Study of Heat Transfer Characteristics and Economic Analysis of a Closed Deep Coaxial Geothermal Heat Exchanger Retrofitted from an Abandoned Oil Well
Abstract
:1. Introduction
2. Abandoned Oil Well Structure and Working Principles of a CGHE
3. Development of Numerical Heat Transfer Model
3.1. Model Assumptions
- The thermal properties of ground, pipe, water, and backfilling material are dependent on their specific temperatures.
- The properties of the ground are homogeneous in each ground layer.
- The penetration of groundwater is neglected.
- The lateral and bottom boundaries of the borehole model are set to be temperature constant.
- The geothermal heat flux in the ground is assumed to be uniform and constant.
3.2. Initial and Boundary Conditions
3.3. Heat Transfer Governing Equation
3.4. Numerical Discretization
3.5. Equation Solving Algorithm
3.6. Model Validation
4. Thermal Performance Analysis of the Retrofitted CGHE
4.1. Parameter Settings of the Retrofitted CGHE
4.2. Average Heat Extraction Rate in the Heating Season
4.3. Orthogonal Test Scheme Design and Test Results
4.4. Analysis of Orthogonal Test Results
5. Case Study and Economic Analysis
5.1. Actual Operation of the Ground Source Heat Pump (GSHP) System with Retrofitted CGHE
5.2. Initial System Investment
- (1)
- Drilling cost
- (2)
- Pipe material cost
5.3. Total Operation Cost of the GSHP System with Retrofitted CGHE
- (1)
- Water pump cost
- (2)
- Heat pump operation cost
6. Conclusions
- (1)
- A sensitive analysis was carried out by using the orthogonal experimental method to investigate the influence of four critical variables on the thermal performance of CGHE. The results show that the inlet water temperature plays the most important role in heat extraction rates, followed by types of CGHE retrofitted from abandoned wells, flow rate, and inner tube thermal conductivity.
- (2)
- An economic analysis of the retrofit, considering the operation costs of the water pump and heat pump, was performed. Given the fact that the CGHE in this study was retrofitted from the abandoned oil wells, the drilling cost can be reduced by up to CNY 1800 thousand. The pump operation cost of CGHE retrofitted from three-section abandoned oil wells is significantly increase with the increase in the flow rate, while the heat pump operation cost decreases because the COP increases.
- (3)
- This study integrates actual project considerations to determine the most appropriate circulation flow based on building load requirements. From an economic perspective, a flow rate of 35 m³/h is selected as the optimal choice, which aligns with both the economic efficiency and operation stability of the middle and deep CGHEs.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
Abbreviations
AOGW | abandoned oil and gas wells |
CGHE | coaxial geothermal heat exchanger |
DBHE | deep borehole heat exchanger |
EGS | enhanced geothermal systems |
FDM | finite difference method |
FVM | finite volume method |
GSHP | ground source heat pump |
Nomenclature
a | thermal diffusivity (m2/s) |
c, cf, cg | specific heat capacity of the circulating fluid (J/(kg·K)) |
