Indirect Solar Power I: Hydroelectric

Indirect Solar Power I: Hydroelectric

Dr. HarrisPhys 105Ch 5.1-5.2

3/25/13

History of Hydropower

• The use of flowing water to perform work dates back thousands of years. Horizontal and vertical waterwheels in numerous variation were invented to reduce human labor.– The were used to mine, crush rock, grind corn and grain, saw wood, etc.

Hydrological Cycle

• As our nation developed, dams were built to impound flowing water.

• Turbines now replace waterwheels, and electricity produced by hydropower accounts for 9% of the power we use in this country.

• Water power is a consequence of the natural cyclical transport of water between the Earth’s surface and the atmosphere. – This hydrological cycle involves:

• evaporation of surface water when heated by sun light• precipitation• downward flow of water

• Essentially, water is the working fluid of an enormous heat engine powered by sunlight. Thus, hydropower is an indirect form of solar.

Hydrological Cycle

How A Dam Works

Horizontal Turbine at A Hydropower Plant

Energy Transformations

• Water stored in a dam possesses potential energy (Ep = mgh)– m is the mass (kg) of water, g is the acceleration of gravity (9.8 m/s2),

and h is the height difference (also called the “head”)between the reservoir water level and the downstream water level (m)

• When the gates open, gravity forces the water to flow downward. The potential energy is converted into kinetic energy

• The water spins the turbine, converting kinetic energy into mechanical energy, which then produces electrical energy.

• Hydropower is the most efficient form of electric generation, with a conversion efficiency of 90%

Example

• Hoover dam has a hydraulic head of 590 ft. The maximum flow rate through the penstock is 5.67 million liters of water per second. Given that the density of water is 1 kg/liter, calculate the theoretical maximum power, in GW, of the dam. (1 ft = 0.3048 m; )

[ 5.67 𝑥106 𝐿𝑠 𝑥 1𝑘𝑔𝐿 𝑥 9.8𝑚𝑠2

𝑥 590 𝑓𝑡 𝑥 0.3048𝑚𝑓𝑡 ]𝑥 𝐺𝑊109𝑊

𝑥 0.90

mass per sec g h

Ep per sec (W)

90% efficiency

9 GW

Power Losses

• In the previous example, I intentionally underlined the word “theoretical”.

• In actuality, a substantial fraction of the potential energy is lost as heat due to friction in the penstock

• There are also frictional and mechanical losses at the turbine.

Consumption of Hydropower

• Hydropower is widely considered to be the most important form of renewable energy because it is the most widely used. – 96% of the renewable energy consumed in the U.S. is hydropower.

• 19% of the world’s electricity is produced by hydropower

• China is the largest producer, followed by Canada, Brazil, and the U.S.

U.S. Consumption

• Fluctuations are caused by variations in annual precipitation.

Advantages of Hydropower

1. Hydropower uses the energy of running water, without reducing its quantity, to produce electricity. Therefore, all hydropower is renewable.

2. Hydropower plants don't release pollutants into the air. Currently, hydropower prevents the emission of GHG corresponding to the burning of 4.4 million barrels of petroleum per day worldwide.

3. There is still a significant capacity of untapped or under-utilized hydropower in this country.

Hydropower Potential in the U.S. (GW)

Region Potential Developed % Developed

New England 6.3 1.9 30.1

Middle Atlantic 9.8 4.9 50.0

East North Central 2.9 1.2 41.3

West North Central 6.2 3.1 50.0

South Atlantic 13.9 6.7 48.2

East South Central 8.3 5.9 71.1

West South Central 7.3 2.7 36.9

Mountain 28.6 9.5 33.2

Pacific 64.4 38.2 59.3

TOTALS 147.7 74.1 50.2

Advantages of Hydropower

4. Hydropower is more responsive than other forms of energy for meeting peak demands.– Demands for power vary greatly during the day and night. These

demands vary considerably from season to season, as well. – Nuclear and fossil fuel plants are not efficient for producing power for

the short periods of increased demand during peak periods (long startup)

