Energy, Power, and Climate Change 8.6 Hydroelectric Power We can divide hydroelectric power production into two groups: sun-derived (gravitational energy), and moon-derived (tidal energy). THE HYDROELECTRIC DAM The traditional hydroelectric dam uses water at a high potential energy to drive a turbine as the water descends under the gravitational pull to a lower level: © 2006 By Timothy K. Lund

Energy, Power, and Climate Change 8.6 Hydroelectric Power We can divide hydroelectric power production into two groups: sun-derived (gravitational energy), and moon-derived (tidal energy). THE HYDROELECTRIC DAM Calculate the energy yield for the water shown in the reservoir below: Since each m3 of water has a mass of 1000 kg, we have the total mass of water in the reservoir given by © 2006 By Timothy K. Lund m = 1000(1000)(2000)(25) m = 51010 kg Since each kilogram of water drops an average of (75 + 100)/2 = 87.5 meters, the total potential energy relinquished is given by U = mgh = (51010)(10)(87.5) = 4.381013 J

Energy, Power, and Climate Change 8.6 Hydroelectric Power We can divide hydroelectric power production into two groups: sun-derived (gravitational energy), and moon-derived (tidal energy). THE HYDROELECTRIC DAM If the water flow rate is 10 m3 per second, what is the power provided by the moving water? Since each m3 of water has a mass of 1000 kg, we have the total mass of water each second given by © 2006 By Timothy K. Lund m = 1000(10) m = 10000 kg/s Since each kilogram of water drops an average of (75 + 100)/2 = 87.5 meters, the total energy per second is given by U = mgh = (10000)(10)(87.5) = 8.75106 J FYI: Since 1 J / s is a watt, the power provided by the water is 8.75 MW.

Energy, Power, and Climate Change 8.6 Hydroelectric Power FYI: At night, when power demand is at its lowest, power plants can keep running and use excess power to pump water into a dam's reservoir. This has the effect of storing energy for future use. This is called the PUMPED STORAGE SCHEME. Energy, Power, and Climate Change 8.6 Hydroelectric Power We can divide hydroelectric power production into two groups: sun-derived (gravitational energy), and moon-derived (tidal energy). THE HYDROELECTRIC DAM If the water flow rate is 10 m3 per second and the reservoir is NOT replenished, how long will it provide power? The total volume of water in the reservoir is given by V = 1000(2000)(25) © 2006 By Timothy K. Lund V = 5107 m3 5107 m3 1 s 10 m3 = 5106 s 1 h 3600 s 1 d 24 h = 579 d FYI: Unlike nuclear and coal power plants, hydroelectric plants can be turned off and on just by controlling the water flow.

The Hoover Dam in Colorado can generate 1.5109 watts. © 2006 By Timothy K. Lund

Energy, Power, and Climate Change 8.6 Hydroelectric Power We can divide hydroelectric power production into two groups: sun-derived (gravitational energy), and moon-derived (tidal energy). THE TIDAL BARRAGE In some rivers and on some coastlines, the tidal range (the difference between high and low tide) can be up to 10 to 15 meters. A dam, called a barrage, is built across the bay, or river, and takes advantage of a two-way turbine scheme: Once when the dammed water is rising, and once when it is falling. © 2006 By Timothy K. Lund

The tidal barrage in Rance, France. After the tide has brought water into the estuary it is held back and slowly released through 24 turbines producing 240 MW. The tidal range is up to 13.5 meters at his location. © 2006 By Timothy K. Lund