thermal performance analysis of the standing-column-well...

AASCIT Journal of Energy 2015; 2(3): 29-35

Published online May 10, 2015 (http://www.aascit.org/journal/energy)

Keywords Standing Column Well Type,

Thermal Response Test,

Effective Thermal

Conductivity,

Thermal Resistance

Received: April 16, 2015

Revised: April 27, 2015

Accepted: April 28, 2015

Thermal Performance Analysis of the Standing-Column-Well (SCW) Geothermal Heat Exchanger

Changhee Lee1, Sanghoon Lee

2, Kyungbin Lim

3, *

1Department of Mechanical and Automotive Engineering, Songwon University, Gwangju, Korea 2Department of Energy Engineering, Jeonju University, Jeonju, Korea 3Department of Mechanical Design Engineering, Hanbat National University, Daejeon, Korea

Email address [email protected] (K. Lim)

Citation Changhee Lee, Sanghoon Lee, Kyungbin Lim. Thermal Performance Analysis of the Standing-

Column-Well (SCW) Geothermal Heat Exchanger. AASCIT Journal of Energy.

Vol. 2, No. 3, 2015, pp. 29-35.

Abstract Thermal performance is the most important factor for development of borehole heat

exchanger utilizing geothermal energy. The thermal performance is affected by many

different design parameters such as configuration type and pipe size of borehole heat

exchanger. And this eventually determines operation and cost efficiency of borehole heat

exchanger system. Main purpose in this study is assessing the thermal performance of

open standing column well type geothermal heat exchanger. For this, thermal response test

rig is established with line-source theory. The test rig is composed of sub-circulation pump,

a boiler, temperature sensors, flow-meters, and data logger system. The thermal response

test is performed with additional constant input heat source. Effective thermal conductivity

and thermal resistance are obtained from measured data. From the measurement, the

effective thermal conductivity is found to have rather high value indicating active heat

transfer for the open standing column well type heat exchanger. Meanwhile, the thermal

resistance has lower value compared to compute results from design programs and this

suggests modification for computing procedure.

1. Introduction

Protected agriculture depends on large-scale energy consumption which is mostly

provided by fossil fuel. Agriculture related energy demand keeps its increasing trend as

consumption pattern of agriculture product becomes steady all year round rather than

seasonal. In Korea, size of the protected agriculture has expanded its area from 1,500ha in

year 1990 to 58,000ha in year 2010. Most of energy spent in agricultural industry is for

heating and energy cost already takes around 30~40% of total cost in the protected

agriculture business. This fact calls for immediate energy saving measure for agriculture

industry. Improvement in energy efficiency is essential measure to save energy, which

required effective and optimized components and systems as well as advanced cultivation

technology. Meanwhile, application of renewable energy can be one of plausible

alternatives. The renewable energy is known as clean and secure due to its less or zero

production of carbon dioxide. It has also its advantage in supply over limited fossil fuel.

Common renewable or alternative energy sources for agriculture industry can be named as

solar energy, heat pump system using air-source energy, and geothermal energy among

many others.The heat pump using geothermal energy is becoming favorable choice in the

agricultural industry. The geothermal energy is extracted from underground to utilize

temperature difference even though it is small. This can apply to the small size facility

such as the protected agriculture unit. The geothermal energy utilization requires only

AASCIT Journal of Energy 2015; 2(3): 29-35 30

small initial investment for its facility which has semi-

permanent lifetime with 40~50 years. The geothermal energy

is also free of producing the carbon dioxide and high price of

fossil fuel with limited resources. With these advantages, the

geothermal heat pump system is expected to expand its

application. However, research on the geothermal system is

insufficient to have efficient and optimized system.

In an effort to utilize the geothermal energy, different types

of heat pump systems are introduced. Each heat pump system

requires the borehole heat exchanger system and it has been

installed in Korea over 1,000 places nationwide until 2009.

The geothermal heat exchanger system is composed of a

number of components to extract energy from underground

and supplies this energy as heat source to the heat pump. The

system is basically circulating fluid to transfer energy and is

designed to have high thermal conductivity. Designs of the

borehole heat exchanger are classified as vertical closed type,

horizontal closed type, standing column well (SCW) type and

dual well type along with their characteristics. The most

common type of the borehole heat exchanger in KOREA is

vertical closed type and share of different designs is shown in

Fig. 1. [1].

