Hydraulic Energy of Waterfalls and its Application in Electric Power Generation

Hydro-electric power is the power harnessed from the energy of falling water. The hydroelectric power plant can be regarded as one of the oldest source of electric energy in commercial quantity. It relies on the potential energy of water at a high level for the generation of electrical energy.

^{Static Pexels Free Image: Waterfalls}

It is situated where there is the highest fall or large quantity of water, i.e., at the bottom of a deep and steep-sided valley or gorge, or near the base of dam having large catchment areas. Dams can be of two types, viz: impounding (non-overflow) and spillway (overflow). If impounding dams are built, means must be provided to release excess flow, by providing spillway gates.Spillway dams are always concrete and for low head power stations.

A hydroelectric generating unit consists of steel penstock with motor operated steel headgates (water-inflow), water turbine (hydraulic turbine) to convert the water-energy into mechanical energy, an electric generator to convert mechanical energy into electrical energy and governor for regulating water pressure. After the water has given up its energy to the turbine, it is discharged through tailrace. The amount of energy available from water depends on both the quantity of water and its pressure at the turbine. The force depends upon the water head.

^{Wikimedia Creative Commons: Hydroelectric Dam}

It is an attractive source of electrical power because it is renewable and free. It is also one of the cleanest sources of power as it produces no smoke. Quite unlike in nuclear power plant that has dangerous waste material or by-products in the form of alpha, beta and gamma rays, hydro has no such waste, hydro has no such waste product as such quite safe and can be sited close to where people reside. Although, there is some some level of thermal pollution which depletes the level of oxygen thereby affecting aquatic life of plants and animals, its advantages more than compensate that single drawback.

Sun is the primary source of energy. As the sun shines, it touches down on the earth surface, seas, rivers, oceans. The heat energy of sun through the process of evaporation takes up part of the water in the earth surface and initiates the formation of cloud. This cloud will precipitate and can come back as rain, ice, sleet, dew, hail or frost.

^{Staticflickr Free Image: Water Cycle}

Based on the above fact, it is still in place to describe a hydroelectric power as a form of solar energy because the sun plays a significant role in renewing the water and placing it in a vantage position that gives the potential energy which we harness to generate the electricity.

The process of renewal starts by evaporating the water from the oceans with the cloud providing the mechanism of transport to the position where it later precipitates on the land masses, finding its way to the ocean through the river. It is when it travels in this way that we can tap the potential energy before it returns to the ocean.

Water stored in the dam has potential energy when it is approximately unleashed from its elevated reservoir, by so doing, the potential energy of water is converted to kinetic energy (energy of motion).It is in this way that the potential energy of water is first transformed into mechanical energy and then to electrical energy.

When you consider that large bodies of water are abundant in most places, you will wonder why these people still suffer from an epileptic power supply.

Is it that the technology available to harness this hydropower is not available to them? We will find out soon. To answer this question, we need to understand that the water must have the potential energy or kinetic energy before it can do any useful work. A stagnant pool with no head or current can hardly drive the water turbine.

Hydroelectric Power Generation

Hydroelectric power generation involves the storage of water, conversion of the hydraulic energy of water into mechanical energy in a water turbine and conversion of mechanical energy in an electric generator.

The first hydroelectric plant came into existence in the 1880´s and now constitutes more than 33% of the world´s installed capacity. Hydroelectricity is an important source of renewable energy and provides significant flexibility as a baseload power plant, peaking and energy storage applications (pumped water storage).

While the initial capital erection of the hydroelectric power plant is high, the inherent simplicity of the hydroelectric plants coupled with their low operating cost makes them quite attractive. Its low maintenance cost, long service life and high reliability, makes them a very cost-effective and flexible source of electricity generation.

Especially valuable is their operating characteristics of the fast response for startup, loading, unloading and its ability for load following other useful attributes include their ability to start without the availability of power system voltage (black start capability). It can also move from generation mode, to supply reactive MVAR (asynchronous mode) and to pump storage applications. We will review all these attributes in turn.

