Geothermal Energy - Civil Engineering, Imperial College

• Geothermal Energy is being presented for Year 1 MEng Civil Engineering at Imperial College London by David M. G. Taborda on 06th March 2025, Geotechnics Section

What is Geothermal Energy?

• Geothermal energy systems come in a wide variety of forms and are generally classified according to their operating temperatures
• High temperature systems inject water as vapour into turbines, generating electricity
• Water flows into an area closer to a heat source, increasing its temperature

Iceland and Azores

• Iceland and Azores (Portugal) are locations that have geothermal energy

Enhanced Geothermal Systems

• Enhanced geothermal systems can be implemented in locations that aren't Iceland
• Moderately high temperatures (i.e. above 80-100 degrees) still means vapor can be used to produce electricity (e.g. Cornwall, Los Alamos)

Low Temperature Geothermal

• Low temperature geothermal resources cannot be used to produce electricity but can be explored for providing heating
• Examples include Paris Geothermal District Heating Scheme (established 1970s, temperatures from 50 to 80 deg G at depths 1500-2000 m)
• Southampton District Heating Scheme (established 1980s, depths of 1800 m has brine at 76 deg C)

Ground Source Heat

• Lower temperature systems still rely on the existence of "anomalies”
• Most of the heat extracted for shallow depths is recharged through solar and, if managed, possibly renewable & sustainable
• The temperature is located "at depth" and "at surface"
• Soil acts as heat sink and heat source

Shallow Ground-Source Heat

• Shallow ground-source heat resources are located in Oxford, UK and Sweden
• Summer months are used for heating using Heat flux
• A heat exchanger is used for Heat storage at ~ 11°C

Exchanging Heat

• Heat is exchanged with the buildings using Heat Pumps
• -3°C Compressor turns into to 62°C, 10°C Evaporator turns into 45°C then 5°C Ground Heat Exchanger turns into 35°C using a Condenser

Exchanging Heat with the Ground

• Heat can be exchanged with the ground using closed loops, open loops, and thermo-active structures (i.e. geotechnical structures equipped with heat exchangers)
• Closed loops have Vertical closed loops (borehole heat exchangers) and Horizontal closed loops

Open Loops

• Convection is extremely important in open loop systems because they rely on direct water extraction from the ground
• A well screen is used and there is restriction of water flow between aquifers

Thermo-Active Structures

• Heat is exchanged with the ground using thermo-active structures (i.e. geotechnical structures equipped with heat exchangers)

Applications of Geothermal Energy

• Cooling down the Tube, London Underground System
• Provide heating, Cooling

Applications - Cooling Down the Tube

• Green Park Station uses abstraction borehole, water filtration and submerged pump
• This is done using a Heat exchanger to provide Warmed water and Cool water

Applications - Providing Heating

• Thermo-active tunnels are used for access and the access points at 500m centres for each tunnel

Applications - Provide heating / cooling

• Thermo-active tunnels, approach to Brenner Base tunnel

Applications - Transfer Heat

• Ice Rinks + Heating with Heat extracted from ice
• Residential/Commercial building with Hot water & space heating
• Heat Pump connected with a Ground loop

Applications - De-icing

• Bridge & Road de-icing will allow Fluid Flows from Energy Piles and Embankment to the Bridge Deck

Applications - The Geothermal City

• Geothermal energy can be used for:
• Heating and cooling for residential buildings
• Heating and cooling for commercial buildings
• District heating
• Heat exchangers in infrastructure – transport & sewers
• Thermo-active foundations
• As storage for intermittent sources of energy

Understanding Heat Transfer

• An equation is used to transfer heat and understand the phenomenon

Thermal Conductivity

• Fourier's law and Unit: W/(mK) are used
• "heat flux through an element of 1m length when subjected to a temperature gradient of 1K"
• Warm plate is kept at 20°C and Cold plate is kept at 10°C

Heat Capacity

• Specific heat capacity, c's Unit: J/kg/K
• "energy required by 1 kg of material to increase its temperature by 1 K"
• Volumetric heat capacity, (pc)'s Unit: J/m³/K and "energy required by 1 m³ of material to increase its temperature by 1 K" are also used

