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Ocean Thermal Energy Conversion
Oceans Vast natural reservoir: Receiving and Storing Solar Energy
The oceans take in solar energy in proportion to their surface area, where is nearly three times that of land.
Ocean Land
Ocean Thermal Energy Conversion
Water near the surface of tropical and subtropical seas is maintained by this solar radiation a higher temperatures than the water at greater depth or at higher latitudes.
Ocean Current: The warm surface water From the equatorial regions either to the north or to the south.
Gulf Stream or Japanese Current
Tapping the energy in the ocean currents
Gigantic underwater turbines/like windmill
Relatively steady velocity/intermittent of wind,solar radiationsNo need for energy storageThe energy available is enormous
Serious proposals have put forth to construct ocean current turbines with electricity produced then cabled to shore.
Economically? Competing with conventional power plant.
Have not been advanced beyond the discussion stage.
Through the use of heat engines
Warm tropical surface 22 oC
Cold water at depths of about 1000 m, 2 oC
Heat source, Thot
Heat sink, Tcold
Electricity
T difference is steady, different time, different season.So no need for energy storage system.
Figure 5.8 Globe distribution of the OTEC resource. The temperature difference (degrees Celsius ) is shown between the surface and 1000 meter depth. (Figure supplied by the National Renewable Energy Laboratory.)
Florida, Puerto Rico, Hawaii and other islands. Electric E is now obtained primarily from imported fossil fuels.
Two types of heat engines have been considered, and demonstrated, for ocean thermal energy conversion(OTEC)
Closed-cycle system.
A working fluid such as ammonia to evaporate into gas by energy from warmer waterSimilar to the ordinary steam-powered electric generating plant,
in which water as working fluid.
Open cycle system where the working fluid is seawater
Warm water
VacuumVapor turbine
Coldwater
Drive Condensedby
Figure 5.9 An OTEC heat engine using ammonia as a working fluid. The turbine is driven by the ammonia vapor and is connected to a generator to produce electricity. The warm water is drawn from the ocean surface; the cold water from a depth of 1000 meters. (Source: Figure supplied by the National Renewable Energy Laboratory.)
Closed-cycle system.
eg. Calculate the thermodynamic efficiency, η, for an ideal heat engine operating between surface waters and water at 1000 m depth if the surface water temperature is 25 oC and the deeper is 5 oC.
%7.6067.0)298/2781()/1(29825
2785
==−=−===
==
hc
h
c
TTKCT
KCT
η
Solution:
Of course, the efficiency of an actual heat engine will be less than that.
Efficiency
The Carnot efficiency is only 7%, and the net efficiency to be expected in practice, about 2.5%
Very large volumes of both warm and cold water must be circulated past the heat exchangers to produce useful amounts of power.
Estimated: 25×106 liters/sec of both warm and cold water would be needed for 100MW of the electric output.For 40MW plant, 10 meters diameter pipe needed.
However, more power was required to operate it than it produced.
Open cycle system to generate 22kW of electric power, 1930 Cuba
In the 1970s the US DOE financed the design of large floating OTEC power plants that were fixed in position by cables to the ocean floor.
Small test plant of 10-15kW was built off the Kona coast of the island of Hawaii.
No useful electric output was intended.
Goal : 100MWe
Figure 5.10 A design concept for a 100 MWe OTEC power plant. One of the 25 MWepower modules is shown in the cutaway portion. The platform has a diameter of 100 meters. Design by TRW Systems Group, Inc.
Other possibilities:
Generation of Hydrogen
Conversion of ocean water to fresh water.
Biomass as an Energy Feedstock
Solar energy -> Biomass energy
Photosynthesis
CO2, H2O, Radiant energy -> Carbohydrate, O2
Electromagnetic radiation with a spectrum that ranges from 0.3 to 3 microns in wavelength.From the near-ultraviolet through the visible to the infrared
What are the details of the process by which plants use the energy of sunlight to form vegetable matter?
Quantum description of electromagnetic radiation.
First understood about a hundred years ago: Quanta, separate little bunches of energy. E=hν
Ultraviolet light, sufficient to break a chemical bond
Middle portion of the solar spectrum where intensity is at its peak where most of the photosynthesis takes place. Visible light, has sufficient energy to raise an atom to excited states.It is possible for bonding to take place between neighboring atoms, thus forming new compounds.
CO2+ 2H2O+light -> CH2O + H2O +O2 +QQ = -112kcal/mole, 1 mole: 6.02×1023
6CO2+ 6H2O+light -> C6H12O6 + 6O2 +Q, glucoseQ = -674kcal/mole
Carbohydrates, represented by formula Cx(H2O)yGlucose C6H12O6, sucrose C12H22O11
Photosynthesis, multi-step reactionAt least 2 pigments, Chlorophyll(叶绿体), absorbs light in the red part 0.7 µm and the
blue 0.4/0.5 µm. Looks green.The light absorbs lead to formation of oxidants and reductants.Energy-rich adenosine triphosphate ATP
Carotenoids, gather light energy and transfer it to chlorophyll.
Phycobilins(藻胆素), another pigments, in marine photosynthetic organisms
Figure 5.11 The energy on which life depends enters the biosphere in the form of light. The light energy is converted to stored chemical energy by photosynthesis. These are four-year-old eucalyptus trees on Bioenergy Development Corporation land on the island of Hawaii. (Source: Oak Ridge National Lab/Dept. of Energy/National Renewable Energy Laboratory)
The exact photosynthetic process is complex; it differs for different plants and involves a variety of enzymes and chemical steps.
Pigments allow it to adapt to the light it happens to receive.eg. Below the surface of the sea, the light is mostly green and red algae thrive because they can absorb the green light, allowing the photosynthesis to take place. Green algae, which would reflect rather than absorb green light, cannot grow.
