There are many ways to maximise wind power generation at a particular site. One of the prime solutions is bigger rotors and blades, which can cover a wider area. The benefits of using longer blades are twofold. One, these can tap the wind speeds at higher altitudes. Two, large rotor diameters can help in harnessing more wind energy.
In European countries, blades of up to 88.4 metres (almost as long as a football field) have been deployed. The largest blades deployed in the US measure around 67 metres. With technological improvements, some wind turbine developers are manufacturing turbines with longer blades that will reportedly increase the capacity utilisation factor (CUF) to over 63 per cent. GE has plans to manufacture wind turbine generators (WTGs) with blade sizes up to 107 metres. In India, Suzlon’s prototype S128 turbine has the largest blades measuring 63 metres. However, the increase in blade sizes comes with its own challenges.
Engineering and logistical constraints
The biggest challenge with longer blades is associated with manufacturing. Most blades tend to bend and flex when faced with higher wind speeds, hence the turbines are at the risk of collapse. Thus, engineers need to find designs and materials that can withstand the stresses that come with high wind speeds. Moreover, in order to ensure the reliability of large and complex blades, high quality standards need to be maintained. This process is costly and labour intensive.
Bigger rotors are used for larger blades. This poses another engineering challenge. The increase in rotor size adds to the aerodynamic and gravity loads. In this scenario, increasing the rotor size to a point that it maximises the CUF while limiting its impact on other components in the turbine is a challenge.
The growing logistical constraints are an impediment in the transportation of large blades. Some blades have become too large for the existing transportation infrastructure to accommodate. The logistical challenges include limited road and rail connectivity, low highway underpass heights, fewer trucks for carrying large blades, and manoeuvring trucks at sharp turns. These constraints can reduce the number of developable sites and increase the cost of setting up plants at predetermined sites, making them economically unviable.
A recent study by DNV GL, “R&D Pathways for Supersized Wind Turbine Blades”, evaluates the various solutions for dealing with logistical constraints. It concludes that the use of lighter-than-air cargo airships and bending of blades to navigate curves in rail transport may become the most preferred solutions for the delivery of large blades to project sites at a neutral or reduced overall levellised cost of energy (LCoE). However, as lighter-than-air airships are not yet commercially available and rail transport with blade bending is also not tested, these options will not find greater uptake in the near future.
On-site assembly of blade pieces is the most promising as turbine manufacturers already have some experience with this solution. However, it increases the LCoE and require cost reductions in other aspects of the wind systems. On-site blade manufacturing is another solution. While it makes transportation costs negligible, it will no doubt be very expensive. This study was undertaken for the US wind energy segment, but the research results are applicable for the Indian industry as well, as it will also be introducing WTGs with longer blades going forward.
Biodiversity and man-made challenges
As the size of blades increases, more instances of bird deaths will be reported. To mitigate this issue, the industry, government, environmentalists and researchers need to work together. Some wind developers are taking initiatives to organise field studies with independent researchers to gauge the extent of risk. Recently, experts from the Bombay Natural History Society and Salim Ali Centre for Ornithology and Natural History carried out a thorough study to investigate bird mortalities in wind farms in Kutch, Gujarat, and Davangere, Karnataka. The study reported 47 bird deaths from 11 different species in Kutch and seven deaths from three different species in Davangere. The proximity of the wind plants to four biodiversity parks is a key reason for higher death tolls in the Kutch region. In general, WTGs and blades could be painted to increase their visibility and avoid bird collision. Some believe that the use of larger turbines and blades could spoil the natural landscape. This is a major concern in Western countries. In India, commercial aspects and efficiency gains will continue to be favoured over aesthetics.
To address public concerns, the wind industry can continue to engage with communities and policymakers at the local and state levels to build trust and generate acceptance. A major concern is that the deployment of blades with large rotor diameters will require amendments to the regulations concerning airspace. Further, some offshore and onshore wind projects set up near international boundaries (especially for India’s western neighbours) could be stalled due to national security concerns. To this end, some of the overcautious regulations can be simplified with coordinated effort among concerned agencies, which will be the toughest challenge.
Innovations under way
The industry at large is undeterred by the challenges. It is already investing in new technologies that are being deployed from the manufacturing stage to the project commissioning stage. At the manufacturing stage, the increase in the size of blades has brought with it a need for stronger composite materials. Therefore, new composite materials are currently being deployed that will increase the lifespan of a turbine blade, improve the manufacturing process, and contribute to the overall efficiency of turbine systems. In addition, new heating techniques are being developed that will work well with the new composite material. Moreover, focused research and development is being undertaken to reduce the time taken to manufacture a single turbine blade by almost 40 per cent, that is, from 38 hours to 24 hours. Further, advanced carts and material handling systems are being designed to rotate a blade 270 degrees, which reduces the number of times blades must be moved throughout the manufacturing process. Meanwhile, new equipment handling systems are being designed to aid the installation of larger blades. Further, 3D-projected blueprints will be used in the assembly of large blades in order to reduce the amount of time taken by workers to complete the job.
The innovations undertaken with respect to longer blades are well appreciated, particularly in India, where old WTGs need to be repowered. Plants commissioned in the early 1990s will near their end-of-life in a few years. About 19,500 old WTGs with less than 1 MW capacity rating have a repowering potential of 10.5 GW. This statistic is sufficient to drive developers to install new variants of WTGs with longer blades and 2-3 MW of capacity rating.
By Sarthak Takyar