MODULE 4a
TIDAL POWER
BASIC PRINCIPLES OF TIDAL POWER(Characteristics of Tidal
Energy)
Tides are produced mainly by the gravitational attraction of the moon and the
sun on the water of solid earth and the oceans.
About 70 per cent of the tide producing force is due to the moon and 30 per
cent to the sun.
The moon is thus the major factor in the tide formation.
Surface water is pulled away from the earth on the side facing the moon, and
at the same time the solid earth is pulled away from the water on the opposite
side.
Thus high tides occur in these two areas with low tides at intermediate points.
As the earth rotates, the position of a given area relative to the moon changes,
and so also do the tides.
There are thus a periodic succession of high and low tides.
Two tidal cycles (i.e. two high tides and two low tides) occur during a lunar day
of 24 hours and 50 minutes.
The time between high tides and low tide at any given location is a little over 6
hours.
A high tide will be experienced at a point which is directly under the moon.
At the same time, a diametrically opposite point on the earth's surface also
experiences a high tide due to dynamic balancing.
In a period of 24 hrs 50 minutes, there are therefore, two high tides and two low
tides.
The rise and fall of the water level follows a sinusoidal curve, shown with
point A indicating the high tide point and point B indicating the low tide
point.
The average time for the water level to fall from A to B and then rise to C is
approximately 6 hours 12.5 min.
The difference between high and low water levels is called the range of the
tide.
The tidal range R is defined as :R = water elevation at high tide—water
elevation at low tide.
Because of the changing positions of the moon and sun relative to the earth,
the range varies continuously.
Fig: Relative high and low tides showing variation in range during lunar month
At times near full or new moon, when sun, moon and earth are approximately
in a line, the gravitational forces of sun and moon enhance each other.
The tidal range is then exceptionally large, the high tides are higher and low
tides are lower than the average. These high tides are called springtides
Near the first and third quarters of the moon, when the sun and moon are at
right angles with respect to the earth, neap tides occur.
The tidal range is then exceptionally small, the high tides are lower and the
low tides higher than the average. Hence the range is not constant.
It varies during the 29.5 day lunar month being maximum at the time of new
and full moons, called the spring tides, and minimum at the time of the first
and month, third quarter moons, called the neap tides.
The spring-neap tidal cycle lasts one-half of a lunar month.
Operation methods of Utilization of Tidal Energy (Harnessing
Tidal Energy)
1)Single Basin Arrangement
In a single basin arrangement there is only one basin interacting with the sea.
The two are separated by a dam (or barrage) and the flow between them is
through sluice ways located conveniently along the dam.
Potential head is provided by rise and fall of tidal water levels, this is usually
accomplished by blocking the mouth of a long narrow estuary with a dam
across it, thereby creating a reservoir.
The dam or barrage constitutes of a number of sluice gates and low head
turbine sets.
The generation of power can be achieved in a single basin arrangement either
as a
(a) Single ebb-cycle system, or
(b)Single tide-cycle system, or
(a) Double cycle system.
a)Single Ebb cycle system.
When the flood tide (high tide) comes in, the sluice gates are opened to permit sea-
water to enter the basin or reservoir, while the turbine sets are shut.
The reservoir thus starts filling while its level rises, till the maximum tide level is
difference between the full reservoir level and the falling tide level.
To run the turbines, they are started and keep working until the rising level of the
next flood tide and the falling reservoir level together reduce the effective head on
the turbines to the extent where it can no longer work safely and efficiently.
The turbines are then closed and the sluice gates opened again , to repeat the cycle
of operations.
At the beginning of the ebb tide the sluice gates are closed.
Then the generation of power takes place when the sea is ebbing (flowing back of
tide) and the water from the basin flows over the turbines into the lower level sea
water.
After two or three hours when there is sufficient estuary, the ebb tide has a long
duration than the flood tide, the ebb operation provides an increased period of actual
work.
b)Single tide cycle system
In single tide cycle system, the generation is affected when the sea is at flood
tide.
The water of the sea is admitted into the basin over the turbines.
As the flood tide period is over and the sea level starts falling again, the
generation is stopped.
The basin is drained into the sea through the sluice ways.
Flood operation scheme needs larger size plant, operating, for shorter period
and hence less efficient as compared to ebb tide operation.