Fig 1. Current status of geothermal heat exchanger application in Korea

(2008).

Recently, research on thermal performance has been

reported for the large diameter geothermal heat exchanger

system to have low operating cost and high efficiency [2].

However, its result is still far from practical application and

yet requires further improvement. The SCW type of

geothermal heat exchanger is expected to follow active

development procedure for its design and commercialization

due to high feasibility. As demand for useful geothermal

energy system is gradually increased, further study is

necessary to improve optimized design and effective

borehole heat exchanger system.

In this research, the thermal performance of the SCW type

of geothermal heat exchanger is analyzed by measuring

thermal conductivity and thermal resistance using standard

measurement technique. The basic schematics of geothermal

heat exchanger system are shown in Fig. 2 and current study

is done for open SCW type of Fig. 2(a). This type has direct

heat transfer procedure in the borehole between circulating

fluid and underground environment through SCW instead

having indirect contact through additional supplying pipe.

The SCW geothermal heat exchanger has large diameter

column of over 8in (203.2mm) and its installation and

construction procedure is completely different from existing

vertical-closed type. However, energy extraction from the

heat source in geothermal facility is known as most effective.

Fig 2. Classification of geothermal heat exchanger (a: open type, b, c and d:

Close type).

The proposed open SCW heat exchanger system can be

simulated for its design and analysis with existing analysis

software. These include softwares such as GLHEpro program

for closed concentric tube type geothermal heat exchanger as

shown in Fig. 3 and EED program for coaxial type [3-4]. The

open SCW geothermal heat exchanger is designed using

these design tools after simple modification.

In this research, thermal response test rig is built and

applied for the open SCW heat exchanger system to analyze

the thermal performance. The thermal performance is

evaluated with effective thermal conductivity and thermal

resistance.

31 Changhee Lee et al.: Thermal Performance Analysis of the Standing-Column-Well (SCW) Geothermal Heat Exchanger

Fig 3. Design program for geothermal heat exchanger (GLHEpro.).

2. Experimental Methodology

The thermal performance for the geothermal heat

exchanger is usually measured with thermal response test rig

[5-10]. Similar measurement procedure is applied to the open

SCW type heat exchanger to find the effective thermal

conductivity and thermal resistance.

2.1. Thermal Response Test Rig

The borehole collector pipes are connected to the

equipment with quick couplings at the back of the trailer and

the heat carrier fluid is pumped through the system in a

closed loop. The fluid passes through the heater, and the inlet

and outlet fluid temperatures are recorded every second

minute by a data-logger. Also the power supply is recorded

during the measurements in order to determine the actual

power injection. The power supply was shown to be stable

during the measurements. The test is fully automatic

including the recording of measured data, and takes about

three days to execute. The groundwater level is determined

manually with a separate fluid alarm during the

measurements.

The thermal response test rig is composed of various

components as described in Table 1 and set up on trailer bed

[11,16] as shown in Fig. 4. The schematic diagram of the test

rig is also presented in Fig. 5. Examples of components

include electric and control units, data logger, measuring

program and flowmeters. The inlet and outlet pipes, boiler

and filter are equipped with 4-wire RTD type temperature


sensor.

Table 1. Specifications of thermal response test rig.

No Component Manufacturer Specification Remarks

1 Boiler Kyungdong 35,300~36,500kcal/h

2 Sandfilter FeelanTek 400LPM -

3 Pump WILLO 8m3/h -

4 Oil Tank - 400 Liter -

5 Flowmeter (Water) BLUE-WHITE 20~200LPM, ±1%

6 Flowmeter (Oil) MACNAUGHT 35~830cc/min, ±1%

7 Temp. Sensor Pt100Ω, 4-wire ±0.5%

8 Data Logger AGILENT 34790A,34902MUX ±1%

9 Inverter LS SV015iG5A,380V,30Hz~60Hz

10 Measurement Program National Instruments LabView 8.6 -

Fig 4. Thermal response test rig.

Fig 5. Schematic diagram of thermal response test rig.

2.2. Theory of Thermal Response Test

Line-source theory has been well established and applied

to thermal performance test since 1980 [12, 13]. It is even

expanded to comparison study for energy loss in different

heat exchanger arrays [14, 15, 17]. This line-source theory is

also applied to the thermal response test for open SCW type

heat exchanger in this research.

