ARENA DAM

The Arena Dam is located on the Arena River in the north central region of Trinidad

(Refer To Figure 1.0), it forms a 35,000 acre feet reservoir which serves as the main raw water

storage facility for Trinidad, augmenting the dry season. Water from the Arena is impounded

during the wet season for release systematically during the dry season .The Arena River

provides about 40% of the water needed to fill the reservoir , the remainder is obtained from

the Tumpuna River. A weir is located downstream of the confluence of the Arena and Tumpuna

Rivers backs water up the Arena River channel to the dam toe .Consequently During the wet

season a pumping station at the base of the dam pumps water back into the reservoir and

During the dry season water is released through the outlet works and flows down the Arena

and Tumpuna Rivers to the Caroni River an there it is withdrawn for water treatment. This

system allow for an approximate 75 million gallons per day of treated drinking water year

round about half of the island supply.

The earth fill embankment is approximately 1.6 million cubic yards and

has a crest elevation of about 80 feet above the original streambed (Refer to Figure 2.0 & 3.0).

The upstream sloping core is composed of dispersed clay, and the shells are composed of

compacted fine sand and silty fine sand. The Arena Dam is built upon deep, stiff, fissured clay

soil interbedded with sand .The Dam is located approximately 12 miles from the El Pilar Fault (a

major Caribbean fault with seismic activity which can be compared to that of the San Andres

Fault).At the dam Site, a flood plain approximately 800 feet wide lies at an elevation of an

approximate 73 feet above the mean sea level. This flood plain is underlain by up to 50 feet of

alluvium. Prior to construction, the Arena River flowed in a channel approximately 20 feet deep

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in the flood plain. The reservoir consist of rolling hills with relief generally ranging from 150 feet

to 200 feet.

Figure 1.0 showing the vicintiy map

Figure 2.0 showing Maximum Dam Section

Figure 3.0 showing the outlet conduit profile

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PUMPED STORAGE CONCEPT

The principle governing the pumped storage concept is fairly simple, it utilized

gravitational potential to store energy .In perspective there are two bodies of water, one

located at a higher elevation than the other, and a system of tunnels and piping connects them

both. When the demand is low (referred to as the off peak time) and electricity is cheap the

plant uses energy to pump water from the lower reservoir to the upper reservoir (Refer To

Figure 5.0). Subsequently, when Demand is high (known as the Peak Time ) and electricity is

more expensive water from the elevated reservoir is released back into the lower reservoir

through the same system of pipes , at this point turbines as they would normally in a traditional

hydroelectric plant generates electricity (Refer To Figure 4.0) . The system is mainly housed

within man made caverns inside mountains to reduce the environmental impacts. This type of

plant actually has a net use of energy, the advantage come from the fact that once the facility is

operational it can respond quickly to energy demands. Nearly all facilities use the height

difference between the two reservoirs or natural bodies of water, however there is a slight

difference , a “pure pumped “storage plant just shifts the water between the reservoirs while

the “pump-back “approach is a combination of pumped storage and conventional hydroelectric

plants that use a natural steam-flow. When taking into account evaporation losses from the

exposed water surface and conversion loses, 70 to 80 percent of the electrical energy used to

pump the water form to lower reservoir to the higher reservoir can be regained .This technique

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is currently one of the most effective means of storing large amounts of electrical energy

however capital cost and the appropriate geography are to be taken into careful consideration.

Figure 4.0 showing the generation of electricity when demand is high .

Figure 5.0 shows the storage of energy by pumping water to elevated reservoir

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WATER TURBINES

A turbine extracts energy from a fluid which possesses high head .In general there are

two types, reaction and impulse, the difference lies in the manner of head conversion. In the

reaction turbine, the fluid fills the blade passages, and the head change or pressure drop occurs

within the impeller. Reaction Designs are of the radial flow, mixed flow and axial flow types and

are basically dynamic devices designed to convert the high energy fluid to a form of momentum

.An impulse turbine first converts the high head through a nozzle into a high velocity jet, which

then strikes the blades at one position as they pass by. The impeller passages are not fluid –

filled, and the jet flow past the blades is basically at constant pressure.

REACTION TURBINES

Newton’s third law is used to describe the transfer of energy for reaction turbine.

Reaction turbines are low head, high flow devices, the flow is opposite that in a pump entering

at the larger diameter section and discharging through the eye after giving up most of its

energy to the impeller .The first inward flow turbine was built by James B. Francis an now all

radial or mixed flow designs are now called Francis turbines however ,at lower heads a turbine

can be designed with only axial flow which is termed as a propeller turbine , the propeller may

either be fixed blade or adjustable which is known as the Kaplan turbine .

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THE FRANCIS TURBINE

These are adaptable to varying heads and flows and may be run in reverse as a pump

such as on a pumped storage set up. They operate in a water head from 10 to 650 meters and

are normally used for electrical power production, the speed range of the turbine is from 83 to

1000 rpm. These turbines are most time mounted with the shaft vertical to keep water away

from the attached generator and to accommodate installation and maintenance access to it and

the turbine. The turbine is noted to be an inward flow device as said before with water entering

around the periphery and moving to the center before exhausting .the rotor is contained in a

casing that spreads the flow and pressure evenly around the periphery.

Figure 6.0 shows a typical large Francis turbine in which water is fed radially to the runner from guide vanes

which are disposed around the full circumference

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THE KAPLAN TYPE TURBINE

The Kaplan turbine is a pure reaction turbine, the main feature is that all the flow energy

and pressure is exhausted over the rotor and not in the supply nozzle .Kaplan turbines are more

suited for low pressure heads and large flow rates such as on dams and tidal barrage schemes.

Figure 7.0 Shows details of a large Kaplan turbine through which the water flow is essentially axial.IMPULSE TURBINES

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The impulse turbine utilizes the concept of Newton’s Second law and is suitable in

applications requiring high head and relatively low power. Impulse turbines changes the

direction of flow of a high velocity fluid, the resulting impulse then spins the turbine and leaves

the fluid flow with a lower kinetic energy than its initial. Before reaching the turbine the fluid’s

pressure head is changed to velocity head by accelerating the fluid with a nozzle, because of

this these turbines do not require a pressure casement around the rotor since the fluid jet is

created by the nozzle before reaching the blade on the rotor which normally has an elliptical

split-cup shape .the Pelton wheel is an example of an impulse turbine and was named after

Lester A. Pelton who produced the first efficient design.

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Figure 8.0 shows a typical design of A Pelton Wheel

DIFFERENCE BETWEEN IMPULSE AND REACTION TURBINE

N.O Impulse Turbine Reaction Turbine

1. The fluid flows through the nozzle and impinges on the moving

The fluid first flows through the guide mechanism and then through the moving blades

2. The fluid impinges on the buckets with kinetic energy

The fluid glides over the moving vanes with pressure and kinetic energy

3. The fluid may or may not be admitted over the whole circumference

He fluid pressure is reduced during its flow through the moving blades

4. The blades are symmetrical The blades are not symmetrical

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REFERENCES

Websites

https://mospace.umsystem.edu/xmlui/bitstream/handle/10355/34526/P%200651-%20Design

%20and%20Peformance%20of%20Arena%20Dam.pdf?sequence=1

http://www.technologystudent.com/energy1/pstr1.htm

http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity

http://www.mech.uq.edu.au/courses/mech7350/lecture-notes-in-pdf/mech7350-10-hydraulic-

turbines.pdf

http://www.freestudy.co.uk/fluid%20mechanics/t8a203.pdf

http://large.stanford.edu/courses/2010/ph240/boysen2/

Books

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Fluid Mechanics Frank M White 4th edition

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