Offshore wind turbines are some very impressive pieces of engineering. But building and deploying them is only half of the story…
What are offshore wind turbines and how are they different from onshore ones?
Offshore wind turbines, as the name suggests, are wind turbines constructed and located in bodies of water, such as the ocean or river estuaries. While the term is usually used to describe wind farms at sea, offshore wind farms can be located fairly close to shore.
There is no technical limit to the depth of water they can be built in; however, while it is theoretically possible to build in any depth, the maximum is probably closer to 1,970 feet (600m), practically speaking.
The methods used to construct them and anchor them will vary, however, for obvious reasons.
One of the main differences between the onshore and offshore wind turbines, apart from their location, is the fact that onshore turbines have had a longer period of time for development.
“Offshore wind power has been developing rapidly for the last ten years,” John Olav Giæver Tande, Norwegian Chief Scientist at SINTEF Energy Research, told IE in an interview. Tarde heads the sub-program for offshore wind energy within the European Energy Research Alliance and is a member of the European Technology Platform on Wind Energy.
By global capacity, offshore wind power is still relatively small when compared to land-based wind energy. According to Tande, the reason why is it being a relatively new technology. “In terms of the capacity of offshore wind, it is something like the capacity of land-based wind in 2001. It is, more or less, twenty years behind in development,” he explained.
This is for a variety of reasons, including cost, but the gap is closing. Offshore wind power is now quite cost-competitive, thanks to the rapid development of the technology.
“Dogger Bank, which is the largest offshore wind farm to be built to date [has a capacity of] 3.6 GW with a cost of 40 Euros ($48) per MWh,” Tande gave as an example. This project cost somewhere around 150 Euros ($182) per MWh.
To put that into perspective, a new coal plant can cost around $123 to $135 per MWh, and a new nuclear power plant anywhere between $60 to $143 per MWh.
While this is not bad, the costs are set to drop further. By 2030, it would not be unreasonable to expect costs to be around 40 Euros ($48) to 60 Euros ($73) per MWh for offshore wind power, Tande said.
How are offshore wind turbines built?
Offshore wind turbines are very similar to land-based wind turbines in terms of the design itself. The only real difference is the fact that they are “marinized.”
This means that the turbines are modified slightly to enable them to better resist corrosion and prevent excessive damage from humidity. Offshore wind turbines also tend to be much larger than their onshore counterparts.
This is because it is actually easier to install larger wind turbines offshore. However, this is where the similarity ends.
“The real difference [between the two], is obviously the foundations,” Tande said. “The most common foundation is [what is termed] bottom fixed. This is a hollow, steel cylinder similar to the tower, but piled deep into the seabed, typically 30 meters (98 feet) or more.”
But the depth to which they extend will vary depending on the local geology, the water depth, and the size of the turbine, he added. This kind of installation is usually the go-to for wind farms cited up to 82 to 98 feet (25 to 30 meters) in water depth.
Another method is called “jacket foundations” and can be applied in deeper waters.
This is a well-established construction technique developed by the oil and gas industry. Some of these installations were developed for those in up to several hundred feets of water but, as Tande explained to IE, the industry has begun to question if this was really justifiable.
As a result of that, floating systems were developed instead. Since bottom-fixed solutions are not considered viable in most cases for wind turbines in the water depth of more than 230 feet (70 meters), the adoption of floating platforms were needed.
“These can be either semi-submersible platforms, or they can be spar type platforms,” Tande explained. Both of these types of platforms are anchored to the seafloor using mooring lines.
But this setup does mean that floating platforms are best suited for deep-water installations of between 655 and 984 feet (200 and 300 meters). This is because “if [the water] is too shallow the anchor lines need to be fairly widespread and they become longer and more expensive. If you can keep them in deeper water the [mooring lines] can go directly down.”
Some tension leg platforms also exist, with one stiff anchoring point underneath the platform, but in general, according to Tande, the industry is generally moving towards using slack anchor lines for floating-type platforms.
