# «Nikhil Manikonda Faculty of Graduate and Postdoctoral Studies Department of Civil Engineering University of Ottawa Ontario, Canada. ©Nikhil ...»

## PERFORMANCE OF DEEP GEOTHERMAL

## ENERGY SYSTEMS

Dissertation submitted in partial fulfillment for the award of the

degree of Master of Applied Sciences

In

Environmental Engineering

By

Nikhil Manikonda

Faculty of Graduate and Postdoctoral Studies

Department of Civil Engineering

University of Ottawa

Ontario, Canada.

©Nikhil Manikonda, Ottawa, Canada, 2012.

## ACKNOWLEDGEMENTS

First and foremost, I offer my obeisance to the Almighty for having bestowed his grace. I humbly present this research as a part of my curriculum, which contains the details of research work on the “Performance of deep geothermal energy systems”.I sincerely wish to acknowledge and thank my supervisor Dr. Majid Mohammadian for his deep sincerity, support, and guidance throughout the research work, whose kind cooperation helped me collect sufficient information for the completion of this work I am very thankful to all my friends and family members for their support and encouragement during the dissertation. Most importantly, I thank my girlfriend for pushing me forward and reminding me that I was capable of doing it, even when I was down.

Nikhil Manikonda i Abstract Geothermal energy is an important source of clean and renewable energy. This project deals with the study of deep geothermal power plants for the generation of electricity. The design involves the extraction of heat from the Earth and its conversion into electricity.

This is performed by allowing fluid deep into the Earth where it gets heated due to the surrounding rock. The fluid gets vaporized and returns to the surface in a heat pipe. Finally, the energy of the fluid is converted into electricity using turbine or organic rankine cycle (ORC). The main feature of the system is the employment of side channels to increase the amount of thermal energy extracted. A finite difference computer model is developed to solve the heat transport equation. The numerical model was employed to evaluate the performance of the design. The major goal was to optimize the output power as a function of parameters such as thermal diffusivity of the rock, depth of the main well, number and length of lateral channels. The sustainable lifetime of the system for a target output power of 2 MW has been calculated for deep geothermal systems with drilling depths of 8000 and 10000 meters, and a financial analysis has been performed to evaluate the economic feasibility of the system for a practical range of geothermal parameters. Results show promising an outlook for deep geothermal systems for practical applications.

** ii Table of Contents Overview: **

Objectives of the present study

Methodology

Structure of thesis

Chapter 1: Introduction

1.1 Working pattern of geothermal power plant

1.2 Closed loop versus open loop geothermal systems

1.2.1. Closed loop systems

1.2.2. Open loop systems

1.3. Geothermal power plant types

1.3.1 Dry steam power plant

1.3.2 Flash steam power plant

1.3.3 Binary steam power plant

1.3.4 Hybrid power plant

1.4. Different uses geothermal energy

1.5. Drilling technologies

1.5.1 Diamond drilling

1.5.2. Rotary steerable PDC drill

Chapter 2: Literature review

2.1. Deep geothermal power plants

2.1.1. Geothermal electricity generation in Soultz-sous Forets

2.2 Shallow geothermal systems:

Chapter 3: A modified design for deep geothermal systems and numerical modeling.......... 53

3.1. Governing equation

3.2. Numerical scheme

3.3. Calculation of the source term

3.4. Computer program

3.5. Computational grid and boundary conditions

** iii 3.6. Typical output power **

4.1. Budgeting

4.2. Need for budgeting

4.3. Budgeting for the geothermal power plant

4.4. Calculations for cost analysis

4.4.1. Total drilling length 8,000m

4.4.2. Total drilling length 10000m

Chapter 5: Sensitivity analysis and numerical results

5.1. Model Parameters

5.2. Sensitivity analysis

5.2.1. Sensitivity to the grid size

5.2.2. Sensitivity to the time step size

5.2.3. Sensitivity to the depth of main well

5.2.4. Sensitivity to the vertical distance between lateral channels

5.2.5. Sensitivity to the number and length of side channels:

5.3. Numerical results

5.3.1. Total drilling length of 10,000m

5.3.2. Total drilling length of 8,000 m

Chapter 6: Summary and Conclusions

7. References

Appendix I

I.1 System lifetime with respect to distance between side channels

Appendix II

** II.1 Present value (P) analysis: **

Figure1.1: First geothermal power plant in Italy [1]

