Glistening under the bright rays of the sun, rows of spinning white turbines dot both sides of the National Highway 44 in Kanyakumari district of Tamil Nadu. Located in one of the highest wind potential regions near the southern-most tip of the sub-continent, some of these wind turbines are also among country’s oldest, and are now nearing their end-of-life.
“The first of these wind mills was set up here in 1994. At that time, these 250 KW missions could generate up to 4.5 lakh units per annum. In the last 28 years, their capacity has plummeted down to 2.5 lakh units per annum, mainly due to aging,” says SV Subbaraj, AGM at the Ramco Wind Farms located in Muppandal, where winds from the Arabian Sea gusts through the mountain passes rotating the blades.
The village in Kanyakumari district houses country’s largest operational onshore wind farm generating capacity of 1,500 MW, and is integral to the overall installed capacity of 9,600 MW of Tamil Nadu. The state is the hub of wind energy in India and significant if it is to meet its 2030 renewable energy target of 500 GW — announced by Prime Minister Narendra Modi in Glasgow last year. And a key part of this target has been to push for repowering of the old missions which have outrun their design life with high capacity missions — a transition also necessitated by a government policy for repowering of wind projects released in 2016.
DECISION TO REPOWER
An average lifespan of a wind turbine is 25 years, after which its efficiency begins to reduce. But with most of these old turbines still operational in the most wind-rich sites, also referred to as Class I sites, it has left little space for the high-capacity new missions to be set up, which have to be now moved to less windy sites. The government estimates suggest more than 10GW of old wind turbines carrying less than 1 MW capacity are installed in wind-rich class-1 sites across states.
“When it comes to clean energy, we must look at the bouquet of solutions, and we might lose out if we don’t capitalise on the benefits of repowering,” says Deepak Sriram Krishnan, Associate Director for Energy Program of World Resources Institute (WRI), India. “Tamil Nadu has been a pioneer in the wind energy installations, and it is in the best interest of the state to get cracking on repowering of existing missions, provided there is accommodation of additional capacity and due price for it.”
OLD VS NEW
The oldest wind turbines date back to 1990s, when small capacity missions of 250 KW were set up at some of the best windy sites. Their towers were not more than 25-30 metres high – much less than the current 100-metre-high towers, which produce anywhere between 1-3 MW of power. The diameter of old turbines was also roughly 26 metres compared to roughly 86 metres of a 1.65 MW mission currently in use.
Some of the developers have also decided to re-engineer a few of the existing missions on trial basis and doing the research and development (R&D) work. “One of the missions towering over the palm trees is about 114-metre-tall and generates 950 KW power. As the height was increased, the efficiency has also gone up,” says Subbaraj. “If we put high capacity missions in this class one site, then we can perhaps become the number one site in the whole world.”
Repowering – or replacing the existing old missions with the latest ones is the government goal for better utilisation of wind resources – crucial if India has to meet its RE goals. The plan, however, is yet to take off, with the existing transmission capacity unable to support the increased energy output and lack of infrastructure to handle higher load. Wind mill owners further rue lack of incentives amid high cost involved in the transition.
With higher wind speed at nearly 100 metres above the surface, increasing the height of the existing missions is another way. But that requires more land, considering each turbine should be located at a distance which is five times (or seven times) the diameter of the turbine (5D-7D concept). As of today, a 1.65 MW mission requires about 2.5 acres of land.
Another challenge is the impact on neighbouring wind mills — a phenomenon called the wake effect — the tendency of the wind to slow down, lose its energy and become turbulent once it passes from one turbine to another behind it. According to developers, the cost of repowering — replacing an existing mission with a high-capacity mission is a costly venture, with an amount roughly equal to what’s required to increase the height of the mission.
With further concerns over climate change, Francis Jayasurya, Director-India, Global Wind Energy Council, however, says it’s important that government reassess the wind speed across different regions of the state, and determines wind potential which can guide the new developers. “If they can do the wind mapping, and publish the data, it can guide future missions,” he added. “And, for repowering, we need to be prepared on all four scales – from land, technology, grid as well as the economy.”
CONCERNS OVER CLIMATE CHANGE
Favoured by the seasonal monsoon winds, the turbines here generate a speed of 32 m/second during the peak season which lasts from May to September when 80% of the wind generation is recorded. While a minimal 20% generation is recorded during October-March when wind speed drops.
While the developers rule out any significant change in wind patterns over the years, there continues concerns over fluctuating monsoonal winds, its accurate forecasting and any long-term impacts of climate change in near future. This is important as 80-90% of wind generation happens during the monsoon months.
The wind mill owners are dealing with changes in wind variability with wind mills carrying pitch control, where the latter enables the blades to move a few degrees every time the wind changes in order to keep the rotor blades at the optimum angle in order to maximise output for all wind speeds.
According to meteorologists, the wind speed does not undergo much variation at least till 100 metres above the ground surface, which is where the wind mills tend to operate, and variability in wind speed is mostly believed to be due to local factors like heating of land, development of low pressure areas, they add. But, uncertainties due to climate change need to be accounted for in long-term stability of these projects.
With the highest installed capacity among all states, Tamil Nadu is currently at the forefront of India’s clean energy transition and could lead the way for India to meet its 2030 renewable energy goals. The progress, however, depends on how it capitalises on the opportunities before it, removes the bottlenecks in repowering which can pave the way for a clean energy future, and help fetch the government its target of 60 GW of wind energy.