UNIT-4

(Q&A)

1. List relative advantages and limitations of floating and shore based OTEC plants? (7M)
Remember (June 2022)

Ocean Thermal Energy Conversion (OTEC) is a method of generating electricity by exploiting

the temperature difference between warm surface water and cold deep water in the ocean. There
are two main types of OTEC plants: floating and shore-based. Each has its own set of advantages
and limitations:

Floating OTEC Plants:

Advantages:

1. Location Flexibility: Floating OTEC plants can be located farther offshore, allowing access to
larger thermal gradients. This can result in higher energy production potential.
2. Reduced Environmental Impact: Floating plants have the potential to reduce environmental
impact because they can be positioned away from sensitive coastal areas, minimizing disturbance
to coastal ecosystems.
3. Ease of Deployment: Floating platforms can be constructed and deployed relatively quickly
compared to shore-based plants, which require significant construction on land.
4. Less Grid Dependency: Floating OTEC plants can potentially operate independently of onshore
power grids, making them suitable for remote and island communities.

Limitations:

1. Maintenance Challenges: Maintenance and repair of floating platforms can be more

challenging due to their offshore location, particularly in rough sea conditions.
2. Wave and Storm Vulnerability: Floating plants are more susceptible to damage from storms
and rough sea conditions, which can be a significant risk factor.
3. Higher Initial Costs: The construction and deployment of floating platforms tend to be more
expensive compared to shore-based OTEC plants.
4. Limited Scalability: The size of floating OTEC plants is constrained by the availability of
suitable deep-water locations and the capacity to anchor the platform securely.

Shore-Based OTEC Plants:

Advantages:

1. Stability and Security: Shore-based plants are typically more stable and secure than floating
platforms, as they are anchored to solid ground.
2. Easier Maintenance: Maintenance and repair activities are generally easier and less risky for
shore-based plants due to their onshore location.
3. Integration with Existing Infrastructure: Shore-based plants can be integrated more easily
with existing infrastructure, including electrical grids and desalination facilities.
4. Potentially Lower Initial Costs: Initial construction costs for shore-based plants can be lower
compared to floating platforms, especially in areas with suitable coastline.

Limitations:

1. Limited Thermal Gradient: Shore-based OTEC plants are limited by the availability of a
significant temperature gradient between surface and deep water, which may be less pronounced
near the coast.
2. Environmental Impact: These plants are often located in coastal areas, which can have a
greater impact on local marine ecosystems and coastal communities.
3. Space Constraints: Availability of suitable coastal land for OTEC plants can be limited,
particularly in densely populated or developed coastal regions.
4. Permitting and Regulatory Challenges: Shore-based plants may face more stringent permitting
and regulatory processes due to their proximity to populated areas.

2. What are the main advantages and disadvantages of OTEC system? And explain the various
technologies available for OTEC. (7M) Remember (June-2022)

Advantages and disadvantages of OTEC

Ocean Thermal Energy Conversion (OTEC) is a promising renewable energy technology, but
like any energy source, it comes with its own set of advantages and disadvantages.

Advantages:

1. Renewable and Sustainable: OTEC utilizes the temperature difference between warm surface
water and cold deep ocean water to generate electricity, making it a renewable and sustainable
source of energy.
2. Stable and Reliable: Unlike some other renewable sources like solar and wind, OTEC provides
a stable and continuous source of energy because it is not subject to weather fluctuations or
seasonal changes.
3. Reduces Greenhouse Gas Emissions: OTEC does not produce greenhouse gas emissions
during electricity generation, contributing to efforts to combat climate change.
4. Potential for Dual Use: OTEC can be integrated with desalination processes, producing fresh
water in addition to electricity, which is valuable in water-scarce regions.
5. Can Provide Energy Independence: Particularly for island nations or coastal communities,
OTEC can reduce reliance on imported fossil fuels, enhancing energy security.
6. Low Environmental Impact (Compared to Some Alternatives): OTEC systems typically
have lower environmental impacts compared to some other forms of energy generation.
However, thorough environmental assessments are still required.

