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Ocean thermal energy conversion (OTEC) and Geothermal energy Renewable sources III 1 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= Renewable sources III ΔT Th − Tc = Th Th This is the maximum (ideal) efficiency. In reality it will be lower 2 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 Renewable sources III 3 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 Renewable sources III 4 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 Renewable sources III 5 References ! Most of the material of this lecture is coming from ! Ref4: “Renewable energy resources”, J. Twidell and T. Weir, second edition, 2006 Renewable sources III 6