Study Notes on Geothermal Energy

GEOTHERMAL ENERGY

INTRODUCTION

  • Definition: The term "geothermal" is derived from Greek, where "geo" means earth and "thermal" means heat. Thus, geothermal energy refers to energy extracted from the earth.
  • Geological Structure:
    • The Earth consists of three primary layers:
    1. Core: Innermost layer.
    2. Mantle: Middle layer.
    3. Crust: Outermost layer.
  • Heat Transfer:
    • The transfer of heat from the Earth's core to its crust leads to convective currents.
    • These currents move molten rocks beneath the crust in circular motions, akin to boiling water in a pot. As magma rises and cools, it sinks back, forming loops.
  • Geothermal Energy Potential:
    • Highest near tectonic plate boundaries where temperature gradients are more pronounced.
  • Convective Currents:
    • These currents also exist in the mantle, causing flow patterns that result in tectonic activity (e.g., mid-ocean ridges from solidified mantle).
  • Magma: The molten rock that reaches the earth's surface.
  • Applications:
    • Geothermal energy can be harnessed directly for heating, cooling, drying, or hot water, or utilized for electricity generation.

GLOBAL OVERVIEW

  • Installed Capacity:
    • As of 2021, global geothermal installed capacity was 15,854 MW (Megawatts).
  • Top Geothermal Countries:
    • Information pertaining to the leading countries in geothermal energy harnessing is provided.

GEOTHERMAL RESOURCES

  • Different geothermal resources:
    • Geothermal Gradient: Refers to the temperature increase with depth.
    • Hot Dry Rock: Areas with high-temperature rock.
    • Hot Water Reservoirs: Underground aquifers filled with hot water.
    • Natural Steam Reservoirs: Areas where steam is naturally available.
    • Molten Magma: The source of geothermal heat.
    • Geopressured Reservoirs: High-pressure zones containing hot water and gas.

DIRECT USE OF GEOTHERMAL ENERGY

  • Various types of applications:
    • Geothermal Heat Pumps: Systems for heating and cooling.
    • Geothermal Residential Heating: Utilization in homes.
    • Geothermal District Heating: Heating systems for multiple buildings.

ELECTRICITY GENERATION

  • Methodology:
    • Similar principles as conventional thermal power plants.
    • Uses steam from geothermal resources to power turbines for electricity generation.
  • Types of Power Plants:
    1. Dry Steam Power Plants:
    • Direct steam usage for electricity production with minimal processing.
    1. Flash Steam Power Plants:
    • Hot water vaporizes and the resultant steam powers the turbines.
    • Separators are used to prevent liquid from entering turbines, which can cause damage.
    1. Binary Cycle Power Plants:
    • Hot water from geothermal reservoirs heats a secondary working fluid in a vaporizer, without direct contact with the turbine.

THEORY

  • Energy Exchange in Heat Pumps:
    • Energy exchanged between the soil and working fluid through a ground loop system.
  • Thermal Energy Equation:
    • For building or district heating applications:
      \dot{Q} = \dot{m} cp (Ti - T_o) where
    • \dot{Q} = Heat energy transferred,
    • \dot{m} = Mass flow rate,
    • c_p = Specific heat capacity,
    • T_i = Inlet temperature,
    • T_o = Outlet temperature.
  • Carnot Efficiency:
    • Maximum efficiency of a power plant:
      \eta{Carnot} = 1 - \frac{TL}{T_H} where
    • T_H = Hot reservoir temperature,
    • T_L = Cold reservoir temperature.
  • Thermal Efficiency:
    • Defined as:
      \eta{th} = \frac{\dot{W}{turb.actual}}{m{supply}(h{supply}-hf@T{amb})}

EXAMPLES

  • Carnot Efficiency Calculation Example:
    • Given 220°C fluid and condensation pressure of 20 kPa.
    • T_H = 220 + 273.15 = 493.15 K;
    • Calculate TL using saturation temperature table for 20 kPa: TL = 60.06°C = 333.21 K.
    • Compute Carnot efficiency:
      \eta_{Carnot} = 1 - \frac{333.21}{493.15} = 0.324 or 32.4%.
  • Thermal Efficiency of Geothermal Power Plants:
    • Range Depending on Technology:
    • Dry steam: 18%-22%
    • Double flash: 15%-20%
    • Binary plants: 8%-15%.

APPLICATIONS AND CASE STUDY

  • Single Flash Steam Power Plant:
    • Ulubelu Geothermal Plant in Lampung, Indonesia
    • Operated by: PLN and PGE with a total capacity of 220 MW.
  • Binary Cycle CHP Plant:
    • Svartsengi Geothermal Power Plant in Keflavik, Iceland
    • Heating capacity of 190 MW, with an electricity generation capacity of 75 MW.
  • District Heating System:
    • Balcova-Narlidere District Heating in Izmir, Turkey
    • Hot water capacity of 2020 m³/h at temperatures ranging from 90-144 °C.

ECONOMICS

  • Cost Breakdown of Geothermal Power Plant:
    • Capital Cost: Construction expenses including land, labor, and overhead.
    • Operating and Maintenance Cost: Utilities, wages, health insurance, and maintenance.
    • Fuel Cost: Geothermal plants have stable energy costs as they rely on free natural heat.

LEVELIZED COST OF ELECTRICITY (LCOE)

  • Cost Comparison Table (Example 8.1):
    • Geothermal energy's total LCOE (including tax credits) is $34.49/MWh with a high capacity factor of 90%.

SUMMARY

  • Geothermal energy can be harvested through direct application (heating) or indirect application (power generation).
  • Different types of geothermal plants exist for electricity generation.
  • Geothermal resources are best located near tectonic plate boundaries but come with risks such as the potential for earthquakes.

ADDITIONAL NOTES

  • Geothermal energy represents one of the most reliable and efficient renewable energy sources available with considerable economic benefits due to low operational costs.
  • While the theoretical maximum efficiency is 32.4%, actual efficiencies vary based on technology used.