The Rs 12-cr units will have a range of a km
Experts from the Indian Institute of Technology (IIT)-Bombay and Delhi are giving final touches to a proposal they will submit to the Centre on the installation of air purifiers in the Capital. The proposal is a follow up of the Supreme Court’s recent directive where it asked the government authorities to install anti-smog towers in Delhi, on the lines of that built in China to combat toxic air. The proposed air purifier will be 20 metres tall and will use a large-scale throughput filtration system that will clean the polluted air and release it back into the atmosphere. The units will suck polluted air, pass it through the filter system, which will separate the particles, and release the clean air. Each tower will cover up to a kilometre. The pollutant particles will be collected stage-wise using filters that would be efficient to clean even particles as small as few nanometres or micrometres. “These are tested across five nanometres to beyond 10micrometres. Filters will be arranged to increase the operational surface area, to achieve high efficiency and output of the air so that it starts operating in such a complex system as the atmosphere. That is a challenge for us. The basic principles are already well known. They are routinely used in indoor air cleaners. Here the complexity is the scale and specificity and meteorological conditions of the atmosphere,” said one of the team members on anonymity. The Rs 12-cr purifiers will be designed as a downdraft unit, which means cleaning would be more in the downstream direction where the wind flows. The units will be designed to release 75 million meter cubic of clean air per day. “This is called the clean air delivery rate (CADR), which is the engineering measure of cleaning unit. This means that this much clean air is released into the atmosphere,” he said. China installed a 100-meter high air purifying tower at XI’ian in Shaanxi province last year, in collaboration with the University of Minnesota. According to the media reports, the tower made substantial improvements to Xi’ian’s air quality. Indian scientists, however, are looking at a different model. “China built a very large tower which sucks air from the bottom and releases on top. We found that its efficiency is not good. Their CADR is also five to six times higher to ensure the clean air reaches 10 km. We realised that is not correct in a populous city like Delhi. So, in our design, air will be drawn from the top and released just three to four metres above the ground,” he explained. Experts involved in the project are not sure how effective the anti-smog towers will be as once released, the clean air stands the chance of getting polluted again. They say it will at best be effective locally. “We can say that it will halve the severity. It won’t be 100 per cent cleaning. Suppose the concentration of PM2.5 is 100 microgramme per cubic metre (ug/m3). So the tower will bring it down to 50 ugm3. However, these are all modelling calculations right now, numbers that we will have to measure and then we will know,” he said. Currently, there is no data on the effectiveness of the project in China. “They have not published the measurements. There is only computational data,” he added. A lot would depend on meteorological conditions such as wind speed and direction. Clean air would be released in all directions but the natural wind will try to push it in one direction, so the clean air would be seen moving towards the wind flow direction. Suppose the wind flows from north to south, then it will be cleaner in the southern direction of the tower as opposed to other directions. “It is not exactly the solution. It’s a clean air zone. It’s a local severity-mitigation technology,” he said. Another team member said that once the numbers of their prototypes are out, they will decide how many need to be installed and where. “We can then decide whether we want it for severe air pollution days, or for sensitive areas like hospitals or schools or at pollution hotspots,” he said. He adds that the team will take 8-10 months to make the first unit after it receives the funds. “We are hoping that by next winter season, when pollution levels peak again, the units would be built and we would have some substantial data,” he said.
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