The ebb operation plant will be of smaller size, but will operate over a large
period.
The aim and effort should be to obtain as long a period of operation as
possible at the beginning and finishing the work at the minimum operating
head.
c)Double cycle system
In Double cycle system,the power generation is affected during the ebb as
well as in flood tides.
The direction of flow through the turbines during the ebb and flood tides
alternates, but the machine acts as a turbine for either direction of flow.
In this method, the generation of power is accomplished both during
emptying and filling cycles.
Both filling and emptying processes take place during short periods of time,
the filling when the ocean is at high tide while the water in the basin is at low
tide level, the emptying when the ocean is at low tide and the basin at high-
tide level.
The flow of water in both directions is used to drive a number of reversible
water turbines, each driving an electrical generator.
Electric power would thus be generated during two short period during each
tidal period of 12 h, 25 min. or once every 6h, 12.5 min.
Though the double cycle system has only short duration interruptions in the
turbine operation, yet a continuous generation of power is still not possible.
Furthermore the periods of power generation coincide only occasionally with
periods of peak demand.
However, a fundamental drawback to all methods for generating tidal power
is the variability in output caused by the variations in the tidal range.
2)DOUBLE BASIN ARRANGEMENT
It requires two separate but adjacent basins.
In one basin called "upper basin" (or high pool), the water level is
maintained above that in the other, the low basin (or low pool).
Because there is always a head between upper and lower basins, electricity
can be generated continuously, although at a variable rate.
In this system the turbines are located in between the two adjacent basins,
while the sluice gates are as usual embodied in the dam across the mouths of
the two estuaries.
At the beginning of the flood tide, the turbines are shut down, the gates of
upper basin A are opened and those of the lower basin B are closed.
The basin A is thus filled up while the basin B remains empty.
As soon as the rising water level in A provides sufficient difference of head
between the two basins, the turbines are started.
The water flows from A to B through the turbines, generating power.
The power generation thus continues simultaneously with the filling up the
basin A.
At the end of the flood tide when A is full and the water level in it is the
maximum, its sluice gates are closed.
When the ebb tide level gets lower than the water level in B, its sluice gates
are opened whereby the water level in B, which was arising and reducing the
operating head, starts falling with the ebb.
This continues until the head and water level in A is sufficient to run the
turbines. With the next flood tide the cycle repeats itself.
With this twin basin system, a longer and more continuous period of genera-
tion per day is possible.
The operation of the two basin scheme can be controlled so that there is a
continuous water flow from upper to lower basin.
However since the water head between the basins varies during each tidal
cycle, as well as from day to day, so also does the power generated.
As in the case with single basin scheme, the peak power generation does not
often correspond in time with the peak demand.
One way of improving the situation is to use off-peak power, from the tidal
power generators or from an alternative system, to pump water from the low
basin to the high basin.
An increased head would then be available for tidal power generation at
times of peak demand.
ADVANTAGES AND LIMITATION OF TIDAL ENERGY
Advantages:
(1) The biggest advantage of the tidal power is besides being inexhaustible, it is
completely independent of the precipitation (rain) and its uncertainty. Even a
continuous dry spell of any number of years can have no effect whatsoever on the tidal
power generation.
(2)Tidal power generation is free from pollution, as it does not use any fuel and also
does not produce any unhealthy waste like gases, ash, atomic refuse.
(3)These power plants do not demand large area of valuable land because they are on
the bays (sea shore).
(4) Peak power demand can be effectively met when it works in combination with
thermal or hydroelectric system.
Limitations of Tidal energy(Problems faced in exploiting
Tidal Energy)
(1)The fundamental drawback to all methods of generating tidal power is the
variability in output caused by the variations in the tidal range.
(2)The tidal ranges is highly variable and thus the turbines have to work on a
wide range of head variation. This affects the efficiency of the plant.
(3)Since the tidal power generation depends upon the level difference in the sea
and an inland basin, it has to be a intermittent operation, feasible only at a
certain stage of the tidal cycle. This intermittent pattern could be improved to
some extent by using multiple basins and a double cycle system.
(4)The duration of power cycle may be reasonably constant but its time of
occurrence keeps in changing, introducing difficulties in the planning of the load
sharing every day in a grid.
(5)Sea water is corrosive and it was feared that the machinery may get corroded.
(6)Construction in sea or in estuaries is found difficult.
