🌋Geothermal Systems Engineering Unit 6 – Direct Use Applications in Geothermal Systems

Key Concepts and Terminology

• Geothermal energy originates from heat within the Earth's core and radiogenic decay of minerals
• Direct use applications harness geothermal energy for heating and cooling purposes without generating electricity
• Heat exchangers transfer heat from the geothermal fluid to a secondary fluid, such as water or air
  • Plate heat exchangers consist of thin plates stacked together with alternating channels for hot and cold fluids
  • Shell and tube heat exchangers have a bundle of tubes inside a shell, with geothermal fluid flowing through the tubes and the secondary fluid flowing through the shell
• Geothermal fluid is the hot water or steam extracted from a geothermal reservoir
• Thermal efficiency measures the effectiveness of a heat exchange system in transferring heat from the geothermal fluid to the secondary fluid
• Cascaded use involves utilizing geothermal energy for multiple applications in a sequential manner, with each subsequent use requiring lower temperatures
• Geothermal heat pumps use the stable temperature of the Earth's shallow subsurface to heat and cool buildings

Geothermal Resource Types

• Low-temperature geothermal resources have temperatures below 90°C (194°F) and are suitable for direct use applications
  • These resources are often found in sedimentary basins or near the edges of tectonic plates
• Moderate-temperature geothermal resources range from 90°C to 150°C (194°F to 302°F) and can be used for both direct use and binary cycle power generation
• High-temperature geothermal resources exceed 150°C (302°F) and are primarily used for electricity generation
• Hydrothermal systems contain naturally occurring hot water or steam trapped in permeable rocks or fractures
• Enhanced Geothermal Systems (EGS) involve injecting water into hot, dry rock formations to create artificial geothermal reservoirs
• Sedimentary basin geothermal resources are found in deep aquifers with high permeability and porosity
• Shallow geothermal resources, such as those used by geothermal heat pumps, rely on the relatively constant temperature of the Earth's subsurface

Direct Use Applications Overview

• Space heating and cooling for residential and commercial buildings
  • Geothermal district heating systems distribute heat from a central geothermal source to multiple buildings
• Agricultural applications include greenhouse heating, crop drying, and aquaculture
  • Geothermal energy can extend growing seasons and improve crop yields in greenhouses
  • Fish farming benefits from the stable temperatures provided by geothermal water
• Industrial processes, such as food processing, paper production, and chemical extraction
  • Geothermal heat can be used for pasteurization, sterilization, and evaporation in food processing
• Balneology and spa therapy utilize geothermal water for its therapeutic properties
• Snow melting and de-icing on sidewalks, driveways, and roads
• Desalination of seawater or brackish water using geothermal heat
• Absorption cooling systems use geothermal heat to drive a refrigeration cycle, providing cooling without the need for electricity

Heat Exchange Systems

• Direct heat exchange systems transfer heat from the geothermal fluid directly to the end-use application
  • Open-loop systems circulate geothermal water directly through the heating system and back into the reservoir
  • Closed-loop systems use a secondary fluid to transfer heat from the geothermal fluid to the end-use application
• Indirect heat exchange systems use a heat exchanger to transfer heat from the geothermal fluid to a secondary fluid
  • This approach minimizes corrosion and scaling issues in the end-use equipment
• Downhole heat exchangers are installed directly in the geothermal well, eliminating the need for surface heat exchangers
  • This design reduces heat loss and improves overall system efficiency
• Plate heat exchangers are compact and efficient, making them suitable for smaller-scale applications
• Shell and tube heat exchangers are more robust and can handle higher pressures and temperatures
• Heat pump systems can be used to boost the temperature of the geothermal fluid for higher-temperature applications
• Hybrid systems combine geothermal heat with other renewable energy sources, such as solar thermal or biomass, to meet peak demand or provide backup heating

