India has achieved a cumulative installed renewable energy capacity of 106.37 GW as of March 31, 2022. Of this, wind energy accounts for about 40.12 GW, contributing to 37 per cent of the total renewable energy generation capacity in the country. Although wind energy holds a considerable share of the total energy generation, the total potential capacity is much higher that what we have achieved. According to the National Institute of Wind Energy (NIWE), the total wind energy potential is around 302 GW at 100 metre hub height and 695.5 GW at 120 metre hub height. After concluding a high-level study on the estimated potential of wind energy in the country, the Ministry of New and Renewable Energy (MNRE), in 2018, set a target of installing 60 GW of wind capacity by 2022 and 140 GW by 2030. The target set by the Government of India looks ambitious. However, the overall progress has been slow considering the yearly variation in installed capacity post 2018, despite the sector receiving significant investment in the windy states.
One of the key solutions to sluggish growth is repowering. Over the years, the wind market has witnessed a significant improvement in design features. Wind turbines of 225-500 kW unit size were the preferred choice in the 1990s. Today, the most popular wind turbine unit size in India is 2.5-3.5 MW and the hub height of wind turbines, which was initially 26 metres, has increased to 120-150 metres today.
The country has enormous possibilities for repowering with an estimated potential of the order of 10 GW. There are many old sites operating at 500-800 kW capacity in states such as Tamil Nadu, Gujarat, Madhya Pradesh, and Maharashtra. Under these circumstances, repowering may be a preferred choice, as most of the older, low rated capacity turbines are operating at Class I sites. These are high potential sites that remain underutilised. Therefore, it is natural to explore the option of replacing old units with modern high capacity wind turbine technology that could offer better returns and more power output.
Tamil Nadu is home to about 25 per cent of India’s total installed wind power capacity of 40.12 GW. Approximately 25 per cent of turbines in India are rated < 500 kW with Tamil Nadu being the state with the highest number of old generation turbines. The state has the best wind energy sites such as Muppandal, Tirunelveli, Chittipalayam, Kethanur, Gudimangalam, Poolavadi, Mrungappatti, Sunkaramudaku, Kongal Nagaram, Gomangalam and Anthir. In Muppandal, the first private sector wind farm was set up in 1992 with two wind turbine generators (WTGs) of 250 kW each. The rating of WTGs in the wind pocket is 220-500 kW. The repowering of wind farms offers several benefits such as efficient utilisation of potential wind sites to deliver higher quantum of energy, improved plant load factors (PLFs), higher efficiency, better project economics, better land utilisation per megawatt of installed capacity, lower operations and maintenance costs and better integration with the grid.
In a preliminary research analysis carried out for the assessment of the repowering potential, Muppandal in the Kanyakumari district of Tamil Nadu was taken as a suitable example to understand how repowering could add to the existing installed capacity, while considering the trade-off between the performance of wind turbines and land utilisation per megawatt of capacity installed. Muppandal is regarded as one of the oldest wind pockets of the country as it uses first-generation WTGs, which have completed 20 years.
The wind pocket, which is considered a suitable example, has a total area coverage of 9.1 square km, with an installed capacity of 55.4 MW. Vestas RRB has the highest capacity of wind turbines in the region. The represented wind pocket is characterised by small to moderate complexity with non-contiguous land. The elevation is 70-100 metres above the mean sea level with an average wind speed of 7.5-8 metres per second at 100 metre height. The potential capacity of the area of interest was assessed considering the trade-off between the performance of wind turbines and land utilisation per megawatt of capacity installed. The GE 2.7 MW-132-50 Hz WTG model was selected as it represents the dominant wind technology at present and is an appropriate WTG class suited for the site. This WTG model is a mainstream IEC-61400 IIIA fully certified turbine. The wind turbine has a rotor diameter of 132 metres, and the hub height of the tower is 130 metres with a rated power capacity of 2,700 kW.
The potential capacity was assessed considering a spacing criterion of 5D x 3D (where D is the rotor diameter) between the turbines as per IEC standards and existing terrain. A machine modelling wind flow was created to arrive at an optimal layout of the wind farm. The model optimised the positioning of WTGs considering the performance of wind turbines, array efficiency and land utilisation per megawatt of capacity installed. The optimal layout of the wind farm was prepared keeping in view that one machine does not cause strong interference with other machines in the vicinity. Using this criterion, the capacity per square km was estimated at 10 MW. The total potential capacity in the area of interest increased from 55.4 MW to 94.5 MW, with an indicative net generation PLF of 42.5 per cent at P75 level of confidence, considering standard losses of machine and grid availability and transmission losses. The various uncertainties drawn in the wind flow model pertaining to data modelling, data use and terrain are assessed. The total uncertainty is applied to the energy yield for estimating PLFs at 75 per cent probabilities of exceedance. The research further indicates that if the same area of interest is repowered, the potential installed capacity will increase 1.7x fold, the number of turbines will decrease by 84 per cent, with an increased energy yield production of approximately 50 per cent. The reduction in the number of turbines will result in efficient cost of operation and maintainence. This depicts the efficient utilisation of the potential wind pocket. It is important to note that there is no benchmark in the capacity estimation of a repowered project. For every wind pocket across the country, it may come out different. The estimated potential capacity is a factor of free long-term wind speed prevailing at the site, terrain characteristics, and the least interference value achieved between one wind turbine and other machines in the vicinity.
While repowering manifests better economic incentives, it also entails multiple challenges/barriers, which may affect the project return calculations, specially looking at the current reverse bidding processes. One of the challenges pertain to new PPAs. In old wind farms, there used to be multiple owners in a single wind pocket, and the entire wind pocket was operated by different agencies and OEMs. Another primary barrier to re-powering is the general lack of economic incentives to replace the older WTGs. To compensate for the additional cost of repowering, appropriate incentives are necessary. Grid augmentation is another barrier considering congestion of grid lines at the pass. The MNRE needs to intervene aggressively with an appropriate policy that could cover up the additional project cost, perhaps on the lines of the generation-based incentive scheme introduced some years ago. Such additional funds even for a period of five years will be helpful in meeting the initial interest portion of the debt obtained.