** Figure1.2: Plant view and working employees - 1st Geothermal Power Plant (1904) [1].**

....... 4 Figure1.3: Closed loop system with power cycle [7]

Figure1.4: Typical heat pipe design [9]

Figure1.5: Open loop system [10]

Figure1.6: Dry steam geothermal power plant [13]

Figure1.7: Flash system power plant [15]

Figure1.8: Binary steam power plant cycle [17]

Figure1.9: Diamond cutter drill bit [21]

Figure1.10: Rotary steerable PDC drill sample [22]

Figure2.1: ORC system of a geothermal power plant [23]

Figure2.2: Temperature versus depth profile [24]

Figure2.3: Performance of the system at a depth of 5000m [24]

Figure2.4: Cross section temperature profile oriented to the main groundwater flow direction [25]

Figure2.5: A schematic view of a geothermal power plant U-pipe system [26]

** Figure2.6: Geothermal power gain versus velocity of the system shown in Fig.**

2.5 [26]........ 35 Figure2.7: Sketch of a deep BHE with varying pipe diameter [27]

Figure2.8: Cross section of a typical double-U shaped BHE [27]

Figure2.9: Temperature distribution around two running BHEs which are extracting heat [27]

Figure2.10: Sketch of double U-tube BHE cross-section [28]

Figure2.11: Temperature (0C) change versus time (months) [33]

Figure2.12: Temperature distribution and BHE workloads for non-optimized equal load [33]

** Figure2.13: Schematic diagram of boreholes in GHE.**

(a) Double U-tube and (b) Single Utube [35]

Figure2.14: Borehole thermal resistance versus thermal conductivity [35]

** Figure2.15: Function for four interacting boreholes separated by a distance B [37].**

............... 47 Figure2.16: 3D image of the resulting mesh [39]

Figure2.17: Temperature contours after 5, 10, 20, and 30 years [42]

Figure 2.18: 3-D finite element model top view [43]

Figure3.1: Flowchart showing the system model

Figure3.2: X-Y view of a mesh

** Figure3.3: Typical simulation result where colours show the temperature difference.**

Heat pipes are identified by the light blue colour.

v Figure3.4: Typical output power versus time (days)

Figure3.5: Demand curve performance

Figure3.6: Sinusoidal performance of the system-Tecplot image

Figure3.7: 2D image of the simulation using Tecplot

Figure4.1: Expected average power generation with respect to time (months)

Figure4.2: Total cost versus time (months) for a total drilling length of 8,000m

** Figure4.3: Total cost versus time (months) for a total drilling length of 10,000m.**

................. 77 Figure5.1: Depth of main well vs. time

Figure5.2: System lifetime with respect to the distance between channels for a geothermal gradient of 350C/km and a total drilling length of 10,000m

** Figure5.3: System lifetime with respect to the distance between channels for a geothermal gradient of 450C/km and a total drilling length of 10000m.**

Figure5.4: System lifetime with respect to the distance between channels for a geothermal gradient of 550C/km and a total drilling length of 10,000m

Figure5.5: System lifetime with respect to the distance between channels for a geothermal gradient of 650C/km and a total drilling length of 10,000m

Figure5.6: System lifetime with respect to the distance between channels for a geothermal gradient of 350C/km and a total drilling length of 8,000m

Figure5.7: System lifetime with respect to the distance between channels for a geothermal gradient of 450C/km and a total drilling length of 8000m

Figure5.8: System lifetime with respect to the distance between channels for a geothermal gradient of 550C/km and a total drilling length of 8,000m

Figure5.9: System lifetime with respect to the distance between channels for a geothermal gradient of 650C/km and a total drilling length of 8,000m

Figure6.1: Optimal lifetime with respect to thermal diffusivity for total drilling length of 8,000m for various geothermal gradients (shaded region shows the economically unfeasible area)