Design Considerations

• Resource temperature and flow rate determine the heat output and efficiency of the system
  • Higher temperatures and flow rates allow for more heat extraction and a wider range of applications
• Heat demand and load profile of the end-use application
  • Systems should be designed to meet peak demand while maintaining efficiency during periods of low demand
• Distance between the geothermal resource and the end-use location
  • Longer distances require more insulation and larger pipe diameters to minimize heat loss
• Corrosion and scaling potential of the geothermal fluid
  • Proper material selection and water treatment strategies can mitigate these issues
• Environmental regulations and permitting requirements
  • Geothermal projects must comply with local, state, and federal regulations regarding water use, disposal, and environmental impact
• Economic feasibility, including capital costs, operating costs, and payback period
  • Factors such as drilling costs, heat exchanger size, and energy prices influence the economic viability of a project
• Integration with existing heating and cooling systems
  • Retrofitting existing buildings to use geothermal energy may require modifications to the distribution system and end-use equipment

Environmental Impact and Sustainability

• Geothermal direct use applications have a low carbon footprint compared to fossil fuel-based heating systems
  • They do not produce greenhouse gas emissions during operation
• Proper management of geothermal reservoirs ensures long-term sustainability
  • Monitoring reservoir pressure, temperature, and chemistry helps prevent overexploitation and depletion
• Reinjection of spent geothermal fluid maintains reservoir pressure and minimizes subsidence
  • This practice also reduces the environmental impact of wastewater disposal
• Geothermal systems have a small land footprint compared to other renewable energy technologies
  • Most of the infrastructure is located underground, allowing for multiple land uses above the surface
• Low noise and visual impact make geothermal systems suitable for urban and residential areas
• Potential environmental concerns include water use, induced seismicity, and thermal pollution
  • Careful site selection, monitoring, and mitigation strategies can minimize these risks
• Life cycle assessment (LCA) studies have shown that geothermal direct use applications have a lower environmental impact than conventional heating systems over their entire lifespan

Case Studies and Real-World Examples

• Reykjavik, Iceland: Geothermal district heating system serving over 200,000 residents
  • The system has been in operation since the 1930s and meets 99% of the city's space heating needs
• Boise, Idaho, USA: Geothermal district heating system serving over 90 buildings in the downtown area
  • The system, which began operation in 1983, has resulted in significant energy savings and reduced greenhouse gas emissions
• Paris, France: Geothermal heat pump system at the Paris-Saclay research campus
  • The system, which consists of 36 boreholes and 75 heat pumps, provides heating and cooling for over 1 million square meters of buildings
• Taupo, New Zealand: Geothermal heating for the Huka Prawn Park aquaculture facility
  • The facility uses geothermal water to maintain optimal temperatures for prawn growth, resulting in higher yields and reduced energy costs
• Xianyang, China: Geothermal greenhouse complex covering over 1 million square meters
  • The greenhouses use geothermal heat to extend the growing season and improve crop quality, contributing to local food security and economic development
• Larderello, Italy: Geothermal heat used for industrial processes, including chemical extraction and food dehydration
  • The geothermal resource has been utilized for industrial purposes since the early 1900s, demonstrating the long-term viability of geothermal direct use applications

Challenges and Future Developments

• High upfront capital costs associated with drilling and infrastructure development
  • Innovative financing mechanisms and government incentives can help overcome this barrier
• Limited awareness and understanding of geothermal direct use applications among policymakers and the public
  • Education and outreach efforts are needed to promote the benefits and potential of geothermal energy
• Lack of standardized regulations and permitting processes across jurisdictions
  • Streamlining and harmonizing regulatory frameworks can facilitate the development of geothermal projects
• Need for improved exploration and resource characterization techniques
  • Advanced geophysical and geochemical methods can help identify and assess geothermal resources more accurately
• Development of advanced heat exchanger materials and designs
  • Materials with higher corrosion and scaling resistance can improve the efficiency and longevity of heat exchange systems
• Integration of geothermal direct use applications with other renewable energy technologies
  • Hybrid systems that combine geothermal with solar thermal, biomass, or waste heat can provide more flexible and reliable heating solutions
• Expansion of geothermal district heating networks in urban areas
  • Retrofitting existing buildings and infrastructure to accommodate geothermal heating can significantly reduce urban carbon footprints
• Utilization of low-temperature and co-produced geothermal resources
  • Tapping into these underutilized resources can expand the potential for geothermal direct use applications in new regions and sectors