MET445- RENEWABLE ENERGY ENGINEERING

MODULE IV

OCEAN ENERGY

Geothermal energy

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Module 4

• Ocean Energy – Devices for Wave Energy conversion
• Ocean Thermal Energy Conversion (OTEC): Principle of OTEC system
• Methods of OTEC power generation
• Open Cycle (Claude cycle), Closed Cycle (Anderson cycle) and Hybrid cycle (block diagram description of OTEC)
• Geothermal energy: Introduction, hot dry rock resources, magma resources, vapor and liquid dominated systems, binary cycle, advantages and disadvantages

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Ocean energy

• Tremendous amount of energy is available in oceans and seas due to waves and tides, which are mechanical forms of energy related to kinetic and potential energy of the ocean water.
• Oceans also have thermal energy due to solar heating of ocean water.

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Wave energy

• Like most other renewable energy sources, wave energy is ultimately caused by solar energy.
• Ocean and sea waves are caused by wind, and wind is caused by uneven solar heating of earth and water bodies and thus resulting temperature fluctuations.

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Wave energy conversion

• Waves with an amplitude of 2 m and period of 10 s are of considerable interest for power generation with wave energy fluxes averaging between 50 kW and 70 kW per metre width of oncoming wave.
• Wave energy can be better concentrated than the solar energy.
• Ocean wave energy is the primary energy.

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Wave energy conversion

• A demonstration plant of 150 kW capacity for conversion of wave energy into electrical energy has been built at Vizhinjam near Trivandrum.
• This site was selected considering its good wave power potential, easy access to deep water, away from cyclonic zone and nearness to the available infrastructural facilities.
• This plant was commissioned in October 1991.

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Principle of Wave energy Plant

• The wave energy plant utilizes an “oscillating water column” chamber and a self rectifying air turbine to produce power.
• The device works similar to the operation of a bellow.
• Ocean wave enters the chamber inside the caisson and cause the water mass to move up and down producing a bidirectional air flow through an opening at the top of the caisson.

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Principle of oscillating water column device giving bidirectional air flow to unidirectional rotating turbine

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• The special design of turbine makes it rotate unidirectionally even though actuating air flow is bidirectional.
• The turbine devices an induction generator connected to the grid.

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Wave energy conversion Devices

• Wave energy is a combination of kinetic and potential energies available in sea waves.
• The forward motion of sea water can easily be seen on sea beaches, lashing up to 100 meters.
• In deep sea this forward motion of the wave strikes the ships, depicting the presence of kinetic energy.

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• The potential energy is due to rise of sea water at the wave crest.
• The difference of head between the crest and the through of sea wave is the potential energy.

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Wave energy conversion Devices

• 1. Buoy type machine
• The buoy is a floating part of a system which rises and falls with rise and fall of sea waves.
• The buoy oscillates up and down with the wave, the energy can be exhibited on a pulley with a string and counterweight arrangement.
• The up and down motion of the counterweight can be converted into to and fro motion of a piston which can operate a machine or a generator.

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Oscillating buoy with a string, pulley and counterweight

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Wave energy conversion Devices

• 2. Dolphin type wave energy generator
• Designed in Japan.
• A supporting structure is built in the sea bed to provide a firm position for the equipment.
• The structure is erected on pile foundations.
• One generator is installed on rolling motion.
• The gear arrangement with stationary generator rotates the rotor to generate electric power.

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Dolphin type wave energy generator

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Wave energy conversion Devices

2. Dolphin type wave energy generator

• The buoy is at the other end of the connecting rod floats and has two motions, namely rolling motion and oscillatory motions represented by N and M respectively.
• The floating generator collects wave energy from the buoy through a gear arrangement and continuously generates power.

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Normally one Dolphine type wave energy generator is of 100 kW capacity.

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Wave energy conversion Devices

3. Oscillating Ducks

• Designed in Scotland.
• It is a float type wave energy conversion plant in which several duck shaped devices (each 25 m long) are installed in a linear width-wise array along a line which is perpendicular to the direction of the wave.
• The system consists of a long cylindrical spine of 15 m diameter on which cam shaped ducks are installed in an array to form an assembly.

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(a) Phases of duck motion, (b) oscillating ducks with a floating powerhouse

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Wave energy conversion Devices

3. Oscillating Ducks

• It responds to the incoming wave with a nodding action.
• When the forward moving wavefront strikes the head on the face of the ducks, wave energy is passed on and the ducks starts to oscillate.
• The face of the duck is designed for maximum wave energy absorption.

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Wave energy conversion Devices

3. Oscillating Ducks

• Power is generated by the relative motion of the ducks where the wave energy is converted into mechanical energy.
• The cylindrical spine transfers motion through linkages and gears to the generator rotor.
• The overall length of the cylindrical spine varies between 100 m and 500 m.