(7)Cost is not favorable compared to the other sources of energy.
(8)It is feared that the tidal power plant would hamper the other
natural uses of estuaries such as fishing, or navigation.
(9)Usually the places where tidal energy is produced are far away from the
places where it is consumed. This transmission is expensive and difficult
Problems faced in exploiting Tidal Energy
1.Usually the places where tidal energy is produced are far away from the
places where it is consumed. This transmission is expensive and difficult.
2. Intermittent supply: Cost and environmental problems, particularly barrage
systems are less attractive than some other forms of renewable energy.
3. Cost: The disadvantages of using tidal and wave energy must be considered
before jumping to conclusion that this renewable, clean resource is the answer
to all our problems. The main detriment is the cost of those plants.
4. Altering the ecosystem at the bay: Damages such as reduced flushing,
winter icing, and erosion can change the vegetation of the area and disrupt the
balance. Similar to other ocean energies, tidal energy has several prerequisites
that make it only available in a small number of regions.
5.For a tidal power plant to produce electricity effectively (about 85%
efficiency), it requires a basin or a gulf that has a mean tidal amplitude (the
differences between spring and neap tide) of 7 m or more.
6. A barrage across an estuary is very expensive to build and affects a very wide
area—the environment is changed for many miles upstream and downstream
PRINCIPLE AND WORKING OF WAVE ENERGY
The energy in sea waves mainly comes in an irregular and oscillating forms
at all times of the day and night.
Solar energy causes winds to blow over vast sea areas, which in turn causes
waves to form.
The wave height, period and direction are primarily dependent on the wind
properties and also the geometry of sea.
As long as sun shines, wave energy never will be depletes, it varies in
intensity available all the times.
When the wind blows across the water surface, air particles from the wind grab the
water molecules they touch.
Stretching of the water surface by force or friction between the air and water creates
capillary waves.
Surface tension acts on these ripples to restore the smooth surface and thereby
waves are formed.
Energy in the waves is harnessed basically in the form of mechanical energy using
wave energy converters, also known as wave devices or wave machines.
A wave device may be placed in the ocean in various possible situations and
locations.
The fluctuating mechanical energy obtained is modified/smoothed out to drive a
generator.
SEA WAVE FORMATION BY STORMS
Advantages and limitation of wave energy
Advantages
1.Sea waves have high energy densities and provide a consistent stream of
electricity generation capacity.
2. Wave energy is clean source of renewable energy with limited negative
environmental impacts.
3. It has no greenhouse gas emissions or water pollutants.
4. Operating cost is low and operating efficiency is optimal.
5. Damage to ocean shoreline is reduced.
Limitations
1.High construction costs.
2. Marine life is disrupted and displaced.
3. Damage to the devices from strong storms and corrosion create problems.
4. Wave energy devices could have an effect on marine and recreation
environment.
MODULE 4b
OCEAN THERMAL ENERGY
CONVERSION
BASIC PRINCIPLES OF OTEC PLANT
It is an indirect form of solar energy at sea, collection and storage are free.
The surface of the water acts as the collector for solar heat while the upper
layer of the sea constitutes infinite heat storage reservoir.
Thus heat contained in the oceans, which is solar in origin could be converted
into electricity by utilizing the fact that the temperature difference between
the warm surface water of the tropical oceans and the colder waters in the
depths is about 20-25°C.
Warm surface water could be used to heat some low boiling organic fluid, the
vapour of which would run a heat engine.
The exit vapour would condensed by pumping cold water from the deeper regions.
The amount of energy available for ocean thermal power generation is enormous,
and is replenished continuously.
Considering deep water in general, the high temperatures are at the surface, whereas
deep water remains cool.
In the tropics, the ocean surface temperature often exceeds 25°C, while 1 km below,
the temperature is usually not higher than 10°C.
Water density decreases with an increase in temperature .
Thus there will be no thermal convection currents between the warmer, lighter water
at the top and deep cooler, heavier water.
Thermal conduction heat transfer between them across the large depths, is too
low, so the warm water stays at the top and the cool water stays at the bottom.
It is said, therefore, that in tropical waters there are two essentially infinite
heat reservoirs, a heat source at the surface at about 27°C and a heat sink,
some 1 km directly below, at about 4°C ; both reservoirs are maintained
annually by solar incidence.