– Hydroelectric generators can be started or stopped instantly, so it acts as a great supplement to nuclear and coal plants. This also provides energy stability and an immediate source of power after a blackout.• Additional power is supplied by water which is pumped uphill

during night hours (low demand) and stored in a reservoir. During peak demand, this stored water is released to generate power

Hydraulic Rams

• Water is pumped uphill using a hydraulic ram. This uses no electricity or fuel. The momentum of the water itself is used to force it uphill, a phenomena known as the “water hammer” effect

Advantages of Hydropower

5. Hydropower contributes to the storage of drinking and farming water. – Reservoirs collect rainwater, which can then be used for consumption or

for irrigation. This adds protection against floods and droughts.

6. Hydropower plants have very long lifetimes.

7. Dams are scenic and can provide a source of tourism revenue to a state. They also draw fisherman.– Over 1 million people toured the Hoover dam last year.

8. Hydropower is cheap– Hydropower is immune to rising fuel costs because it consumes none.

There is no waste to store or transport.

Disadvantages

1. Dam failure– The failure of a dam is catastrophic. The sudden release of massive

volumes of water from upstream can inflict massive damage and cause death.

– There have been numerous domestic dam failures over the last 100 years. The most recent was at Hope Mills Dam in NC (2010). Some have resulted in loss of life (# of deaths in parentheses). • Lower Otay Dam, San Diego, CA ; 1916 (40) • St. Francis Dam, Santa Clarita, CA; 1928 (600)• Baldwin Hills, Los Angeles, CA; 1963 (3)• Spaulding Pong, Norwich, CT; 1963 (6)• Buffalo Creek, West Virginia; 1972 (125)• Teton Dam, Idaho; 1976 (11)• Kelly Barnes Dam, Georgia; 1977 (39)• Lawn Lake Dam, Rocky Mountain National Park; 1982 (3)

Banqiao Reservoir Dam, China

• Heavy rains caused the dam to exceed capacity. 171,000 people died. 11 million were homes lost

Disadvantages of Hydropower

2. Hydropower is completely dependent on precipitation, which fluctuates vastly. Therefore, it is impossible to predict the annual availability of hydropower (see slide 13)

3. Soil Erosion. Dams hold back the sediment load normally found in a river flow, depriving the downstream of this. In order to make up for the sediments, the downstream water erodes its channels and banks. This lowering of the riverbed kills vegetation and river wildlife.

4. Disease. Dam reservoirs in tropical areas, due to their slow-movement, are literally breeding grounds for mosquitoes, snails, and flies. This leads to enhanced spread of disease.

Disadvantages of Hydropower

Disadvantages of Hydropower

5. Depletion of aquatic life and “flood-dependent” species– Native salmon return to the river where they were hatched to spawn

another generation of salmon. This cycle has been interrupted by dams because they block the migration path. Salmon are also killed by turbine blades.

– Salmon populations in the Pacific Northwest and Alaska are in serious decline

– Reservoirs cause drastic shifts in water temperature, oxygen and nitrogen content, and nutrient concentration. These conditions are not always suitable for neighboring ecosystems (http://www.treehugger.com/corporate-responsibility/water-released-by-dam-killing-100000-fish-a-day.html)• Many species of fish, bird and plant have become extinct due to

dams, as well as the loss of vast areas of forest and the loss of the land which was flooded to accommodate the reservoir.

Disadvantages of Hydropower

6. Due to EPA mandates and environmentalist protest, much of the available hydropower capacity of this country will never be used. Thus, significant expansion of hydropower many never actually occur.

7. Accumulation of sediment in reservoirs fed by rivers that flow over soil reduces the reservoir capacity drastically over time. Without expensive maintenance, the lifetime of the dam can be as low as 50 years.