renewable energy

1. CHAPTER 14 –Hydroelectric Energy
Introduction
Hydro energy (from “hydro” meaning “water”) is energy that comes from the
force of moving water. This force of moving water or the energy of elevated
water can be significant. The fall and movement of water is part of a continuous
natural cycle called the water cycle. The cycle consists of four basic steps :
1. Energy from the sun evaporates water in the Earth’s oceans and rivers and
draws it upward as water vapor.
2. When the water vapor reaches the cooler air in the atmosphere, it condenses
and forms clouds.
3. The moisture eventually falls to the Earth as rain or snow, replenishing the
water in the oceans and rivers.
4. Gravity then drives the water, moving it from high ground to low ground.

2. Introduction
Hydroelectricity is the term referring to electricity generated by hydro- power.
The production of electrical power “arises” using the gravitational force of
falling or flowing water. It is the most widely used form of renewable energy.
The major advantage of hydroelectricity is the elimination of the cost of fuel.
The cost of operating a hydroelectric plant is not affected by increases in the
cost of fossil fuels such as oil, natural gas, or coal.
These plants also have long lives, with some plants still in service after 50+
years.
Operating labor cost is also usually low since many of the plants are
automated and have few personnel onsite during normal operation.

3. Introduction
The equation describing electric power production at a hydroelectric plant is:
P = (ρqgΔh)η
Where:
P = power, W
ρ = density of water, 1,000 kg/m3
q = flow rate, m3/s
g = acceleration due to gravity, 9.8 m/s2
Δh = decrease in height, m
η = fractional conversion of efficiency, 0.0–1.0
Hydropower is the most economical way to generate electricity today. No
other energy source, renewable or nonrenewable, is comparable to it. It costs
less than one cent per kilowatt·hour (kW·h) to produce electricity at a typical
hydro plant.

4. Early History
The earliest use of hydropower was with water mills several thousand years
ago. The Greeks used water wheels to grind wheat into flour more than 2,000
years ago.
By the late nineteenth century, the electrical generator was developed and
could now be coupled with hydraulics.
At the beginning of the twentieth century, many small hydroelectric power
plants were being constructed by commercial companies in mountains near
metropolitan areas. Grenoble, France, held the International Exhibition of
Hydropower and Tourism with over one million visitors.
As the power plants became larger, their associated dams developed
additional applications that included flood control, irrigation, and navigation.

5. Availability/Distribution
Brazil, Canada, New Zealand, Norway, Paraguay, Austria, Switzerland, and
Venezuela are the only countries in the world where most of the internal
electric energy production is from hydroelectric power. Paraguay produces 100
percent of its electricity from hydroelectric dams and exports 90 percent of its
production to Brazil and Argentina. Norway also produces nearly 99 percent
of its electricity from hydroelectric sources
At the domestic level, hydropower today only represents approximately 7
percent of the electricity generated in the United States.

6. Characterization
A lake is located at the top of a mountain. A power plant has been constructed
at the bottom of the mountain. The potential energy of the water traveling
downhill to the base of the mountain is used to generate electricity. This is the
operating mode in the daytime during peak electrical demand. At night, when
demand is reduced, the water is pumped back up the mountain. Assume the
lake is located at an elevation of 3,000 ft above the power plant. The flow rate
of water is 500,000 gpm. The turbine efficiency is 30 percent.
In order to solve for the power in watts, convert the height and flow rate to SI
units:

9. Extraction
The turbines in a hydroplant are basically divided into two main classes:
impulse turbines and reaction turbines. These perform a continuous
transformation of both the potential and kinetic energy of the water into useful
work. Of these, practically only three types are now being utilized: the Pelton,
Francis, and Kaplan turbines. The Pelton turbine or Pelton wheel, the runner of
which is driven by a free jet of water and rotates in the air, belongs to the
impulse turbine class; in effect, it converts the available head into kinetic
energy by a contracting nozzle. The Francis and Kaplan turbines, whose
runners are subject to the complete flow of water flowing through them,
belong to the reaction turbine class; the Francis turbine is essentially a reversed
radial pump, whereas the Kaplan turbine is a reversed axial pump