Zone of Interest for Saturated Materials

• Since horizontal closed loops are superficial, they may not be necessarily located within saturated soils
• The different soil types include: Clay, Silt, Sand and Gravel

Summary of Properties Needed

• Specific heat capacity – measures the amount of energy needed to increase the temperature of the material
• Thermal conductivity – defines the rate at which heat is transfer by conduction
• Permeability – controls the amount of heat transferred through convection
• These properties are fairly well established but of difficult determination

Laboratory Measurements

• Direct laboratory measurements of thermal conductivity & heat capacity are used

Field Techniques

• Field techniques includes needle probes and Thermal response test (TRT) – normally coupled with a thermal recovery test
• Equations and values are associated for measure

Design Procedures

• Numerical methods (e.g. FD, FE) and Simplified expressions (Carslaw & Jaeger, 1959) are used:
  • T(z,t) = Tave – Tamp exp
  • Tave - average soil temperature (deg C) and Tamp thermal amplitude (deg C)
  • • depth (m) and t - time instant (day)
  • • phase constant (day) and α - thermal diffusivity (m²/day)

Closed Loop Systems

• Main design criterion is to prevent depletion of heat and Design consists of determining a unit extraction rate q (W/m)
• Available methods for borehole heat exchangers: ASHRAE, VDI 4640 (Germany), SIA 284-6/2010 (Switzerland) and MCS MIS 2500 (UK)
• UK Guidelines – MCS MIS 2500 has equations
• UK Guidelines – MCS MIS 2500 (UK) has examples

Open Loop Systems

• Main design criterion is to prevent thermal breakthrough (i.e. abstraction of hot water during cooling or cold water during heating)
• Simplified methods (e.g. Banks, 2009)

Open Loop Equations

• Distance is above which risk of thermal breakthrough is minimal
• Used If there is regional hydraulic gradient for breakthrough

Open Loop Systems Example

• Aquifer is often used

Thermo-Active Structures

• Main design criterion is the safety of the structure:
• Stresses in the structural elements
• Strength of adjacent soil
• Foundation displacements and ground movements should be taken into account to establish structural safety
• Temperature field should be estimated for peak injection and peak extraction in order to guarantee good thermal performance
• Simplified procedures (e.g. Bourne-Webb et al., 2013) or coupled thermo-hydro-mechanical numerical methods (e.g. FE, FD)

Thermo-Hydro-Mechanical Analysis

• Needed to:
• check Are thermal properties enough?
• check Are soil properties – e.g. strength and stiffness – affected by temperature?
• Quantify this effect?
• Advanced temperature-controlled lab testing is needed!

Pile Foundations

• Total shaft capacity: Qs = Alateral · qs where qs = a • Su,average
• Total base capacity: Qb = Abase · (Nc • Su,base + y • L) where Nc = 9.0

Additional items

• Mechanical behaviour such as Shear Stiffness, Shear stress's affects,
• Hydraulic behaviour such as Depth, Permeability, Initial pore pressure will effect the piles

Finite Element method for coupled THM analyses

• Force Equilibrium is equal to stiffness, HM, TM times Disp. to get Reaction
• Continuity Condition is equal to HM, Darcy's law, TH times Pore pressure to get Water flow
• Energy Conservation is equal to TM, TH, Heat content & diffusion times Temp. to get Heat flux

Summary

• The term "Geothermal Energy" incorporates several technologies that rely on very different operation principles.
• Shallow geothermal, i.e. ground source energy, can be integrated in most Civil Engineering developments
• If adequately managed it can be considered renewable
• Knowledge of heat transfer is necessary – development and implementation of new laboratory and field techniques
• Design for closed loops is based on codes or simplified rules but for complex situations numerical simulations may be needed
• Design of open-loops may be carried out using techniques adapted from hydrogeology; coupled hydro-mechanical analysis can be employed
• Design of thermo-active structures is currently done based on very basic procedures – additional research clearly needed, focussing on temperature effects on soils and on methods of analysis