by David Hoyle
Two tidal energy projects that have been featured in the news recently nicely illustrate the two primary types of tidal hydropower generation systems: tidal barrage and tidal stream.
Tidal barrage systems are analogous to historical tide mills: seawater is impounded behind a dam at high tide; when the tide has partially ebbed outside the dam, water is released through sluices to drive turbines that generate electricity. Modern tidal barrage plants are typically able to generate power on both ebbing and flooding tides. Currently, there are only two large-scale commercial tidal power plants in the world. One is in La Rance, France, and the other is in Sihwa Lake, South Korea. Each generates approximately 250 MW, enough to power 250,000 homes.
Tidal stream systems don’t require a dam, and instead harvest energy from tidal streams, much as wind turbines harness the power of moving air. One challenge is that the speed of the tidal current (or ocean current) needs to be fast enough to drive a turbine; 1.5 knots is a typical “cut-in velocity” for a tidal turbine. A tidal stream project, described below, involves a fascinating technology specifically designed to work in low velocity tidal currents or ocean currents.
Barrage Power Generation Example – Mersey Tidal Power Project
Authorities in Liverpool, England, are planning a tidal barrage power plant in the River Mersey using the UK’s second highest tides, which range from 13 to 33 feet. The plant would have at least 1,000 MW capacity (enough to power 1 million homes). The video linked below shows what the plant would look like in a detailed fly-through.
Video: Mersey barrage flythrough
Tidal Stream Power Generation Example – Minesto Dragon Kites
The tidal stream generation project involves what are known as tidal kite generators. A tidal kite generator is designed to work efficiently in low speed tidal streams or ocean currents. The kite “flies” in the tidal stream just as a child’s kite flies in the wind. The kite is automatically controlled to follow a figure-eight path as it glides at high speed through the water (you can see this in the video, linked below). The high relative speed between the kite and the water (up to 10x the natural speed of the current) drives the turbine propeller on the kite to efficiently generate electricity. Again, it’s the force of the current that flies the kite, and the movement of the kite that drives the propeller, not the other way around! Electricity is fed down the kite’s tether cable to its seabed anchor and from there over to shore.
Swedish tidal energy company Minesto began testing its first 100 kW Dragon 4 tidal power kite off of the Faroe Islands between Scotland and Iceland in early summer 2022 and added a second Dragon 4 kite in 2023. Minesto plans to deploy tidal kite arrays at four sites by 2030, generating a total of 120 MW and supplying 40% of Faroe’s electricity. The kite arrays will use Minesto’s Dragon 12 kites which produce 1.2 MW each. Installation and testing of the first Dragon 12 kite began in February 2024; please take a look at the article linked below for details.
Video: Minesto’s Deep Green technology – unlocking renewable baseload power
Article: Minesto’s Dragon 12 functionality verified | Energy Global
David,
Thanks again for keeping us up to date on tidal power happenings. These are good articles. –Mark