Choppy waters ahead for offshore wind

This story was originally published by Hakai Magazine and appears here as part of the Climate Desk collaboration.

When it’s completed, Norway’s Hywind Tampen will be the world’s largest floating offshore wind farm. Compared with most wind farms — even other offshore wind farms — the Hywind Tampen is unusual: the 88-megawatt operation is located farther out to sea than almost any other wind farm to date. Floating 140 kilometres offshore, the turbines will sit in water between 260 and 300 metres deep.

The vast majority of offshore wind farms are located in shallow water near the coast. But new advancements in fixed-foundation and floating turbines are giving developers tools to build in even deeper water far out on the continental shelf, the part of the ocean between the coast and the shelf break, a steep drop-off point after which lies the deep, open ocean. These developments have huge potential: as much as 80 per cent of future offshore wind power will likely come from installations farther from the coast.

The venture into deeper water, though, is raising concerns among some scientists about the unintended consequences of putting wind farms in continental shelf waters. Ben Lincoln, a physical oceanographer at Bangor University in Wales, and his colleagues describe in a new paper how offshore wind power in these waters could potentially disrupt vital natural processes. The key concern, Lincoln says, is turbulence.

Compared with nearshore coastal water, which is highly energetic and well mixed, the deeper reaches over the continental shelf are quite calm. In some places, the water over the continental shelf becomes seasonally stratified, meaning the water settles into distinct layers based on its temperature and salinity. In the spring, often in May and April, increased sunlight causes warmer and less salty water to rise and the colder, saltier water to sink. Between them, a kind of buffer layer called the seasonal thermocline develops.

The stratification of the water is an essential physical process. Without it, the cycle of life as we know it would not occur. The stratification of the continental shelf causes plankton and nutrients from the seabed to be ferried up to the higher strata of water, triggering the spring phytoplankton bloom — an annual feast relied upon by myriad marine creatures. Despite making up less than 10 per cent of the ocean’s total area, these continental shelf seas account for between 10 and 30 per cent of the ocean’s primary production. Since 2009, more than 90 per cent of fish caught globally has come from these regions and the shallower coastal waters.

Seasonal stratification also helps pace out how quickly phytoplankton can chew through the nutrients in the water. While the water column stratifies, the water whisks up plankton and nutrients from the seafloor into the upper layer and the seasonal thermocline. Exposed to direct sunlight, the plankton in the warm top layer will bloom and burn through the layer’s nutrients. Meanwhile, the plankton in the thermocline will receive less light from the sun, meaning they won’t use up the layer’s nutrients as quickly and will be a stable source of phytoplankton throughout the summer months.

However, according to Lincoln’s research, putting structures in this seasonally stratified water complicates things.

Lincoln and his colleagues’ research shows how water flowing past the submerged element of a wind turbine, such as the cables that keep it affixed to the seafloor, creates turbulence that pushes the water up and down and mixes the layers.

According to Lincoln, if the added turbulence kicks up too many nutrients from the bottom to the higher layers, phytoplankton could use up the nutrients too quickly. More mixing could also upset the stratification process, causing the seasonal algae bloom to happen later in the year. These blooms are essential food for zooplankton, which, in turn, support larger species. Birds, fish, and other marine life have evolved so that their own life cycles align with these seasonal phytoplankton blooms. This means that increasing turbulence could cause effects that cascade through the food web.

So far, this concern is unique to turbines on the continental shelf, Lincoln says. “Basically, nobody’s been too worried about the impact of the shallow-water wind farms that make up 99 per cent of wind farms in the world because they’re already in such energetic places, the extra turbulence really has a limited impact.”

While Lincoln’s paper shows that the presence of wind turbines increases turbulence, the researchers can’t say by precisely how much. Depending on the numbers, this turbulence could cause problems. Or, potentially, benefits.

If this increased mixing stirs up just enough nutrient-rich matter from the seafloor, it could make the water more productive in a sustainable way. Doing this intentionally was even proposed in 1986 in the so-called induced mixing experiment. With climate change causing stratification — and, thus, the spring phytoplankton bloom — to happen earlier each year, the enhanced turbulence caused by standing structures in these regions could even counteract this effect, Lincoln says.

Previous research by Jeff Carpenter, a fluid physicist at the Helmholtz Centre for Environmental Research in Germany, provides one look at the scale of the effect. Carpenter and his colleagues travelled to the DanTysk offshore wind farm, located on the continental shelf off Germany. The team determined the ambient levels of turbulence in the ocean and the turbulence in the wake of the turbine, calculating that the turbine’s presence increased the mixing of the waters — how fast the stratification near the turbine was destroyed — by seven to 10 per cent.

This increase could be enough to cause some of the phenomena described in Lincoln’s paper, Carpenter says. However, those numbers are for a fixed-base turbine. “You would get different effects depending on the foundation structure,” Carpenter says.

Right now, there are very few offshore wind turbines over continental shelves. However, the number is expected to explode in the coming years. As Lincoln and his colleagues write, offshore wind capacity is set to increase 600 per cent over the next decade.

According to Jackie Harrop, the marketing manager for HR Wallingford, a United Kingdom–based non-profit research organization, the ocean has only seen small-scale floating wind operations so far. Yet many countries are looking to enter or expand in this field. Scotland, for instance, is planning to add 11 floating wind farms, with a cumulative capacity of 15 gigawatts, to its energy mix — though Harrop notes that the organization is unsure if all of the floating turbines are over the continental shelf. The group estimates that there are 50 gigawatts of capacity planned elsewhere around the globe and that there are more than 40 different varieties of floating foundations being tested.

“Floating wind power is on course to become an essential part of the renewables mix, as the additional power generated by installing turbines in deeper seas will be key in meeting carbon reduction targets,” Harrop says by email.

In their paper, Lincoln and his colleagues are careful to note that to combat climate change offshore wind production must grow. They say it’s not a matter of stopping the field’s expansion, but rather looking further into the potential benefits and impacts so we can maximize the former while minimizing the latter. More work needs to be done to understand how offshore wind turbines will affect marine ecosystems before these developments pick up speed, Lincoln says. “It’s important to do the research now so that the right decisions are made.”