What are the Pros and Cons of Hydropower and Tidal Energy

Welcome back! This is post three of our four-part series on renewable energy. Our first post introduced the idea of using renewable energy as a way to keep up with economic demand while reducing our impact on the environment and our second post explored the benefits and limitations to solar and wind energy. Today we discuss two new forms of renewable energy, hydro and tidal power. Continue reading to learn more! 

What is hydropower?

Hydropower is energy produced by moving water through a turbine. Hydropower was the first large-scale renewable energy source used in the U.S., accounting for one third of the nation’s total electrical needs in the 1940’s. Dam construction peaked in the 1960’s and has since gradually declined. Today, hydroelectric generation provides roughly 6% of the United States’ electricity. The decline in electrical generation from hydropower was a result of increased investment in other sources of energy, siting limitations for large-scale hydroelectric plants, and rising environmental concerns about impact of dams on aquatic ecosystems.

What are the pros of hydropower?

1. Clean: Generating electricity from hydro-energy does not produce harmful greenhouse gases.

2. Efficient: Hydropower has a very high energy conversion efficiency. Energy conversion efficiency is the ratio between the output of an energy conversion machine and its energy input – the higher the ratio, the more efficient the power plant. Approximately 90% of the energy captured is able to be converted into electrical energy. The energy conversion efficiencies for solar and wind are much lower, averaging 15% for solar and 60% (theoretical limit) for wind respectively.

3. Reliable: Large reservoirs behind dams mean that power plant operators are able to control the flow and output of electricity to match energy demand.

4. Cost-effective: Hydroelectric energy has a low levelized cost of energy (LCOE) compared to conventional sources (coal and natural gas). Remember, levelized costs of energy are calculated by taking the total cost to build and operate a new power plant over its lifetime divided by its expected energy output – measured in dollars per kilowatt hour ($/kW-hr). The LCOE for hydropower is $0.08/kW-hr, making it a competitive with coal and natural ($.07-$0.14/kW-hr).

What are the cons of hydropower?

1. High environmental impact: Most of the hydroelectric dams constructed in the U.S. were built without considering their impact on ecosystems. Large dams have large environmental footprints. Hydroelectric dams impact a river’s flow, temperature, chemistry, and silt load. International Rivers – an environmental NGO – reports that large dams have led to the extinction of fish species, disappearance of birds in floodplains, losses of forest, wetland, and farmland, erosion of coastal deltas, and other destructive impacts.

2. Effect on fish populations: The negative environmental impacts of dams adversely affect fish populations by reducing thousands of miles of fish habitat by limiting access to spawning grounds. Large dams have greatly reduced Salmon populations of the Pacific Northwest.

3. Siting: Some proposed sites for hydroelectric power plants in the U.S. have been widely contested. The construction of the O’Shaughnessy Dam and Hetch Hetchy reservoir in Yosemite National Park (1923) was seen as a major loss for conservationists. The proposal of the Echo Park Dam at Dinosaur National Monument in the 1950s was met with bitter opposition from environmental groups, like the Sierra Club, who argued the government was prioritizing development over the region’s natural treasures.

4. Displacement: Large scale hydroelectric plants can also have a negative impact on human populations. The Three Gorges Dam in China displaced more than 1.2 million people, flooding 13 cities, 140 towns, and 1,350 villages.

What is tidal energy?

Tidal energy is a form of hydropower that converts energy from the tides. Tidal energy, while still in early stages of development, may be a viable method of electricity generation for coastal communities.

What are the pros of tidal energy?

1. Clean: Tidal energy does not produce greenhouse gas emissions.

2. Reliable: Tidal currents are predictable. Knowledge of high and low tide cycles can help a tidal power plant construct an efficient system for generating electricity.

3. Effective at low speeds: Water has a density 1,000 times greater than air, making it possible to generate electricity at low speeds.

4. High Potential: The U.S. Department of Energy estimates that tidal and wave energy hold up to 1,400 Terawatt hours (TWh) of potential energy each year. The U.S. requires about 4,000 TWh of energy each year. Unfortunately, the best sites are located in Alaska, far from consumers.

What are the cons of tidal energy?
1. Siting: Suitable sites for tidal power plants must meet specific requirements. For example, sites must have a tidal range of at least 7 meters (about 23 feet). Current technology also limits the suitable geographic location of tidal energy plants. The U.S. has only a handful of sites appropriate for an economically competitive commercial plant. A study conducted by the U.S. Department of Energy, estimated that the U.S. (excluding Hawaii and Alaska) could generate only 0.6 Terawatt hours (TWh) of the 4000 TWh needed each year.

2. Intermittent: Tidal energy can only be produce electricity during tidal surges, limiting electricity generation to 10 hours per day.

3. Environmental impact remains unknown: Tidal energy is still in its infancy. Therefore, the environmental effects of tidal energy plants have not been fully studied. For example, tidal barrages rely on manipulation of ocean levels and therefore pose similar environmental effects of hydroelectric dams.

4. Expensive: Technology for generating electricity from tidal energy is relatively new and not yet commercially profitable. Tidal power plants are also expensive to build and maintain. There are very few commercial tidal energy plants operating in the world. Therefore, advances in technology and construction at a larger scale are needed to lower the LCOE of tidal energy.

Want to learn more about renewable energy? Read our entire series here.

Post 1: Why do Renewable Energy Sources Matter?

Post 2: What are the Pros and Cons of Solar and Wind Energy?

Post 3: What are the Pros and Cons of Hydro and Tidal Energy? (this post!)

Post 4: What are the Pros and Cons of Biomass and Geothermal Energy?