C, C1, C2 | heat capacity of the circulating fluid (J/(m·K)) |
d, do, di, d1i, d1o, d2i, d2o | pipe diameter (m) |
pf | frictional pressure drops (Pa) |
ps | local pressure drops (Pa) |
Hj | bottom coordinate of the j-layer |
k, kg, kp1, kp2 | thermal conductivity (W/(m·K)) |
qg | geothermal heat flux (W/m2) |
h1, h2, ha | convective heat transfer coefficients (W/(m2·K)) |
K, K1, K2 | height of the equivalent coarse grain (m) |
l | pipe length (m) |
M | circulating fluid flow rate (kg/s) |
P | power consumption (W) |
Q | total heat transfer rate of the CGHE (W) |
r | radial coordinate (m) |
rb | borehole radius (m) |
rbnd | the radius of the radial boundary (m) |
R1, R2 | thermal resistance ((m·k)/W) |
ta | ambient air temperature (°C) |
u | average section velocity (m/s) |
v | kinematic viscosity (m2/s) |
ρ, ρf, ρg | density (kg/m3) |
λ | friction factor |
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No. | Length of Borehole (m) | Borehole Diameter (m) | External Diameter of the Outer Pipe (m) | Inner Diameter of the Outer Pipe (m) | k of the Ground W/(m·K) | Specific Heat Capacity 106J/(Nm3·K) |
---|---|---|---|---|---|---|
First section | 0~455 | 0.44 | 0.34 | 0.32 | 2.8 | 1.10 |
Second section | 455~2017 | 0.31 | 0.24 | 0.23 | 2.8 | 1.93 |
Third section | 2017~2626 | 0.22 | 0.18 | 0.16 | 3.0 | 1.41 |
Forth section | 2626~2829 | 0.15 | 0.14 | 0.12 | 3.5 | 2.24 |
Operating Condition | Modeling Parameter | |||
---|---|---|---|---|
A: Configuration of CGHE Retrofitted from Abandoned Oil Wells | B: Inlet Temperature | C: Flow Rate | D: Thermal Conductivity of Inner Pipe kp2 | |
(°C) | (m³/h) | (W/(M·K)) | ||
1 | 4-section with variant do of 2829 m | 5 | 25 | 0.17 |
2 | 4-section with variant do of 2829 m | 10 | 30 | 0.25 |
3 | 4-section with variant do of 2829 m | 15 | 35 | 0.34 |
4 | 4-section with variant do of 2829 m | 20 | 40 | 0.43 |
5 | 3-section with variant do of 2626 m | 5 | 30 | 0.34 |
6 | 3-section with variant do of 2626 m | 10 | 25 | 0.43 |
7 | 3-section with variant do of 2626 m | 15 | 40 | 0.17 |
8 | 3-section with variant do of 2626 m | 20 | 35 | 0.25 |
9 | 4-section with constant do of 2829 m | 5 | 35 | 0.43 |
10 | 4-section with constant do of 2829 m | 10 | 40 | 0.34 |
11 | 4-section with constant do of 2829 m | 15 | 25 | 0.25 |
12 | 4-section with constant do of 2829 m | 20 | 30 | 0.17 |
13 | 3-section with constant do of 2626 m | 5 | 40 | 0.25 |
14 | 3-section with constant do of 2626 m | 10 | 35 | 0.17 |
15 | 3-section with constant do of 2626 m | 15 | 30 | 0.43 |
16 | 3-section with constant do of 2626 m | 20 | 25 | 0.34 |
Operating Condition | Combination of Factors | Average Heat Extraction (kW) |
---|---|---|
1 | A1B1C1D1 | 422.18 |
2 | A1B2C2D2 | 385.94 |
3 | A1B3C2D2 | 344.84 |
4 | A1B3C3D3 | 305.00 |
5 | A2B1C2D3 | 383.09 |
6 | A2B2C1D4 | 324.62 |
7 | A2B3C4D1 | 319.11 |
8 | A2B4C3D2 | 269.08 |
9 | A3B1C3D4 | 420.76 |
10 | A3B2C4D3 | 394.05 |
11 | A3B3C1D2 | 326.41 |
12 | A3B4C2D1 | 301.88 |
13 | A4B1C4D2 | 393.67 |
14 | A4B2C3D1 | 349.02 |
15 | A4B3C2D4 | 292.01 |
