Wind energy is the oldest and technologically most established renewable source of energy. While landbased windfarms are widespread, the market for offshore windfarms have seen a rise attributable to the stronger and more stable wind resource at sea, and the size of the current generation turbines that inhibit land-based transportation, causing concerns of visual and audible nuisance. To ensure continued development at current sites and to open up new markets, evolution of technologies for installation in deeper waters has become necessary.
Bottom-fixed foundations inherently have some limited feasibility with increase in depth of water, resulting in the emergence of floating foundations. With more than 30 concepts under development worldwide, Europe is the leader in technology development, working on about two thirds of concepts, closely followed by Japan and the United States. France seems to be particularly interested in semi-submersibles, while Japan and Norway show specific interest in spar buoys. Tension Leg Platform (TLP) designs have mainly originated in the United States.
The offshore industry relies heavily on conventional seabed-fixed foundations, which are believed to be optimal for shallow waters. In fact, a notable 97% of the installed capacity in Europe is supported by monopiles. But in many cases, fixed foundations are simply not an option. A number of countries, including Europe and many others, have a very limited access to shallow waters. Floating foundations can overcome this barrier, allowing developments to move further away offshore. The opportunity is huge as around 80% of Europe’s offshore wind resources are in waters over 60m deep.
Floating foundations offer several opportunities among renewable sources of energy:
Floating foundations have already been proven in harsh operating environments. The main concepts for offshore wind power are well known in the oil and gas sector where they are deployed commercially at a large scale. By eliminating the depth constraint and easing the set-up, floating foundations could open the way for power generation from deeper waters. There are 1693 wind farm projects worldwide. The overall status of projects and projects under construction have been presented in Fig.2 below.
Platform designs for offshore wind require adaptation to accommodate different dynamic characteristics and a distinct loading pattern. This design has been used to a great extent in fixedbottom foundations, including monopoles, jackets and gravity-base designs.
The three main concepts for floating foundations are spar-buoy, semi-submersible and tension leg platform. Variants of these also exist, including multiple turbines mounted onto a single floating foundation. The first floating windfarm at more than 100 meters water depth, having 30 megawatts (MW) of power generation capacity, started operating off the coast of Scotland during end 2017. Considering the activities in the market, it is presumed that three to five additional foundation designs would have been demonstrated at full scale (2 MW or larger) by 2020.
Out of 168 fully commissioned offshore wind projects presented in Figure 2, 14 have been developed on floating foundation structures. The distribution of fully commissioned floating projects (14) is as follows:
In Japan, two projects (2 MW and 12 MW) have been tested, with results showing that only 35% of the capacity has been achieved. Efforts are in progress to maximize the capacities.
France has 3 fully commissioned projects, and is making arrangements such as anchoring and connecting the prototype to network array cable and connection box installations have been completed. However, the projects are getting delayed on account of public opposition.
There are two on-going projects in the United Kingdom: a tender has been issued seeking the supply of a floating LiDAR for providing wind resource data at the Wave Hub offshore site in Cornwall. The second project, having an onshore substation in Peterhead, Scotland, is called Hywind. This has been provided with a Spar Floater (30MW).
Tianjin University in China is continuously working on this technology. It has 25 patents and is focusing on balancing ballast tanks.
Patent No. CN107120234 from Dalian University of Technology discloses an offshore floating double-rotor vertical axis wind powergeneration platform (See Figure 4). The platform comprises two vertical axis wind power generation systems, a floating platform and a mooring system.
Patent application no. CN206801782U from Wuhan University deals with a storm sea-floating type combined generating device with wind power generating device and photovoltaic power plants (See Figure 5). The wind-driven generator through supporting device is set on the main floating body and the photovoltaic power plant is arranged on the auxiliary floating body. When the wind speed is greater than the safe wind speed, the wind power plant stops functioning and the photovoltaic panels will close down. When the wind speed is lower than the safe wind speed, the floating type photovoltaic generating device expands outwards for photovoltaic power generation.
