Hydropower, or hydroelectric power, is the most common and least expensive source of renewable electricity. Hydropower technologies have a long history of use because of their many benefits, including high availability and lack of emissions.
Hydropower is fueled by water, so it is a clean fuel source. Hydropower technologies use flowing water to create energy that can be captured and turned into electricity. Hydropower doesn’t pollute the air like power plants that burn fossil fuels, such as coal or natural gas.
Hydroelectric Power Plant Working
The system consists mainly of a reservoir, a penstock, a turbine, and a generator.
If the water is allowed to flow to a lower elevation through the penstock, the potential energy of the water is converted into KE where some of it is captured by the turbine. After passing through the turbine, the water exits to the stream at the lower elevation. The turbine rotates due to its acquired KE from the flow of water, thus rotating the generator and generating electricity.
Water flows from the upper reservoir down a tunnel called a “penstock” through adjustable gates called “wicket gates,” then through the turbine, and finally exiting into the lower reservoir.
Imagine the unit initially at standstill, and the generator disconnected from the grid. We open the wicket gates a little, so the water begins to accelerate the turbine and generator rotor.
Since the stator current is zero, the electromagnetic developed torque is zero, and the turbine torque is used for pure acceleration. When we reach synchronous speed (say 300 rpm), we adjust the generator field until the generator voltage matches the grid voltage.
At this point we close the generator circuit breaker, connecting the generator to the grid, a process called “synchronizing the generator.”
After synchronizing, we continue to open the wicket gates, which applies more torque to the turbine blades, and hence the generator rotor, which accelerates slightly above 300 rpm.
The speed of the rotating stator field is determined by the grid frequency, which is determined by the combination of all of the synchronous machines in the external system. Hence, the terminal grid behaves like an “infinite” second synchronous machine. Therefore, the generator rotor will stabilize at a speed of 300 rpm.
Hydropower as Energy Storage Facilities
Dammed hydropower projects can also be built as energy storage facilities. During periods of peak electricity demand, these facilities operate much like a traditional hydropower plant. The water released from the upper reservoir passes through turbines, which spin generators to produce electricity.
However, during periods of low electricity use, electricity from the grid is used to spin the turbines backward, which causes the turbines to pump water from a river or lower reservoir to an upper reservoir, where the water can be stored until the demand for electricity is high again.
Equation of Output Electrical Energy
The KE entering the turbine KEt is given by
Where As is the sweep area of the turbine’s blades in one revolution. Thus, the power entering the turbine Pt is
The PE of the water behind the reservoir PEr can be found from
PEr = WH = mgH (Equation 3)
Where W = the weight of the water, kg; H = the elevation of the water with respect to the turbine, m; m = the water mass of the reservoir, kg; g = the gravitational speed, m/s.
If m (kg) is the mass of the water entering the penstock, Eq. (3) can be applied to find the input PE of the penstock as given by
PEp-in = mgH (Equation 4)
The water flow fw inside the penstock is defined as the mass of water m passing through the penstock during a time interval t.
That is, fw = m/t (Equation 5)
Substituting fw into Eq. (4) gives
PEp-in = fwt gH (Equation 6) which is converted to kinetic energy inside the penstock.
Applying Eq. (1), the output KE of the penstock KEp-out can found from
Where t = the duration of water flow, s; v = the speed of water exiting the penstock, m/s; Ap = the cross-sectional area of the penstock, m2 .
Due to losses inside the penstock such as water friction, the KE exiting the penstock KEp-out is less than the PEp-in at the entrance of the penstock. Thus, the efficiency of the penstock is given by
ηp = KE p-out/PE p-in (Equation 8)
The blades of the turbine cannot capture all of the kinetic energy KEp-out exiting the penstock. The ratio of the energy captured by the blades KEblade to KEp-out is known as the power efficiency Cp.
Cp = KEblade/KEp-out (Equation 9)
The energy captured by the blades KEblade is not all converted into mechanical energy entering the generator KEm due to the various losses in the turbine. The ratio of the two energies is known as turbine efficiency ηt .
ηt = KEm/KEblade (Equation 10)
The output electric energy of the generator Eg is equal to its input kinetic energy KEm minus the losses of the generator. Thus, the generator efficiency ηg is defined as
ηg = Eg/KEm (Equation 11)
Using Eqs. (8) through (11) in Eq. (6), we can write the equation of the output electrical energy as a function of the water flow and head.
Eg = fgHt(Cpηpηtηg) (Equation 12)
Classification of Hydroelectric Systems
Hydropower technologies can be classified into three types:
- Small Scale
- Large scale
Micropower: The micropower hydro, which is often known as the run of the river hydro, is used for producing power up to 100 W. The micropower hydro does not require large impoundment dams, but it may require a small, less obtrusive dam. A portion of a river’s water is diverted into a canal or pipe to spin turbines.
Small Scale: The small hydroelectric systems can produce power ratings, up to a few megawatts. There are two versions of hydroelectric systems: diversion-type and reservoir-type.
The reservoir-type may require a small dam to store water at higher elevations. The diversion-type does not require a dam, and relies on the speed of the current to generate electricity.
The diversion-type small hydroelectric system does not require a dam, and therefore is considered more environmentally sensitive. The river for this small hydroelectric system must have strong enough current for a realistic power generation.
Large-scale: Large scale hydropower plants are generally developed to produce electricity for government or electric-utility projects. These plants are more than 30 MW in size.
Advantages and Disadvantages of Hydroelectric Power
Hydropower is fueled by water, so it is a clean fuel source. Hydropower doesn’t pollute the air like power plants that burn fossil fuels, such as coal or natural gas.
The popularity of hydropower systems is due to their mature and proven technology, reliable operation, suitability for sensitive ecology, and capability to produce electricity even in small rivers.
The impoundment hydropower creates reservoirs that offer a variety of recreational opportunities, notably fishing, swimming, and boating. Most hydropower installations are required to provide some public access to the reservoir to allow the public to take advantage of these opportunities. Other benefits may include water supply and flood control.
Many large-scale dam projects have been criticized for altering wildlife habitats, impeding fish migration, and affecting water quality and flow patterns. The new hydropower technologies can reduce these environmental impacts through the use of fish ladders (to aid fish migration), fish screens, new turbine designs, and reservoir aeration.
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