Geothermal Energy Study Notes

SUSTAINABLE ENERGY: ENGINEERING FUNDAMENTALS AND APPLICATIONS

Chapter 8: Geothermal Energy

Introduction

  • Definition of Geothermal Energy: Derived from two words, "geo" meaning Earth and "thermal" meaning heat; thus, geothermal energy is the energy extracted from the Earth.

  • Geological Structure of the Earth:

    • Composed of three layers: core, mantle, and crust (from center to surface).

    • Heat Transfer: Heat flows outward from the core to the crust through convection currents, resulting in molten rocks circulating beneath the crust, akin to boiling water.

    • The geothermal energy potential is highest near tectonic plate boundaries due to larger temperature gradients.

    • The natural convection currents cause fracturing at the ocean floor, leading to mid-ocean ridge formations when molten mantle reaches the ocean and solidifies. The term "magma" refers to the molten rock that escapes from the crust.

  • Utilization of Geothermal Energy: Can be applied directly for heating, drying, cooling, or providing hot water, or to generate electricity.

Geological Structure of the Earth (Figures)

  • Figure 8.1: Depicts the geological structure of Earth, illustrating how the crust is thin relative to the mantle and core.

  • Figure 8.2: Shows the tectonic plates and their interactions: divergence at divergent boundaries, convergence at convergent boundaries, and lateral sliding at transform boundaries.

Global Overview

  • Installed Capacity: By end of 2021, total geothermal installed capacity reached 15,854 MW. (Figure 8.5)

  • Geothermal Gradient: U.S. geothermal temperature gradient shown in degrees Celsius per kilometer depth (Figure 8.6).

Geothermal Resources

  • Types of Geothermal Resources:

    • Geothermal gradient

    • Hot dry rock

    • Hot water reservoirs

    • Natural steam reservoirs

    • Molten magma

    • Geopressured reservoirs

Direct Use of Geothermal Energy

  • Systems:

    • Geothermal heat pump systems (for heating and cooling)

    • Geothermal residential and commercial heating

    • Geothermal district heating

Electricity Generation

  • Process: Similar to conventional thermal power plants, where energy from the working fluid converts into steam to power a turbine.

  • Types of Geothermal Power Plants:

    • Dry Steam Power Plants: Utilizes steam directly from the ground with minimal processing.

    • Flash Steam Power Plants: Hot water vaporizes at lower pressures, a separator prevents liquid from entering the turbine to avoid damage.

    • Binary Cycle Power Plants: Hot water heats a secondary working fluid in a vaporizer without direct contact to the turbine; the geothermal fluid is reinjected.

Theory

  • Geothermal Heat Pumps: Exchange energy between the soil and working fluid in ground loops.

  • Energy Formulas:

    • For building heating: ext{Rate of heat transfer}, \ ext{Q̇} = ext{ṁ} imes cp imes (Ti - T_o) \ ext{(8.1)}

    • Carnot efficiency: ext{Efficiency}, \ ext{η}{Carnot} = 1 - rac{TL}{T_H} \ ext{(8.2)}

    • Thermal efficiency: ext{η}{th} = rac{ ext{Ẇ}{turb, actual}}{ ext{ṁ}{supply} imes (h{supply} - hf@T{amb})} \ ext{(8.3)}

Energy Calculations

  • Actual Work Done by Turbine: ext{Ẇ}{turb, actual} = ηt imes ext{Ẇ}_{turb, ideal} \ ext{(8.4)}

  • Isentropic Efficiency: ηt = rac{hi - he}{hi - h_{e,s}} \ ext{(8.5)}

  • Ideal Work: ext{Ẇ}{turb, ideal} = ext{ṁ} imes (hi - h_{e,s}) \ ext{(8.6)}

Geothermal Power Plant Diagrams

  • Figures 8.12 - 8.15: Schematic and T-S diagrams for various geothermal power plants: Dry Steam, Single Flash, Double Flash, Binary Cycle Power Plant.

Example Calculation

  • Example 8.1: Calculation of Carnot efficiency for a geothermal power plant with fluid temperature at 220°C and condensation pressure at 20 kPa:

    • Conversion: T_H = 220°C + 273.15 = 493.15 ext{ K}

    • Saturation temperature at given pressure: TL = T{sat@20kPa} = 60.06°C ightarrow 333.21 ext{ K}

    • Carnot Efficiency Calculation:

\eta_{Carnot}=1-rac{333.21}{493.15}=0.324ext{ (32.4\exponentialE

Efficient Geothermal Plants

  • Performance metrics for geothermal plants:

    • Dry Steam Plants: ~18-22% net efficiency at temperatures ~240–300 °C.

    • Double Flash Plants: 15-20% net efficiency for temperatures ~200–250 °C, top plants reaching ~20-22%.

    • Binary Plants: 8-15% net efficiency for moderate temperatures ~110-180 °C; advanced setups achieving ~14-16%.

Applications and Case Study

  1. Single Flash Steam Plant: Ulubelu Geothermal Plant in Lampung, Indonesia; 4 units at 55 MW each, total capacity of 220 MW.

  2. Binary Cycle CHP Plant: Svartsengi Geothermal Power Plant in Keflavik, Iceland; heating capacity of 190 MW and electricity generation of 75 MW.

  3. Geothermal District Heating: Balcova-Narlidere district heating system in Izmir, Turkey; hot water capacity of 2020 m³/h, water temperature range of 90-144 °C.

Economics of Geothermal Energy

  • Cost Components:

    • Capital Cost: Total construction cost including land, drilling, equipment, labor; includes interest costs.

    • Operating and Maintenance Costs: Covers utilities, wages, insurance, taxes, and maintenance.

    • Fuel Cost: Unlike fossil-based plants, geothermal costs are stable as the energy source is readily available.

Levelized Costs of Electricity (LCOE)

  • Estimate Table (example data following in the notes):

    • Geothermal (90% CF): LCOE of $34.49/MWh; high capacity factor and low variability.

  • Capacity Factor (CF): Measures actual output compared to potential full power output; high reliability.

Summary

  • Geothermal energy harnessed directly or indirectly via various applications.

  • Direct applications include geothermal heat pumps and district heating.

  • Indirect applications involve power generation using different methods (dry steam, flash steam, binary cycle).

  • Geothermal potential largely exists near tectonic plate boundaries with risks of seismic activity due to the geological context.