Tidal Energy: The New Sustainable Resource

by Matthew J. Woo, P.E., RCDD, LEED AP BD+C

This article is part of Wood Harbinger’s newsletter series.

Identifying and implementing wide–scale, alternative-energy sources may be the greatest global imperative of our time. Why? We are in danger of damaging our earth’s atmosphere and water from global air pollution caused by carbon dioxide emissions and acid rain, exhausting our limited supply of natural resources (e.g. coal, oil, natural gas), and outgrowing the limited capacity of our power plants and electrical infrastructure. Alternative energy sources also give us the potential to free ourselves from dependence on other countries for oil and the associated foreign policies that come with these relationships. Moreover, it gives us the potential to reduce our reliance on nuclear energy, thereby reducing the risk of safety breaches and waste byproduct problems inherent with this energy source.

In past editions of our Sustainability newsletter, I have discussed alternative energy generation systems, such as solar photovoltaics (PV) and other solar technologies, that help reduce the energy demand on our power grid. My colleague Paul Greenwalt has written blog posts about geothermal energy and heat pump systems. Wind turbines represent another technology gaining traction. However, an often-overlooked source of clean, renewable energy comes not from solar, wind, or geothermal sources but from the tides. In this article, we will delve into the details of tidal energy, which has been growing since around the early 2000s and offers great promise for being a clean, renewable, alternative energy source in coastal areas.

What is Tidal Energy?

Tidal energy (or tidal power) is a form of hydropower, using water to create energy. Tidal energy converts the energy obtained from tidal movement into electric power using tidal generators. Tidal power was first conceived in the 1970s, during the oil crisis, when there was an initial emphasis on developing alternative energy sources to distance the U.S. and Europe from the volatility of the oil markets.

The cycle of tidal energy from tidal currents converted to electricity and distributed through the grid to power buildings. Image: Adobe Stock

There are multiple power-generating methods. Tidal stream generators or tidal energy converters (TEC) are the most common, most cost effective, and the least ecologically damaging. Tidal stream generators use the kinetic energy of moving tidal water to power turbines, like wind powering wind turbines. Another less common and more costly method, called tidal barrage, uses the potential energy of temporarily stored tidal water to power turbines, like water flowing over a dam.

Tidal stream generators are like underwater wind turbines. Image: Adobe Stock

Types of tidal stream generators

Currently there is no standard tidal stream generator design, but a large variety of designs installed without extended periods of operation to help gauge performance and rate of return on investment. The European Marine Energy Centre recognizes six principal types of tidal energy converters: horizontal axis turbines, vertical axis turbines, oscillating hydrofoils, venturi devices, Archimedes screws and tidal kites. Compared to an open turbine in free stream, ducted turbines are capable of as much as 3 to 4 times the power of the same turbine rotor in open flow. Power output of current tidal stream generators range in size from about 100kW to as high as about 2MW. Turbines typically have a single rotor set with fixed or adjustable pitch blades and may be able to rotate for optimal heading during tidal exchange.

Benefits of Tidal Energy

Tidal energy is a sustainable resource, which offers many benefits and only a few drawbacks. First off, tidal energy is a clean renewable resource which is more efficient than wind energy due to the density of water and more efficient than solar energy due to its high ~80% conversion efficiency. Tides are also more predictable than wind energy and solar energy, therefore more reliable. Tidal energy is effective at low tidal speeds, which means that the turbines can turn more slowly, minimizing impacts to local ecosystems. It can be a less costly resource, both in terms of construction and maintenance costs, compared to other renewable energy sources. Tidal energy produces no carbon dioxide, greenhouse gases, or waste.

Environmental impacts of Tidal Energy

The main environmental concerns with tidal energy are from the turbine blades striking or entangling marine organisms, as higher speed flowing water increases the risk of organisms being pulled near or through these devices. As with all offshore renewable energies, there is also a concern about how the creation of electromagnetic fields and acoustic outputs may affect marine organisms.

How Tidal Energy Compares with Other Renewables

Tidal stream generators draw energy from water currents in much the same way as wind turbines draw energy from air currents. However, the potential for power generation by an individual tidal turbine can be greater than that of a similarly rated wind energy turbine, due to the higher density of water relative to air (water is about 800 times the density of air), which means that a single tidal generator can provide significantly more power at low tidal flow velocities compared with a similarly sized wind turbine.

While the initial cost of building tidal energy plants is high, tidal energy power plants are expected to run for a very long time, which make them more cost-effective in the long run. Solar panels are fragile, less efficient, and can require costly maintenance.  Wind turbines operate at much higher speeds, which can wear out moving parts more quickly. Geothermal energy can be very costly to install and require deep wells or large open areas.

Depending on geographical location, the cost of tidal energy is about 5-10 cents per kWHr of electricity produced compared to about 3-5 cents per kWHr of electricity produced for solar and about 2.5-4 cents per kWHr of electricity produced for wind.  Future advancements in technology and manufacturing efficiency will help to drive the cost down for these renewable energy sources, making them even more cost competitive with traditional energy sources that come from fossil fuels.

Tidal Energy Usage Worldwide

Tidal stream generators can operate where tide speed is at least 2 knots (1 m/s).  Currently, 19 countries worldwide are investing in tidal power systems. The first major tidal-power project in the United States was installed in April 2007 in the East River between Queens and Roosevelt Island in New York City. Other than the United States, tidal generators are also found in Europe, Australia, Canada, and South Korea.

As with wind power, tidal turbines have specific site requirements. Tidal stream turbines may be arrayed in high-velocity areas where natural tidal current flows are concentrated, such as the west and east coasts of Canada, the Strait of Gibraltar, the Bosporus, and numerous sites in Southeast Asia and Australia. Such flows occur almost anywhere where there are entrances to bays and rivers, or between land masses where water currents are concentrated. Several sites around the world have the needed geography for successful tidal energy generation, including U.S. locations, such as the San Francisco Bay, around Humboldt County in California, the Columbia River in Oregon, and the Piscataqua River in New Hampshire. Internationally, there are locations around Wales, England, and Scotland as well as the Bay of Fundy in Canada.

There are multiple locations around the world with high potential for tidal energy generation. Image credit: Mr. Stevenson’s 7th Grade Electricity—7A Tidal Power via opportunity:energy

Modern advances in turbine technology may eventually enable us to generate large amounts of power from the ocean, especially tidal currents using the tidal stream designs but also from the major thermal current systems such as the Gulf Stream, which is covered by the more general term “marine current power.”

Now is the Time

Now more than ever, we must strive to develop and expand alternative sources of energy that are clean and renewable. Tidal energy can be an important step toward reducing reliance on fossil fuels for energy production, reducing air pollution, and helping to preserve the natural environment of our one and only home planet.