[L18] Geothermal energy

Geothermal energy forms

geothermal energy systems assume a wide variety of forms, generally classified according to the operating temperatures

High temperature geothermal energy functioning

• water flows into an area closer to a heat source (hot rocks or magma) → increase water temperature → creates steam or water reservoirs

• wells are drilled to bring steam to the surface

• water as vapour is injected into turbines → generates electricity

Why do we need to drill to access the vapour

Crystalline rock covered by low conductivity sediments

Closed-loop system Cornwall

Extraction → power production → lithium extraction → reinjection

Advantages / disadvantages for geothermal energy

24/7 electricity generation (deep energy)

renewable (for shallow energy if managed properly but needs electricity for heat pump)

low emissions

but

limited high-temp locations

complex design (thermal + hydraulic + mechanical)

risk of thermal depletion

Shallow ground heat sources

heating only
most heat extracted is recharged through solar → considered renewable & sustainable

heat can be stored in ground during summer (sink) and used in winter

uses heat pumps

Conventional geothermal system

Natural hot water / steam resevoirs, volcanic areas

EGS

Enhanced geothermal systems

Inject water at high pressure → creates fractures → circulate → extract

Closed loop system

fluid circulates in pipes and warms up

• vertical loops (borehole heat exchangers)

• horizontal loops (slinky system)

Heat pump

• heat absorbed from water → refrigerant in pump evaporates

• goes through compressor → increases pressure and temperature

• circulates through heat exchanger

• condenses back to a liquid and passes through expansion valve lowering its pressure and temperature

Open-loop

extract groundwater directly from aquifer

heat exchanged → water reinjected

depends on groundwater flow (convection)

Injection well (hot water in)
Abstraction well (cold water out)

Thermo-active structures

infrastructure with built in heat exchangers

piles, tunnels, retaining walls

provides heating + cooling + structural function

Geothermal energy applications

• District heating (e.g. Paris)

• Building heating/cooling

• Underground heat exchangers (e.g. metro systems)

• Ice rinks (heat recovery)

• Road de-icing

• Energy storage

• “Geothermal city” concept → thermo-active foundations

• Thermo-active foundations

Thermo active tunnel

pipes in tunnel lining

normal heat exchanger, cooler in summer and heater in winter

cheap → no added infrastructure

Ice rink

pipes under ice absorb heat to keep it frozen → heat pump → ground loop

Change in heat content

= diffusion (conduction) + advection (convection)

Fourier’s law

q = rate of heat transfer

in W/(mK)

lambda = thermal conductivity J/S

Conduction

heat transfer through solids

Convection

heat transfer via moving fluids

controlled by permeability

Specific heat capacity

J/kg/K

energy required by 1kg of material to increase t by 1K

Volumetric heat capacity

J/m3/K

energy required by 1m3 of material to increase its temperature by 1K

Thermal conductivity

rate at which heat is transfer by conduction

Permeability

controls the amount of heat transferred through convection

Field vs lab techniques to measure thermal conductivity & heat capacity

Field

• needle probes

• thermal reponse test TRT

Lab

• sensors

Main goal of design principles

Avoid thermal depletion

→ predict temp in ground (numerical or expressions)

Closed loop systems design procedures

• determine ground thermal conductivity

• determine extraction rate as a function of thermal conductivity and ground temperature

• calculate required borehole length

Open loop design

• reinjected water may return too quickly → reduces efficiency

Must consider:

• well spacing (larger = better = longer use)

1. Calculate distance for minimal risk of thermal breakthrough

• flow rate

• permeability

2. Time for breakthrough w or w/out regional hydraulic gradient

• volumetric heat capacity

Thermo-active structures design

Concerns:

• structural safety

• adjacent soil strength & stiffness

• thermal stresses

Heating → expansion → increased stresses

Cooling → contraction

Must estimate temperature field

Take into account foundation displacements and ground movements

Numerical analysis needed

Thermo active pile

expand when heated → soil restrains deformation → increase in axial force

opposite for cooling