Respiration, essentially the opposite of the photosynthesis, provides energy for the plant. “Burning” carbohydrate molecular and release carbon dioxide, water and energy. Basic for both plant and animal.
At what rate can vegetable matter be produced by photosynthesis on the surface of the earth?
How many people can be fed ?Depends on the availability of sunlight, appropriate land, temperature, climate, and nutrients in the soil, as well as plant diseases and insects.
EstimationThe average intensity per unit horizontal area and per unit time at the top of the atmosphere is 0.5 cal/min·cm2. Given 47% of the solar energy incident on the atmosphere reaches the ground, the energy available for food production averages:
daycmcal338
day1hr24
hr1min60)47.0()cmmin/cal5.0( 2
2
⋅=×××⋅
On a typical summer day, a forest or field will have a higher value, about 500 to 700 cal/cm2.
Only about 25%, right wavelength to produce photosynthesis, 60% or 70% will be absorbed by plant. The amount of energy per carbohydrate unit synthesized is about 5eV. The red end of visible spectrum, 1.7eV per photon. 8 photons needed to have a store of 5eV. Rough efficiency 35%.
Overall efficiency 0.25×0.70×0.35=6%
More practical by making direct measurements:Such measurements indicate that if there are 500cal/cm2 ·day incident, the net potential plant production is about 71g/m2 ·day. The gross production is 106 grams, but the respiration loss of 35 grams reduces this to a net yield of 71 grams.
Until about 1880, the main source of energy for heating, transportation, and industrial processes in US was wood. Developed and developing country.
Table 5.3 Net Biomass Production for a Summer Day
To meet the energy demands of a modern nation, wood and other vegetable matter of course, be burned directly to obtain heat energy.Vegetable matter can be converted by well-established processes into liquid or gaseous fuels as substitutes for gasoline, oil, or natural gas.79×1015Btu 98×1015Btu, total energy used in US in 2003
Biomass: Municipal Solid Waste
The majority of this solid waste has its origin in photosynthesis:Paper 36%, food waste 11%, plant waste 12%, wood 6%.Rest: Glass 6%, plastic 11% and metal 8%.
Population growth,Per person, 2.7 pounds in 1960
to 4.4 pounds in 2001.
Before 1960, 63% landfilled, 21% combusted without energy recovery.
Figure 5.12 The Otter Tail Power Plant in South Dakota. This plant burns refuse to generate electricity. Plants such as this can extract up to 700 kWh of electrical energy per ton of processed solid waste. (Source: Philip Shepherd/Dept. of Energy/National Renewable Energy Laboratory)
Figure 5.13 Diagram of the production of gasohol from grain such as corn. In addition to the ethanol product, the stalks and cobs as well as by-products of the fermentation and distillation have economic value. Fossil fuel energy (FFE) is required for almost every stage of the process, including steps not indicated such as transportation.
Geothermal Energy
Two nonsolar source of renewable energy available.
Based on the naturally occurring heat from the interior of the earth.
Tidal EnergyBased on the potential and kinetic energies of the earth-moon-sun system which is bound together by gravitational forces.
Normal Geothermal Gradient Even in dry rock the normal geothermal gradient(30 ̊C/km) produces useful temperatures any place on the globe. Drill holes 20,000 feet deep are achievable, corresponding to a temperature of about 190 ̊C above the surface temperature. This temperatures is adequate for electricity generation.
Layers of our land Cycle of the WATER
Formation of Geothermal Energy
Process of Geothermal Energy
Use dry steam from underground directly to generate units to produce electricity.
eg: A dry steam generation operation is at the Geysers in northern California.
Dry Steam
• Vapour dominated resources where steam production is not contaminated
• Steam is 1050 ̊F - 1220 ̊F, then pass through turbine and expands
• Blades & shaft rotate to generate power
•Most common & commercially attractive
•Cooling towers generate waste heat
•Used in areas where geysers do not exist
• Need water to inject down into rock
• Well is deep, so it takes more time to inject water into well
Process of Geothermal Energy
Pump water up from underground to create steam at surface to produce electricity.
eg: “CalEnergy Navy I” flash geothermal power plant located at the Coso geothermal field.
Flash Steam
• Use very hot steam (>300 ̊F) and hot water resources
• Steam either comes directly from the resource, or the depressurized hot water
• Steam turns turbines to drive generators for producing electricity
• Steam is the only significant emission from these plants. Small emission of carbon dioxide, nitric oxide, and sulfur are present, but ~50 times less than traditional fossil-fuel power plants
Process of Geothermal Energy
Use low-temp underground water to heat up other “working fluid” to turn on turbine
eg: Mammoth Pacific binary geothermal power plant at the Casa Diablo geothermal field.
Binary Steam
• Hot water resources (100 ̊F – 300 ̊F) passed through a heat exchanger in conjunction with a secondary fluid with a lower BP (eg. Isobutane/ isopentane)
• Secondary fluid vaporizes, turns turbines to drive the generators, its fluid recycled through the heat exchanger
• Condensed geothermal fluid returns to the reservoir. Since these plants are self-contained, hence nothing is emitted
•Lower-temperature reservoirs are more common, so binary plant is more prevalent
Figure 5.15 The Geysers, a geothermal steam field in northern California, Showing venting geothermal wells and gathering pipes. In this area, which covers 30 square miles, over 200 wells have been drilled; the deepest of these is more than 10,000 feet deep.
It had an installed generating capacity of 1967 MWe in 1989.
In 2005 the reported total geothermal capacity in the US was 2800 MWe for electric utilities and 5400 MWe for nonutility energy producers.
Thank you!