Disadvantages:

1. High Capital Costs: OTEC systems have high initial capital costs due to the specialized
equipment required and the complexity of deploying and maintaining infrastructure in the marine
environment.
2. Location Specific: OTEC is most feasible in tropical regions where there is a large temperature
difference between surface and deep ocean waters. This limits its applicability to certain
geographic areas.
3. Engineering Challenges: Building and maintaining OTEC systems in the harsh marine
environment presents engineering challenges, including corrosion, biofouling, and the need for
robust materials.
4. Environmental Impact Assessment Required: While OTEC has lower environmental impact
compared to some other energy sources, it still requires thorough assessments to ensure it does
not harm marine ecosystems or wildlife.
5. Potential for Ocean Thermal Pollution: OTEC could potentially cause localized changes in
water temperatures, which may affect marine life in the vicinity of the plant.
6. Competition with Other Uses of Ocean Space: OTEC may compete with other activities like
shipping, fishing, and conservation efforts for access to ocean space.
7. Limited Potential for Large-Scale Energy Generation: OTEC is currently limited in terms of
the scale of electricity generation it can achieve compared to other renewable sources like solar
or wind.

Methods or Power cycle types

Cold seawater is an integral part of each of the three types of OTEC systems: closed-cycle, open-
cycle, and hybrid. To operate, the cold seawater must be brought to the surface. The primary
approaches are active pumping and desalination. Desalinating seawater near the sea floor lowers
its density, which causes it to rise to the surface.
The alternative to costly pipes to bring condensing cold water to the surface is to pump vaporized
low boiling point fluid into the depths to be condensed, thus reducing pumping volumes and
reducing technical and environmental problems and lowering costs.

1. Closed cycle

Closed-cycle systems use fluid with a low boiling point, such as ammonia (having a boiling point
around -33 °C at atmospheric pressure), to power a turbine to generate electricity. Warm
surface seawater is pumped through a heat exchanger to vaporize the fluid. The expanding vapor
turns the turbo-generator. Cold water, pumped through a second heat exchanger, condenses the
vapor into a liquid, which is then recycled through the system.

In 1979, the Natural Energy Laboratory and several private-sector partners developed the "mini
OTEC" experiment, which achieved the first successful at-sea production of net electrical power
from closed-cycle OTEC. The mini OTEC vessel was moored 1.5 miles (2.4 km) off the
Hawaiian coast and produced enough net electricity to illuminate the ship's light bulbs and run its
computers and television.

2. Open cycle
Open-cycle OTEC uses warm surface water directly to make electricity. The warm seawater is
first pumped into a low-pressure container, which causes it to boil. In some schemes, the
expanding vapor drives a low-pressure turbine attached to an electrical generator. The vapor,
which has left its salt and other contaminants in the low-pressure container, is pure fresh water. It
is condensed into a liquid by exposure to cold temperatures from deep-ocean water. This method
produces desalinized fresh water, suitable for drinking water, irrigation or aquaculture.

In other schemes, the rising vapor is used in a gas lift technique of lifting water to significant
heights. Depending on the embodiment, such vapor lift pump techniques generate power from
a hydroelectric turbine either before or after the pump is used.

In 1984, the Solar Energy Research Institute (now known as the National Renewable Energy
Laboratory) developed a vertical-spout evaporator to convert warm seawater into low-pressure
steam for open-cycle plants. Conversion efficiencies were as high as 97% for seawater-to-steam
conversion (overall steam production would only be a few percent of the incoming water). In
May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced close to 80 kW of
electricity during a net power-producing experiment.[46] This broke the record of 40 kW set by a
Japanese system in 1982.
3. Hybrid cycle

A hybrid cycle combines the features of the closed- and open-cycle systems. In a hybrid, warm
seawater enters a vacuum chamber and is flash-evaporated, similar to the open-cycle evaporation
process. The steam vaporizes the ammonia working fluid of a closed-cycle loop on the other side
of an ammonia vaporizer. The vaporized fluid then drives a turbine to produce electricity. The
steam condenses within the heat exchanger and provides desalinated water (see heat pipe).