10. Processing
Most hydroelectric power comes from the potential energy of dammed water
driving a water turbine and generator. The power extracted from the water
depends on the volume flow rate and on the difference in height between the
source and the water’s outflow (see Equation 14.2). This height difference is
referred to as the head. The amount of potential energy in water is
proportional to this head.
A typical hydropower plant is a system consisting of three parts:
1. A power plant where the electricity is produced
2. A dam that can be opened or closed to control water flow
3. A reservoir (artificial lake) where water can be stored

11. To generate electricity, a dam opens its gates to allow water from the reservoir
above to flow down through large tubes called penstocks. A penstock delivers
the water to the turbine. At the bottom of the penstocks, the fast-moving water
spins the blades of turbines. The turbines are connected to generators to
produce electricity.

12. A dam serves two purposes at a hydropower plant. First, a dam increases the
head, or height, of the water. Second, it controls the flow of water. Dams
release water when it is needed for electricity production. Special gates called
spillway gates release excess water from the reservoir during heavy rainfalls.
A tidal power plant makes use of the daily rise and fall of ocean water due to
tides; such sources are highly predictable and, if conditions permit
construction of reservoirs, can also be employed to generate power during
high demand periods.

13. Processing
Existing hydro plants have been described in three categories:
1. Small
2. Micro
3. Pico
• The definition of a small hydro project varies, with a generating capacity of
up to 10 MW generally accepted as the upper limit of what can be termed
small hydro.
• Micro hydro is a term used for hydroelectric power installations that
typically produce up to 100 kW of power.
• Pico hydro is a term used for hydroelectric power generation of under 5 kW.

14. Transportation/ Transmission
Pumped storage.
This method allows electricity to be supplied during high peak demands by
moving water between reservoirs at different elevations. At times of low
electrical demand, excess generation capacity is used to pump water into the
higher reservoir.
When there is higher demand, water is released back into the lower reservoir
through a turbine.
Pumped-storage schemes currently provide the most commercially important
means of large-scale grid energy storage and improve the daily capacity factor
of the generation system.
One of its biggest advantages is its ability to store energy on both a daily and
seasonal basis. Pumped-storage is a very reliable energy storage method.

15. Environmental Issues
Hydropower can potentially cause environmental problems. Several of these
problems are listed here:
1. Damming rivers may permanently alter river systems and wild- life
habitats.
2. Fish may no longer be able to swim upstream.
3. It may affect water quality by churning up dissolved metals that may have
been deposited by discharges from industrial operations.
4. It may increase silting, change water temperatures, and lower the levels of
dissolved oxygen.
5. It may destroy biologically rich and productive lowland and river valley
forests, marshland, and grasslands.

16. Environmental Issues
6. It can be disruptive to surrounding aquatic ecosystems—both upstream
and downstream of the plant site.
7. Water exiting a turbine usually contains very little suspended sediment,
which can lead to scouring of river beds and riverbanks.
8. Flowing water has the ability to transport particles heavier (with respect to
density) than itself downstream.
9. Reduced river flow rates due to drought, climate change, or other
diversions can reduce the amount of water that can be used for
hydroelectricity.

17. Environmental Issues
10. Hydroelectric dams may result in the relocation of the people living where
the reservoirs are planned or located.
11. Dam failures due to poor construction, terrorism, or other causes can be
catastrophic to downriver settlements and infrastructure. (Hurricane
Katrina is but one example.) Dams are an obvious target for wartime attack,
sabotage, and terrorism.
12. In time, silt will accumulate upstream of a dam, causing the storage
capacity to decrease.
It should be noted that most of these problems can be and have been
successfully managed by the technical community.

18. Future Prospects and Concerns
The technical community needs to increase the conversion into electrical
energy in the future. Hydropower plans should attempt to:
1. Utilize the total flow available
2. Increase the total height of the waterfall
3. Reduce any heat losses.
Finally, the procedure for licensing and relicensing dams is presently a lengthy
and expensive process.
Many environmental impact studies must be undertaken and it takes
anywhere from 3 to 7 years to obtain a license to build a hydroelectric dam or a
relicense to continue operations. Hopefully, this time period will be reduced in
the future.