Planning of Hydroelectric Power Facility

Hydroelectric plants are located in geographic areas where they will make economic use of hydraulic energy resources. We must understand that the water must be placed in a position that gives it the potential energy. This potential energy is acquired by placing the water in a kind of overhead reservoir. In this case, we must have a lower reservoir where the water turbine will be mounted and awaits the falling water from the upper reservoir.

This energy could come from the use of a pump to push the water to the upper reservoir. If we resort to the use of pump, it then means that we will have to rely on an external generation or the grid to run the hydroelectric power plant (such adaptation gives rise to pumped water storage scheme). This approach is costly, and as such, it is rarely adopted as a means of providing power continuously throughout the day.

It is only used when there is an excess generation or during the period of light load. At this period, the excess power is used to pump the water to the upper reservoir pending when the peak load will arise. During the period of peak demand, the overhead water is released which drives the turbine.

The best alternative to getting water to the upper reservoir is to locate such plant in a hilly area. A typical hydroelectric power plant consists of a dam which raises the water surface of the stream to create a head. The elevated or high inland location allows the ease of construction of the dam with its associated vast water reservoir. The reservoir is to hold water to avoid power cuts in the event of drought and floods. This water head is created by constructing a dam across river beds, to channel the water towards the powerhouse and to create storage reservoirs.

The water head that can be achieved is dependent upon the topography of the area. A high head offers economy of power generation because it reduces the quantity of water that would be needed to produce a given power. This reduced quantity of water needed puts less need for large reservoirs and reduces the pressure on penstock, turbines, on the quantity of water each will handle, and it will go a long way to reduce the capital cost of construction of the plant.

Head represents the potential energy and the vertical distance through which the fluid falls in the energy conversion process.

The siting of a prospective hydraulic plant requires careful evaluation of technical, environmental, economic and social factors. A significant portion of the project cost may be set aside to lessen the impact it will have on the immediate environment where the site is located. Such effect could be on fish, wildlife and relocation of infrastructure and population from floodplains.

Hydroelectric Plant Scheme

There are four main types of hydroelectric plant arrangement, classified according to the method of controlling the hydraulic flow rate at the site.

Run-of-the water plant: it has a small amount of water storage and has very minimal control of flow through the plant. It uses the water as it comes and may be affected when there is drought or flood, which may lead to less generation.

Pondage (medium head type).

Storage plant (high head type): This type can store water in the reservoir. This is excellent control of water to the plant irrespective of time of the day or season of the year.

Pumped water storage plant: this type of power plant has a reversible turbine that can work as a pump to pump the water to the upper reservoir and a generator that can operate as a motor to drive the pump. The direction of the rotation of the turbine is reversed during the off period and is utilised in pumping water from the lower reservoir, thereby storing energy that will be used during the period of peak demand.

Pumped Water Storage

In pumped storage schemes, water is pumped uphill and stored in a reservoir until when the energy is needed at which time it is discharged through the turbine. The schematic is as shown below. Pumped storage scheme offer economic advantages where sites exist with suitable rainfall, and geological strata and the topography allows the construction of two reservoirs at different levels.

^{Wikipedia Creative Commons:Pumped Storage Plant}

The scheme is usually used to provide electricity during a period of peak demand. During off-peak periods especially at night when there is plenty of generation, excess electricity generated from other sources is used to pump water back into the storage reservoirs and such the scheme is one of the ways of storing electricity.

The operation of pumped water storage plants are quite similar to those of hydroelectric power plants as regards to time of engaging the units from idling; load pick up shedding rate and other characteristics of the equipment operation.

The principal feature of the PSP operation is in its pumping/ generating cycle which is the main factor affecting its efficiency. The PSP efficiency is determined by the ratio of electric power generated during the generating cycles to that consumed during the pumping cycle. The efficiency of up to 70-79 percent is obtainable from pumped water storage, meaning that the cost of [providing such energy is 1.7 times the cost of pumping.

During pumping, the reservoir is filled by consuming the energy of thermal power plants. Pumping is done during the period of light load. When the water is released to generate power, the power thermal power plants of the system are unloaded.

Operating heads make the other classification of a hydroelectric plant. Thus we have:

High-head plants (over 300m). High-head plants have a head which can exceed 300m. Pelton turbines are used for such high heads. The amount of impounded water is minimal in this case.

