Here are some of the key design criteria of wind turbines designed for low wind speed sites and technology options available to improve energy capture on wind farms.
Majority of the sites in India have an average wind speed of 7 m/s or lower at standard hub height of 80 metres. This poses challenges in terms of extracting maximum energy from such sites.
Power extracted from wind can be described in terms of air density, wind speed, rotor diameter and turbine efficiency.
Where the density of the air, D is rotor diameter, Cp is the power coefficient and V is the wind speed. Air density depends on the altitude and temperature of the project site. Higher wind power can be captured by improving the power coefficient, by increasing rotor diameter and by ´increasing the wind speed´.
While significant research has gone into improving the power coefficient (Cp), it has a theoretical limit of 0.593. Most of the modern wind turbines have a Cp of up to 0.52. Wind turbine blades have undergone major developments in order to achieve a high Cp. Wind turbine blade design was originally based on airfoils used in aircraft industries and was not optimised for higher angle of attack frequently employed by a wind turbine blade. Today manufacturers are using airfoils optimised for higher angles of attack.
Increasing rotor diameter is a key factor to capture more energy especially at low wind speeds. Each one metre increase in blade length will allow the wind turbine to capture approximately 2 per cent more energy. Increasing rotor diameter poses certain challenges to the wind turbine design. Total rotor mass increases significantly as rotor diameter increases. This increase in blade weight is approximately 2-4 tonnes for every five metres of increase in blade length.
The additional blade weight can be managed using controls strategies which will help to mitigate increased gravitational and inertial loads. Most of the modern wind turbines today use advanced controls technologies which reduce rotor loads on wind turbines. In addition, new lightweight materials for blade technologies such as carbon can also be used to reduce blade weight.
FUTURE TRENDS IN WIND GENERATION IN INDIA
India has an installed wind generation capacity in excess of 20 GW. India´s wind energy potential, which has been estimated by various organisations, is at least 100 GW. Majority of the undeveloped sites in Indian States are low wind speed sites with the average mean wind speed between 4.5-7.5 m/s. Many of these sites also pose challenges in terms of power evacuation infrastructure and smooth transportation of large wind turbine components.
As the penetration of wind power grows, the grid will also need to be stabilised through forecasting and scheduling techniques. As the generation-based policies get established, there will be added focus on performance and reliability of wind turbine assets. We have already addressed the low wind speed turbine development (including performance and reliability) earlier in the article. Technology development would also be critical to tackle some of the other challenges, thereby allowing the Indian wind industry to achieve its true potential.
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