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Wave energy conversion Devices

3. Oscillating Ducks

• To achieve a highly efficient absorption it is necessary to mount a series of ducks on a non- movable spine.
• If the spine is sufficiently long (more than wavelength), the angular distribution of the waves incident on this structure will produce phase cancellations of translation force components along the spine and the spine will remain stationary.

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150 kW conversion system in Vizhinjan, TVM

• The wave power available at the site is nearly 13 kW/m.
• The wave energy conversion system consists of an oscillating water column, a chamber floating in the sea experiencing wave action through a side opening.
• The chamber size is 10 x 10 m with a height of 15 m.
• The turbine is 2 m diameter and coupled to an

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(a) OWC wave energy conversion system (wave rising) (b) OWC wave energy

conversion system (wave falling)

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• The wave patterns are irregular in amplitude, phase and direction, so the wave energy devices parameters match the irregular slow medium and high amplitude wave motion.

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Ocean Thermal Energy Conversion (OTEC)

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• The oceans and the seas which cover about 70% of the earth are constantly receiving solar radiation and act as the largest natural solar collector.
• An ocean as a collector has an enormous storage capacity.
• Energy from the ocean is available in several forms, such as ocean thermal energy, wave energy and tidal energy.

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• Ocean Thermal Energy Conversion (OTEC) is a new technology, needed to be harnessed especially in India where the coastline is about 6000 km.
• OTEC converts the thermal energy, available due to temperature difference between the warm surface water and the cold deep water, into electricity.
• According to MNRE, the overall potential of ocean energy in the country may be in excess of 50,000 MW.

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Working principle of OTEC system

• A temperature difference of about 20oC between the warm surface water of the sea and cold deep water in equatorial areas between latitude 30^o S and 30^o N.
• Solar heat energy is absorbed by ocean water.
• Lambert’s law of absorption- “Each water layer of identical thickness absorbs an equal fraction of light that passes through it.”
• The intensity of heat decreases with the increase in water depth.

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• The temperature at the surface changes slowly, then remains constant at a depth of about 200 m.
• Subsequently, the temperature decreases asymptotically and approaches a low value of about 4oC at a depth of 1000 m.
• The difference in temperature between the surface and the deeper parts of the ocean is utilised to generate electrical energy.

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• The basic process of OTEC is to bring the warm surface water and the cold water from a certain depth of the sea through pipes so as to act as heat source and heat sink for operating a heat engine.
• Three types of OTEC plants
• Closed
• Open
• Thermoelectric

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Closed Rankine cycle or Anderson closed cycle OTEC system

• Uses a low boiling point working fluid like ammonia or propane.
• Warm water from the surface which is at a temperature of about 26oC is brought in one pipe, and cold water at a temperature of around 5oC is brought in another pipe from a depth of about 1000 m.
• Two water pipes are used in conjunction with a working fluid to generate electric power.

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Closed Rankine cycle or Anderson closed cycle OTEC system

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• Closed Rankine cycle or Anderson closed cycle OTEC system
• The operational activities of the plant are:
• The warm sea water evaporates the liquid ammonia into vapour in a unit called an evaporator. This can be done because ammonia exists in the form of gas at the temperature corresponding to the surface sea water.
• The liquid ammonia which is not evaporated collects in a unit known as separator, which again recirculates through the evaporator.

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Closed Rankine cycle or Anderson closed cycle OTEC system

• The evaporated ammonia is the form of high pressure vapour is made to pass through a turbine where its pressure and temperature make the turbine to rotate, thus converting thermal energy to an electric generator produces electric power.
• The ammonia vapour coming out of the turbine, which is now at the lower pressure than when it entered the turbine is condensed back into liquid ammonia by cooling it with the colder sea water brought up from the deep part.

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• Closed Rankine cycle or Anderson closed cycle OTEC system

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• The liquefied ammonia collects in an ammonia sump. After a few hours of operation, the makeup quantity of ammonia is added from the ammonia storage to make up for the operational loss.
• The liquified amonia is then pumped back to the evaporator, thus completing the cycle. The cycle repeats to run the plant continuously.

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Thermoelectric OTEC

• Works based on thermoelectric principle.
• Semiconductors are used to design two separate packs covered by a thin thermal conducting sheet.
• Warm water from the surface of the ocean is circulated over one device and the cold water pumped from the depth of the ocean is allowed to flow over the other device.

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Thermoelectric OTEC

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Thermoelectric OTEC

• The temperature difference between these two water with the solid state semiconductor devices generates the electric power.
• A part of electric power generated in used to operate the pumps and other equipment.

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Open Cycle (Claude cycle) OTEC

• Similar to flash cycle of geothermal power plants.
• A chamber is maintained at a subatmospheric pressure by a vacuum pump.
• Warm surface water flows into this chamber, where its pressure is reduced.
• As the pressure of warm water decreases, its temperature also decreases, resulting in a liquid vapor mixture.

