L4 - Geothermal energy Flashcards
Why is geothermal energy different to other forms of renewable
- The only form of renewable energy that is independent of the sun.
- It has its ultimate source within the earth
Prerequisites for exploitable geothermal resources
- Aquifer: contains water which can be accessed by drilling
- Cap rock: retains the geothermal fluid, stops water dissipating to other areas
- Heat source
Aquifer
- Natural aquifers are porous rocks that can store water and through which the water can flow
- Porosity is cavities in the rock
- Permeability is the ability to transmit water
Cap rock
- Needed to restrict the liquid from the aquifer from escaping.
- Seals the rock of the aquifer from outside influences
Heat source
- In volcanic areas: heat is abundant
In low enthalpy areas:
- located in deep sedimentary basins where aquifers carry water to depths where it becomes warm enough to exploit
- located in wet rocks
- located in hot dry rocks where natural heat production is high but artificial aquifer must be made
Type of rocks (systems)
- Volcanic
- Sedimentary
- Hot wet rocks
- Hot dry rocks: have heat source but no aquifers.
Hot dry rock technologies and their problems
- All rock systems rely on the presence of water circulating throught the rock to extract heat. However in an aquifer more than 90% of heat can be contained in the rock rather than in the water
Problems:
- Rocks are poor heat conductors
- Rocks are usually poorly permeable
- Exploitable geothermal reservoirs are rare
Types of geothermal technologies
- Electricity generation
- Direct use
- Heat pumps
Electricity generation: Dry steam power plant
This system is used for vapour dominated (not contaminated with liquid) resources
- Reservoir produces superheated steam 180-225 C at 4-8 MPa. Steam reaches the surface at several hundred km/h and passes through turbine.
Efficiency not very high and at typical temps rarely exceed 20%
Electricity generation: Single flash steam power plant
- More common
- The water reaching the surface may get flashed (vaporated) or still remain liquid at high pressure
- Best to avoid flashing followed by rapid scale build-up. Thus the well is kept under pressure to maintain liquid phase.
- Condensing turbines operate under 155-165 C, 0.5-0.6 MPa.
Electricity generation: Binary cycle power plant
These use secondary working fluid with lower boiling point than water (eg pentane, butane) which is vaporised and drives the turbine
- Known as organic ranking cycle (ORC) plant
Adv:
- lower temp resources can be developed where single flash systems prove inefficient
- chemically impure fluids can be exploited
- for temps below 170C, higher generating efficiencies are possible than in low temp steam flash plants
Disadv:
- Relatively low efficiencies
Electricity generation: Double flash power plant
- Good when geothermal fluids contain low levels of impurities. Unflashed liquid remaining after initial flashing flows to a low-pressure tank where another pressure drop provides additional steam
Disadv:
- Large volumes of fluid are needed (x5 more than dry steam plant)
Direct use technology
- Low enthalpy geothermal heat has been used for non-electrical purposes
- Geothermal fluid is under much lower pressure and temp conditions
But:
- Water too saline and corrosive so corrosion resistant heat exchangers are needed and the useful heat load (eg greenhouses) need to be shifted to secondary circuit
Uses of direct use tech
- Balneology
- Agriculture
- Acquaculture
- Industrial uses (product drying, warming etc)
- Residential and district heating
Heat pump technologies
- Heat pumps rely on heat transfer by conduction of hea tin the ground rather than extraction of ground water, unlike other geothermal techniques