3. Explain the working of Open Cycle OTEC plant with a neat diagram. (7M) Understand
(Jan2023)

Open cycle

Open-cycle OTEC uses warm surface water directly to make electricity. The warm seawater is
first pumped into a low-pressure container, which causes it to boil. In some schemes, the
expanding vapor drives a low-pressure turbine attached to an electrical generator. The vapor,
which has left its salt and other contaminants in the low-pressure container, is pure fresh water. It
is condensed into a liquid by exposure to cold temperatures from deep-ocean water. This method
produces desalinized fresh water, suitable for drinking water, irrigation or aquaculture.
In other schemes, the rising vapor is used in a gas lift technique of lifting water to significant
heights. Depending on the embodiment, such vapor lift pump techniques generate power from
a hydroelectric turbine either before or after the pump is used.

In 1984, the Solar Energy Research Institute (now known as the National Renewable Energy
Laboratory) developed a vertical-spout evaporator to convert warm seawater into low-pressure
steam for open-cycle plants. Conversion efficiencies were as high as 97% for seawater-to-steam
conversion (overall steam production would only be a few percent of the incoming water). In
May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced close to 80 kW of
electricity during a net power-producing experiment.[46] This broke the record of 40 kW set by a
Japanese system in 1982.

Wave Energy :

4. What do you mean by Wave Energy and list the advantages of wave power and also list the
Main difficulties encountered in the development of wave power? (7M) Remember (June
2022)

Wave Energy : Ocean wave energy, or just simply Wave Energy, is another type of
ocean based renewable energy source that uses the power of the waves to generate
electricity. Unlike tidal energy which uses the ebb and flow of the tides, wave
energy uses the vertical movement of the surface water that produce tidal waves.
Wave power converts the periodic up-and-down movement of the oceans waves into
electricity by placing equipment on the surface of the oceans that captures the energy
produced by the wave movement and converts this mechanical energy into electrical
power.
Wave energy is actually a concentrated form of solar power generated by the action of
the wind blowing across the oceans surface. As the suns rays strike the Earth’s
atmosphere, they warm it up. Differences in the temperature of the air masses around the
globe causes the air to move from the hotter regions to the cooler regions, resulting in
winds.
As the wind passes over the surface of the oceans, a portion of the winds kinetic energy is
transferred to the water below, generating waves.
 In fact, the ocean could be viewed as a vast storage collector of energy transferred by the
sun to the oceans, with the waves carrying the transferred kinetic energy across the
surface of the oceans. Then we can say that waves are actually a form of energy and it is
this energy and not water that moves along the ocean’s surface.

Wave Energy Advantages

 Wave energy is an abundant and clean energy resource as the waves are generated by the
wind.
 Pollution free as wave energy generates little or no pollution to the environment compared
to other green energies.
 Reduces dependency on fossil fuels as wave energy consumes no fossil fuels during
operation.
 Wave energy is relatively consistent and predictable as waves can be accurately forecast
several days in advance.
 Wave energy devices are modular and easily sited with additional wave energy devices
added as needed.
 Dissipates the waves energy protecting the shoreline from coastal erosion.
 Presents no barriers or difficulty to migrating fish and aquatic animals.