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Open Cycle (Claude cycle) OTEC

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Open Cycle (Claude cycle) OTEC

• Low pressure vapor is directed to a steam turbine while the liquid is discharged from the chamber.
• The vapor exits to a direct contact condenser, which is maintained at a much lower pressure.
• Cold deep water is supplied to the condenser by a pump, and mixing of this cold water with the vapor from the turbine outlet turns the vapor to the liquid, which is discharged.

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Hybrid cycle OTEC cycle

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Bio-fouling

• The raw ocean water which is pumped in for evaporator and condenser, contains micro-organisms which stick on the water side of both the heat exchangers.
• This biological impurities of sea water that deposits and grows on the evaporator and condenser metal surfaces, creating thermal resistance for heat transfer, is known as bio- fouling.

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Location of OTEC plants

• The selection of suitable site for an OTEC plant needs a temperature difference of about 20oC between the surface and the deep sea ocean water.
• If the temperature difference is higher the site becomes more suitable as it will increase the power output, consequently the per unit cost will reduce.
• Such sites are available in ‘Torrid and Temperature zone’ of the globe between latitudes 30oS and 30oN.

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• Ocean Thermal Energy Conversion (OTEC): Principle of OTEC system, Methods of OTEC power generation – Open Cycle (Claude cycle), Closed Cycle (Anderson cycle) and Hybrid cycle (block diagram description of OTEC)

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Wave power technologies

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1. Oscillating water column technology

• Transferring the energy of wave water to air and using the compressed air to drive a turbine.
• Water moves into a hollow container by the wave motion. This compresses air that was in the container.
• The compressed air drives the turbine, which is connected to a generator where electricity is produced.

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• When the wave moves in the opposite direction, air fills the container from the top, which is open to the atmosphere.
• Air flows through the turbine from both sides, depending on whether the device is breathing in or out. The process is repeated with the next wave.
• The system uses special rotor geometry so that there is no need to change the blade angles or the direction of rotation.

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• These systems must be large , and many of them should be installed to produce significant amount of electricity.
• Oscillating water column technology has been installed off the coast of western Scotland.

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• Oscillating water column technology can be applied in three types of coastal wave power plants.
• Shoreline power plants
• Near-shore power plants
• Breakwater power plants

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Geothermal power generation

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1. Liquid dominated resource

1. Flash steam system
2. Binary cycle system

2. Vapour dominated resource

Liquid dominated resource

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• Geothermal fluid is either available from natural outflow or from a bored well.
• The drilling cost increases greatly with depth and the technically viable depth is 10 km.

Flashed steam system

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• The choice of geothermal power plant is influenced by brine characteristics and its temperature.
• For brine temperature more than 180^oC, the geothermal fluid is used.
• This flashed steam is suitable for power generation.
• Geothermal fluid is a mixture of steam and brine, it passes through a flash chamber where a large part of the fluid is converted to steam.

Flashed steam geothermal power plant

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Flashed steam system

• produce electric power.
• Hot brine from flash chamber and turbine discharge from the condenser are reinjected into the ground.
• Reinjection of the spent brine ensures a continuous supply of geothermal fluid from the well.

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• Dry saturated steam passes

through the turbine coupled with the generator to

Flashed steam system

• Commercially available turbogenerator units in the range of 5-20 Mwe are in use.
• To improve the total efficiency of the system, hot water is utilised for poultry farming in cold regions.

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Binary cycle system

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• A binary cycle is used where geothermal fluid is hot water with temperature less than 100^oC.
• This plant operates with a low boiling point working fluid (isobutane, freon) in a thermodynamic closed Rankine cycle.
• The working fluid is vapourised by geothermal heat in a heat exchanger.

Binary cycle geothermal power plant

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Binary cycle system

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• Vapour expands as it passes through the turbine coupled with the generator.
• Exhaust vapour is condensed in a water cooled condenser and recycled through a heat exchanger.
• Power plants of 11 MW in California and 10 MW in Raft River Idaho USA operate on binary cycle.

Vapour dominated geothermal Electric power plant

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• In a vapour dominated plant, steam is extracted from geothermal wells, passed through a separator to remove particulate contents and flows directly to a steam turbine.
• Steam that operates the turbine coupled with the generator is at a temperature of about 245^oC and pressure 7 kg/cm² (7 bar) which are less than those in conventional steam cycle plants (540^oC and 130 kg/cm²).

Vapour dominated geothermal power plant

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Vapour dominated geothermal power plant

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• Exhaust steam from the turbine passes through a condenser and the water so formed circulates through the cooling tower.
• It improves the efficiency of the turbine and controls environmental pollution associated with the direct release of steam into the atmosphere.
• Waste water from the cooling tower sump is reinjected into the geothermal well to ensure continuous supply.

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Wave power generation

• Heaving Float or Buoy Systems https://youtu.be/8miWW2QyN_4?feature=shared

Oscillating water column device

• https://youtu.be/CNKcSbjCiVw?feature=shared

Surge devices

Thank you

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