The concept of ocean thermal energy conversion (OTEC) is based on the
utilization of this temperature difference in a heat engine to generate power.
OPEN CYCLE OTEC SYSTEM
An open-cycle OTEC uses the warm ocean surface water as working fluid. It is
a non-toxic and environment friendly fluid.
The major components of this system consists of evaporator, low-pressure
turbine coupled with electrical generator, condenser, non-condensable gas
exhaust, and pumps.
Evaporator used in an open-cycle system is a flash evaporator in which warm
sea water instantly boils or flash in the chamber that has reduced pressure than
atmosphere or vacuum.
It results in reduced vaporization pressure of warm sea water.
The low-pressure vapour (steam) expands in turbine to drive a coupled
electrical generator to produce electricity.
A portion of electricity generated is consumed in plants to run pumps and for
other work, and the remaining large amount of electricity is stored as net
electrical power.
A large turbine is required to accommodate large volumetric flow rates of low-
pressure steam, which is needed to generate electrical power, and is used with
other plant components in a similar manner.
During vapourization process in an evaporator, oxygen, nitrogen, and carbon
dioxide dissolved in sea water are separated with the help of deareater and are
non-condensable.
They are exhausted by non-condensable gas exhaust system.
Condenser is used to condense vapour or steam released from steam turbine is
condensed by cold deep sea water and returned back to sea.
If a condenser is used, condensed steam (desalinated water) remains
separated from cold sea water and is pumped into marine culture ponds or
back into the ocean.
To avoid leakage of air in atmosphere and to prevent abnormal operation of
plants, perfect sealing of all components and piping systems is essential.
The cooling water from the deep ocean which is at about 11°C, on reaching
the condenser, its temperature rises to about 15°C, due to heat transfer
between the progressively warmer outside water and cooling water inside the
pipe.
The warm surface water is continuously pumped into evaporator and cycle is
repeated.
CLOSED OR ANDERSON OTEC CYCLE
The closed cycle approach was first proposed by Barjot in 1926, but the most
recent design was by Anderson and Anderson in the 1960s.
The closed cycle is sometimes referred to as the Anderson Cycle. In the cycle
propane was chosen as the working fluid.
The temperature difference between warm surface and cool surface was
20°C. The cool surface was at about 600 m deep.
It has different arrangement when compared to open-cycle OTEC.
Organic fluid with low boiling point is used as working fluid.
Ammonia liquid is the most widely used working fluid. Working fluid flows
in a closed loop and perfectly sealed piping system.
Working fluid circulates around the loop continuously.
Warm ocean surface water flows through completely separate piping system
and discharges in upper surface of ocean.
Warm surface sea water and working fluid piping are placed very closely to
each other in a heat exchanger to transfer warm sea water heat into working
fluid.
The cold deep sea water piping system is in contact with working fluid piping
system in a condenser where working fluid condenses to its liquid state.
Working fluid is pumped through heat exchangers in a closed loop cycle
which is perfectly leakage proof.
Warm sea surface water is pumped through separate pipe in heat exchanger in
close contact with fluid closed loop cycle
Warm sea water transfer its heat energy to working fluid in heat exchanger
and working fluid vapourizes.
The fluid vapour makes the turbine to rotate and drive an electrical generator
to produce electricity.
Fluid vapour leaving the turbine is cooled and condensed as liquid fluid and
is pumped again to repeat cycle.
Cold deep sea water is pumped through a separate pipe in condenser for
providing efficient cooling of working fluid.
The working fluid may be ammonia, propane, or a Freon.
The operating (saturation) pressures of such fluids at the boiler and condenser
temperatures are much higher than those of water and their specific volumes
are much lower, being comparable to those of steam in conventional power
plants.
Such pressures and specific volumes result in turbines that are much smaller
and hence less costly than those that use the low pressure steam of the open
cycle.
The closed cycle also avoids the problems of the evaporator.
It however, requires the use of very large heat exchangers (boiler and
condenser) because, for an efficiency of about 2 percent, the amounts of heat
added and rejected are about 50 times the output of the plant.
Site Selection
In selecting a site for an OTEC facility, the primary consideration is a
significant temperature difference—at least about 20°C—between surface
and deep ocean waters (for 700-900 m depth or more) that will permit year
round operation.