The next consideration when building offshore wind farms is how to connect them to the onshore electrical grid. In most cases, subsea cables are run from the farms to the shore.
But this is only part of the story. Prior to transmitting electricity onshore, the power needs to be collected from the turbines, ready for distribution.
In most cases, all turbines in a farm are connected to a large transformer to convert the electricity to high voltage AC for transmission to land, or if the farms are located more than 62 miles (100km) from shore, the power must be transmitted as DC. This will require high voltage DC converter stations at each end of the DC transmission line.
What are the advantages and disadvantages of offshore wind farms?
One of the main advantages of offshore wind power is that farms can be very large indeed. They can also be built fairly close to where the energy is consumed.
“Most large cities are located close to the sea, like London,” Tande gave as an example. For this reason, there is a huge potential for building offshore wind farms fairly close to meet demands from large cities.
To provide this onshore would require a huge amount of land. Unlike onshore wind farms, where land is at a premium, farms at sea can be larger. However, like any renewable technology, this is something of a trade-off as the turbines can also affect local ecosystems.
Because they are relatively larger, offshore turbines can supply large amounts of “clean” energy with a minimum impact on the environment. Although, it is worth noting that their impact on marine life is not fully known. Being offshore, they are also “out of sight, out of mind” with regards to being seen as an eyesore.
Not only that, but offshore wind resources are very large. This technology “could produce 18 times the global electricity needs if exploited to the full,” Tande said. Most of this potential, around 80%, could come from energy harvesting in the deep sea. For this reason, floating-type wind farms have a potentially very lucrative future.
However, at the best locations on land, onshore wind farms are still cheaper to build (and maintain) than offshore ones, at least for now. So, given a great location, it is probably better to build an onshore farm than offshore, Tande said.
One drawback is that offshore farms can impact the fishing industry, especially during construction of the turbines.
This is especially the case for bottom-fixed platforms that are generally located in depths of 65-98 feet (20-30 meters). That is also usually where fishermen practice their trade, Tande noted, adding that, “you are not allowed to go fishing in between the turbines.” But from the fish’s point of view, wind farms offer a novel form of shelter.
With fishing prohibited, fish stocks have seen something of a boom in recent years in areas around offshore wind farm sites. The foundations of wind turbines may also provide a form of artificial reef too.
However, as Tande explained, this might not lead to a real increase in the fish population, rather mean that fish are attracted to these areas. More research is needed before making a final judgment.
These kinds of issues can be mitigated with good planning prior to construction. With regards to fishing, fishermen must be consulted, for example.
How long do wind turbines last?
A challenge of offshore wind turbines is the ease of maintenance and access for repairs. Being so remote, maintenance and repair costs are significantly higher than onshore ones.
“If you have a problem with a land-based wind, you jump in your truck, drive and you are there,” Tande said. With offshore, the question of whether you can actually get to them to work on them is real.
For example, being at sea, you will often need to check the weather forecast, as rough seas can make travel impossible. If waves are more than 6 feet (2 meter) high, it is often not possible to access the turbines.
This makes planning and scheduling maintenance vitally important for offshore platforms. For this reason, many modern farms have sophisticated modeling techniques to help aid maintenance decisions and scheduling. Most turbines have a special anti-corrosion coating that is good for at least 20 years.
Moreover, if a part is considered likely to fail in 6 months‘ time, and maintenance activities are occurring in the next week, it is probably a good idea to change the part ahead of time rather than months down the line, Tande explained.
Another frequently asked question is, “Do wind turbines really kill birds?”. The answer is yes and no.
If the farms are located in migration paths of birds, then they would present a very serious threat to them. But, as Tande also explained, there are technological solutions to this problem.
For example, there are radar systems that can be used to sense if a flock of birds is en route to the farm. These systems can be used to shut off the turbines temporarily to allow the migrating birds to travel through the farm safely and avoid collisions.
Could offshore wind offer countries a way to become energy independent?