The effective thermal conductivity and thermal resistance

can be obtained from the thermal response test data as

equations (1) and (2) following details of references [12, 13].

λ (1)

R T T ln ln (2)

The thermal resistance is an important design parameter

for geothermal heat exchanger. Since the most of the existing

design program computes this with some approximated

parameter values, properly measured thermal resistance data

can provide useful way to improve its design. Once the

thermal resistance data is obtained by measurement, data can

be used in the design program to update its performance. This

procedure is repeated until update is reached to convergence

to have correct design.

2.3. Experimental Apparatus of the Thermal

Response Test Equipment

The equipment is set up on a small trailer and consists of a

1 kW pump circulating the heat carrier through the borehole

collector and through a cross-flow heater with adjustable and

stable heating power in the range 4.5 kW. The fluid

temperature is measured at the inlet and outlet of the

borehole with a thermostat, with an accuracy of 0.2 _C. The

temperatures are recorded at a set time interval by a data-

logger. The equipment is powered by 16A electricity. The

apparatus in Figs. 1 and 2 were slightly altered from its

original construction in order to obtain self-airing and

automatic pressure control. The thermal insulation has

gradually been improved in order to minimize energy losses

and influence of temperature changes in ambient air.

2.4. Uncertainty of Heat Balance

In order to verify that the experimental measurements are

reasonable, a justifiable means of validation is required. The

approach is to use a heat balance. The simplest expression of

the heat balance equation is

q"# V ∙ C' ∙ T() T"# (3)

Where, qin [W] is the measured heat input to the water

heater elements and pumps. V [LPM] is the flow rate; Cp is


the specific heat of water; Tin and Tout are measured from

the thermostat.

After applying all of the calibration equations to the

measurement devices, the heat transfer rate predicted by the

right hand side of Eq. (3) can be compared to the measured

power input (left hand side of Eq. (3)). The numbers

summarized in Table 1 are the average values over the length

of each test and they are used to compare the instrumentation

uncertainties and total heat input error.

The uncertainties in the temperature measurement are

* 0.01 + for the probes and * 0.04 + for the signal

conditioner of the digital displays with the analog signal.

Adding the error in quadrate gives the total uncertainty for

the temperature measurements given in Eq.(4).

∆T - *0.01"#1 *0.04"#1 - *0.01()1 *0.04()1 3 *0.2872+ (4)

Taking into account that ∆T for each test is approximately

5+ , the uncertainty due to the temperature measurement

becomes

Table 2. Heat balance check.

location Transducer

reading[w]

Average

q[w] Difference[w]

% of average

power

A 2506.6 2657.8 151.2 5.68

B 3207.2 3302.5 95.3 2.89

Table 3. Result from flow-meter calibration.

Actual flow [LPM] Calibration flow [LPM] Error [%]

3.316 3.192 3.73

7.355 7.494 1.85

10.749 10.991 2.20

14.929 17.703 1.51

Error *1.:;1+<+ 3 *5.74% (5)

Using the highest error for the flowmeter taken from Table

2 of * 3.73%, the total uncertainty in the heat balance

equation is

TotalError - *0.05741 *0.0373^2 3 6.85% (6)

3. Results and Investigations

Glasshouse test site is located in Choong-ju of Korea. The

site is for protected agriculture equipped with the open SCW

type heat exchanger. The borehole is located in rock of

granite gneiss and has specifications of 8in diameter, the

depth of 203m (effective depth of 195m), water intake per

day of 190m3/day and steady water level of -9m. The

measurement has been performed twice with 8 days interval

and other experimental conditions are as in Table 4.

Table 4. Experimental condition for thermal response test.

Item Specification

Rate of heat input (kW) 42.0±0.1

Flow circulation rate (LPM) 160±3

Temperature difference between inlet and outlet () Over 3.5

Measured time (h) Over 12

Measuring interval (sec) 12

3.1. Analysis of the Effective Thermal

Conductivity

Inlet and outlet temperature of the open SCW geothermal

heat exchanger are measured with constant circulation rate of

underground water, which has additional heat source in the

test rig. The additional heat source is added in the test rig to

have distinct temperature difference between inlet and outlet

of SCW heat exchanger. Even with the additional heat source,

temperature gradient remains same as case without additional

heat source and this allows application of equation (1) to find

the effective thermal conductivity. Temperature distributions

are shown in Fig. 6. The initial temperatures of circulation

water in inlet and outlet of the geothermal heat exchanger are

16.75 and 16.67 , respectively and show thermal

equilibrium status. Both of the inlet and outlet temperatures

are rapidly increased for around first 40 minutes period of

initial injection and then decreased for around 20 minutes.