Patent No. FR3048409 from IFP Energies Nouvelles deals with stabilizing a system subjected to external stresses. The stabilization system comprises at least three liquid stores and connecting tubes. Liquid reserves are spatially distributed. The connecting tubes ensure the circulation of the liquid between all liquid reserves (See figure 6). Thus, the liquid can move in all directions, to dampen excitations, regardless of the direction of the swell . Stability against fluctuations and strength of the floating body are the major challenges in this technology which are being addressed. Issues related to stability of the power generation capacity during different climatic conditions, and wind direction fluctuations are aspects that have already been addressed.
Self-balancing, water level adjustment in ballast tanks and changing the direction of fan with respect to wind direction are some of the advancements in this technology. With the advancement of technology, aqua culture cages, hybrid energy systems, etc. have been developed by scientists to extract ocean current energy during the wind-off periods. Current research areas include reduced maintenance of support structures for vibration damping, weather alarming systems, self-protection systems, against weather conditions, etc. As deeper waters are explored for installation of wind farms, the size of the installation is increasing and the transportation difficulties of big installations (size of the vessel, handling of the structure) are emerging as major issues.
Global offshore wind investment had risen to record levels of USD 27.6 billion in 2016, but fell to USD 18.9 billion in 2017, and is projected at USD 15.1 billion in 2018, reflecting the ebb and flow of project and policy cycles as well as falling costs.
Given Europe’s commanding lead in offshore wind development, the continent has accounted for the preponderance of investments to date. The USD 8.6 billion spent in 2017 was down from a record USD 20.82 billion in the previous year. China is picking up its pace; 13 projects with a capacity of 3.7 gigawatts (GW) may require USD 10.8 billion. Globally, investments in offshore wind are set to grow substantially over the next several years.
In many industry segments, investors have put their money in start-ups to innovate technologies and make the innovations market-ready. However, this is not the case in the energy sector where there are obstacles for start-ups in operating real assets, such as energy storage or renewable energy. This is mainly due to these areas being capex-intensive from day 1 of their development. A wind start-up for instance needs to pay for development studies, land leases, engineering, equipment and many more.
However, with the federal regulations governing offshore windfarms in place for many counties, there is a boost in the pace of activity in this sector. This has enabled entry of startups such as Deepwater Wind (acquired by Ørsted in 2018), Ocean Energy Company and Kite Power Systems (inventors: Shell, E.On and Schlumberger Limited).
Investment in the energy sector at a commercial scale involves large capital and considerable amount of time. Though pilot projects demonstrate the technical aspects, there are still barriers for commercialization. Existing farms are developed by multinational corporations, whereas small and innovative companies face lack of resources to enter the market with their products. Also, for the opportunities for acquisition, they are not able to prove the maturity of their designs and plans.
Cost (both installation & operational) and maintenance problems are two demanding issues with floating turbines. As the investment and operating costs are more, it takes years to reach profitability, eventually reducing the interest of investors. Environmental issues are other major challenges. The large sweeping area of the turbine’s blades is creating a potential hazard for birds as well as aquatic life. This is inhibiting the installation of windfarms in the areas known for large bird migrations. This problem is very prevalent in Norway, as coasts in Norway have many bird-migration areas.
Although technology commercialization is lagging behind research, innovations are happening right from the construction of foundations to reduction in maintenance. Commercialization of floating windfarms is anticipated between 2020 and 2025. The first full-scale prototype for floating wind turbines has been in operation for several years. Demonstration continues for new floating foundation concepts such as sparsemi sub. Experts expect offshore wind to grow by more than 20 percent year on year, and floating windfarms will open up new growth opportunities.
Offshore windfarm power price is between USD 0.06 and 0.10/KWh for the contract awarded in 2018, which is a significant decline compared to USD 0.14/KWh in 2017. On the other hand, Hywind’s (first floating wind farm in operation) developers are targeting a power price of USD 0.05 to 0.07/kWh by 2030.
Floating turbines are set to overtake all other windfarms in power generation. With increased investments in R&D on floating windfarms, benefits such as reduced cost and improved efficiency will be realized.