While theoretically possible, this is unlikely in the next few decades. However, offshore wind will likely form a large part of a nation’s energy needs eventually and private enterprises are also very excited by the potential for the technology.
For countries with a coastline, the use of offshore wind power is something of a “no brainer,” Tande said.
The U.K., for example, has identified that offshore wind could provide an important part of the country’s future energy supply. It is hoped that by around 2050, at least one third of the U.K.’s energy supply could be met with offshore wind farms. A similar amount is predicted for Europe by around the same time.
But the potential benefits of wind power are also great for private enterprises. The vision of the Ocean Renewable Energy Action Coalition (OREAC) is 1,400 GW of offshore wind by 2050.
To put that into perspective, the currently installed capacity is somewhere in the region of 25-30 GW globally, Tande said. If true, “this would point to offshore wind as being the ‘next big thing’ that could revolutionize the energy system.” It has the potential, therefore, of being a trillion-dollar industry by 2050.
One interesting example comes from Switzerland, which doesn’t have a coastline. “I have colleagues in Switzerland who are quite keen to get into offshore wind. For countries where there is no natural coastline, there are utility companies [which] could invest in an offshore wind farm and buy the electricity from that.” Tande said.
Other examples exist, too, with many oil and gas companies looking into the potential for offshore wind. Equinor, Tande explained, “are now gearing up their business for offshore wind.”
What is the future for offshore wind power?
The future of offshore wind power is one that is only really limited by the time and resources invested in research and development.
In order for the targets of U.K. and Europe to be possible, more work needs to be done to bring the cost per MWh down, as well as mitigating and reducing the environmental impact of such wind farms.
Tande explained that the current issues can be divided into practical, engineering, and fundamental research issues.
For the former, proceedures like changing the blade of a floating turbine at sea can be very technically challenging. As both the turbines and servicing vessels are floating and bobbing around at sea, devising methods to safely complete the procedure without damaging the turbine, or indeed the new blade or vessel, are essential.
“This is an engineering challenge, but it is also a research challenge,” Tande said.
Another potential area of research is how to more efficiently connect wind farms to the energy grid. Moreover, topics such as subsea connectors and lead-free cables are all areas of future research.
In general, however, Tande explained that there are three main large research topics that the research community has been struggling with for some time.
The first one is to understand and model the airflow inside wind farms. This can be done; however, it requires supercomputation. The process is still very slow and not very practical.
In order to make the wind farms as efficient as possible, it will be necessary to know the mechanics of the airflow at each turbine and throughout the farm. This is especially the case for times when a single turbine is altered and the question of how it will affect the others arises.
The second challenge is that engineers need to more fully understand the dynamic interactions between hydrodynamics, mechanics, wind dynamics, and electrical dynamics of huge machines hundreds of feet tall with huge swept areas of the turbine blades.
“We have experts who can understand the hydrodynamics, and the aerodynamics and the electrotechnical and the mechanical [aspects], but to combine these together, and understand all the detailed interactions is [one of] the next big research questions, ” Tande said.
The third one concerns how to operate the power grid with 100% renewables and a very large amount from wind power. Not only that, but how one can integrate that with the whole grid and provide reliable stability needs to be answered.
This could be achieved, Tande explained, through on-site battery storage, pumped-storage hydropower, hydrogen production, load flexibility, and more.
In fact, some researchers have a vision of a future where floating wind turbines in inaccessible and remote places, like the Arctic, that just produce hydrogen or ammonia which then can be picked up by ships and transported to land.
Another interesting area of research is how to best digitalize the systems to help predict the condition of equipment and more. Plans are also afoot to be able to create digital models of the farms to enable operators to zoom in on particular components to check their condition.
Machine learning will be key to this kind of development, Tande said; especially to help produce rapid and reactive models.
All things considered, the future of offshore wind power is looking very bright indeed. While there are some technical challenges to overcome to drive the cost down, it is only a matter of time before the bold targets set by the countries are met with victory.