Geothermal Flashcards
Earth’s heating?
- Much of the internal heat of the Earth is thought to come from the decay of radioactive thorium.
- Thorium-232 is the main natural radioactive isotope. It decays to Lead- 208 via alpha decay* and has a half-life of 14 billion years.
- The Earth still has about 85% of the thorium it had when it formed.
- Convection in mantle
- Emplacement of granites
Iceland and UK heat sources?
Iceland: 30 high temperature systems: T>250 degrees C
UK:
• Only area higher than 120 millawatts per m2 is South West – Cornwall – granites – high abundance of thorium
• Average heat flow in Whin Sill in north
• Average thermal gradient 26 degrees per km
Types of geothermal?
• Shallow • Hydrothermal Systems o Confined to areas of high heat flow on plate boundaries • Engineered geothermal systems o Redruth, Cornwall
Shallow?
Ground source heat pumps:
• An energy efficient heat/cooling device that absorbs heat from or rejects heat to the ground
• Uses the earth as a heat source in winter and sink in summer
• Based on convection cycle
Hydrothermal?
- Water moves down through the ground
- Reaches hot rock
- Heated
- Moves in a convection cycle
Engineered Geothermal Systems?
- Fracture stimulation of hot, dry rock
- Increasing permeability and therefore flow in rock
- Injects water into the rock
- Hot water rises back up production well
- Generates electricity
Europe’s capabilities?
- At 5km if it utilised 5% of the rock volume with greater temperatures than 160 degrees
- Would support 130 GWe of power generation
- Generate 900 TWh
Currently Geothermal Power plants using hydrothermal systems are confined to areas of high heat flow on plate boundaries
Geothermal Power Generation (Dry Steam Power)?
o Only 5 Dry Steam Fields worldwide o Hydrothermal fluids used directly to turn turbine o No need for separation o 20-100 MWe o Temperature above 210 degrees C
Geothermal Power Generation (Flash Power Plant)?
o Steam used for electricity production o Brine and steam separated o High pressure steam contains more energy than brine o 20-100 MWe o Temperature above 210 degrees C o Iceland – Hellisheioi Power Plant • Combined heat and power • 300 MWe and 100 MWt • 50 boreholes – 2000m deep • Re-inject non-condensable gases into ground (H2S)
Binary Power Plant?
o Use in lower temperature fields
o Hydrothermal fluids separate from ‘binary’ fluid
o Binary fluid has low boiling point (butane, pentane) – vaporised in heat exchanger that drives turbine
Weardale?
• NW England
o Modest heat flow
o Mining history from coal mining
o Warm water known from the mines
• Granite proved 1961
• 1962 Woodlands confirmed the high heat flow
• 2004 Eastgate 1 confirmed high fluid flow
• 2010 Eastgate 2 tested the geological model
• Trial pits – proved local till deposits contain huge boulders (erratics) in silty matrix
o Inclined boreholes proved Slitt Vein 8m wide
• Eastgate geothermal gradient 38 degrees per km (UK average of 21)
UK warm water deep basin potential?
- Southampton District Energy Scheme
- Single borehole to 2km into Triassic Sherwood Sandstone
- Temp 75 degrees C
- Power 1.7MW
Triassic Sherwood Sandstone – Outcrop
- High net to gross reservoir
- High porosity
- High permeability
- High kv/kh
Formation resource evaluation:
• Geothermal resource calculation requires:
o A geological model of the aquifer (reservoir)
o Maps of the temperature field
o Estimates of the hydraulic properties of the aquifer, especially porosity
• Permeable, porous rocks are mainly found at depth in the Mesozoic basins
• Five main basins in the UK:
o Northern Ireland
o East Yorkshire and Lincolnshire
o Cheshire
o Worcester
o Wessex
Hydraulic Properties of the Aquifer:
• Porosity
• Permeability
o From interconnections between voids in the rock
o Primary permeability from interconnected pores
o Secondary permeability from fracking
o Pumping test reliable way to measure
• Transmissivity = permeability x thickness
o Transmissivity limits can be used to define the productive reservoir
o Much of the reservoir transmissivity is often contained in a few highly permeable zones