Main difficulties encountered in the development of wave power

The development of wave power faces several challenges, both technical and economic. Here are
some of the main difficulties encountered in the development of wave power:

1. Harsh Marine Environment: Wave energy devices operate in a harsh marine environment,
which subjects them to corrosive saltwater, high winds, and rough seas. This environment can
lead to material degradation and maintenance challenges.
2. Engineering and Technological Complexity: Designing and building wave energy converters
that can withstand the forces of waves and generate electricity efficiently is a complex
engineering task. Developing reliable and cost-effective systems remains a significant challenge.
3. Cost Competitiveness: The capital costs associated with building and deploying wave energy
devices are currently high compared to other renewable energy sources like solar and wind.
Achieving cost competitiveness with these established technologies is a major hurdle.
4. Scaling for Commercial Viability: Transitioning from small-scale prototypes to full-scale
commercial deployments is a substantial challenge. Ensuring the scalability of wave energy
technologies while maintaining efficiency and cost-effectiveness is critical.
5. Resource Variability and Predictability: Wave energy is inherently variable and can be
difficult to predict accurately. This variability poses challenges for grid integration and requires
sophisticated forecasting and energy storage solutions.
6. Environmental Impact and Mitigation: Like any marine-based technology, wave energy
projects require thorough environmental impact assessments to ensure they do not harm marine
ecosystems or wildlife. Mitigating potential impacts is crucial for obtaining regulatory approvals.
7. Grid Integration and Infrastructure: Wave energy projects must be integrated into existing
energy grids, which may require upgrades and adaptations to handle the variability and
intermittent nature of wave energy.
8. Site Selection and Permitting: Identifying suitable locations with consistent wave resources
and obtaining the necessary permits and approvals can be challenging. Competing uses for
coastal and marine areas can also complicate the permitting process.
9. Technological Reliability and Durability: Ensuring the long-term reliability and durability of
wave energy devices is essential for their economic viability. Failures and maintenance issues
can lead to costly downtime.
10. Funding and Investment: Securing funding for research, development, and commercial
deployment of wave energy technologies can be challenging, especially given the long lead times
and risks associated with emerging technologies.
11. Public Perception and Acceptance: Public perception and acceptance of wave energy projects
can be a factor in the success of such developments. Engaging with local communities and
addressing concerns about visual impacts, navigation, and potential impacts on fisheries is
crucial.
12. Regulatory and Policy Frameworks: Clear and supportive regulatory and policy frameworks
are essential for the successful development of wave energy projects. Regulatory uncertainty or
unfavorable policies can hinder progress.
Wave Energy conversion devices
5. Discuss the different types of wave energy conversion devices. (7M) Analyze (Jan 2023)
6. Describe the concepts of converting wave energy into mechanical or electrical energy. (7M)
Create (Jan 2023)

Wave energy conversion devices

Ocean wave energy has many advantages over ocean wind energy in that it is more predictable,
less variable and offers higher available energy densities. Depending on the distance between the
energy conversion device and the shoreline, wave energy systems can be classified as being
either Shoreline devices, Nearshore devices or Offshore devices. So what is the difference
between these three types of energy extraction devices.

Shoreline devices are wave energy devices which are fixed to or embedded in the shoreline, that
is they are both in and out of the water.

Nearshore devices are characterised by being used to extract the wave power directly from the
breaker zone and the waters immediately beyond the breaker zone, (i.e. at 20m water depth).

Offshore devices or deep water devices are the farthest out to sea and extend beyond the breaker
lines utilising the high-energy densities and higher power wave profiles available in the deep
water waves and surges.

One of the advantages of offshore devices is that there is no need for significant coastal
earthworks, as there is with onshore devices.

As most of the energy within a wave is contained near the surface and falls off sharply with
depth. There is a surprising range of designs available that maximise the energy available for
capture. These wave energy devices are either fixed bottom standing designs used in shallow
water and which pierce the waters surface, or fully floating devices that are used to capture the
kinetic energy content of a waves movement and convert each movement into electricity using a
generator.

There are currently four basic wave energy “capture” type methods:
 Point Absorbers – these are small vertical devices either fixed directly to the ocean floor
or tethered via a chain that absorb the waves energy from all directions. These devices
generate electricity from the bobbing or pitching action of a floating device.

