For instance, in the National Capital Region (NCR) centred on Delhi, high pollution levels have hit the efficiency of solar rooftop projects to the extent of 10%, the experts said.

Delhi is among the most polluted cities in the world. The World Health Organization ranked Delhi as the most polluted city in the world in 2014. In 2016, it ranked 11.

“Ninety per cent of our projects are in the metros, the top eight cities of India... They are usually either in centre of city or in industrial areas. And we have already seen, particularly in NCR, that whenever we set up a project we do a generation forecast based on 25 years of meteorological data," said Andrew Hines, co-founder of CleanMax Solar, an on-site rooftop solar power developer. “We have already seen in NCR that the plant underperforms—radiation is consistently lower than what historical average suggests."

There is no explanation other than pollution, he added. “Certainly if you have a look at pollution today or in the last few years versus last 25 years, that’s the biggest factor. It’s already driving generation numbers down in cities. Rural areas’ pollution may not be as much of a factor although pollution doesn’t stay contained. So if it is in NCR, it will be there in neighbouring states," said Hines.

The loss on account of pollution assumes significance as the average efficiency of a solar panel is usually only around 16-22% of total capacity.

Experts said solar power generation is impacted by the dimming effect, a phenomenon wherein the amount of solar radiation reaching the earth’s surface decreases due to the presence of pollutants in the air that absorb solar radiation and reflect it back into space.

“Dimming effect is well established and because of that the solar panels are bound to receive less solar radiation and that ultimately results in those panels generating less energy. If we can eliminate such particles from atmosphere, the efficiency will definitely improve," said Sumit Sharma, a fellow at the Energy and Resources Institute, a Delhi-based think tank working on environmental issues.

The annual mean of particulate matter (PM) under 2.5 micrograms found in every cubic metre of air or PM 2.5 in the data released in 2014 for Delhi was 153 ug/m3. It declined to 122 ug/m3 in 2016.

“In the urban and industrial areas, we clearly see more particulate matter that impacts the generation capacity of solar panels due to soiling. What it means for us is frequent cleaning and possibly longer-term impact on the module health," said Sanjeev Aggarwal, managing director and chief executive of Amplus Energy Solutions Pvt. Ltd, which sets up solar rooftop projects.

This is in the backdrop of solar power projects being bid out at record low tariffs.

“It (efficiency hit) can be as high as 10% in NCR…10% may not sound like a big number but it is a big number from an investment perspective. So if your solar plant is underperforming by 10% from what you forecast, that is very significant... Normally you would see solar power plants fluctuating by +-5% due to natural weather fluctuations but if there is a structural 10% drop across the 25-year life of the project, then that’s a major thing especially if very aggressive bids are going on," said Hines.

Queries emailed to spokespersons of the ministries of environment, forest and climate change; and new and renewable energy remained unanswered at the time of going to press.

Driven by the government’s ambitions for a green economy, India’s solar power generation capacity has more than tripled to over 12,228 megawatts (MW) as of 31 March 2017 from 2,650MW in May 2014.

mayank.a@livemint.com

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