A new version of an old technology has been proposed to harvest electricity from the tides. So far we are yet to see a prototype, let alone a commercial-scale plant, but on paper, the design has many advantages. Among these is the romantic appeal of a revival of the technology that launched the Industrial Revolution, now only preserved as historical landmarks across Europe. More pragmatically, it could be the key to allowing the UK to cut deep into its carbon dioxide emissions at an affordable price.
Independent marine engineer Rod Rainey has published a design in Proceedings of the Royal Society A for the use of lightweight steel tidal power barrages in the Severn Estuary. To non-engineers, steel and lightweight may not go together, but Rainey is comparing his proposal to structures largely made of concrete, like most tidal power alternatives.
Rainey's idea uses water wheels that turn as the tide drives water past them, like those popular for harnessing river flows during the 19th Century. Alternative tidal structures use turbines that turn fast enough to be a menace to fish, but Rainey's wheels would rotate so slowly that aquatic life should be unaffected. Rainey has modeled the system by treating it as if it was an electric circuit, with the barrage as a resistor/inductor combination and the area upstream as a capacitor. Having then found the optimum way to maximize power from such a circuit, he considered the applicability of the model to the real conditions in the Estuary.
Counter-intuitively, Rainey argues that more total power can be generated if water is sometimes pumped against the tide. He also notes this would maintain the ranges in water heights upstream of the barrage, while only having a small effect on tidal flows downstream. Maintaining ranges is important for flood mitigation, as well as for ecosystems that depend on large variations.
The wheels are technically known as breast-shot water wheels, with a cylindrical core and attached blades sited over a curved base or “breast”. If placed across most of the estuary at Porlock, Somerset, Rainey calculates they could produce an average of 4 gigawatts of power, more than 10 percent of the UK's electricity demand, although some gaps would be required to let shipping through. He argues the price would be cheaper than nuclear power.
To demonstrate the viability of the technology, Rainey wants to build a much smaller version, spanning just 50 meters (160 feet) of the estuary and generating 2 megawatts.
Despite the prices of solar and wind-generated electricity falling, often making them cheaper than fossil fuels, the problem of intermittency remains. Even as it becomes cheaper and easier to store electricity, experts fret about how viable it is to power a country through a cloudy or windless week without resorting to polluting sources.
After the 70s oil shocks, the tides were investigated as a source of energy but largely abandoned as more expensive than other options. Only recently has attention returned. Like most renewable sources, tidal energy does not operate 24/7. However, the lulls as the tide turns are fairly short and can be predicted years in advance, making them easy to cover with batteries or other sources.
There are far fewer suitable sites for tidal power than wind or solar, but the Severn Estuary is one of the best. Lying between south Wales and Somerset, its shape serves as a funnel, giving it one of the highest tidal ranges (15 meters or 50 feet) in the world. Unlike most other extreme tide locations, it is close to major electricity consumers.
Ideas for harnessing these giant tides for electricity go back to the 1920s, but have been appearing with renewed frequency in response to the need for clean and reliable power sources. Questions remain, however, about both the economic viability and the environmental effects.
[H/T: The Economist]
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