Jim Conca: Tidal energy — all renewables are not created equal

Most everyone has heard of hydroelectric energy, or hydropower, that uses a dam to store water in a reservoir. Water released from the reservoir flows through turbines, spinning them to generate electricity.

But there are other types of hydropower that harness marine currents, tidal energy and wave energy, also referred to as marine current energy. Tidal stream turbines work much like submerged windmills, but are driven by flowing water rather than by air.

Tidal power is far more reliable than either wind or solar power as the sun doesn’t always shine and the wind doesn’t always blow. Tidal is still intermittent, meaning it isn’t constant and we can’t turn it on or off like a light switch, but it is very predictable. The predictability of tidal power is highly attractive for electric grid management, removing much of the need for back-up plants usually powered by natural gas.

Tidal turbines can be installed in the ocean at places with high tidal current velocities, or in places with strong ocean currents. These flows are literally as predictable as the tides, driven by the phases of the moon. The technology consists of twin axial flow rotors 15 to 20 meters in diameter, each driving a generator via a gearbox much like hydroelectric and wind turbines. The twin power units of each system are mounted on wing-like extensions on either side of a tubular steel central pillar some ten feet in diameter, set into a hole drilled into the ocean floor.

Water is over 800 times denser than air, so the force of the tidal flow, if only several miles per hour in places like Strangford Lough in Ireland, is similar to a 300 mph wind on a wind turbine. So tidal turbines can be significantly smaller than wind turbines, and placed much closer together.

A study by the U.S. Department of Energy identified almost 50 GW of marine current power in the Alaskan coastal waters alone, which are particularly ideal for this source. The World Energy Council estimates that up to 1,000 GW of marine energy could be installed by mid-century. This is pretty significant, equivalent to half of the world’s present coal capacity.

Washington state also has significant potential for tidal energy. The best places — those with kinetic power densities greater than 1,000 watts per square yard — are the mouth of the Columbia River, Spieden Channel, President Channel, San Juan Channel, Middle Channel, Rosario Strait, Bellingham Channel, Guemes Channel and Deception Pass. That’s enough to replace all fossil fuel power in Washington and Oregon.

Marine current energy is still being developed, and costs are coming down. But estimates by the International Renewable Energy Agency range from 20 cents to 27 cents per kilowatt hour, over double what other renewables are at present. Our local nuclear power plant, Columbia Generating Station, provides power at 4.7 cent/kWh, and our existing hydro plants provide power at 2 cent/kWh, so it would have to come down dramatically to become competitive. Or get significant tax credits like wind and solar.

Environmental impact studies of marine energy have shown relatively little impact to marine ecosystems. These studies focused on the effects of the tidal turbines on the integrity of the breeding marine mammal populations, the abundance, diversity, integrity and extent of the sea floor communities associated with the submerged rocky reefs, and the population of breeding seabirds.

But researchers at Cardiff University and Liverpool aren’t so sure. There are concerns that tidal power could have negative impacts on marine life by changing how sedimentation, dissolved oxygen and nutrients work in the area around the turbines.

Barring any strange, unlooked-for negative effects, marine current technologies should grow quickly in this century and join hydro, nuclear, wind and solar as a solid part of our future low-carbon energy mix.

This story was originally published October 14, 2017 at 1:48 PM with the headline "Jim Conca: Tidal energy — all renewables are not created equal."