Tidal energy is a renewable energy that has been used for more than a thousand years and operating commercially for the last 50 years.
Tides are caused by the gravitational pull of the moon and sun on the earth’s oceans. As the earth rotates, the sun and moon pull the oceans around the earth, causing the water levels to rise and fall. When the sun and moon pull together or against each other, we get bigger tidal ranges called spring tides.
As the tide rises and falls the potential energy is transferred into kinetic energy. The energy in the tides is similar to riding a bike up a hill and coasting back down. At the top of the hill, potential energy is stored, and as you accelerate down the hill, the potential energy is transferred to kinetic energy as you go faster. The bigger the hill you climb, the faster you will go on the way down to the bottom. Similarly, the larger the tidal range, the difference between high and low tide, the faster the tidal currents.
Tidal stream and tidal range are different ways to extract energy from the tides. Tidal stream extracts energy from the tidal current as it flows back and forth. Tidal range extracts energy from the change in water levels as the tides rise and fall.
Tide mills date as far back as the early centuries AD with archaeologists suggesting examples in Roman London and in 6th Century Ireland. Electrical power generating tidal barrages have been used for over half a century. The world’s first tidal power station, La Rance Tidal Power Station in Brittany, France, was commissioned in 1966. It has a tidal generation capacity of 240MW (for comparison, BELCO’s peak load is approximately 100MW). The barrage is just under half a mile wide and relies on water falling 18.5ft through its turbines. With a 40% capacity factor, it would be capable of powering the entire island of Bermuda if coupled with suitable energy storage.
Of the global tidal range resource, 90% is distributed among just five countries and Bermuda is not one of them. Bermuda’s tidal range is between 0.4m and 1.2m (1.3 and 4 feet) which is six times less than the Solway Firth in Scotland, and 10 times less than the tidal range found at the Bay of Fundy, Canada which can reach 12m, or 38 feet!
The most visible example of tidal action in Bermuda is at Flatts Bridge where water flows in and out of Harrington Sound. Interestingly, the flow in and out of the bridge is not the same. The flow in and out of Harrington Sound also finds its way through the vast cave networks connected to the sound.
As water will always follow the path of least resistance, if a tidal device was installed at Flatts bridge, then the resistance at the bridge would increase and more flow would find its way through the caves. This makes it difficult to predict the available tidal resource at the bridge. An estimate of the available energy ranges from 25 to 100 kW (1 to 4 houses), however only a fraction of this can be extracted. In 1977 it was calculated that 756 thousand m3 equivalent to 7.5 million wheelbarrows of water flow out of Harrington sound every tide. Assuming that the water level difference across the bridge is approximately 1 ft or 0.3m, the available power flowing through the bridge is approximately 50kW or 75 brake horsepower (BHP), equivalent to a small car. However, the extractable power is only 59% of that, but we cannot block the entire passage under the bridge to marine traffic and marine life.
If marine life and marine traffic is allowed to pass through Flatts Inlet and a 2m (6ft) turbine is installed to one side of the flow, at a peak flow of 3 knots, the maximum power extracted by a tidal turbine with a good power coefficient of 0.42, would be only 5kW, enough energy to power one clothes dryer.
The pros of tidal energy include being predictable, clean and energy dense. The cons include operating in a harsh environment and high loads from the weight of so much water required to be held back as the tides fall to then power generators.
While tidal energy is a clean, renewable energy source, it is unlikely to ever be considered a viable option for Bermuda. Bermuda has such a small tidal range and no substantial areas with the potential to capture and store the rise and fall in tides.