Geothermal and hydroelectricity - lecture 7 Flashcards
Sustainable energy sources
- Not substantially depleted by continued use
- No significant pollutant emissions or other environmental problems
- Does not perpetuate substantial health hazards or social injustices
- most renewable sources come from the sun (solar energy to drive wind, waves etc)
- Global use is somewhere around 600 Ej/ yr
The system- processes
numerous processes that are impacted by the sun
- photosynthesis - 1260 Ej/yr
- ocean tides 93.5 EJ/yr
- convection in rocks 1008 EJ/yr
- wind, waves convection currents (<1%) 11.7 thousand Ej/yr
- hydro electric power
- hydrological cycle (evapo/ precipitation 23%)- 1.26 million Ej/yr
- convenction in volcanoes and hot springs 9.36 Ej/yr
- direct conversion to heat in air, earth and oceans (47%) 2.55 million Ej/yr
- solar radiation 5.4 millo Ej/ yr but short wave radiation, direct reflection to space (30%) 1.64 million Ej/yr
- all components are much bigger than our consumption
- ocean tides & hydro electrics and geothermals can produce less
- all form the sun except convention in volcanoes & hot springs
Terminology on size of resource
Available resource
- Total annual energy delivered by resource
Technical potential
- Maximum annual energy extractable from accessible resource using current mature technology ( smaller than available resource)
Practicable potential
-Technical potential reduced to account for distribution constraints, public acceptability ( distribution constraints ‘socially acceptable’ to access.
Economic potential
-Amount of the practicable potential that is economically viable - can it compete with others on the market?
- not all ‘available resource ‘ as shown by processes impacted by sun are actually accessible
Key questions to think about
- how much renewable resources are available?
- are they available where we want them?
- are they available when we want them?
Top 15 countries using geothermal energy (IEA, 2010)
- Geothermal is a small system in terms of global electricity production, is only 0.3% of global electricity - twice as much is used for direct heating as is used for electricity.
- Geothermal ‘direct use’ is to heat buildings, for agriculture etc
- country with the highest geothermal production in 2013 was US with 16,603 GWh/yr
- highest geothermal direct use is China with 20,932 GWh/yr
Tectonic plates
- only one of renewables not fuelled by the sun, bases for geothermal is the heat stored at the center of the earth from when it was formed ( center of earth 7000 oc and is gradually cooling).
- part of geothermal is the amount of isotopes present in the earths crust, so uranium 238, potassium 40 all decaying over lifetime and as they decay they give off heat energy which is emitted out .
- center is moved by convection currents, so rocks are fluid enough to pass heat around by movement
- outer shell- solid crust it becomes conduction which is much less efficient at moving around so limitation on heat can get out of earth.
- tectonic plates moving, locally can get much more active areas heat fluxes and that becomes economically viable to tap into.
- geothermal located on all the plates where we have geothermal activity eg. on south american plate, caribbean plate, Eurasian plate etc .
A hydrothermal geothermal resource
- to get resources, need resoviour rock (aquifer), a porous rock containing water.
- a heat source ( magmatic intrusion) which is gradually cooling down
- cap rock (across top, have an impermeable rock preventing material from escaping).
- need to tap into system& take energy out so need a recharge zone, when aquifer is exposed so when its under cap rock, its called a ‘confined aquifer’.
- when it reaches the surface it becomes exposed, so u get water come down and heated by magmatic intrusion.
- greater pressure cap rock imposes, higher temp it has to be for steam production
- look at diagram
Types of geothermal resources:
- Hydrothermal – hot water or steam in confined aquifers, under pressure (process discussed)
- Geopressurized – hot, high-pressure brines (saline water) with dissolved methane - salty solution- systems hot water sitting in minerals will dissolve those minerals more actively than standard ground water, so get high salt solution, those brines are associated with dissolved methane so can either tap off heat and use it as a geo-thermal resource or tap off methane and use it as a natural gas resource.
- Hot dry rock - where no water present, much more extensive, globally rocks are warmed, things like granite have much higher levels of radio decay occuring.
- Magma- can tap down few km down/ typically can bore down up to 5km) below earths surface maga cab be 1000s0c.
3 Uses of Geothermal Energy
Direct use: Geothermal heat found near the surface of the Earth can be used directly for heating buildings like the programmes in Paris and Southampton - 55 district heating schemes, 35-40 left in operation now.
- greenhouses in iceland are geated by direct heat from geothermal energy - efficient use of low grade heating.
- bring water up and use it for heating buildings (low-level heat).
- water that comes up has high salt solution so don’t tend to stick it directly into buildings to heat, you have a heat exchange that takes heat away, cleaned up and then piped around buildings.
Idealized geothermal power plant
Need: HEAT (TEMPERATURE) and fluid (Permeability).
- have plants in ICELAND, JAPAN, MEXICO, ITALY, US + 12 other countries
- only a few resovoirs produce dry steam, most produce hot water.
- to get geothermal energy, we need much higher temps 180oc +
- geothermal ranges from 55oc to 300oc
- once we get above 180oc can use it to generate electricity
- geothermal gives off water vapour/ water/ dry steam resovoirs
**from resovoir, natural steam rushes up the well, piped into a turbine, force of steam spins the blades, blades turn generator producing electricity. The steam is condensed and water is put back in resovoir to be heated again (like drax).
Single flash geothermal power
As steam comes up and goes into the turbine, behind is a condensor that collapses the steam down to water, like at drax
- cooling tower so lose some heat but also get to put water back into the system.
- geothermal isn’t renewable!!!!!!!!!!!!!
- tapping heat out, can’t do that forever!- slowly being depleted over time and not many have steam has to be low pressure to get steam, most systems are pressurised so super heated use flash technology to bring water up under pressure as it reaches the surface goes into a expander area that allows the pressure to drop, reduces steam and drives the turbine.
- look at diagram
Temperatures encountered at a depth of 5 km in Europe
geothermal at plate boundaries but more generally have a resource of hot rocks.
- Cornwall has broadly hard rocks, bedrock& granite so temps elivated due to radioactivity thats similar across large areas of Europe but not all will have aquifer material over them.
data from (2006, Trans-Mediterranean Interconnection for Concentrating Solar Power, Final Report, GAC)
Enhanced Geothermal Systems (EGS)
Enhanced permeability by causing existing fractures to slip and propagate or creating new tensile cracks by raising fluid pressure.
- hot dry rocks when we have no water & are interested in trying to extract the energy/ heat from rocks at a broader basis- to do this there is an invetsigation in EGS which is essentially fracking in rocks.
- cold water pumped down, passed through fractures and then 2nd well will pump water back up heated.
Hot Dry Rocks- The Concept
- use natural fracture system in basement rocks
- enlarge their transitivity and connectivity through water injections
- install a multi- well system
- under submersible pumps to adjust the water pressure in the fracture system
- through pumping and lifting, force the water to migrate through the enhanced fracture system and capture the heat
- similar to hydraulic fracturing for natural gas/shale gas
- same problems of seismic & micro seisminisy potential for ground water to be contaminated also.
- BUT because hot dry rocks are distributed across the globe, there is lots of interest
- not truly renewable but large reserves of it, can last 10-50 years before being depleted
Ground source heat pumps(Randolph & Masters, 2008)
- look at diagram
- trying to extract heat out of ground at localised level
- essentially fridge, acts like an ‘air heat pump’ fridge (inside) and to pump heat back out, so it is taking heat away from inside+ pumping into environment.
- can turn fridge around and use it to push heat back into centre so air heat pump wll essentially use a little electricity to heat across a boundary and either do cooling or heating
