Offshore wind projects are often measured in terms of capacity. Hornsea 1, which lies along the coast of Yorkshire, can deliver a max output of 1.3GW. Siemens Gamesa’s SG 14-222 DD, the largest turbine in operation, can generate up to 15MW on its own.
Yet the operative word here is “can”. The wind won’t always be blowing, even out at sea where there are no land structures to stand in its way. The unpredictability of weather conditions still means that offshore farms only see 50 to 55 percent utilisation, according to Kashish Shah, an analyst at the Institute for Energy Economics and Financial Analysis.
Some seasons can be downright dismal. Last September, unseasonably calm weather caused a 60 percent drop in production. From the UK’s 22GW wind capacity the grid was only able to eke out 2GW of output.
This intermittence holds back the transition to renewable resources. The UK’s National Grid remains tethered to fossil fuels, falling back on these systems to make up for the difference when demand peaks.
To truly wean ourselves off from hydrocarbons, alternative renewable sources will need to offset offshore wind and solar’s dips. One emerging technology looks at one of the most constant forces on Earth: the rise and ebb of tides.
What Is Tidal Power?
Tides produce power through kinetic energy. The moon exerts forces that raise tides, while the Earth’s gravity pulls it back. It’s a dynamic that’s linked to the evolution of life itself, with high tidal cycles creating a nutrient-rich ocean ripe for life.
Now, we have the means to turn that primordial cycle into electricity. There are a number of ways to harness tidal power. One would be through a tidal stream generator. These generators operate on the same principle as wind turbines, except the turbines can be found underwater.
Another would be through Dynamic Tidal Power (DTP) technology, which uses the hydrostatic height of tides to funnel water through a barrage. The barrage is essentially a dam that houses potentially thousands of turbines, which produce energy as water streams back and forth.
Why Aren’t We Hearing More About Tidal?
The short answer: money. Compared to solar and wind energy, tidal technology is at its infancy. It was only in 2016 that the world’s first large-scale tidal farm broke ground in Scotland. Most of the major tidal projects today are in early stages.
Building underwater turbines remains expensive. The wind industry has spent decades chipping at the cost of building the machinery while increasing efficiency. Tidal turbines are relatively new. Extreme conditions beneath the surface require these machines to withstand brutal forces, further increasing cost. Tidal’s funding amounts to roughly 1 percent of what goes into offshore wind projects.
Another factor to consider is the environmental impact of underwater turbines. Hoisted far up into the air, turbine blades only pose a risk to migratory birds. Even then, the impact isn’t as great as it’s often made out to be. Offshore farms cause an average of 0.4 bird deaths per GWh of energy generated. Fossil fuel-powered platforms kill 5.2 birds per GWh.
Because there are few operational sites, we still don’t fully understand how tidal installations affect marine wildlife and habitats. Some studies show porpoises and seals may go out of their way to avoid the machines. Data from another study contradicts those findings, attributing the disruption to installation activities, not the operation of turbines themselves.
How Big Is Tidal’s Potential?
Despite the higher capital expenditure needed to get tidal projects off the ground, researchers and renewable energy companies remain convinced that tides will play an important role in the energy mix of the future. Chiefly for one reason: predictability.
We know that tides change at least twice a day–the UK itself is semi-diurnal, which means it has two low tides and two high tides. The moon won’t be changing its behaviour anytime soon. These constant variables make it easier to forecast supply with a greater degree of accuracy. In theory, stable sources should help stop us from scrambling to turn on coal plants when lulls in wind sap the grid.
Calmer conditions are also less of an obstacle to tidal turbines than wind turbines. Water is approximately 800 times denser than air. That means turbines still receive massive amounts of kinetic energy, despite slow moving streams. An underwater turbine can create energy from streams moving at a languid 2.2mph. Wind turbines need more than thrice that force, only sparking to life at wind speeds of 7mph to 9mph.
Not only can tidal turbines operate better in placid conditions. Theoretically, they can generate more power, too. Strong flows allow these machines to generate more energy, even when using blades that are the same size as their topside counterparts.
In the UK, who leads the world in offshore wind development, tidal finds a modest yet promising foundation. Half of Europe’s capacity for tidal power comes from the region. One study estimates that tidal can supply 11 percent of the UK’s annual energy demands.
How Can Tidal Augment Offshore Wind?
Tidal installations need relatively smaller space, compared to the sprawls of offshore wind farms that encroach on miles of seabed. Hybrid tidal and wind projects can raise energy output per square metre whilst scaling back environmental impact.
Some companies are redesigning turbines altogether, incorporating systems that can harness wave and flow energy even as a traditional turbine spins above the surface. A single EXCIBUOY platform from manufacturer Excipio Energy can generate 29MW. At least five regular offshore wind turbines will be required to create the same output. Two, if you were using the world’s largest turbines.
One hybrid project goes beyond renewables and takes aim at oil and gas rigs. Canadian based Waterotor International Corp. recently unveiled “The Big Cajun”, a system that uses wind and water to generate large amounts of energy–energy that the company wants to use to decarbonise oil and gas. “The Big Cajun’s first application allows Big Oil to reduce its carbon footprint and drastically reduce costs by replacing diesel-generated electricity production on platforms”, according to the company.
Merging offshore wind with tidal power also benefits the latter. Tidal is expensive, but offshore wind is becoming cheaper by the year, buoyed by generous subsidies and decades of trying to make the entire supply chain more cost efficient.
Sean Parsons, external affairs director for renewable energy company Simec Atlantis Energy, believes tidal can ride the wave into the next phase of development: “If the cost of supporting infrastructure and development could be shared with other local technologies, like floating wind, then this would be a great benefit to the projects.”
Will tidal solve wind energy’s reliability problems? It’s too early to tell. Early concept experiments give us a glimpse at how wind and tidal can create hybrid energy islands that will pick up the slack from wind. However, before any of these projects are deployed at a larger, commercially viable scale, more support in the way of investments and subsidies needs to go into tidal power.
