Hydroelectric Power Systems

hyro electric dam
Hydro electric dam

Hydroelectric power plants convert potential and kinetic energy of water into electricity.

In hydroelectric production, potential energy of water is created by the construction of large dams. The dams hold the water at a greater height than the water level at the turbines.

Water is the greatest contributor of renewable energy to the world electricity generating system.

The common unit of measurement used to indicate the installed capacity of a hydroelectric power station is Megawatts.

Types of hydroelectric power plants

Based of design, hydro electric power plants are broadly divided into 2 types;

  1. Impoundment power plant
  2. Diversion power plant.

In the impoundment type of power plant, All water coming into the plant is dammed. The dammed water creates the head needed to power the hydro plant.

Impoundment  type of Hydro dam
Impoundment type of Hydro dam

In the diversion type, the water coming to the plant is partially dammed, some water is allowed to flow back into the river immediately while the rest is used to power the hydro plant.

Head and tail water

 In a hydro power plant system, the dammed water upstream is known as Head water and the water on the other side of the dam is known as Tail water. The difference between the top of the head water and that of the tail water is called pressure head.

Components of hydroelectric power plant

Trash rack; when water is coming fresh from the dam it needs to be cleared of big debris such as wood , plastic bags etc. the duty of the trash rack is to remove these.

Inlet gate; after water have been processed to remove debris, it then passed through the inlet gate. The inlet gate is used to regulate water flow into the turbines

Water conductors; water conductors are equipment used to convey water within the hydo electric power system. Example include; penstock, draft tube, scroll case, spill way, canals etc.

Penstock; the main pipe that conveys water to the turbine is known as penstock. It is a pressurized water conductor.

Spill way;  a spillway is a water conductor used to prevent overflow of dammed water

Runner; the runner is part of the turbine system and it rotates as water from the penstock passes through it at high pressure

Power house; the power house houses the turbine, electric generators, transformers and other electric generating equipment

Open air switch yard; when the electrical transmission equipment is installed in open air, it is called an open air switch yard.

How a hydroelectric turbine works

Water with high pressure head on the upstream side of the dam is lead through large penstock pipe. Before the water get to the turbine it passes through a scroll case. The scroll case is a large curved pipe that helps the flowing water to efficiently rotate the turbine. The turbine is connected to the generator through a shaft. so  as the turbine rotates, the connected generator also rotates  producing electricity.

Advantages of hydroelectric power plant.

  1. It is a completely green process
  2. It is a very efficient way of generating electricity.

Disadvantages of hydroelectric power plant

1. It is always necessary for settlement around the dammed area relocate. This is because water required to run hydroelectric plants is extremely large and dammed water will cause flooding of the settlement.

2. It is capital intensive. This is because the hydro dams constructed are extremely large.

Types of hydro power plants

  1. Impoundment type power plant. This type of power plant utilizes a dam to impound large amount of flowing river water. The dammed water is the potential energy necessary to power the hydro plant. The different type of dams that could be used include; gravity, reinforced concrete, buttress, rock filled or earth dams.
  2. Diversion type power plant( run of the river). In this case, the river water is partially dammed, only part of the flowing water is utilized to produce electricity.
  3. Tidal water plant. This process works based on tidal action of water. it helps create a height difference on either side of the turbine, this pressure difference cause water to flow through the turbine rotating it.
  4. Pumped storage power plant. This type of plant basically consists of very large reservoir of water held at a very high level. The stored water is then made to flow downwards through a penstock pipe to run the turbine and generate electricity. After reservoir is depleted, the water is pumped back again and the whole process is repeated. Pumped storage power plant is a type of standby power generator.Its acts as a supplement and is usually made available to the national grid during peak hours

Types of turbines

Broadly of two types;

  1. Reaction turbine
  2. Impulse turbine

The reaction turbine are pressure turbines. When water flow through this turbine, it creates a pressure differential which causes the turbine to rotate.

Impulse turbine are pressure-less turbines. It rotates due to the kinetic energy given to it by a jet of water as its hits the blades.

Types of turbine runners

There are many types of turbine runners, however, the 3 most commonly used are;

  1. Kaplan runner
  2. Francis runner
  3. Pelton runner

Kaplan runners are types of turbine runners that can operate with medium to high water flows and efficient at low pressure heads. Kaplan runners are reaction  runners because the pressure difference of the water flowing through the blades causes it to rotate.

Kaplan runner
Kaplan runner

Francis runners  are reaction runners and can operate within a wider range of pressure heads than the Kaplan runners.

Francis runner
Francis runner

Pelton runners are only adequate for very high flows and operates within a narrower range compared to the Kaplan and Francis runners.

pelton runner
Pelton runner

How to estimate the number of Blocks required for wall construction

On a construction site it may become necessary to estimate the number of blocks required for the construction of a wall. To do it correctly you will need to read the construction plan or building plan and section drawing. From the building plan you will need to know;

  1. The length of the wall
  2. The height of the wall
  3. The thickness of the wall
  4. Window or door spaces within the wall unit.