Medium head plants (30m-300m). Such head requires a more significant quantity of discharge as compared to high head plants. Francis turbine is used for such heads.

Low head plants (less than 30m). Kaplan or Francis turbine can be used in this type of head. This type of turbine is designed to handle large volumes of water.

Power and Energy of Water Flow

The power which can be obtained from any river or stream can be determined from the following variables:
The available head (H)
The rate of discharge of water (Q m³ /s)
The efficiency of the system ( ʅ_overall)

Q is the quantity of water that can be measured in either m³/s or litres/s. Assuming that this quantity of water falls through a height, (H) in meters from the upper reservoir to a lower reservoir at zero velocity, the work done by the falling water equals force × distance.

We know that force = mg
Where m = mass
g = acceleration due to gravity
mass, in this case, is equal to the product of the quantity of water and the specific gravity of water which is represented by symbol ρ
ρ is equivalent to 1000kg/ m³ for Q in m³/s
ρ is equivalent to 1kg/ litre, for Q in litres/s

Work done by the falling water = Q ρ g × H = ρQgH
Head (H) is the vertical distance through which water falls
Weight of the water in Newton = volume of water or discharge (Q) × ρ (specific gravity of water) × g (acceleration due to gravity)

Weight of water available in Newton (W) = Q ρ g

Electrical energy available in Newton = weight of water available in Newton (W) × head (H) × overall efficiency of the system ( ʅ_overall )× time (s)
Thus electrical available = W × H × ʅ_overall
In order words, the electrical energy available is Q ρ gH ʅ_overallt

We know that g has the value of 9.81 m²/s and p = 1000kg/ m³ (for pure fresh water)
The power available from the scheme in SI unit is given by

P = ρ g ʅ_overallQH

If E is the energy, then E = ρ g ʅ_overallQHt

Where the energy is in watt-hour

Substituting g = 9.81 m²/s and P = 1000 kg/ m³
( for pure fresh water), we can write the power in kilowatts may as;

P = 9.81 ʅ_overall QH…………….(1)

So much information can be deduced from equation (1) above:
The same power can be produced by a large discharge and a low head (which is typical of lowland rivers).

Thus, the potential generating capacity of any hydroelectric plant depends on:

The volume of water available for use
The regulation of water flow
The available head, that is, height of the waterfalls

The power generating capacity of pumped water storage plant with the reversible machines operating as turbines is same as in equation (1)

P = 9.81 ʅ_overall QH

In pumping the water from the lower pond into the upper pond, the power is given by

P_pump = 9.81QH/ ʅ_pump
Where ʅ_pump is the pumping efficiency

^{Wikimedia Creative Commons: Irrigation canal from a Dam}

At some time of the year, it can happen that the availability of water is low or when the generation is not required from the hydro plants, it may be advantageous to let the hydro plant operates as motor supplied from the power system. They now function as synchronous compensators.

To reduce the amount of power needed to drive the machine, the water is pushed below the turbine runner by compressed air. This feat is achieved by closing the water inlet valve and injecting compressed air, which drives the water toward the lower reservoir. The runner now rotates in the air and thus requires much less motive power in water.

Conclusion

Hydropower is one of the cleanest means of power generation with very low running cost. It is an abundant and emission-free renewable source of energy that is currently available. The only draw-back in its erection is the huge civil engineering work needed for the construction of the dam. It is more reliable when compared to solar and wind as it is not inherently intermittent, although droughts and floods can cause power cuts with a reservoir and good catchment area, such occurrence is most unlikely. Hydropower does not only produce energy, but with the construction of dams, it can also provide new drinking water sources, irrigation for crops, flood control and new waterways for transport.

Its low maintenance cost, long service life and high reliability, makes them a very cost-effective and flexible source of electricity generation. Especially valuable is their operating characteristics of the fast response for startup, loading, unloading and its ability for load following. Other useful attributes include their ability to start without the availability of power system voltage (black start capability). It can also move from generation mode, to supply reactive MVAR (asynchronous mode) and to pump storage applications.

References

1. Hydropower Plants For the Future
2. Planning of Hydropower Projects
3. Pumped Hydroelectric Storage
4. How Hydro energy Works