclusions

The study has shown that even though the manufacturers estimate the life span of the

GSHP to be about 20 years, it is highly unlikely that its current coefficient of performance

would be sustained over the projected operating life. There was however not enough evidence

to support creeping soil degradation process in the borehole over such a relatively short

period of operation of the GSHP system. The specific findings are highlighted as follows;

• The theoretical mean differential inlet and outlet fluid temperature were obtained as

2.5o C for January and 4o C for July. These compare well with the experimental results

which were obtained as 2.9o C - 3o C for January and 3.87o C- 4.2o C for July.

• The amount of energy dissipated was obtained as 449.3 GJ – 487.3 GJ as against

101.1 GJ - 104.5 GJ per year of energy extracted thus creating about 77-78.6% heat

retention rate in the surrounding soil.

The study has given further insight into the thermal behaviour of borehole heat

exchangers and the need for an integrated heat mitigation approach in the design of cooling

dominated ground coupled heat pumps systems. Some of the design concepts could for

instance include secondary heat recovery and storage systems depending on the location and

end use application. In the absence of any known method for establishing the life span of

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bore heat exchangers and the fact that the performance of each ground-coupled heat

exchanger appears to be influenced by location, more research is needed to acquire better and

wider understanding of the effect of heat dissipation and soil degradation processes in

borehole systems.

Acknowledgments

The authors wish to thank the Ningbo Science and Technology Bureau, China for

supporting this research under the Key Laboratory of Integrated Thermal Energy Storage

Technologies (ITEST) for buildings project.

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[25] http://www.dcd.com/insights/novdec_2005_13.html [26] http://nesoil.com/properties/texture/sld011.htm [27] Inc., A., ed. Fluent 6.3 User Guide. 2007. Chapter 13.2.1 [28] Inc., A., ed. Fluent 6.3 User Guide. 2007. Chapter 9.2 [29] [Inc., A., ed. Fluent 6.3 User Guide. 2007. Chapter 12.4.3 [30] Inc., A., ed. Fluent 6.3 User Guide. 2007. Chapter 12.2.4 [31] Inc., A., ed. Fluent 6.3 User Guide. 2007. Chapter 12.4.7 [32] J. Darkwa, G. Kokogiannakis, G. Suba, Effectiveness of an intensive green roof in a

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Research Highlights

• Theoretical and experimental fluid differential temperatures were in good agreement

• Theoretical results were obtained as 2.5o C/ 4o C for January/July

• Experimental results were 2.9o C - 3o C/ 3.87o C- 4.2o C for January/July

• Mean annual energy dissipated was 4.5 times more than the amount extracted

• There was not enough evidence to support creeping soil degradation in the borehole

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Figure captions

Figure 1: Ground coupled heat pump system

Figure 2: Soil particle size profiles at various depths in the ground

Figure 3a: Plan view of borehole

Figure 3b: View of section X-X

Figure 4: Temperature variation along the pipe

Figure 5: Inlet and outlet water temperature profiles in Jan. 2009

Figure 6: Inlet and outlet water temperature profiles in July 2009

Figure 7: Energy extracted/dissipated on selected days for Jan/July 2009

Figure 8: Energy extracted/dissipated on selected days for Jan/July 2010

Figure 9: Energy extracted/dissipated on selected days for Jan/July 2011

Figure 10: Annual energy extracted/dissipated for 2009-2011

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Table captions

Table 1: Soil particle size

Table 2: Soil thermal properties

Table 3: Equipment Specification

Table 4: Simulation Data

Table 5: Summary of energy extracted and dissipated

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*D50 represents the mean particle size as measured by the Particle Analyser equipment.

It means that 50% by volume of the sample are finer and 50% are larger than D50.

Table 1: Soil particle size Sample Depth (m)

10 25 45 70

Grain size (*D50 µm)

7 8.5 7.5 9

Soil type Silt Silt Silt Silt

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Table 2: Soil thermal properties

Soil sample

depth (m)

Conductivity (k)

W/(m·K)

Resistivity (r)

°C·cm/W

Thermal diffusivity

(α) mm²/s

Volumetric specific heat

(Cp ) MJ/m³·K

% Error

Mean Temp. (°C)

10 1.293 77.3 0.281 4.606 0.16 22.93 25 1.097 91.2 0.266 4.125 0.21 22.82 45 1.270 78.8 0.330 3.843 0.42 20.85 70 1.259 79.4 0.461 2.729 0.62 23.34

Mean value 1.229 81.68 0.334 3.839 0.35 22.48

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Table 3: Equipment Specification

Equipment Type Measured parameters Resolution Accuracy

Bettersize Particle Analyser

2000 Particle size 1-2600µm

<±3% <±3%

Thermal Properties Analyser

KD2 Pro

Conductivity (k) W/(mK) Resistivity (r) °C·cm/W Volumetric specific heat (Cp ) MJ/m³·K Thermal diffusivity (α) mm²/s

±0.01 W/(mK) 50°C·cm/W 0.5 MJ/(m3K) 0.1mm2/s

± 10% ± 10% ± 10% ± 10%

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*High density polyethylene

Table 4: Simulation Data Item Internal

dia.(di) m

Bore dia.(db) mm

Borehole depth (D) m

Thermal conductivity (k) W/mK

Mass Flow rate (ṁ) kg/s

Density

(�)

kg/m3

Thermal diffusivity (α) mm²/s

Specific heat capacity (Cp) kJ/(kg K)

2 x Pipe *(HDPE)

25 - - 0.46 - 950 - -

Soil - - - 1.23 - 1900 0.33 2.02 Fluid (Water)

- - - 0.614 0.12 1000 - 4.2

Grout - - - 2.25 - 2500 0.52 1.39 20 No. Bore holes

- 900 70 - - - - -

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Table 5: Summary of energy extracted and dissipated

Year Daily energy extracted

(kWh)

Daily energy dissipated

(kWh)

Yearly energy extracted (GJ/year)

Yearly energy dissipated (GJ/year)

Energy Dissipation ratio

2009 484 676.8 104.5 487.3 4.7:1 2010 484 672.8 104.5 484.4 4.6:1 2011 468 624 101.1 449.3 4.4:1

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Figure 1: Ground coupled heat pump system

Heat Pump

Cooling/ Heating radiant ceiling

20 Nos. Boreholes

Geothermal Pumps Loa

d

circ

ulat

ing

pu

mps

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(a) 10m in the ground (b) 25m in the ground

(c) 45 m in the ground (d) 70 m in the ground

Figure 2: Soil particle size profiles at various depths in the ground

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Figure 3a: Plan view of borehole Figure 3b: View of section X-X

Grout

Outlet pipes

X

Inlet pipes

X

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Figure 4: Temperature variation along the pipe

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Figure 5: Inlet and outlet water temperature profiles in Jan. 2009

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Figure 6: Inlet and outlet water temperature profiles in July 2009

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Figure 7: Energy extracted/dissipated on selected days for Jan/July 2009

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Figure 8: Energy extracted/dissipated on selected days for Jan/July 2010

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Figure 9: Energy extracted/dissipated on selected days for Jan/July 2011

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Figure 10: Annual energy extracted/dissipated for 2009-2011