Download Ocean thermal energy conversion (OTEC) and Geothermal energy

Ocean thermal energy conversion (OTEC)
and
Geothermal energy
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Principle
!   In both cases we use the fact that there is a natural temperature
difference
!   OTEC: difference of temperature between surface (hot) and deep sea (cold)
!   Geothermal: difference of temperature between surface (cold) and deep in
the Earth’s crust (hot)
!   Both will use Carnot’s engine principle
During an energy process heat  work, an ideal Carnot engine (reversible process)
will have an efficiency η equal to
η=
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ΔT Th − Tc
=
Th
Th
This is the maximum (ideal) efficiency.
In reality it will be lower
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OTEC
Sun radiation warming up water of ocean
surface is creating a temperature difference
This difference ΔT is fairly low
!   About 10K
!   25K at best in tropical seas (surf – 0.5km/1km)
Needs a low boiling point fluid:
!   To be vaporised at T=15C
!   Ammonia close circuit
!   Expansion will then be transformed in
mechanical and electrical energy
η=
10
25
= 3.4% generally η =
= 8.6% at best
288
288
In a perfect heat exchanger, the power from a volume flow Vf is
P0 = ρ CV f ΔT
C: water heat capacity
Taking into account Carnot’s principle
(everything perfect)
" CV f ( #T )
P=
Th
2
Ex: assuming ΔT=20C, in order to get P=1MW, we would need Vf=650 m3 of water per hour
 Fairly substantial flow  require large and expensive pumps
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Geothermal power
1000C
4000C
The inner part of the Earth is at about 4000C
Due to conduction and convection (Mantle is liquid) there
is a temperature gradient across the Earth
This gradient varies according to regions:
!   Hyperthermal: > 80C/km – tectonic plate boundaries
!   Semithermal: 40-80C/km away from tec. plate bound.
!   Normal: <40C/km
Main ways for heat to be obtained:
!   Hydrothermal circulation: existing underground water is heated and transformed into
vapour (Geyser)
!   Hot igneous systems: heat from magma – Experimental in Hawaii (3MW electricity)
!   Dry rock: Poorly conductive dry rock heats up over millions of year – Injection of water
to extract the heat
Temperature difference between Th and Tc will be high enough to get a good efficiency η
Typical thermal power extracted:
!   Hydrothermal circulation: 10 – 50MW/km2
!   Dry rock: 200 – 300MW/km2
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Conclusion
!   OTEC
!   Many ideas but still at the experimental level (100s of paper studies)
!   Advantage
•  Large amount of renewable clean energy available (oceans cover 70% of the Earth’s surface)
!   Disadvantages
•  Technically not mature
–  Needs very high efficiency (heat exchangers, pumps….)
•  High cost/power unit according to studies
–  Cost could be decreased if could be used for other purposes (building cooling, desalination…)
•  Unknown impact on ecosystem
!   Geothermal
!   It has been used at large scale from the 60s-70s
•  Between 1970 and now the world production has been multiplied by about 15, reaching an
electricity production of about 15 GW
!   Typical power plant is producing an electric power of 100-150 MW
!   Advantages
•  Large quantity of clean energy available
•  Competitive price
!   Disadvantages
•  Drilling in hot rocks / direct access to hot water  not fully renewable
•  Location dependant
•  Need to go deep to get high temperature (above 150C) for electricity production
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References
!   Most of the material of this lecture is coming from
!   Ref4: “Renewable energy resources”, J. Twidell and T. Weir,
second edition, 2006
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