Typical wave energy devices include, floating buoys, floating bags, ducks, and
articulated rafts, etc. These devices convert the up-and-down pitching motion of the
waves into rotary movements, or oscillatory movements in a variety of devices to
generate electricity. One of the advantages of floating devices over fixed devices it that
they can be deployed in deeper water, where the wave energy is greater.
 Wave Attenuators – also known as “linear absorbers”, are long horizontal semi-
submerged snake-like devices that are oriented parallel to the direction of the waves. A
wave attenuator is composed of a series of cylindrical sections linked together by flexible
hinged joints that allow these individual sections to rotate and yaw relative to each other.
The wave-induced motion of the device is used to pressurise a hydraulic piston, called a
ram, which forces high pressure oil through smoothing accumulators to turn a hydraulic
turbine generator producing electricity. Then wave attenuators convert the oscillating
movement of a wave into hydraulic pressure.
 Oscillating Water Column – is a partly submerged chamber fixed directly at the
shoreline which converts wave energy into air pressure. The structure could be a natural
cave with a blow hole or a man made chamber or duct with an wind turbine generator
located at the top well above the waters surface. The structure is usually constructed
perpendicular to the waves so that the ebbing and flowing motion of the waves force the
trapped water inside the chamber to oscillate in the vertical direction. As the waves enter
and exit the chamber, the water column moves up and down and acts like a piston on the
air above the surface of the water, pushing it back and forth. This air is compressed and
decompressed by this movement and is channelled through a wind turbine generator to
produce electricity. The speed of air in the duct can be enhanced by making the cross-
sectional area of the duct much less than that of the column.
 Overtopping Devices – also known as “spill-over” devices, are either fixed or floating
structures that use ramps and tapered sides positioned perpendicular to the waves. The
sea waves are driven up the ramp and over the sides filling-up a small tidal reservoir
 which is located 2 to 3 metres above sea level. The potential energy of the water trapped
inside the reservoir is then extracted by returning the water back to the sea through a low
head Kaplan turbine generator to produce electricity.

Then overtopping devices convert the potential energy available in the head of water into
mechanical energy. The disadvantage of onshore overtopping schemes is that they have a
relatively low power output and are only suitable for sites where there is a deep water
shoreline and a low tidal range of less than about a meter.

Tides:
Basic principle of Tide Energy
7. Explain about double basin arrangement in tidal power generation. (7M) Understand (Jan
2023).
Double basin type
In this arrangement, the turbine is set up between the basins as shown in figure. One basin is
intermittently filled tide and other is intermittently drained by the ebb tide. Therefore, a small
capacity but continuous power is made available with this system as shown in figure. The
main disadvantages of this system are that 50% of the potential energy is sacrificed in
introducing the variation in the water levels of the two basins.

Double basin with pumping

In this case, off peak power from the base load plant in a interconnected transmission system
is used either to pump the water up the high basin. Net energy gain is possible with such a
system if the pumping head is lower than the basin-to-basin turbine generating head.

Components of Tidal Energy

8. Explain how the tides are originated and give its uses in power generation. (7M) Understand
(June 2022)
What are Tides?
Tides are the unusual conditions that occur over the sea at least twice in a lunar year. The
gravitational pull of the sun and the moon along with the magnetic fields of the Earth contribute
to rising sea levels. This rise in the sea level at any particular period of time is termed a high tide,
whereas, if the sea level falls, then it is considered as the low tides.

When the gravitational field of both the moon and the Earth are in a straight line, their influence
becomes very strong which leads to the flow of over gallons of water to the shore causing the
low tide condition. When the moon perfectly aligns with the earth and the sun, the gravitational
pull on Earth becomes stronger as a result of which high tides become higher while the low tide
becomes lower during each tidal cycle. These tides are known as spring tides.

Types of Tides

There are high tides and low tides which are defined specifically as follows.

