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How hydroelectric power works and why there’s debate over whether it helps slow climate change.
Edward C. Hyatt hydroelectric plant was forced to shut down due to low water levels in Lake Oroville reservoir.
Frank Schulenburg / Wikimedia Commons
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The changing climate has created an urgent need to switch to renewable, emission-free sources of energy—an ambitious goal that California, by state law, must reach by 2045. At the same time, California finds itself caught in a dangerous Catch-22 because climate change is also making the transition to clean energy more difficult to achieve. Climate change itself is making climate change harder to combat.
If that seems confusing, let’s narrow it down to one word—water.
Water is in short supply in California and throughout most of the western United States. More than 70 percent of territory in the 11 westernmost states was under at least “moderate” drought conditions as of August 2022, according to the U.S. Drought Monitor, with about 52 percent under “severe” drought.
What causes the drought? The factors are complex, but climate change makes droughts strike more often, with greater intensity, and with longer-lasting effects, according to the Center for Climate and Energy Solutions, an independent environmental policy think tank.
What does this have to do with clean energy? Drought means less water for electricity, and water-power—aka hydroelectric energy—produces about 17 percent of the world’s electricity. In California, however, due to seemingly endless drought conditions, hydro accounts for just 7.49 percent of energy generated in-state. That’s still more than the country as a whole, which gets 6.3 percent of its power from hydro.
However, California also pulls in out-of-state hydroelectric power, mostly from the Hoover Dam in Nevada, one of the six largest hydroelectric plants in the country. Operating at full capacity, Hoover Dam generates 2,074 megawatts of power. About half of that electricity goes to California.
Drought Drives Hydroelectric Down, Emissions Up
The western drought has lowered the Hoover Dam’s power output by 33 percent. And in 2021, California’s in-state monthly generation of hydroelectric power fell to 48 percent below normal, according to data from the U.S. Energy Information Administration. In 2021, Edward C. Hyatt hydro plant (pictured above) at Lake Oroville reservoir in Butte County was forced to shut down due to low water levels.
It was the first shutdown of the plant in eight decades, and experts say that more hydroelectric plants around the state risk going offline as drought conditions persist.
The drought-driven cutbacks in water-power cause pollution to go up. Hydroelectric is one of the lowest-emitting power sources when it comes to greenhouse gasses. A 2011 report by the World Nuclear Association found the average amount of greenhouse gas emissions from hydroelectric plants to be 26 tons for every gigawatt-hour (GWh) of electricity produced.
That emissions figure is small compared to natural gas, which emitted an average 499 tons per GWh, and of course the leading polluter, coal, with 888 tons.
But there’s one more problem. California does not count large hydroelectric power plants as clean energy. Only hydro plants that produce 30 megawatts or fewer count toward the state’s renewable energy goals.
So is hydroelectric an important tool in the effort to slow down climate change, or just another industrial energy source? The answer appears to be—it’s complicated.
What Is Hydroelectric Power?
Along with wind, flowing water is one of world’s oldest sources of power. Way back in the second century BC, wheels turned by the force of rushing water drove hammers to pound grain, break rocks and even to make paper. Centuries later, water powered the beginnings of the industrial revolution. The Cromford cotton mill in Derbyshire, England, began operations in 1771 and is considered the world’s first factory, its cotton-spinning machines powered by water.
A little more than a century later, in 1880, Grand Rapids, Mich. became the first U.S. city to power electric lights with hydroelectric energy. That event came 31 years after British-American inventor James Francis developed the Francis Turbine, a type of water-powered turbine still in wide use today, albeit with some significant improvements on the original.
The power of water ultimately derives from two factors: gravity and the sun. Solar heat causes water on the Earth’s surface to evaporate. Once it rises into the atmosphere, it condenses into precipitation—rain and snow—and falls back to Earth, collecting in rivers and streams which then flow into lakes and oceans, following the quickest path gravity gives them.
The energy of a flowing river causes a wheel or turbine to spin, which in turn activates an attached generator which produces electricity. This type of hydroelectric power is known as “diversion,” because it involves diverting water from a river directly through a facility housing a turbine and generator.
But rivers alone do not provide enough power. Modern hydroelectric energy relies on dams to block water flow, storing the water in reservoirs—and keeping its kinetic energy pent up. This process is called “impoundment.” A dam is equipped with penstocks, or floodgates, that open and allows the stored water to rush out, releasing the kinetic energy and turning the turbines.
As of 2020, there were 2,300 dams used for power generation in the United States, out of 90,000 dams in the country.
A Third Type of Hydro Power
The diversion and impoundment methods rely solely on gravity to force water through a power plant’s turbines. A third type of hydro plant adds its own electricity to the mix, as a way to store energy in the form of water, because electricity itself cannot be stored. But the energy source used to produce electricity can. This third variety of hydro power is referred to as “pumped storage,” and it uses stored water as, in effect, a giant battery.
Pumped storage hydropower is used mainly to fill in when other power sources may come up short, such as peak energy consumption periods—the hottest days of summer, or coldest times of winter—or intervals when nuclear, wind or solar facilities are slowed or shut down.
A pumped storage facility consists of not one reservoir but two, each at a different elevation. When demand for electricity goes up, the water from the upper reservoir pours down into the lower one, through the turbines that generate power. But instead of releasing that water into a river, the pumped storage facility recharges its power supply by reversing the turbines and, using standard electricity from the grid, pumping the water back into the upper reservoir.
There are 43 pumped storage plants in the United States, of which four are located in California. Those four plants combined—two in Fresno County, one in Los Angeles County and one in San Diego County—can generate over 2,700 megawatts of power during times of high need.
A single megawatt of electricity is generally estimated to be enough for 750 to 1,000 typical American homes.
Is Hydroelectric Clean Energy, or Not?
Hydroelectric energy does not use fossil fuels, emits low amounts of carbon dioxide and other greenhouse gasses, and of course the fuel it uses, water, is renewable and non-polluting. Nonetheless, there is a reason why California doesn’t count large hydroelectric plants as clean energy producers. The environmental damage caused by the massive dams required to make both impounded and pumped storage facilities work, environmentalists say, makes hydroelectric power “neither ‘cheap’ nor ‘clean.’”
According to the environmental news site Eco Watch, hydro advocates who claim that water-power is less expensive than other energy sources are dishonestly disregarding “environmental full-cost accounting,” a way of assessing costs that looks not only at the price of construction and operation of a power plant, but the “indirect” costs to the surrounding environment.
Beyond the cost of basic construction, dams require new roadways and power lines, both of which can wreak havoc on the environment. The reservoirs created by dams flood areas where there was previously no large body of water, destroying the natural habitats of wildlife in the area.
Stagnant reservoir water and the ever falling and rising water levels kill vegetation, which then decomposes and emits methane, a gas that according to the U.S. Environmental Protection Agency is 25 times more powerful than carbon dioxide when it comes to trapping heat in the atmosphere. In calculating how well it’s doing in reaching its climate goals, California deals with methane by simply not counting it.
But reservoirs nationwide are estimated to emit 13.4 million metric tons of methane annually, with about half that total coming from reservoirs used for hydroelectric power.
“People are right to think of hydro as a low-carbon resource, but the variability is very high and there are some reservoirs that have lifecycle emissions of greenhouse gasses that are higher per unit of electricity produced than a fossil plant,” John Parsons, an energy economist at the MIT Center for Energy and Environmental Policy Research, told the MIT Climate Portal site. “You don’t want to just be advocating hydro everywhere.”
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