Since 1 hour after initial injection, both temperatures are

progressively increased to 32.84 (Inlet) and 29.20

(Outlet). The reason for early temperature fluctuation is

believed due to mixing of heated incoming water and

unheated underground water inside the geothermal heat

exchanger. Initial operation forces supply of heated water

from the test rig into the heat exchanger. The supplied heated

water tends to stay in upper region of heat exchanger and

flows out first in early operation. However, this heated

incoming water eventually mixes with existing water in

lower part of the heat exchanger to reach another thermal

equilibrium condition.

Figure 7 is semi-logarithmic temperature distribution.

Average value of inlet and outlet temperature is linearized

with natural log of time and its slope is deduced to obtain the

effective thermal conductivity along equation (1). The

effective thermal conductivity of the open SCW type

geothermal heat exchanger are summarized in Table 3 and

appeared rather high values.

Fig 6. Inlet and outlet temperature variation of circulation water in SCW

geothermal heat exchanger.


Fig 7. Linearization of inlet and outlet average temperature.

Table 5. Evaluation of effective thermal conductivity.

Item 1st measurement 2nd measurement

Heat input (kWh) 503.0 489.8

Slope (κ) 4.26 4.15

Effective thermal

conductivity, λeff

[W/(m.K)]

3.76 3.86

3.2. Thermal Resistance Variation

Figure 8 shows the variation of thermal resistance of the

open SCW type geothermal heat exchanger. The thermal

resistance is calculated by equation (2) using measured data.

Value of the thermal resistance is rapidly decreased for

around 40 minutes and is gradually converged to constant as

time elapses. It indicates the thermal resistance can reach

steady level even in rather short time of operation. The

thermal resistances are compared with those of design

programs (GLHEpro and EED). The thermal resistance

obtained using measured data are smaller than those of

computed by design program. The design programs

overestimate the thermal resistance by 5.93 times (GLHEpro

Program) and 1.11 times (EED Program) compared with

experimental result. As a result, the thermal resistance value

in the design program should be revised with the measured

value for optimized design. With this update of the thermal

resistance, it is possible to have optimum design of the open

SCW type geothermal heat exchanger and eventually reduce

the cost of construction and installation.

Fig 8. Variation of thermal resistance.

Table 6. Comparison of thermal resistance.

Item Experiments Computed by Design Program

GLHEpro EED

Thermal resistance (kWh/m) 0.0118 (average)

(1st measurement; 0.0095, 2nd measurement; 0.0141) 0.07 0.02

Difference Ratio (Computed value/Measured

value) - 5.93 1.11

4. Conclusions

The thermal performance of the open SCW type

geothermal heat exchanger is measured with the thermal

response test rig. The test rig is built on trailer bed and the

measured data are analyzed with line-source theory. Using

measured data, the effective thermal conductivity and the

thermal resistance are evaluated. The effective thermal

conductivity yields rather high value indicating high heat

transfer ability. Meantime, the measured thermal resistance

has lower value compared to the predicted values by design

programs. This trend suggests that the predicted numerical

values in the design software need modification for better

design of geothermal heat exchangers.

As viewed in consideration of the above experiment result,

the important factor of the geothermal heat exchanger design

of SCW is the value of thermal resistance. The thermal

resistance value should reflect on the resistance value

evaluated experimental method more than that of numerical

method.

Acknowledgement

This research was supported by the research fund of

Hanbat National University in 2011

Nomenclature

Q heat injection [W]

H borehole depth (SCW) [m]

R thermal resistance [K/(W/m)]

T temperature of fluid []

r radius of borehole [m]

t elapsed time [hr]


κ slope of fluid temperature against ln(t)

α thermal diffusivity [m2/s]

λ thermal conductivity [W/m.K]

γ Euler's constant (=0.5772)

Subscript

eff effective sur surface b borehole o outside f fluid

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