In this post we are going to be covering Hydro power hydel energy. Both terms imply extraction of potential energy from running fresh water or falling water. The term is also connected mainly to fresh water not seawater. The process involves creating a barrier in the path of flowing water. The water is than channelled and made to drop, where its potential energy converts into kinetic energy and this energy is then extracted by means of a turbine.
The scale of hydropower is extremely vast. Energy can be extracted from a few watts to Gigawatts. In fact the world’s largest power plant is a hydro energy project called the Three Gorges Dam in China It has a nameplate capacity of 22 GW.
Large scale hydro projects cannot just power towns and cities, but whole country. For example, the “3 Gorges Dam” has the capacity to produce 23 GW of Power. In comparison, Pakistan requires only 19 GW at the most demanding of times.
Scale of Hydro Power
The scales of hydro power are as follows:
Pico Hydro Energy = 5 kW
Micro Hyrdo = 5KW to 100 KW
Small Scale Hydro = 10 Megawatts or less
Anything above 10 Megawatts is rated as Large Scale Hydro
There are many countries that are heavily reliant on hydropower. For example Norway gets more than 98% of its energy through hydropower. Even to this day, Hydropower is one of the cheapest modes of energy generation. Furthermore the energy extracted is renewable energy. Water is constantly replenished by nature because of the water cycle. There are no CO2 emissions when converting water’s kinetic energy into electricity. This does not however mean, that hydro energy does not have any negative environmental impact as will be explored later in the video.
Burns, streams, rivers all have different levels of water flow. The flow rate of fresh water also varies across the year. And therefore to extract energy, there a different types of turbines. Some more suitable than the others depending upon the head of water and the flow rate.
Head of water is the measure of hydrostatic energy of water. It is simply the height of water above a certain point. For hydro power it is the measure of water height that will be available above the turbine. And there is also the flow rate. The flow rate is the volume of water crossing a certain point in a second.
Based on the head of water and the flow rate, the turbines type is selected.
Types of Turbines
There are four major types of turbines. They are as follows.
- Kaplan Turbine
- Francis Turbine
- Cross flow turbines or Banki Turbines
- The Pelton wheel.
The chart here shows the operating envelopes of all three turbines based on flowrate and head of water.
For example when there is high head and low flow rate than Pelton wheeel is used. On the other hand when the head of water is low and flow rate is high, than Kaplan turbines are used. Pelton wheel is a an impulse turbine while Kaplan turbine is a reaction turbine. There is also Frances turbine which falls in the middle. Francis turbines are used for medium head levels and medium flow rate. The operational envelope of Frances turbine is very wide. It is a hybrid turbine in that it utilizes both impulse of force and reaction of force. Another advantage of Frances turbine is that it can also act as a pump for pumped storage systems which will be discussed later. Note that both Kaplan and Frances turbines can reach operational efficiency of over 90%, making them one of the most efficient renewable energy devices.
Lastly there is the cross flow turbine or the banki turbine. It is a slow moving machine which is well suited for locations with a low head but high flow. Being a slow mover the turbine is easier to maintain as the bearings don’t need to be replace often. Furthermore the turbine is self-cleaning and gets clogged up less frequently compared to other turbines. Compared to Frances and Kaplan turbines both cross flow turbines and Pelton wheels have lower efficiency.
There are other variety of turbines also available particularly at the microscale. There are turbines that can even run in the shallowest and slowest of running water (as slow as 2 mph). For example motor rotor can utilize water as slow as 2 mph.
One of the advantage of hydropower is their usage as pump storage hydro projects. The idea of these projects is to meeting the high electricity demand during peak times. It should be noted that our electricity requirements change through the day. Pump storage provides and energy buffer. During the times of lull when energy is not required by a hydro power plant, water is pumped back from lower elevation downstream to higher position upstream. This is done by using grid electricity and running the turbine in reverse which allows it to act as a pump. Water pumping increases the reservoir capacity and the water head. This water is then used during the peak energy demand times, which occur during the day. The water that was pumped back can then be utilized alongside the existing and incoming water to run the turbines to their peak electricity.
Disadvantages of Hydro Power
Despite the many positives of hydro power, there are also a few negatives.
To harness hydro power particularly at large scale, dams have to be constructed that come at a huge cost.
Firstly creating a barrier across flowing water floods the upstream land. This means natural environment is destroyed and the habitat of not only plants and animals is affected but also people living in the vicinity have to be moved.
There are many fish species that spawn upstream in the river. The building of a dam blocks their pathway, although by providing a fish ladder, this problem can be alleviated to a large extent. Similarly many fresh water fish species cannot migrate downstream.
Accumulation of silt, debris and dead wood can reduce the capacity overtime. Furthermore, decaying plantation that is stagnates upstream of the barrier also produces emissions.
Building a large dam can also alter the natural water table level.
It should be noted that due to environmental legislation, it is difficult to build large scale hydropower projects in the modern times. The best practice is therefore have several small scale run of the river projects than a few large ones. In run of the river project, portion of the water slow from a river or stream is channelled into pipes, that transport the water to a powerhouse. A turbine is located in the power house that utilizes this siphoned water to generate electricity. The powerhouse is located at a much lower elevation than the point at which the water is bled from the river or stream.
Run of the river schemes are relatively environmentally friendly, although their ROI is lower than large scale projects. The other advantage of Run of the river schemes is that they are relatively easy to build and commission within days or weeks as opposed to large scale projects that can take several years.
Energy can be very efficiently extracted from water. For example if Kaplan or Frances Turbines used almost 80% of the energy can be converted.
Likewise if Francis turbine is used, than more than 90% of energy can be extracted.
The total amount of power that can be extracted by a turbine can be determined by a simple formula.
- P P is power in watts
- η is the dimensionless efficiency of the turbine
- ρ is the density of water in kilograms per cubic meter
- Q is the flow in cubic meters per second
- g is the acceleration due to gravity
- h is the height difference between inlet and outlet in meters
P = η * ρ * Q* g* h
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