The greater the difference, the lower will be the cost of generating electricity.
The best sites are in the tropical belt between about 20°N and 20°S latitude.
In choosing a site, consideration should be given to the potential for bio-
fouling effects.
As a general rule, an OTEC plant would be located offshore in order to
provide access to the deep colder water.
However, an ideal situation might be one where the shoreline dropped steeply
to a considerable depth.
Most of the installation could then be more conveniently build on land.
Energy Utilization
An OTEC plant should be less than about 30 km from shore.
The electricity generated could then be transmitted inexpensively to land by
submarine cable.
If the plant is so far from shore that these costs become prohibitive, the electricity
generated can be utilized at the plant site to produce energy-intensive materials.
Direct electric current has to be used to decompose sea water by the process
of electrolysis, the main products would be hydrogen and oxygen.
The hydrogen could be liquified and transported by tanker to a point where it
could be used as fuel.
Alternatively, the hydrogen could be combined with atmospheric nitrogen to
form ammonia for use as fertilizer, thereby saving natural gas which is
presently the main source of hydrogen for this purpose.
Problem associated with OTEC
The main problem with this renewable energy is the necessity of a
temperature gradient higher or equal to 20ºC, between the hot and cold
reservoir.
Higher the temperature difference, the better efficiency it will have. This
requirement is only fulfilled in tropical and equatorial zones during the whole
year.
The OTEC systems efficiency is not quite high because of the little
temperature difference.
Although the ideal energy conversion using 26 ºC and 4 ºC warm and cold
seawaters is 8%, due to several losses final 3-4% efficiency is got.
The small land based OTEC plants need kilometres of piping to move a high
volume of cold water from deep ocean. Its cost could be up to the 75 % of the
total power plant costs.
Higher cost than other energy sources (hydroelectric, wave energy and diesel)
in islands.
Although floating OTEC plants could apparently be a solution, maintenance
and repair costs would also be high.
Floating plants and piping of land based plants must withstand high stresses
during storms.
The ocean thermal plant might appear as though it is floating on the water,
but it has an enlarged and massive construction beneath the water.
Large ships might experience difficulties in navigation with the floating body
nearby.
The ocean thermal energy plant might need large sizes of turbines.
The disadvantage comes in when the prices of turbines might be unaffordable
and huge.
The deposition and growth of microorganisms inside the pipes of evaporator
and condenser heat exchanger is known as biofouling. Biofouling reduces the
heat transfer efficiency and thereby lowers the performance.
Environmental Impacts of OTEC plants
It is feared that biota including eggs, larvae and fish could be entrained and
destroyed due to intake and expulsion of large volumes of water.
Changes in local temperature and salinity might also effects the local
ecosystem, impact coral and influence ocean currents and climate.
In open cycle OTEC system, CO2 dissolved in warm water is released to
Atmosphere. However, the quantity of CO2 released is very small and under
worst conditions would be only 1/15 that of oil or 1/25 that of coal based
generation of same power.
Release of large quantities of cold water into warmer surface environment
will also have biological effects.
Availability of OTEC plant worldwide
A 50 kW floating closed cycle test plant was installed off Hawaii in 1979.
The Tokyo Electric Power Co. built and operated a 100-kW shore based
closed cycle plant in the Republic of Nauru.
The Japanese Government is designing a 10 MW floating plant and also
considering a land-based plant
Currently the only continuously operating OTEC system is located in
Okinawa Prefecture, Japan.
Makai Ocean Engineering’s ocean thermal energy conversion (OTEC) power
plant in the US is the world’s biggest operational facility of its kind with an
annual power generation capacity of 100kW, which is sufficient to power 120
homes in Hawaii.
In India, conceptual studies on OTEC plant for Kavaratti (Lakhshadweep island),
Andaman Nicobar Islands and at Kulasekharapatnam (Tamil Nadu) were initiated
in 1980.
A preliminary design for 1 MW (gross) closed Rankine cycle floating plant was
prepared by IIT Chennai in 1984.
In 1997, National Institute of Ocean Technology (NIOT) signed a memorandum
with Saga University, Japan for joint development of 1 MW plant, near the port
of Tuticorin (Tamil Nadu).
The objective is to demonstrate the OTEC plant for one year, after which it could
be moved to the Andaman Nicobar Islands for power generation.