1. Spring Tides: It usually occurs on full moon nights when the sun, moon, and the earth
are aligned in a straight line, resulting in high tides due to the strong gravitational fields.
2. Neap Tides: It usually occurs on quarter moon nights when the gravitational pull of the
sun and the moon are extremely weak due to the counteracting effects of each other. This
results in the generation of the weak tides marked by the negligible difference between
the high and the low tides.
The main force behind the generation of the tides is the gravitational pull of the moon which
remains under the influence of the rotation of the earth.

 As the gravitational force of the sun and moon become stronger, the water in the sea rises
upwards forming a bulge-like structure.
 The rotation of the earth causes the rise and the fall in the sea level and the greater the
difference between the high and the low tides, the greater shall be the tidal current.
 The tidal current so generated is transformed into usable electricity with the help of the
tidal generator or through the turbine.
 This tidal energy is generated via tides which are the result of the force of the sun and
moon and the action of the earth, so, this can be considered as a non-exhaustible and
clean source of energy.

Methods For Generating Tidal Energy

Tidal energy can be generated by the following methods.

 Tidal Stream Generator: The moving water, depending upon its velocity, is used by
such kinds of generators to generate electricity. The places like bridges that can capture
the high velocity of the running water are used for installing the generators.
 Dynamic Tidal Power: It works on the phenomenon which involves the usage of long
dams built from coastal regions to that of the oceans.
 Tidal Barrage: This directly involves the use of the high and the low tides with the help
of recurring potential energy. In this, the water is channeled out via the dams during the
high tides and this potential energy which is stored, later, gets converted into electricity.
 Tidal Lagoon: This involves the development of the proper artificial infrastructure in
terms of the circular retaining walls. These walls are installed with the turbines to store
the potential energy.

9. List and explain the various tidal energy conversion schemes. (7M) Remember (June 2022)

The tidal power plants are generally classified on the basis of the number of basins used for
the power generation. They are further subdivided as one-way or two-way system as per the
cycle of operation for power generation.

The classification is represented with the help of a line diagram as given below.
Working of different tidal power plants

1. Single basin-one-way cycle

This is the simplest form of tidal power plant. In this system a basin is allowed to get filled
during flood tide and during the ebb tide, the water flows from the basin to the sea passing
through the turbine and generates power. The power is available for a short duration ebb tide.

Figure: (a) Tidal region before construction of the power plant and tidal variation

Figure: (b) Single basin, one –way tidal power plant

Figure (a) shows a single tide basin before the construction, of dam and figure (b) shows the
diagrammatic representation of a dam at the mouth of the basin and power generating during
the falling tide.

2. Single-basin two-way cycle

In this arrangement, power is generated both during flood tide as well as ebb tide also. The
power generation is also intermittent but generation period is increased compared with one-
way cycle. However, the peak obtained is less than the one-way cycle. The arrangement of
the basin and the power cycle is shown in figure.

Figure: Single –basin two-way tidal power plant

The main difficulty with this arrangement, the same turbine must be used as prime mover as
ebb and tide flows pass through the turbine in opposite directions. Variable pitch turbine and
dual rotation generator are used of such scheme.

3. Single –basin two-way cycle with pump storage

In this system, power is generated both during flood and ebb tides. Complex machines
capable of generating power and pumping the water in either directions are used. A part of
the energy produced is used for introducing the difference in the water levels between the
basin and sea at any time of the tide and this is done by pumping water into the basin up or
down. The period of power production with this system is much longer than the other two
described earlier. The cycle of operation is shown in figure.
Figure: Single-basin, two-way tidal plant coupled with pump storage system.

4. Double basin type

In this arrangement, the turbine is set up between the basins as shown in figure. One basin is
intermittently filled tide and other is intermittently drained by the ebb tide. Therefore, a small
capacity but continuous power is made available with this system as shown in figure. The
main disadvantages of this system are that 50% of the potential energy is sacrificed in
introducing the variation in the water levels of the two basins.

5. Double basin with pumping

In this case, off peak power from the base load plant in a interconnected transmission system
is used either to pump the water up the high basin. Net energy gain is possible with such a
system if the pumping head is lower than the basin-to-basin turbine generating head.