In most cases, the thickness of the wall is not required unless the wall thickness is greater than the width or thickness of the block.

How to estimate the number of blocks using AREA METHOD

This method requires that you know the number of blocks required to construct 1m2 of wall. For a normal block size of length 450mm, height 225mm, and width 225mm, with thickness of wall the same as the block, there is approximately 8 blocks per 1m2. Simply put; 8 blocks per m

The above result was obtained practically, however other calculations report a value of between 8-10 blocks per m2 .

How to practically obtain the number of blocks per unit area

calculating the no of blocks per square meter

You can obtain the result practically, look for an already constructed wall of similar type you want to build. The wall should still show the exposed block work, or is not yet rendered. Then with a builders square or builder’s level, draw a square of 1m dimension on the wall using a chalk or erasable material. The square you draw should have an area of 1m2 .Next, count the number of blocks that fall within the square. The result you get is the number of blocks per square meter. This method works for any size of block or brick.

Now that you have the amount of blocks per m2 , look at the building and section plan then calculate the area of the wall using the area formula of length x height .  From this wall area, deduct any area of door and window spaces. Finally multiply the wall area by number of blocks per unit area or per m2 . (note the unit)

Formula for number of blocks required = (Area of wall) X (no. of blocks per unit area)

Simple example;

length and height of wall to calculate area

A wall of length 5m and height 3m will have an area of

5m X 3m = 15m2

Therefore number of blocks = 15m2 X 8 = 120 Blocks.

How to estimate the number of blocks using DIMENSION METHOD

This method involve using the block dimensions to estimate the number of blocks required for a wall construction.

Lets assume you are required to estimate the number of blocks in foundation required to fence a plot of land. Block will be laid in stretcher bond. The height of wall in foundation being about 450mm. and we are to use block size of length 450mm, height 225mm, and width 225mm. The dimensions of the plot of land being 30m by 15m. Here is what you do;

Perimeter of plot of land;

2 X (30+15)m = 90m

Since the wall will be constructed in stretcher bond, you will just divide the perimeter of the plot of land by the length of the block;

90m / 0.45m = 200 blocks          ( note; 450mm is converted to m by dividing it by 1000)

This 200 blocks is the approximate number of blocks required for the 1st layer in foundation.

Since the height of wall in foundation is 450mm, it means we will have 2 layers of blocks in foundation i.e. 450mm / 225mm (height of block) = 2

Hence total blocks required for foundation is 200 blocks X 2 = 400 blocks

Load Combinations in Reinforced Concrete Design.

Load combination.

The load combination for reinforced concrete design is 1.4Gk + 1.6Qk where Gk and Qk represent dead and imposed loads respectively. 1.4 and 1.6 is the factor of safety for dead and 1.6 is the factor of safety for imposed loads.

Other local combinations include; 1.2 Gk+1.2Qk+1.2Wk where Wk represents wind load.

For full information on load combinations see table 2.1 page 9 of BS 8110 part 1, 1997.

In reality however, various load combinations of dead, imposed and wind loads should be considered. This is to ensure that the structure is designed for the worst possible case.

Load cases:

While it is ok to load all parts of the structure using 1.4Gk + 1.6Q, it is also to be appreciated that theoretically, these loads can vary i.e from 1.0 Gk to 1.4 Gk and 0.0 Qk to 1.6 Qk, in the light of these, various load cases need to be considered to achieve an accurate design.

For example, assuming we are working on a 3 span beam with Gk of 25kN/m and Qk of 10kN/m. It is good practice to consider the following load cases;

Case 1: loaded even spans with 1.4Gk + 1.6Qand odd spans with 1.0Gk.

Even Span Loaded 1
even span loaded 2

Case 2: loaded odd spans with 1.4Gk + 1.6Qand even spans with 1.0Gk.

odd spans loaded 1
odd spans loaded 2

Case 3: All spans with 1.4Gk + 1.6Q 

All spans loaded 1
All spans loaded 2

Each load case is analyzed for bending moments and shear forces.

Bending moment diagrams for each load case. (all values in kNm);

CASE 1. (Even spans loaded):

Moment diagram for even span loaded

CASE 2. (Odd spans loaded):

moment diagram for odd spans loaded

CASE 3. (All spans loaded)

moment diagram for all spans loaded

The results of load cases are then compared. So for any beam span, the highest value of bending moment and shear forces of the 3 load cases is taken for design.

Final Result.

Final result

Reference.

Mosley & Bungey, Reinforced concrete design. 5th Ed. Page 30.

BS 8110 part 1, 1997.