Figure6.2: Optimal lifetime with respect to thermal diffusivity for total drilling length of 10,000m for various geothermal gradients (shaded region shows the economically unfeasible area)

** viList of Tables:**

Table1.1: Drilling factors and their performances [21]

Table2.1: Results illustrating the performance of the system [24]

Table4.1: System components and their costs

Table 4.2: Cost analysis for a total drilling length of 8000m

Table4.3: Cost analysis for a total drilling length of 10000m

Table5.1: Range of Initial Parameters

Table5.2: Depth of main well

Table5.3: Length and number of channels used in optimization for total drilling length of 10,000 m

Table6.1: Optimal design conditions for geothermal gradient of 550C/Km

Table6.2: Optimal conditions and system lifetime for geothermal gradient of 350C/km (shaded region shows the economically unfeasible area)

Table6.3: Optimal conditions and system lifetime for geothermal gradient of 450C/km (shaded region shows the economically unfeasible area)

Table6.4: Optimal conditions and system lifetime for geothermal gradient of 550C/km (shaded region shows the economically unfeasible area)

Table6.5: Optimal conditions and system lifetime for geothermal gradient of 650C/km (shaded region shows the economically unfeasible area)

Table I.1: System lifetime with respect to distance between side channels for total drilling length of 10000(m) and geothermal gradient of 350C/km (shaded region shows the economically unfeasible area)

Table I.2: System lifetime with respect to distance between side channels for total drilling length of 8000(m) and geothermal gradient of 350C/km (shaded region shows the economically unfeasible area)

Table I.3: System lifetime with respect to distance between side channels for total drilling length of 10000(m) and geothermal gradient of 450C/km (shaded region shows the economically unfeasible area)

Table I.4: System lifetime with respect to distance between side channels for total drilling length of 8000(m) and geothermal gradient of 450C/km (shaded region shows the economically unfeasible area)

Table I.5: System lifetime with respect to distance between side channels for total drilling length of 10000(m) and geothermal gradient of 550C/km (shaded region shows the economically unfeasible area)

Table I.6: System lifetime with respect to distance between side channels for total drilling length of 8000(m) and geothermal gradient of 550C/km (shaded region shows the economically unfeasible area)

vii Table I.7: System lifetime with respect to distance between side channels for total drilling length of 10000(m) and geothermal gradient of 650C/km (shaded region shows the economically unfeasible area)

Table I.8: System lifetime with respect to distance between side channels for total drilling length of 8000(m) and geothermal gradient of 650C/km (shaded region shows the economically unfeasible area)

BHE: Borehole heat exchanger EGS- Enhanced geothermal system FD- Finite difference FE- Finite element GCHP- Ground coupled heat pump GHE- Ground heat exchanger GHP- Geothermal heat pump GSHP: Ground source heat pump HDR- Hot dry rock ORC- Organic rankine cycle PDC- Polycrystalline diamond compact TRCM- Thermal resistance and capacity models UGCHE- U vertical ground coupled heat exchanger

T = Fluid temperature in the in-pipe u = refrigerant fluid velocity ɸ = specific heat transfer co-efficient z = vertical coordinate Ts = vertical soil temperature D = depth t = time step size T = temperature Ti,nj,k = temperature at time t nt at the location represented by the gird point (i, j,k) Ti,nj,1 = temperature at time t n 1 t at the location represented by the gird point (i, j,k) k K x, K y and K z = respectively the thermal diffusion coefficient in x, y, and z directions t = the time S = the heat source or sink v x y z = volume of the computational cell

Objectives of the present study

**The following objectives are considered in this study:**

To design a geothermal power plant that can generate 2 MW of electricity and

To estimate system lifetime under various conditions using numerical simulations.

To obtain the optimal conditions which lead to maximum system life time.

To conduct a financial analysis in order to make sure that the system is economical.

Methodology A numerical approach has been employed in this project to design an economical geothermal power plant system. Initially, an imaginary geothermal power plant design is prepared. All the parameters that feature in order to develop the power plant are considered and a numerical model is developed. The equations are solved with the help of the computer program written in FORTRAN. The grid independency and time step independency analysis are also performed.