The demand for electricity in the U.S. has increased tremendously over the past decades. While the net electricity generation in 1950 was about 334 megawatt-hours (MWh), it reached a record high of 4,178 million MWh in 2018, according to the U.S. Energy Information Administration (EIA). This number does not even account for the estimated 30 million kWh generated by rooftop solar panels and other small renewable sources.
As the world is moving towards more sustainable energy sources to lessen the effects of climate change and to reduce air, water, and other forms of pollution, about 17 percent of U.S. electricity generation came from renewable sources in 2018. In the U.S. and many other countries, the fastest-growing energy sources for electricity generation are renewables such as solar and wind.
What is Hydropower?
Hydropower harnesses the kinetic energy of falling or fast-running water. The force of water can be used as mechanical energy or to generate electricity. Hydropower is one of the oldest energy sources used by man to mechanically power grain and lumber mills. The first hydroelectric power plant in the U.S. opened in 1881 at Niagara Falls, New York. Today, hydroelectric power is the largest renewable electricity source in the United States.
Hydroelectric power plants use running water, typically diverted from a river or a reservoir, to spin a turbine. The spinning turbine drives a generator to produce electricity. The amount of energy in the water is determined by the water volume and its flow or fall. The primary use of hydropower today is to generate electricity. According to EIA data, hydroelectric power accounted for about 7 percent of the total U.S. electricity generation and 41 percent of electricity generation from renewable sources in 2018.
Types of Hydropower
There are basically four major types of hydropower plants. The most common type of hydroelectric power plants in the U.S., with an installed capacity of almost 80,000 megawatts (MW), are storage hydro plants. Storage (or impoundment) hydropower plants use reservoirs, typically with a dam, to store water, which can be released as needed to meet electricity demand. Electricity generation can be started up at short notice and adjusted to meed base and peak loads. Storage hydropower plants can also operate independently of water inflow to cover periods of low water supply.
Pumped-storage hydropower (PSH) is a form of hydroelectric energy storage using two water reservoirs. Water is released through a turbine from a reservoir at a higher elevation to a separate reservoir at a lower elevation to generate electricity during peak demand times. At times of low electricity demand, typically at night, the water is cycled back to the upper reservoir to store energy like a battery. PSH plants consume more energy to pump the water back to the upper reservoir than they produce, making them net energy consumers.
Diversion or run-of-river (ROR) power plants divert water directly from a river and have little or no water storage available. A portion of the river is channeled through a penstock to a turbine. Run-of-river plants provide a continuous power supply with little flexibility and are mainly used to maintain baseload generation.
Offshore hydropower uses the kinetic energy of ocean waves and tidal currents in oceans to generate electricity. Tides cause variations in water levels of up to 40 feet, which can be used to channel the water through turbines with a couple of different technologies.
Tidal barrages work similar to dams and hold the water from incoming tides in a tidal basin such as a bay or lagoon. The basin empties through turbines on the outgoing tide. Two-way systems can generate electricity from incoming and outgoing tides.
Tidal turbines are placed on the ocean floor in areas of strong tidal flow. They work similarly to wind turbines, using a consistent water current directly. These turbines are typically placed in rows like fences.
Tidal fences also work as a system of turbines mounted on a vertical axis on the seabed. These technologies are not yet established in the U.S., but they have great potential for renewable energy generation.
Other forms of offshore hydropower use the energy of waves to generate electricity in many different ways to channel the power of waves.
Benefits of Hydroelectric Power Generation
Hydropower is a renewable energy source that contributes to lower carbon dioxide (CO2) emissions and reduces the impact of climate change. Hydropower does not produce any immediate air or water pollution, and no pollution is caused by mining or transportation of fuel to the plants. Hydroelectric power is also one of the cheapest energy sources available.
Additionally, hydropower is a primary contributor to stabilizing the U.S. power grid. Able to increase or decrease power output quickly, hydropower plants follow loads and provide flexibility reserves. They can respond rapidly to system events and provide power for periods of peak demand.
Is Hydropower the Best Green Energy Source?
Renewable energy sources are typically categorized in hydroelectric and non-hydroelectric generation. While hydropower is one of the best renewable energy sources for effectively balancing the electric power grid, it is considered a clean, but not necessarily a green, energy source.
Hydropower significantly affects the environment in multiple ways, while it also introduces social impacts. Dams for reservoirs not only destroy river ecosystems by altering water temperature, chemical composition, and dissolved oxygen levels, they are also barriers disturbing the movement of migratory river animals. Dams lower water tables along the river and intensify erosion due to a reduced sediment load downstream. Reservoirs for storage hydropower cover large areas, displace people and animals and destroy the underlying land ecosystems.
Recent studies also indicate that reservoirs are significant sources of methane emissions on top of their carbon dioxide emissions. Emissions vary based on various factors such as water and air temperatures, vegetation and soil types, and other factors. While the exact amount of greenhouse gases (GHG) emitting from hydropower reservoirs is unclear, these emissions are not included in the total U.S. greenhouse gas statistics. Storage hydropower also produces significant emissions during construction, including manufacturing the concrete, steel, and equipment used in building dams.
While hydropower has several benefits over fossil-fueled power plants, its usefulness depends on the technology used. Big storage dams and reservoirs can cause significant damage to the environment, while run-of-river and pumped-storage plants can be built in less destructive ways.