16 | A4B4C1D3 | 247.68 |
Analysis of Range | A | B | C | D |
---|---|---|---|---|
n1 | 364.5 | 404.9 | 330.2 | 348.0 |
n2 | 324.0 | 363.4 | 340.7 | 343.8 |
n3 | 360.8 | 320.6 | 345.9 | 342.4 |
n4 | 320.6 | 280.9 | 353.0 | 335.6 |
Ri | 43.9 | 124 | 22.7 | 12.5 |
Performance Parameter | Number |
---|---|
Nominal heating capacity | 500 kW |
Designed inlet and outlet water temperatures of the evaporator | 15/7 °C |
Design inlet and outlet water temperatures of the condenser | 40/45 °C |
Nominal power consumption | 115 kW |
Rated COP | 4.48 |
Pipe Type | Hydraulic Diameter (m) | Re | Pr | Nu | h (W/m2·K) |
---|---|---|---|---|---|
Annular pipe section 1 | 0.32 | 16,287.79 | 9.02 | 112.02 | 315.78 |
Annular pipe section 2 | 0.23 | 20,760.25 | 9.02 | 136.02 | 705.69 |
Annular pipe section 3 | 0.16 | 25,844.40 | 9.02 | 162.07 | 2113.49 |
Inner pipe | 0.09 | 83,431.80 | 9.02 | 413.88 | 2819.27 |
Pipe Type | Fitted Curve | |
---|---|---|
Annular pipe section 1 | y = 79.86 ± 15 + (9.43 ± 0.4) × x | y: convection heat transfer coefficient (W/m2·K) x: flow rate (m3/h) |
Annular pipe section 2 | y = 178.53 ± 34 + (21.07 ± 1.0) × x | |
Annular pipe section 3 | y = 548.87 ± 162 + (62.40 ± 4.9) × x | |
Inner pipe | y = 712.90 ± 135 + (84.19 ± 4.0) × x |
Buried Depth | 1000 m | 1500 m | 2000 m | 2500 m | 3000 m |
---|---|---|---|---|---|
Drilling cost (103CNY) | 600 | 900 | 1200 | 1500 | 1800 |
Buried Depth | 1000 m | 1500 m | 2000 m | 2500 m | 3000 m |
---|---|---|---|---|---|
Pipe material cost (103CNY) | 440 | 660 | 880 | 1100 | 1320 |
Pipe Type | Fitted Curve | |
---|---|---|
Annular pipe section 1 | y = −8.07 ± 0.82 + (0.51 ± 0.02) × x | y: total pressure drop (m) x: flow rate (m3/h) |
Annular pipe section 2 | y = −27.69 ± 2.82 + (1.75 ± 0.09) × x | |
Annular pipe section 3 | y = −10.80 ± 1.10 + (0.68 ± 03) × x | |
Inner pipe | y = 4.72 |
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Liu, R.-J.; Jia, L.-R.; Zhang, W.-S.; Yu, M.-Z.; Zhao, X.-D.; Cui, P. Study of Heat Transfer Characteristics and Economic Analysis of a Closed Deep Coaxial Geothermal Heat Exchanger Retrofitted from an Abandoned Oil Well. Sustainability 2024, 16, 1603. https://doi.org/10.3390/su16041603
Liu R-J, Jia L-R, Zhang W-S, Yu M-Z, Zhao X-D, Cui P. Study of Heat Transfer Characteristics and Economic Analysis of a Closed Deep Coaxial Geothermal Heat Exchanger Retrofitted from an Abandoned Oil Well. Sustainability. 2024; 16(4):1603. https://doi.org/10.3390/su16041603
Chicago/Turabian StyleLiu, Rui-Jia, Lin-Rui Jia, Wen-Shuo Zhang, Ming-Zhi Yu, Xu-Dong Zhao, and Ping Cui. 2024. "Study of Heat Transfer Characteristics and Economic Analysis of a Closed Deep Coaxial Geothermal Heat Exchanger Retrofitted from an Abandoned Oil Well" Sustainability 16, no. 4: 1603. https://doi.org/10.3390/su16041603
APA StyleLiu, R. -J., Jia, L. -R., Zhang, W. -S., Yu, M. -Z., Zhao, X. -D., & Cui, P. (2024). Study of Heat Transfer Characteristics and Economic Analysis of a Closed Deep Coaxial Geothermal Heat Exchanger Retrofitted from an Abandoned Oil Well. Sustainability, 16(4), 1603. https://doi.org/10.3390/su16041603